Plc & Its Application in Drive Control

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PLC & ITS APPLICATION IN

DRIVE CONTROL


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CONTENTS

Introduction

History of PLC

Block diagram

Components

Operation

Response time

Replacing relays

2Programmable Logic Conrollers


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Introduction

Device that was invented to replace the

necessary sequential relay circuits for

machine control.

The PLC works by looking at its inputs and

depending upon their state, turning on/off its

outputs.

The user enters a program, usually via

software, that gives the desired results.



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History of PLC

First design 1968

Implemented 1969

Goals

Eliminate high cost associated with frequent

replacement of relay based system

Reduce production downtime

Provide expandability



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Block diagram

I/O SYSTEM POWER SUPPLY

PROCESSOR

MEMORY UNIT

PROGRAMMING

DEVICE

Inputs

Outputs



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Cont..



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In PLC



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Input relays :

Connected to the outside world

They physically exist and receive signals

from switches, sensors, etc.

These do not receive signals from theoutside world nor do they physically

exist.

Internal utility relays :



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Counters :

Do not physically exist

They are simulated counters and they can

be programmed to count pulses

do not physically exist

on-delay type and off-delay type

retentive and non-retentive types

Timers :



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Output relays :

These are connected to the outside world

They physically exist and send on/off signals

to solenoids, lights, etc

can be transistors, relays, or triacs depending

upon the model chosen

Assigned to store data

Volatile or non volatile

Data storage :



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PLC Operation

A PLC works by continually scanning a program

Consisting of 3 important steps



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Check input status :PLC takes a look at each input to determine if it is

on or off

It records the data into its memory to be used

during the next step

Execute program :

PLC executes the program one instruction at a

time

It will store the execution results for use later

during the next step 12Programmable Logic Conrollers


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Update output status :

It updates the outputs based on which inputswere on during the first step and the results of

executing your program during the second

step

One scan time is defined as the time it takesto execute the 3 steps listed above.



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Response time :

PLC takes a certain amount of time toreact to changes

Total response time = (Input response

time + Program execution time + Outputresponse time )



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Cont



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Pulse stretch function :

Extends the length of of the input signaluntil the plc looks at the inputs during

next scan.



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Interrupt function :

interrupts the scan to process a specialroutine

As soon as the input turns on, regardless of

where the scan currently is, the plcimmediately stops what its doing and

executes an interrupt routine.

Goes back to the point it left off at andcontinues on with the normal scan process.



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Cont



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Programming languages

Program is a set of instructions that arearranged in a logical sequence to control

the actions of process or machine

Most common PLC language is Ladder

logic



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Ladder diagram ladder logic diagram



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Relays :

A relay is an electrically operated switch

Many relays use an electromagnet to

operate a switching mechanism



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Cont

Relay (electromagnetic switch)



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Replacing relays

Contact

Coil

Ladder logic



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Basic instructions

Loadpresence of input signal is needed toturn an o/p on

Laodbarabsence of referenced signal is

needed Outif path has logic continuity, the

referenced o/p is energized

Outbarif path of false instructions preceding Latch coilfor an output to remain energized

Unlatch coilreset a latched output

Programmable Logic Conrollers 24


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Cont

Load

Loadbar

Out

Outbar



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Level application


We are controlling lubricating oil beingdispensed from a tank. This is possible by using

two sensors. We put one near the bottom and

one near the top, as shown in the picture


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Cont

Both of our inputs will be NC (normallyclosed) fiber-optic level sensors

We are using an internal utility relay



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Scanning



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Advantages

Reduces the size of system

Less expensive

Easy maintenance

Easy to modify



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References

Programmable controllers (2nd edition) byThomas A Hughes

www.plcs.net

www.mech.co.uk



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THANK YOU

Plc & Its Application in Drive Control

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