Advantages of PLCs

Cost effective for controlling complex systems. Flexible and can be reapplied to control other systems quickly and easily. Computational abilities allow more sophisticated control. Trouble shooting aids make programming easier and reduce downtime. Reliable components make these likely to operate for years before failure.

CHAPTER - INTRODUCTION TO PLCS

PLC Origin

- 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

no rm a llyo p e n

no rm a llyc lo se d

inp ut c o il

O R

O R

Logical Control

with Relays

115VACwall plug

relay logic

input A(normally closed)

input B(normally open)

output C(normally open)

ladder logic

A B C

Relay Logic

In a PLC

ladder

powersupply

+24V

com.

inputs

outputs

push buttons

logic

PLC

AC power

115Vac

neut.

A B C

light

Reading Ladder Logic

HOT NEUTRAL

INPUTS OUTPUTS

A B X

C D

E F

G

H

Y

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

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.

A

B

B

Note: The line on the right is being left off intentionally and is implied in these diagrams.

Mnemonics

0 0 0 0 00 0 0 0 10 0 0 0 20 0 0 0 30 0 0 0 40 0 0 0 50 0 0 0 6

L D NL DA N DL DL DA N DO R

AB

CD

A B

C D

X

E N D

the m ne m o nic c o d e is e q uiva le nt tothe la d d e r lo gic b e lo w

S T0 0 0 0 7 XE N D0 0 0 0 8

N o te : T he no ta tio n sho w n a b o ve isno t s ta nd a rd A lle n- B ra d le yno ta tio n. T he p ro gra m to theright w o uld b e the A - B e q uivale nt.

S O RB S TX IC AX IO BN X BX IO CX IO DB N DO T E XE O RE N D

SFCs

Start

End

power up

power down

flash

Execution followsmultiple paths

Structured Text

i := 0 ;R E P E A Ti := i + 1 ;U N T IL i > = 1 0E N D _ R E P E A T;

PROCESS

Feedback fromsensors/switches

PLC

Connections toactuators

PLC in a Control Loop

Read inputs

PLC program changes outputsby examining inputs Set new outputs

Process changes and PLC pauseswhile it checks its own operation

THECONTROLLOOP

Power turned on

The Four Stages of a PLC Scan

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

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

x

x

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

xIIT

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

Ladder Logic Inputs

When power is applied (on) the output x is activated for the left output, but turned

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

x x

OSRx

(this is also known as a one shot relay)

off for the output on the right.

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

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

LU

IOT

end before setting an output. (Note: This instruction will only update the outputs using

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

x

xx

the output table, other instruction must change the individual outputs.)

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.

Ladder Logic Outputs

Try to develop (without looking at the solution) a relay based controller that will allow three switches in a room to control a single light.

Sample Problem: