chapter9 plc july 08 v1

8/8/2019 Chapter9 PLC July 08 v1

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Topic 2Topic 2-- 22EEB5223/EAB4223 Industrial Automation & Control Systems

This course cover topics related to measurements(PLTF) and automation in the process industry. Theseinclude a study on industrial sensors and actuators,

industrial controllers such as computer-based control,PLC, DCS and FF.

This course cover topics related to measurements(PLTF) and automation in the process industry. Theseinclude a study on industrial sensors and actuators,

industrial controllers such as computer-based control,PLC, DCS and FF.

Synopsis

SynopsisSynopsis Learning Outcomes

Learning OutcomesLearning Outcomes

that students are expected to achieved aftersuccessfully completing the course

that students are expected to achieved aftersuccessfully completing the course

The learning outcomes related toIndustrial instruments andMeasurements

Have knowledge andunderstanding of the variousprocess industry instruments,

concept of measurements,calibration and configurationrequirements and their applications.

Be able to design and develop acontrol loop consisting the processinstruments, based on a prescribed

requirement.

The learning outcomes related toAutomation and the implementation ofProgrammable logic controller

Have knowledge and

understanding of the PLCarchitecture and its importance inautomation.

Be able to develop a PLC programto perform sequential, and batchcontrol.

The learning outcomes related toDCS and Foundation Fieldbus usein process industries.

Have knowledge and

understanding of a DCS and itsuse in process industries, andthe features and architecture of aFoundation Fieldbus system, andits improvement over theconventional DCS.

This topic covers the key and base areas of automation. The learning approach

used is the mixed-mode delivery comprises of lecture-tutorial-lab activities..

Process , Industrial Instr.& Meas.

Process , IndustrialProcess , Industrial InstrInstr.& Meas..& Meas. Automation

AutomationAutomation DCS and Foundation Fieldbus

DCS and FoundationDCS and Foundation FieldbusFieldbus


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The focus of this topic:

develop sequence diagram for a given problemdescription involving electro-pneumatic actuators, sensorsand electrical actuators (motor and etc), and field devices.

develop and implement a ladder diagram for a process ina manufacturing plant, batch process, and robotic system.

Batch process

Electro-pneumatic Robotic System

Manufacturing system

Sen

sors

PLC

Programming!!!

Objectives


4/94Topic 2Topic 2-- 44EEB5223/EAB4223 Industrial Automation & Control Systems

CONTENTSCONTENTS

Overview of AutomationProgrammable Logic Controller

Development of Ladder diagram

and,

Several examples



What isWhat is AutomationAutomation??

Automation is the ability of a systemand/or devices to perform workintelligently with minimum or without

human supervision or intervention. It isa system designed to extend thecapacity of machines to perform tasksformerly done by human, and to controlsequences of operation without human

intervention.



What isWhat is AutomationAutomation??

Continue

The term Automation also been used to describenon-manufacturing system in which programmed orautomatic devices can operate independently or

nearly independently of human control. In the fieldsof communications, aviation, or astronautics, forexample, such devices as automatic switching

equipment, automatic pilots, and automatic guidancecontrol systems are used to perform variousoperations much faster or better that could be

accomplished by humans.



Why needWhy need AutomationAutomation??

To increase product standards with consistentquality.

- increased production and lowered costs,

thereby making goods available to more peoplewith better quality.

To gain higher throughput.

- more can be produced at lower costs, thusallows wages to increase which leads to theincrease of workers motivation.

To reduce labor dependencies.



Who needWho need AutomationAutomation?? Small and medium scale industries (SMIs) in the

manufacturing sectors.SMI could benefit from low cost automation toreplace repetitive process usually carried out by

human.

Multinational companies (MNC).

Current technology allows almost every aspect ofmanufacturing process can be automated, hencereducing dependencies on production workersand at the same time increasing productivity and

quality.



Where doesWhere does AutomationAutomation apply to?apply to?

Assembly line Test and finishing packaging (repetitive

process)

Hazardous environment

High speed and precision process, and

Computerized-plant management.



Benefits of automation:Benefits of automation: Cost reduction in terms of labor requirement

Increase efficiency in terms of Quality and

Quantity Flexible to market demand (faster and flexible).

i.e., changes in product and changes in process

Reliable automation processes can sustain longhours but still maintaining its desired performance(consistent quality and durability).

High repeatability

Safe

A solution for labor shortage



Examples ofExamples of AutomationAutomation??

Industrial:Camera inspection

Mark and Lead Test inspection

Pick and place system

Conveyor system

Automatic packer sorter

Automatic bonding machine

Die-attached machine.

Domestic:Automatic gate

Washing machine

Public utilities:

Traffic light

Commercial:

Office Automation

Intelligent building

Lifts and escalators

Automatic Teller Machines (ATM)


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A PLC is a computer, havingconnections to external inputs andoutputs.

The program of a PLC has the task toset the outputs, i.e. depending on the

inputs, the outputs, and the program.

What is a PLC?


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PROGRAMMABLE LOGICCONTROLLER

A substitute for hardwired relay panels.

A digitally operating electronic apparatus which uses aprogrammable memory for the internal storage of

instructions by implementing specific functions such aslogic sequencing, timing, counting, and arithmetic tocontrol, through digital or analog input/output modules,

various types of machines or processes. The digitalcomputer which is used to perform the functions of a

programmable controller is considered to be within thisscope. Exclude are drum and other similar mechanical

sequencing controllers.

National Electrical Manufacturing Association (NEMA) -1968


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The Design criteria for the first PLC,specified in 1968- Hydraulic Division of

General Motors Corporation

Purpose - To eliminate thehigh cost associated withinflexible relay-controlled

systems.

-The Specification-

A solid-state system withcomputer flexibility

Capable of survivingindustrial environment

Able to be easily

programmed andmaintained by plant

personnel

To be re-usable.


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The first PLC appeared in early 1970s.Merely relay replacement machines.

Other capabilities were later added, e.g.,Counters, Timers, Arithmetic, Text handling,Analogue signal handling, and PID control

TRADITIONAL AREA OF PLC APPLICATION ISTHAT OF SEQUENTIAL CONTROL


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PLC improves production efficiency. The present stateof intense industrial competition requires that:

The speedof theproductionequipment andproduction line canbe set up.

Lowering materialand labor costof aproduct

Improving qualityand lowering rejects.

Minimisingdowntime ofproduction

equipment. Low cost production

equipment.


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PLC meets most of the above needs

Traditionally, automation is only applicableto single item high volume production

Now, necessary to automate production ofmultiple variety of products, in moderatequantity, as well as achieving higher overallproductivity and requiring minimuminvestment in plant and equipment.

FMS answers these needs. The systemincludes, NC machines, industrial robots,automatic transports and computerizedcontrol of production. PLC in the use of mostautomated production equipment.


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Electromechanical Control

Advantages Disadvantages

Standardisedcomponents

Insensitive tointerference

Inexpensive for small

systemShort-term overloadcapability

High operating costs

Modifications are

difficultUnclear system layout

Wear (maintenance

therefore required)High spacerequirements

Expensive components


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Electronic ControlAdvantages Disadvantages

High reliability afterearlier developmentproblems.

Extremely high operatingspeed.

Low space requirements.

Low power consumptions.

No moving parts.

Low costs.

Frequent failures in theinitial phase.

Unclear layout

Modifications are difficult.

Highly paid specialists arerequired.


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Programmable Controls (PLCs)Advantages Disadvantages

High reliability after earlier

development problems.

Simple trouble-shooting, due to plug-in

design.

Simple installation.No standardisation required (means

more flexibility).

Low space requirements.

Low power consumption.

No moving parts.

Rapid modification of program.

Production and programming can becarried out in parallel.

No uniform programming language.

Slower than parallel logic system since

processing is carried out cyclically.

C i Wi d l i d PLC


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Comparison Wired logic and PLC

Wired Logic PLC

Control device

(Hardware)

Control Scale

Change or addition ofspecification

Delivery period

Maintenance (by makers andusers)

Reliability

Economic efficiency

Specific purpose

Small and medium

Difficult

Several days

Difficult

Depend on design andmanufacturer

Advantage on small scale

operation

General purpose

Medium and large

Easy

Almost immediate

Easy

Very high

Advantage on small,

medium and large scalescale operation


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A typical control system

Selector switchPushbuttons

Photoelectric switchesLimit switches

Circuit breakers

Proximity switchesLevel switches

Motor starter contactsRelay contacts

etc.

RelaysTimers

Counters

Logic UnitsMechanical Cams

etc.

AlarmControl relays

FansLights

HornsValvesMotor starters

Solenoidsetc.

CONTROLLERINPUT DEVICES OUTPUTS

REPLACEDBY

REPLACEDBY

ProgrammableLogic Controller

ProgrammableLogic Controller


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ApplicationsControl Type Functions

Sequence control Auto/Semi-auto manualcontrol of machine/process

Sophisticated Control Analog control (T,P,F,L, etc)

Servo motor controlStepper motor control

Supervisory control Process monitoring and alarmFault diagnosis andmonitoring F.A., F.M.S.,C.I.M. etc.

TRADITIONAL VS PROGRAMMABLE CONTROL


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SYSTEMSConventional

controller

Controlled

plant

+

-

S1

S2

K1

Programmable

Logic Controller

Controlled

plant

S1

S2

K1

+

-

+

Input 1

Input 2

Output 1

Its behaviour depends on the wiringarrangements

Its behaviour depends on theinstruction stored in memory

Memory

holdsacontrolprogram

PLC replaces most of the relay panel wiring by

software programming


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Electrical connections to a PLC

Note: Each PLC has three sets of terminals throughwhich it connects to the plant/industrial processes.

P>

N

L

PE

N

L

COMMON OUTPUT 1 OUTPUT 2 OUTPUT 3

INPUT 1 INPUT 2 INPUT 3

Input devices

Output devices

Inputterminals

Output

terminals

240 VAC

Powerterminals

PLC


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INPUT DEVICES

>

K

a. pushbutton b. Selectorswitch

c. Thermostat d. Pressureswitch

e. Level switch f. Relaycontact

g.Proximitysensor

h. Photoelectricsensor.


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OUTPUT DEVICES

a. Contactorcoil

b. Relay coil c. Valve coil

d. Signal lamp e. Hooter

TYPES OF SWITCHES


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TYPES OF SWITCHES

Selector switch

Pushbutton switches

Photoelectric switches

Limit switches

Proximity switchesLevel switches

Thumbwheel switches

Slide switches

RATING:24 Volts AC/DC

48 Volts AC/DC

120 Volts AC/DC

230 Volts AC/DC

TTL level(Transistor-to-transistor +/- 5V)

Isolated Input

Ho PLC o k ?


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Conventional relay/contactor control systems perform allcontrolling processes at the same time.

In a PLC the program sequence is executed step by step and is

repeated cyclically.

How PLC works?

Reads ininputs andtransfers the

value to amemory area

The givencommands(program) will

be executed stepby step

The outputs areset according toresults of the

processor

Input levelInput level

(Reactions)

ProcessingProcessing

(Details)

Output levelOutput level

Common Structure of a PLC


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Common Structure of a PLC

Input circuits

Output circuits

Micropro

cessor

(CPU)

Memory

Powersup

ply

The power supply

serves the

controller with a

variety of stable

direct voltages

Output devices receive signals from the controller

Input devices send signals

into the controller

Theprocessor

takes its

instructions

from thememory


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PLC COMPONENTS

1. Processor Microprocessor based, may allow arithmetic operations,

logic operators, block memory moves,computer interface,

local area network, functions, etc.

2. Memory Measured in words (2 bytes)

ROM

RAM

PROM

EEPROM

EPROM

EAPROM

and Bubble Memory.

THE DIFFERENT AREAS IN MEMORY , AND


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THE DIFFERENT AREAS IN MEMORY , ANDTHE FUNCTIONS

1. SYSTEM MEMORY

Executive- A permanently stored collection of

supervisory programs to direct system activitiese.g., program execution, communication withother devices, and fault diagnosis.

Scratch pad- A temporary store used by theCPU to store small amount of data generatedduring program execution.

THE DIFFERENT AREAS IN MEMORY , AND


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THE DIFFERENT AREAS IN MEMORY , ANDTHE FUNCTIONS

2. APPLICATION MEMORY

Data table area- This holds the User Program, aData Table for counter and timer preset valuesand the Input/Output Table.

INPUT TABLE

OUTPUT TABLE

INTERNAL STORAGE BITS

STORAGE REGISTERS

CONTROL PROGRAMINSTRUCTIONS

Also known as Markers , Flags, Internaloutputs, Internal coils, Internal controlrelays.

They do not operate any external devices,instead used for latching and interlockingpurposes in the control program.

A group of bits holding information in binary, BCD,or ASCII format. Values e.g., timer presets andaccumulated values, counter presets and

accumulated values, compare set points, maths

operation results and ASCII characters.

PLC COMPONENTS continue


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PLC COMPONENTS continue

3. I/O Modular plug-in peripheral

AC voltage input and output

DC voltage input and output

numerical input and output

special-purpose modules, for eg. High speedtimers, and stepping motor controllers.

4. Power supply AC Power

5. Peripheral Hand-held programmer (HHP)

Operator console

Printer

Graphic processor

Simulator

Network communication interface

ANALOG I/O


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ANALOG I/Os

Analog inputs:

Flow sensors, humidity sensors,

potentiometers, pressure sensors,temperature sensors.

Analog outputs:

Analog meters, analog valves and

actuators, DC and AC motor drives.

SOME SPECIAL I/Os


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Thermocouple input : Low level analog signal,filtered, amplified, and digitized beforesending to the processor through I/O bus.

Fast input: 50 to 100 microsecondpulse signal detection.

ASCII I/O: Communicates with ASCII devices.

Stepper motor output: Provide directly control of

stepper motor.

Servo interface:Control DC servo motor for point-to-point controland axis positioning.

PID control : The Proportional Integral Derivative is usedfor closed loop process control.

Network module.

The Technical features


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Input a. Number of external inputs

b. Types of external inputs

Output a. Number of external outputs

b. Types of external outputs

Auxiliary relays a. Number of output relays/holding relays etc.

Counters a. Number of counters

Timers a. Number of timers

Memory a. RAM

b. EPROM

c. EEPROM

Peripherals a. Handheld programming console

b. L.S.S.- Ladder Support Software

c. G.P.C. Graphic Programming Console

d. F.I.T. Factory Intelligent Terminal

PLC WIRING DIAGRAM


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01

02

03

+V

11

12

01 HR20

HR20

HR20 11

02A

B

External

switches

Stored program

MTR

-V

+ -

DC Power supply

PLC

Isolation


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Inputs and outputs are electrically separated (noelectrical connection), using

- opto-electronic coupler

Or

-or electromechanical relay

INPUT CONNECTIONSDC Power SupplyDC Power Supply


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L1 L2

1

23

COMMON

1

2

3

GND

+ - + -

+V

-V

1

2

3

pp yDC Power Supply

(a)

(b) (c)AC 120 or 230VAC, e.g.,AC input modules convertsignals to logic 1 or 0state

DC

High DC

TTLLow level DC(compatible with TTL)

AC inputdevices

DCinputdevices

OUTPUT CONNECTIONSDC Power SupplyDC Power Supply


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L1 L2

L1

12

GND

12

3

+ - + -

+V

-V

1

2

3

DC Power Supply

(a) (b) (c)

DC output module interfacelogic signal from processorwith a DC output field device :voltage > +5 V.

TTL

3

-V

+V

TTL output moduleinterface logic signalfrom processor with aTTL compatible

voltages

AC output module interfacelogic signal from processor witha AC output field device :

voltage 120 or 230 VAC.

CONNECTING FIELD DEVICES TO


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CONNECTING FIELD DEVICES TODC Input/Output modules.

Two common types of field circuits operate in theON/OFF switching condition.

a. Contact circuits- e.g., relays operate in theON/OFF condition as their

mechanical contacts open or close.

b. Solid state circuits- common use of

transistor (ON- saturation) (OFF-cutoff)

Sinking input module


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Sinking input module

+VOutputDC powersupply

DC common

DC input module

1 state: +DC voltage

SwitchingelementPNP

Sinkinginputterminal

+

-

Turning ON PNP produces a positive DC atoutput lead of field device

Sourcing input module


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g p

Output

DC commonDC powersupply

DC input module

1 state:

near ground potential

Switchingelement NPN

Sourcing

inputterminal

+

-

+VDC

Conventionalcurrent

Turning ON NPN produces a very low DC at

output lead of field device.

Types of outputs


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a. Open collector NPN current sinking

b. Open collector PNP current sourcing

Current sinking: the load (field device) is connected

bet. the output and the positive lead of the P/S.

+VDC

Output

DCCommon

Field

device

To +V DC

To DCcommonSwitching element

NPN transistor

Conventionalcurrent

DC PowerSupply

Sinking output

terminal

Current sourcing: the load (field device) is connected bet.


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g ( )the output and the negative common lead of the P/S.

+ V DC

Output

DCCommon

Fielddevice

To +V DC

To DCcommon

Switching element

PNP transistor

Conventionalcurrent

Sourcing output terminal


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Scan TimeScan time: The process of reading the inputs, executingthe program and updating the outputs.

Scan time is normally a continuous and sequentialprocess of reading the status of inputs, evaluating thecontrol logic and updating the outputs.

Scan time indicates how fast the controller can react tofield inputs and correctly solve the control logic.

I/O Update Program scan

What is a SCAN Cycle?A PLC resolves the logic of a ladder diagram (program) rung by rung


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A PLC resolves the logic of a ladder diagram (program) rung by rung,from the top to the bottom.

Usually, all the outputs are updated based on the status of the internalregisters.

Then the input states are checked and the corresponding input registers

are updated.Only after the I/Os have been resolved, is the program then executed.This process is run in an endless cycle.

The time it takes to finish one cycle is called the scan time. In somecontrollers the idle state is eliminated. In this case, the scan time variesdepends on the program length.

Begin

SCANcycle

Output

Resolvelogic

IdleInput

Factors influencing scan time


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Factors influencing scan time

Scan time (time required to make a single scan) varybetween 1 ms to 30 ms.

The use of remote I/O subsystems increases scantime: having to transmit the I/O update to remotesubsystem.

Monitoring of the control program adds overheadtime to the scan: the micro has to send the status ofthe coils and contacts to the display (CRT) or other

monitoring devices.

Types of PLC Programming Language


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The IEC 1161-3 Standard

a. Structured text programming (ST)- A high level language that has somesimilarities to Pascal. Statements can be used to assign values tovariables

b. Logic diagram Agraphical language, and most widely used. UseBoolean mnemonics to represent the process, before converting intologic diagram.

c. Functional Block Diagram Programming (FBD) Agraphical language.Used in applications involving the flow of signals between controlblocks.

d. Instruction List (IL)- A low level programming language, much like

assembly language programming.

e. Sequential Function Chart (SFC)- Agraphical programming method.Very useful for describing sequential type processes.

LADDER DIAGRAM


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A ladder diagram is a means of graphically representing thelogic required in a relay logic system, representing +V and0V

Consists of a number of rungs connecting two vertical lines.

+V

Rail

Rung

PB1 PB2 R1

R1

R1 A

0V

Rail

Programming a PLC


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The General Rule

SET RESET OUTPUT

LATCH

Well use the general rule to create the ladderdiagram. More examples in lecture and tutorial

PLC Ladder Diagram INSTRUCTIONS


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1. Relay

2. Timer and Counter3. Program Control

4. Arithmetic

5. Data Manipulation

6. Data Transfer, and

7. Others, such as sequencers.

RELAYS


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A switch whose operation is activated by an electromagnet.

contact

coil

input

R1

R1

Relay coil

Output contact

A Relay consists oftwo parts, the coil and

the contact.

Contacts:

a. Normally open

b. Normally closed

Coil:

a. Coil

b. Negative coil

LOGIC STATES


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ON: TRUE, contact closure, energize, etc.

OFF: FALSE, contact open, de-energize, etc.

Do not confuse the internal relay andprogram with the external switch and relay.Internal symbols are used for programming,

External devices provide actual interface.

(In the notes we use the symbol ~ torepresent negation. AND and OR are logicoperators).

AN EXAMPLE OF RELAY LOGIC


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For a process control, it is desired to have the processstart (by turning on a motor) five seconds after a parttouched a limit switch. The process is terminatedautomatically when the finished part touches a

second limit switch. An emergency switch will stopthe process any time when it is pushed.

LS1

PB1 (EMERGENCY STOP)

LS2

TIMER5

R1

R2

(MOTOR)

Rung 1: R1=(LS1+R1).(~PB1.~LS2)

R ng2 R1(dela ed5seconds)


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R1

R2TIMER

PR=5

R1

R1

LS1PB1

LS2

Rung 2: R1 (delayed 5 seconds)

EXAMPLE: CONTROLLING A CONVEYOR BELT


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FREE STANDING CONVEYOR

PARTS

BIN

CONVEYOR

MOTOR

INFRA RED

THROUGH

BEAM

INFRA RED

REFLECTIVE

REJECT

SOLENOID

st

Programming a PLC


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The General Rule

SET RESET OUTPUT

LATCH

A very very important rule!!!

As well see.

Program 1:

When START switch (st) is energized conveyor will


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When START switch (st) is energized, conveyor will

move. If switch (st) is de-energized, the belt will stillmoving until sensor (infra red through beam) isblocked.

mct HRMC mc-

mc

st

s0

HRMC

mct

mc-

SET=st

RESET=s0HRMC=(st+HRMC).s0

HRMC

HRMC

st s0 HRMC

HRMC

HRMC mct

TIMING

DIAGRAM

LADDER

DIAGRAM


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FLOW CHART

A i i l i f h f i


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A pictorial representation of the sequence of operations tobe carried out.

Different shaped boxes are used to represent different

actions.PROCESS

BOX

DECISION

BOX

TERMINATIONBOX

CONNECTOR

ACTIVATE

RELAY R1

LS1 active

?

START

A

START Translate into

Flow Chart example.


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START

L1 active

?

L2 active

?

L3 active

?

P1 active

?

ACTUATE

RELAY R1

A

L1

L2 L3

P1 R1

no

no

no

yes

yes

Translate into

ladder diagram

LOGIC DIAGRAM


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The International Standards Organisation (ISO) symbols:

&

>1

=1

>1

&

&

SR

YY

1NOT

AND

OR

EX-OR

NOR

NAND

Inhibition

R-S Flip-flop

T=A

T=A.B

T=A+B

T=A.B+B.A

T=(A+B)

T=(A.B)

T=A.B

Example: The use of a logic diagram


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A process:If limit switch L1 AND key P1 are activated, OR,NOT limit switch L2 AND limit switch L3 are

activated, then relay R1 is activated.Expressed by a single Boolean equation: L1.P1 + L2.L3 = R1

The logic implementation of this function into a formrequired for input into PLC.

&

&1

1

L1

P1

L2

L3

R1

SEQUENCE CHARTS


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Also known as:

Time motion diagrams, state diagrams, or bar chart.

Used for:

Visualising the operation of switching systems.

Describing the step-by-step operation of relay

systems, pneumatic systems, or any other type ofswitching systems.

Example: PLC control of actuators (solenoidvalves)


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ACTUATOR - A ACTUATOR - B

S2 S3 S4 S5

Y1Y2

START

S1

ACTUATOR - A

Example: (a) energisingY1, (b) de-energisingY1


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S2 S3 S2 S3

Y1Y1

(a) Cylinder extended (b) Cylinder retracted

Wiring diagram (Electro-pneumatic system)


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00000

0000100002

00003

00004

00005

COM

.

.

.

10000

1000110002

10003

10004

10005

COM

.

.

.

+ -+12VDC

+-

S1

S2

S3

S4

S5

+

-

Y1

Y2

TIMER

A Timer consists of an internal clock, a count value register, and an


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g

accumulator. It is used for timing purposes.

Clock

Accumulator

Contact

reset

output

Input

Reset

Output

Count0 1 2 3 4 5

Register

PLC TIMER

ON delay Timer:


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Example: If a timer in the circuit has a preset of 10 seconds, after a10-second delay, the timer ON-delay energise contact closes andpower is supplied to an output device via the contact.

Time delay

Timing coil input

On-delay energise

PLC TIMER

OFF delay Timer:


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Example: If a timer in the circuit has a preset of 10 seconds, after a10-second delay, the timer OFF-delay de-energise contact opensand power is removed from the controlled device.

Time delay

Timing coil input

On-delay de-energise

COUNTER

Digital counters output in the form of a relay contact when a pre-


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assigned count value is reached.

Register

Accumulator

Contact

input

reset

output

5

Input

Reset

Output

Count0 1 2 3 4 5 0 1

PLC COUNTER

CNT (Counter) is a preset decremental counter.


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It decrements one count every time an input signal goes from OFFto ON. The counter must be programmed with a count input, areset input, a counter number and a set value (SV).

The set value can range from 0 to 9999.

CNT 01

003

Count

input

Reset

input

Counter

number

SV

Example: A Traffic light control


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A B

C D

Traffic Light Control

The sequence diagram for routes A,B,C and D.


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24s 4s 2s 24s 4s 2s 24s 4s 2s 24s 4s 2s120s

ROUTE A

Red

Amber

Green

ROUTE B

Red

Amber

Green

ROUTE C

Red

Amber

Green

ROUTE D

Red

Amber

Green

Solution: Consider routes A and B ONLY.

24s 4s 2s 24s 4s 2s

TIM0


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TIM1

TIM2

TIM3

TIM4

TIM5

tim0

tim1

tim2

tim3

tim4

tim5

Solution example: Consider routes A and B ONLY.

24s 4s 2s 24s 4s 2s


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tim3+tim4

[(tim5+tim0+tim1).tim2]+[tim4.tim5]ROUTE A

Red

Amber

Green

ROUTE B

Red

AmberGreen

tim2+tim3

(tim1+tim2+tim3+tim4).tim5

tim0+tim1

tim5+tim0

Solution example: Routes A and B ONLYTIM5

TIM0

TIM3 TIM4

TIM2 TIM3

AMBER-A

GREEN-A

TIM0

24S


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END

TIM1

TIM2

TIM3

TIM4

TIM5 TIM2

TIM0

TIM1

TIM4 TIM5

TIM1

TIM2

TIM3

RED-B

TIM4

TIM5

TIM0 TIM1

TIM5 TIM0

TIM1

4S

TIM2

2S

TIM324S

TIM4

4S

TIM5

2S

RED-A

AMBER-B

GREEN-B

EXAMPLE: CAR PARK CONTROL

This is a simple car park control system that allow only a


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maximum of 100 parking spaces. Every time a car comes in,the LC will automatically add one through sensor S1. Any carthat goes out will automatically be subtracted by one throughsensor S2. When 100 cars are registered, the car park full sign

will be lighted to inform oncoming vehicles not to enter.

Car coming in

Car going out

CAR PARK

FULL

S1 (00000)

S2 (00001)

(00200)

Examples of DIFU and DIFD


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DIFU and DIFD turns an output ON for one scan time.DIFU turns its output ON when it detects an OFF-ONtransition in its input signal. DIFD turns its output ON

when it detects an ON-OFF transitions in its inputsignal.

Timing diagram:

INPUT

DIFU

DIFD

Examples of CMP


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Compare (CMP) is used to compare the data in a specificchannel, with the data in another channel, or a four-digit,

hexadecimal constant. Therefore, two data must bespecified immediately after the CMP instruction. One ofthe data must be a channel.

Ladder symbols:

CMP

source

destination

Examples of ADD

ADD (Add )


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ADD (Addition)

ADD totals the data in two different channels, or onechannel and a constant and then outputs the sum to a third

channel.

Three data must be specified: an augend, an addend and aresult channel.

Ladder symbols:

ADD

Augend

Addend

Results

Examples of SUB

SUB (S b )


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SUB (Substract)SUB finds the difference between the data in one channeland the data in another channel or a constant, and then

result to a third channel.Three data must be specified: an minuend, ansubstracthend and a result channel.

Ladder symbols:

SUB

Minuend

Substracthend

Results

1. Sense car coming in

2 Add 1 f

00000

01000

DIFU

01000

ADD

Example: Parking control


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2. Add 1 for every car3. Sense car going out

4. Subtract 1 for every car

5. Compare with 100

6. Indicator ON for CAR PARK FULL

ADDHR00

#0001

DIFU

01001

00001

SUB

HR00

#0001

00200

01001

CMP

HR00

#0100

AR01

AR01 AR02

AR03

Note: Auxiliary relay -: contains flagsand bits for special functions.

AR01 always turns ON).

If constant equal 100, AR02 turns ON.

AR03 turns ON if constant greaterthan 100.


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Sequence Function Chart (GRAFCET)

SFC is an approved means of organising the program in a PLC. Aprogram based on SFC has an inherently stable structure, has

h t ti d i t t bl h t


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shorter scan time, and is easy to troubleshoot.

0

1

2

Double box: Initial step

Single box: Step

Between two steps is a

transition(enables the next step to become active,

and the preceeding step inactive).

Sequence Function Chart (GRAFCET)Example:

When FORWARD PB switch is pressed and released, the

conveyor will move It will stop 1 5 sec after the infra red through


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0

1

2

Motor stopped

Sensor S1 is blocked?

Forward pushbutton?

Motor runs forward

Motor runs forward

Motor runs for 1.5 s?

conveyor will move. It will stop 1.5 sec after the infra red throughsensor, S1, is blocked by the part on the belt.


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Sequence Function Chart (GRAFCET)

Parallel branching : allows for two or more sequences toproceed simultaneously.

Example:


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53

54

55

56

57

58

q

r

s

t

p

59

Example:

The main sequence diverges into three sub-sequences after step 53.

Steps 54, 55Steps 57, 58

Step 59

If 53 is active and p is true, 54, 57, and 59 areactivated together.

When 55, 58, and 59 are active and r is true, 56is activated.

Note: Two or more steps can be active at the same time.

End of Lecture notes on PLCEnd of Lecture notes on PLC

chapter9 plc july 08 v1

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