PLC AND ITS APPLICATION

BY

SOUVIK GANGULI

ASSISTANT PROFESSOR, EIED

THAPAR UNIVERSITY, PATIALA

PLC (Programmable Logic Controller)

It is basically a controller

Not continuous but logic state i.e. on-off controller

Sequence of logic states are programmable

All operations are to control a process

FOR WHAT TYPE OF PROCESSES PLC IS SUITABLE ?

Automatic bottle filling system

An example of discrete state process control

Example of Discrete State Process Control

Liquid Level Monitoring

Discrete State Process Control and PLC

• In majority of industrial process control, simply a controlled variable is regulated , but there are many processes where not a variable is controlled but sequence of events are to be controlled These are called discrete state process control.

• So discrete state process control system works on the basis of sequence of events.

• These sequence of events are programmed and controlled by Programmable Logic Controller (PLC).

Relay and Switch

• Relay as primary control element of

Discrete State Process Control

Use of relay and switch to start a motor Use of relay for a latch

When entire control system is implemented using relays , the system is called relay sequencer.

LADDER DIAGRAM

• A special schematic representation of the hardware and its connection has been developed that makes combination of hardware and event sequence description clear. This schematic is called ladder diagram.

• An industrial control system typically involves electric motors, solenoids, heaters etc. which are operated from the a.c. power line. e.g. when a conveyor motor is turned on it may mean starting of a 50 H.P. motor. This will not be done by single toggle switch but a small switch may be used to energize a relay with contact ratings that can handle a heavy load as shown in previous fig.

• When a relay panel is wired to implement a ladder diagram , we say that it has been programmed

to satisfy the ladder diagram.

Advantages of Computer Based Controller or Programmable Logic Controller (PLC) over Relay Logic

Controller

• After wiring of relay panel if the event sequence is to be changed , it is necessary to rewire all or part of the panel.

• It may even be necessary to add more relays to the system than the previous. This is quite trouble some.

• The I/O variables of discrete state process control systems are binary in nature just as with computer.

• Many of the control relays of the ladder diagram can be replaced by software which means less hardwire failure.

• It is easy to make change in programmed sequence of events when it is only a change in software.

Advantages of Computer Based Controller or Programmable Logic Controller (PLC) over Relay Logic

Controller

• Special functions such as time delay actions and counters are easy to form in software.

• Solid state devices such as SCRs , TRIACs are available that can control high power ac/dc in response to low level commands from a computer.

BASIC BLOCK DIAGRAM OF PLC

• Processor executes a program to perform operations such as arithmetic or logical operations on input variables.

• The processor is usually able to accommodate a number of I/P s called channels.

INPUT AND OUTPUT MODULE OF PLC

• I/P module examines the state of physical switches and other I/P devices and put their states into a form suitable for the processor.

• O/P module supplies a.c power to external devices such as motors, lights, solenoids etc.

• Internally O/P module accepts 1 or 0 I/P from the processor and use this to turn on/off an a.c. power control device such as triac.

I/P module O/P module

PLC details

• PLC can actually be considered to be a box full of hundreds or thousands of separate relays, counters, timers and data storage locations .

• These counters timers etc. don’t really exist but rather they are simulated and can be considered software counters, timers etc.

• I/P relays and O/P relays are connected to the outside world , they physically exist .

• There are some internal internal utility relays (contacts ) . They don’t receive signals from the outside world or not physically exist. They are simulated relays.

PLC OPERATION (I/O SCAN MODE AND EXECUTION MODE)

• The operation of the PLC can be

considered in two modes,

I/O Scan Mode & Execution Mode

In I/O Scan mode the processor

updates all outputs and inputs,

the state of all inputs one channel

at a time. The time required for this

depends on the speed of the

processor.

CHECK I/P STATUS

EXECUTE PROGRAM

UPDATE O/P STATUS

PLC EXECUTION MODE

• During this mode the processor evaluates each rung of the ladder diagram program that is being executed sequentially, starting from the first rung and proceeding to the last rung.

• As a rung is evaluated the last known state of each switch and relay contact in the rung is considered and if any true path to the o/p device is detected , then the o/p is indicated to be energized, I.e. set on.

• At the end of the ladder diagram the I/O mode is entered again and all O/P devices are provided with on or off state determined from execution of the ladder diagram program.

• All I/P s are sampled and execution mode is entered again.

PLC SCAN TIME

• An important characteristics of the PLC is how mach time is required for one complete cycle of I/O scan and execution .

• This depends on how many I/P and O/P channels are involved and on the length of the ladder diagram program.

• The speed of the controller is dependent on the clock frequency of the processor. The higher the clock frequency the greater is the speed and faster is the scan and execution time.

SYMBOLS FOR CONTROL ,TIMER RELAYS, etc

TYPICAL EXAMPLE OF PLC LADDER DIAGRAM

LADDER DIAGRAM DURING START AND STOP OF THE ELEVATOR SYSTEM

DURING START DURING STOP

FULL LADDER LIAGRAM FOR THE ELEVATOR SYSTEM