Introduction to PLCIntroduction to PLC

CHAPTER - INTRODUCTION TO PLCS

Advantages of PLCs

Cost effective for controlling complex systems.Flexible and can be reapplied to control other systems quickly and easilysystems quickly and easily.Computational abilities allow more sophisticated control.T bl h ti id k i i dTrouble shooting aids make programming easier and reduce downtime.Reliable components make these likely to operate for years before failure.

PLC Origin

l d l l i h l- Developed to replace relays in the late 1960s

- Costs dropped and became popular by 1980s

- Now used in many industrial designs

Relays and

Schematic

Symbols

input coil

normallyopen

normallyclosed

OR

open

OR

Logical ControlLogical Control

with Relays115VACwall plug

relay logic

input A input Boutput C(normally open)p(normally closed) (normally open) (normally open)

l dd l i

A B C

ladder logic

Relay Logicpush buttons

Relay Logic

In a PLC powersupply+24V

com.

inputs

PLC

ladder

inputs

outputs

logicA B C

115Vac lightAC power

neut.

Reading Ladder Logicg g

HOT NEUTRALHOT NEUTRALA B X

C D G Y

E F H

INPUTS OUTPUTS

Note: Power needs to flow through some combination of the inputs (A,B,C,D,E,F,G,H) to turn on outputs (X,Y).

A Ladder Logic Example

A B

Note: When A is pushed, the output B will turn on, and the input B will also turn on and keep B on permanently - until power is removed.

B

N Th li h i h i b i l f ff i i llNote: The line on the right is being left off intentionally and is implied in these diagrams.

Mnemonics

000000000100002000030000400005

LDNLDANDLDLDAND

AB

CD

the mnemonic code is equivalent tothe ladder logic below

0000500006

ANDOR

A B X

ST00007 XEND00008

C D

END

Note: The notation shown above isnot standard Allen-Bradleynotation. The program to theright would be the A-B equiva

SORBSTXIC AXIO Bg q

lent. NXBXIO CXIO DBNDOTE XOTE XEOREND

Start

power up Execution followslti l th

power downflash

multiple paths

End

S d TStructured Text

i := 0;REPEATi := i + 1;UNTIL i >= 10END_REPEA T;

PLC in a Control Loop

PROCESS

Feedback fromsensors/switches

Connections toactuators

PLC

PLC h

The Four Stages of a PLC Scan

PLC program changes outputsby examining inputs Set new outputs

THE Power turned on

Read inputsProcess changes and PLC pauseswhile it checks its own operation

CONTROLLOOP

Power turned on

p p

Ladder Logic InputsLadder Logic Inputs

Normally open, an active input x will close the contactand allow power to flow.

x

Normally closed, power flows when the input x is not open.

x

xIIT

immediate inputs will take current values, not those fromthe previous input scan. (Note: this instruction is actually

IIT

an output that will update the input table with the currentinput values. Other input contacts can now be used toexamine the new values )examine the new values.)

Wh i li d ( ) th t t i ti t d f th l ft t t b t t d

Ladder Logic OutputsWhen power is applied (on) the output x is activated for the left output, but turned

x x

off for the output on the right.

An input transition on will cause the output x to go on for one scan

OSRx

(this is also known as a one shot relay)

When the L coil is energized, x will be toggled on, it will stay on until the U coil

LU

is energized. This is like a flip-flop and stays set even when the PLC is turned off.xx

Some PLCs will allow immediate outputs that do not wait for the program scan to

U

end before setting an output. (Note: This instruction will only update the outputs usingthe output table, other instruction must change the individual outputs.)

IOTx

Note: Outputs are also commonly shown using parentheses -( )- instead of the circle. This is because many of the programming systems are text based and circles cannot be drawn.

Sample Problem:

Try to develop a relay based controller that will allow three switches in a room to control a single light

p

three switches in a room to control a single light.