computer integraged manufacturing.pdf

COMPUTER INTEGRATED

MANUFACTURING

COMPUTER INTEGRATED

MANUFACTURING

Dr. C. ELANCHEZHIANProfessor

Dr. B. VIJAYA RAMNATHProfessor

S. ARUNPRASADAssistant Professor

SRI SAI RAM ENGINEERING COLLEGE,West Tambaram, Chennai - 44

ANURADHA PUBLICATIONSKUMBAKONAM CHENNAI

COMPUTER INTEGRATED MANUFACTURING

© 2011, Anuradha Publications

This book or part thereof cannot betranlsated or reproduced in any formwithout the written permission of theauthor and the publisher.

ISBN :

Price : `***

Head Offi ce

PREFACE

UNIT I: COMPUTER AIDED DESIGN 9Concept of CAD as drafting and designing facility, desirable features of

CAD package, drawing features in CAD - Scaling, rotation, translation, editing, dimensioning, labeling, Zoom, pan, redraw and regenerate, typical CAD command structure, wire frame modeling, surface modeling and solid modeling (concepts only) in relation to popular CAD packages.

UNIT II: COMPONENTS OF CIM 9CIM as a concept and a technology, CASA/SME Model of CIM, CIM II,

benefi ts of CIM, communication matrix in CIM, fundamentals of computer communication in CIM - CIM data transmission methods - seriel, parallel, asynchronous, synchronous, modulation, demodulation, simplex and duplex. Types of communication in CIM - point to point (PTP), star and multiplexing. Computer networking in CIM - the seven layer OSI model, LAN model, MAP model, network topologies - star, ring and bus, advantages of networks in CIM.

UNIT III: GROUP TECHNOLOGY ANDCOMPUTER AIDED PROCESS PLANNING 9

History of Group Technology - role of G.T. in CAD/CAM Integration - part families - classifi cation and coding - DCLASS and MCLASS and/OPTIZ coding systems - facility design using G.T - benefi ts of G.T - cellular manufacturing. Process planning - role of process planning in CAD/CAM Integration - approaches to computer aided process planning - variant approach and generative approaches - CAPP and CMPP systems.

UNIT IV: SHOP FLOOR CONTROL AND INTRODUCTION TO FMS 9Shop fl oor control - phases - factory data collection system - automatic

identifi cation methods - Bar code technology -automated data collection system. FMS - components of FMS -types - FMS workstation - material handling and storage system - FMS layout - computer control systems - applications and benefi ts.

UNIT V: COMPUTER AIDED PLANNING ANDCONTROL AND COMPUTER MONITORING 9

Production planning and control - cost planning and control - Inventory Management - material requirements planning (MRP) - shop fl oor control. Lean and Agile Manufacturing. Types of production monitoring systems - structure model of manufacturing - process control and strategies - direct digital control.

Total = 45 Periods

SYLLABUS

Unit - I : INTRODUCTION

1.1 Concept of CAD as drafting and Design Facility ........................................1.1

1.1.1 CAD Defi nition ..................................................................................1.2

1.1.2 Design Process ...................................................................................1.2

1.2 Desirable Features of CAD package ...........................................................1.2

1.2.1 Application software ..........................................................................1.2

1.2.2 AutoCAD ...........................................................................................1.3

1.2.3 Features of AutoCAD.........................................................................1.3

1.2.4 AutoCAM...........................................................................................1.4

1.2.5 Features of AutoCAM system ............................................................1.4

1.3 Drawing features in CAD ............................................................................1.5

1.3.1 Two Dimensional Transformation ...................................................1.10

1.3.2 Basic Modelling Transformations ....................................................1.12

1.4 Typical CAD command structure ..............................................................1.17

1.5 Wire Frame Modelling ..............................................................................1.17

1.5.1 Wireframe with Linear Edges ..........................................................1.18

1.5.2 Wireframe with Curvilinear Edges ..................................................1.19

Contents

1.5.3 Advantages of wireframe modelling ................................................1.20

1.5.4 Disadvantages of wireframe modelling ...........................................1.20

1.6 Surface Modelling .....................................................................................1.21

1.6.1 Advantages ........................................................................................1.26

1.6.2 Disadvantages ...................................................................................1.26

1.7 Solid Modelling .........................................................................................1.26

1.7.1 Solid Model Construction Techniques ..............................................1.28

1.7.2 Feature-based modelling ...................................................................1.32

1.7.3 Advantages of Solid Modelling ........................................................1.33

1.8 Comparison of Various Modelling ............................................................1.33

Review Questions ...............................................................................................1.34

Unit - II : COMPONENTS OF CIM

2.1 Introduction - Computer integrated manufacturing .....................................2.1

2.1.1 Computer-Integrated Manufacturing (CIM) as a concept and a Technology .........................................................................................2.3

2.2 CASA/SME Model of CIM .........................................................................2.7

2.3 Development of CIM (CIM II) ....................................................................2.8

2.4 Benefi ts of CIM .........................................................................................2.10

2.5 Communication Matrix in CIM .................................................................2.10

2.6 Fundamentals of Computer Communication in CIM ................................2.12

2.7 CIM Data Transmission Method ...............................................................2.13

2.7.1 Serial and Parallel data transmission ................................................2.13

2.7.2 Asynchronous data transmission ......................................................2.14

2.7.2.1 Advantages and Disadvantages ofAsynchronous Transmission ...........................................................2.15

2.7.3 Synchronous Transmission ..............................................................2.15

2.7.3.1 Advantages and Disadvantages ofSynchronous Transmission .............................................................2.15

2.7.4 Pulse Code Modulation and Demodulation .....................................2.16

2.7.5 Type of Communication Systems ....................................................2.17

2.8 Types of communication in CIM ...............................................................2.18

2.8.1 Point-to-Point communications........................................................2.18

2.8.2 Star Network ....................................................................................2.19

2.8.3 Multiplexing .....................................................................................2.20

2.9 Computer Networking in CIM ..................................................................2.22

2.9.1 Principles of Networking .................................................................2.22

2.9.2 Private Computer Communication Networks ..................................2.24

2.9.3 Public Switched Data Networks ......................................................2.24

2.9.4 Local Area Network (LAN) .............................................................2.24

2.9.5 Network Techniques .........................................................................2.25

2.9.6 Components of a Network ...............................................................2.26

2.9.7 Network Wiring Methods .................................................................2.27

2.9.8 Network Topologies .........................................................................2.28

2.10 Open System Inter Connection (OSI) ........................................................2.29

2.10.1 Seven Layers of OSI model ............................................................2.30

2.11 Local Area Network (LAN model) ............................................................2.34

2.11.1 Elements of LAN ..........................................................................2.35

2.11.2 Computer Network Architecture .....................................................2.36

2.12 Manufacturing Automation Protocol (MAP model) ..................................2.41

2.13 advantages of Networking in CIM .............................................................2.42


Unit - III : GROUP TECHNOLOGY AND COMPUTER AIDED PROCESS PLANNING

3.1 Introduction to Group Technology ..............................................................3.1

3.1.1 History of Group Technology ............................................................3.2

3.2 Role of GT in CAD/CAM Integration .........................................................3.2

3.3 Benefi ts of Group Technology .....................................................................3.4

3.4 Part Family ..................................................................................................3.4

3.4.1 Defi nition ...........................................................................................3.4

3.4.2 Types of part family ............................................................................3.4

3.5 Methods of Group Parts into families ..........................................................3.9

3.5.1 Visual Inspection ................................................................................3.9

3.5.2 Production Flow Analysis (PFA) .......................................................3.9

3.5.2.1 Steps to be followed to carry out production fl ow analysis .............3.9

3.5.2.2 Procedure for production fl ow analysis .........................................3.10

3.6 Classifi cation and Coding ..........................................................................3.14

3.6.1 Parts Classifi cation ...........................................................................3.14

3.6.1.1 System based on part design attributes .........................................3.14

3.6.1.2 System based on manufacturing attributes ...................................3.14

3.6.1.3 System based on design and Manufacturing system attributes ....3.15

3.6.2 Coding ..............................................................................................3.15

3.6.2.1 Hierarchical coding .......................................................................3.16

3.6.2.2 Decision tree coding ......................................................................3.17

3.7 Coding System ...........................................................................................3.17

3.7.1 OPTIZ Coding System .....................................................................3.18

3.7.2 Machine Class Coding System .........................................................3.21

3.7.3 K-K-3 System ...................................................................................3.22

3.7.4 Code System .....................................................................................3.23

3.7.5 D-Class System.................................................................................3.24

3.7.6 RNC System .....................................................................................3.25

3.8 Facility Design using Group Technology ..................................................3.26

3.8.1 Machine cell types ............................................................................3.26

3.8.2 Cell Layouts ......................................................................................3.27

3.8.3 Key machine concept ........................................................................3.28

3.9 Advantages of Group Technology .............................................................3.28

3.10 Disadvantages of Group Technology .........................................................3.29

3.11 Cellular Manufacturing ..............................................................................3.29

3.11.1 Defi nition ........................................................................................3.29

3.11.2 Objectives .......................................................................................3.30

3.11.3 Composite Part Concept .................................................................3.30

3.11.4 Machine Cell Design ......................................................................3.32

3.11.4.1 Types of machine cells and layouts ..............................................3.32

3.11.4.2 Factors accounted for cell design ................................................3.36

3.11.4.3 Key machine concept ...................................................................3.37

3.12 Process Planning ........................................................................................3.37

3.12.1 Basic functions of process planning ...............................................3.38

3.13 Computer aided Process Planning (CAPP) ...............................................3.38

3.13.1 Role of process planning in CAD/CAM integration ......................3.39

3.13.2 Approaches to CAPP ......................................................................3.40

3.13.2.1 Manual Approach ........................................................................3.40

3.13.2.2 Variant Approach ........................................................................3.43

3.13.2.3 Generative Approach ...................................................................3.46

3.13.2.4 Hybrid Approach .........................................................................3.50

3.13.3 Product Development through Computer-AidedProcess Planning (CAPP) ...............................................................3.52

3.13.4 Benefi ts of CAPP ............................................................................3.54

3.13.5 Economics of CAPP .......................................................................3.55

3.13.6 CAPP steps used for Machining Operation ...................................3.55

3.13.7 Advantages of CAPP ......................................................................3.56

3.14 Computerized Manufacturing Process Planning (CMPP) .........................3.57

3.15 Computer-Aided Production Management (CAPM) .................................3.58

3.15.1 Bill Of Material (BOM) ..................................................................3.59

3.15.2 Stock Control ..................................................................................3.60

3.15.3 Material Requirement Planning (MRP) ..........................................3.61


Unit - IV : SHOP FLOOR CONTROL AND INTRODUCTION TO FMS

4.1 Introduction - Shop Floor Control ...............................................................4.1

4.1.1 Phases in Shop Floor Control .............................................................4.2

4.1.1.1 Order Release ..................................................................................4.2

4.1.1.2 Order scheduling .............................................................................4.3

4.1.1.3 Order Progress ................................................................................4.5

4.2 Types of Scheduling ....................................................................................4.6

4.2.1 Reasons for shop fl oor scheduling process .........................................4.7

4.2.2 Types of scheduling techniques ..........................................................4.8

4.2.3 Stages in Scheduling ...........................................................................4.9

4.2.4 Types of Loading ..............................................................................4.10

4.2.5 Load Charts and Machine Loading Charts ......................................4.12

4.3 Activities of CIM based SFC .....................................................................4.14

4.4 Factory Data Collection System ................................................................4.15

4.4.1 Types of factory data collection system. ...........................................4.15

4.4.2 Numbers and Arrangement of Keyboard-BasedTerminals Possible in the Factory ....................................................4.19

4.5 Automatic Identifi cation Methods .............................................................4.20

4.5.1 Reasons for using automatic identifi cation techniques .....................4.21

4.5.2 Technology of AIS ............................................................................4.22

4.5.2.1 Bar code .........................................................................................4.22

4.5.2.2 Radio frequency system .................................................................4.22

4.5.2.3 Magnetic stripes ............................................................................4.24

4.5.2.4 Optical Character Recognition (OCR) ..........................................4.24

4.6 Bar Code Technology ................................................................................4.25

4.6.1 Bar Code Symbol ..............................................................................4.26

4.6.2 Bar Code Readers .............................................................................4.29

4.6.2.1 Contact bar code reader ................................................................4.29

4.6.2.2 Non contact bar code reader .........................................................4.30

4.6.3 Bar Code Printers ..............................................................................4.31

4.7 Automated Data Collection System ..........................................................4.31

4.7.1 Data Acquisition System ..................................................................4.32

4.7.2 Data Logger ......................................................................................4.33

4.7.3 Multilevel Scanning ..........................................................................4.34

4.8 Automated Data Collection Technologies .................................................4.35

4.8.1 Bar Code (explained in 4.5.2.1) ........................................................4.35

4.8.2 Optical Character Recognition .........................................................4.35

4.8.3 Machine Vision System ...................................................................4.37

4.8.4 Radio Frequency Identifi cation ........................................................4.39

4.8.5 Magnetic Identifi cation .....................................................................4.40

4.8.6 Voice Technology .............................................................................4.40

4.9 Introduction to Flexible Manufacturing System (FMS) ............................4.41

4.9.1 Types of Flexibility ...........................................................................4.42

4.9.2 Types of FMS ...................................................................................4.45

4.9.3 Components of FMS .........................................................................4.49

4.9.3.1 Workstations ..................................................................................4.49

4.9.3.2 Material Handling and Storage System .........................................4.51

4.9.3.3 Computer Control System ..............................................................4.58

4.9.3.4 Human Resources ..........................................................................4.59

4.9.4 Applications of FMS .........................................................................4.59

4.9.5 Benefi ts of FMS ...............................................................................4.60


Unit - V: COMPUTER AIDED PLANNING AND CONTROL AND COMPUTER MONITORING

5.1 Introduction - Production Planning and Control .........................................5.1

5.1.1 Effects of Production Planning and Control ......................................5.3

5.2 Computer-Integrated Production Management System...............................5.5

5.2.1 Engineering and manufacturing data base ..........................................5.8

5.2.2 Material requirements planning (MRP) ..............................................5.8

5.2.3 Capacity planning ...............................................................................5.9

5.2.4 Inventory management .....................................................................5.11

5.2.5 Shop fl oor control .............................................................................5.11

5.2.6 Cost planning and control .................................................................5.11

5.2.6.1 Cost planning .................................................................................5.12

5.2.6.2 Cost control ...................................................................................5.13

5.3 Production Planning Process .....................................................................5.13

5.3.1 Functions of PPC ..............................................................................5.14

5.4 Material Requirements Planning (MRP) ...................................................5.18

5.4.1 Benefi ts of MRP ...............................................................................5.23

5.5 Shop Floor Control (SFC) System.............................................................5.24

5.5.1 Introduction ......................................................................................5.24

5.5.2 Various Activities of SFC .................................................................5.29

5.5.3 Scheduling Techniques for SFC .......................................................5.29

5.5.4 Scheduling and Controlling Production for deliveryschedules - Line of Balance (LOB) Method ....................................5.37

5.6 Lean and Agile Manufacturing ..................................................................5.45

5.6.1 Lean Production and Waste in Manufacturing .................................5.45

5.6.2 Agile manufacturing .........................................................................5.49

5.6.3 Just-In-Time Approach .....................................................................5.53

5.7 Production Monitoring System ..................................................................5.55

5.7.1 Data logging systems ........................................................................5.56

5.7.2 Data acquisition systems ..................................................................5.57

5.7.3 Multilevel scanning ..........................................................................5.58

5.8 Structural Model of a Manufacturing Process ...........................................5.58

5.9 Process Control Strategies .........................................................................5.60

5.9.1 Feedback control ...............................................................................5.61

5.9.2 Regulatory control ............................................................................5.62

5.9.3 Feedforward control .........................................................................5.62

5.9.4 Preplanned control ............................................................................5.63

5.9.5 Steady-state optimal control .............................................................5.65

5.9.6 Adaptive control ...............................................................................5.66

5.10 Direct Digital Control (DDC) ....................................................................5.69


1.1 CONCEPT OF CAD AS DRAFTING AND DESIGN FACILITY

Computer Aided Design (CAD) is the technology concerned with the integrated

design activities using a digital computer. This includes creation and modifi cation

of graphic images on a video display, printing these images on a printer or plotter

as a hard copy, analyzing and optimizing the design and storing and retrieving of

design information for further process as database.

CAD can be described as any design activity that involves the effective use

of computer to create and modify an engineering design. The use of a computer in

the design of a product is to increase the productivity of the designer and to create

a database for manufacturing. In this unit, we shall study the various activities of

design process with the aid of computer.

1UNIT

Introduction

1.2 COMPUTER INTEGRATED MANUFACTURING

1.1.1 CAD Defi nition

CAD is the term which means Computer Aided Design. CAD can be defi ned the computer is utilized in the creation of model, modifi cation and analysis of a design to get the optimum model.

It requires computer with proper display and input / output devices as hardware. To create and modify the design and to analyze proper software viz., AutoCAD, ProE, Catia, Ansys, Nastran etc. are required.

1.1.2 Design Process

The design process is the pattern of activities that is followed by the designer in arriving at the solution of a technological problem is generated. The design progresses in a step-by-step manner from identifi cation of the problem to give the better solution to the problem.

There are different models are available in the design process. They are shigley, Pahl and Beitz. Ohsuga and Earle.

1.2 DESIRABLE FEATURES OF CAD PACKAGE

1. Managing various fi le manipulation in the computer

2. Loading computer programs into memory and controlling the execution of program.

3. Create environment torun the application softwares.

Windows, OS/2, UNIX, and Linux are some of the wellknown Operating systems

1.2.1 Application software

The application softwares in CAD include the following.

1. Software to create and modify 2D and 3D models of components.

2. Software for engineering analysis in the created model.

INTRODUCTION 1.3

3. Comparability between the softwares.

AutoCAD, Pro-E, IDEAS, UniGrpahics, CADian, Solid works, CAD Key and CATIA are some of the wellknown application softwares used in computer aided design.

1.2.2 AutoCAD

AutoCAD is a drawing software package developed by the company Autodesk Inc., USA. It is one of the most widely uses softwares for creating engineering drawings easily and quickly. The important features of AutoCAD are listed below.

1.2.3 Features of AutoCAD

1. Creating basic geometrical objects line, circle, and rectangle, etc. can be easily drawn.

2. We can easily modify the size, shape, and location of objects by using AutoCAD commands.

3. We can erase, move, and rotate the selected objects.

4. We can create duplicates of objects by using COPY. ARRAY, OFFSET, and MIRROR features.

5. We can change the size of objects by using commands like TRIM, EXTEND, LENGTHEN, STRETCH, SCALE, etc. It is also possible to create FILLET, CHAMFER and BREAK in objects.

6. The Zooming feature enables to magnify the details in a drawing.

7. The Layering feature, various portions of a drawing can be drawn on different layers, which can be superimposed according to the need.

8. Dimensioning of the facility improves the details of the drawing.

9. Hatching feature is used to fi ll area of a drawing with a predefi ned pattern. The pattern is used to differentiate components of an object. It is also possible to create our own hatch patterns.


10. AutoCAD supports 3D modeling such as wireframe, surface, and solid modeling. Each type has its own creation and editing techniques.

11. We can split the drawing area into two or more adjacent rectangular areas and display different view of the model using viewport.

12. The surfaces of 3D models have been viewed with realistic effects. It is also possible to create hidden-line or shaded image of model.

13. Plotting the drawing is very easy.

1.2.4 AutoCAM

A part program is needed to produce a component in a CNC machine. Traditionally, these part programs are written manually by part programmer by using G-codes and M-codes. Nowadays, various graphical user interface (GUI) based systems are available for developing the CNC part programs. In these systems, a dedicated CAM software helps in developing the CNC part programs.

The CAM systems may be linked to any major CAD systems such as AutoCAD, Solid works, CADKey, etc. Typical examples of such CAM systems are MasterCAM, Virtual Gibbs, SmartCAM, SurfCAM, EdgeCAM, AlphaCAM etc. Out of these, MasterCAM is widely used in industry as well as in educational institutions.

1.2.5 Features of AutoCAM system

The features of AutoCAM systems are given below.

1. Geometry of the part can be drawn easily by using the available geometric entities. It is also possible to modify the part as per our requirements. The dimensions and other annotations required for drafting can also be defi ned.

2. Standards are available to convert the CAD database into manufacturing database.

INTRODUCTION 1.5

3. Most AutoCAM system provides a complete cutting tool database for cutting process parameter selection. This helps in customizing the cutting tool database of individual operations to suite their requirements.

4. Tool path modules are available for tool path generation. These modules utilize the geometry of the part and the built in cutting tool database to generate the optimum tool path. The tool path can be verifi ed before fi nishing the part program.

5. Simulation facilities are available to check the generated CNC programs.

Advantages of AutoCAM1. Creation of part program is easy.

2. The time required to create the part program is minimized.

3. The error in part program is minimized.

4. The part program can be easily modifi ed.

5. The overall productivity of CAD/CAM system is increased.

6. To ensure effi cient, reliable and user friendly link between CAD and CAM.

1.3 DRAWING FEATURES IN CAD

Editing CommandsSome of the important editing commands are discussed below:

CHANGE: Alters properties of selected objects

Command: CHANGE (enter)

Select objects or window or Last (select objects to be changed)

Properties/<Change point>: (type P)

Change what property (Color/Elev/LAyer/LType/Thickness)? (type Layer)


New Layer: (enter new layer name and press enter)

ERASE: Erases entities from the drawing.

Command: ERASE (enter)

Select objects or Window or Last: (Select objects to be erased and press enter when fi nished)

EXTEND: Lengthens a line to end precisely at a boundary edge.

Command: Extend (enter)

Select boundary edge(s)...

Select Objects (pick the line which represents the boundary edge which lines will be extended to)

(press enter when fi nished selecting cutting edges)

<Select object to extend>/Undo: (pick the line(s) that need to be extended

TRIM: Trims a line to end precisely at a cutting edge.

Command: Trim (enter)

Select cutting edge(s)...

Select Objects (pick the line which represents the cutting edge of line in which objects will be trimmed to)

(press enter when fi nished selecting cutting edges)

<Select object to trim>/Undo: (pick the line(s) that need to be trimmed)

GRIPS:You can edit selected objects by manipulating grips that appear at defi ning

points on the object. Grips is not a command. To activate grips simply pick the

INTRODUCTION 1.7

object. Small squares will appear at various entity-specifi c positions. By selecting an end grip you can stretch the entity to change its size. By selecting the center grip you can move the entity to a new location. To remove grips press CTL-C twice. You can perform the following using grips: Copy, Multiple Copy, Stretch, Move, Rotate, Scale, and Mirror.

DIMENSIONING:

Dimensioning of an object usually involves four or fi ve stages:

• Select the type of dimensioning

• Select the object

• Choose a position for the dimension

• Decide on the dimension text.(optional)

• Choose a position for the text

You are now going to be shown how to dimension the top line and one of the angles of the second triangle.

LABELLING:

Labelling which specifi es a number of the bars to which it relates is used by Master RC for the production of schedules. The remaining labelling types, that is, those which indicate only which mark a bar is, or where it begins and ends, have no effect on the schedule.

Terminology

In this section the following terms are used:

Distribution lineUsed for groups of bars, running from the start of the fi rst group to the end of

the last group.


LeaderThe line connecting linking the label itself to the distribution line.

Labelling setThe arrows, short bar symbols, distribution lines, blobs, annotation and the

label text itself which make up one complete set of graphics labelling the bar(s). For some types of labelling these are consolidated into a BLOCK.

Label itselfThe text at the end of the label leader (sometimes placed directly on the

distribution line)

Labelling command summaryThere follows a brief summary of the purpose of each labelling command,

after which each is discussed in more detail:

Label Single BarThis is used to label one or more bars of the same mark, represented on the

drawing by a single bar. For example, the longitudinal bars in the elevation of a beam.

ZOOMOne single command will give you the versatility to move around your

drawing. This is the ZOOM

Zoom ExtentsThis option will display all the graphics that are contained in the drawing

(referred to as the drawing extents) with the largest image possible

Zoom WindowThis option (also a ‘hidden’ default) prompts the user to pick two corners of a

box on the existing view in order to enlarge that area to fi ll the display.

INTRODUCTION 1.9

Zoom PreviousThis option restores the displayed view prior to the current one. For the

purpose of this option, up to 10 views are saved so that the last ten views can be recalled. This option includes every time you use the scroll bar, which is one reason to avoid the scroll bars for panning a lot in your drawing.

Zoom RealtimeZoom Realtime provides interactive zooming capability. Pressing <ENTER>

(after entering zoom) on the command line automatically places you in Realtime mode. Hold the left mouse button down at the midpoint of the drawing and move the cursor vertically to the top (positive direction) of the window to zoom in up to 100% (2x magnifi cation). Hold the left mouse button down at the midpoint of the drawing and move the cursor vertically to the bottom (negative direction) of the window to zoom out to 100% (.5x magnifi cation). You cannot zoom out beyond the extents of the current view.

When you release the pick button, zooming stops. You can release the pick button, move the cursor to another location in the drawing, and then press the pick button again and continue zooming from that location. To exit Realtime Zoom mode, press <ENTER> or (ESC).

Zoom AllThis option causes AutoCAD to display the whole drawing as far as its

drawing limits or drawing extents (whichever is the greater of the two).

PAN

Another useful command is PAN. These are both quicker than using the scroll bars on the side of the drawing area, unless you have a very short distance to move your drawing (and can make your scroll bars obsolete and thereby create more drawing space)..


REDRAWWhen BLIPMODE is on, marker blips left by editing commands are removed

from the current viewport.

Before REDRAW After REDRAW

REGENERATE

REGEN regenerates the entire drawing and recomputes the screen coordinates for all objects in the current viewport. It also reindexes the drawing database for optimum display and object selection performance.

1.3.1 Two Dimensional Transformation

Geometric transformations have numerous applications in geometric modeling e.g., manipulation of size, shape, and location of an object. In CAD, transformation is also used to generate surfaces and solids by sweeping curves and surfaces, respectively. The term ‘Sweeping’ refers to parametric transformations, which are utilized to generate surfaces and solids. When we sweep a curve, it is transformed through several positions along or around an axis, generating a surface. The appearance of the generated surface depends on the number of instances of the transformation. A parameter t or s is varied from 0 to 1, with the interval value equal to the fraction of the parameter. For example, to generate 10 instances, the parameter will have a value t/10 or s/10. Ti develop an easier understanding of transformations, we will fi rst study the two-dimensional transformations and the extend it to the study of three-dimensional transformations. Until we get to the discussion of surfaces and solids, we will limit our discussion of transformation to only the simple cases of scaling, translation, rotation, and the combinations of

INTRODUCTION 1.11

these. Applications of transformations will become apparent when we discuss the surface and solid modeling.

There are two types of transformations:

Modeling TransformationIn this transformation alters the coordinate values of the object. Basic

operations are scaling, rotation and, combination of one or more of these basic transformations. Examples of these transformations can be easily found in any commercial scaling, translation, and rotation transformations, respectively.

Visual TransformationIn this transformation there is no change in either the geometry or the

coordinates of the object. A copy of the object is placed at the desired sight, without changing the coordinate values of the object. In Auto CAD, the ZOOM and PAM commands are good example of visual transformation.

1.3.2 Basic Modeling TransformationsThere are three basic modeling transformations: Scaling, Translation, and

Rotation. Other transformations, which are modifi cation or combination of any of the basic transformations, are Shearing, Mirroring, copy, etc.

Let us look at the procedure for carrying out basic transformations, which are based on matrix operation. A transformation can be expressed as

[P*] = [P] [T]

where [P*] is the new coordinates matrix, [P] is the original coordinates matrix, or points matrix, [T] is the transformation matrix

With the x-terms set to zero, the P matrix can be written as,


.

.

.

.

.

.

.

.

.

P

x

x

x

x

y

y

y

y

0

0

0

0n n

1

2

3

1

2

3

=

R

T

SSSSSSSSS

6

V

X

WWWWWWWWW

@

The size of this matrix depends on the geometry of the object, e.g., a point is defi ned by the single set of coordinates (x1, y1, z1), a line is defi ned by two sets of coordinates (x1, y1, z1) and (x2, y2, z2), etc. Thus a point matrix will have the size 1 × 3, line will be 2 × 3, etc.

A transformation matrix is always written as a 4 × 4 matrix, with a basic shape shown below,

T

1

0

0

0

0

1

0

0

0

0

1

0

0

0

0

1

=

R

T

SSSSS

6

V

X

WWWWW

@

Values of the elements in the matrix will change according to the type of transformation being used, as will see shortly. The transformation matrix changes the size, position, and orientation of an object, by mathematically adding, or multiplying its coordinate values. We will now discuss the mathematical procedure for scaling, translation, and rotation transformations.

1.3.2 Basic Modelling Transformations

(a) Scaling

In scaling transformation, the original coordinates of an object are multiplied by the given scale factor. There are two types of scaling transformations: uniform and non-uniform. In the uniform scaling, the coordinate values change uniformly along the x, y, and z coordinates, where as, in non-uniform scaling, the change is not necessarily the same in all the coordinate directions.

INTRODUCTION 1.13

Uniform ScalingFor uniform scaling, the scaling transformation matrix is given as

T

s

s

s

0

0

0

0

0

0

0

0

0

0

0

0

1

=

R

T

SSSSS

6

V

X

WWWWW

@

Here, s is the scale factor.

Non-Uniform ScalingMatrix equation of a non-uniform scaling has the form:

T

s

s

s

0

0

0

0

0

0

0

0

0

0

0

0

1

x

y

z

=

R

T

SSSSS

6

V

X

WWWWW

@

where, sx, sy, sz are the scale factors for the x, y, and z coordinates of object.

(b) Homogeneous CoordinatesBefore proceeding further, we should review the concept of homogeneous

coordinate system. Since the points matrix has three columns of the x, y, and z values, and a transformation matrix is always 4 × 4 matrix, the two matrices are incompatible for multiplication. A matrix multiplication is compatible only if the number of columns in the fi rst matrix equals the number of row in the second matrix. For this reason, a points matrix is written as,

.

.

.

.

.

.

.

.

.

.

.

.

T

x

x

x

x

y

y

y

y

z

z

z

z

1

1

1

1n n n

1

2

3

1

2

3

1

2

3

=

R

T

SSSSSSSSS

6

V

X

WWWWWWWWW

@


Here, we have converted the Cartesian coordinates into homogeneous coordinates by adding a 4th column, with unit value in all rows. When a fourth column, with values of 1 in each row, is added in the points matrix, the matrix multiplication between the [P] and [T] becomes compatible. The values (x1,y1, z1, 1) represent the coordinates of the point (x1, y1, Z1), and the coordinates are called as homogeneous coordinates. In homogeneous coordinates, the points (2,3,1), (4,6,2), (6,9,3), (8,12,4), represent the same point (2, 3, 1), along the plane z = l, z = 2, z = 3 and z = 4, respectively. In our subsequent discussion on transformation, we will use homogeneous coordinates.

Example 1:If the triangle A(l, 1), B(2,1), C(l, 3) is scaled by a factor 2, fi nd the new

coordinates of the triangle.

SolutionWriting the points matrix in homogeneous coordinates, we have

P

1

2

1

1

1

3

0

0

0

1

1

1

=6 >@ H

and the scaling transformation matrix is,

T

2

0

0

0

0

2

0

0

0

0

2

0

0

0

0

2

s =

R

T

SSSSS

6

V

X

WWWWW

@

The new points matrix can be evaluated by the equation

[P*] = [P] [T], and by substitution of the P and T values, we get

P

1

2

1

1

1

3

0

0

0

1

1

1

2

0

0

0

0

2

0

0

0

0

2

0

0

0

0

1

2

4

2

2

2

6

0

0

0

1

1

1

*= =

R

T

SSSSS

> >

V

X

WWWWW

H H

INTRODUCTION 1.15

Figure 1.1: Scaling technique

(c) Translation TransformationIn translation every point on an object translates exactly the same distance.

The effect of a translation transformation is that original coordinate values increase or decrease by the amount of the translation along the x, y, and z-axis. For example, if line A(2,4), B(5,6) is translated 2 units along the positive x axis and 3 units along the positive y axis, then the new coordinates of the line would be.

A’(2 + 2, 4 + 3), B’(5 + 2, 6 + 3) or

A’(4, 7), B’(7, 9)

The transformation matrix has the form:

T

x y

1

0

0

0

1

0

0

0

1

0

0

0

0

1

1 =

R

T

SSSSS

6

V

X

WWWWW

@

where, x and y are the values of translation in the x and y direction, respectively. For translation transformation, the matrix equation is

[P*] = [P] [Tt]

where, [Tt] is the translation transformation matrix.


Example 2:Translate the rectangle (2,2), (2,8), (10,8), (10,2) 2 units along x-axis and 3

units along y-axis.

SolutionUsing the matrix equation for translation, we have

[P*] = [P] [Tt], substituting the numbers, we get

P

x y

2

2

10

10

2

8

8

2

0

0

0

0

1

1

1

1

1

0

0

0

1

0

0

0

1

0

0

0

0

1

4

4

12

12

5

11

11

5

0

0

0

0

1

1

1

1

*= =

R

T

SSSSS

R

T

SSSSS

R

T

SSSSS

6

V

X

WWWWW

V

X

WWWWW

V

X

WWWWW

@

RotationWe will fi rst consider rotation about the z-axis, which passes through the

origin (0,0,0), since it is the simplest transformation for understanding the rotation transformation. Rotation about an arbitrary axis, other than an axis passing through the origin, requires a combination of three or more transformations, as well see later.

When an object is rotated about the z-axis, all the points on the object rotate in a circular arc, and the center of the arc lies at the origin. Similarly, rotation of an object about an arbitrary axis had the same relationship with the axis, i.e., all the points on the object rotate in a circular arc, and the centre of rotation lies at the given point through which the axis is passing.

INTRODUCTION 1.17

1.4 TYPICAL CAD COMMAND STRUCTURE

1.5 WIRE FRAME MODELLING

The word “wireframe” is related to the fact that one may imagine a wire that is bent to follow the object edges to generate the model. Typically, a wireframe model consists entirely of points, lines, arcs and circles, conics, and curves. Wireframe modeling is the most commonly used technique and all commercial CAD/CAM systems are wireframe-based.

The simplicity of the geometrical concepts based on wireframe modeling makes them attractive to use to introduce users to the CAD/CAM fi eld. In addition, at early design stages, designers might just need a sketch pad to try various ideas.


Wireframes are ideal to provide them with such a capability. From an industrial point of view, wireframe models may be suffi cient to many design and manufacturing needs. From a practical point of view, many companies have large amounts of wireframe databases that are worth millions of dollars and man-hours and therefore make it impossible to get rid of wire frame technology. (Refer fi gure 1.2)

Wireframe models are also considered lengthy when it comes to the amount of defi ning data and command sequence required to construct them. For example, compare the creation of a simple box as a wireframe and as a solid. In the latter, the location of one corner, the length, width, and height are the required input while in the former the coordinates of at least four corners of one face, the depth, and the edge connectivity are required, considering the box as a two-and-a-half-dimensional object In other words, both topological and geometrical data are needed to construct wireframe models while solids require only.

From an application, and consequently engineering, point of view, wireframe models are of limited use. Unless the object is two-and-a-half dimensional, volume and mass properties, NC tool path generation, cross-sectioning, and interference detections cannot be calculated. The model can, however, be used in manual fi nite element modeling and tolerance analysis.

Figure 1.2: Displaying holes and curved ends in wire frame models

1.5.1 Wireframe with Linear Edges

Wireframe with linear edges are the simplest type. It consists of straight line edges joining pair of points. For example a tetrahedron consists of four vertex points (P1, P2, P3, P4) with six linear edges (A, B, C, D, E).

INTRODUCTION 1.19

Example:Joining pair of these points as shown in Table 3.1 model of tetrahedron is

formed. The geometry of the tetrahedron is represented by the co-ordinates of its vertices. It is also shown in table 1.1.

Table 1.1: Linear wireframe model of a tetrahedron

Vertex List Edge List EdgeType

P1 (0,0,0) A<P1, P2> linear

P2 (0,0,1) B <P2, P3> linear

P3 (1,0,0) C <P3, P4> linear

P4 (0,1,0) D<P3, P1> linear

E<P1, P4> linear

F<P4, P2> linear

1.5.2 Wireframe with Curvilinear Edges

In wire frame model the curved boundaries are represented by curvilinear edges. Best example is cone. It consists of a single apex point and a circular base. The apex is joined to the base by an infi nite set of straight line edges known as generators. (Refer fi grue 1.3)

Figure 1.3

To under stand this , fi rst the cone is considered that it contains three vertices.


One at the apex (Pj) and another two (P2 and P3) at the base as shown in the table 1.2

Table 1.2: Curvilinear wireframe model of a cone

Vertex List Edge List Edge Type

P1 (0,0, 3) A<P1, P2> linear

P2 (-1,0,0) B<P1, P3> linear

P3 (0, 0, 0) C<P2, P3> semi-circular

D<P2, P3> semi-circular

The straight edges A, B, C, D connect the vertices as shown in the table 1 .2

Like this considering more number of vertices on its base and corresponding edges connecting these base vertices and apex (generators) a wire frame model of cone with realistic look is created.

Like this wireframe model with curved and linear edges are formed.

1.5.3 Advantages of wireframe modelling

1. Simple to construct.

2. Designer needs little training.

3. It needs less memory space.

4. It takes less manipulation time.

5. It is best suitable for manipulations as orthographic, isometric and perspective views.

1.5.4 Disadvantages of wireframe modelling

• These models are usually ambiguous representations of real objects and rely heavily on human interpretation. Models of complex designs having many edges become very confusing and perhaps even impossible to interpret.

INTRODUCTION 1.21

• The lack of visual coherence and information to determine the object profi le. In most systems, the holes are displayed as two parallel circles separated by the hole length.

• Representations of the intersection of plane faces with cylinders and that of cylinders with cylinders, or tangent surfaces in general is usually a problem in wire frame modeling and requires user manipulations.

• Despite its many disadvantages, the major advantages of wireframe modelling are its simplicity to construct. Therefore, it does not require as much computer time and memory as does surface or solid modeling. However, the user or terminal time needed to prepare and/or input data is substantial and increases rapidly with the complexity of the object being modeled. Wireframe modeling is considered a natural extension of traditional methods of drafting.

• The images of wire frame model cause confusion to the viewer.

• Cannot get required information from this model.

• There is ambiguity (doubt) in identifying the surfaces.

• Hidden line removal feature not available.

• Not possible for mass, volume calculations, NC part programming, cross sectioning etc.

1.6 SURFACE MODELLING

A surface model is an object of less ambiguous representation than a wire frame model. Surface models are necessary several areas of mechanical engineering designs,

• Body panels of automobiles

• Aircraft structural members

• Marine vehicles

• Consumer products etc


The surface model is constructed from surfaces as shown fi gure 1.3 (a). The geometric entities used to construct surface model is curves and surfaces. The mathematical techniques available for handling curves are Bezier and of B-splines. The CAD system provide variety of surfaces as explained below.

Flat plane surfaces: It is defi ned in number of ways, such as between parallel lines, through three point or through a line and a point.

Figure 1.3 (a)

Curved surfaced: It includes the following

1. Surfaces fi tted to arrays of data points

2. Surfaces based on curves

a) Tabulated cylinder b) Rules surface

c) Surface of revolutions d) Swept surfaces

3. Sculptured or curve mesh surfaces

Surfaces fi tted to arrays of data pointsThe surface is fi tted on arrays of data points called control points. The surface

is generated either to pass through or to interpolate the points. Bezier type of surface is shown in fi gure 1.3 (b).

INTRODUCTION 1.23

Figure 1.3 (b): Bezier surface

Tabulated Cylinder: It is defi ned as projecting a generating curve along a vector. This surface is shown in fi gure 1.3 (c)

Figure 1.3 (c): Tabulated cylinder

Rules surface: It is produced by linear interpolation between tow different generating or edge curves. This surface is shown in fi gure 1.3 (d).


Figure 1.3(d): Ruled surface

Surface or revolution: It is produced by revolving a generating curve about a center line. It is shown in fi gure 1.3 (e).

Figure 1.3 (e): Surface or revolution

Figure 1.3 (f): Swept surface

INTRODUCTION 1.25

Figure 1.3 (g): Sculptured surface

Swept Surface: It is produced by sweeping the defi ning curve along an arbitrary spine curve instead of a circular are. It is shown fi gure 1.3 (f).

Sculptured or curve mesh surface: It is produced by a grid of generating curves which interest to form a patch work of surface patches. It is shown is fi gure 1.3 (g).

Fillet surfaces: It is the curve which interpolate between different surfaces, (eg) Chamfer surface. It is shown in fi gure 1.3 (h),

Figure 1.3 (h): Filet surface


1.6.1 Advantages

1. They are less ambiguous

2. Useful for specifi c non-analytical surfaces, called sculptured surfaces such as those used for modeling car bodies, and ship hulls.

3. More complex surfaces are easily identifi able by the model.

4. Hidden line removal feature is available to add realism to the model.

1.6.2 Disadvantages

1. Construction not so simple as wire frame model

2. No information regarding the interior of the model is possible.

3. Mass properly calculation is diffi cult

4. It takes more time to create

5. It requires more memory space

6. It requires more manipulation time.

1.7 SOLID MODELLING

It is the more complete representation of objects than surface and wire frame models. In this, the model is displayed as solid look of an object. Adding colour to the images the model becomes more realistic. These type of models can be quickly created and modifi ed.

Following are the same approaches to defi ne a solid mode.

1. Pure primitive instancing

2. Generalized sweeps

3. Spatial occupancy enumeration

4. Cellular decomposition

INTRODUCTION 1.27

5. Constructive solid geometry (CSG or C-rep)

6. Boundary representation (B-rep)

7. Hybrid scheme.

Out of these, Constructive Solid Geometry approach (CSG), boundary representation approach (B-rep) and hybrid scheme are generally followed today to create solid models. They are explained below.

Constructive solid geometry (CSG or C-rep approach)The CSG system is also called as building block approach. It allows the

designer to build the model with solid graphic primitives such as rectangular blocks, cubes, spheres, cylinders and pyramids.

The solid primitives are combined to form the required solid model by Boolean operations. The different Boolean operators and their operation are given below.

Boolean operator Meaning Purpose/operation

, or + Union To give a shape equal the combination or addition together the two primitives.

+ or | Intersection To give a shape equal to the common volume of the combined primitives.

— Difference To give a shape equal to the volume obtained by subtraction of one primitive from other.

(i) Two dimensionalConsider one rectangular primitive (A) and other circular primitive (B) in

two dimension and position them as in fi gure 1.4 (a) the shape for different Boolean operation is given in fi gure 1.4 (b), (c), (d), (e), they got shapes as shown in shaded form.


Figure 1.4: Boolean operations for two dimensional model

(ii) Three dimensional

Consider one rectangular block primitive (A) and another cylindrical block

(B) and position them as shown in fi gure 1.5 (a). The shape for different Boolean

operation is given in fi gure 1.5 (b) (c), (d), (e).

Figure 1.5: Boolean operation of 3 dimensional object

1.7.1 Solid Model Construction Techniques

Four methods for constructing a 3-D object within a CAD solid modelling

systems are considered the most signifi cant:

• Pure Primitive Instancing (PPI) (Figure 1.6): PPI involves recalling

already-stored descriptions of primitive solids.

• Sweeping (S) (Figure 1.7): Sweeping technique is used in creating solid

models of two-and-a-half-dimensional objects. The class of two-and-

a-half-dimensional objects includes both solids of uniform thickness

in a given direction and axisymetric solids. The former are known as

INTRODUCTION 1.29

extruded solids and are created via linear or translational sweep; the

latter are solids of revolution which can be created via rotational sweep.

Sweeping is used in general as a means of entering object descriptions

into B-rep or CSG-based modellers.

Figure 1.6: Pure primitive instancing

• Boundry representation (B-rep): B-rep construction consists of

entering all bounding edges for all surfaces. This is similar or copying

an engineering drawings into the computer, line by line, surface by

surface, with one important qualifi cation: The lines must be entered

and surfaces oriented in such a way that they create valid volumes.

• Constructive Solid Geometry (CSG): CSG technique uses Boolean

combinations of primitive solids to build a part. The Boolean operations

are addition (+), subtraction(-), and intersection (*), as illustrated in

three dimensions in fi gure 1.8.


Figure 1.7: Sweeping

• A hybrid construction technique is utilised in Pro/Engineer: Pro/E

solid modelling package have a CSG - compatible user input and

therefore provide users with a set of building blocks, called features.

Features are simple basic shapes and are considered solid modelling

entities which can be combined by a mathematical set of Boolean

operations to create the solid. Features themselves are considered valid

“off-the-shelf’ solids. In addition, Pro/E supports sweeping operations

that permit users to utilise wireframe entities (sketched sections) to

create faces that are swept later to create solids. Also, in Pro/E, PPI

INTRODUCTION 1.31

technique is supported to duplicate features that are topologically

identical but vary, in size from the nominal features.

Figure 1.8: Three-Dimensional Boolean Operations

Constructive solid geometry (CSG or C - rep approach)

The CSG system is also called as building block approach. It allows the

designer to build the model with solid graphic primitives such as rectangular blocks,

cubes, spheres, cylinders and pyramids.

The solid primitives are combined to form the required solid model by boolean

operations (Refer fi gure 1.9). The different boolean operators and their operation

are given below.


Figure 1.9: Boolean Operator

1.7.2 Feature-based modellingFeatures generally fall into one of the following categories:

• Base Features: The basic feature may be either a sketched feature or datum plane(s) referencing the default coordinate system (MCS). The base feature is important because all future model geometry will reference this feature directly or indirectly; it becomes the root feature. Changes to the base feature will affect the geometry of the entire model.

• Sketched Features: In general, sketched features are created by extruding, revolving, blending, or sweeping a sketched cross section. Material may be added or removed by protruding or cutting the feature from the existing model.

• Referenced Features: Referenced features reference existing geometry and employ an inherent form; they do not need to be sketched. Some examples of referenced features are rounds, drilled holes, and shells.

• Datum Features: Datum features, such as planes, axes, curves, and points, are generally used to provide sketching planes and contour references for sketched and referenced features. Datum features do not have physical volume or mass, and may be visually hidden without affecting solid geometry.

INTRODUCTION 1.33

1.7.3 Advantages of Solid Modelling

• It is complete and unambiguous.

• Suitable for automated applications like creating part program without much human involvement.

• Creation is fast.

• It gives more information.

1.8 COMPARISON OF VARIOUS MODELLING

S. No.

Wire frame modelling Surface modelling Solid modelling

1. Confusion to the viewer Less confusion No confusion2. More ambiguity in

identifying the surfacesLess ambiguity No ambiguity

3. Connot get required information

Can get required information

More informations

4. Not suitable for automated applications

To some extent suitable for automated applications

Best suitable for automated application

5. Not possible for mass, volume calculations, NC part programming, cross sectioning etc.

Not possible possible

6. Need less memory More memory Still more memory7. Manipulation takes less

timeMore time Still more memoiy

8. Construction simple Diffi cult than wire frame

Diffi cult than wire frame and surface models

9. No realistic look Realistic surface look Realistic solid look


10. Represented as a collection of corner points and edge lines

Represented as a collection of corner points and edge lines point, edge lines and face surfaces

Represented as a collection of corner points and edge lines points, edge lines, face surface and internal volume.

REVIEW QUESTIONS:

1. Explain about drawing features in CAD.

2. What do you understand from Two Dimensional Transformation?

3. Explain in detail about Basic Modelling Transformer.

4. Defi ne Sealing.

5. Defi ne Homogenous Coordinates.

6. What is Rotation?

7. What are the typical CAD command structures?

8. What do you understand from wireframe modelling?

9. Discuss briefl y about the wireframe modelling.

10. State the advantages of wireframe modelling.

11. Discuss the disadvantages of wireframe models are manifold.

12. State surface modelling and explain in detail.

13. What are the advantages of surface modelling?

14. What are the disadvantages of surface modelling?

15. Explain solid modelling and explain in detail with neat sketch.

16. What are solid modelling Techniques? Explain with neat sketch?

17. Describe briefl y about the feature based modelling.

18. What are the advantages of solid modelling?

19. Discuss and compare about the various models.

2.1 INTRODUCTION - COMPUTER INTEGRATED MANUFACTURING

Computer-integrated manufacturing (CIM) is the manufacturing approach of using computers to control the entire production process. This integration allows individual processes to exchange information with each other and initiate actions. Through the integration of computers, manufacturing can be faster and less error-prone, although the main advantage is the ability to create automated manufacturing processes. Typically CIM relies on closed-loop control processes, based on real-time input from sensors. It is also known as fl exible design and manufacturing.

The term “computer-integrated manufacturing” is both a method of manufacturing and the name of a computer-automated system in which individual engineering, production, marketing, and support functions of a manufacturing enterprise are organized. In a CIM system functional areas such as design, analysis,

2UNIT

Components of CIM


planning, purchasing, cost accounting, inventory control, and distribution are linked through the computer with factory fl oor functions such as materials handling and management, providing direct control and monitoring of all the operations.

As a method of manufacturing, three components distinguish CIM from other manufacturing methodologies:

• Means for data storage, retrieval, manipulation and presentation.

• Mechanisms for sensing state and modifying processes.

• Algorithms for uniting the data processing component with the sensor/modifi cation component.

CIM is an example of the implementation of information and communication technologies (ICTs) in manufacturing.

CIM implies that there are at least two computers exchanging information, e.g. the controller of an arm robot and a micro-controller of a CNC machine.

Some factors involved when considering a CIM implementation are the production volume, the experience of the company or personnel to make the integration, the level of the integration into the product itself and the integration of the production processes. CIM is most useful where a high level of ICT is used in the company or facility, such as CAD/CAM systems, the availability of process planning and its data.

There are three major challenges to development of a smoothly operating computer-integrated manufacturing system:

• Integration of components from different suppliers: When different machines, such as CNC, conveyors and robots, are using different communications protocols. In the case of AGVs, even differing lengths of time for charging the batteries may cause problems.

• Data integrity: The higher the degree of automation, the more critical is the integrity of the data used to control the machines. While the CIM system saves on labor of operating the machines, it requires extra

COMPONENTS OF CIM 2.3

human labor in ensuring that there are proper safeguards for the data signals that are used to control the machines.

• Process control: Computers may be used to assist the human operators of the manufacturing facility, but there must always be a competent engineer on hand to handle circumstances which could not be foreseen by the designers of the control software.

2.1.1 Computer-Integrated Manufacturing (CIM) as a concept and a Technology

A number of defi nitions have been developed for computer integrated manufacturing (CIM). However a CIM system is commonly thought of as an integrated system that an compasses all the activities in the production system from the planning and design of a product through the manufacturing system, including control. CIM is an attempt to combine existing computer technologies in order to manage and control the entire business. CIM is an approach that very few companies have adopted at this time, since surverys show that only 1 or 2% of U.S. manufacturing companies have approached full-scale use of FMS and CAD/CAM, let alone CIM systems.

As with traditional manufacturing approaches, the purpose of CIM is to transform product designs and materials into salable goods at a minimum cost in the shortest possible time. CIM begins with the design of a product (CAD) and ends with the manufacture of that product (CAM). With CIM, the customary split between the design and manufacturing functions is (supposed to be) eliminated.

CIM differs from the traditional job shop manufacturing system in the role of the computer plays on the manufacturing process. Computer-integrated manufacturing systems are basically a network of computer systems tied together by a single integrated database. Using the information in the database, a CIM system can direct manufacturing activities, record results, and maintain accurate data. CIM is the computerization of design, manufacturing, distribution, and fi nancial functions into one coherent system.


Figure 2.1 presents a block diagram illustrating the functions and their relationship in CIM. These functions are identical to those found in a traditional production (planning and control) system for job shop MS. With the introduction of computers, changes have occurred in the organisation and execution of production planning and control through the implementation of such systems as mrp, capacity planning, inventory management, shop fl oor control and cost planning and control.

The following fi gure 2.1 shows the major functions in the CIM.

Figure 2.1: Major Functions in CIM

The society of manufacturing Engineers (SME) defi ned as “CIM is the integration of the total manufacturing enterprise through the use of integrated systems and data communications coupled with new managerial philosophies that improve organizational and personal effi ciency.

CIM basically involves the integration of all the functions of an enterprise. The new CIM wheel (Figure 2.2) of Society of Manufacturing Engineers illustrates this concept well and demonstrates the interrelationship among the various segments of


the enterprise. CIM is generally considered as a new approach to manufacturing, management and corporate operation. It is generally interpreted that CIM includes most of the advanced manufacturing technologies such as computer aided design, computer numerical control, robots, just in time production, etc. However, CIM goes beyond all those technologies and provides new way of doing business that includes commitment to customer satisfaction, total quality, and continuous improvement. As discussed in the earlier chapters, a single enterprise based database, that supports all the information needs for manufacturing in every department becomes an essential part of CIM. This database removes the communication barriers between various departments of an enterprise allowing for complete integration of all departments.

Figure 2.2: The New Manufacturing Enterprise Wheel Suggested bySociety of Manufacturing Engineers

(Courtesy: of the Society of Manufacturing Engineers, Dearborn, Michigan)


The SME Manufacturing enterprise wheel has 6 defi ned areas. They are:

1. The success of an enterprise depends on the customer, and thus customer becomes the hub of the wheel. With a clear understanding of the customer requirement and the market place, the enterprise will succeed.

2. The next level focuses on the organisational structure of the enterprise. This deals with the organizing people, training, motivation and cooperation in teamwork. There are a number of techniques used to achieve these goals such as organisational learnings, leadership, standards, quality circles, and rewards.

3. The third level in the wheel focuses on the shared knowledge of the enterprise. This will include all the databases and archival knowledge and experience, all of which can be utilised to support the people and the processes.

4. All the systems that are actually used in the total enterprise are present in this part of the wheel. All the processes are grouped into three major categories, namely product and process, manufacturing and customer support. Each of these have the components that actually perform the necessary functions.

5. Resources and responsibilities of the enterprise are included in this section. The resources are the people, materials, tools, information, technology and suppliers. The responsibilities will be to the employees, investors and the communities that it will be serving while undertaking the statutory, ethical and environmental safeguards.

6. The fi nal part of the wheel is the actual manufacturing infrastructure. This would include all the infrastructure such as customers and their needs, suppliers, distributors, prospective workers, natural resources, fi nancial markets, educational and research institutions and competitors.


There should be a tight integration between all the segments as shown to

achieve the benefi ts of CIM.

In the earlier chapters many of the components of CIM have been dealt in

suffi cient detail. In order to achieve the CIM, it therefore becomes necessary to

integrate all the functions (either automatic or manual) through some means such that

the benefi ts of automation can be achieved. For example, FMS may be considered

as “mini CIM” since a number of functions of CIM are already available within an

FMS. If the FMS control program is linked to the business data processing unit, CIM

can be realised. However one difference is that FMS relies on complete automation

with very little manual intervention save for the work and tool preparation areas.

However, in CIM there can be a large number of manual operations present besides

the automated equipment, all of which will have to be taken into account while

planning and implementing CIM.

2.2 CASA/SME MODEL OF CIM

CIM is the integration of totally manufacturing enterprise by using integrated

system and DATA communication with new managerial philosophies that improve

organizational and personnel effi ciency.

Beginning of the 80’s was developed that CIM Wheel (CIM wheel) of the

CASA/SME (Refer fi gure 2.3) [“computer and Automated of system Association”

OF the “Society OF Manufacturing Engineers” OF the United States OF America).

Main idea was the holistic view of the enterprise, on the basis of the CIM. In center

of the CIM Wheel stands the integrated architecture (integrated system architecture)

with a common database (common DATA) and the information administration and -

communication (information resources management & communication].


Figure 2.3: CASA model of CIM

On the second level enterprise functions became from the ranges factory automation, product & processes and production planning and - control over the components of the integrated architecture links with one another.

At this concept it was new that over it going out administrative tasks, on a third level, were considered. It concerns management & personnel management, marketing, strategic planning and fi nancial system. The advancement CIM of the Wheel is the Manufacturing Enterprise Wheel.

2.3 DEVELOPMENT OF CIM (CIM II)

CIM is an integration process leading to the integration of the manufacturing enterprise. Figure 2.4 indicates different levels of this integration that can be seen


within an industry. Dictated by the needs of the individual enterprise this process usually starts with the need to interchange information between the some of the so called ISLANDS OF AUTOMATION. Flexible manufacturing cells, automatic storage and retrieval systems, CAD/CAM based design are the examples of islands of automation i.e computer based automation has been achieved completely in a limited spheres of activity in an enterprise. This involves data exchange among computers, NC machines, robots, gantry systems etc. Therefore the integration process has started bottom-up. The interconnection of physical systems was the fi rst requirement to be recognised and fulfi lled.

Figure 2.4: Levels of integration against evolution of CIM

The next level of integration, application integration in Figure 2.4 is concerned with the integration of applications; applications are used in the data processing sense. Application integration involves supply and removal of information, communication between application users and with the system itself. Thus the application integration level imposes constraints on the physical integration level. There also has to be control of the applications themselves.


2.4 BENEFITS OF CIMThe following is the list of a few of the benefi ts that can be achieved by use

of CIM.

1. CIM improves the operational control by means of reduction in the number of uncontrollable variables, reducing dependents on human communication.

2. CIM improves the short run responsiveness.

3. CIM improves long run accommodations by means of changing product volumes, different part mixes.

4. CIM reduces the inventory through reducing lot sizes, improving inventory turn overs for the particular company.

5. CIM increases machine utilization by means of eliminating or reducing machine setup, utilizing automated features.

2.5 COMMUNICATION MATRIX IN CIM

The communication matrix is a tool for pro actively planning communications on a project. Firstly, you list all the individuals and groups who will need to be told


things, or to whom you must listen (Stakeholder Analysis) along one axis; and all the topics or the information that will need to be communicated along the other axis).

This tool is excellent for pre-planning with groups; they can write on blank proformas or contribute as a whole onto a fl ipchart or laptop projection.

Once you have produced the matrix, send it out to all the participants (with any politically sensitive items removed) so people know who, how and when you are planning to communicate and the part they will play. You may get a lot of calls from people who feel that they, or others, have been left out of important parts, but these conversations are shorter and much more productive than contact after the event complaining about lack of communication.

The following Table 2.1 shows the working and procedure used in communication matrix.

Table: 2.1: Procedures of communication

To FromProject

Manager

Project Team

Members

Control Board

CustomerSub-

suppliers

Other Stake- holders

Project Manager × ?Project Team

Members ×

Control Board ×

Customer ? ×


Sub-suppliers ×

Other Stack

holders? ×

2.6 FUNDAMENTALS OF COMPUTERCOMMUNICATION IN CIM

Data Communication is the exchange of information of the data between the parties intended for communication. The data involved may be a piece of information in a simple text fi le or an audio fi le. It may even be the video conferencing between the parties.

What ever may be the medium for communication, the important thing is that the data exactly reaches the destination it is intended for. The most happening thing today in data communication is VoIP i.e. Voice over Internet Protocol.

The physical integration of industrial controllers with Computer Aided Design (CAD) systems and manufacturing management systems has become one of the most important issues in the fi eld of Computer Integrated Manufacture (CIM). Communications links between these intelligent, computer based systems are a vital part of all modern, manufacturing organisations endeavouring to integrate management systems and production systems into a more effi cient, responsive and cohesive unit.

Communications within a manufacturing organisation can take on many forms. At a basic level it is often necessary to reliably transfer data or programs, developed on a Computer, to a Computer Numerical Control (CNC) machine tool, robot or Programmable Logic Controller (PLC). At a higher level it may be necessary to integrate CAD workstations, industrial controllers (CNCs & PLCs) and manufacturing management computer systems through a Local Area Network (LAN). However, in order to establish links and networks that can function with


industrial equipment, there needs to be an understanding of the basic mechanisms and problems of data communications and the special needs of the manufacturing environment.

Contrary to the (often misguided) industry faith in turn-key solutions, the selection, installation and maintenance of a network requires a good deal of in-house expertise. A large proportion of high-technology manufacturing systems fail simply because companies place too much reliance on external consultants and vendors instead of developing this vital, in-house expertise. Inevitably, manufacturers need to realise that “all-embracing”, “ever-lasting”, “future-proofed”, “turn-key” solutions simply do not exist in the world of reality. This is particularly true for industrial communications networks which live in an environment of widely differing, incompatible and constantly changing computer technologies and standards.

In many disciplines of engineering, there are diffi culties in fi nding standards that cover a specifi c technology. This is certainly not the case in terms of industrial networking. In fact there are an enormous number of standards that cover the area. The bulk of these standards do not, in isolation, provide a mechanism for reliable data communication in the factory. A range of cohesive standards needs to be selected in order to realise a viable link or network. However, many standards are incompatible with one another or unsuitable for the industrial environment. Some communications standards are even irrelevant to the applications to which they are now applied.

In order to select communications equipment or develop communications protocols for the manufacturing environment, one needs to understand both the networking technology and the standards themselves.

2.7 CIM DATA TRANSMISSION METHOD

2.7.1 Serial and Parallel data transmission

The need to provide data transfer between a computer and a remote terminal has led to the development of serial communication.


Serial data transmission implies transfer data transfer bit by bit on the single (serial) communication line .

In case of serial transmission data is sent in a serial form i.e. bit by bit on a single line. Also, the cost of communication hardware is considerable reduced since only a single wire or channel is require for the serial bit transmission. Serial data transmission is slow as compared to parallel transmission.

However, parallel data transmission is less common but faster than serial transmission. Most data are organized into 8 bit bytes. In some computers, data are further organized into multiple bits called half words, full words. Accordingly data is transferred some times a byte or word at a time on multiple wires with each wire carrying individual data bits. Thus transmitting all bits of a given data byte or word at the same time is known as parallel data transmission.

Parallel transmission is used primarily for transferring data between devices at the same site. For eg : communication between a computer and printer is most often parallel so that entire byte can be transferred in one operation.

2.7.2 Asynchronous data transmission

Serial data communication generally employs either synchronous or asynchronous communication scheme. This two scheme used different techniques for synchronizing in the circuits in sending and receiving end.

In asynchronous transmission each character is transmitted separately, that is one character at a time. The character is preceded by a start bit, which tells the receiving end where the character coding begins, and is followed by a stop bit, which tells the receiver where the character coding ends. There will be intervals of ideal time on the channel shown as gaps. Thus there can be gaps between two adjacent characters in the asynchronous communication scheme. In this scheme, the bits within the character frame (including start, parity and stop bits) are sent at the baud rate.

The START BIT and STOP BIT including gaps allow the receiving and sending computers to synchronise the data transmission. Asynchronous communication


is used when slow speed peripherals communicate with the computer. The main disadvantage of asynchronous communication is slow speed transmission. Asynchronous communication however, does not require the complex and costly hardware equipments as is required for synchronous transmission.

2.7.2.1 Advantages and Disadvantages of Asynchronous TransmissionThe advantage of asynchronous transmission is that it does not required any

local storage at the terminal or the computer and is thus cheaper to implement.

Major disadvantage of asynchronous transmission is that the transmission lines is idle during the time intervals between transmitting characters.

2.7.3 Synchronous Transmission

In Synchronous communication scheme, after a fi xed number of data bytes a special bit pattern is send called SYNC by the sending end.

Data transmission take place without any gap between two adjacent characters., however data is send block by block. A block is a continuous steam of characters or data bit pattern coming at a fi xed speed. You will fi nd a Sync bit pattern between any two blocks of data and hence the data transmission is synchronized.

Synchronous communication is used generally when two computers are communicating to each other at a high speed or a buffered terminal is communicating to the computer.

2.7.3.1 Advantages and Disadvantages of Synchronous TransmissionMain advantage of Synchronous data communication is the high speed. The

synchronous communications required high-speed peripherals/devices and a good-quality, high bandwidth communication channel.

The disadvantage include the possible in accuracy. Because when a receiver goes out of Synchronization, loosing tracks of where individual characters begin and end. Correction of errors takes additional time.


2.7.4 Pulse Code Modulation and DemodulationPulse Code Modulation (PCM) forms the heart of the modern telephone

system. To understand PCM, let’s consider how multiple analog voice signals are digitized and combined on to a single digital trunk.

The analog signals are digitized in the end offi ce by a device called a codec (coder-decoder) producing a 7 or 8 bit number. The coder makes 8000 samples per second (125μsec/sample) because the Nyquist theorem say that is suffi cient to capture all the information from the 4-KHz telephone channel bandwidth. At a lower sampling rate, information would be generated/gained. This technique is called PCM (pulse Code Modulation). As a consequence, virtually all time intervals within the telephone system are multiplies of 125μsec.

Demodulation is the act of extracting the original information-bearing signal from a modulated carrier wave. A demodulator is an electronic circuit (or computer program in a software defi ned radio) that is used to recover the information content from the modulated carrier wave.

These terms are traditionally used in connection with radio receivers, but many other systems use many kinds of demodulators. Another common one is in a modem, which is a contraction of the terms modulator/demodulator.

In electronics and telecommunications, modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted. This is done in a similar fashion to a musician modulating a tone (a periodic waveform) from a musical instrument by varying its volume, timing and pitch. The three key parameters of a periodic waveform are its amplitude (“volume”), its phase (“timing”) and its frequency (“pitch”). Any of these properties can be modifi ed in accordance with a low frequency signal to obtain the modulated signal. Typically a high-frequency sinusoid waveform is used as carrier signal, but a square wave pulse train may also be used.

In telecommunications, modulation is the process of conveying a message signal, for example a digital bit stream or an analog audio signal, inside another


signal that can be physically transmitted. Modulation of a sine waveform is used to transform a baseband message signal into a passband signal, for example low-frequency audio signal into a radio-frequency signal (RF signal). In radio communications, cable TV systems or the public switched telephone network for instance, electrical signals can only be transferred over a limited passband frequency spectrum, with specifi c (non-zero) lower and upper cutoff frequencies. Modulating a sine-wave carrier makes it possible to keep the frequency content of the transferred signal as close as possible to the centre frequency (typically the carrier frequency) of the passband.

A device that performs modulation is known as a modulator and a device that performs the inverse operation of modulation is known as a demodulator (sometimes detector or demod). A device that can do both operations is a modem (modulator-demodulator).

2.7.5 Type of Communication Systems

The communication system can be classifi ed into three categories

1. Simplex

2. Full Duplex

3. Half Duplex

SimplexA simplex system is a communication system in which the message can be

send in one direction only.

Radio and TV boardcasting are eg User - Transmitter - Receiver - User

Full DuplexA full duplex system is one in which the link is capable of transmitting in both

the direction, at the same.

Eg : telephone system.


Half DuplexIn a half duplex system, each end may transmit, but only one at a time. This

requires both transmitting and receiving circuitary at each end. but the actual link between the two ends may be shared.

Eg : A citizen’s band radio where a frequency channel is shared and each party has to say “over” to switch the direction of the communication.

2.8 TYPES OF COMMUNICATION IN CIM

2.8.1 Point-to-Point communications

Point-to-point communications generally refers to a connection restricted to two endpoints.

Point-to-point is sometimes referred to as P2P or Pt2Pt. This usage of P2P is distinct from P2P referring to peer-to-peer fi le sharing networks.

Point-to-point is distinct from point-to-multipoint where point-to-multipoint also refers to broadcast or downlink.

Basic point-to-point data linkA traditional point-to-point data link is a communications medium with exactly

two endpoints and no data or packet formatting. The host computers at either end had to take full responsibility for formatting the data transmitted between them. The connection between the computer and the communications medium was generally implemented through an RS-232 interface, or something similar. Computers in close proximity may be connected by wires directly between their interface cards .

When connected at a distance, each endpoint would be fi tted with a modem to convert analog telecommunications signals into a digital data stream. When the connection used a telecommunications provider, the connections were called a dedicated, leased, or private line. The ARPANET used leased lines to provide point-to-point data links between its switching nodes, which were called Interface Message Processors.


Modern point-to-point linksMore recently (2003), the term point-to-point telecommunications relates to

wireless data communications for Internet or Voice over IP via radio frequencies in the multi-gigahertz range. It also includes technologies such as laser for telecommunications but in all cases expects that the transmission medium is line of sight and capable of being fairly tightly beamed from transmitter to receiver. Today (2009) there are online tools to help users fi nd if they have such line of sight, one example is the PTP estimator from AlphiMAX.

The telecommunications signal is typically bi-directional, either time division multiple access (TDMA) or channelized.

In hubs and switches, a hub provides a point-to-multipoint (or simply multipoint) circuit which divides the total bandwidth supplied by the hub among each connected client node. A switch on the other hand provides a series of point-to-point circuits, via micro segmentation, which allows each client node to have a dedicated circuit and the added advantage of having full-duplex connections.

2.8.2 Star Network

Star networks are one of the most common computer network topologies. In its simplest form, a star network consists of one central switch, hub or computer, which acts as a conduit to transmit messages. This consists of a central node, to which all other nodes are connected; this central node provides a common connection point for all nodes through a hub. Thus, the hub and leaf nodes, and the transmission lines between them, form a graph with the topology of a star. If the central node is passive, the originating node must be able to tolerate the reception of an echo of its own transmission, delayed by the two-way transmission time (i.e. to and from the central node) plus any delay generated in the central node. An active star network has an active central node that usually has the means to prevent echo-related problems.

The star topology reduces the chance of network failure by connecting all of the systems to a central node. When applied to a bus-based network, this central hub


rebroadcasts all transmissions received from any peripheral node to all peripheral nodes on the network, sometimes including the originating node. All peripheral nodes may thus communicate with all others by transmitting to, and receiving from, the central node only. The failure of a transmission line linking any peripheral node to the central node will result in the isolation of that peripheral node from all others, but the rest of the systems will be unaffected.

It is also designed with each node (fi le servers, workstations, and peripherals) connected directly to a central network hub, switch, or concentrator.

Data on a star network passes through the hub, switch, or concentrator before continuing to its destination. The hub, switch, or concentrator manages and controls all functions of the network. It is also acts as a repeater for the data fl ow. This confi guration is common with twisted pair cable. However, it can also be used with coaxial cable or optical fi bre cable

2.8.3 Multiplexing

Multiplexing is a form of data transmission in which one communication channel carries several transmissions at the same time. The telephone lines .that carry our daily conversations can carry thousands or even more of conversations at a time using multiplexing concept. The exact number of simultaneous transmission depends on the type of communication channel and the data transmission rate.

Economics of scale play an important role in the telephone system. It costs essentially the same amount of money to install and maintain a high-bandwidth trunk as low-bandwidth trunk between two switching offi cers. Consequently, telephone companies have developed elaborate schemes for multiplexing many conversations over a single physical trunk.

Accordingly, the communication channel is shared in such a way as to maximum the utilization of the channel capacity. Thus the method of dividing a single channel into many channels so that a number of independent signals may be transmitted on it is known as Multiplexing.


Multiplexing schemes can be divided into two basic categories1. Frequency Division Multiplexing FDM

2. Time Division Multiplexing TDM

Frequency division multiplexing ( FDM) is the technique used to divide the bandwidth available in a physical medium into a number of smaller independent logical channels with each channel having a small bandwidth. The method of using a number of carrier frequencies each of which is modulated by an independent speech signal is in fact frequency division multiplexing.

The following fi gure depict how three voice-grade telephone channels are multiplexing using FDM. When many channels are multiplexed together, 400Hz is allocated to each channel to keep them well separated. First the voice channels are raised in frequency, each by a different amount. Then they can be combined, because no two channels how occupy the same portion of the spectrum. Notice that even though there are gaps(guard bands) between the channels, there is some overlap between adjacent channels, because the fi lters do not have sharp edges. This overlap means that a strong spike at the edge of one channel will be felt in the adjacent one as non-thermal noise.

Frequency-division multiplexing works best with low-speed devices. The frequency division multiplexing schemes used around the world are to some degree standardized. A wide spread standard is 12 400-Hz each voice channels ( 300Hz for user, plus two guard bands of 500Hz each) multiplexed into the 60 to 108 KHz band. Many carriers offer a 48 to 56 kbps leased line service to customers, based on the group. Other standards upto 230000 voice channels also exist.

Example:The allocated spectrum is about IMHz, roughly 500 to 1500 KHz. Different

(stations, each operating in a portion of the spectrum. With the interchannel separation great enough to prevent interference. This system is an example of frequency division multiplexing.


Advantages of FDM1. Here user can be added to the system by simply adding another pair of

transmitter modulator and receiver domodulators.

2. FDM system support full duplex information fl ow which is required by most of application.

3. Noise problem for analog communication has lesser effect.

Disadvantages of FDM1. In FDM system, the initial cost is high. This may include the cable

between the two ends and the associated connectors for the cable.

2. In FDM system, a problem for one user can sometimes affect others.

3. In FDM system, each user requires a precise carrier frequency.

2.9 COMPUTER NETWORKING IN CIM

Data is defi ned as the raw, unreduced information that is available on each component of a CIM system like a PC, Robot, Workstation or CNC. Normally, each component wants access to all the necessary data to make decisions. This means each component in a CIM system, can take advantage of all available information to achieve higher reliability and more optimal processing or manufacturing. Networks allow channels of communications to exist among various sections of a manufacturing system. Peal-time modifi cations to business plans can be effected via communications through the network. Networks are essential to move information faster.

Networks such as the ones used in CIM systems make data sharing easy, peripheral changing easy and information sharing possible.

2.9.1 Principles of Networking

As technology has been putting more processing power into smaller computers the trend has been to move away from massive, centralised processors which handle


every tasks to networks of smaller processors, each closely tailored to particular applications. From the manufacturing standpoint, this eliminates the possibility of shutting down the entire plant if a large central computer fails. Networking also allows piece-wise growth of the overall system with minimal investment and disruption.

Networking provides the means for tying together the various “islands of automation” and in the process makes integration possible by allowing high speed data exchange between related functions.

Networking is essential for several activities. Some examples are:-

i) Design and Development of VLSI components, aircrafts etc.

ii) Airline or train reservation.

iii) Electronic mail.

Communication networks can be classifi ed into four categories depending upon the physical separation of the communicating devices.

a) Miniature (<50 mm) such networks are concerned with the interconnection of multiple computational elements which are implemented on the same I.C.

b) Small (<500 mm) these are concerned with the interconnection of multiple computational units which are located on a single rack.

c) Medium (<1 km) these networks are concerned with the interconnection of multiple computational units (offi ce workstation, CAD systems, shop fl oor terminals, CNC systems, Robots etc). These are connected through a LAN.

d) Large (>1 km) Large networks involve connection of remote mainframes, networking of a minicomputer system to a remote, mainframe or terminals etc. It can be city wide (Metropolitan Area Network (MAN)) or countrywide or Worldwide (WAN)


Typical wide area networks in India are:

INDONET INET

NICNET RABMN

ERNET SATCOM

ICNET VSNL

DOT

2.9.2 Private Computer Communication Networks

A manufacturing industry may have a number of computer systems located at different sites around a country. There is a requirement for them to communicate each other to share resources and exchange information. An autonomous data communication network for this purpose is called a private computer communication network.

2.9.3 Public Switched Data Networks

Initially organisation implemented their own private nationwide data networks using communication lines based from public telephone authorities. As computer technology grew and computer became varied it became necessary to communicate between different computers in different organisations. This led to the development of public switched data networks (PSDN).

More recently the need for effi cient transmission of voice and data has led to the development of Integrated Services Digital Networks (ISDN).

2.9.4 Local Area Network (LAN)

A network is a group of computers that are linked to communicate with each other and share resources like hard disk, printers etc via the cables and interfaces that connect the computers and peripherals. Application softwares used in a network allows several users access the same program and data at the same time.


Figure 2.5: Typical Network

As the name implies, a LAN is a system that covers short distances. Usually LAN is limited, to a single department or a single building or a single campus. Figure 2.5 shows a typical network.

A LAN consists of a number of computers connected to a fi le server. Common resources of the network include a bank of printers and facilities for disc mirroring and disc duplexing.

2.9.5 Network Techniques

NETWORK BASICS : A given network technology falls into one of two categories: Local area network (LAN) or Wide area networks (WAN). LANs are intended to serve a number of users who are physically located close together. WANs are more akin to telephone network, tying different people in different buildings, cities or even countries. A message is routed through several interim points before reaching its fi nal destination; a WAN may also incorporate the. ability


to automatically change to an alternate message routing path if the computer at one location fails. A LAN (local area network) has 2 to 10 times more traffi c on it than a wide area network (WAN). Each gate node gateway or system, if well designed, reduces communication requirement.

Each individual point within a network that can communicate through the network is called a node . Each node is assigned a unique address. This way, a destination address can be put into each message and it can be sent to correct recipient.

2.9.6 Components of a Network

A LAN is a system comprising the following basic components:

i) Computers (PC’s, Graphics Workstations, Minis etc)

ii) Network interface card (NIC): This is a communications hardware in the form of an add-on card for sending and receiving messages. This is also called network adapter. NIC is plugged into one of the slots of the PC expansion slots and the transmission cable is attached to the connector provided on the card.

iii) Network Cable: A transmission cable is attached to each device (computer/ peripheral) to enable the transmission of messages from one device to another. The details of cables commonly used are given in Table 2.2.

Table 2.2: Cables used in Networking

TypeData

transmission rate

Distance Remarks

Twisted pair 1 M bit/sec short distance

Least expensive base band, single channel


Coaxial Cable

base band 10 M bit/sec upto 4 km Multi Channel Capability

broad band 5 M bit/sec upto 50 km

Fibre Optics Multi Channel Large Capacity Expensive

iv) Network Server: This computer is used to manage shared resources. Server is a combination of hardware and software. The fi le server does the following tasks:

• Manages the shared hard disk.

• Makes sure that multiple requests do not confl ict each other.

• Protect data.

• Prevent unauthorized access.

• Maintain a list of privilege and authorisations.

v) Central Mass storage: The hard disc of the fi le server showed have suffi cient capacity (usually in term of Gigabytes).

2.9.7 Network Wiring Methods

There are two basic ways that three or more nodes can be incorporated in a network; these are point-to-point and multidrop. (Refer Figure 2.6)

Figure 2.6: Types of Network Arrangement


2.9.8 Network Topologies

Given that one or the other or a combination of wiring methods will be used when interconnecting network nodes, there are several commonly used network topologies, or ways of routing the interconnections. (Refer Figure 2.7)

Figure 2.7: Classifi cation of Networks

i) Star Network:

This means running a separate cable or line between fi le server and each node. This is useful when a master slave relationship exists between the fi le server and the nodes. For sending data and fi les from one node to another request should be made to the fi le server which establishes a dedicated path between the nodes. The data can be transmitted through this path.

ii) Ring Network:

This involves connecting all nodes in series. The cable will normally loop back to form a full circle. This is some times used when nodes are widely separated, as each node can act as a repeater (amplifi er) for message destined for downstream nodes. The data will have to pass through other workstations before reaching the


fi le server. The data is sent in the form of a packet which contains both source and destination addresses of the data. As the packet circulates through the ring the destination station copies the data into its buffer and the packet continues to circulate until it goes back to source workstation as an acknowledgement.

iii) Bus Networks:

Allow all nodes to share the same cable. Any message that travels on the cable is “seen” by every node on the cable. This topology uses both base band and broad band transmission.

iv) Hybrid Networks:

This includes features of more than one topology to achieve the optimal trade-off of reliability, performance, fl exibility and cost.

2.10 OPEN SYSTEM INTER CONNECTION (OSI)

The OSI model is based on a proposal developed by the International organization for Standardization (ISO) as a fi rst step toward international standardization of the Protocols used in the various layers (day and Zimmermann. 1983). It was revised in 1995 (day,1995). The model is called the ISO OSI (Open System Interconnection) Reference because it deals with connecting open systems-that is, systems that are open communication with other systems. We will just call it the OSI model for short. (Refer fi gure 2.8)

The OSI model has seven layers. The principles that were applied to arrive, at the seven layers can be briefl y summarized as follows:

1. A layer should be created where a different abstraction is needed.

2. Each layer should perform a well-defi ned function.

3. The function of each layer should be chosen with an eye toward defi ning internationally standardized protocols.

4. The layer boundaries should be chosen to minimis the interfaces.


5. The number of layers should be large enough that distinct functions need not be thrown together in the same layer out of necessity and small enough that the architecture does not become unwiedly.

The OSI model itself is not a network architecture because it does not specify the exact services and protocols to be used in each layer. It just tells what each layer should do. However, ISO has also produced standards for all the layers, although these are not part of the reference model itself Each one has been published as a separate international standard.

2.10.1 Seven Layers of OSI model

1. The physical layerThe physical layer is concerned with transmitting raw bits over a communication

channel The design issues have to do with making sure that when one side sends a 1 bits, it is received by the other side as a 1 bits, not as a 0 bit. Typical questions here are how many volts should be represent a 1 and how many for a 0, how many nanoseconds a bit lasts, whether transmission may proceed simultaneously in both directions, how the initial connection is established and how it is torn down when both sides are fi nished, and how many pins the network connector has and what each pin is used for. The design issues here largely deal with mechanical, electrical, and timing interfaces and the physical transmission medium which lies below the physical layer.

2. The data link layerThe main task of the data link layer is to transform a raw transmission facility

into a line that appears free of undetected transmission errors to the network layer. It accomplishes this task by having the sender break up the input data into data frames (typically a few hundred or a few thousand bytes) and transmit the frames sequentially. If the service is reliable, the receiver confi rms correct receipt of each frame by sending back m acknowledgement frame.

Another issue that arises in the data link layer (and most of the higher layers as well) is how to keep a fast transmitter from drowning a slow receiver in data.


Some traffi c regulation mechanism if often needed to let the transmitter know how much buffer space the receiver has at the moment Frequently, this fl ow regulation and the error handling are intonated.

Figure 2.8: The OSI reference model

Broadcast networks have an additional issue in the data link layer how to control access to the shared channel. A special sublayer of the data link layer, the medium access control sublayer, deals with this problem.

3. The network layerThe network layer controls the operation of the subnet. A key design issue is

determining how packets are routed from source to destination. Routes can be based on static tables that are “wired into” the network and rarely changed. They can also


be determined at the start of each conversation, for example, a terminal session (example, a login to a remote machine). Finally, they can be highly dynamic, being determined a new for each packet, to refl ect the current network load.

If too many packets are present in the subnet at the same time, they will get in one another’s way, forming bottlenecks. The control of such congestion also belongs to the network layer. More generally, the quality of service provided (delay, transit time, jitter, etc.,) is also a network layer issue.

When a packet has to travel from one network to another to get to its destination, may problems can arise. The addressing used by the second network may be different from the fi rst one. The second one may not accept the packet at all because it is too large. The protocols may differ and so on. It is up to the network layer to overcome all these problems to allow heterogeneous networks to be interconnected.

In broadcast network, the routing problem is simple, so the network layer is often thin or even nonexistent.

4. The transport layerThe basic function of the transport layer is to accept data from above, split it

up into smaller units pass these to the network layer and ensure that the pieces all arrive correctly at the other end. Furthermore, all this must be done effi ciently and in a way that isolates the upper layers from the inevitable changes in the hardware technology.

The transport layer also determines what type of service to provide to the session layer and ultimately to the users of the network. The most popular type of transport connection is an error-free point-to-point channel that delivers messages or bytes in the order in which they were sent. However, other possible kinds of transport service are the transporting of isolated messages with no guarantee about the order of deli very and the broadcasting of messages to multiple destinations. The type of service is determined when the connection is established.


The transport layer is a true end-to-end layer, all the way from the source to the destination. In other words, a program on the source machine carries on a conversation with a similar program on the destination machine using the message headers and control messages. In the lower layers, the protocols are between each machine and its immediate neighbours, and not between the ultimate source and destination machines, which may be separated by many routers. (Refer fi gure 2.8)

5. The session layerThe session layer allows users on different machines to establish sessions

between them. Sessions offer various services including dialogue control token management (preventing two parties from attempting the same critical operation at the same time), and synchronization (check pointing long transmissions to allow them to continue from where they were after a crash).

6. The presentation layerUnlike lower layers, which are mostly concerned with moving bits around the

presentation layer is concerned with the syntax and semantics of the information transmitted. In order to make it possible for computers with different data representations to communicate the data structures to be exchanged can be defi ned in an abstract way along with a standard encoding to be used “on the wire”. The presentation layer manages these abstract data structures and allows higher-level data structures (example, banking records), to be defi ned and exchanged.

7. The application layerThe application layer contains a variety of protocols that are commonly

needed by users. One widely-used application protocol is HTTP (Hyper Text Transfer Protocol), which is the basis for the World Wide Web. When a browser wants a Web page, it sends the name of the page it wants to be server using HTTP. The server then sends the page back. Other application protocols are used for fi le transfer, electronic mail and network news.


2.11 LOCAL AREA NETWORK (LAN MODEL)

A Local Area Network (LAN) is an arrangement of hardware and software that permits logically related devices to communicate with each other distances up to 20 miles. In the CIM environment, these devices may be machining centres. CAD/CAM workstations, AG Vs NC equipment, robots PLCs, data acquisition systems, bar code readers and so on. LAN is usually proprietary for a single organisation.

Developed by Datapoint Corp., the fi rst LAN, implemented in 1977, was called an Attached Resource Computer (ARC) rather than a local area network. Early promotions for this product prophesied that it would “dramatically alter the way the business world thinks and uses computers. “By 1983,5000 units were in place in the U.S. alone. Early computer networks used star confi guration with “dumb” (non-programmable) terminals accessing a host’s computing power. This centralized approach was cost-effective during the 1970s. As minis and micros evolved and became inexpensive, they are replaced dump terminals. Minis and micros encouraged distributed processing and are now hosts or nodes in a network. A LAN deploys switching technology to transfer data.

Special Features of LAN1. Shared transmission medium.

2. Peer-to-peer communication (i.e., a device can communicate directly with another one at the same level).

3. High-speed communication, up to 10 Mbps, as compared to a wide area network (WAN), which uses standard communication facilities such as telephone and leased lines.

Thus, LANs permit a large number and variety of computer systems and other digital devices, including machining centers, to share peripherals and exchange information at high speed over limited distances. This fulfi lls the communications needs of most CIM plants.

LANs have become popular in recent years. The main reason for this is the microcomputer’s low cost and enhanced capabilities. As a network node a micro


can share expensive computer resources such as databases and large printers. LANs are also expandable as the need arises, so investment is not wasted.

Several factors should be considered in designing and implementing LAN. For example, the speed of the LAN should be approximately equal to the fasted computer or device on the input/output bus. The goals are reliability, maintainability, cost, fl exibility, compatibility, and extensibility. LANs should allow networking of a variety of devices, even of different makes. For security, the fi les and records should be lockable.

A wide variety of LANs are available. Prospective users should discuss their particular needs with application engineers of companies engaged in the networking business. Such a company is sometimes called a system integrator.

2.11.1 Elements of LAN

LANs comprise the following elements:

i) NodesIn a CIM environment, the usual nodes are PCs, PLCs. CNC machines, and

simi equipment and other digital devices.

ii) Network communication cards (adaptor cards)The network cards provide communications paths between a node and the

network.

iii) Network softwareThe network software is usually a package from the vendor that provides all

the necessary protocol to communicate from the node environment, say DOS, to all network environment.

iv) Transmission media (cabling)Depending on the type of network, the transmission media may be a telephone

line twisted-pair cable, coaxial cable, or fi ber-optic cable.


v) Server (PC or dedicated)The server is a special node with a hard disk where the network software

“resides.” It is the junction of fi le transfers and mass storage.

A dedicated server offers several storage and performance advantages over a PC used part time as a server.

vi) PeripheralsPrinters, plotters, modems, and so on.

2.11.2 Computer Network Architecture

When selecting computer networks for CIM, questions to be answered include:

• What network architecture, or confi guration?

• What protocol?

• Which transmission standards?

• Which media to use?


Figure 2.3: Typical distributed system architectures


The computer network is a collection of nodes that can communicate with each other via digital transmission lines.

The network architecture is the set of functions that should be performed by the network with its nodes, or computers. (Typical digital network architectures are shown in fi gure 2.9).

The network protocol (discussed below) is the set of rules stating how two or more nodes should intermit during a communications session.

To be able to communicate with a computer or any device the fi rst step is to ensure that the physical transmission facilities (i.e., physical lines) exist between the desired nodes.

Figure 2.10: “Bus” type local area network architecture.

Once the physical link is established the next step is to make sure that the network is not used as a point-to-point line. There are several different techniques


available to interleave the “random” traffi c of such lines, including the multi-dropping, multiple access techniques, packet switching, or fast circuit switching and different other forms of time division multiplexing (line sharing) techniques. The last step before a successful communication can start on the line is to make sure that the bit stream sent has correctly arrived.

Typical computer network and Local Area Network (LAN) architecture include:

• The “bus” or “open ring” structure, [fi gure 2.9(a) and fi gure 2.10]

• The “star” or “hierarchical” structure, [fi gure 2.9(b) and fi gure 2.11]

• The “loop” or “ring” structure [fi gure 2.9(c) and fi gure 2.12]

The above listed architectures are widely used in different Local Area Networks (LANs). ALAN is a private data communications system operating in hostile environment (Example: factory shop-fl oor) making use of the distributed processing concept in a limited geographical area. LANs are capable of accomplishing shop-fl oor communication and control between a number of different machine controllers, micro, and minicomputers. FMS cells and workstations.

In the “bus” or “open ring” structure a master scheduler controls the data traffi c. If data is to be transferred the requesting computer sends a message to the scheduler, which puts the request into a queue. The message contains an identifi cation code which is broadcast to all nodes of the network. The scheduler works out priorities and notifi es the receiver as soon as the bus is available. The identifi ed node takes the message and performs the data transfer between the two computers. Having completed the data transfer the bus becomes free for the next request in the scheduler’s queue.

The benefi t of this architecture is that any computer can be accessed directly and messages can be sent in a relatively simple and to assign frequencies and priorities to organize the traffi c.,

In the “star” confi guration each computer at each level has its specifi c assignment corresponding to the tasks to be solved. At that level. If all computers


are the same only one type of interface and communications package is required. Unfortunately in practice most “stat” structures grow to an irregular shape utilizing a variety of computers and controllers.

The “star” architecture is vulnerable in the case of a computer switch. A further diffi culty is that twisted pair wires limit communications distance and bandwidth and are sensitive to electrical interferences.

In the “ring” architecture an intelligent interface is required for each node. The data fl ow within the ring can be controlled by a scheduler or by sending the messages at pre-described intervals. As soon as an intelligent interface receives a message via the ring, it investigates it to determine whether the address in the packet.

Figure 2.11: “Star” type local area network architecture


Figure 2.12: “Ring” type local area network architecture

2.12 MANUFACTURING AUTOMATION PROTOCOL(MAP MODEL)

MAP is a hardware/software protocol developed jointly by a group of industries and vendors of computers and PLCs. It follows the ISO OSI model. MAP was developed as a result of the plans of General Motors to automate its factories.

MAP uses a broad band LAN, with a token ring protocol for traffi c control. Since it is broadband, all devices in the LAN like computers. CNC machines, robots, PLC’s etc., share the same cable, but different groups of devices can be placed on separate “channels” on the line. Additionally, closed-circuit TV (video) channel can also be accommodated on same cable. MAP physical level is based on the IEEE 802.4 token-bus standard. At the data link level, it uses the IEEE 802.2 logical control standard. MAP also uses 8473 network layer protocol for connection less-mode network service.


2.13 ADVANTAGES OF NETWORKING IN CIM

1. SpeedNetworks provide a very rapid method for sharing and transferring fi les.

Without a network, fi les are shared by copying them to fl oppy disks, then carrying or sending the disks from one computer to another. This method of transferring fi les in this manner is very time-consuming.

2. CostThe network version of most software programs are available at considerable

savings when compared to buying individually licensed copies. Besides monetary savings, sharing a program on a network allows for easier upgrading of the program. The changes have to be done only once, on the fi le server, instead of on all the individual workstations.

3. Centralized Software ManagementOne of the greatest benefi ts of installing a network at a school is the fact that all

of the software can be loaded on one computer (the fi le server). This eliminates that need to spend time and energy installing updates and tracking fi les on independent computers throughout the building.

4. Resource SharingSharing resources is another area in which a network exceeds stand-alone

computers. Most schools cannot afford enough laser printers, fax machines, modems, scanners, and CD-ROM players for each computer. However, if these or similar peripherals are added to a network, they can be shared by many users.

5. Flexible AccessSchool networks allow students to access their fi les from computers throughout

the school. Students can begin an assignment in their classroom, save part of it on a public access area of the network, then go to the media center after school to fi nish their work. Students can also work cooperatively through the network.


6. Security

Files and programs on a network can be designated as “copy inhibit,” so that you do not have to worry about illegal copying of programs. Also, passwords can be established for specifi c directories to restrict access to authorized users.

Advantages and disadvantages of a Wireless LAN

Wireless LANs have advantages and disadvantages when compared with wired LANs. A wireless LAN will make it simple to add or move workstations, and to install access points to provide connectivity in areas where it is diffi cult to lay cable. Temporary or semi-permanent buildings that are in range of an access point can be wirelessly connected to a LAN to give these buildings connectivity. Where computer labs are used in schools, the computers (laptops) could be put on a mobile cart and wheeled from classroom to classroom, providing they are in range of access points. Wired network points would be needed for each of the access points.

Advantages

• It is easier to add or move workstations

• It is easier to provide connectivity in areas where it is diffi cult to lay cable

• Installation can be fast and easy and can eliminate the need to pull cable through walls and ceilings

• Access to the network can be from anywhere in the school within range of an access point

• Portable or semi-permanent buildings can be connected using a wireless LAN

• Where laptops are used, the ‘computer suite’ can be moved from classroom to classroom on mobile carts


• While the initial investment required for wireless LAN hardware can be similar to the cost of wired LAN hardware, installation expenses can be signifi cantly lower

• Where a school is located on more than one site (such as on two sides of a road), it is possible with directional antennae, to avoid digging trenches under roads to connect the sites

• In historic buildings where traditional cabling would compromise the façade, a wireless LAN can avoid drilling holes in walls

• Long-term cost benefi ts can be found in dynamic environments requiring frequent moves and changes

• They allows the possibility of individual pupil allocation of wireless devices that move around the school with the pupil.

Disadvantages

• As the number of computers using the network increases, the data transfer rate to each computer will decrease accordingly

• As standards change, it may be necessary to replace wireless cards and/or access points

• Lower wireless bandwidth means some applications such as video streaming will be more effective on a wired LAN

• Security is more diffi cult to guarantee, and requires confi guration

• Devices will only operate at a limited distance from an access point, with the distance determined by the standard used and buildings and other obstacles between the access point and the user

• A wired LAN is most likely to be required to provide a backbone to the wireless LAN; a wireless LAN should be a supplement to a wired LAN and not a complete solution


• Long-term cost benefi ts are harder to achieve in static environments that require few moves and changes

• It is easier to make a wired network ‘future proof’ for high data transfer.

REVIEW QUESTIONS:

1. Explain briefl y CIM as concept and technology.

2. State the CASA/SME in CIM.

3. What do you mean by communication matrix?

4. Explain CIM data transmission method.

5. Discuss about serial and parallel data transmission.

6. Explain about Asynchronous data transmission also explain about synchronous data transmission method.

7. Explain the types of communication system.

8. What are the communication in CIM.

9. What do you understand about

(i) PTP.

(ii) Star Netwrok.

(iii) Multiplexing.

10. How can you differentiate the multiplexing schemes?

11. Explain computer networking in CIM, also state its principle.

12. Explain briefl y Network topologies with neat sketch.

13. Describe:

(a) MAP

(b) LAN

3.1 INTRODUCTION TO GROUP TECHNOLOGY

Group Technology (GT) is a manufacturing philosophy in which similar parts

are identifi ed and grouped together to take the advantage of their similarities in

design and production. Similar parts are arranged into part families. Each family

possesses similar design and/or manufacturing characteristics.

Example:

A plant producing 5000 different parts may be able to group into 25-35 part

families. Since each member of a family have almost similar processing activities,

grouping of machines required for the processing of all the members of a part family

leads to an effi cient manufacturing method. This groups of machines are known as

cells (GT cells). Manufacturing method is known as cellular manufacturing.

3UNIT

Group Technologyand Computer Aided

Process Planning


3.1.1 History of Group Technology

In 1925, R. Flanders presented a paper in the United States before the American Society of Mechanical Engineers in which he described a way of organizing manufacturing at Jones and Lamon Machine Company that would today be called group technology.

In 1937, A. Sokolovskiy of the Soviety Union described the essential features of group technology by proposing that parts of similar confi guration be produced by a standard process sequence, thus permitting fl ow line techniques to be used for work normally accomplished by batch production

In 1949, A. Korling of Sweden presented a paper (in Paris, France) on “group production,” whose principles are an adaptation of production line techniques to batch manufacturing. In the paper, he describes how work is decentralized into independent groups, each of which contains the machines and tooling to produce “a special category of parts.”

In 1959, researcher S. Mitrofanov of the Soviet Union published a book entitled Scientifi c Principles of Group Technology. The book was widely read and is considered responsible for over 800 plants in the Soviet Union using group technology by 1965. Another researcher, H. Opitz in Germany studied work parts manufactured by the German machine tool industry and developed the well-known parts classifi cation and coding system for machined parts that bears his name.

In the United States, the fi rst application of group technology was at the Langton Division of Harris-Intertype in New Jersey around 1969. Traditionally a machine shop arranged as a process type layout, the company reorganized into “family of parts” lines each of which specialized in producing a given part confi guration. Part families were identifi ed by taking photos of about 15% of the parts made in the plant and grouping them into families. When implemented, the changes improved productivity by 50% and reduced lead times from weeks to days.

3.2 ROLE OF GT IN CAD/CAM INTEGRATION

An important aspect of Group Technology is that it often helps to minimize unnecessary variety of components in a manufacturing plant by making designers

GROUP TECHNOLOGY AND COMPUTER AIDED PROCESS PLANNING 3.3

aware of existing similar components. Often design engineers are unaware of the existence of similar designs in current production, perhaps because the part numbering system does not carry suffi cient information to allow them to retrieve designs from the CAD system. In these circumstances parts effectively tend to be duplicated, perhaps with minor differences which are unnecessary to the parts role in the end product. Among other problems, unnecessary part numbers lead to a proliferation of paperwork and increased stock.

The use of GT codes to retrieve data is also useful when it comes to process planning. Process planners rather than starting from zero with each new part to be planned can review the process plan for a similar part (i.e., a part with a similar GT code) and modify it to develop the process plan for the new part.

Group technology and cellular manufacturing are applicable in a wide variety of manufacturing situations. GT is most appropriately applied under the following conditions:

• The plant currently uses traditional batch production and a process type layout.

• The parts can be grouped into part families.

Each machine cell is designed to produce a given part family or limited collection of part families. So it must be possible to group parts made in the plant into families. However, it would be unusual to fi nd a mid-volume production plant in which parts could not be grouped into part families.

There are two major tasks that a company must undertake when it implements group technology. These two tasks represent signifi cant obstacles to the application of GT.

1. Identifying the part families

If the plant makes 10,000 different parts, reviewing all of the part drawings and grouping the parts into families is a substantial task that consumer a signifi cant amount of time.


2. Rearranging production machines into machine cells

It is time consuming and costly to plan and accomplish this rearrangement, and the machines are not producing during the changeover.

3.3 BENEFITS OF GROUP TECHNOLOGY

• GT promotes standardization of tooling, fi xturing, and setups.

• Setup times are reduced, resulting in lower manufacturing lead times.

• Worker satisfaction usually improves when workers collaborate in a GT cell

• Material handling is reduced because parts are moved within a machine cell rather than within the entire factory.

• Process planning and production scheduling are simplifi ed.

• Higher quality work is accomplished using group technology.

3.4 PART FAMILY

3.4.1 Defi nition

A part family is nothing but the collection of parts which are similar in geometric shape and size or similar steps of manufacturing process are required in the production.

3.4.2 Types of part family1. Design part family.

2. Manufacturing part family.

(1) Design Part FamilyA part family with similar design characteristic and features are grouped in a

family is known as design part family.

The basic thinking of design engineers will be interm of function and performance and the design should be creative. Surveys in manufacturing industries


have repeatedly indicated that there is a considerable amount of similarities available in the part manufacturing. (Refer fi gure 3.1)

Figure 3.1: Part family

According to one important survey is found that 90% of the 3000 part made by a manufacturing industry fall into only fi ve major families of the part.

Example:A pump has the basic components such as motor, housing, shaft, seals and

fl anges. Inspite of variety of pumps manufactured, each of these components is basically same interms of design and manufacturing methods. Consequent all shafts can be placed in one family of shafts.


Creating new parts and introducing new parts are quit expensive. Therefore the design of the part should be modifi ed to a common structure, that will reduced the cost considerably. Otherwise for the new parts, the design engineer should prepare one or more NC programs, new process plans, new fi xtures and new tools.

The fi gure 3.1 (a) and (b) illustrates examples of two parts from the same family. This parts are placed in same family due to its similarity in size and other design features. They have exactly same shape and size but the area of production is differ because different fi nishing process.

Similarly from the fi gure 3.1 (c) and (d), it is observed that the shape and size are different but the operations are same. That is three holes must be provided. It has same manufacturing characteristics but different shapes even though these parts are grouped with respected manufacturing process. (Refer fi gure 3.2 & 3.3)

Figure 3.2: Thirteen parts with similar manufacturing process requirementsbut different design attributes.


Figure 3.3: Grouping parts according to their geometric similarities

(2) Manufacturing Part FamilyManufacturing part family is really grouping the manufacturing machines

into separate workcells. That is traditional arrangement of the machines have to be arranged according to function.

For batch production, if the machines of the same type are arranged in group of lathe, group of milling machine, group of drilling machine and grouping of grinding machine, then there is a considerable random movement. Now this will create unnecessary movement of workpiece

During machining of a given part, the workpiece may visit several time to the particular machine. This results in an improper material handing, a large in process inventory, more manufacturing lead time, more loading time and high cost. Such a arrangement is not effi cient due to time wastage and effort


Therefore, manufacturing part families are used to arrange machines are arranged into cells. Each cell is organised to specialize in the manufacture of a particular part family.

In the fi gure 3.4, all similar machines are arranged in a single way in functional layout of machine tool in a traditional plant. Arrows indicate the fl ow of material and parts in various stages of completion, and it shows that there is a complexity in transportation of fi nished parts. In group technology layout, the machines are arranged based on the fl ow of product.

(a) Functional layout of machine tools in a traditional plant

(b) Group Technology

Figure 3.4: Layout of Machine tools


3.5 METHODS OF GROUP PARTS INTO FAMILIES

There are three general methods available for change over to group technology from traditional production process. These methods are time consuming and involve the analysis of much data by properly trained personnel.

1. Visual inspection.

2.. Production Flow Analysis (PFA).

3. Classifi cation and coding by examination of design and production data.

3.5.1 Visual Inspection

Visual inspection method is the least sophisticated, simplest and least expensive method. The visual inspection involves the classifi cation of parts into families by looking at either the physical part, photographs of the parts or drawing of the parts and arranging them into similar groupings. But this method is less accurate to compare with other methods.

3.5.2 Production Flow Analysis (PFA)

Production fl ow analysis is a technique for identifying part families and associated grouping of machine tools. It does not use a classifi cation and system and part drawing to identify families. Production fl ow analysis makes the use of information contained on route sheets instead of part drawing. Work part with identical or similar routings are classifi ed into part families. These groups can then be used to form logical machine cells in a group technology layer.

3.5.2.1 Steps to be followed to carry out production fl ow analysisi) Route sheet for all the parts to carryout production fl ow analysis.

ii) The matrix shows operation numbers and the component number is prepared showing which component requires which operations.

iii) Any particular part is included only in one group. For facility grouping, one machine type should be only is one group.


iv) If any operation is required by only one or very few components or if some operation is required by all the components, then these operations should not be taken in account while deciding the groups.

3.5.2.2 Procedure for production fl ow analysis

Procedure for production fl ow analysis can be organised into four categories.

i) Data collection.

ii) Sorting of process routing.

iii) PFA chart.

iv) Analysis.

i) Data collectionThe fi rst step in the PFA procedure is to decide on the scope of study and

to collect the necessary data. The scope defi nes the population of the parts to be analysed.

The minimum data required for PFA are part numbers and machine routing.

This data can be obtained from routing sheet additional data, such as lot size, time standards and annual production rate, might be useful for designing machine cells of the desired production capacity.

ii) Sorting of process routingThe second strip is to arrange the parts into groups according to the similarity

of their process routing. The computer card format for organising the process routing data in PFA allows space for the part number, sequence of code and other data like lot size that identify particular machines in routing. (Refer fi gure 3.5)


Figure 3.5: Card format for organizing process routing data in PFA.

A sorting procedure is used on the cards to arrange them into “Packs”. The packs are nothing but a group of parts with identical process routing. Some packs may contain only one part number. A pack identifi cation number or letter given on each pack.

Table 3.1: Possible code numbers to indicated processes and machines(Highly simplifi ed).

Process Code Process Code

Cut-off 01 Shaper 13

Lathe 02 Planer 14

Turret lathe 03 Broach 15

Chucker 04 Deburr 16

Drill manual 05 Polish 17

NC drill 06 Buff 18

NM 07 Clean 19

Bore 08 Paint 20

Grind-surface 09 Plate 21

Grind-exterior cylinder 10 Assemble 22

Grind-interior cylinder 11 Inspect 23

Grind-centerless 12 Package 24


iii) PFA chartThe PFA chart is graphical representation of the process used for each pack.

A simplifi ed version of PFA chart is shown in fi gure 3.6. The chart is merely a plot of the process code number for all the packs that have been determined.

Figure 3.6: PFA chart (Highly simplifi ed).

iv) AnalysisIt is the most diffi cult and most subjective method in PFA. Due to the crucial

slip in procedure, the data exhibited in PFA chart has to be analysed and similar groups have to be identifi ed. If any similar groups are found in the PFA chart, must be rearranged to a new pattern which brings together packs with similar ratings. The possible re-arranged PFA chart is shown in fi gure 3.7.


Figure 3.7: Rearranged PFA chart, indicating possible machine groups.

The different groups are identifi ed with in blocks. The machines identifi ed with in blocks would be synthesised is to logical machine cell. This can be analysed to develop a revised process sequence. Sometimes, there is no necessary of rearrangement of PFA chart. Then, these parts must continue its own PFA chart.

Disadvantages of PFA1. It provides no mechanisms for rationalizing the manufacturing routings.

2. There is no consideration being given to routing sheet whether the routing are optimal or consistent or even logical.

3. The main weakness of PFA is that data used in the analysis are derived from production route sheet, but the process sequence from these sheet


have been prepared by different process planner, so there is a chances of differences which may refl ect on routing sheet This will cause non-optimal illegical and unnecessary slips.

3.6 CLASSIFICATION AND CODING

In group technology, the parts are identifi ed and grouped into families by classifi cation and coding (C/C system) systems,

3.6.1 Parts Classifi cation

1. System based on part design attributes.

2. System based on manufacturing attributes.

3. System based on both design and manufacturing attributes.

3.6.1.1 System based on part design attributesIn pertain to similarities in geometric features and consist of the following:

i) External and internal shapes and dimensions.

ii) Aspect ratio. (Length-to-width or length-to-diameter).

iii) Dimensional tolerance.

iv) Surface fi nishes.

v) Path functions.

vi) Material type.

vii) Major dimensions.

3.6.1.2 System based on manufacturing attributesIn pertains to similarities in the methods and the sequence for the manufacturing

operations performed on the part. The selection of manufacturing process depends on many factors such as shapes, dimensions and other geometric features of the part. The manufacturing attributes of a part consist of the following:

i) The primary production processes used.


ii) The secondary and fi nishing process used.

iii) The dimensional tolerance and surface fi nish.

iv) The sequence of operations performed.

v) The tools, dies fi xtures and machinery used.

vi) The production quality.

vii) The Production rate

viii) Production time.

ix) Major dimensions.

x) Basic external shape.

3.6.1.3 System based on design and Manufacturing system attributes

Figure 3.8

This system contains the best characteristics of both design and manufacturing attributes. There is a vital link between the design and manufacturing system attributes, by means of group technology as shown in fi gure 3.8.

3.6.2 Coding

The coding of parts can be based on a particular company’s own system or it can be based on commercial classifi cation and coding system. The classifi cation and coding system (C/C system) must be highly competent to the other system. The


code structure generally consist of numbers, letters or combinations of two. Each specifi c component of product is assigned a code. The code may pertain less than twelve digit design attributes or less than twelve digit manufacturing.

In most advanced system it may pertains thirty digit attributes of both design and manufacturing.

1) Hierarchical coding.

2) Poly code.

3) Decision-Tree coding.

3.6.2.1 Hierarchical coding• It is also called monocode.

• This code, the interpretation of each succeeding digit depends on the value of preceding digit. (Refer fi gure 3.9)

Figure 3.9: Hierarchical coding.

• Each symbol amplifi es the information contained in the preceding digit.

• Therefore a digit code cannot be interpreted alone.

• The main advantages of this system is a short code contains large amount information. But it is complicated to apply in a computerized system.


3.6.2.2 Decision tree coding

• It is also known as hybrid code.

• This is most advanced system. It combines both design and manufacturing attributes a as shown in fi gure 3.10 & 3.11.

Figure 3.10

Figure 3.11: Decision-tree classifi cation for a sheet-metal bracket.

3.7 CODING SYSTEM

The six major industrial coding system are:

1. OPITZ system.


2. KK-3 system.

3. DCLASS system.

4. Multi-class system.

5. CODE system.

6. RNC-6 digit Mono code system.

3.7.1 OPTIZ Coding System

It is oldest scheme of classifi cation and coding system. It was developed by H.OPTIZ university of Aachen is West Germany.

OPTIZ system uses the following sequence as shown in fi gure 3.12.

Figure 3.12: OPTIZ system

The form codes (1 2 3 4 5) describes the primary design attributes of the part. The supplementary codes (6 7 8 9) describes the manufacturing attributes like dimensions, work material, shape of raw material and accuracy of material.

The secondary code (A B C D) describes the type of production and sequence of production. The secondary code may be designed by the fi rm to serve its own particular needs. The basic structure of the OPTIZ system of part classifi cation and coding is shown in fi gure 3.13.

In the form code the fi rst digit indicates whether the part is rotational or not. The other digits indicates the general shape and proportions of the part.

The survey of rotational part is limited with code value 0,1,2 as shown in fi gure 3.14 because there is no unique features

For general, the classifi cation of work parts, the coding of fi rst fi ve digits is also mentioned in the fi gure 3.14.


Figu

re 3

.13:

Bas

ic st

ruct

ure

of th

e op

tiz sy

stem

of p

arts

cla

ssifi

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ding

.


Figure 3.14: Form code (digits 1 through 5) for rotational parts in the optiz system. Part classes 0.1, and 2.


3.7.2 Machine Class Coding System

Figure 3.15: Code determined for part-Multiclass system.


• It was developed under the name of MICLASS (Metal Institute CLASSifi cation) system, Netherland.

• This system was developed to help automated and standardize several design, production and management function.

• It involves the range from 12 to 13 digits The fi rst 12 digits are universal code and remaining 18 digits are used to code data that are specifi c to the particular company or industry.

• This system is used interactively with computer and asks the user many questions On the bases of the answers, the computers automatically assigns a code number to the part. The software is available in modules that can be linked. (Refer fi gure 3.15)

3.7.3 K-K-3 System

• It is a general purpose coding system for the parts that are to be machined.

• It is 21 digits decimal system

• This code is much lengther than the previous systems.

• It classifi es the dimensions and aspect ratio of the parts

• It was developed by Japan Society for the promotion of machine industry. (Refer fi gure 3.16)

Digit Items (Rotational components)1.

Parts nameGeneral classifi cation

2. Detail classifi cation3.

MaterialsGeneral classifi cation

4. Detail classifi cation5.

Major dimensionsLength

6. Diameter7. Primary shapes and ratio of major of dimensions


8.

External surface

External surface and outer primary shape

9. Concentric screw threaded parts10. Functional cut-off parts11. Extraordinary shaped parts12. Forming13. Cylindrical surface14.

Shap

e de

tails

and

kin

ds o

f pr

oces

ses

Internal surface

Internal primary shape 15. Internal curved surface16. Internal fl at surface and cylindrical

surface17. End surface18.

Nonconcentric holesRegularly located holes

19. Special holes20. Noncutting process21. Accuracy

Figure 3.16: The structure of a KK-3 system for rotational components.

3.7.4 Code System

• It consists of eight digits.

• Every digit in eight digit contains 16 possible values (zero through 9 and A through F) which are used to describe the part’s design and manufacturing attributes.

• The initial digit position indicates the basic geometry of the part is called the major division of the CODE system.

• This digit would be used to specify whether the shape was a cylinder, fl at piece, block or other.

• The interpretation of the remaining seven depends on the value of the fi rst digit, but these digits form a chain type structure. (Refer fi gure 3.17)


Figure 3.17: CODE system

2nd and 3rd digits Basic geometry and principle manufacturing process.

4th, 5 th and 6th digit Secondary manufacturing process, example, threads, grooves, slots, etc.,

7th and 8th digits Overall size of the part.

3.7.5 D-Class System

• It also consists of eight digits and they are divided into fi ve code segments. (Refer fi gure 3.18)

• The fi rst segment composed of three digits, which is used to denote the basic shape of the part.

• The second segment denotes the form features of the code

• The features may be such as complexity of the part which includes (such as holes and slots) heat treatments, and special surface fi nishes

• The complexity is determined by the number of special features.

• The third segment indicates the over all size envelop of the coded part which composes one-digit-size code.


• The fourth segment indicates the precision which also composes one digit in length. The fi nal segment comprises two code digit, which is used to denote the material type.

Figure 3.18: D - CLASS system

3.7.6 RNC System

Figure 3.19: RNC system.


3.8 FACILITY DESIGN USING GROUP TECHNOLOGY

Design of the machine cell is critical in cellular manufacturing. The

performance of a GT cell depends on its design. An effective cell design is governed

by the following.

i) Machine cell types.

ii) Cell layouts.

iii) Key machine concept.

3.8.1 Machine cell types

1. Single machine cellIt consists of one machine with necessary fi xtures and tooling. This type of

cell is suitable to work parts with attributes that allow them to be processed on

turning or milling.

2. Group machine cell with manual handlingIt consists of more than one machine with manual handling of material between

the machines. This cell is organized in to a U shaped layout as shown in fi gure 3.20

(a). This allows a) variations in the workfl ow b) Using of multifunction workers and

c) To use the existing process-type layout machines without re-arrangement.

3, Group machine cell with semi-integrated handlingIt consists of more than one machine with the use of conveyor to move the

parts between machines in the cell

4. Flexible Manufacturing System (FMS)It is the combination of fully automatic processing machines and automated

material handling systems.


3.8.2 Cell Layouts

• Different cell layouts are U - shape (fi gure 3.20 (a)), in - line (fi gure 3.20 (b)), loop (fi gure 3.20(c)), and rectangular (fi gure 3.20 (d)).

• U-shape layout make use of manual material handling and others semi automated material handling.


Figure 3.20: Machine cells with semi-integrated handling.

3.8.3 Key machine concept

In a GT machine cell there is certain machine which is more expensive or doing critical operations. This is known as key machine. Other machines are known as supporting machines. The arrangement of supporting machines are in such a way that they must keep the key machine busy. That is, the utilization of key machine must be more than that of supporting machines.

3.9 ADVANTAGES OF GROUP TECHNOLOGY

1. Speed of the throughput: The time taken by components moving from raw materials to fi nished part is greatly reduced.

2. “Work in process” in clearly substantially reduced because of increase in speed of throughput.

3. Scrap-the rate of scrap is dropped and familiarity with limited range of components seems the logical reason.

4. Material handling: Special equipment can be devised for the family and group and the level of automation rises signifi cantly.

5. Design retrieval can be possible.


6. Control over production: When dealing with the administration of the group of machines, control is easier than when all production in under functional layout.

7. Application to other spheres of activities is possible like shipping inspection etc.,

8. Rate fi xing becomes easier.

9. Special tools and fi xtures can be economically devised.

3.10 DISADVANTAGES OF GROUP TECHNOLOGY

1. The cost of implementation is generally high. This is because an outside consultant is often required since in house expertise on GT is rarely a available. It requires a long set up time and painful debugging.

2. Group technology may not be suitable for a factory with a very large variety of products.

3. The entire production of the company cannot be put under the GT and hence GT will have to coexist with the conventional layouts.

4. There are too many GT codes in use and there is no one GT code that suits all applications.

5. It is often diffi cult to conceive all the operations for a group of components being taken care of in the cell created for it.

6. The range of product mix in a plant may be under constant change in which case the GT cells may need constant revision which is impractical.

3.11 CELLULAR MANUFACTURING

3.11.1 Defi nition

Cellular manufacturing is an application of group technology in which dissimilar machines or processes have been aggregated into cells, each of which is dedicated to the production of a part or product family or a limited group of families


3.11.2 Objectives

The typical objectives in cellular manufacturing are similar to those of group technology:

1. To shorten manufacturing lead times: Shorten manufacturing lead times, by reducing setup, workpart handling, waiting times, and batch sizes.

2. To reduce work-in-process inventory: Smaller batch sizes and shorter lead times reduce work-in-process.

3. To improve quality: This is accomplished by allowing each cell to specialize in producing a smaller number of different parts. This reduces process variations.

4. To simplify production scheduling: The similarity among parts in the family reduces the complexity of production scheduling. Instead of scheduling parts through a sequence of machines in a process-type shop layout, the parts are simply scheduled though the cell.

5. To reduce setup times: This is accomplished by using group tooling (cutting tools, jigs, and fi xtures) that have been designed to process the part family, rather than part tooling, which is designed for an individual part. This reduces the number of individual tools required as well as the time to change tooling between parts.

3.11.3 Composite Part Concept

Part families are defi ned by the fact that their members have similar design and/or manufacturing features

The composite part concept takes this part family defi nition to its logical conclusion. It conceives of a hypothetical part, a composite part for a given family, which includes all of the design and manufacturing attributes of the family

An individual part in the family will have some of the features that characterize the family but not all of them. The composite part possesses all of the features.


There is always a correlation between part design features and the production operation required to generate those features. Round holes are made by drilling, cylindrical shapes are made by turning, fl at surfaces by milling, and so on.

A production cell designed for the part family would include those machines required to make the composite part. Such a cell would be capable of producing any member of the family, simply by omitting those operations corresponding to features not possessed by the particular part.

The cell would also be designed to allow for size variations within the family as well as feature variations.

a) The composite part for a family of machined rotational parts.

b) The individual features of the composite part

Figure 3.21: Composite part concept.

Consider the composite part in fi gure 3.21 (a). It represents a family of rotational parts with features defi ned in fi gure 3.21 (b). Associated with each feature is a certain machine operation as summarized in table 3.2. A machine cell to produce this part family would be designed with the capability to accomplish all seven operations required to produce the composite part (the last column in the table

To produce a specifi c member of the family, operations would be included to fabricate the required features of the part. For parts without all seven features,


unnecessary operations would simply be omitted. Machines, fi xtures, and tools would be organized for effi cient fl ow of workparts through the cell

In practice, the number of design and manufacturing attributes is greater than seven and allowances must be made for variations in overall size and shape of the parts in the family. Nevertheless, the composite part concept is useful for visualizing the machine cell design problem.

Table 3.2: Design features of the composite part and the manufacturing operations required to shape those features.

Lable Design featureCorresponding manufacturing

operation1. External cylinder Turning2. Cylinder face Facing3. Cylindrical step Turning4. Smooth surface External cylindrical grinding5. Axial hole Drilling6. Counterbore Counterboring7. Internal threads Tapping

3.11.4 Machine Cell Design

• Design of the machine cell is critical in cellular manufacturing.

• The cell design determines to a great degree the performance of the cell. In this subsection, we discuss types of machine cells, cell layouts, and key machine concept.

3.11.4.1 Types of machine cells and layoutsGT manufacturing cells can be classifi ed according to the number of machines

and the degree to which the material fl ow is mechanized between machines.

Types of machine cells:a. The single machine cell


b. Group machine cell with manual handling (type IIM generally, type III M less common).

c. Group machine cell with semi-integrated handling (type IIM generally, type III M less common).

d. Flexible manufacturing cell or fl exible manufacturing system (type IIA generally, type III A less common).

a. The single machine cell• The single machine cell consists of one machine plus supporting fi xtures

and tooling. This type of cell can be applied to workparts whose attributes allow them to be made on one basic type of process such as turning or milling:

• For example, the composite part of fi gure 3.21 could be produced on a conventional turret lathe with the possible exception of the cylindrical grinding operation.

2. Group machine cell with manual handling• The group machine cell with manual handling is an arrangement of more

than one machine used collectively to produce one or more part families.

• There is no provision for mechanized parts movement between the machines in the cell Instead, the human operators who run the cell perform the material handling function.

• The cell is often organized into a U-shaped layout (as shown in fi gure 3.22). This layout is considered appropriate when there is variation in the work fl ow among the parts made in the cell

• It also allows the multifunctional workers in the cell to move easily between machines

• The group machine cell with manual handling is sometimes achieved in a conventional process type layout without rearranging the equipment.


This is done simply by assigning certain machines to be included in the machine group and restricting their work to specifi ed part families

• This allows many of the benefi ts of cellular manufacturing to be achieved without the expense of rearranging equipment in the shop. Obviously, the material handling benefi ts of GT are minimized with this organization.

Figure 3.22: Machine cell with manual handling between machines. Shown is a U-shaped machine layout

3. Group machine cell with semi - integrated handling

The group machine cell with semi-integrated handling system such as a conveyor, to move parts between machines in the cell.

4. Flexible manufacturing cell or fl exible manufacturing system

The fl exible manufacturing system (FMS) combines a fully integrated material handling system with automated processing stations. The FMS is the most highly automated of the group technology machine cells.

Types of layouts

A variety of layouts are used in GT cells. The U-shape, as in fi gure 3.22, is a popular confi guration in cellular manufacturing. Other GT layouts include in-line, loop, and rectangular, shown in fi gure 3.23 for the case of semi-integrated handling.


Figure 3.23: Machine cells with semi-integrated handling

Figure 3.24: Four types of part moves in a mixed model production system. The forward fl ow of work is from left to right.


Determining the most appropriate cell layout depends on the routing of parts produced in the cell.

Four types of part movement can be distinguished in a mixed model part production system. They are illustrated in fi gure 3.24 and are defi ned as follows, where the forward direction of work fl ow is defi ned as being from left to right in the fi gure:

1. Repeat operation, in which a consecutive operation is carried out on the same machine, so that the part does not actually move.

2. In-sequence move, in which the part moves from the current machine to an immediate neighbour in the forward direction.

3. By-passing move, in which the part moves forward from the current machine to another machine that is two or more machines ahead.

4. Backtracking move, in which the part moves from the current machine in the backward direction to another machine.

When the application consists exclusively of in-sequence moves, then an in-line layout is appropriate.

A U-shaped layout also works well here and has the advantage of closer interaction among the workers in the cell.

When the application includes repeated operations multiple stations (machines) are often required.

For cells requiring by-passing moves, the U-shape layout is appropriate.

When backtracking moves are needed, a loop or rectangular layout is appropriate to accommodate recirculation of parts within the cell.

3.11.4.2 Factors accounted for cell design

(a) Quantity of work to be done by the cell This includes the number of parts per year and the processing (or assembly)

time per part at each station. These factors determine the workload that must


be accomplished by the cell and therefore the number of machines that must be included as well as total operating cost of the cell and the investment that can be justifi ed.

(b) Part size, shape, weighty and Other physical attributesThese factors determine the size and type of material handling and processing

equipment that must be used.

3.11.4.3 Key machine conceptIn some respects, a GT machine cell operates like a manual assembly line

and it is desirable to spread the workload evenly among the machines in the cell as much as possible. On the other hand, there is typically a certain machine in a cell (or perhaps more than one machine in a large cell) that is more expensive to operate than the other machines or that performs certain critical operations in the plant.

This machine is referred to as the key machine. It is important that the utilization of this key machine be high even if it means that the other machines in the cell have relatively low utilization.

The other machines are referred to as supporting machines and they should be organized in the cell to keep the key machine busy.

In a sense, the cell is designed so that the key machine becomes the bottleneck in the system.

3.12 PROCESS PLANNING

Process planning is an important task in discrete part manufacturing. In the production process, there is a sequence of production operations through which the raw material is effectively converted into fi nished product. So, there must be a perfect production planning and production design is necessary to complete the product. This process of decision making is called process planning. The sequence of operations are recorded on a route sheet. The route sheet consists of the following informations.


• Cutting conditions (feed and spindle speed).

• Total time of production and production cost.

• Planning the process.

Current trends in routing sheet is to store relevant data in computers and affi x a barcode to the part, that serves as a key into the database of parts information. Process planning depends upon the

• Type of the product.

• Quantity of product.

• Type of raw material and parts.

• Production facilities and technology on hand.

3.12.1 Basic functions of process planning

Process planning is earned out in two stages:

1. Process design.

2. Operation design.

(1) Process designProcess design is macroscopic decision-making of an overall process route

for converting the raw material into fi nished product.

(2) Operation designOperation design is microscopic decision-making of an individual operations

contained in the process route.

3.13 COMPUTER AIDED PROCESS PLANNING (CAPP)

• CAPP is an automatic process planning functions by means of computers.

• CAPP accomplishes the complex task of production planning by viewing the total operations as an integrated system, so that the individual


operations and steps involved in production are co-ordinated perfectly with other system and are performed effi ciently and reality with the help of computers.

• CAPP requries extensive software and good co-ordinates with CAD/CAM and it is a powerful tool for a effi ciently planning and scheduling the manufacturing operations.

• CAPP is effective in small volume, high variety parts production requiring machining forming and assembly operations.

• CAPP requires vast amount of knowledge and experience in manufacturing methods and technology.

• It is necessary to fi nd technical loss concerning the sequence of operations associated with each given shape to be manufactured based upon past experiences of process design and to make a process fi le.

3.13.1 Role of process planning in CAD/CAM integration

The process planning system involves a series of the following steps in manufacturing process.

1. Interpretation of product design data.

2. Selection of machining process and tools, jigs and fi xtures.

3. Determination of processing and geometrical shape.

4. Determination of datum surfaces.

5. Determination of sequence of operations.

6. Determination of production tolerance

7. Determination of cutting conditions.

8. Calculation of production time.

9. Generation of route sheet.


3.13.2 Approaches to CAPP

There are four basic important approaches to perform the task of process planning. They are:

1. Manual approach.

2. Variant approach.

3. Generative approach.

4. Hybrid approach.

Among these, the Hybrid approach is an approach to perform the task of process planning which combines both variant and generative type. Each approach is appropriate under certain conditions. Therefore, knowledge of nature, advantages and limitations are important.

3.13.2.1 Manual Approach• The traditional manual approach involves examining an engineering part

drawing and developing manufacturing process plans and instructions.

• These are based upon knowledge of process and machine capabilities, tooling, materials, related costs and shop practices etc.

• This approach requires very skilled manufacturing analyst to develop process plans which are feasible, low cost and consistent with plans for similar parts.

• If the part to be produced belongs to an existing product design, of similar parts, then the process planning involves recalling that existing process plans of a similar part and modifying them to create a new routing for the new product.

• Work books or other “Data management” methods are often used to manually classify, store and retrieve that information.

• If the part to be produced is new, the planner may have to generate a routing as a unique plam.


• The manual approach is highly subjective, labour intensive, time consuming, tedious, and often boring.

• Further more, the task requires personnel well trained and experienced in manufacturing shop fl oor activities.

There are two levels of process planning which is applicable for both manual as well as CAPP approaches. They are:

i) High-level planning.

ii) Low-level planning.

i) High-level planningIn high-level planning the planner identifi es the machinable features (surfaces)

of the part, groups them into setups, and orders these setups.

Each setup is listed in the order in which it is to be done, the features to be cut in each of the setups, and the tools for cutting each feature.

ii) Low-level planningIn low-level planning, the planner specifi es the details of performing the each

step that results from the fi rst level (choosing machines, cutting conditions, type fi xturing, cost and time estimates, etc.,).

Planner through manual process will follow a less consistent set of steps to develop process plans for new product. These steps involve primarily stock preparation, plan preparation, and specifi cation. In detail, it has follows

1. Get orientedThe process planner will go through the engineering drawing to identify the

basic structure, and checks for any major problems.

2. Recognize outer envelope of the fi nished partWith the help of the engineering drawing, the planner can easily recognize the

outer, or bounding envelope of the part. The recognition includes both the geometric


shape and the surface fi nish of the envelope, which determines the optimal shape of stock in order to produce the fi nished part.

3. Choose the optimal stockWhile selecting the stock, the dimensions of the stock are about 1/4 in larger

than the fi nished part’s dimensions which could be selected based on the above step.

4. Recognize part featuresThe second component of the fi nished part recognition is listing its feature

and sub features that are subtracted from its outer envelope.

5. Choose a stock preparation planThe next step is for the planner to outline all methods for getting the raw

material (stock) into an accurate shape with minimum scrap, and them it is graphed.

6. Consider alternative methods for producing each feature

7. Generate a plan by exploring feature interactionsThe diffi culty in generation of plan is that fi nding an order to machine the

features in which no sub goal interferes too seriously with achieving the others. The fi nal generation plan of the part may be expressed graphically in what may be called a “Feature interaction graph”.

8. Integrate the squaring graph with the feature integration graphThe graphs obtained in steps 5 and 7 are merged with as much overlap between

the steps as possible so that we can get a compact sequence. The more overlap, the better and more concise the fi nal plan. The resulting graph is the fi nal plan outline represents the fi nal ordering for the desired process plan.

9. Verify the planIn this, the planner typically verifi es the plan by checking the setups are

actually feasible, that the clamps are not in the way of the tools, etc.


10. Elaborate the process planThe above nine steps represent the high-level planning of the process plan.

This step represents the low-level planning. Here, the planner generates more details for producing each individual feature, choosing feeds and speed, estimating costs and standard times etc.,.

After this, the planner releases the process plan to the various department for execution.

The manual approach is the best approach for small companies with few process plans to generate. A good analyst can create process plans which a accurate, farily consistent and cost effective with minimum scrap.

Advantages of manual approach1. Good fl exibility.

2. Investment cost is low.

Disadvantages of manual approach1. This becomes rapidly ineffi cient and unmanageable when the number

of process plans and revisions to those plans increases.

2. It requires large time for planning.

3. Inconsistent plans.

4. This approach always refl ects the personal experiences and preferences, prejudices of the process planner.

3.13.2.2 Variant Approach• Variant process planners use existing process plans, then allow the user to

edit the plan for their new parts.

• The variant CAPP systems are based on GT and parts classifi cation and coding.


• ln this system, a standard process is stored in computer fi les for each part code number and the process plan for new part is created by identifying recalling and retrieving and existing plan for similar part and the plan is edited for any modifi cation.

• The standard plans may be based on current routings or ideal plan is prepared for each family. The basic variant approach to process planning with group technology (GT) is,

i) Go through normal group technology setup procedures.

ii) After part families identifi ed, develop standard process plan for each.

iii) When a new plan has been designed, prepare a GT-code for each part.

iv) Use the GT system to lookup which part family is the closest match, and retrieve standard plan for that family.

v) Edit standard plan so that values now match the new design parameters, and add or delete steps are required.

The basic workstructure of variant CAPP (is shown in fi gure 3.25).

Figure 3.25: Basic workstructure of variant CAPP.


Figure 3.26: Operation of a retrieval computer-aided process planning system.

• The user begins by identifying group technology board for the component for which the process plan is to be determined. (Refer fi gure 3.26)

• A search is made of the part family fi le to determine, if a standard route sheet exists for a given part code .

• If a fi le contains a process plan for a part, it is retrieved and displayed for the use.

• The standard process plan is examined to determine if there is any modifi cation is necessary.

• Although the new part has the same code number, minor differences in this process might be required to make the part.

• The standard is edited accordingly If the fi le does not contain a process plan for the given code number, the user may search the fi le for a similar code number for which a standard routing exists.

• By editing the existing process plan or by starting from scratch check the develops the process plan for the new part.

• This becomes the standard process plan for the new part code number.

• The fi nal step is the process plan formatter, which prints the route sheet in the proper format. The formatter may call other application programs,


determining cutting conditions for machine tool operations, calculating standard times for machining operations or computing cost estimates. MIPLAN is an example of retrival type CAPP system.

Advantages of variant approach1. Investment cost is low.

2. Development time is less.

3. It is well suited to medium to low product mixes.

4. It can be rapidly developed for various companies and various parts.

5. It can be interfaced with other CIM operations.

6. One program can be used in radically different industries.

Disadvantages of variant approach1. GT codes cannot be used for a longer period.

2. Planning operations are comparatively slow.

3. More chances of error than generative systems.

3.13.2.3 Generative Approach• Generative process planners should create a new process plan without the

use of any existing plans. This does not imply that the process planner is automatic. It is an alternative systems to variant CAPR.

• A generative CAPP creates the process plan using systematic procedure rather than retrieving and editing the existing plans form a database. The structure of generative CAPP is shown in fi gure 3.27.

• Generative plans are generated by means of decision logics, formula technology algorthims and geometric based data used for converting a part from the raw material to fi nished state.


• The rules of manufacturing and equipment capabilities are stored in a computer system. The process sequence is planned without human assistance and predefi ned standard plans.

• The design of generative CAPP is a problem in the fi eld of expert system which is a branch of artifi cial intelligence.

• The artifi cial intelligence techniques used in GCAPP are PROPEL, GAGMAT, SAPT, XPLANE, STRIPS, TWEAK, EXCAP and the algorthmieal system like LUPRA-TOUR for turned parts, PRICAPP and ICAPP systems for milled parts.

• The expert system are computers programs that are capable of solving complex problems that normally requires a human who has years of experience and education.

Stages of generative CAPP systems i) Knowledgebase.

ii) Computer comparable part description.

iii) Inference engine.

i) Knowledge baseThe technical knowledge of manufacturing and the logic used by successful

process planners must be captured and coded into a computer program.

An expert system is applied for this process planning and it is incorporated into a knowledge has to solve process planning problems and to create route sheets.


Figure 3.27: Structure of a generative CAPP system.


ii) Computer comparable part description• Generative CAPP requires computer comparable description of the part.

The description contains the following data needed to plan the process sequence.

1. The geometric part of the model can be developed on a CAD system during product design.

2. A group technology code number of the part defi ning its features in signifi cant detail.

iii) Inference engine

Figure 3.28: General structure of generative CAPP.


• The generative CAPP system requires the capability to apply the process planning logic with calculus algorithm and process knowledge contained in the knowledge based to a given part description. CAPP system applies its knowledge base for problem solving and this problem solving procedure is referred to as the “Inference Engine”.

• By using inference engine Generative CAPP system synthesis a new process plan for each new part present to it. The general structure of Generative CAPP system is shown in fi gure 3.28.

• The several Generative CAPP system have been such as GENPLAN, CIMx APPS, METCAPP, APPAS, CMPP, EXCAP, XPALN and so on.

Advantages1. Flexibility and consistency for process planning for new parts.

2. Higher overall planning quality.

3. Planning operations are comparatively fast.

4. Generative CAPP is fully automatic.

5. It is suitable for large companies.

DisadvantagesIt requires more extensive setup.

3.13.2.4 Hybrid ApproachAn approach which combines the characteristics of both variant and generative

CAPP is known as hybrid approach. (Refer fi gure 3.29)

An example of this type of approach is “stock preparation plan”. By having standard shock scopes and knowing all the possible ways, we can prepare a given stock such type of information can be made available to the decision logic in order to shorten it.


In hybrid, object-oriented programming techniques is used i.e., Object Oriented Database Administrative System (OODBAS). The main concept used is the object.

It is described by a complex of properties which constitute the object attributes and methods.

The input information for OODBAS are graphical as well as alphanumeric. The CAPP module’s also contains the following data.

• Graphical data related to the parts (shape, size, dimensions, as basic types of object-oriented models).

• Alpha numerical data which contain technological information.

• Methods base, which comprises optimization and calculation algorithms.

Figure 3.29: Hybrid approach.


3.13.3 Product Development through Computer-Aided Process Planning (CAPP)

Process planning involves defi ning the operations required in the manufacture of a product. More specifi cally, this could involve such decisions as

a) Types of manufacturing process required.

b) Sequence of operations.

c) Machining speeds, feeds and operation times.

d) Tooling requirements.

e) Work-holding arrangements.

f) Machine selection and routing requirements.

Figure 3.30: Product development via CAPP.

An interactive C APP system, used in conjuction with a DBMS, computerises routine data-gathering, helps automate manufacturing decision-making, and makes available more of the planner’s time for methods improvement and cost-reduction programmes.

Figure 3.30 Shows how CAPP fi ts into the development of the product in a CIM organisation, and effectively provides the vital interface between design and manufacture.


Figure 3.31: Database structure of a CAPP system.

A more comprehensive representation is shown in fi gure 3.31. This shown the inputs and outputs to a common database using a commercial CAPP package called CAPES (Computer-Aided Planning and Estimating System). A system such as this can provide the following facilities:

a) “Where-used” capability, allowing the user to list parts and operations using specifi c machines, materials and tools from within the database.

b) Rapid estimating by direct generation or from simple modifi cation of “similar-to” products or jobs.

c) Speedy response to changes in materials, machines, tools and conditions.

d) Costing facilities for labour, materials, and overheads.

e) Method analysis and “what- if” simulation.

f) Graphical analysis of machine manipulation, loading, gauging and set-up.

g) Automatic documentation of route cards and operation plan sheets in either VDU display or hard-copy form.


h) Automatic release of operational data to production control, CNC part-programing, and manufacture via the common database.

i) Instant retrieval of information, assisted by coding and classifi cation systems as described in the previous section. The CAPES package uses a retrieval system called Finder. This operates via an hierarchical “tree” classifi cation system with ten possible levels.

3.13.4 Benefi ts of CAPP

CAPP has the following advantages.

i) Increase in productivity of process plannersThe systematic approach and availability of process plan in the data permits

more process plans to be developed by user. It gives more complete and detailed process plan.

ii) Product rationalisation and StandardizationCAPP leads to more logical and consistent process planning than conventional

process planning. It results better product quality and reduces the manufacturing cost.

iii) Improved legibilityCAPP improved legibility compared to manual prepared route sheets and it

reduces manual effort in preparation if routing sheet.

iv) Integrated with other applicationThe ability of CAPP programs to be interfaced with other application programs

such as cost estimating, work standards and others.

v) Effective inventoriesMore-effective use of inventories of tools, gauges, fi xtures and other tools are

possible.


vi) Reduced lead timeCAPP reduced leads time to propare process plans.

vii) Updation and AccessThe part drawing can be revised and every planner can access the same

database (central database).

viii) Other improvementsCAPP reduces calculation errors.

ix) ResponseCAPP provides faster response to engineering changes.

3.13.5 Economics of CAPP

In a detailed survey of twenty two large and small manufacturing industries using generative type CAPP, the following estimated cost saving were obtained.

i) 5 8% reduction in process planning effort.

ii) 10% saving in direct labour.

iii) 4% savings in material.

iv) 10% saving in scrap.

v) 6% saving in work-in-process.

3.13.6 CAPP steps used for Machining Operation

The important step involving computer aided process planning for machining operations are;

1. Interpretation of part design data.

2. Selection of maching process.

3. Selection of machine tools and fi xture.


4. Machining optimization.

5. Decomposition of machinable volumes.

6. Selection of machinable volumes.

7. Generation of precedence constraints.

8. Sequence of machinable volumes.

Factors to be considered for designing CAPP in Engine block manufacturing system.

1. Product geometry, material, tolerance, weight, etc.,.

2. Available process.

3. Available machine tools and fi xtures.

4. Manufacturing skill.

5. Inventory.

Computer aided process planning begins with engineering drawings, specifi cations, parts or material lists and a forecast of demand.

3.13.7 Advantages of CAPP

1. It reduces process planning and production lead time.

2. It has faster response to engineering changes, this will leads to revise or create new plans according to technology development.

3. It access to up-to-date information in a central database.

4. It improves cost estimating procedures and reduces the calculation error.

5. It gives more complete and detailed process plans.

6. It gives improvement in production scheduling and capacity utilization.


7. It improves the ability to introduce new manufacturing technology,

8. It is easily incorporate with other manufacturing application programs.

9. It reduces the effort of process planning.

10. It reduceds expenses of direct labour.

11. It reduces the scrap.

12. CAPP provides cost saving in material, scrap and work-in progress.

13. It reduces inaccuracies in manufacturing.

14. It provides greater control of management in all levels.

15. It provides great effort on optimization technique in manufacturing.

3.14 COMPUTERIZED MANUFACTURING PROCESS PLANNING (CMPP)

Process planning is performed in all industries it signifi cant is greatest in small batch discrete part metal fabrication and manufacturing industries.

A completed manufacturing processes includes the complete transformation from a raw material to a desired product. In general Computerized Manufacturing Process Planning (CMPP) refers to either Machining process planning or Assembly process planning. Machining process planning is concerned with how each single workpiece is machined on individual machine or manufacturing cells.

While assembly process planning is concerned with how several workpieces can be assembled together to form a Machine part, Machining process planning is often called as process planning.

A process planning is an important document for production management. It can be used for a management of production the assurance of product quality and optimization of production scheduling. It is a key link for integrating design and manufacturing.


The following essential informations are necessary of CMPP.

1. Design data.

2. Quality requirement data.

3. Production type data.

4. Raw material data.

5. Company capacity and capability data.

3.15 COMPUTER-AIDED PRODUCTION MANAGEMENT (CAPM)

The function of a CAPM system is to monitor and control the organisation of production and materials requirements of the manufactured product.

A good example of a CAPM system is shown in fi gure 3.32. This is called the Micro’s Manufacturing System and is discussed by kind permission of Kewill Systems pic. This CAPM package contains a number of modules which may be categorised as either links between CAD, stock control and purchasing departments through the DBMS.

a) Material Control (including stock control, bill of material, material requirement planning, and purchasing order printing and progressing), or

b) Production Control (including work in progress, scheduling, shop documentation, and job costing).

Designers can thus have ready access to purchase data fi les for raw materials and bought-out items. Typical purchase information could include: item availability, cost, and delivery; alternative suppliers; and previous purchase history. Designs may therefore be effi ciently optimised at an early stage in the process.

Standard purchase notes may be printed and dispatched via interface with stock control data. Information includes: item descriptions, quantities, supplier, unit prices, carnage price, date ordered, date due, and delivery instructions.


Once the item has been delivered, the form may be modifi ed to include the data of receipt and the information returned to the database as a “goods received” fi le. The REC (received) dates of the roadster form reveals that two of the ordered items have been delivered, although the QT Y OUTST (quantity outstanding) column indicated that these were not supplied in the fall quantities required.

Figure 3.32: ACAPM system: the Micro’s Manufacturing System.

3.15.1 Bill Of Material (BOM)

• BOM is a list of assemblies, compound components, component elements, raw materials, and bought-out items which make up the complete product.

• When created via the DBMS of a CIM system, the BOM allows the user to quickly explode down through the various levels of product constituents, from intermediate components and assemblies (high level) down to raw materials (low level).

• This lists the intermediate components of a “roadster” bicycle produced with the aid of the Micro’s CAPM system.

• The works orders is effectively the instructions for the shop fl oor to manufacture or assemble whatever is listed on the BOM display.


• A CIM-based BOM system can provide the following facilities:

a) Fast creation of BOMs via DBMS interface with CAD and stock control databases.

b) Simplifi cation of new BOM creation by rapid cross-reference with existing BOMs.

c) Creation of costed BOMs with automatic updating of current costs.

d) Accurate and effi cient updating of stock records via stock control interface.

e) Automatic issuing of works orders to production control via the DBMS.

f) The creation of “Trial Kitting Lists” to determine if suffi cient stock is available to meet the requirements of the works order. This helps to minimise shortages and work in progress.

3.15.2 Stock Control

• A computer-aided stock control system provides a database record of the movement of all stock in and out of the stores. Via the VDU, it accepts enquiries and produces reports on the status of all stock items.

• The stock record of each item could include the following information:

• Stock number; description; stock-on-hand; stock-on-order; allocated stock; reorder level; lead time; supplier code; bin location; material, labour, and overhead cost; selling price.

• Such a computerised stock control facility helps to minimise stock levels and redundant stock; provides faster updating of stock records and early warning of shortages for assembly programmes; and allows instant access to current order information via DBMS interface with the purchasing system.


• This type of computer-aided purchasing system also enables engineers to compile priority lists for the purchase of items, and so assist in bringing forward the implementation of the works order.

3.15.3 Material Requirement Planning (MRP)

The function of MRP is to take the total sales and production demand explode it through the BOM structures and, not only calculate the requirement for new purchase orders and works orders, but also identify any existing orders which need to be brought forward, delayed, or cancelled. In a CAPM system, MRP should link directly with BOM data and stock

REVIEW QUESTIONS:

1. Explain Briefl y about group Technology.

2. What is the Role of GT in CAD/CAM?

3. What are the Benefi ts, & disadvantages of GT?

4. Explain Types of part family.

5. Explain Briefl y about PFA.

6. What are the types of coding system?

7. Explain the Opitz coding system.

8. Explain KK- 3 & D class coding system.

9. Explain about composite part concept.

10. Explain Briefl y with Suitable example about cellular manufacturing.

11. Defi ne CAPP.

12. Explain about CAPP.

13. Write a short note on generative CAPP.

14. Explain Briefl y on variant CAPP.


15. Write advantages and disadvantages of CAPP.

16. Explain computerized manufacturing process planning.

17. Explain Briefl y about material requirement planning.

4.1 INTRODUCTION - SHOP FLOOR CONTROL

• Shop fl oor control (SFC) deals with managing the work-in-process.

• This consists of the release of production orders to the factory controlling the progress of the orders through the various work stations, and getting the current information of the status of the orders.

• This can be shown in the form of a factory information system. (Refer fi gure 4.1).

• The input to the shop fl oor control system is the collection of production plans

• These can be in the form of master schedule, manufacturing capacity planning and MRP data. The factory production operations are the processes to be controlled.

4UNIT

Shop Floor Control and Introduction to FMS


4.1.1 Phases in Shop Floor Control

A typical shop fl oor control system consists of three phases:

1. Order release.

2. Order scheduling.

3. Order progress.

The three phases and their connections to other functions in the production management system are pictured in fi gure 4.1. In today’s implementation of shop fl oor control, these phases are executed by a combination of computer and human resources with a growing proportion accomplished by computer automated methods.

Figure 4.1: Three phases in a shop fl oor control system.

4.1.1.1 Order Release• The order release phase of shop fl oor control provides the documentation

needed to process a production order through the factory. The collection of documents is sometimes called the shop packet. It consists of,

SHOP FLOOR CONTROL AND INTRODUCTION TO FMS 4.3

1. The route sheet, which documents the process plan for the item to be produced.

2. Material requisitions to draw the necessary raw materials from inventory.

3. Job cards or other means to report direct labor time devoted to the order and to indicate progress of the order through the factory.

4. Move tickets to authorize the material handling personnel to transport parts between work centers in the factory if this kind of authorization is required.

5. Parts list, if required for assembly jobs.

• In the operation of a conventional factory, which relies heavily on manual labor, these are paper documents that move with the production order and are used to track its progress through the shop.

• In a modern factory, automated identifi cation and data capture technologies are used to monitor the status of production orders, thus rendering the paper documents (or at least some of them) unnecessary.

• The order release module is driven by two inputs, as indicated in fi gure 4.1. The fi rst is the authorization to produce that derives from the master schedule.

• The authorization proceeds through MRP which generated work orders with scheduling information.

• The second input to the order release module is the engineering and manufacturing data base which provides the product structure and process planning information needed to prep the various documents that accompany the order through the shop.

4.1.1.2 Order scheduling• The order scheduling module follows directly from the order release

module and assigns the production orders to the various work centers in the plant.


• In effect, order scheduling executes the dispatching function in PPC.

• The order scheduling module prepares a dispatch list which indicates which production orders should be accomplished at the various work centers. It also provides information about relative priorities of the different jobs.

• For example, by showing due dates for each job. In current shop fl oor control practice, the dispatch list guides the shop foreman in making work assignments and allocating resources to different jobs so that the master schedule can best be achieved.

• The order scheduling module in shop fl oor control is intended to solve two problems in production control: 1) Machine loading and 2) Job sequencing

• To schedule a given set of production orders or jobs in the factory, the orders must fi rst be assigned to work centers. Allocating orders to work centers is referred to as machine loading

• The term shop loading is also used, which refers to the loading of all machines in the plant. Since the total number of production orders usually exceeds the number of work centers, each work center will have a queue of orders waiting to be processed.

• Job sequencing involves determining the sequence in which the jobs will be processed through a given work center.

• To determine this sequence, priorities are established among the jobs in the queue and the jobs are processed in the order of their relative priorities. Priority control is a term used in production control to denote the function that maintains the appropriate priority levels for the various production orders in the shop.

• As indicated in fi gure 4.1, priority control information is an important input in the order scheduling module. Some of the dispatching rules used to establish priorities orders in the plant include:

1. First-come-fi rst serveJobs are processed in the order in which they arrive at the machine. One might

argue that this rule is the most fair.


2. Earliest due dateOrders with earlier due dates are given higher priorities.

3. Shortest processing timeOrders with shorter processing times are given higher priorities.

4. Least slack timeSlack time is defi ned as the difference between the time remaining until due

date and the process time remaining. Orders with the least slack in their schedule are given higher priorities.

5. Critical ratio• The critical ratio is defi ned as the ratio of the time remaining until due date

divided by the process time remaining. Orders with the lowest critical ratio are given higher priorities.

• When an order is completed at one work center, it enters the queue at the next machine in its process routing.

• That is, the order becomes part of the machine loading for the next work center, and priority control is utilized to determine the sequence of processing among the jobs at the machine.

• The relative priorities of the different orders may change over time. Reasons behind these changes include:

1) Lower or higher than expected demand for certain product.

2) Equipment breakdown that cause delays in production.

3) Cancellation of an order by a customer.

4) Defective raw materials that delay an order.

4.1.1.3 Order ProgressThe order progress module in shop fl oor control monitors the status of the

various orders in the plant, WIP, and other characteristics that indicate the progress


and performance of production. The function of the order progress module is to provide information that is useful in managing the factory based on data collected from the factory.

The information presented to production management is often summarized in the form of reports, such as the following:

1. Work order status reportsThese reports indicate the status of production orders. Typical information in

the report includes the current work center where each order is located, processing hours remaining before completion of each order, whether the job is on-time or behind schedule, and priority level.

2. Progress reportsA progress report is used to report performance of the shop during a certain time

period (example, week or month in the master schedule). It provides information on how many orders were completed during the period, how many orders should have been completed during the period.

3. Exception reportsAn exception reports indicates that deviations from the production schedule

(example, overdue jobs) and similar exception information.

These reports are useful to production management in making decisions about allocation of resources, authorization of overtime hours, and other capacity issues and in identifying problem areas in the plant that adversely affect achieving the MPS.

4.2 TYPES OF SCHEDULING

Process-focussed production systems produce many non-standard products in relatively small batches that fl ow along different routes or paths through the production facility and require frequent machines change-overs. Such production systems are also known as intermittent production systems or job shops fi gure 4.2.


Figure 4.2: Scheduling and Shop fl oor decisions in process-focusedproduction system.

In such production systems, the departments or work centres are organized around the types of equipment or operations. (Example, drilling, welding, soldering, etc.,). Products fl ow through work centres in batches corresponding to individual customer orders or batches of economic batch quantities in produce to stock situations.

4.2.1 Reasons for shop fl oor scheduling process

a) Job shops have to produce products against customer orders for which delivery dates have to be promised .

b) Production lots tend to be quite small and may require numerous machine change-overs.

c) Possibility of assigning and reassigning workers and machines to many different orders due to fl exibility.


d) In such a fl exible, variable and changing environment, schedules must be specifi c and detailed work centre-wise to bring orderliness.

The type of scheduling technique used in job shop depends on the volume of orders, the nature of operations and the job complexity.

4.2.2 Types of scheduling techniques

1. Forward Scheduling.

2. Backward Scheduling.

1. Forward scheduling

Figure 4.3: Forward scheduling.

• In this approach, each task is scheduled to occur at the earliest time that, the necessary material will be on hand and capacity will be available.

• It assumes that procurement of material and operations start as soon as the customers, requirements are known. The customers place their orders on a ‘needed-as-soon-as possible’ basis. (Refer fi gure 4.3)

• The earliest completion date assuming that everything goes as planned could be quoted to the potential customer.

• Some buffer time may be added to determine a data that is more likely to be achievable, if it is acceptable to the customer.

• Forward scheduling is used in many companies such as steel mills and machine tool manufacturers where jobs are manufactured to customer orders and delivery is requested on ‘as early as possible’, basis.


• Forward scheduling is well suited where the supplier is usually not able to meet the schedules.

• This type of scheduling is simple to use, gets jobs done in shorter lead times but accumulates high work in process inventories.

2. Backward scheduling

Figure 4.4: Backword scheduling

• This scheduling technique is often used in assembly-type industries and in job shops that commit in advance to specifi c delivery dates.

• After determining the required schedule dates for major sub-assemblies, the schedule uses these required dates for each component and works backward to determine the proper release data for each component manufacturing order.

• The job’s start date is determined by “setting back’ from the fi nish date, the processing time for the job.

• By assigning jobs as late as possible, backward scheduling minimizes inventories, since each job is not completed until it is due nut not earlier. Backword scheduling is also known as reverse scheduling. (See fi gure 4.4)

4.2.3 Stages in Scheduling

1. Loading.

2. Dispatching.


1. Loading• Loading or shop loading is the process of determining which work centre

receives which job.

• It involves assigning a job or task to a particular work centre to be preformed during a scheduling period (such as a week).

• Loading of work centres depends on the available capacity and the expected availability of the material for the job. The jobs are assigned to machines or work centres taking into consideration the priority sequencing and machine or work centre utilization.

2. Dispatching• Dispatching is sequencing and selecting the jobs waiting at a work centre

(i.e., determining which job to be done next) when capacity becomes available

• It is actually authorising or assigning the work to be done. The dispatch list is a means of priority control.

• lt lists all jobs available to a work centre and ranks them by a relative priority. When priorities have been assigned to specifi c jobs, scheduling gets implemented through the dispatch list.

4.2.4 Types of Loading

Loading procedures are categorised as either fi nite loading or infi nite loading. In fi nite loading, jobs are assigned to work centres by comparing the required hours for each operation with the available hours in each work centre for the scheduling period. In infi nite loading, jobs are assigned to work centres without regard to capacity (as if the capacity were infi nite).


a) Finite loading

Figure 4.5: Finite loading.

Finite loading systems start with a specifi ed capacity for each work centre and a list of jobs to be processed at the work centre (sequencing). The work centre’s capacity is allotted to the jobs by simulating job starting times and completion times. The fi nite loading system combines loading, sequencing and detailed scheduling. It creates a detailed schedule for each job and each work centre, based on the capacity of the work centre. (Refer fi gure 4.5)

b) Infi nite loadingThe process of loading work centres with all the jobs, when they are required

without regard to the actual capacity available at the work centre is called infi nite loading. Infi nite loading indicates the actual released order demand (load) on the work centre, so as to facilitate decision about using overtime, sub-contracting or using alternative routing and delaying selected orders. (Refer fi gure 4.6)


Figure 4.6: Infi nite loading.

4.2.5 Load Charts and Machine Loading Charts

a) Load chart or Load scheduleA load schedule or load chart is a device for comparing the actual load (labour

hours and machine hours) required to produce the products as per the MPS against the available capacity (labour hours and machine hours) in each week.

Figure 4.7, illustrates the load schedule or chart shown graphically for a particular work centre having a weekly capacity of 100 standard hours and the weekly load for six weeks period. The load against each time period (i.e., week) is as shown:


Figure 4.7: Load schedule.

Machine loading chart (Gantt load chart)

Figure 4.8: Gantt load chart drawn for a particular week of a particular month.


Gantt charts are used to display graphically the work loads on each work centre. There are two types of Gantt charts,

i) Gantt load chart.

ii) Gantt scheduling chart or progress chart.

Gantt charts are simple to devise and easy to understand. The Gantt load chart offers the advantage of ease and clarity in communicating important shop information.

Activity Week number

Scheduling 1 2 3 4 5 6 7 8 9

Engg. release

Procurement

Fabrication

Receipt of materials

Inspection

Assembly

Shipping

Figure 4.9: Gantt scheduling chart.

4.3 ACTIVITIES OF CIM BASED SFC

a) Assigning a priority to each order which helps in setting the sequence of processing orders at work centres.

b) Issuing dispatching lists to each work centre. These lists indicate which orders are to be produced at a work centre, their priorities and completion dates/ times.

c) Providing input-output control on all work centres.


d) Up-dating the work-in-progress inventory. Information such as number of good parts coming out of each processing step (operation), amount of scrap, amount of rework required and number of units short on each order.

e) Measuring the effi ciency, utilization and productivity of workers and machines at each work centre.

4.4 FACTORY DATA COLLECTION SYSTEM

• The Factory Data Collection (FDC) system consists of the various paper documents, terminals, and automated devices located throughout the plant for collecting data on shop fl oor operations plus the means of compiling and processing the data usually by computer.

• The factory data collection system serves as an input to the order progress module in shop fl oor control. Using our feedback control system analogy of fi gure the FDC system is the sensor component of the shop fl oor control system.

• Examples of the types of data on factory operations collected by the FDC system include piece counts completed at certain work center, direct labor time expended on each order, parts that are scrapped, parts requiring rework, and equipment downtime.

• The data collection system can also include the time clocks used by employees to punch in and out of work.

4.4.1 Types of factory data collection system.

i) On-Line Versus Batch Systems• The purpose of the factory data collection system is twofold: to supply

data to the order progress module in the shop fl oor control system and to provide current information to production foreman, plant management and production control personnel.


• To accomplish this purpose, the factory data collection data collection system must input data to the plant computer system.

• This can be done in either an on-line or off-line mode. In an on-line system the data are entered directly into the plant computer system and are immediately available to the order progress module.

• The advantage of the on-line data collection system is that the data fi le representing the status of the shop can be kept current at all times.

• As changes in order progress are reported, these changes are immediately incorporated into the shop status fi le. The personnel with a need to know can access this status in real time and be confi dent that they have the most up-to-date information on which to base any decisions.

• In the off-line collection system, the data are temporarily stored in either a storage device or a stand-alone computer system to be entered and processed subsequently by the plant computer in a batch mode. In this mode of operation, there is a delay in the data processing.

• Consequently, the plant computer system cannot provide real-time information on shop fl oor status.

• This delay and the requirements for a separate data storage system are Hie principle disadvantages of this confi guration.

• The advantage of an off-line collection system is that it is generally easier to install and implement.

ii) Data Input Techniques• The techniques of factory data collection include manual procedure,

computer terminals located in the factory and other technologies.

• The manually oriented techniques of factory data collection are those in which the production workers must fi ll out paper forms indicating order progress data.


• The forms are subsequently turned in and compiled using a combination of clerical and computerized methods.

The manual/clerical techniques include:

1. Job Traveller• This is a log sheet included in the shop packet that travels with the order

through the factory. Workers who spend time on the order are required to record their times on the log sheet together with other data such as the data, piece counts, defects and so on.

• The job traveller becomes the chronological record of the processing of the order.

• The problem with this method is its inherent incompatibility with the principles of real-time data collection. Since the job traveller moves with the job, it is not readily available for compiling current order progress.

2. Employee time sheets• In the typical operation of this method, a daily time sheet is prepared for

each worker and the worker must fi ll out the form to indicate the work that was accomplished during the day.

• Data entered on the form include the order number, operation on the route sheet, the number of pieces completed during the day, time spent and so on.

• Some of these data are taken from information contained in the shop packet for the order. The time sheet is turned in daily and order progress information is compiled.

3. Operators tear strips• With this technique, the shop packet includes a set of preprinted tear strips

that can easily be separated from the packet

• The preprinted data on each tear strip include order number, route sheet details and so on.


• When a worker fi nishes an operation or at the end of the shift, one or the tear strips is torn off, piece count and time data are recorded by the worker and the form is turned in to report order progress.

4. Prepunched cards• This is essentially the same technique as the tear strip method, but

prepunched computer cards are included with the shop packet instead of tear strips.

• The prepunched cards contain the same type of order data and the workers must write the same kind of production data onto the card.

• The difference in the use of prepunched cards as that in compiling the daily order progress, mechanized data processing procedures can be used to record some of the data.

• There are problems with all of these manually oriented data collection procedures. They all rely on the co-operation and clerical accuracy of factory workers to record data onto a paper document.

• There are invariably errors in this kind of procedure. Error rates associated with hand-written entry of data average 1/30.

• Some of the errors can be detected by the clerical staff that does the compilation of order progress.

• Examples of detectable errors include wrong dates incorrect order numbers (the clerical staff knows which orders are in the shop and they can usually determine when order number has been entered by a worker and incorrect operation numbers on the route sheet (if the worker enters a certain operation number but the preceding operation number has not been started, an error has been made).

• Other errors are more diffi cult to identify.

• If a worker enters a piece count of 130 pieces which represents the work completed in one shift when the batch size is 230 parts, this is diffi cult for the clerical staff to verify.


• If a different worker on the following day complete the batch and also enters a piece count of 130, it is obvious that one of the workers overstated his / her production.

• Another problem is the delay in submitting the order progress data for compilation. There is a time lapse in each of the methods between when events occurs in the shop and when the data representing those events are submitted.

• The job traveller method is the worst offender in this regard.

• Here the data might not be compiled until the order has been completed too late to take any corrective action. This method is of little value in a shop fl oor control system.

• The remaining manual methods described above suffer a one-day delay since the shop data are generally submitted at the end of the shift and a summary compilation is available until the following day at the earliest.

• The data-entry methods also include more automated input technologies such as magnetic card readers or optical bar code readers.

• Certain types of data, such as identifi cation of order, product, and even operation sequence number, can be entered with the automated techniques using magnetised or bar-coded included with the shop packet.

4.4.2 Numbers and Arrangement of Keyboard-Based Terminals Possible in the Factory

1. One centralized terminal• In this arrangement there is a single terminal located centrally in the part.

This requires all works to walk from their workstations to the central when they must enter the data. If the plant is large, this becomes inconvenient.

• Also, use of the terminal increase at the time of a shift change, and this results in over time for the workers.


2. Satellite terminals• In this confi guration, there are multiple data collection terminals located

throughout the plant.

• The number and locations are designed to strike a balance between minimizing the investment cost in terminals and maximizing the convenience of the workers in the plant.

3. Workstation terminals• The most convenient arrangement for the workers is to have a data

collection terminal at each workstation.

• This minimize the time lost in walking to the satellite terminals. However, it seems to be justifi ed only when the number of data transactions is relatively large and when the terminals are also designed for collecting certain data automatically.

• The trend in industry is toward more use of automation in factory data collection systems. Although the term automation is used, many of the techniques require the participation of human workers.

4.5 AUTOMATIC IDENTIFICATION METHODS

• The fi eld of automatic identifi cation is often associated with the material handling industry. In fact, the industry trade association called the Automatic Identifi cation manufacturers (AIM) is an affi liate of the Material Handling Institute, Inc. Many of the applications of this technology relate to material handling.

• Automatic identifi cation is a term that refers to various technologies used in automatic or semiautomatic acquisition of product data for entry into a computer system.

• These technologies are mostly sensor-based methods that provide a means of reading data that are coded on a document, product, component,


container and so on without the need for human interpretation of the data. Instead, the computer system interprets and processes the data for some useful application.

• The applications of automated identifi cation systems are numerous they include retail sales, warehousing (semi-automated storage and picking), product sortation and tracking shipping and receiving and shop fl oor control.

• Some of the automated identifi cation applications require workers to be involved in the data collection procedure, usually to operate the identifi cation equipment in the application.

• These techniques are therefore semi-automated rather than automated methods. Other applications accomplish the identifi cation procedure with no human participation.

• The same basic sensor technologies may be used in both cases. For example, certain types of bar code readers are operated by people while other types are operated automatically.

4.5.1 Reasons for using automatic identifi cation techniques

1. There are some very good reasons for using automatic identifi cation techniques. First and foremost, the accuracy of the data collected is improved, in many cases by a signifi cant margin. To illustrate, the error rate in bar code technology is approximately 10,000 times lower than in manual keyboard data entry. The rate of 1/3,000,000 is used an error rate of comparison with the handwritten and keyboard entry methods. The error rates of most of the other technologies is not as good as for bar codes but still better than manual-based methods.

2. A second reason for using automatic identifi cation is to reduce the time required by human workers to make the data entry. The speed of data entry handwritten documents in approximately documents is


approximately 5 to 7 characters per second and it is 10 to 15 characters per second (at best) for keyboard entry.

4.5.2 Technology of AIS

Automatic identifi cation methods are capable of reading hundreds of characters per second. This comparison is certainly not the whole story in a data collection transaction, but the time savings in using automatic identifi cation techniques can mean substantial labour cost benefi ts for large plants with many workers.

The technologies available for use in automatic identifi cation systems

• Barcodes.

• Radio frequency systems.

• Magnetic stripe.

• Optical character recognition

• Machine vision.

4.5.2.1 Bar codeThe use of bar codes in factory data collection system is predominant and

growing. The other techniques are either used in special applications in factory operations or they are widely applied outside the factory.

4.5.2.2 Radio frequency system• Radio-Frequency (RF) systems rely on the use of radio frequency signals

similar to those used in wireless television transmission.

• Although the type of signal is the same, there are differences in the use of RF technology in product identifi cation

• One difference is that the communication is in two directions rather than one direction (as in TV).

• Also, the signal power is substantially lower in factory identifi cation applications (ranging from several milliwatts to 7 watts).


• Radio-frequency identifi ed, an antenna at some location where data are to be read and a reader the interprets the data

• The identifi cation tag is a transponder, a device that is capable of emitting a signal of its own when it receives a signal from an external source. It is attached to the product, truck, railway car, or other item.

• The term “tag” is misleading, since the term refers to a small but rugged boxlike container that houses the electronics for data storage and RF communication.

• The container may be as much as 2.5 × 2.5 × 7.5 inch in size and be capable of withstanding temperatures from –40 to +400°F. The tags are usually read-only devices that contain up to 20 characters of data representing the item identifi cation and other information that is to be communicated.

• Recent developments in the technology have provided much higher data storage capacity and the ability to change the data in the tag (read/write tags).

• This opens many opportunities for incorporating much more status and progress information into the automatic identifi cation system.

• The antenna is located at an identifi cation station and listens for the RF signal from the identifi cation tag that uniquely the item to which it is attached.

• The signal is then fed to a reader that decodes and validates the signal prior to transmission of the associated data to the collection computer system.

• The hardware required for an RF identifi cation system has tended to be more expensive than for most data collection technologies.

• For this reason, RF systems have generally been appropriate for data collection situations in which environmental factors preclude the use of optical techniques such as bar codes.


• For example, RF systems are suited for identifi cation of products with high unit values in manufacturing processes (such as spray painting) that would obscure any optically coded data.

• They are also used for identifying railroad cars and in highway trucking applications where the environment and conditions make other methods of identifi cation infeasible.

4.5.2.3 Magnetic stripes• Magnetic stripes (the term magnetic strip is also used) attached to the

product or container can also be used for item identifi cation in factory and warehouse applications.

• These are the same kinds of magnetic stripes that are used to encode identifi cation data onto plastic access cards for use in automatic bank, tellers.

• Their use seems to be declining for ship fl oor control applications because they are more expensive than bare codes and cannot be scanned remotely.

• Two advantages they possess is their larger data storage capacity and the ability to alter the data contained in them.

4.5.2.4 Optical Character Recognition (OCR)• Optical Character Recognition (OCR) techniques refer to a specially

designed alphanumeric character set that is machine readable by an optical sensor device.

• The substantial benefi t offered by OCR technology is that the characters and associated text can be read by human beings as well as machines.

• The list of disadvantages, at least for factory and warehouse applications includes the requirement for near-contact scanning, lower scanning rates and a higher error rate compared to bar code scanning.


• Machine vision systems are used for automated inspection tasks. The applications also include certain classes of automatic identifi cation problems and these applications may grow in number as the technology advances.

• For example, machine vision systems are capable of distinguishing between a limited set of products moving down a conveyor so that the products can be sorted.

• The recognition task is accomplished without requiring that a special identifi cation code be placed on the product. The recognition by the machine vision system is based on the inherent geometric features of the object.

4.6 BAR CODE TECHNOLOGY

• Bar code technology has become the most popular method of automatic identifi cation in retail sales and in factory data collection

• The bar code itself consists of a sequence of thick and spaces is coded to narrow spaces separating the bars. The pattern of bars and spaces is coded to represent alphanumeric characters. Bar code readers interpret the code by scanning and decoding the sequence of bars. The reader consists of the scanner and decoder.

• The scanner emits a beam of light that is swept past the bar code (either manually or automatically) and sense light refl ections to distinguish between the bars and spaces.

• The light refl ections are sensed by a photo detector that converts the spaces into an electric signal and the bars into absence of an electrical signal.

• The width of the bars and spaces is indicated by the duration of the corresponding signals. The decoder analyses the pulse train to validate and interpet the corresponding data. (Figure 4.10).


Figure 4.10: Conversion of bar code into pulse train of electrical signals.

• Certainly, a major reason for the acceptance of bar codes is their widespread use in grocery and other retail store.

• In 1973, the grocery industry adopted the Universal Product Code (UPC) as its standard for item identifi cation. This is a 10 digit bar code that uses fi ve digits to identify the product and fi ve digits to identify the manufacturer.

• The U.S. Department of Defence provided another major endorsement in 1982 by adopting a bar code standard (Code 39) that must be applied by vendors on product cartons supplied to the various agencies.

4.6.1 Bar Code Symbol

• The Universal Product Code is only one of many bar code formats in commercial use today.

• The bar code standard adopted by the automotive industry the Department of Defence the General Services Administration and many other manufacturing industries os Code 39, also known as AIM USD-2 (for automatic Identifi cation Manufacturers Uniform Symbol Description-2), although this is actually a subset of Code 39. We describe this format as an example of bar code symbols [2,3,5].


Table 4.1: Character structure, USD-2.

• Code 39 uses a uniquely defi ned series of wide and narrow elements (bars and spaces) to represent 0-9, the 26 alpha characters and special symbols.

• The wide elements are equivalent to a binary value of one and the narrow elements are equal to zero.

• The width of the narrow bars and spaces called the X dimension provides the basis for a scheme of classifying bar codes into codes into three code densities.

* High density: X dimension is 0.010 inch or less.

* Medium density: X dimension is between 0.010 and 0.030 in.

* Low density: X dimension is 0.030 in. or greater.

• For bar codes with X ≥ 0.020 in., the wide elements must be printed with a width of anywhere between 2 x and 3 x (two to three times the X dimension).

• For bar codes will X < 0.020 in., the wide elements must have a width between 2.2 x and 3 x. Whatever the wide-to-narrow ratio, the width must


be uniform throughout the code in order for the reader to consistently interpret the bar code. Figure presents the character structure for USD-2, and fi gure illustrates how the character might be developed in a typical bar code.

• Denotes a start/stop code which must precede and follow every bar code message Note that is used only for the start/stop code.

Figure 4.11: Atypical grouping of characters to form a bar code in Code 39.

• In addition to the character set in the bar code, there must also be a so-called “quiet zone” both preceding and following the bar code, in which there is no printing that night confuse the decoder. (Refer fi gure 4.11)

• The reason for the name Code 39 is that nine elements (bars and spaces) are used in each character and three of the elements are wide elements.

• The placement of the wide spaces and bars in the code is what uniquely designates the character. Each code begins and ends with either a wide or narrow bar.

• The code is sometimes referred to an code three-of-nine.


4.6.2 Bar Code ReadersBar code readers come in a variety of confi gurations; some require a human

being a operate them and others are stand-alone automatic units. They are usually classifi ed as contact or non-contact readers.

4.6.2.1 Contact bar code reader• Contact bar code readers are hand-held wands or light pens operated by

moving the tip of the wand quickly past the bar code on the object or document. The wand tip must be in contact with the bar code surface or in very close proximity during the reading procedure.

• In a factory data collection application, they are usually part of a keyboard entry terminals. Figure illustrates this type of terminal, which allows the worker to input data both by using the bar code reader and by keystroke entry

• The terminal is sometimes referred to as a stationary in the sense that it is placed in a fi xed location in the shop. When a transaction is entered in the factory, the data are communicated to the computer system in an on-line or batch.

• In addition to their use in factory data collection systems stationary contact bar code readers are widely used in retail establishments to enter the item identifi cation in a sales transaction. Bar codes (Universal Product Codes) are include on the labels for many types of products sold commercial today.

• Contact bar code readers are also available as portable units which can be carried around the factory or warehouse by a worker

• They are battery-powered units that include a solid-state memory device capable of storing data acquired during operation.

• The data can subsequently be transferred to the computer system. Portable bar code readers often include a keypad that can be used by the operator to input data that cannot be entered via bar code


• These portable units are used for order picking in a warehouse and similar applications which require a worker to move large distances in a building.

4.6.2.2 Non contact bar code reader• Noncontact bar code readers do not use a contacting want read the bar

code. Instead, they locus a light beam on the bar code and a photodector reads the refl ected signal to interpret the code.

• The reader probe is located a certain distance from the bar code (several inches to several feet) during the read procedure.

• Noncontact readers are classifi ed as fi xed beam and moving beam scanners.

1. Fixed beam readers are stationary units that use a fi xed beam of light They are usually mounted beside a conveyor and depend for their operation on the movement of the workpiece typically in large warehousing and material handling operations where large quantities of materials must be identifi ed as they fl ow past the scanner on conveyors. Fixed beam scanners in these kinds of operations represent some of the fi rst applications of bar codes in industry, and they date back to the 1950s.

2. Moving beam scanners use a highly focused beam of light actuated by a rotating mirror to traverse an angular sweep in search of the bar code on the object. Lasers are often used to achieve the highly focused light beam. A span is defi nes as a single sweep of the light beam through the angular path. The high rotational speed of the mirror allows for very high scan rates-up to 1440 scans per second. This means that many scans of a single bar code can be made during a typical reading procedure, thus permitting verifi cation of the reading. Moving-beam scanners can be either stationary or portable units. Stationary moving beam scanners are located in a fi xed position to read bar codes on objects as they move past on a conveyor or


other material handling equipment. They are used in warehouses and distribution centers to automate the product identifi cation and sortation operation.

4.6.3 Bar Code PrintersPreprinted bar codes are produced using traditional techniques such as

letterpress and lexographic printing. These methods are used for printing labels in large quantities for product cartons.

Bar codes can also be prepared by other methods in which the process is controlled by microprocessor to achieve more individualize printing of the bar-coded document or item label. Examples of applications of these individualized bar code printing methods include keyboard entry of data for inclusion in the bar code for each item that is labeled automated weighing scales and other inspection procedures in which unique grading and labelling of product is required, unique identifi cation of production lots for pharmaceutical products and preparation of route sheets and other documents that are included in a shop packet traveling with production order.

Production workers use bar code readers to indicate order number and completion of each step in the operation sequence. The various printing technologies used in these applications include dot matrix, ink jet, and electronic printing.

A relatively new technology for bar code making of metal parts in a factory makes use of a laser etching process. The process provides a permanent identifi cation mark on the item which is not susceptible to damage in the harsh environments that are encounted in many manufacturing processes. The laser etching process is beginning to be used in the automotive industry.

4.7 AUTOMATED DATA COLLECTION SYSTEM• The trend in factory data collection systems is towards the use of more

automation technologies. Some of the bar code reading methods and other automatic identifi cation techniques discussed in the two preceding sections can be operated in a fully computer-automated mode.


• These fully automated methods fall within the scope of what we are calling process monitoring.

• As indicated earlier; computer process monitoring involves a computer system which is directly connected to the manufacturing process for the purpose of collection data on the process and associated equipment.

• There is no direct control of the manufacturing process as a result of the computer process monitoring installation.

• The process either remains under the control of human operators or its automatically controlled by a process controller that is separate from the computer process monitoring system.

• The hardware components used in a computer monitoring system axe sensors and transducers, analog-to-digital converters, simple limit switches and photo detectors to indicate presence or absence of an object (example, a workpart), pulse generators (example, optical encoders), multiplexers and so on.

• Devices used to output data from the computer to the manufacturing process (example, digital-to-analog converters) are typically not part of the process monitoring application.

4.7.1 Data Acquisition System• A Data Acquisition System (DAS) is a computer system used to

automatically collect data from a process or piece of equipment.

• Data acquisition systems either perform an analysis of the data or transmit the data to another computer for processing and analysis.

• A microcomputer is typically used as the controller/processor for a current technology DAS. Other possible controller/processors include microcomputers, microprocessors, and single-board computers.

• The functions of the controller/processor include synchronizing the data sampling and storage tabulating the data for presentation and statistical and


other analysis. Other components of the DAS include-analog transducers, analog-to-digital converters (ADCs), digital transducers and digital input interfaces (example, pulse counters).

• A standard microcomputer can be converted to use as a DAS by adding the necessary ADSs, multiplexers, and other circuitry as a separate front-end unit or in the form of I/O cards in the computer.

• The means of sampling the data can be either synchronous or asynchronous process. Synchronous sampling involves the use of software to drive the sampling process.

• In this case, the frequency of the sampling rate is determined by the computer processor timer, and sampling must be performed synchronously with the time.

• Asynchronous sampling involves the use of hardware to perform the sampling task. This is generally laster and more accurate but more expensive

• Also the hardware-based system cannot be re programmed thus reducing its fl exibility to meet changing application requirements.

4.7.2 Data Logger• A data logger is a system that is sometimes compared with a data acquisition

system. A data logger is a device that automatically collects and stores data for later off-line (batch) analysis. No data analysis capability is available on the data logger.

• Before microprocessors became so common, data loggers represented a viable means of data collection. Today, almost all devices that collect data posses a micro-processor-based control unit that permits some type of analysis to be performed.

• Consequently, the distinction between data loggers and data acquisition systems has become blurred and most systems commercially available today fall into the DAS category.


4.7.3 Multilevel Scanning

Figure 4.12: Multilevel scanning in computer process monitoring.

• In computer process monitoring, it is possible for the numbers of monitored variables to becomes quite large. Although it is technically feasible to monitor all of the variables, it may result in ineffi cient use of the computer since some of the variables would not be needed under normal operating conditions.

• In this kind of situation, it is appropriate to utilize a computer process monitoring confi guration referred to as multilevel scanning. (Figure 4.12)

• In multilevel scanning, there are two (or more) levels of process scanning performed by the computer system, a high-level scan and a low-level scan.

• During normal operation of the process, only the high-level scan is performed. In this monitoring procedure, only the key variables and status data are scanned.

• When these variables indicate that the process is operating abnormally, the computer switches to a low-level scan for the affected operations and equipment.

• This low-level scan involves a more complete data logging and analysis procedure to ascertain the source of the malfunction. A low-level scan can also be initiated on request of the operator.


• Many of the same functions and capabilities discussed can be accomplished in computer process monitoring. These include the use of program interrupts, machine breakdown diagnosis and certain aspects or error detection and recovery.

4.8 AUTOMATED DATA COLLECTION TECHNOLOGIES

4.8.1 Bar Code (explained in 4.5.2.1)

4.8.2 Optical Character Recognition

• Like bar codes, optical character recognition (OCR) recognizes and processes symbols. But unlike the bare code system, which interprets data coded in a series of bars and spaces, OCR devices interpret human readable characters for computers. As wand or slot readers, OCR scanners collect character information in the form of pixels. (Refer fi gure 4.13)

• Data can be scanned either from the left or the right. Typical character fonts are OCR-B, found at the bottom of the UPC symbol on grocery items and OCR-A, found on DOD bar code labels, paperback books and retail clothing tags.

• The average OCR scanner can read 20-200 characters per second, high-speed systems as many as 1,200 characters per second. (Refer fi gure 4.14)

• Feature extraction or template comparison are the two techniques for decoding data scanned by OCR systems.

• The former method compares character features such as vertical, horizontal, or diagonal lines and loops with those stored in the computer memory. The latter method compares the character, pixel by pixel, after it has been decoded in brinary data form.

• For each character, the computer memory contains an array of pixels called a character template.


Figure 4.13: Bar code contact wands and Slot readers.


Figure 4.14: Hand-held bar code scanners.

4.8.3 Machine Vision System

In vision or image processing systems, computers analyze and interpret images, Though there may be different approaches to analysis, most vision system begin the task with a camera scene divided into pixels.

The computer compares the pixels to identify prominent object features such as edges or holes. Comparing these features with those of the images stored in memory allows recognition.


Figure 4.15: Stationary bar code scanners.

Cameras used in vision systems are either vidicon similar to a commercial TV camera, or CCD (Charge-Coupled Device). A third type is the CCPD (Charge-Coupled Photo-Diode) image sensor. Both CCD and CCPD cameras are based on solid state electronics.

Vision systems can carry out a variety of tasks in seven general categories:

a) Gaging.

b) Verifying.

c) Identifying.


d) Recognizing.

e) Locating.

f) Detecting fl aws and.

g) Multimedia integrating.

Identifying includes all the tasks in which symbols determine an object’s identity, whereas recognizing uses observed features of the object. Multimedia integrating combines the image data with word processing, database, graphic and communication systems. This versatility of integration is attractive for CIM applications. Vision system applications in manufacturing include sorting, material handling, process control, machine monitoring, safety, guidance.

4.8.4 Radio Frequency Identifi cation

When no line of sight exists between the scanner and the identifi cation tag as in some material handling applications or when read/write capability is required, Raido Frequency Identifi cation (RFID) is the answer.

The object being tracked has a transponder that transmits a specifi c radio frequency representing a unique signature or data stream that the transmitter or reader can interrogate. The antenna picks up the signal

RFID systems are based on transmission of a radio signal and its obstruction by the object if in the capture windows. The basic components are control unit (reader), antenna, and coded identifi cation .

The antenna continuously transmits a low-wattage (1-7 mW) microwave signal. When a tag enters the fi eld of view, the refl ected signal gets frequency-modulated. Coded tag may be active of passive.

The active tag is battery-operated can store several Kbytes of data and can add, delete or change the data in the tag on the basis of the key code received. RF tags carry predetermined process information that local scanning logic stations at each workstation can read.


After completing the programmed work, the station adds a relevant message to the tag. At the end of an assembly line, a computer can off-load all the information, thus producing a complete record of the assembly and test functions.

4.8.5 Magnetic Identifi cation

Magnetic Ink Character Recognition (MICR) uses stylised OCR fonts. The fonts are printed with a magnetic ink to permit readability after being overprinted or even smudged. MICR is used to read smaller documents of size 7 to 20 cms. Like OCR, these also require precise orientation and registration.

4.8.6 Voice Technology

Speech is the most natural way of communication. This eliminates the need of the user to understand a computer system. Voice technology is intelligently packaged and applied in several applications.

Moreover the training can be minimised and the key board entry can be eliminated and hand and eye co-ordination is no longer needed.

Voice recognition (VR) is of two types

i) Speaker dependent.

ii) Speaker independent.

Most voice recognition systems are speaker independent systems. VR systems recognise the user’s vocabulary and stores a computer image of each utterance and compares later the input words to the computer stored words. If the input matches the stored pattern, recognition is achieved.

This provides larger vocabulary and accurate recognition. Commercial VR systems are having around a few hundred words in active vocabulary and skilful programming can develop application dependent vocabularies.

Real application of VR systems rests on the fact that user need not be trained to use the system. Speaker independent system uses recognition template from memories of the previously recorded images.


The templates represent speech patterns of both male and female speakers. These are now available with limited vocabularies.

4.9 INTRODUCTION TO FLEXIBLE MANUFACTURING SYSTEM (FMS)

Figure 4.16: Flexibility and Automation.

Flexibility has become a key consideration in the design of manufacturing systems. It is the essential feature of fl exible manufacturing systems. Flexibility is a collection of properties of manufacturing system that support changes in production activities or capabilities. It is defi ned as the ability of a system to cope with changing circumstances. It describes the ability of manufacturing system to be applied for different tasks which may be due to changes in product demand, changes in production system and changes in product itself If the system is not fl exible then it may not be able to operate effectively as situation changes. The general relationship


between fl exibility and automation is shown in fi gure 4.16, which shows that the most highly automated systems usually have the least fl exibility and vice versa.

Flexibility is not a single homogeneous property, but is composed of various different components.

A FMS is a group of NC machine tools that can randomly process a group of parts, having automated material handling and central computer control to dynamically balance resource utilisation so that the system can adopt automatically to change in part production, mixes and levels of output.

FMS is a randomly loaded automated system based on group technology manufacturing linking integrated computer control and a group of machines to automatically produce and handle (move) parts for continuous serial processing.

FMS combines microelectronics and mechanical engineering to bring the economies of scale to batch work. A central online computer controls the machine tools, other workstations, and the transfer of components and tooling. The computer also provides monitoring and information control. This combination of fl exibility and overall control makes possible the production of a wide range of products in small numbers.

4.9.1 Types of Flexibility

a) Machine fl exibilityThis is the ease of making the changes to process a given set of part types. The

setup time required for a machine tool to switch from from one part type to another includes: cutting tool preparation time, part positioning and releasing time, and the NC program change-overtime.

The fl exibility can be attained by:

i) Technological progress, example sophisticated tool loading and part loading devices.

ii) Proper operation assignment, so that there is no need to change the tools that are in the tool magazines, or they are changed less often.


iii) Having the technological capability of bringing both the part and required cutting tools to the machine tool together.

b) Process fl exibilityThis is the ability to produce a given set of part types (each possibly using

different materials in several ways), i.e., the capability to absorb changes in product mix. It is also known as “mix fl exibility”. Process fl exibility increases as machine set up costs decrease.

This fl exibility can be attained by having:

i) Machine fl exibility.

ii) Multipurpose, adaptable, CNC machining centres.

c) Product fl exibilityThis is the ability to change to process new part types, i.e., the capability

to absorb economically and quickly the production of new product designs. It can be measured by the time required to switch from one part mix to another, not necessarily of the same part types. It can be attained by having:

i) An effi cient and automated production planning and control system.

ii) Machine fl exibility.

d) Routing fl exibilityThis is the ability to process a given set of parts on alternative machines, i.e.,

not to experience a dramatic decrease in production rate when breakdown occur. This fl exibility can be attained by:

i) Allowing for automated and automatic rerouting of parts.

ii) Pooling machines into machine groups.

iii) Duplicating operation assignments.


e) Volume fl exibilityThis is the ability to operate profi tably, at different production volumes. The

lower the volume is, the more volume fl exible the system must be. This fl exibility can be attained by having:

i) Multipurpose machines.

ii) A layout that is not dedicated to a particular process.

iii) A sophisticated, automated materials handling system.

iv) Routing fl exibility.

f) Expansion fl exibilityIt is the ability to easily add capability and capacity into the system, as needed.

This fl exibility is attained by having

i) A non-dedicated, non-process driven layout.

ii) A fl exible materials handling system consisting of, say, wire guided carts.

iii) Modular, fl exible machining cells with pallet changers.

iv) Routing fl exibility.

g) Operation fl exibilityIt is the ability to interchange ordering of operations on part. The fl exibility

is attained by:

i) Keeping the routing options open.

ii) Making the decision like the ‘next’ operation or the ‘next’ machine in real time and depends upon current system state.

h) Production fl exibilityIt is the universe of part types that can be processed. It is attained by:


i) Increasing the level of technology and the versatility of the machine tools.

ii) Having all above mentioned fl exibilities.

Figure 4.17, displays the relationship between the different fl exibilities. The arrow signify “necessary for”. An ideal FMS would possess all of the defi ned fl exibilities. However, the cost would be quite high.

Figure 4.17: Relationship among fl exibility types.

There are two basic types of FMS, namely dedicated FMS and random FMS. A dedicated system machines fi xed set of part types with well defi ned manufacturing requirements over a known time horizon. The random FMS, on the other hand, machines a greater variety of parts in random sequence.

4.9.2 Types of FMS

The FMS can be classifi ed based on:

1. Kinds of Operation.

2. Number of machines.

3. Flexibility level.

1. Kinds of operationBased on the kinds of operations the FMS can be classifi ed as:

1. Processing operations.

2. Assembly operations.


An FMS is designed to perform one or the other. The system may process rotational parts or non rotational parts.

Flexible manufacturing systems with multiple stations that process non rotational parts one much higher than systems that process rotational parts.

2. Number of machinesBased on the number of machines in the system, the FMS can be classifi ed

as follows:

i) Single machine cell.

ii) Flexible manufacturing cell.

iii) Flexible manufacturing system.

i) Single Machine Cell (SMC)• It consists of one CNC machining center.

• It has parts storage system for an attended pertion.

• The raw workparts are loaded into the parts storage unit and completed parts are unloaded from it, (Refer fi gure 4.18)

• The cell can be designed to operate in either fl exible mode or a batch mode or in combination.

Sl. No.

Batch mode Flexible mode

1. The Machine processes parts of a single style is specifi ed lot sizes.

The system satisfi es the following fl exibility test.a) Processing different part style.b) Responding to changes in production schedule.c) New parts are introduced.

2. It is changed over to process a batch Error recovery can not be satisfi ed.


Figure 4.18: Single machine cell consisting of one CNC machining center and parts storage unit.

ii) Flexible Manufacturing Cell (FMC)

Figure 4.19: A fl exible manufacturing cell consisting of three identical processing stations (CNC machining centers), a load/unload station and a part handling

system.


iii) Flexible manufacturing systemsIt has four or more processing workstation which are connected by a

common part handling system (mechanical) and by a distributed computer system (electronics).

Sl. No.

FMS FMC

1. It has four or more machines. It has two or three machines.

2. It includes non processing workstation.

It does not include non processing workstations.

3. Computer control system is larger and more sophisticated.

Computer control system is smaller.

3. Flexibility levelBased on the level of fl exibility, the FMS can be classifi ed on:

i) Dedicated FMS.

ii) Random-order FMS.

i) Dedicated FMS• It is designed to produce a limited variety of part styles.

• The product design is stable and the system can be designed with a certain amount of process specialization to make the operations more effi cient.

• To make limited part family, the machines can be designed for the specifi c processes.

ii) Random-order FMS• It is more appropriate when the part family is large.

• There are some variations in part confi gurations.


• New part designs are introduced into the system.

• Engineering changes in parts are produced.

• It uses general purpose machines for producing variations in product and it is capable of processing parts in various sequences.

• More sophisticated computer control system is required.

4.9.3 Components of FMS

The components of FMS are listed below;

1. Workstations.

2. Material handling and storage system.

3. Computer control system.

4. Human resources.

4.9.3.1 Workstations

Depends on the type of work accomplished by the system, the various processing or assembly equipments are used in an FMS.

Types of workstations

1. Machining stationsThe CNC machine tool is used as a workstations in FMS. Most common is

the CNC machining center. The horizontal machining center is commonly used.

CNC machining centers possess features that make them compatible with the FMS, including automatic tool changing and tool storage, palletized workparts, CNC, capacity distributed Numerical Control.

Machining centers are having automatic pallet changers. It is interfaced with the FMS part handling system. Machining centers are used for non rotational parts. Turning centers are used for rotational parts.


Mill-turn centers are used for parts that are mostly rotational but require multitooth rotational cutters like milling and drilling. For achieving higher production in milling than machining center, special milling machine modules can be used. The milling module can be horizontal spindle, vertical spindle or multiple spindle.

2. Load and unload stationsThe load and unload station connects the FMS and the rest of the factory. In

this station raw workparts enters the system and fi nished parts exit the system.

Loading and unloading can be carried out either manually or by automated handling system. The mechanized cranes and other handling devices are used to lift the heavy parts. Air hoses or other washing facilities are required to remove chips and ensure clean mounting and locating points.

The loading and unload station should include a data entry unit and monitor for communication between the operator and the computer system.

The instructions are sent to the operator regarding which part to load on to the next pallet. When different pallets are required for different parts, the correct pallet must be supplied to the station. All of these require communication between the computer system and the operator at load and unload station.

3. Other processing stationsSheet metal fabrication processing stations are used in addition to machining.

The processing workstations consist of press working operations such as bending, shearing, punching, notching, and forming processes.

Therefore workstations consist of heating furnace, a forging press and trimming station.

4. Assembly operationFor batch production, the fl exible automated assembly system are developed

instead of manual labour. Robots are used for assembly operation.


The robots are programmed to perform various tasks with sequence variation and different product styles assembled in the system. In electronics assembly system, programmable component placement machines are used.

5. Other stations and Equipment1. Inspection - Co-ordinate measuring machine. Special inspection probes

in a machine tool spindle, Machine vision.

2. Cleaning parts and pallet fi xtures.

3. Central coolant delivery system.

4. Chip removal system.

4.9.3.2 Material Handling and Storage System

This systems used to transfer parts from one station to other station.

1. FunctionsThe material handling and storage system performs the following functions.

1. The material handling system is used as a temporary storage of parts. In FMS, the number of parts waiting for processing exceeds the number of parts being processed at any movement.

2. The material handling system includes the locations for loading and unloading stations.

3. The material handling system must be capable of being controlled by the computer system.

4. It carries variety of workpart confi gurations.

5. The parts must be capable of moving from any one machine in the system to any other machines to provide various routing alternations for the different parts.


2. Material handling equipment1. Conventional material transport equipment Industrial trucks, Automated

guided vehicle system, Conveyors, Cranes.

2. In line transfer mechanisms.

3. Robots.

3. Material handling systems1. Primary handling system - Transfer of workparts between station in the

system.

2. Secondary handling system.

It consists of automatic parallel changes, transfer devices and other similar mechanisms.

Transfer of workparts from the primary system to the machine tool or other processing stations.

4. Other purpose1. Reorientation of the workpart.

2. Used as a buffer storage.

5. FMS LayoutThe material handling system establishes the FMS layout. Most layout

confi gurations found in today’s FMSs can be divided into fi ve categories

1. In-lint layout.

2. Loop layout.

3. Ladder layout.

4. Open fi eld layout.

5. Robot-centered cell.


1. In the in-line layout, the machines and handling system are arranged in a straight line. In its simplest form, the parts progress from one workstation to the next in a well-defi ned sequence, with work always moving in one direction and on back fl ow. The operation of this type of system is similar to a transfer line, except that a variety of workparts are processed in the system. Since all work units follow the same routing sequence, even though the processing varies at each station, this system is classifi ed as type III A in our manufacturing systems classifi cation system. For in-line systems requiring greater routing fl exibility, a linear transfer system that permits movement in two directions can be installed. One possible arrangement for doing this is shown in fi gure 4.20, in which a secondary work handling system is provided at each workstation to separate most of the parts from the primary line. Because of the variation in routings, this is a type II A manufacturing system.

Figure 4.20: In-line FMS layouts.


Table 4.2: Material handling equipment.

Layout confi guration

Typical material handling system(Chapter or Section)

In-line-layout In-line transfer system

Conveyor system

Rail guided vehicle system

Loop layout Conveyor system

In-fl oor towline carts

Ladder layout Conveyor system

Automated guided vehicle system

Rail guided vehicle system

Open fi eld layout Automated guided vehicle system

In-fl oor towline carts

Robot-centered layout

Industrial robot

2. In the loop layout, the workstation are organized in a loop that is served by a part handling system in the same shape, as shown in fi gure 4.21 (a). Parts usually fl ow in one direction around the loop, with the capability to stop and be transferred to any station. A secondary handling system is shown at each workstation to permit parts to move without obstruction around the loop. The load/unload station(s) are typically located at one end of the loop. An alternative form of loop layout is the rectangular layout. As shown in fi gure 4.21 (b), this arrangement might be used to return pallets to the starting position in a straight line machine arrangement.


Figure 4.21: FMS loop layout.

3. The ladder layout consists of a loop with rungs between the straight sections of the loop, on which workstations are located, as shown in fi gure 4.22. The Rings increase the possible ways of getting from one machine to the next and obviate the need for a secondary handling system. This reduces average travel distance and minimizes congestion in the handling system, thereby reducing transport time between workstations.


Figure 4.22: FMS ladder layout.

4. The open fi eld layout consists of multiple loops and ladders and may include sidings as well as illustrated in fi gure 4.23. This layout type is generally appropriate for processing a large family of parts. The number of different machine types may be limited, and parts are routed to different workstations depending on which one becomes available fi rst.

5. The robot-centered cell (fi gure 4.24) uses one or more robots as the material handling system. Industrial robots can be equipped with grippers that make them well suited for the handling of rotational parts, and robot-centered FMS layouts are often used to process cylindrical or disk-shaped parts.


Figure 4.23: Open fi eld FMS Layout.

Figure 4.24: Flexible machining cell with turning centres and a Robot serving as the material handling unit.


4.9.3.3 Computer Control SystemThe FMS consist of a computer system which is interfaced to the material

handling system, workstations.

The FMS computer system consist of central computer controlling the individual machines and other computer components.

1. Distributing the control instructions to various machinesThe central computer co-ordinate the individual machines through the

instructions. In FMS, part programs must be downloaded to machines, The DNC system stores the programs, editing existing programs, developing new programs and perform other function.

2. Workstation controlIn a fully automated FMs, the individual machines operate under computer

control.

3. Production controlCo-ordinating various production operations of the FMS modules by direct

communication with their controllers like CNC etc.,

4. Primary material handling system controlIt is also called as traffi c control. If refers to the management to primary

material handling system that moves parts between machines. This can be achieved by activating switches at various branches and points at machine tool locations.

5. Secondary material handling system controlIt is also called shuttle control. It controls the secondary material handling

system at each workstation.

6. Monitoring of workpartsThe computer must monitor the station of each cart or pallet in the primary

and secondary handling system.


7. Tool managementArrange the availability of the right tool in the right conditions at the right

time in the right place. Tool life is also monitored by the computer system.

8. Performance reporting and MonitoringThe computer control system is programmed to collect all the data on the

operation and performance of the system and reports can be prepared based on the performance of FMS.

9. Machine diagnostics to obtain any malfunctions of the FMS modules.

4.9.3.4 Human ResourcesHuman are needed to manage the operations of FMS.

i) Loading the workparts and unloading the fi nished parts.

ii) Selecting tools and setting tools.

iii) Maintenance and repair the machines.

iv) Writing part programs.

v) Operating computer systems.

4.9.4 Applications of FMS

1. All types of machining operations.

2. Assembly works.

3. Sheet metal press-working.

4. Forging.

5. Welding.

6. Surface-treatment.

7. Inspection and Testing.


4.9.5 Benefi ts of FMS

a) To provide fl exible manufacturing facilities for a family of work pieces, that is, there will be a greater potential to make changes in terms of products, technology etc., as the need arises. Necessity depends on market needs and not when the production deems it so. It means that it provides more fl exible response to the customers.

b) It reduces both direct and indirect labour cost; machines and material handling systems associated with FMS are unmanned and are under the control of a supervisory computer. As there is automatic handling and automatic gauging and inspection by the use of Robots, the need for manual labour is minimised. The cost of labour per unit of production is less which in turn is responsible for reduced unit cost.

c) It increases or improves the utilisation of equipment and facilities through its inherent fl exibility. Utilisation in FMS is as high as 85% as compared to 50% or even less in conventional case.

d) It provides reduced manufacturing lead time, reduced inventory of parts (both stock and work in progress), since time spent by the material on shop fl oor is reduced drastically and responsible for rapid throughput of work on the part.

e) It can maximise the combination of operations at a single location.

f) FMS minimises the requirement of tooling as in most of the cases specialised tooling is software.

g) It provides better and more consistent quality products.

h) It provides a better management control by integration of computers, NC and automated handling. It provides the scheduling fl exibility too.

In general, there will be an increase in productivity of quality goods and a reduction in manufacturing cost by decreasing both transfer time and production time. (Refer fi gure 4.25)


Figure 4.25

Figure 4.26: Benefi ts derived from FMS

• Reduced cycle time.

• Lower Work-In-Process (WIP) inventory.


• Low direct labour costs.

• Ability to change over to different parts quickly.

• Improved quality of product (due to consistence).

• Higher utilization of equipment and resources (Utilisation) better than standalone CNC machines).

• Quicker response to market changes.

• Reduced space requirements. Ability to optimise loading and throughput of machines. Expandability for additional processes or added capacity. Reduced number of tools and machines required.

• Motivation for designers to add variations and features to meet customer requirements.

• Compatible with CIM.

REVIEW QUESTIONS:

1. Explain about (SFC) Shop Floor Control.

2. Explain the scheduling techniques and stages in scheduling.

3. What are the steps involved in the factory data collection system? Explain.

4. What re the methods used for Automatic Identifi cation.

5. Explain the Bar code Technology.

6. Explain Bar code Technology in detail.

7. Explain various Bar code Reader.

8. Explain Bar Code printers.

9. Describe Automated Data Collection System.

10. Describe Multi level scanning.


11. Explain components of FMS in detail.

12. Describe FMS with neat diagram.

13. Explain material handling and storage system in detail.

14. Give FMS layout with neat diagram.

15. Give the application of FMS in detail.

16. Explain machine vision system in detail.

17. Explain voice Technology in detail.

18. Give detail description of OCR with diagram.

19. State the benefi ts of FMS.

20. Explain various workstations in FMS.

5.1 INTRODUCTION - PRODUCTION PLANNING AND CONTROL

Any manufacturing activity require resource input in terms of men, materials, capital and machines. In any business that produced a product or service, production activity must be related to market demands indicated by the continuous stream of customer’s orders. For maximum effectiveness, this must be in such a way that customer demands are satisfi ed, but at the same time production activities are carried on in an economic manner. The process of developing this kind of relationship between market demands and production capability is the function of Production Planning and Control or sometimes referred to as production control.

Production planning and control can be effected principally through the management of work fl ow inventories and backlogs and changing levels of operation. The set of policies and procedures that are used to manage work fl ow,

5UNIT

Computer Aided Planning and Control and

Computer Monitoring


inventories backlogs and changes in the level of production rate comprise what called a production planning and control system.

PPC is factory’s nervous system. The functions of PPC in a factory can be easily compared with the nervous system in human organism. It serves to co-ordinate the activities of a plant just as the nervous system regulates muscular movements. When simple repetitive operations are performed, this production control is accomplished more or less subconsciously in the same manner that the nervous system automatically regulates one’s breathing. When less repetitive activity is involved, more conscious direction is necessary, both in the plant and in the human system.

Customer demands are likely to differ in quantities and delivery schedules and this will lead to large fl uctuations in the production levels. So to meet any demand, it is desirable to have planning for production in future time periods for inventories of fi nished goods and meet part of market demands from such fi nished goods inventories. Furthermore, the lead times involved in procurement of manufacturing inputs warrant planning for production in advance. This is particularly so, in the Indian context, with specifi c reference to industrial raw materials. Also, requirements of skilled manpower necessitate such planning where time factor involved in training personnel is rather large. Also the social political structure in India makes it quite diffi cult for an organisation to have varying manpower levels. This, again, necessitates production planning in order to smooth out the needs for manpower.

Another reason why PPC is necessary, is the need to meet changes in demands due to trend, cyclical and reasonable factors. Long-run changes in demand are taken care of by change in overall capacity by expansion and or new facilities. However, in short run, these will have to be taken care of by such factors as sub-contracting, using overtime and building up inventories. Needless to say, in planning production for these purposes, one should take into consideration the changes in production levels over future periods in order to economise on cost of production. This is must factor which necessaries planning for production and exercising control.

COMPUTER AIDED PLANNING AND CONTROL AND COMPUTER MONITORING 5.3

Moreover, production operations are subject to variety of uncertainties such as emergency order, breakdown, material shortages and various other contingencies. PPC provides a way to take these factors into considerations.

5.1.1 Effects of Production Planning and Control

The effects of PPC can be grouped in two captions:

a) Material factors

b) Human factors

a) Material factorsUnder this following categories are included:

i) Quality of the output:An improvement in volume of output within quality and safety limits

laid down by management is most common objective of PPC.

ii) Plant utilisation:With ever increasing capital investment per producer in industry, making

complete use of plant is of growing importance. Experience and research has shown that in many types of plant the capital saving due to improved load factors are proving the most substantial of all. These improvements are also being achieved through better labour effectiveness.

iii) Use of services:Again economics in the use of steam, water, air and electricity may be

paramount factors.

iv) Quality of product:It may be sometimes desirable for economic or other reasons, to improve

the quality of product to new or more consistent standard.


v) Process effi ciency:An operator can have a far more signifi cant, effect on process effi ciency

than that was previously possessed.

vi) Standard of safety:In dealing with many products quite apart from the normal good

standards a particularly high level of safety may be important, which is being achieved by it.

vii) Works cleanliness:It is another objective of management.

b) Human factorsUnder this following may be included.

i) Effectiveness of work:The work should be such that it meets the ego and emotion of the worker

and he feels the pride over it. In other words, the objective of management is to choose right man for right job at right place at right time on right wages and salaries.

ii) Interest in work:The worker should take interest in work and he will put the heart and

hand in performing the task is another prime aim of good management.

iii) Waiting time:The waiting time should remain minimum for the want of materials,

tools, equipment, supervision, inspection, delivery etc. It can only be achieved when the worker on the work will help fully and take interest in it.

iv) Need for supervision:To make the worker expert and self-dependent in normal day to day

work is the other aim of the management. The supervisory time should be


reduced. The supervisors should be left, to perform the task of planning, coordination, motivation, control and feedback informations only.

v) Ideas for new methods:Workers, working on the machine are said to be the best man for new

idea and suggestions, as he knows the various aspect of work fully. To give encouragement to the worker for new ideas and new method, PPC is brought in picture.

vi) Team spirit:To develop the team spirit and feeling of brotherhoodness among

workers is another aim. The workers should do the work as a team, and should recognise their value and status in company as a group not individuals.

vii) Absentieeism:To minimise and regulate the absenteeism, PPC may be introduced.

viii) Labour turnover:It helps the turnover to its minimum.

5.2 COMPUTER-INTEGRATED PRODUCTION MANAGEMENT SYSTEM

There have been several factors working over the last several decades to cause the evolution of a more modern and effective approach to the problems of production planning and control cited above. The most obvious of these factors was the development of the computer, a powerful tool to help accomplish the vast data processing and routine decision-making chores in production planning that had previously been done by human beings.

In addition to the computer, there were other factors which were perhaps less dramatic but equally important. One of these was the increase in the level of professionalism brought to the fi eld of production planning and control. Production


planning has been gradually transformed from what was largely a clerical function into a recognized profession requiring specialized knowledge and academic training. Systems, methodologies, and even a terminology have developed to deal with the problems of this professional fi eld.

Important among the methodologies of production planning and control, and another signifi cant factor in the development of the fi eld, is operations research. The computer became the important tool in production planning, but many of the decision-making procedures and software programs were based on the analytical models provided by operations research. Linear programming, inventory models, queueing theory, and a host of other techniques have been effectively applied to problems in production planning and control.

Another factor that has acted as a driving force in the development of better production planning is increased competition from abroad. Many American fi rms have lost their competitive edge in international and even domestic markets. Increasing U.S. productivity is seen as one important way to improve our competitive position. Better management of the production function is certainly a key element in productivity improvement.

Finally, a fi fth factor is the increase in the complexity of both the products manufactured and the markets that buy these products. The number of different products has proliferated, tolerances and specifi cations are more stringent, and customers are more particular in their requirements and expectations. These changes have placed greater demands on manufacturing fi rms to manage their operations more effi ciently and responsively.

As a consequence of these factors, companies are gradually abandoning the traditional approach in favor of what we are calling computer-integrated production management systems. There are other terms which are used to describe these systems and their major components. IBM uses the term “communications-oriented production information and control system—COPICS”—to identify the group of system elements. George Plossl integrates the various system concepts under the name “manufacturing control”. Computer-Aided Manufacturing— International,


calls its development effort in this area the “factory management project”. Oliver Wight refers to the use of MRP II, or manufacturing resource planning, to consolidate the manufacturing, engineering, and fi nancial functions of the fi rm into one operating system. All of these terms refer to computerized information systems designed to integrate the various functions of production planning and control and to reduce the problems.

Figure 5.1: of activities in a computer-integrated production management system.

Figure 5.1 presents a block diagram illustrating the functions and their relationships in a computer-integrated production management system. Many of these functions are nearly identical to their counterparts in traditional production planning


and control. For example, forecasting, production planning, the development of the master schedule, purchasing, and other functions appear die same in fi gure 5.1. To be sure, modern computerized systems have been developed to perform these functions, but the functions themselves remain relatively unchanged. More signifi cant changes have occurred in the organization and execution of production planning and control through the implementation of such schemes as MRP, capacity planning, and shop fl oor control. What follows is a brief description of some of the recently developed functions in a CIPMS. We will neglect those functions which are nearly the same as their conventional counterparts. The newer functions are highlighted in fi gure 5.1 bold blocks.

5.2.1 Engineering and manufacturing data base

This data base comprises all the information needed to fabricate the components and assemble the products. It includes the bills of material (assembly lists), part design data (either as engineering drawings or some other suitable format), process route sheets, and so on. Ideally, these data should be contained in some master fi le to avoid duplication of records and to facilitate update of the fi les when design engineering changes are made or route sheets are updated. As shown in fi gure 5.1, the design engineering and process planning functions provide the inputs for the engineering and manufacturing data base.

5.2.2 Material requirements planning (MRP)

MRP involves determining when to order raw materials and components for assembled products. It can also be used to reschedule orders in response to changing production priorities and demand conditions. The term priority planning is now widely used in describing computer-based systems for time-phased planning of raw materials, work-in-progress, and fi nished goods.

We will devote most of the following chapter to the subject of material requirements planning.


5.2.3 Capacity planning

MRP is concerned with the planning of materials and components. Capacity planning, on the other hand, is concerned with determining the labour and equipment resources needed to meet the production schedule.

Capacity planning will often necessitate a revision in the master production schedule. It would be infeasible, and counterproductive in all likelihood, to develop a master schedule that exceeds plant capacity. Therefore, the master schedule is checked against available plant capacity to make sure that the schedule can be realized. If not, either the schedule or plant capacity must be adjusted to be brought into balance. Capacity planning has always been of concern in traditional production planning and control. However, it is an area of planning whose recognition has been growing in recent years due to its impact on the ability to achieve the master production schedule.

The term “plant capacity” is used to defi ne the maximum rate of output that the plant can produce under a given set of assumed operating conditions. The assumed operating conditions refer to the number of shifts (one, two, or three shifts per day), number of days of plant operation per week, employment levels, and whether or not overtime is included in the defi nition of plant capacity. Capacity for a production plant is traditionally measured in terms of output units of the plant. Examples would be tons of steel for a steel mill, number of automobiles for a car assembly plant, and barrels of oil for a refi nery. When the output units of a plant are nonhomogeneous, input units may be more appropriate for measuring plant capacity. A job shop, for instance, may use labor hours or available machine hours to measure capacity.

Capacity planning is concerned with determining what labor and equipment capacity is required to meet the current master production schedule as well as the long-term future production needs of the fi rm. Capacity planning is typically performed in terms of labor and or machine hours available.

The function of capacity planning in the overall production planning and control system is shown in fi gure 5.1. The master schedule is transformed into


material and component requirements using MRP. Then these requirements are compared with available plant capacity over the planning horizon. If the schedule is incompatible with capacity, adjustments must be made either in the master schedule or in plant capacity. The possibility of adjustments in the master schedule is indicated by the arrow in fi gure 5.1 leading from capacity planning to the master schedule.

Capacity adjustments can be accomplished in either the short term or the long term. Capacity planning for short-term adjustments would include decisions on such factors as the following:

1. Employment levels. Employment in the plant can be increased or decreased in response to changes in capacity requirements.

2. Number of work shifts. Increasing or decreasing the number of shifts per week.

3. Labour overtime hours or reduced workweek.

4. Inventory stockpiling. This would be used to maintain steady employment during temporary slack periods.

5. Order backlogs. Deliveries of product to customers would be delayed during busy periods.

6. Subcontracting. Letting of jobs to other shops during busy periods, or taking in extra work during slack periods.

Capacity planning to meet long-term capacity requirements would include the following types of decisions:

1. Investing in more productive machines or new types of machines to manufacture new product designs

2. New plant construction

3. Purchase of existing plants from other companies


4. Closing down or selling off existing facilities which will not be needed in the future

5.2.4 Inventory management

In the manufacturing environment, inventory management is closely tied to material requirements planning. The objectives are simple to keep the investment in inventory low while maintaining good customer service. The use of computer systems has provided opportunities to accomplish these objectives more effectively.

5.2.5 Shop fl oor control

The term “shop fl oor control” refers to a system for monitoring the status of production activity in the plant and reporting the status to management so that effective control can be exercised. We examine shop fl oor control and the use of computers to monitor production.

5.2.6 Cost planning and control

The cost planning and control system consists of the data base to determine expected costs to manufacture each of the fi rm’s products. It also consists of the cost collection and analysis software to determine what the actual costs of manufacturing are and how these actual costs compare with the expected costs. The following section is devoted to this important area in the operations of a computer-integrated production management system.

The cost planning and control function encompasses most of the other functions within the computer-integrated production management system. It receives data from all of the other CIPMS modules and reduces them to a lowest common denominator: money. The objectives of the cost planning and control system are to help answer the following questions:

1. What are the expected costs to manufacture and sell each of the company’s products?

2. What are the actual costs to manufacture and sell each of the company’s products?


3. What are the differences between what it should cost and what it does cost, and how are these differences explained?

The underlying basis for attempting to answer these questions is the objective of minimizing the costs of manufacturing the fi rm’s products.

5.2.6.1 Cost planningCost planning is concerned with the fi rst of the three questions: What are

the expected costs of manufacturing a product? An attempt is made to answer the question by determining the standard cost for the product. The standard cost for the product is the aggregate cost of labor, materials, and allocated overhead costs. The standard costs are compiled from various data sources and other modules in the CIPMS. The following list includes several standard data sources:

1. The bill of materials gives the components and materials used in the product.

2. Process route sheets list the manufacturing operations used for each component in the product.

3. Time standards specify the operation times for each operation listed on the route sheets.

4. Labor and machines rates allow the time standards to be converted into dollar costs for each operation.

5. Material quotations from purchasing provide information on material costs, based on catalog price data or direct quotes from potential vendors.

6. Accounting data determine appropriate overhead rates.

With so much data collected from many different sources, the computation of a standard cost for a product is not an insignifi cant task. To accomplish the task and determine a meaningful cost value, the current approach involves use of a data base which is common for engineering, manufacturing, and accounting. In this way, all


departments have access as needed to the same information fi les and there is greater consistency and accuracy of computations based on data in these fi les.

Development of standard costs for all of the company’s products provides a yardstick against which the actual production cost performance can be measured. Determining the actual costs is the function of cost control.

5.2.6.2 Cost controlCost control is concerned with the second and third questions: What are the

actual costs of manufacturing? And what are the differences between actual costs and expected costs?

In any manufacturing activity, there will be differences between the standard costs computed in cost planning and the actual costs that occurred during production. The reasons why these differences happen comprise a never-ending list. Actual prices of raw materials increase above quoted prices, machines break down, differing lot sizes infl uence production costs, actual process sequences deviate from the planned route sheets, and a vast collection of other reasons result in variances between actual costs and standard costs.

Cost control involves the collection of data from which the actual costs of the product can be calculated. Data on material costs can be compiled through the purchasing department. Data on labour costs can be collected by means of the shop fl oor control system. Over-head costs are usually excluded from consideration because they do not represent an actual expense of the product but rather an allocation of general factory and corporate expenses. Included within the cost control function is the preparation of reports that document actual product costs and variances from standard costs.

5.3 PRODUCTION PLANNING PROCESS

The production planning process starts with a good sales forecast for the next year that discounts as many of the variables in the marketplace as possible. The demand management issues, such as interplant transfers, distribution requirements,


and service parts must also be factored into the production plan. Changes in inventory or backlog levels that affect the overall production rate must also be considered.

Effective production planning processes have reviews at regular intervals with a time fence for changes requested in the aggregate production levels. For example, successful fi rms often review the production plan monthly and make changes quarterly. The time fence frequently sets limits on how late in the planning cycle, changes in the aggregate levels can be made. For example, the time fence may indicate that no changes can be made in the current or closest period and that not more than a 10 percent change can be made in the nearest future period. Routine reviews of the production plan keep the communication alive between top management and manufacturing.

5.3.1 Functions of PPC

The highest effi ciency in production is obtained by manufacturing the required quantity of product, of the required quality, at the required time by the best and cheapest method. To attend this objective management employs production planning and control, the tool that coordinates all manufacturing activities. (Fig. 5.2)

The main functions of PPC can be classifi ed in ten categories:

i) Materials:Raw materials, as well as standard fi nished parts and semi-fi nished products

must be available when required to ensure that each production of operation will start on time. Duties include the specifi cation of materials (both with respect to dimensions and quality) quantities and availability; delivery dates standardization and reduction of variety, procurement and inspection. This function also covers the procurement of semi-fi nished products from subcontractors.

ii) Methods:1. The purpose of this function is to analyse possible methods of

manufacture and try to defi ne the best method compatible with a given set of circumstances and facilities. This analysis covers both the general


study and selection of production processes (for the manufacture of components or assemblies) and the detailed development and specifi cations of methods of application.

2. Such a study results in determining the sequence of operations and the division of the product in to assemblies and sub-assemblies, modifi ed by the limitations of existing layout and work fl ow.

iii) Machines and equipments:Methods of manufacture have to be related to available production facilities,

coupled a detailed study of equipment replacement policy, Maintenance policy, procedure and schedules are also functions connected with managerial responsibility for equipment, since the whole problem of breakdowns and reserves can be seriously refl ected in halts in production, tool management, as well as problems in both design and economy of jigs and fi xtures, which constitutes some of the major duties of production planning and control.

iv) Routing:Once the overall methods and sequence of operations have been laid down,

each stage in production is broken down to defi ne each operation in detail, after which the issue of production orders can be planned. Routing prescribes the fl ow of work in the plant and is related to considerations of layout of temporary storage locations for raw materials and components of material handling systems. Routing is fundamental production function on which all subsequent planning is based.

v) Estimating:When production orders and detailed operation sheets available with

specifi cations, feeds, speeds and use of auxiliary attachments and methods, the operation times can be worked out. This function involves the extensive use of operation analysis in conjuction with methods and routing as well as work measurement in order to set up performance standards. The human element fi gures prominently in work measurement because it is sensitive to systems of work rating and wage incentive schemes. Hence it may consequently result in a wide scatter of


operation times and in unduly large fl uctuations and perhaps instabilities in time schedules.

vi) Loading and Scheduling:Machines have to be loaded according to their capability performing the given

task and according to their capacity Machine loading is carried out in conjuction with routing to ensure smooth work fl ow; and with estimating, to ensure that the prescribed methods feeds and speeds are best utilised. Scheduling is perhaps the toughest job facing a production manager because it determines the utilisation of equipment and manpower and hence the effi ciency of the plant. Scheduling must ensure that operations are properly dovetailed, that semi-fi nished components arrive at their next station in time, that assembly work is not delayed, and that on the other hand the plan is not unnecessary loaded with physically and fi nancially with work in process, i.e., with semi-fi nished components waiting for their next operation. This calls for a careful analysis of process capacities, so that fl ow rates along the various production lines can be suitably co-ordinated. In machine loading, appropriate allowances for set up of machines, process adjustments, and maintenance down-time have to be made, and these allowances form vital part of the data constantly used by the scheduling function.

vii) Despatching:This function is concerned with the execution of the planning function.

Despatching is the routine of setting productive activities in motion, through release of orders and instruction and in accordance with previously planned times and sequences as embodied in route sheets and loading schedules. Despatching authorizes the start of the production operations by releasing materials, components, tools, fi xtures and instruction sheets to the operator and ensures that material movement is carried out according to the planned routing sheets and schedules.

viii) Expediting:This control tool is the executive arm that keeps a close watch on the progress

of the work expediting or ‘follow up’ or ‘progress’ as it is some times called, is


logical step after dispatching. Despatching maintains them and sees them through to their successful completion. This function has to keep close liaison with scheduling, in order to provide effi cient feed-back and prompt review of targets and schedules.

Figure 5.2: The ten functions of production planning and control cycle

ix) Inspection:Another major control function is that of inspection. Although the control of

quality is often detached from the production planning and control department, its fi ndings and criticisms are of supreme importance both in the execution of current plans and in the planning stage of future undertakings. When the limitations of


processes, methods and manpower are known. These limitations can form a basis for future investigations in evaluating, with the view to improving production methods, or indicating the cost implications of quality at the design stage.

x) Evaluating:Perhaps the most neglected function, but on an essential link between control

and future planning, is that of evaluating. The executive tasks of despatching and expediting are concerned with the immediate issues of production and with measures that will as certain the fulfi lment of set targets. Valuable information is gathered in this process, but the feedback mechanism is rather limited in nature and unless provision is made so that all this accumulated information can be properly digested and analysed, valuable data may be irretrievably lost. Thus here the evaluating function comes in, to provide a feedback mechanism on a longer term basis so that past experience can be evaluated with the view to improve utilisation of methods and facilities. Many fi rms consider this function important enough to divorce part of it from production planning and control and to establish it as a separate department in its own right in which wider aspects of production management can be studied, using modern tools of operations research.

5.4 MATERIAL REQUIREMENTS PLANNING (MRP)

The material requirements planning system is essentially an information system consisting of logical procedures for managing inventories of components assemblies, sub-assemblies, parts and raw materials in a manufacturing environment. The primary objective of an MRP system is to determine how many of each item in the bill of materials must be manufactured or purchased and when. The key concepts used in determining the material requirements are:

1. Product structure and bill of materials

2. Independent versus dependent demand

3. Parts explosion

4. Gross requirements


5. Common-use items

6. On-hand inventories

• Scheduled receipts

• Net requirements

7. Planned order releases

8. Lead time

Brief discussions of these concepts are given below:

1. Product structure and bill of materialsProduct is the single most important identity in an organization. The product

is what a company sells to its customers. The survival of a company depends on the profi t on the sales of the products. A product maybe made from one or more assemblies, sub-assemblies and components. The components are made from some form of raw materials. To manufacture the products, it is therefore important to understand the product structure and have correct information on the components, subassemblies, and assemblies.

A bill of materials is an engineering document that specifi es the components and subassemblies required to make each end item (product). It can be represented as a symbolic exploded view of the end item structure. Consider a hypothetical product called end item E1 (at level 0), which is made up of two subassemblies S1 and S2 at level 1 as shown in Figure 5.3 . Subassemblies S1 and S2 at level 1 consist of two and three components at level 2, respectively A complete product structure for product E1 is shown in Figure 5.3. The end item E1 is called a parent item to subassemblies S1 and S2, which are called component items. Similarly, subassembly S1 is a parent item to components C1 and C2 and S2 is a parent item to C3, C4 and C5. At level 3, the raw material becomes input to the components at level 2.


Figure 5.3: Product structure for hypothetical product and bill of materials

2. Independent versus Dependent demandThe demand for the end items originates from customer orders and forecasts.

Such a demand for end items and spare parts is called independent demand. The demand by a parent item for its components is called dependent demand. For example, if the end-item demand is X number of units and one unit of end item requires Y units of a subassembly, then the demand of that subassembly is XY units.

3. Parts explosionThe process of determining gross requirements for component items, that

is, requirements for the subassemblies, components and raw materials for a given number of end-item units, is known as parts explosion. Therefore, parts explosion essentially represents the explosion of parents into their components.

4. Gross requirements of component itemsTo compute the gross requirements of component items, it is necessary to

know the amounts of each component item required to obtain one parent item. This information is available from the product structure and the bill of materials (as indicated in parentheses beside each component item). For example, if the demand for end product El from a market survey in period 7 is 50 units (for product structure and the bill of materials, see Figure 5.3), we can determine the dependent demand (gross requirements) for the subassemblies and the components as follows:


Demand of S1 = 1 × demand of E1 = 50 units

Demand of S2 = 2 × demand of E1 = 100 units

Demand of C1 = 1 × demand of S1 = 50 units





Figure 5.4: Product structure for end product E2

5. Common-use itemsMany raw materials and components may be used in several subassemblies

of an end item, and several end items. For example, consider the product structure for end products E1 and E2 given in Figure 5.3 and Figure 5.4, respectively. Components C2 and C4 are common to both E1 and E2. In the process of determining net requirements, common-use items (C2 and C4 in this case) must be collected from different products to ensure economies in manufacturing and purchasing these items.


6. On-hand inventory, Scheduled receipts and Net requirementsIn some cases, when there is ongoing production activity, initial inventory

for some of the component items is available from previous production runs. Also, to maintain continuous production from one planning horizon to another, some inventory is planned to be available at the end of the planning horizon. This inventory is referred to as on-hand inventory for the current planning period. Further more, it takes some time for orders to arrive. Therefore, the orders placed now are delivered into some future periods. These are known as scheduled receipts. The net requirements in a period are thus obtained by subtracting the on-hand inventory and items already on order to be available in that period for these component items from the gross requirements.

7. Planned order releasesPlanned order releases refer to the process of releasing a lot of every component

item for production or purchase. The question is how the economic lot sizes of component items are determined. Because shortages are not permitted in an MRP system, the lot sizes are determined by trading off the inventory holding costs and setup costs. Although the manufacturing system is a multistage production system, the demand at each stage (level) is deterministic and time varying. Lot sizes in an MRP system are determined for component items for each stage sequentially starting with level 1, then level 2 and so on.

8. Lead time and Lead time offsettingThe lead time is the time it takes to produce or purchase a part. In manufacturing,

the lead time depends on the setup time, production time, lot size, sequence of machines on which operations are performed, queuing delays, and so forth. The purchasing lead time is the time that elapses between placing an order with a vendor and receipt of the order.

We know from the parts explosion and gross to net requirements how many of each component item (subassemblies, components and raw materials) are needed


to support a desired fi nished quantity of an end item. Information on the sequence in which the operations must be done and the amount of time it takes to perform these operations for a given lot size, is required to schedule the component items. The manufacture or purchase of component items must be offset by at least their lead times to ensure availability of these items for assembly into their parent items at the desired time.

5.4.1 Benefi ts of MRP

There are many advantages claimed for a well-designed, well managed material requirements planning system. Among these benefi ts reported by MRP users are the following.

(1) Reduction in inventoryMRP mainly affects raw materials, purchased components and work-in-

process inventories. Users claim a 30 to 50% reduction in work-in-process.

(2) Improved customer serviceSome MRP proponents claim that late orders are reduced to 90%.

Quicker response to changes in demand and in the master schedule.

(3) Greater productivityClaims are that productivity can be increased by 5 to 30% through MRP.

Labour requirements are reduced correspondingly.

(4) Reduced setup and product changeover costs

(5) Better machine utilization

(6) Increased sales and reductions in sales price


5.5 SHOP FLOOR CONTROL (SFC) SYSTEM

5.5.1 Introduction

Figure 5.5: Three Phases in a Shop Floor Control System

Shop fl oor control system refers to a system for monitoring the status of manufacturing activities on the plant fl oor and reporting the status to management so that effective control can be exercised. Production Activity Control (PAC) and Factory Co-ordination (FC) provide a frame work which integrates the requirements planning functions of RP type systems and the planning and control activities on a shop fl oor, and in doing so close the loop between the tactical and operations layers of the production planning and control hierarchy.


Production management systems are concerned with planning and control of the manufacturing operations. The functions of production planning, development of the master schedule, capacity planning and MRP all deal with the planning objective. Systems that accomplish the control objective are often referred to as “Shop Floor Control” (SFC).

The input to the SFC system is the collection of production plants like results of process planning, Material Requirement Planning (MRP), capacity planning. The SFC system consists of the following elements. (Refer fi gure 5.5)

i) Master schedule.

ii) Engineering database.

iii) Manufacturing database.

iv) MRP.

v) Capacity planning

The fi gure 5.5 shows the various phases in a shop fl oor control systems.

Computer Integrated Manufacturing (CIM) has recently a great deal of attention as an effective strategy to improve manufacturing responsiveness and quality. CIM seeks to integrate the entire manufacturing enterprise using a network of computer systems. However, the evolution to CIM has been slower than expected. This can be directly attributed to the high software development and maintenance costs and the diffi culty in achieving the required levels of integration between system (Ayres, 1989; Merchant, 1988). These problems are especially evident in the development of the Shop Floor Control System (SFCS). The SFCS is responsible for planning, scheduling, and controlling the equipment on the shop, fl oor.


Table 5.2: Planning, Scheduling, and Execution activities for each level in the SFCS control architecture.

Planning Scheduling ExecutionEquipment Operation-level

planning (e.g. tool path planning).

Determining the start/fi nish times for the individual tasks. Determining the sequence of part processing when multiple parts are allowed.

Interacting with the machine controller to initiate and monitor part processing.

Workstation Determining the part routes through the workstation, (e.g. selection of processing equipment). Includes replanning in response to machine breakdowns. Allocation of shared tools.

Determining the start/fi nish times for each part on each processing machine in the workstation.

Interacting with the equipment level controllers to assign’ and remove parts and to synchronize the activities of the devices (e.g. as required when using a robot to load a part on a machine tool).

Shop Determining part routes through the shop. Splitting part orders into batches to match material transport and workstation capacity constraints. Managing shared tools between workstations.

Determining the start/fi nish times for part batches at each workstation. Scheduling the delivery of shared tools.

Interacting with the workstation controllers and the Resource Manager to deliver/pickup parts.


Figure 5.6: Cycles of activities in a computer - integrated manufacturing system based SFC.

The shop processes orders which are part of the master production schedule for the facility. Shop level planning uses the shop level process plan and the current shop loading to determine the workstation route that the individual parts will take. Further, the order may be broken up into batches and part groups to


facilitate production, transport, and/or handling. Shop level scheduling convents the batches and workstation assignments from planning into a list of tasks to be issued by execution. This list provides a priority ranking for the order within the shop. Additionally, start/end times will be set for tasks at each workstation. Shop level execution co-ordinates the production of the current open orders in the shop by issuing the tasks to the individual workstations as specifi ed by the scheduler. The shop also facilitates interactions between the individual workstations where necessary.

Processing workstations produce parts in the form of patches or part groups. Workstation level planning uses the workstation level process plan and the current workstation loading to determine the actual pieces of equipment that the parts will be processed on. This also includes the determination of the specifi c work content that is to be achieved by the equipment. For example, a milling machine might perform the drilling operation during the next production run. The planning module will evaluate the workstation load and attempt to balance the processing requirements across all equipment in the workstation. Scheduling converts the sequence of equipment operations for all parts into a list of tasks that is issued by execution. This task list represents current scheduling policy being utilized by the workstation. Execution co-ordinates the movement of the batch or part group though the workstation by issuing the tasks to the individual equipment controllers and co-ordinating the interaction between equipment.

Processing equipment performs operations on part groups. Equipment level planning uses the work content assigned by the workstation to determine a sequence of operations to be performed on that piece of equipment. In the case of machining, planning might optimize machining parameters based on the machine and tool status. Scheduling converts this sequence of operations into a list of specifi c tasks to be issued by execution. The list represents a priority ranking of the part groups currently assigned to the equipment. Execution co-ordinates the production of part groups by performing the tasks provided by scheduling. Equipment level execution provides the direct interface between the control system and the physical machines.


The fi gure 5.6 shows the cycle of activities in a modern manufacturing system using shop fl oor control.

5.5.2 Various Activities of SFC

The various activities included in shop fl oor planning and control are

1. Assigning a priority to each order which helps in setting the sequence of processing orders at work centres.

2. Issuing dispatching lists to each work centre. These lists indicate which orders are due to be produced at a work centre, their priorities and completion dates/times.

3. Up-dating the work-in-progress inventory. Information such as number of good parts coming out of each processing step (operation), amount of scrap, amount of rework required and number of units short on each order.

4. Providing input-output control on all work centres.

5. Measuring the effi ciency, utilization and productivity of workers and machines at each work centre.

5.5.3 Scheduling Techniques for SFC

Process-focussed production systems produce many non-standard products in relatively small batches the following different routes or paths through the production facility and require frequent machines change-overs. Such production systems are also known as intermittent production system or job shops.

In such production systems, the departments or work centres are organized around the type of equipments or operations, (e.g., drilling, welding, soldering, etc.) Products fl ow through work centres in batches corresponding to individual customer orders or batches of economic batch quantities in produce - to stock situations.


Figure 5.7: Scheduling and Shop fl oor decisions in process - Focused production system.

Figure (5.7) illustrates scheduling and shop-fl oor decision in process-focusses operations or job shop. The following reasons shop fl oor scheduling process in quite complex.

a) Job shops have to produce products against customer orders for which delivery dates have to be promised.

b) Production lots tend to be quite small and may require numerous mahcine change-overs.

c) possibility of assigning and reassigning workers and machines to many different orders due to fl exibility.

d) In such a fl exible, variable and changing environment, schedules must be specifi c arid detailed work centre-wise to bring orderliness.

The type of scheduling technique used in job shop depends on the volume of orders, the nature of operations and the job complexity. Two types of scheduling techniques used are.


1. Forward Scheduling

2. Backward Scheduling.

1. Forward Scheduling:

Figure 5.8: Forward Scheduling.

In this approach, each task is scheduled to occur at the earliest time that, the necessary material will be on hand and capacity will be available. It assumes that procurement of material and operations start as soon as the customers, requirements are known. The customers place their orders on a ‘needed-as-soon-as possible’ basis. The earliest completion date, assuming that everything goes as planned, could be quoted to the potential customer. Some buffer time may be added to determine a date that is more likely to be achievable, if it is acceptable to the customer. Forward scheduling is used in many companies such as steel mills and machine tool manufacturers where jobs are manufactured to customer orders and delivery is requested on ‘as early as possible’, basis. Forward scheduling is well suited where the supplier is usually not able to meet the schedules. This type of scheduling is simple to use, gets jobs done in shorter lead times but accumulates high work in process inventories. Figure 5.8 illustrates forward scheduling.

2. Backward Scheduling:

This scheduling technique is often used in assembly-type industries and in job shops that commit in advance to specifi c delivery dates. After determining the


required schedule dates for major sub-assemblies, the schedule uses these required dates for each component and works backward to determine the proper release date for each component manufacturing order. The job’s start date is determined by letting back’ from the fi nish date, the processing time for the job.

Figure 5.9: Backward Scheduling

By assigning jobs as late as possible, backward scheduling minimizes inventories, since each job is not completed until it is due, but not earlier. Backward scheduling is also known as reverse scheduling. (See fi gure 5.9).

Stages in Scheduling:Scheduling is performed in two stages, viz.:

1. Loading.

2. Dispatching.

Loading:Loading or shop loading is the process of determining which work centre

receives which job. It involves assigning a job or task to a particular work centre to be performed during a scheduling period (such as a week). Loading of work centres depends on the available capacity (or determined by load schedules) and the expected availability of the material for the job. The jobs are assigned to machines or work centres taking into consideration the priority sequencing and machine or work centre utilization.


Dispatching:Dispatching is sequencing and selecting the jobs waiting at a work centre

(i.e., determining which job to be done next) when capacity becomes available. It is actually authorising or assigning the work to be done. The dispatch list is a means of priority control. It lists all jobs available to a work centre and ranks them by a relative priority. When priorities have been assigned to specifi c jobs, scheduling gets implemented through the dispatch list.

Finite loading and Infi nite loading:Loading procedures are categorised as either fi nite or loading or infi nite

loading. In fi nite loading, jobs are assigned to work centres by comparing the required hours for each operation with the available hours in each work centre for the scheduling period. In infi nite loading, jobs are assigned to work centres without regard to capacity (as if the capacity were infi nite).

a) Finite Loading:Finite loading systems start with a specifi ed capacity for each work centre

and a list of jobs to be processed at the work centre (sequencing). The work centre’s capacity is allotted to the jobs by simulating job starting times and completion times. The fi nite loading system combines loading, sequencing and detailed scheduling. It creates a detailed schedule for each job and each work centre, based on the capacity of the work centre.

Figure 5.10 shows a fi nite capacity load profi le for a work centre having a capacity of the work centre.


Figure 5.10: Finite Loading

b) Infi nite Loading:The process of loading work centres with all the jobs, when they are required

without regard to the actual capacity, available at the work centre is called infi nite loading. Infi nite loading indicates the actual released order demand (load) on the work centre, so as to facilitate decision about using overtime, sub-contracting or using alternative routings and delaying selected orders.

Figure 5.11 illustrates the infi nite loading profi le.

Figure 5.11: Infi nite Loading


Load Charts and Machine Loading Charts:

a) Load Chart or Load Schedule:A load schedule or load chart is a device for comparing the actual load (labour

hours and machine hours) required to produce the products as per the MPS against the available capacity (labour hours and machine hours) in each week.

Figure 5.12 illustrates the load schedule or chart shown graphically for a particular work centre having a weekly capacity of 100 standard hours and the weekly load for six weeks period. The load against each time period (i.e., week) is as shown:

Week Number 1 2 3 4 5 6Load (std hours) 100 100 80 60 60 70

Figure 5.12: Load Schedule

b) Machine Loading Chart (Gantt Load Chart):Gantt charts are used to display graphically the work loads on each work

centre. There are two types of Gantt charts viz,

i) Gantt load chart and ii) Gantt scheduling chart or progress chart.

Figure 5.13(a) illustrates a Gantt load chart drawn for a particular week of a particular month.


Figure 5.13: (a) Gantt load chart drawn for a particular weekof a particular month.

Gantt charts are simple to devise and easy to understand. The Gantt load chart offers the advantage of ease and clarity in communicating important shop information.

Figure 5.13 (b) illustrates a Gantt scheduling chart.

ActivityWeek number

1 2 3 4 5 6 7 8 9SchedulingEngg. releaseProcurementReceipt of materialsFabricationAssemblyInspectionShipping

Figure 5.13: (b) Gantt Scheduling Chart.


5.5.4 Scheduling and Controlling Production for deliveryschedules - Line of Balance (LOB) Method

It is quite common that production system often produce products as per the commitment to a delivery schedule promised to the customers. These delivery schedules can be part of a purchase order.

In order to ensure that the actual product deliveries match with the planned delivery schedules, a system must be devised to schedule and control all the processing steps of the production system.

Quite often, the fi rm may be in schedule in terms of deliveries, but may default soon on deliveries because the production pipeline may run out of products sooner or later. When this happens, it may be too late to take corrective action, because the deliveries get affected until the pipeline can again be refi lled with products.

Line-of-Balance(LOB) technique has been used successfully to schedule and control upstream processing steps in a variety of production systems (LOB) producing goods and services.

LOB Technique:The Line of Balance technique is used in production scheduling and control

to determine, at a review date, not only how many (quantity) of an item should have been completed by that date, but also how many should have passed through the previous (upstream) operation stages (processing steps) by that time so as to ensure the completion of the required delivery schedule.

LOB is a charting and computational technique for monitoring and controlling products and services that are made to meet specifi c delivery schedules. The concept of LOB is similar to the time phased order point system (TPOP) and MRP system (Material Requirements Planning). Starting from the delivery schedule (date) for the fi nal product and the quantity, the product structure tree is drawn on a horizontal scale, off-setting lead time on a time scale, refl ecting the previous processing steps or stages of production. The processing steps or production stages may include


purchased parts, machined parts, sub-assemblies and major assembly operations to support delivery schedule for the fi nished product. The LOB chart shows the quantity of parts, components, sub-assemblies, major assemblies and end products produced at every stage and at any given review date. It indicates the quantity of goods or services that should have been completed at every production stage or processing step and at any given time, so as to meet the delivery date for the end product.

The LOB technique shows the desired progress as well as the actual progress achieved on the LOB chart. The LOB technique can be best explained with an example as below:

Example:XYZ Company has received customer orders to deliver a product for which

the operations program and the delivery schedules are given below:

Delivery Schedule:

Week No.Quantity of end product to be

delivered1 52 103 104 105 15

Figure 5.14: Operations Programme


Develop a LOB chart and determine the quantities that should have passed through the upstream processing step/stages during the review point at the end of 2nd week.

Solution:Method: The fi ve stages required to be followed in LOB technique are

1. Preparation of operation programme or assembly chart.

2. Preparation of cumulative completion / delivery schedule or objective chart.

3. Construction of LOB chart.

4. Construction of program progress chart.

5. Analysis of progress and corrective action. These stages are illustrated below:

Stage 1: Preparation of operation programme or assembly chart:The operation programme or assembly chart shows the Mead time’ for each

operation stage/ processing step. The ‘lead time’ is the length of time prior to the completion of the fi nal operation/ processing step by which, intermediate operations must be completed.

Figure 5.15 illustrates an operation programme chart or assembly chart

Figure 5.15: Operation Programme Chart or Assembly Chart.


The delivery lead time for the fi nished product (end item) is zero and the time scale indicating ‘lead time’ runs from right to left, the operation programme chart indicates that the purchased part A must be combined with the item B in operation stage/processing step 4, three days before the completion of end item.

Item B, prior to combination, has undergone a conversion operation which has to be completed fi ve days before the completion of end item. The purchased part for item B must be available 10 days prior to the delivery date for the end item, which means the longest lead time is 10 days.

Stage 2: Preparation of Completion schedule (cumulative) or objective chart

The quantities of the end item to be completed, week by week and cumulatively, are indicated in the cumulative completion schedule and shown in the table below:

Table 5.3: Cumulative completion schedule

Week No.

Qty. of end item to be completed

Cumulative quantity to be completed

1 5 Nos. 5 Nos.

2 10 Nos. 15 Nos.

3 10 Nos. 25 Nos.

4 10 Nos. 35 Nos.

5 15 Nos. 50 Nos.

The cumulative completion schedule is shown graphically in the objective chart in fi gure 5.16:


Figure 5.16: Cumulative completion schedule graph or objective chart

Stage 3: Construction of line of balance chart:The line of balance shows the quantity of item that should have been completed

at each operation stage/processing step in a particular week at which, progress will be reviewed so as to meet the delivery schedule for the fi nished product and to meet the completion schedule.

The line of balance chart can be constructed graphically as illustrated below:

The following steps are required to construct the line of balance chart graphically

Step (a): Draw the cumulative completion schedule graph as shown in fi gure 5.17 (a).


Figure 5.17

Step (b): Draw a vertical line AB on the cumulative completion schedule graph at the week at which the review is to take place (say 2nd week in this example).

Step (c): Draw the line of balance schedule on the right hand side of the cumulative completion schedule graph [refer fi gure 5.17(b)]. Show by means of vertical bars, the 5 operation stages on the LOB schedule and indicate the quantities of the item that should have been passed through the operation stages/processing steps 1 to 5, by means of height of the vertical bars for each stage/processing step. In this example, it is done as below:

Let line A B cut the cumulative completion schedule graph at point ‘C’ From ‘C’ draw a horizontal line upto the vertical bar at operation stage/processing step No. 5. The height of the vertical bar indicates the quantity of the item that should have been completed at operation stage No. 5(i.e., completion of end product). In this example, this quantity of end product that should have been completed by the end of week number two is 15 numbers.

Step (d): For each of the other operation stages/processing step (i.e., operation stages 1 to 4), fi nd out how many should have been completed at the end of week No.2. This will be the total of not only the requirements for the completed end item


by the two-week review date, but also the quantity to be completed in the lead time for that operation. This is determined graphically as follows:

Draw a horizontal line C, D1 from the line AB, such that length C, D,, indicates the lead time (i.e., 3 days) for operation stages 4 and 3. From D,, draw a vertical line to cut the cumulative completion schedule graph at E,. Draw a horizontal line from E{ extending it upto the vertical bars drawn at operation stage No.4 and 3 (note both operations 4 and 3 indicate the quantities of the item that should have passed through these two stages. In this example, it is 21 numbers. (Analytically calculated as 15 + (3/5) x10 = 15 + 6 = 21 numbers).

Similarly, for operation stage no.2, draw a horizontal line CD2, such that the length CD9 indicates a lead time of Sjjays. Draw a vertical line D2E2 to cut the cumulative completion graph at E2. Draw a horizontal line from E2 upto the vertical bar drawn at operation stage No.2 at the 2nd review week, (in this case quantity is 25 numbers).

For operation stage No. 1, draw a horizontal line CD3 such, that the length CD3 indicates the lead time for operation stage No.l (in this example it is 10 days or 2 weeks). Draw a vertical line D3E3 to cut the cumulative completion graph at E3. Draw a horizontal line from E3 upto the vertical bar drawn at operation stage No.1 on the LOB schedule. The height of the vertical bar indicates the quantity of the item that should have been completed at operation stage no.1 (in this case the quantity is 35 i.e., 35 numbers. of purchased part B should have been received by the end of 2nd review week).

Step (e): Draw the line of balance (a stair-case step like line) by joining the tops of the vertical bars for each operation stage.

Step 4: Construction of Programme Progress Chart:The programme progress chart for the review week (week No.2 in this

example) is shown in Figure 5.18.


Figure 5.18: Programme Progress Chart

In this graph, the actual number of items produced at each operation stage against the quantities that should have been produced as indicated by line of balance are shown on the LOB chart. This chart indicates clearly, the excess or shortage in the quantities of the item at the operation stages, which is illustrated in the fi gure 5.18. The actual quantities are shown by hatched vertical bars.

Step 5: Analysis of Progress and Corrective Action:By referring to the programme process chart which is prepared every week,

the difference between the desired production (as indicated by line of balance) for the review week (week no.2 in this example) can be compared with the actual production achieved at the end of the review week (shown by height of patched vertical bars for each operation stage). The excess production or short fall in production can be found out (as shown in fi gure 5.18) and appropriate corrective action such as expediting delivery of bought-out item (item B) or production of in-house made items or reducing the production to bring it in line with the line of balance.

Benefi ts of LOB Technique1. LOB is a simple planning and control technique, which like network

analysis, formalizes and enforces planning discipline and enables control to be exercised at each stage of the production line.


2. LOB prevents any feeling of false security which might be engendered if the delivery of an item is on schedule but unappreciated shortfalls at early stages are building up trouble.

3. LOB enables identifi cation of shortfalls or even excessive production or purchasing levels, so that corrective action can be taken in good time.

4. LOB achieves its greatest benefi ts when products or services are produced to specifi c delivery schedules, production involves many processing steps and production lead times are long.

5.6 LEAN AND AGILE MANUFACTURING

5.6.1 Lean Production and Waste in Manufacturing

Lean production means doing more work with fewer resources. It is an adaptation of mass production in which work is accomplished in less time, in a smaller space, with fewer workers, and with less equipment, and yet achieves higher quality levels in the fi nal product. Lean production also means giving customers what they want and satisfying or surpassing their expectations. The term “lean production” was coined by researchers in the International Motor Vehicle Program at the Massachusetts Institute of Technology to describe the way in which production operations were organized at the Toyota Motor Company in Japan during the 1980s. Toyota had pioneered a system of production that was quite different from the mass production techniques used by automobile companies in the United States and Europe. summarizes most of the comparisons between mass production and lean production.

The Toyota Production System had evolved starting in the 1950s to cope with the realities of Japan’s postwar economy. These economic realities included (1) a much smaller automotive market than in the United States and Europe, (2) a scarcity of Japanese capital to invest in new plants and equipment, and (3) an outside world that included many well-established automobile companies determined to defend their markets against Japanese imports. To deal with these challenges, Toyota developed a production system that could produce a variety of car models with


fewer quality problems, lower inventory levels, smaller manufacturing lot sizes for the parts used in the cars, and reduced lead times to produce the cars. Development of the Toyota production system was led by Taiichi Ohno, a Toyota vice president, whose efforts were motivated largely by his desire to eliminate waste in all its various forms in production operations.

The ingredients of a lean production system can be visualized as the structure shown in Figure 5.19. At the base of the structure is the foundation of the Toyota system: elimination of waste in production operations. Standing on the foundation are two pillars.

Table 5.3: Comparison of Mass Production and Lean Production

Mass Production Lean ProductionInventory buffers Minimum wasteJust-in-case deliveries Just-in-time deliveriesJust-in-case inventory Minimum inventoryAcceptable quality level (AQL) Perfect fi rst-time qualityTaylorism (workers told what to do) Worker teamsMaximum effi ciency Worker involvementInfl exible production systems Flexible production systemsIf it ain’t broke, don’t fi x it Continuous improvement

Figure 5.19: The structure of a lean production system.


• Just-in-time production

• Autonomation (automation with a human touch).

The two pillars support a roof that symbolizes a focus on the customer. The goal of lean production is customer satisfaction. Between the two pillars and residing inside the structure is an emphasis on worker involvement: workers who are motivated, fl exible, and continually striving to make improvements. Table identifi es the elements that make up just-in-time production, worker involvement, and autonomation in the lean production structure. (Refer fi gure 5.19)

Table 5.4: The Elements of Just-in-Time Production, Worker Involvement, and Autonomation in the Lean Production Structure

Just-in-Time Production

Worker Involvement Autonomation

Pull system of production control using kanbans Setup time reduction for smaller batch sizes Production leveling On-time deliveries Zero defects Flexible workers

Continuous improvement (kaizen) Quality circles Visual management The 5S system Standardized work procedures Participation in total productive maintenance by workers

Stop the process when something goes wrong (e.g., a defect is produced) Prevention of overproduction Error prevention and mistake proofi ng Total productive maintenance for reliable equipment

The underlying basis of the Toyota production system is elimination of waste, or in Japanese, muda. The very word has the sound of something unclean (perhaps because it begins with the English word “mud”).

• Actual work that consists of activities that add value to the product. Examples include processing steps to fabricate a part and assembly operations to build a product.

• Auxiliary work that supports the actual value-adding activities. Examples include loading and unloading a production machine that performs processing steps.


• Muda, activities that neither add value to the product nor support the value adding work. If these activities were not performed, there would be no adverse effect on the product.

Ohno identifi ed seven forms of waste in manufacturing that he wanted to eliminate by means of the various procedures that made up the Toyota system. Ohno’s seven forms of waste are

1. Production of defective parts

2. Production of more than the number of items needed (overproduction)

3. Excessive inventories

4. Unnecessary processing steps

5. Unnecessary movement of people

6. Unnecessary transport and handling of materials

7. Workers waiting.

Eliminating production of defective parts (waste form 1) requires a quality control system that achieves perfect fi rst-time quality. In the area of quality control, the Toyota production system was in sharp contrast with the traditional QC systems used in mass production. In mass production, quality control is typically defi ned in terms of an acceptable quality level or AQL, which means that a certain minimum level of fraction defects is tolerated. In lean production, by contrast, perfect quality is required. There is little or no inventory in a lean system to act as a buffer. In mass production, inventory buffers are used just in case these quality problems occur. The defective work units are simply taken off the line and replaced with acceptable units. However, such a policy tends to perpetuate the cause of the poor quality. Therefore, defective parts continue to be produced. In lean production, a single defect draws attention to the quality problem, forcing the company to take corrective action and fi nd a permanent solution. Workers inspect their own production, minimizing the delivery of defects to the downstream production station. (Refer fi gure 5.20)


Figure 5.20: Three categories of activities in manufacturing.

Overproduction (waste form 2) and excessive inventories (waste form 3) are correlated. Producing more parts than necessary means that there are leftover parts that must be stored. Of all of the forms of muda, Ohno believed that the “greatest waste of all is excess inventory.” Overproduction and excess inventories cause increased costs in the following areas:

• Warehousing (building, lighting and heating, maintenance)

• Storage equipment (pallets, rack systems, forklifts)

• Additional workers to maintain and manage the extra inventory

• Additional workers to make the parts that were overproduced

• Other production costs (raw materials, machinery, power, maintenance) to make the parts that were overproduced

• Interest payments to fi nance all of the above.

5.6.2 Agile manufacturing

Agile manufacturing is an approach to manufacturing which is focused on meeting the needs of customers while maintaining high standards of quality and controlling the overall costs involved in the production of a particular product. This approach is geared towards companies working in a highly competitive environment, where small variations in performance and product delivery can make a huge difference in the long term to a company’s survival and reputation among consumers.


This concept is closely related to lean manufacturing, in which the goal is to reduce waste as much as possible. In lean manufacturing, the company aims to cut all costs which are not directly related to the production of a product for the consumer. Agile manufacturing can include this concept, but it also adds an additional dimension, the idea that customer demands need to be met rapidly and effectively. In situations where companies integrate both approaches, they are sometimes said to be using “lean and agile manufacturing.”

Companies which utilize an agile manufacturing approach tend to have very strong networks with suppliers and related companies, along with numerous cooperative teams which work within the company to deliver products effectively. They can retool facilities quickly, negotiate new agreements with suppliers and other partners in response to changing market forces, and take other steps to meet customer demands. This means that the company can increase production on products with a high consumer demand, as well as redesign products to respond to issues which have emerged on the open market.

Agile manufacturing is a term applied to an organization that has created the processes, tools, and training to enable it to respond quickly to customer needs and market changes while still controlling costs and quality.

An enabling factor in becoming an agile manufacturer has been the development of manufacturing support technology that allows the marketers, the designers and the production personnel to share a common database of parts and products, to share data on production capacities and problems — particularly where small initial problems may have larger downstream effects. It is a general proposition of manufacturing that the cost of correcting quality issues increases as the problem moves downstream, so that it is cheaper to correct quality problems at the earliest possible point in the process.

Agile manufacturing is seen as the next step after LEAN in the evolution of production methodology. The key difference between the two is like between a thin and an athletic person, agile being the latter. One can be neither, one or both. In manufacturing theory being both is often referred to as leagile. According to Martin


Christopher, when companies have to decide what to be, they have to look at the Customer Order Cycle (the time the customers are willing to wait) and the leadtime for getting supplies. If the supplier has a short lead time, lean production is possible. If the COC is short, agile production is benefi cial.

Agile Manufacturing should primarily be seen as a business concept. Its aim is quite simple - to put our enterprises way out in front of our primary competitors. In Agile Manufacturing our aim is to develop agile properties. We will then use this agility for competitive advantage, by being able to rapidly respond to changes occurring in the market environment and through our ability to use and exploit a fundamental resource -knowledge.

One fundamental idea in the exploitation of this resource is the idea of using technologies to lever the skills and knowledge of our people. We need to bring our people together, in dynamic teams formed around clearly identifi ed market opportunities, so that it becomes possible to lever one another’s knowledge. Through these processes we should seek to achieve the transformation of knowledge and ideas into new products and services, as well as improvements to our existing products and services.

The concept of Agile Manufacturing is also built around the synthesis of a number of enterprises that each have some core skills or competencies which they bring to a joint venturing operation, which is based on using each partners facilities and resources. For this reason, these joint venture enterprises are called virtual corporations, because they do not own signifi cant capital resources of their own. This, it is believed, will help them to be agile, as they can be formed and changed very rapidly.

Central to the ability to form these joint ventures is the deployment of advanced information technologies and the development of highly nimble organisational structures to support highly skilled, knowledgeable and empowered people.

Agile Manufacturing enterprises are expected to be capable of rapidly responding to changes in customer demand. They should be able to take advantage


of the windows of opportunities that, from time to time, appear in the market place. With Agile Manufacturing we should also develop new ways of interacting with our customers and suppliers. Our customers will not only be able to gain access to our products and services, but will also be able to easily assess and exploit our competencies, so enabling them to use these competencies to achieve the things that they are seeking.

Some Key Issues in Agile Manufacturing

The “I am a Horse” SyndromeThere is an old saying that hanging a sign on a cow that says “I am a horse”

does not make it a horse. There is a real danger that Agile Manufacturing will fall prey to the unfortunate tendency in manufacturing circles to follow fashion and to relabel everything with a new fashionable label. The dangers in this are two fold. First, it will give Agile Manufacturing a bad reputation. Second, instead of getting to grips with the profound implications and issues raised by Agile Manufacturing, management will only acquire a superfi cial understanding, which leaves them vulnerable to those competitors that take Agile Manufacturing seriously. Of course this is good news for the competitors!

The Existing Culture of ManufacturingOne of the important things that is likely to hold us back from making a

quantum leap forward and exploring this new frontier of Agile Manufacturing, is the baggage of our traditions, conventions and our accepted values and beliefs. A key success factor is, without any doubt, the ability to master both the soft and hard issues in change management. However, if we are to achieve agility in our manufacturing enterprises, we should fi rst try to fully understand the nature of our existing cultures, values, and traditions. We need to achieve this understanding, because we need to begin to recognise and come to terms with the fact that much of what we have taken for granted, probably no longer applies in the world of Agile Manufacturing. Achieving this understanding is the fi rst step in facing up to the pain of consigning our existing culture to the garbage can of historically redundant ideas.


Understanding AgilityAgility is defi ned in dictionaries as quick moving, nimble and active. This is

clearly not the same as fl exibility which implies adaptability and versatility. Agility and fl exibility are therefore different things.

Leanness is also a different concept to agility. Sometimes the terms lean and agile are used interchangeably, but this is not appropriate. The term lean is used because lean manufacturing is concerned with doing everything with less. In other words, the excess of wasteful activities, unnecessary inventory, long lead times, etc are cut away through the application of just-in-time manufacturing, concurrent engineering, overhead cost reduction, improved supplier and customer relationships, total quality management, etc.

5.6.3 Just-In-Time Approach

Just-In-Time (JIT) is a management philosophy that strives to eliminate sources of manufacturing waste by producing the right part in the right place at the right time. Waste results from any activity that adds cost without adding value, such as moving and storing. JIT (also known as lean production or stockless production) should improve profi ts and return on investment by reducing inventory levels (increasing the inventory turnover rate), reducing variability, improving product quality, reducing production and delivery lead times, and reducing other costs (such as those associated with machine setup and equipment breakdown). In a JIT system, underutilized (excess) capacity is used instead of buffer inventories to hedge against problems that may arise.

JIT applies primarily to repetitive manufacturing processes in which the same products and components are produced over and over again. The general idea is to establish fl ow processes (even when the facility uses a jobbing or batch process layout) by linking work centers so that there is an even, balanced fl ow of materials throughout the entire production process, similar to that found in an assembly line. To accomplish this, an attempt is made to reach the goals of driving all queues toward zero and achieving the ideal lot size of one unit.


The basic elements of JIT were developed by Toyota in the 1950’s and became known as the Toyota Production System (TPS). JIT was fi rmly in place in numerous Japanese plants by the early 1970’s. JIT began to be adopted in the U.S. in the 1980’s.

Toyota was able to meet the increasing challenges for survival through an approach that focused on people, plants and systems. Toyota realised that JIT would only be successful if every individual within the organisation was involved and committed to it, if the plant and processes were arranged for maximum output and effi ciency, and if quality and production programs were scheduled to meet demands exactly.

JIT manufacturing has the capacity, when properly adapted to the organisation, to strengthen the organisation’s competitiveness in the marketplace substantially by reducing wastes and improving product quality and effi ciency of production.

Just-In-Time Production SystemsJust-in-time (JIT) production systems were developed to minimize inventories,

especially work-in-process (WIP). Excessive WIP is seen in the Toyota production system as waste that should be minimized or eliminated. The ideal just-in-time production system produces and delivers exactly the required number of each component to the downstream operation in the manufacturing sequence just at the moment when that component is needed. This delivery discipline minimizes WIP and manufacturing lead time, as well as the space and money invested in WIP. At Toyota, the just-in-time discipline was applied not only to the company’s own production operations but to its supplier delivery operations as well.

While the development of JIT production systems is attributed to Toyota, many U.S. fi rms have also adopted the philosophy of just-in-time. Other terms are sometimes applied to the American practice of JIT to suggest differences with the Japanese practice. For example, continuous fl ow manufacturing is a widely used term in the United States that denotes a just-in-time style of production operations. Continuous fl ow suggests a method of production in which workparts are processed


and transported directly to the next workstation one unit at a time. Each process is completed just before the next process in the sequence begins. In effect, this is JIT with a batch size of one work unit. Prior to JIT, the traditional U.S. practice might be described as a “just-in-case” philosophy; that is, to hold large in-process inventories to cope with production problems such as late deliveries of components, machine breakdowns, defective components, and wildcat strikes.

The just-in-time production discipline has shown itself to be very effective in high-volume repetitive operations, such as those found in the automotive industry. The potential for WIP accumulation in this type of manufacturing is signifi cant, due to the large quantities of products made and the large numbers of components per product. The principal objective of JIT is to reduce inventories. However, inventory reduction cannot simply be mandated to happen. Certain requisites must be in place for a just-in-time production system to operate successfully. They are (1) a pull system of production control, (2) setup time reduction for smaller batch sizes, and (3) stable and reliable production operations.

5.7 PRODUCTION MONITORING SYSTEM

All the factor, data collection systems described in the preceding section required some form of human participation. Computer process monitoring (also sometimes called computer production monitoring) is a data collection system in which the computer is connected directly to the workstation and associated equipment for the purpose of observing the operation. The monitoring function has no direct effect on the mode of operation except that the data provided by monitoring may result in improved supervision of the process. The industrial process is not regulated by commands from the computer. Any use that is made of the computer to improve process performance is indirect, with human operators acting on information from the computer to make changes in the plant operations. The fl ow of data between the process and the computer is in one direction only—from process to computer.

The components used to build a computer process monitoring system include transducers and sensors, analog-to-digital converters (ADC), multiplexers, realtime


clocks, and other electronic devices. These components are assembled into various confi gurations for process monitoring. We discuss three such confi gurations:

1. Data logging systems

2. Data acquisition systems

3. Multilevel scanning

A particular computer process monitoring system is highly custom designed and may consist of a combination of these possible confi gurations. For example, a data acquisition system may include multilevel scanning.

5.7.1 Data logging systems

A data logger (DL) is a device that automatically collects and stores data for offl ine analysis. Strictly speaking, the data could be analyzed by a person without the aid of computer. Our interest here is in data logging systems that operate in conjunction with computers. Data loggers can be classifi ed into three types:

1. Analog input/analog output

2. Analog input/analog and digital output

3. Analog and digital input/analog and digital output

Type 1 can be a simple one-channel strip chart recording potentiometer for tracking temperature values using a thermocouple as the sensing device. Types 2 and 3 are more sophisticated instruments which have multiple input channels and make use of multiplexers and ADCs to collect process or experimental test data from several sources. The DL can be interfaced with tape punches, magnetic tape units, teletypes and printers, plotters, and so on. They can also be interfaced with the computer for periodic transfer of data.

A programmable data logger (PDL) is a device that incorporates a microprocessor as part of the system. The microprocessor serves as a controller to the data logger and can be programmed by means of a keyboard . The programmable


data logger can easily accommodate changes in rate or sequence of scanning the inputs. The PDL can also be programmed to perform such functions as data scaling, limit checking (making certain the input variables conform to prespecifl ed upper and lower bounds), sounding alarms, and formatting the data to be in a compatible and desirable format with the interface devices.

5.7.2 Data acquisition systems

The term data acquisition system (DAS) normally implies a system that collects data for direct communication to a central computer. It is therefore an on-line system, whereas the data logger is an off-line system. However, the distinction between the DL and the DAS has become somewhat blurred as data loggers have become directly connected to computers.

Figure 5.21: Multilevel scanning in computer process monitoring

Data acquisition systems gather data from the various production operations for processing by the central computer. The basic data can be analog or digital data which are collected automatically (transducers, ADC, multiplexers, etc.). It is a factory-wide system, as compared with data loggers, which are often used locally within the plant. The number of input channels in the DAS is therefore typically greater than in the DL system.

For the data logger the number of input channels might range between 1 and 100, while the data acquisition system might have as many as 1000 channels or more.


The rate of data entry into the DL system might be 10 readings per second for multiple-channel applications. By contrast, the DAS would have to be capable of a data sampling rate of up to 1000 per second. Because of these differences, the data acquisition system would be typically more expensive than the data logger.

5.7.3 Multilevel scanning

In the data acquisition system, it is possible for the total number of monitored variables to become quite large. Although it is technically feasible for all these variables to be monitored through multiplexing, some of the signals would not be needed under normal operating conditions. In such a situation it is convenient to utilize; a multilevel scan confi guration, as illustrated schematically in Figure 5.21.

With multilevel scanning, there would be two (or more) process scanning levels, a high-level scan and a low-level scan. When the process is running normally, only the key variables and status data would be monitored. This is the high-level scan.

When abnormal operation is indicated by the incoming data, the computer switches to the low-level scan, which involves a more complete data logging and analysis to ascertain the source of the malfunction. The low-level scan would sample all the process data or perform an intensive sampling for a certain portion of the process that might be operating out of tolerance.

5.8 STRUCTURAL MODEL OF A MANUFACTURING PROCESS

Most production operations are characterized by a multiplicity of dynamically interacting process variables. These variables can be cataloged into two basic types, input and output variables. However, there are different kinds of input variables and different kinds of output variables. Let us fi rst consider how the input variables might be classifi ed. There are three categories as follows:

1. Controllable input variables. These are sometimes called manipulative variables, because they can be changed or controlled during the


process. In a machining operation, it is technologically possible to make adjustments in speed and feed during the operation. In a chemical process, the controllable input variables may include fl ow rates, temperature set-tings, and other analog variables.

2. Uncontrollable input variables. Variables that change during the operation but which cannot be manipulated are defi ned as uncontrollable input variables. In chemical processing, variations in the starting raw chemicals may be an uncontrollable input variable for which compensation must be made during the process. In machining, examples would be tool sharpness, work-material hardness, and workpiece geometry.

3. Fixed variables. A third category of input to the process is the fi xed variable. These are conditions of the setup, such as tool geometry and workholding device, which can be changed between operations but not during the operation. Fixed inputs for a continuous chemical process would be tank size, number of trays in a distillation column, and other factors that are established by the equipment confi guration.

The other major type of variable in a manufacturing process is the output variable. It is convenient to divide output variables into two types:

1. Measurable output variables. The defi ning characteristic of this fi rst type is that it can be measured on-line during the process. Examples of variables that can be measured during process operation include fl ow rate, temperature, vibration, voltage, and power.

2. Performance evaluation variables. These are the measures of overall process performance and are usually linked to either the economics of the process or the quality of the product manufactured. Examples of performance evaluation variables in production include unit cost, production rate, yield of good product, and quality level.


The structural relationships between these different input and output variables are illustrated in Figure 5.22. The measurable output variables are determined by the input variables. The performance of the process, as indicated by the performance evaluation variable, is determined by the measurable output variables. To assess process performance, the performance evaluation variable must be calculated from measurements taken on the output variables.

The problem in process control is to control the measurable output variables so as to achieve some desired result in the performance evaluation variable. This is accomplished by manipulating the controllable inputs to the process.

There are various ways to implement computer process control of manufacturing operations, both in terms of hardware confi gurations and in terms of software programs. Consideration of hardware confi gurations includes the number and types of computers and how they are interconnected. Software programming is concerned with selecting among the available control strategies to regulate or optimize process performance. Let us fi rst discuss the various process control strategies.

Figure 5.22: Structure of a manufacturing process.

5.9 PROCESS CONTROL STRATEGIES

There are a variety of control strategies that can be employed in process control. The choice of strategy depends on the process and the performance objectives to be achieved. In this section we discuss the following control strategies:


1. Feedback control

2. Regulatory control

3. Feedforward control

4. Preplanned control

5. Steady-state optimal control

6. Adaptive control

5.9.1 Feedback control

In the manufacturing process model shown in Figure 5.23 it is often feasible from a controls viewpoint to relate the behavior of a particular output variable to one corresponding input variable. This matching of one input variable to one output variable forms a single open-loop system. By measuring the output variable and comparing it to the input variable, it is possible to close the control loop, thereby forming an automatic feedback control system. This arrangement is illustrated schematically in Figure 5.23. In the conventional concept of a feedback control system, the value of the controlled variable is subtracted from the value of the input variable and any difference between them is used to drive the controlled variable toward its desired value. In Figure 5.23, y is the controlled variable and x is the input variable. In process control applications, the input variable is often referred to as the set point. The difference between the set point and the measured y value is called the error and becomes the input to the process controller. The manner in which the process controller is designed to operate depends on the physical nature of the process and the feedback measurement device/Consideration of process control theory goes beyond the scope of the current chapter. The feedback control system is also called a closed-loop system because the block diagram takes on the general appearance of a closed loop. By contrast, an open-loop system is one without feedback.


Figure 5.23: Feedback control system.

5.9.2 Regulatory control

Regulatory control is analogous to feedback control except that the objective in regulatory control is to maintain .the overall performance evaluation variable at a certain set-point level. In feedback control, the objective is to control the individual output variables at their respective set-point values.

In many industrial processes it is suffi cient to maintain the performance evaluation variable at \ certain level or within a given tolerance band of that level. This would be appropriate in situations where performance was measured in terms of product quality and it was desired to maintain the product quality at a particular level. In a chemical process, this quality level might be the concentration of the fi nal chemical product. The purpose of process control is to maintain that quality at the desired constant value during the process. To accomplish this purpose, set points would be determined for individual feedback loops in the process and other control actions would be taken to compensate for disturbances to the process.

5.9.3 Feedforward control

The trouble with regulatory control (the same problem is present with feedback control) is that compensating action is taken only after a disturbance has affected the process output. An error must be present in order for any control action to be initiated, but this means that the output of the process is different from the desired value.

In feedforward control the disturbances are measured before they have upset the process, and anticipatory corrective action is taken. In the ideal case, the


corrective action compensates completely for the disturbance, thus preventing any deviation from the desired output value. If this ideal can be reached, feedforward control represents an improvement over feedback control.

Figure 5.24: Feedforward control system (combined with feedback control.)

The essential features of a feedforward control system are illustrated in Figure 5.24. The feedforward control concept can be applied to the individual measurable output variables in the process or to the performance evaluation variable for the entire process. The disturbance is measured and serves as the input to the feedforward control elements. These elements compute the necessary corrective action to anticipate the effect of the disturbance on the process. To make this computation, the feedforward controller contains a mathematical or logical model of the process which includes the effect of the disturbance. Feedforward control by itself does not include any mechanism for checking that the output is maintained at the desired level. For this reason, feedforward control is usually combined with feedback control, as illustrated in Figure 5.24. The feedforward loop is especially helpful when the process is characterized by long “response times” or ‘’dead times” between inputs and outputs. Feedback control alone would be unable to make timely corrections to the process.

5.9.4 Preplanned control

The term preplanned control refers to the use of the computer for directing the pro-’ cess or equipment to carry out a predetermined series of operation steps. The control sequence must be developed in advance to cover the variety of processing conditions that might be encountered. This control strategy usually requires the use of feedback control loops to make certain that each step in the operation sequence is


completed before proceeding to the next step. However, feedback information may not be necessary in every control command provided by the computer.

The name “preplanned control” is not universally applied throughout all areas of industry. Other terms are used to describe control strategies which are either identical or similar to preplanned control. What follows is a listing of some of the terms most frequently used.

Computer Numerical Control: Essentially, it involves the use of the computer to direct a machine tool through a program of processing steps. As such, it is a form of preplanned control. Direct numerical control (DNC), although not the same as CNC, involves a similar control sequence.

Program Control: This term is used in the process industries. It involves the application of the computer to start up or shut down a large complex process, or to guide the process through a changeover from one product grade to another. It also refers to the computer’s use in batch processing to direct the process through the cycle of processing steps. With program control the object is to direct the process from one operating condition to a new operating condition and to accomplish this in minimum time. There are often constraints on this minimum time objective, so the strategy of program control is to determine the best trajectory of set-point values that is compatible with the constraints. In batch processing, there may be a sequence of operating conditions or states through which the process must be commanded.

The paper industry provides an example of program control. In the manufacture of various grades of paper, a slightly different operating cycle is required for each grade. The process control computer is programmed to govern the process through each phase of the operating cycle for any grade of paper produced.

Sequencing Control: This class of preplanned control consists of guiding the process through a sequence of on/off-type steps. The variables under command of the computer can take on either of two states, typically “on” or “off.’’ In sequencing control, the process must be monitored to make sure that each step has been carried out before proceeding to the next step.


An example of the application of sequencing control is in automated production fl ow lines. The sequence of workstation power feed motions, parts transfer, quality inspections which may be incorporated in the line, and so on, are all included under computer control. In addition, the computer may be programmed to perform diagnostic subroutines in the event of a line failure, to help identify the cause of the downtime occurrence. Tool change schedules may also be included as one of the computer functions. The operators are directed by the computer when to change cutters.

5.9.5 Steady-state optimal controlThe term “optimal control” refers to a large class of control problems. We

shall limit its meaning in this discussion to open-loop systems. That is, there is no feedback of information concerning the output. Instead, two features of the system must be known in advance:

1. Performance evaluation variable. This measure of system performance is also called the objective function, index of performance, or fi gure of merit. Basically, it represents the overall indicator of process performance that we desire to optimize by solving the optimal control problem. Among the performance objectives typically used in optimal control are cost minimization, profi t maximization, production-rate maximization, quality optimization, least-squares-error minimization, and process-yield maximization. These objectives are general and must be specifi ed to suit the particular application.

2. Mathematical model of the process. The relationships between the input variables and the measure of process performance must be mathematically defi ned. The model is assumed to be valid throughout the operation of the process. That is, there are no disturbances that might affect the fi nal result of the optimization procedure. This is why we refer to the problem as steady-state optimal control. The mathematical model of the process may include constraints on some or all of the variables. These constraints limit the allowable region within which the objective function can be optimized.


With these two attributes of the process defi ned, the solution of the optimal control problem consists of determining the values of the input variables that optimize the objective function. To accomplish this task, a great variety of optimization techniques are available to solve the steady-state optimal control problem. These techniques include differential calculus, linear programming, dynamic programming, and the calculus of variations. All of these mathematical approaches have been applied to the class of problems in this category of steady-state optimal control.

5.9.6 Adaptive control

Adaptive control possesses attributes of both feedback control and optimal control. Like a feedback system, measurements are taken on certain process variables. Like an optimal system, an overall measure of performance is used. In adaptive control, this measure is called the index of performance (IP). The feature that distinguishes adaptive control from the other two types is that an adaptive system is designed to operate in a time-varying environment. It is not unusual for a system to exist in an environment that changes over the course of time. If the internal parameters or mechanisms of the system are fi xed, as is the case in a feedback control system, the system might operate quite differently in one environment than it would in another. An adaptive control system is designed to compensate for the changing environment by monitoring its performance and altering, accordingly, some aspect of its control mechanism to achieve optimal or near-optimal performance. The term “environment” is used in a most general way and may refer to the normal operation of the process. For example, in a manufacturing process, the changing environment may simply mean the day-to-day variations that occur in tooling, raw materials, air temperature, and humidity (if these have any infl uence on the process operation). An adaptive system differs from a feedback system or an optimal system in that it is provided with the capability to cope with this time-varying environment. The feedback and optimal systems operate in a known or deterministic environment. If the environment changes signifi cantly, these systems might not respond in the manner intended by the designer.


On the other hand, the adaptive system evaluates the environment. More accurately, it evaluates its performance within the environment and makes the necessary changes in its control characteristics to improve or, if possible, to optimize its performance. The manner of doing this involves three functions which characterize adaptive control and distinguish it from other modes of control. It may be diffi cult, in any given adaptive control system, to separate out the components of the system that perform these three functions; nevertheless, all three must be present for adaptation to occur. The three functions of adaptive control are:

1. Identifi cation function. This involves determining the current performance of the process or system. Normally , the performance quality of the system is defi ned by some relevant index of performance. The identifi cation function is concerned with determining the current value of this performance measure by making use of the feedback data from the process. Since the environment will change over time, the performance of the system will also change. Accordingly, the identifi cation function is one that must proceed over time more or less continuously. Identifi cation of the system may involve a number of possible measurement activities. It may involve estimation of a suitable mathematical model of the process or computation of the performance index from measurements of process variables. It could include a comparison of the current quality with some desired optimal performance.

2. Decision function. Once the system performance is determined, the next function is to decide how the control mechanism should be adjusted to improve process performance. This decision procedure is carried out by means of a preprogrammed logic provided by the system designer. Depending on the logic, the decision may be to change one or more of the controllable inputs to the process; it may be to alter some of the internal parameters of the controller, or some other decision.

3. Modifi cation function. The third adaptive control function is to implement the decision. While the decision function is a logic function,


modifi cation is concerned with a physical or mechanical change in the system. It is a hardware function rather than a software function. The modifi cation involves changing the system parameters or variables so as to drive the process toward a more optimal state.

Figure 5.25 illustrates the sequence of the three functions in an adaptive controller applied to a hypothetical process. The process is assumed to be infl uenced by some time-varying environment. The adaptive system fi rst identifi es the current process performance by taking measurements of inputs and outputs. Depending on current performance, a decision procedure is carried out to determine what changes are needed to improve system performance. Actual changes to the system are made in the modifi cation function.

Figure 5.25: General confi guration of an adaptive control system

Collecting data from factory operations can be accomplish by any of several means. Shop data can be entered by workers through manual terofi nals located throughout the plant or can be collected automatically by means of limi/switches, sensor systems, bar code readers, or other/services. The collection and use of production (Ma in factory operations for scheduling and tracking purposes is called shop fl oor control.


5.10 DIRECT DIGITAL CONTROL (DDC)

DDC was certainly one of the important steps in the development of computer process control. DDC is a computer process control system in which certain components in a conventional analog control system are replaced by the digital computer. The regulation of the process is accomplished by the digital computer on a time-shared, sampled-data basis rather than by the many individual analog components working in a dedicated continuous manner. With DDC, the computer calculates the desired values of the input parameters and set points, and these values are applied through a direct link to the process, hence the name “direct digital” control.

The difference between direct digital control and analog control can be seen by comparing Figure 5.26 and 5.27. The fi rst fi gure shows the instrumentation for a typical analog control loop. The entire process would have many individual control loops, but only one is shown here. Typical hardware components of the analog control loop include the sensor and transducer, an instrument for displaying the output variable (such an instrument is not always included in the loop), some means for establishing the set point of the loop (shown as a dial in the fi gure, suggesting that the setting is determined by a human operator), a comparator (to compare set point with measured output variable), the analog controller, an amplifi er, and the actuator that determines the input parameter to the process.

Figure 5.26: A typical analog control loop.


Figure 5.27: Components of a DDC System

In the DDC system (Figure 5.27), some of the control loop components remain unchanged, including (probably) the sensor and transducer as well as the amplifi er and actuator. Components likely to be replaced in DDC include the analog controller, recording and display instruments, set point dials, and comparator. New components in the loop include the digital computer, analog-to-digital and digital-to-analog converters (ADCs and DACs), and multiplexers to share data from different control loops with the same computer.

DDC was originally conceived as a more effi cient means of performing the same kinds of control actions as the analog components it replaced. However, the practice of simply using the digital computer to imitate the operation of analog controllers seems to have been a transitional phase in computer process control. Additional opportunities for the control computer were soon recognized, including:

• More control options than traditional analog. With digital computer control, it is possible to perform more complex control algorithms than with the conventional proportional-integral-derivative control modes used by analog controllers; for example, on/off control or nonlinearities in the control functions can be implemented.

• Integration and optimization of multiple loops. This is the ability to integrate feedback measurements from multiple loops and to implement optimizing strategies to improve overall process performance.


• Ability to edit the control programs. Using a digital computer makes it relatively easy to change the control algorithm when necessary by simply reprogramming the computer. Reprogramming the analog control loop is likely to require hardware changes that are more costly and less convenient.

These enhancements have rendered the original concept of direct digital control more or less obsolete. In addition, computer technology itself has progressed dramatically so that much smaller and less expensive yet more powerful computers are available for process control than the large mainframes available in the early 1960s. This has allowed computer process control to be economically justifi ed for much smaller scale processes and equipment. It has also motivated the use of distributed control systems, in which a network of microcomputers is utilized to control a complex process consisting of multiple unit operations and/or machines.

REVIEW QUESTIONS:

1. Explain MRP with fl ow charts.

2. Explain shop fl oor control system.

3. Distinguish lean and agile manufacturing.

4. Explain JIT Approach in detail.

5. Explain LOB methods with diagram.

6. Explain various benefi ts of MRP.

7. Explain terms

a. Cost planning

b. Cost control

8. Explain the benefi ts of LOB technique.

9. Explain Lean Production & Waste in manufacturing.


10. Explain Agile manufacturing.

11. Describe Computer - integrated Production management system.

12. Give the effect of production planning & control in detail.

13. Explain inventory management in detail.

14. Describe various process control strategies.

15. Give detail description of feedback control.

16. Explain Adaptive control in detail.

17. Explain direct digital control in detail.

18. Explain

i) Regulatory control.

ii) Feed forward control.

19. Give detail description of various activities of SFC.

20. Describe scheduling techniques of SFC.

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