PLC Programming

LADDER LOGIC

Prepared by Gagandeep

FUNCTIONS OF CONTROLLERS 1) on-off control, 2) sequential control, 3) feedback control, and 4) motion control.

CONTROL DEVICES 1) mechanical control - cam, governor, governor, etc., 2) pneumatic control - compressed air, valves,etc. 3) electromechanical control - switches, relays, a timer, counters, etc, 4) electronics control - similar to electromechanical electromechanical control, except uses electronic switches. 5) computer control.

SWITCHE S Non-l -lo c king

Lo c king

No rma lly O pe n

No rma lly Cl C lo s e d

DPST

P1

SPDT

P2 Multiple Th ro w

Multiple P o le

Bre a k- b e f o re - m a ke ke

Ma ke -be f o re -b re a k

TYPES OF SWITCHES

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RATING: Selector switches Pushbutton switches 24 Volts AC/DC 48 Volts AC/DC Photoelectric 120 Volts AC/DC 230 Volts AC/DC switches TTL level Limit Switches (Transistor-to-transistor ±5V) Proximity switches Isolated Input Level switches Thumbwheel switches Slide switches • •

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RELAY S A switch whose operation is activated by an electromagnet is called a "relay" contact

coil

input

Relay coil R1 Output contact

R1

 AN EXAMPLE OF RELAY RELAY LOGIC For a process control, it is desired to have the process start (by turning on a

motor) five seconds after a part touched a limit switch. The process is terminated automatically when the finished part touches a second limit switch.  An emergency emergency switch will will stop the process any time when when it is pushed. L1 LS1

PB1

LS2

R1

R1

R1

TIMER

R2

PB1 LS1

LS2

PR=5

TIMER 5

R1

Motor R2

LADDER DIAGRAM  A ladder diagram (also (also called contact contact symbology) symbology) is a means of graphically representing representi ng the logic required in a relay logic system.

Rail start PB1

emergen emergency cy stop PB2 R1

Rung

R1

R1 A

PLC WIRING DIAGRAM

A

Input 01

B

02

Output

PLC 01

02

20

11 12

20

03 20

11

External switches

Stored program

C

PLC Programming 

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Ladder Diagram - most common Structure Text Text Programming (ST) Functional Block Programming (FB) Instruction List (IL) Sequential Function Chart (SFC)

 AND and OR LOGIC PB1

PB2

R1 R1 = PB1.AND.PB2

AND

PB3

PB4

R2 R2 = PB2.AND.~PB4

PB1

R1 R1 = PB1 .OR. PB2

OR

PB2

COMBINED AND & OR R1 = PB1 .OR. (PB2 .AND. PB3)

R1

PB1

PB2

pb3

RELAY A Relay consists of two parts, the coil and the contact(s). Contacts: a. Normally open

-| |-

b. Normally closed

-|/|-

c. Positive transition sensing

-|P|-

d. Negative transition sensing

-|N|-

()

Coil: a. Coil

-( )-

b. negative coil

-(/)-

c. Set Coil

-(S)-

d. Reset Coil

-(R)-

Relay (continue) Coil: e. Retentive memory Coil

-(M)-

f. Set retentive memory Coil

-(SM)-

g. Reset retentive memory Coil

-(RM)-

h. Positive Transition-sensing Coil

-(P)-

h. Negative Transition-sensing Coil

-(N)-

(set coil latches the state, reset coil deenergize the set coil. retentive coil retain the state after power failure.)

TIMERS AND COUNTERS

Input

Timers:

True

a. Retentive on delay

-(RTO)-

b. Retentive off delay

-(RTF)-

c. Reset

-(RST)-

False

RTO counting stop RTF

stop

True counting resume

counting stop

RTO reach PR value, output ON RTF reach PR value, output OFF

Counter: a. Counter up

-(CTU)-

b. Counter down

-(CTD)-

c. Counter reset

-(CTR)-

PR value in 0.1 second

SEQUENCE R

Sequencers are used with machines or processes involving repeating operating cycles cycles which can be segmented segmented into steps. Output Step

A

B

C

Dwell time

1

ON

OFF OFF

5 sec.

2

ON

ON OFF

10 sec.

3

OFF

OFF ON

3 sec.

4

OFF

ON OFF

9 sec.

PROGRAMMING EXAMPLE 1 Bar code reader

microswitch

Stopper

Conveyor

Part

Robot

id MSI R1 C1 R2 R3 C2 R4 C3 C4

Machine

description microswitch output to bar code reader input from bar code reader output robot output robot input from robot output to stopper input from machine input from machine

state 1 1 1 1 1 1 1 1 1

explanation part arrive scan the part right part loading cycle unloading cycle robot busy stopper up machine busy task complete

SOLUTION Input MS1

Output

01

11

C1 C2

02

12

03

13

C3

04

14

C4

05

01

Programmable Controller PLC

11

14

14

02

14

04

05

03

03

12

13

R1 R2 R3 R4

15

Rung 1. If part part arrives arrives and no part is stopped, trigger the bar code reader. Rung 2. If it is a right part, part, activate the stopper. Rung 3. If the stopper is up, the machine is not busy and the robot is not busy, load the part onto the machine. Rung 4. If the task is completed and the robot is not busy, unload the machine.