Seminar onProgrammable Logic Controller (PLC)

Presented By :

Mahesh J. Vadhavaniya (122511)M. E. – Regular, 2012 – 14

NITTTR, Chandigarh

Subject Faculty : Prof. Ram Murat Singh

Process Control & Automation

Process control

Rules & guidelines

Recognizing Process the Actuate the

the status Information control elements

Why Automation ?

History of Process Control & Automation

PLC Control

Electronics

Control Hard-Wire

Control

Manual Control

1. PLC - Introduction

What does PLC stand for?

Programmable Logic Controllers( Definition according to NEMA standard ICS3-1978)

1. PLC - Introduction

How does a PLC differ from a computer?

PLC Origin

Historical Background

Historical Background

Programmable Controller Development

Programmable Controller Development

INTRODUCTION TO PLCS

INTRODUCTION TO PLCS

1. PLC - Introduction

Leading Brands Of PLC

Leading Brands Of PLC

Areas of Application :

PLC Size :

Major Components of a Common PLC

POWER SUPPLY

I MN OP D

O MU OT D

PROCESSORU UT

L E

P UU LT E

PROGRAMMING DEVICE

From SENSORS

Pushbuttons, contacts,

limit switches, etc.

To OUTPUTSolenoids, contactors,

alarms.Motors

etc.

Major Components of a Common PLC

Major Components of a Common PLC

I/O Module

IS NEEDED TO:•Prevent voltage transients from damaging the processor.•Helps reduce the effects of electrical

USE TO DROP THE VOLTAGE TO LOGIC LEVEL

I/O Module

OPTO- ISOLAT OR

noise

Current Limiting Resisto

r

FROM INPUT

DEVICE

Buffer, Filter,

hysteresi s

Circuits

TO PROCESSOR

IS NEEDED TO:•Prevent voltage transients from damaging the processor.•Helps reduce the

CONVERTS THE AC INPUT TO DC AND DROPS THE VOLTAGE TO LOGIC LEVEL

I/O Module

OPTO- ISOLAT OR

effects of electrical noise

Rectifier, Resistor Network

FROM INPUT

DEVICE

Buffer, Filter,

Hysteresis Circuits

TO PROCESSOR

Light emittingdiode

Ph010-trartsist

orPLC

module

InrxJt ..•

to PLC

1 8Signalto CPU

81

.2

7

a3

4---;---

Figure 1.6aDual-optoooupler IC

in 8-pin DIP

Figure 1.6bBasic DC input circuit

29

Input Connections

DC power sl.pplv

[a)

AC

.--------1 +

[b)

DC

DC PC7N9f supply.--------1

+

l k n

-V

(C)

TTL

IS NEEDED TO:•Prevent voltage transients from damaging the processor.•Helps reduce the effects of electrical noise

I/O Module

OPTO-

ISOLA TOR

FROM PROCESSOR

TTLCircuits

Amplifier RELAY TRIAC

X’SISTO R

TO OUTPU

T DEVICE

L1DC Output

Module120V ac

·, . . . - - - - - -

AC Output Module

OUT

O

OUT

1

OUT2

OUT3

OUT4

I 0 OUTO0 OUT 10 OUT20 OUT 30 OUT40

0

0

ACCQm

Figure 3.9 .AC OUTPUT WIRING CONNECTIONS

m

Figure 3.8, DC OUTPUT WIRING CONNECTIONS

l + DC Power

Supply

-

0

000

0

0

0

0

DC Co

I/O Circuits

I/O Circuits

I/O Circuits

I/O Circuits

OUTPUTS

INPUTSMOTOR

CONTACTORLAMP

PUSH BUTTONS

PLC

INPUT DEVICES:

Push Button

Limit Switch

Thumbwheel SW

Level SW

Flow SW

OUTPUT DEVICES:

Motor

Solenoid

LED Display

Heater Coil

Lamp

L1

L2

P. B SWITCH

I:2

0

I= Input

Moduleslot # in rack

Module Terminal #

Allen-Bradley 1746-1A16

INPUT

MODULE WIRING

DIAGRAM

Address I:2.0/0

LADDER PROGRAM

N. O

C

L2

L1

L2 L1

MOTOR

•SOLENOI D•VALVES•LAMP•BUZZER

CONTACTOR

FIELD WIRING

OUTPUT MODULE WIRING

O:4

0CONTACTOR

LADDER PROGRAM

L1 L

2

Discrete Input

A discrete input also referred as digital input is an input that is either ON or OFF are connected to the PLC digital input. In the ON condition it is referred to as logic 1 or a logic high and in the OFF condition maybe referred to as logic o or logic low.

Normally Open

Pushbutton Normally

Closed Pushbutton

Normally Open

switch Normally

Closed switch

Normally Open

contact Normally

closed contact

OFFLogic 0

IN

PLC

Input Module

24 V dc

OFFLogic 1

IN

PLC

Input Module 24 V dc

Analog Input

An analog input is an input signal that has a continuous signal. Typical inputs may vary from 0 to 20mA, 4 to 20mA or 0 to10V. Below, a level transmitter monitors the level ofliquid in the tank. Depending on the level Tx, the signal to the PLC can either increase or decrease as the level increases or decreases.

IN

PLCAnalog Input Module

Tank

Level Transmitter

OUT

PLCLamp

Digital Output Module

Digital Output

A discrete output is either in an ON or OFF condition. Solenoids, contactors coils, lamps are example of devices connected to the Discrete or digital outputs. Below, the lamp can be turned ON or OFF by the PLC output it is connected to.

OUT

Analog Output

An analog output is an output signal that has a continuous signal. Typical outputs may vary from 0 to 20mA, 4 to 20mA or 0 to10V.

Electric to pneumatic transducer

PLC

Analog Output Module

EP

Pneumatic control valve

Supply air0 to 10V

ProcessorThe processor module contains the PLC’s microprocessor, its supporting circuitry, and its memory system.

The main function of the microprocessor is to analyze data coming from field sensors through input modules, make decisions based on the user’s defined control program and return signal back through output modules to the field devices. Field sensors: switches, flow, level, pressure, temp. transmitters, etc. Field output devices: motors, valves, solenoids, lamps, or audible devices.

The memory system in the processor module has two parts: a system memory and an application memory.

Memory Map Organization

SYSTEM

•System memory includes an area called the EXECUTIVE, composed of permanently-stored programs that direct all system activities, such as execution of the users control program, communication with peripheral devices, and other system activities.•The system memory also contains the routines that implement the PLC’s instruction set, which is composed of specific control functions such as logic, sequencing, timing, counting, and arithmetic.•System memory is generally built from read-only memory devices.

APPLICATION •The application memory is divided into the data table

area and user program area.•The data table stores any data associated with the user’s control program, such as system input and output status data, and any stored constants, variables, or preset values. The data table is where data is monitored, manipulated, and changed for control purposes.•The user program area is where the programmed instructions entered by the user are stored as an application control program.

•Data Table•User Program

Memory DesignsVOLATILE.A volatile memory is one that loses its stored information when power is removed.

Even momentary losses of power will erase any information stored or programmed on a volatile memory chip.

Common Type of Volatile Memory

RAM. Random Access Memory(Read/Write)Read/write indicates that the information stored in the memory can be retrieved or read, while write indicates that the user can program or write information into the memory.

Memory Designs

The words random access refer to the ability of any location (address) in the memory to be accessed or used. Ram memory is used for both the user memory (ladder diagrams) and storage memory in many PLC’s.

RAM memory must have battery backup to retain or protect the stored program.

Memory DesignsSeveral Types of RAM Memory:

1.MOS2.HMOS3.CMOS

The CMOS-RAM (Complimentary Metal Oxide Semiconductor) is probably one of the most popular. CMOS-RAM is popular because it has a very low current drain when not being accessed (15microamps.), and the information stored in memory can be retained by as little as 2Vdc.

Memory DesignsNON-VOLATILEHas the ability to retain stored information when power is removed, accidentally or intentionally. These memories do not require battery back-up.

Common Type of Non-Volatile Memory

ROM, Read Only MemoryRead only indicates that the information stored in memory can be read only and cannot be changed. Information in ROM is placed there by the manufacturer for the internal use and operation of the PLC.

Memory DesignsOther Types of Non-Volatile Memory

PROM, Programmable Read Only MemoryAllows initial and/or additional information to be written into the chip.

PROM may be written into only once after being received from the PLC manufacturer; programming is accomplish by pulses of current.

The current melts the fusible links in the device, preventing it from being reprogrammed. This type of memory is used to prevent unauthorized program changes.

Memory DesignsEPROM, Erasable Programmable Read Only Memory

Ideally suited when program storage is to be semi-permanent or additional security is needed to prevent unauthorized program changes.

The EPROM chip has a quartz window over a silicon material that contains the electronic integrated circuits. This window normally is covered by an opaque material, but when the opaque material is removed and the circuitry exposed to ultra violet light, the memory content can be erased.

The EPROM chip is also referred to as UVPROM.

Memory Designs

EEPROM, Electrically Erasable Programmable Read Memory

Only

Also referred to as E2PROM, is a chip that can be programmed using a standard programming device and can be erased by the proper signal being applied to the erase pin.

EEPROM is used primarily as a non-volatile backup for the normal RAM memory. If the program in RAM is lost or erased, a copy of the program stored on an EEPROM chip can be down loaded into the RAM.

Programmable Logic Controller

Power supply

Inputmodule

CPUProgram memory

Outputmodule

Field Control Input

Elements Process / Machine

PLC Operating Principle

Self testStart

Communicati

on an

In-put scan

PLC OperationBasic Function of a Typical PLC

Read all field input devices via the input interfaces, execute the user program stored in application memory, then, based on whatever control scheme has been programmed by the user, turn the field output devices on or off, or perform whatever control is necessary for the process application.

This process of sequentially reading the inputs, executing the program in memory, and updating the outputs is known as scanning.

While the PLC is running, the scanning process includes the following four phases, which are repeated continuously as individual cycles of operation:

PHASE 1Read

Inputs Scan

PHASE 2Program

Execution

PHASE 3Diagnostics/

Comm

PHASE 4Output Scan

PHASE 1 – Input Status scan

• A PLC scan cycle begins with the CPU reading the status of its inputs.

PHASE 2– Logic Solve/Program Execution

• The application program is executed using the status of the inputs

PHASE 3– Logic Solve/Program Execution

•Once the program is executed, the CPU performs diagnostics and communication tasks

PHASE 4 - Output Status Scan•An output status scan is then performed, whereby the stored output values are sent to actuators and other field output devices. The cycle ends by updating the outputs.

As soon as Phase 4 are completed, the entire cycle begins again with Phase 1 input scan.

The time it takes to implement a scan cycle is called SCAN TIME. The scan time composed of the program scan time, which is the time required for solving the control program, and the I/O update time, or time required to read inputs and update outputs. The program scan time generally depends on the amount of memory taken by the control program and type of instructions used in the program. The time to make a single scan can vary from 1 ms to 100 ms.

PLC CommunicationsCommon Uses of PLC Communications Ports

Changing resident PLC programs - uploading/downloading from a supervisory controller (Laptop or desktop computer).

Forcing I/O points and memory elements from a remote terminal.

Linking a PLC into a control hierarchy containing several sizes of PLC and computer.

Monitoring data and alarms, etc. via printers (OIUs).

or Operator Interface Units

PLC CommunicationsSerial Communications

PLC communications facilities normally provides serial transmission of information.

Common Standards

RS 232

Used in short-distance computer communications, with the majority of computer hardware and peripherals. Has a maximum effective distance of approx. 30 m at 9600 baud.

PLC CommunicationsLocal Area Network (LAN)

Local Area Network provides a physical link between all devices plus providing overall data exchange management or protocol, ensuring that each device can “talk” to other machines and understand data received from them.

LANs provide the common, high-speed data communications bus which interconnects any or all devices within the local area.

LANs are commonly used in business applications to allow several users to share costly software packages and peripheral equipment such as printers and hard disk storage.

PLC CommunicationsRS 422 / RS 485

Used for longer-distance links, often between several PCs in a distributed system. RS 485 can have a maximum distance of about 1000 meters.

PLC CommunicationsProgrammable Controllers and Networks

Dedicated Network System of Different Manufacturers

Manufacturer Network

Allen-Bradley Data Highway

Gould Modicon Modbus

General Electric GE Net Factory LAN

Mitsubishi Melsec-NET

Square D SY/NET

Texas Instruments TIWAY

SpecificationsSeveral factors are used for evaluating the quality and performance of programmable controllers when selecting a unit for a particular application. These are listed below.

NUMBER OF I /O PORTS

This specifies the number of I/O devices that can be connected to the controller. There should be sufficient I/O ports to meet present requirements with enough spares to provide for moderate future expansion.

Working of PLC

II

CPU•User Program memory•Internal timers

•Internal countersInput

ModuleOutput Module

I/O Bus I/O Bus

IV

Field signals

Field Controls

I III

PLC Programming

Addressing Inputs & Outputs

Slot numbers0 1

23 4----------------

30 31

CPU 3

Channel Nos.

01

2

CPU

567

PROGRAMMING

Normally Open (NO)

Normally Closed (NC)

Power flows through these contacts when they are closed. The normally open (NO) is true when the input or output status bit controlling the contact is 1. The normally closed (NC) is true when the input or output status bit controlling the contact is 0.

Coils

Coils represent relays that are energized when power flows to them. When a coil is energized it causes a correspondingoutput to turn on by changing the state of the status bit controlling the output to 1. That same output status bit maybe used to control normally open or normally closed contact anywhere in the program.

Boxes

Boxes represent various instructions or functions that are Executed when power flows to the box. Some of these Functions are timers, counters and math operations.

AND OPERATION

Each rung or network on a ladder program representsa logic operation. In the rung above, both inputs A and B must be true (1) in order for the output C to be true (1).

Rung

A B C

OR OPERATION

Rung

B

A C

In the rung above, it can be seen that either input A or Bis be true (1), or both are true, then the output C is true (1).

NOT OPERATION

Rung

A C

In the rung above, it can be seen that if input A is be true (1), then the output C is true (0) or when A is (0), output C is 1.

Writing Program• Ladder Diagram (LAD)

– Use relay logic symbols to formulate the control task

• Control System Flowchart (CSF)– Use digital graphical symbols to

formulate the control task

&

• Statement List (STL)– Use mnemonic

abbreviation in programming.

1. PLC - Introduction

Data Flow in the PLC

1. PLC - Introduction

n One of the advantages of PLC is that it can be

programmed by non-specialists

n Program can be written either in the form of a

statement list: a set of mnemonic instructions

representing a function of the CPU

or a

ladder diagram: a graphical language resembling the

electrical relay diagrams

1. PLC - Introductionstatement list

1. PLC - IntroductionLadder diagram

Writing Program

• Circuit Diagram

• Ladder Diagram (LAD)

I 1.0

I 1.1

I 1.2

I 1.3

I 1.1

I 1.0

Q 4.1

Q 4.1

I 1.3

I 1.2

Writing Program

• Circuit Diagram

• Control System Flowchart (CSF)

I 1.0

I 1.1

I 1.2

I 1.3

&

I 1.0

I 1.1

Q 4.1

>=1

Q 4.1

=&

I 1.2I 1.3

Writing Program

• Circuit Diagram

• Statement List (STL)

A I 1.0 A I 1.1

OA I 1.2 A I 1.3= Q 4.1

I 1.0

I 1.1

I 1.2

I 1.3

Q 4.1

Ladder logic (1)The ladder logic is the oldest programming language for PLC it bases directly on the relay intuition of the

electricians.it is widely in use outside Europe.

It is described here but not recommended for new projects.

(Kontaktplansprache, langage à contacts)

Ladder Logic (2)

01

(contact

travail)02

03

50

relay coil (bobine)

break contact (contact repos)

make contactorigin:

electrical

circuit

01

02 5

003

corresponding ladder diagram

50

05 4

4

rung

"coil" 50 is used to move other contact(s)

Ladder logic (3)The contact plan or "ladder logic" language allows an easy transition from the traditional relay logic diagrams to the programming of binary functions.

It is well suited to express combinational logic

It is not suited for process control programming (there are no analog elements).

The main ladder logic symbols represent the elements:

make contact

break contact

relay coil

contact travail Arbeitskontakt

contact repos Ruhekontakt

bobine

Spule

Ladder logic (4)Binary combinations are expressed by series and parallel relay contact:

+ 01 02

50

Coil 50 is active (current flows) when 01 is active and 02 is not.

01

02 50

Series

ladder logic representation “logic" equivalent

+ 01

40

02

Coil 40 is active (current flows) when 01 is active or 02 is not.

Parallel

01

02 40

Ladder logic (5)The ladder logic is more intuitive for complex binary expressions than literal languages

textual expression

501 2 3 4

5 6

!N 1 & 2 STR 3 & N 4 STR N 5& 6 / STR & STR = 50

500 1 4 5

6 72 3

10 11

12

!0 & 1 STR 2 & 3 / STR STR 4& 5 STR N 6 & 7

& 11 / STR & 12 = 50

Ladder logic (6)Ladder logic stems from the time of the relay technology.As PLCs replaced relays, their new possibilities could not be expressed any more in relay terms.

The contact plan language was extended to express functions:

literal expression:!00 & 01 FUN 02 = 200

200FUN 02

0100

The intuition of contacts and coil gets lost.

The introduction of «functions» that influence the control flow itself, is problematic.

The contact plan is - mathematically - a functional representation.

The introduction of a more or less hidden control of the flow destroys the freedom of side effects and makes programs difficult to read.

Ladder logic (7)Ladder logic provides neither:• sub-programs (blocks), nor• data encapsulation nor• structured data types.

It is not suited to make reusable modules.

IEC 61131 does not prescribe the minimum requirements for a compiler / interpreter such as number of rungs per page nor does it specifies the minimum subset to be implemented.

Therefore, it should not be used for large programs made by different persons

It is very limited when considering analog values (it has only counters)

→ used in manufacturing, not process control

Operation of Mixer

IF sensor 2 is not trippedTHEN energize solenoid 1 ( ingr A)

IF sensor 2 is tripped AND sensor 1is not tripped

THEN energize solenoid 2 (ingr B)

IF sensor 1 is trippedTHEN enable timer (1 0 seconds )

IF sensor 1 is tripped AND the timer is

not done tim ingTHEN energize motor

IF the timer is done timingTHEN energize solenoid 3 (drain)

Criteria for selecting a PLC• How many control inputs to be processed – Nos.

of Input•

How manyoutput

devices or controllingelements arecontrolled – Nos. of Output.

• What memorycapacityis needed to store the`user program’ ?

• What speed of processing andoperational capabilities desire?

• What are the communication requirements ?• Are thereany specialor specificrequirements

including that of safety, reliability, expandability etc.

• System voltage available for auxiliary supply of PLC.

On getting this information checkfor the specification of the available

PLCs.

Selecting a PLC

Criteria

•••••

Number of logical inputs and outputs. MemoryNumber of special I/O modules Scan TimeCommunications

• Software

A Detailed Design Process

1. Understand the process2. Hardware/software selection3. Develop ladder logic4. Determine scan times and memory requirements

SpecificationsSeveral factors are used for evaluating the quality and performance of programmable controllers when selecting a unit for a particular application. These are listed below.

NUMBER OF I /O PORTS

This specifies the number of I/O devices that can be connected to the controller. There should be sufficient I/O ports to meet present requirements with enough spares to provide for moderate future expansion.

SpecificationsOUTPUT-PORT POWER RATINGS

Each output port should be capable of supplying sufficient voltage and current to drive the output peripheral connected to it.

SCAN TIME

This is the speed at which the controller executes the relay-ladder logic program. This variable is usually specified as the scan time per 1000 logic nodes and typically ranges from 1 to 200 milliseconds.

Specifications

MEMORY CAPACITY

The amount of memory required for a particular application is related to the length of the program and the complexity of the control system. Simple applications having just a few relays do not require significant amount of memory. Program length tend to expand after the system have been used for a while. It is advantageous to a acquire a controller that has more memory than is presently needed.

PLC Status Indicators

•Power On

•Run Mode

•Programming Mode

•Fault

Troubleshooting

1. Look at the process2. PLC status lights

HALT - something has stopped the CPURUN - the PLC thinks it is OK (and probably is)ERROR - a physical problem has occurred with the PLC

3. Indicator lights on I/O cards and sensors4. Consult the manuals, or use software if available.5. Use programming terminal / laptop.

List of items required when working with PLCs:

1. Programming Terminal - laptop or desktop PC.2. PLC Software. PLC manufacturers have

their own specific software and license key.

3. Communication cable for connection from Laptop to PLC.

4. Backup copy of the ladder program (on diskette, CDROM, hard disk, flash memory). If none, upload it from the PLC.

5. Documentation- (PLC manual, Software manual, drawings, ladder program printout, and Seq. of Operations manual.)

Examples of PLC Programming Software:

1. Allen-Bradley – Rockwell Software RSLogix5002. Modicon - Modsoft3. Omron - Syswin4. GE-Fanuc Series 6 – LogicMaster65. Square D- PowerLogic6. Texas Instruments – Simatic6. Telemecanique – Modicon TSX Micro

Summary• Overview

– A dedicated computer for rapid processing of simple logic instructions in a defined time

– Used in automated processes (rollercoaster)– Used a lot in automated industry– Logic control and sequencing approaches– Cost: $60-$400– Machine vision commonly used as supporting

technology– Dr. Red = Good reference

• Use in Industry– Applications include simple and, or, not

diagrams/programs– Few limitations for its function– Main known vendors: Honeywell, Rockwell, Sharp,

Unitronics– Standards: IEC 61131. Trying to standardize PLC

programs• Application examples

– Modeling Programs and Diagnosable functions for PLCs

– Useful in planning and determining structure and diagram

– Parking garage video– Design your own street light system– End of the line