introduction to programmable logic controller(plc)

Introduction to Programmable Logic

Controller(PLC)

Akram Hossain, Professor, Purdue University Calumet

Hammond, IN 46323

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Definition of PLC

Programmable Logic Controllers are solid state devices that can be programmed to performed sequential and discrete state operation on external equipment

They are designed to perform the logic functions previously accomplished by electromechanical relays, drum switches, mechanical and electronic timers and counters, standalone digital PID controllers etc.

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Major Manufacturer of PLC

Allen-Bradley (A-B)

General Electric Gould-Madicon Texas

Instruments Square-D

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Reliance Electric

Siemens

Areas of PLC Applications

Annunciators Injection MoldingAuto Insertion AssemblyBagging Motor

WindingBaking Oil FieldsBlending PaintingBoring PalletizersBrewing PipelinesCalendaring PolishingCasting ReactorsChemical Drilling RobotsColor Mixing RollingCompressors Security SystemsConveyors Stretch Wrap

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Cranes SlittingCrushing SortingCutting StackersDigesters StitchingDrilling Stack

PrecipitatorsElectronic Testing ThreadingElevators Tire BuildingEngine Test Stands Traffic

ControlExtrusion Textile

MachineForging TurbinesGenerators TurningGluing WeavingGrinding Web HandlingHeat Treating Welding

History of PLC

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The Hydramatic Division of the General Motors Corporation specified the design criteria for the first programmable controller in 1968. Their primary goal was to eliminate the high costs associated with inflexible, relay-controlled systems. The specifications required a solid-state system with computer flexibility able to (1) survive in an industrial environment, (2) Be easily programmed and maintained by plant engineers and technicians, and(3) Be reusable. Such a control system would reduce machine downtime and

provide expandability for the future. Some of the initial specifications included the following:

• The new control system had to be price competitive with the use of relay systems.• The system had to be capable of sustaining an industrial environment.• The input and output interfaces had to be easily replaceable.• The controller had to be designed in modular form, so that subassemblies could be removed

easily for replacement or repair.• The control system needed the capability to pass data collection to a central system.• The system had to be reusable.• The method used to program the controller had to be simple, so that it could be easily understood

by plant personnel.

PLC and Computer

A PLC and a computer both are electronic processor unit. The architecture of a PLC’s CPU is basically same as that of a general purpose computer; however, some important characteristics set them apart.

Unlike computer, PLCs are specifically designed to survive the harsh conditions of the industrial environment. A well-designed PLC can be placed in an area with substantial amounts of electrical noise, electromagnetic interference, mechanical vibration, and noncondensing humidity.

Distinction of PLCs is that their hardware and software are designed for easy use by plant electricians and technicians. The hardware interfaces for connecting field devices are actually part of the PLC itself and are easily connected.

The modular and self-diagnosing interface circuits are able to pin point malfunctions and moreover, are easily removed and replaced.

Software programming uses conventional relay ladder symbols, or other easily learned languages, which are familiar to plant personnel.

A computer can execute a complex programming task and also multitasking. An standard PLC is designed to executes a single program in an orderly fashion. As PLCs are rapidly changing, modern PLCs have multitasking capabilities.

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Why PLCs ?

Soft Manufacturing Process Flexible Manufacturing Process Retrofit Existing Process Less Maintenance Easy to Debug

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Why PLCs are so Popular?

Programmable logic controller have made it possible to precisely control large process machines and driven equipment with less physical wiring and wiring time than it requires with standard electro-mechanical relays, pneumatic system, timers, drum switches, and so on.

The programmability allows for fast and easy changes in the relay ladder logic to meet the changing needs of the process or driven equipment without the need for expensive and time consuming rewiring process.

Modem PLCs are "electrician friendly", PLC can be programmed and used by plant engineers and maintenance electricians without much electronic and computer programming background. They can programmed by using the existing ladder diagrams.

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Advantages of PLC

A.     Flexibility     Universal Controller - can replace various independent/ standalone

controller. B.     Implementing Changes and Correcting Errors

     Do not have to rewiring relay panel.     Change program using keyboard.

 C.     Large Quantity of Contact

     Large number of' Soft Contact' available. D.    Lower Cost

     Advancement in technology and open architecture of PLC will reduce the market price.

 E.     Pilot Running (Simulation Capability)

     A program can be simulated or run without actual input connection.

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Advantages of PLC

F. Visual Observation. Can observe the opening and closing of contact switch on CRT . Operator message can be programmed for each possible malfunction.

G. Speed of Operation Depends on scan time -millisecond. Asynchronous operation.

 H. Ladder or Boolean Programming Method.

F. Easy for 'Electrician ,  Reliability

F. In general -very reliable  Simplicity of Ordering Control Sys. Components

One package with Relay, Timers, Control Block, etc. 

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Advantages of PLC

K. Documentation Printout of ladder logic can be printed easily

L. Security Software lock on a program (Password)

M. Ease of Changes by Programming Ability to program and reprogram, loading and down loading

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Disadvantages of PLC Disadvantages of PLC

A. New Technology Change from ladder and relay to PLC concept

B. Fixed program Application Not cost effective for single- function application

C. Environment Consideration Not adapted for very high temperature, high humidity level, high vibration, etc.

A. Fail-safe operation Does not start automatically when power failure ( can be programmed into ) Not "Fail-safe" -Fail-shorted rather than OPEN

Fixed-circuit operation Fixed control system -less costly

What is a Ladder Diagram?

A complete control scheme normally drawn as a series of contacts and coils arranged between two vertical control supply lines so that the horizontal lines of contacts appear similar to rungs of a ladder. The control contacts (input devices) are to left and coils (output devices) on the right. Ladder diagrams are an industrial standard for representing relay-logic control system

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PLC Series 5 Input/Output Chassis

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Allen-Bradley PLC Series 5 Processor Module

Allen-Bradley PLC Series 5 Processor Module

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PLC 5/25 Processor Mode of Operation

PLC 5/25 Processor Mode of Operation

A-B Series 5 Power Supply

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120 VAC Input Module Connection

120 VAC Input Module Connection

120 VAC Output Module Connection

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Two Slot Addressing

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PLC Scan and Update Sequence

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How PLC Works?

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01

02

03

04

00

04

03

02

01

000

1

0

1

0

0

1

0

0

0

0001

I:001O:001

O:001/01

01

I:001

0301

O:001O:001

03

01

00

01

03

04

Processor MemoryInputModule

Output Module

120 VAC120 VAC

InputMap

OutputMap

Ladder Logic in

PLC Input & Output Mapping

00

O:001

I:001

O:001/01

04

I:001

01

O:001

00

03

01

01

03

O:001

01

02

00

01

02

03

04

START

STOP

MOTORRELAYCOIL

ON LIGHTMOTOR

START SW

STOP SW

M

M

L1

L1 N

N

SELF HOLDING FOR "M"

RELAY COIL

CONTROLPOWER

ON LIGHT

INPUT RACK SLOT ADDRESSI: 0 01 / 03

OUTPUT RACK SLOT ADDRESSO: 0 01 / 03

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PLC Program and Data Files

PLC Program and Data Files

Program Files

-Reserved Files (File # 0)

-SFC file (File # 1 )

-Ladder file (File #2- 999) -File #2: Main Ladder File

-Subroutine file (File #3 -999)

-Selectable Interrupt file (File #3 -999)

-Fault Routine File (File #3 -999)

Data Files

-Files which store data of the I/0 module.

-Can be integer data, floating point (real) data, timer

data, counter data, discrete input data, discrete output data, etc.

Input/Output Addressing

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Memory Organization for Data Files

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Memory Map of File Types

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A-B PLC Series-5 Data Organization

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Floating Point Files

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Timer and Counter Data Files

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Control Data File

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PLC Logical Addressing

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General Instruction Format

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General Instruction Format

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Status File

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TIME ON DELAY (TON)

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