programmable logical controller(PLC)

PLC
(Programmable logical controller)
PRESENTED BY
Hemadri
B.Tech (EE) 3 yr

Content
What is plc ?
History of plc.
Architecture
Operation
Classification of plc
Ladder logic
Plc timer
Plc counter
Advantages
Plc applications

PLC
–The PLCs are devices that use
programmable memory for storing the
instructions and implementing the
functions like timing, counting , logic
sequencing and arithmetic functions
for controlling the different processes
and machines.

network digital inputs
digital outputs
analog inputs / outputs

History of PLC
 Invented in 1968 by Morley and a group of fellow
"geeks,“.
 First commercial & successful Programmable Logic
Controllers was designed and developed by Modicon as a
relay replacer for General Motors.
 Earlier, it was a machine with thousands of electronic
parts.
 In 1970s PLCs were introduced with sequencing
implemented in software instead of relays.

Architecture
PROCESSOR
POWER
SUPPLY
I M
N O
P D
U U
T L
E
O M
U O
T D
P U
U L
T E
PROGRAMMING
DEVICE
From
SENSORS
Pushbuttons,
contacts,
limit switches,
etc.
To
OUTPUT
Solenoids,
contactors,
alarms ,
Relays
etc.

Main components
 POWER SUPPLY
Provides the voltage needed to run the primary PLC components
 I/O MODULES
Provides signal conversion and isolation between the internal logic-
level signals inside the PLC and the field’s high level signal.
 PROCESSOR
Provides intelligence to command and govern the activities of the entire
PLC systems.
 PROGRAMMING DEVICE
Used to enter the desired program that will determine the sequence of
operation and control of process equipment or driven machine.

Operation of plc
1)Input scan: (checking the status of inputs)
If there are no problems, PLC will copy all the inputs and
copy their values into memory.
2)Logic solve/scan: (executing a programme)
Using inputs, the ladder logic program is solved once and
outputs are updated.
3)Output scan:(Updating the status of output While solving
logic the output values are updated only in memory when
ladder scan is done, the outputs will be updated using
temporary values in memory.

Ladder logic
The ladder logic is the oldest programming
language for PLC.
 Ladder diagram doesn’t include power supply
 Every line of ladder diagram is called rung.
 There should be atleast one output on every
rung.
 Main symbols used in ladder programming.

NO contact
Plc programming
LOAD I1
OUT 01
I1 O1
LOAD I2
OUT 02
NC contact
Plc programming
I2 O2

Ladder logic for basic gates
A B Logic(Y)
OFF
OFF
ON
ON
OFF
ON
OFF
ON
OFF
OFF
OFF
ON
AND Gate
A B Y
A B Logic(Y)
OFF
OFF
ON
ON
OFF
ON
OFF
ON
OFF
ON
ON
ON
OR Gate
A
Y
B

A B Y
A B Logic(Y)
OFF
OFF
ON
ON
OFF
ON
OFF
ON
ON
ON
ON
OFF
A B Logic(Y)
OFF
OFF
ON
ON
OFF
ON
OFF
ON
ON
OFF
OFF
OFF
Y
A
NOR Gate NAND Gate
B

X2X1
Y1
Y1
Ladder Logic Program
for
Start/Stop of Motor :

PLC Timer
 Timer is an important parameter that needs to be
controlled in industrial control system e.g. a motor to be
operates for a specific time.
 PLCs have in-built timers that counts seconds or
fractions of seconds with the help of clock.
 Diff types of timers :- 1.ON Delay
2.OFF Delay
3. PULSE timers

PLC counter
 The PLC counter are provided in PLCs for counting e.g.
counting the no. of revolution in a shaft, counting no. of
elements passing along a conveyor belt.
 A counter is set for a specific value and when this value
is achieved the contacts will function i.e. NO will be
closed and NC will be open.
 Two types of timers :- 1. UP- counters [CTU]
2. down- counters [CTD]
 Up counters counts from 0 to preset value 0,1,2…. .
 Down counters count from a set value to 0.
 There are also up-down counters that count up and/or
down , called UDC (up- down counters).

AMERICAN:
1. Allen Bradley 5. Westinghouse
2. Gould Modicon 6. Cutter Hammer
3. Texas Instruments 7. Square D
4. General Electric
Leading brands of PLCs
EUROPEAN:
1. Siemens
2. Klockner & Mouller
3. Festo
4. Telemechanique
JAPANESE:
1. Toshiba
2. Omron
3. Fanuc Leading Brands Of PLC
4. Mitsubishi

Advantages
 Reliability.
 Flexibility in programming and reprogramming.
 Cost effective for controlling complex systems.
 Small physical size, shorter project time.
 High speed of operation.
 Ability to communicate with computer systems in the
plant.
 Ease of maintenance /troubleshooting.
 Reduced space.
 Energy saving.

Disadvantages
 PLC devices are proprietary it means that part or
software of one manufacturer can’t be used in
combination with parts of another manufacturer.
 Limited design and cost option
 Fixed Circuit Operations.
 PLCs manufacturers offer only closed architectures.

Applications
 Wherever automation is desired the PLCs are best
suited to meet the task.
 Few examples of industries where PLCs are used :
1) Robots manufacturing and control
2) Car park control
3) Train control station system
4) Food processing
5) Materials handling
6)Machine tools
7)Conveyer system etc.

programmable logical controller(PLC)

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