PLC Slides

8/10/2019 PLC Slides

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1. PLC - Introduction

What does PLC stand for?

PLC - programmable logic controllerPLC implements logic control functions by meansof a program

PLC are basically microprocessor devices that are

used in industry to control plant operations



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Features



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PLCs



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An application example 1: Gate Control

PLC can sense a vehicle at the entrance or exit, andopen and close the gate automaticallyThe current vehicle count is easily determined byprogramming a simple counter



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An application example 2: Conveyor System

PLC can be used to start/stop latching logic for motorcontrolCounters can be used for monitoring product amounts



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1. PLC - IntroductionComparing traditional and programmable control systems - 1



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Comparing traditional and programmable control systems - 2

In traditional control, the switches S1, S2 and S3 must close for

K1 to be turned on - the wir ing

makes the rule @ “hard wired” In PLC systems, the program is written to perform the logic “when S1 is closed AND S2 is closed AND S3 is closed, THEN turn onK1” - the program makes the rule

It is much simpler to change program then wiring!



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How does a PLC differ from a computer?

A computer is optimized for calculation and display tasks

A computer is programmed by specialists

A PLC is designed for (logic) control and regulation tasks

A PLC is programmed by non-specialists

A PLC is well adapted to industrial environment



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Why are PLCs so common?

They are cost-effectiveThey are flexible, reliable and compact

They have significant advantages over traditionalcontrol systems based on relay or pneumatics



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Where are PLCs used?

In every industry where automation is involved,from individual machines to whole processes



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What tasks do PLCs perform?

The logic control tasks such as interlocking,sequencing, timing and counting (previouslyundertaken with relays or pneumatics)

In addition, PLCs can perform a variety ofcalculation, communication and monitoring tasks



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Inputs

Outputs & Power Supply

C o m m u n i c a t i o n

P o r t s

( R S - 4 8 5

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lecture note 9 PLC 20

1. PLC - IntroductionStructure of a PLC



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The PLC processor



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PLC Input/Output Devices



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PLC Input Devices

Push buttonsSwitches (limit switches, level switches, etc.)

Sensors



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PLC Output Devices

Relay contactsSolenoid valves

Signal devices (such as lamps, alarms, etc.)

Motors



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Programming terminal



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Programming is done through programming

terminal

Programming terminal translates engineeringlanguage (logic control) to machine language

(binary code)



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Programming through standard computer

Most PLC manufacturers offer software packages

that allow a standard computer to be used as aprogramming terminal



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Programming through standard computer



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Relating the program to inputs and outputs

The CPU reads the data from the inputs

The program in the CPU uses the inputs to

evaluate the control logic. As the program runs,

the CPU updates the data

The CPU writes the data to the output



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Relating the program to inputs and outputs



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Data Flow in the PLC



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One of the advantages of PLC is that it can be programmed by

non-specialists

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 theelectrical relay diagrams



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Ladder diagram



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Relay Ladder schematic diagram



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Programming symbols and terminology

Special instruction



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Ladder equivalent of electrical circuits



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Ladder equivalent of electrical circuits (AND)



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Ladder equivalent of electrical circuits (OR)



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Ladder equivalent of electrical circuits



PLCInstruction code mnemonics

LD – Load operand into result register : Start a rung with open contacts

LDI – Load negative operand into result register : Start rung with closed contacts

AND – Boolean AND : A series element with open contacts

ANI – Boolean AND with negative operand : A series element with closed contacts

OR – Boolean OR : A parallel element with open contacts

ORI – Boolean OR with negative operand : A parallel element with closed contacts

OUT – Store result register into operand : An output from rung

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PLC Ladder rung instructions

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AND

OR



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PLC instruction list

NOR

NAND



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PLC program instructions

0 LD X400

1 LD Y430

2 ORB3 ANI X401

4 OUT Y430



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0 LD X400

1 OR Y430

2 OUT Y430

3 OUT T450

4 K 50 Delay on timer




Device a circuit that can be used to start a motor and then aftera delay of 100 s start a pump. When the motor is switched offthere should be a delay of 10 s before the pump is switchedoff.


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