lect_11_21_plc

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By

Nazeer Ahmad Anjum

[email protected]

300-5397864

Spring 2007

(Mechatronics)

Department of Mechanical Engineering.

UET Taxila

PLC (Lecture # 21)
mailto:[email protected]:[email protected]

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PLCLecture No 21

Week 11th

29-05-2007

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PROGRAMMABLE LOGIC CONTROLLER

A digital Electronic Device

Uses a programmable memory to store instructions & toimplement functions as

Logic

Sequencing

Timing Counting &

Arithmetic

In order to control machines & Process

I/P devices as switches & o/p devices as motors areconnected to PLC.

controller monitors the i/ps & o/ps according to the programstored in the PLC

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Programmable Logic Controller

1. A control device that uses a programmable

microprocessor and is typically programmed.

2. PLCs are often categorized by the number of I/O

ports they provide.3. PLCs are often RISC based and are designed for

real-time and rugged industrial environments

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Controller

PLCs have been gaining popularity on the factory floor and will

probably remain predominant for some time to come. Most of this isbecause of the advantages they offer.

Cost effective for controlling complex systems. Flexible and can be reapplied to control other systems

quickly and easily.

Computational abilities allow more sophisticated control.

Trouble shooting aids make programming easier and reducedowntime.

Reliable components make these likely to operate for years

before failure. Easy to modify a control system without rewiring the

connections to i/p & o/p devices.

Much faster than relay-operated systems.

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Uses Programmable Logic Controller

Widely used for the implementation of logic control functions.

Easy to use

Reliability of PLC is greater

Maintenance is easier

PLCs take less floor space than do relay control panels.

Can perform a greater variety of control functions than relaycontrol

Virtual real-time control becomes feasible

Used for implementing automatic control of manufacturing

systems Logical information is rapidly and repeatedly processed and

immediately responded to with appropriate actions

A series of logical decisions have to be made and a variety of

actions taken on the basis of input that is obtained from appropriate

sensors

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Features of PLC

1. They are rugged and designed to withstand vibrations,Temperature, Humidityand Noise.2. The interfacing for i/p and o/p is inside the controller.3. They are easily programmed.

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Programmable Logic Control (PLC)

Definition - Dedicated computer for rapidprocessing of simple logic instructions in a

defined time.

Purpose - Send and read signals that canbe used to control and monitor devices.

Process- One of scanning all the devices(sensors, timers, etc.) in a cyclical time

period.

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PLC Control Approaches

Logic Control Method -Thisclosed-loop method usesconditions and events to signal completion of a given step, andthen triggers the execution of some other event. This is anasynchronous method of process control, because it does notalways proceed in a constant time period.

Sequencing Method -This open-loop method uses timers totrigger the completion of one step and the beginning of the next.This is a synchronous control method.

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Discrete Process Control

Discrete process control systems deals with parameters and variables

that change at discrete moments in time.Parameters and variables are also discrete in binary form.They can have either of two possible values, 1 or 0, values mean on/off,True/False, Object present or not present, high voltage value or lowvoltage value.

The binary variables in DPC are associated with i/p signals to thecontroller & o/p signals from the controller.

I/p signals are generated by binary sensors, such as limits switches,Timer, Relay or photo-detector sensors that are interfaced to the process.

O/p signals are generated by the controller to operate the process inresponse to the i/p signals & as a function of time.These o/p signals turn on or off switches, motors, valves, solenoid andother binary actuator related to the process.

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1. Executed by the controller in

response to some event.

2. The change can be to initiate an

operation or terminate an

operation, start a motor or stop,

open a valve or close

3. Has no memory.

4. Does not consider any previous

values of i/p

1. Executed by the control

system either at a specific

point in time or after a certaintime lapse has occurred

2. Uses internal timing devicesto determine when to initiatechanges in o/p variables.

3. Memory element is present.

4. Consider any previous values

of i/p

Logic Control or

Event Driven Changes

Sequencing or

Time Driven Changes

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1. A robot loads a work part into

the fixture, the part is sensed bythe limit switch. Sensing theparts presence is the event thatalter the system state.

2. Counting parts moving along a

conveyor past an optical sensoris an event driven system.

1. Starting times & ending times

for the shift & uniform breakperiods for all the workers.

2. Heat treating operationscarried out for certain lengthof time.

Logic Control

Examples

Sequencing

Examples

A robot is programmed to pickup a raw work part from a known stopping point

along a conveyor & place it into a forging press. In this three conditions must be

satisfied

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1. The raw work part must be at the stopping point

2. The forge press must have completed the process on the

previous part3. The previous part must be removed from the die

The first condition can be indicated by means of a simple limit

switch that sense the presence of the part at the conveyor,

stop & transmits an On signal to the robot controller. The 2nd condition can be indicated by the forge press, which

sends an On signal after it has completed the previous cycle.

The 3rd condition might be determined by the photo-detector

located so as to sense the presence or absence of the part inthe forging die.

When the finished part is removed from the die, an ON signal is

transmitted by the photocell. All three of these ON signals

must be received by the ROBOT controller to initiate the next

work cycle.

Three conditions and Solution

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Uses of Discrete Process Control

1. Widely used in discrete manufacturing as well as process

industries.2. In discrete manufacturing it is used to control the operation

of:

a. Conveyors

b. Automated Storage Systemsc. Automated Transfer Lines

d. Automated Assembly Systems

3. In process industries, discrete control is associated more

with :a. Batch processing than with continuous processes

b. Possible flow from one container to another during the

cycle.

c. Finally packaging

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PLC

CPU

System

User LadderDiagram

Workingmemoryregisters

Input

Flag

Output

InputModule

OutputModule

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PLCLecture No 25

Week 13th

16-06-2007

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Boolean Logic Operators

INPUT

Normally Open

OUTPUT

AND

OR

COMBINED AND & OR

INPUT

Normally Closed

TMR

CTR

Timer

Counter

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Logic Control ElementsPLC's are typically programmed using Boolean logic, shown in

figures by logical AND, OR, and NOT gates.

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AND, OR , & NOT GATEAND, OR, and NOT gates are then used to define two more

logical elements - the NAND and NOR gates - also shown with

their truth tables.

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RELAYS

contact

coil

input

Relay coil

R1

R1Output contact

A switch whose operation isactivated by an electromagnet iscalled a "relay"

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AN EXAMPLE OF RELAY LOGIC

For a process control, it is desired to have the process start (by turning ona motor) five seconds after a part touched a limit switch. The process isterminated automatically when the finished part touches a second limitswitch. An emergency switch will stop the process any time when it is

pushed.

L1

LS1PB1 LS2 R1

R1

R1TIMER

R2

PR=5

LS1

PB1

LS2

R1

TIMER

5MotorR2

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Example of a relay in a simple control

In this system the first relay on the left is used as

normally closed, and will allow current to flow untila voltage is applied to the input A. The second relayis normally open and will not allow current to flowuntil a voltage is applied to the input B. If current is

flowing through the first two relays then currentwill flow through the coil in the third relay, andclose the switch for output C. This circuit wouldnormally be drawn in the ladder logic form. This

can be read logically as C will be on ifA is off andB is on.

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A Simple Relay Controller

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Try to develop a relay based controller that

will allow three switches in a room to controla single light.

Problem:

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Solution: There are two possible approaches to thisproblem. The first assumes that any one of the switches onwill turn on the light, but all three switches must be off for

the light to be off.

The second solution assumes that each switch can turn

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The second solution assumes that each switch can turnthe light on or off, regardless of the states of the otherswitches. This method is more complex and involvesthinking through all of the possible combinations of

switch positions. You might recognize this problem asan exclusive or problem.

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COUNTER

Digital counters,

output in the form of arelay contact when apre-assigned count

value is reached.

Register

Accumulator

contact

input

reset

outpu

Input

Reset

Output

Count 0 1 2 3 4 5 0 1

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TIMERClock

Accumulator

contact

reset

outpu

Register

Contact

Time 5 seconds.

Clock

Reset

Output

Count 1 2 3 40 5

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Counters and Timers

Counters can be used in manufacturing to

measure quantities such as production stock,inventory, and packaging

Timers are used specifically to count clock

pulses Timers and counters greatly expand the

versatility of a PLC and allow the handling of

some variable-type questions Virtually all PLCs on the market today include

countering and timing capability

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PLC ARCHITECTURE

Programmable controllers replace most of the

relay panel wiring by software programming.

ProcessorI/OModules

M emoryPowerSupply

ProgramLoader

Printer

CassetteLoader

EPROMLoader

Switches

Machines

Peripherals External Device

PC

A typical PLC

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PLC COMPONENTS

1. Processor Microprocessor based, may allow arithmeticoperations, logic operators, computer interface, local areanetwork, functions, etc.

2. Memory Measured in words.

ROM (Read Only Memory),

RAM (Random Access Memory),

PROM (Programmable Read Only Memory),

EEPROM (Electric Erasable Programmable ROM),

EPROM (Erasable Programmable Read Only Memory),

EAPROM (Electronically Alterable ProgrammableRead Only Memory.

PLC COMPONENTS

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PLC COMPONENTS

3.I/O Modular plug-in periphery

AC voltage input and output,DC voltage input and output,

Low level analog input,

High level analog input and output,

Special purpose modules, e.g., high speed timers,

Stepping motor controllers, etc.

4. Power supply AC power

5. Peripheral hand-held programmer (HHP)

CRT programmeroperator consoleprintersimulatorEPROM loadergraphics processor

network communication interface

PLC SELECTION CRITERIA

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PLC SELECTION CRITERIA

1. What I/O is required, i. e. the number of I/O, capability ofexpansion for future needs.

2. What types of I/O are required, i. e. signal conditioning, on-board power supply for inputs, outputs, etc.

3. What size of memory is required? This is linked to thenumber of I/O & a complexity of program used.

4. What speed & power is required of CPU.

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Logical Control

Control actions are taken by making decisions

depending on the values associated with variousinputs or variables and the control logic in the

program

If a decision can be made by answering yesornoto

a given question, it is referred to as a decision byattributes Is a part loaded in the machine?

Is the tool path unobstructed?

Is the AGV carrying a part?

Decisions that cannot be made by answering yesor

noare referred to as decisions by variables How long is the bar stock?

What is the feed rate?

What is the art tem erature?

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Ladder Logic Diagram

Well established in industry in relation to the traditional

electromechanical logic devices. Makes use of representations similar to electrical circuits in

which a ser ies connect ionrepresents a logical and and

a parallel connect ionrepresents a logical or.

Made up of inputs, outputs connected according toappropriate logic.

Each rung in the ladder represents a set of logical

relationships between the inputs that leads to a particular

output. The output from one rung of the ladder could be used as an

input in another rung of the same ladder.

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Ladder Logic Diagram

Except when special provisions are made, itis considered that all rungs in a given ladder

logic diagram are executed simultaneously,

so the order of the rungs on the ladder in

general does not matter.

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LADDER DIAGRAM

A ladder diagram is a means of graphicallyrepresenting the logic required in a relay logic system.

A

R1

PB1 PB2

R1

R1

start emergency stop

Rail

Rung

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PLC WIRING DIAGRAM

Externalswitches

Stored program

01 02 20

20

20 11

01

02

03

11

12

C

PLCInput OutputA

B

CO O C S

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CONTROL DEVICES

1) Mechanical Con trol - cam, governor,

etc.,

2) Pneumatic Con trol - compressed air,valves, etc.

3) Electromechan ical Con trol - switches,relays, timers, counters, etc,

4) Electronics Control - similar toelectromechanical control, except useselectronic switches.

5) Compu ter Contro l

PLC E l ti

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PLC Evolution

Programmable Logic Controllers

A

B

P2

P1

I1

Analog WorldBinary World

C

continuous processes

Regulation, controllers

discrete processes

combinatorial sequential

Pneumatic and electromechanicalcontrollersRelay controlpneumatic sequencer

Continuous and Discrete Control (comparison)

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Continuous and Discrete Control (comparison)

A B

Out = A B

B

NOT CA

Out = (A + B) C

"sequential""combinatorial"

ladderlogic

e.g. ladder logic

P2

P1

I1

analog

building

blocks

Ladder Logic

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Ladder Logic

01 02

50

0102

03 50

03

relay coil

break contact

make contact

correspondingladder diagram

origin:

electrical

circuit

50 05

44

rung

"coil" 50 is used to move

other contact(s)

L dd L i

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Ladder Logic

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

0250

Series

+ 01

40

02

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

Parallel

ladder logic representation "CMOS" equivalent

01

02 40

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PLCLecture No 26

Week 14th

19-06-2007 Ladder logic example

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Ladder logic example

Process will mix ingredients, first adding ingredient A until

level reaches sensor 2, then adding ingredient B until levelreaches sensor 1. The mix is then stirred for a period of time,

and tank is emptied. Investigate the control logic required.


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How would you w r ite the ladder log ic for th is problem?

IF sensor 2 is not tripped

THEN energize solenoid 1(Ingr A)

IF sensor 2 is tripped AND

sensor 1 is not tripped THEN

energize solenoid 2 (ingr B)

IF sensor 1 is tripped THENenable timer (10 Sec)

IF sensor 1 is tripped AND

the timer is not done timing

THEN energize motor

IF the timer is done timingTHEN energize solenoid 3

(drain)

Solution X1, X2 = sensors

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Solution

TMR

T1

S3

S1

TMR

T2

S3

T1

X2 S3

S2

X1 X2

M

M

X1

S1, S2, S3 = solenoids (relays)

M = motor (relay)

T1 = tank stir period

T2 = drain period

IF sensor 2 is not tripped THEN energize solenoid 1

(Ingr A)

IF sensor 2 is tripped AND sensor is not tripped THEN

energize solenoid 2 (ingr B)

IF sensor 1 is tripped THEN enable timer (10 Sec)

IF sensor 1 is tripped AND the timer is not done timing

THEN energize motor

IF the timer is done timing THEN energize solenoid 3

(drain)

Programming a PLC

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Programming a PLC

Ladder Logic for Tank

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Ladder Logic for Tank

Address

Low = 0000

High = 0001

Motor = 0500

Relay = 1000

Logic for Ladder Solution

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Logic for Ladder Solution

A B C

D E F

H d it k?

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How does it work?

PROGRAMMING EXAMPLE

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PROGRAMMING EXAMPLE

id description state explanation

MSI microswitch 1 part arrive

R1 output to bar code reader 1 scan the part

C1 input from bar code reader 1 right partR2 output robot 1 loading cycle

R3 output robot 1 unloading cycle

C2 input from robot 1 robot busy

R4 output to stopper 1 stopper up

C3 input from machine 1 machine busyC4 input from machine 1 task complete

Part

microswitch

Bar code reader

Stopper

Conveyor

Machine

Robot

SOLUTION

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SOLUTION

Rung 1. If part arrives andno part is stopped, triggerthe bar code reader.

Rung 2. If it is a right part,activate the stopper.

Rung 3. If the stopper is up,the machine is not busyand the robot is not busy,load the part onto themachine.

Rung 4. If the task iscompleted and the robot

is not busy, unload themachine.

01

02

03

04

05

11

12

13

14

15

InputOutput

ProgrammableControllerPLC

MS1

C1

C2

C3

C4

R1

R2

R3

R4

01 14 11

02

14 04 03

14

12

1305 03

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Light Switch Examples Test

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Light Switch Examples TestSwitch_1

X0

Switch_2

X1

Switch_1

X0

Switch_1

X0

Switch_1

X0

Switch_1

X0

Switch_1

X0

Switch_1

X0

Switch_3

X2

Switch_2

X1

Switch_2

X1

Switch_2

X1

Switch_3

X2

Switch_3

X2

Switch_3

X2

Light_1

Y0

Light_2

Y1

Light_3

Y2

END

Motor Control Examples Test

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Motor Control Examples TestStop

X0

Start

X1

Motor_FBX2

Start

X1

Stop

X0

Intrlk_2

X4

Motor_FB

X2

Intrlk_3X5

Motor

Y0

Motor

Y0

Motor

Y0

END

Start

X1

Motor

Y0

Stop

X0

Intrlk_1

X3

lect_11_21_plc

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