PLC

IEEE HIT STUDENT BRANCH
Workshop on
PLC and SCADA
Presentation By: DEBAYON SAHA

What is PLC?
• PLC – Programmable Logic Controller
• PLC is a digital computer designed for controlling multiple inputs and
output arrangements, extended temperature ranges, immunity to
electrical noise, and resistance to vibration and impact.
• A PLC is an example of a real time system.
• PLC works on the principle of Relay Logic.

History of PLC
PLC was introduced in late 1960’s
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.
Later ,in late 1970’s,the microprocessor became reality & greatly enhanced
the role of PLC permitting it to evolve form simply relay to the
sophisticated system as it is today.

Leading Brand of PLC
• Siemens - Germany
• AB (Allen Bradley) – US
• Schneider Electric (Modicon) – France
• Mitsubishi – Japan.
• ABB – Germany.
• SAIA – Switzerland
• GE Fanuc – US.
• B&R (Bernecker & Reiner) – Austria.
• Teco – Czech Republic.

Types of PLC
• NANO PLC: Latest and smallest plc
Consist of less than 10 input & Output ports
• MICRO PLC: Smaller PLC
Consists of15-20 input & 10-12 output ports
• MINI PLC: Medium size
Consists of 100-200 input & output ports
• RACK PLC: Large size
Consists of 1000 of input & output ports

PARTS OF PLC
PLC
HARDWARE SOFTWARE

HARDWARES & SOFTWARES OF PLC
MAKER’S NAME HARDWARE SOFTWARE
SIEMENS S7-300, S7-200, S7-400 etc. Siematic Manager, Logo
Allen Bradley MicroLogicx LogicxPro
ABB AC500, S500 AC500, S500

MAJOR COMPONENTS OF PLC
• POWER SUPPLY MODULE: 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.
It consists of AI/AO cards & DI/DO cards.
• CPU or PROCESSOR: Provides intelligence to command and govern the activities of the
entire PLC systems.
• IFM MODULE: IFM module helps a PLC to communicate with other PLCs. It also
manages the communication between other modules and processor.

BASIC BLOCK DIAGRAM OF A PLC
I M
N O
P D
U U
T L
E
POWER
SUPPLY
O M
U O
T D
P U
U L
T E
PROCESSOR
IFM MODULE
To
OUTPUT
Solenoids,
contactors,
alarms
etc.
From
SENSORS
Pushbuttons,
contacts,
limit switches,
etc.
To Other PLC

Input & Output of PLC
Outputs
LEDs
Inputs

Programming Languages of PLC
• Ladder Logic
• Functional Block Diagram
• STL or Statement List
• Sequential Function Chart
• Boolean mnemonics

Advantages of PLCs
 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 of PLCs
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.

Ladder Logic
• Ladder logic is the oldest programming language for PLC.
• It is mostly used logic for controlling PLC. language for PLC.
• It is well suited to express Combinational logic.

Concept of NO, NC, EO & EC
• Any Electrical Switch have 2 conditions: 1) Normal or OFF state
2) Excited or ON state
• Again on the basis of contact position any electrical switch have 2 positions:
1) Open Position
2) Closed Position

• NO: Normally Open Position. When switch is pressed then its become
closed. After releasing of switch its again go back to open position.
• NC: Normally Closed Position. When switch is pressed then its become
open. After releasing of switch its again go back to close position.
• EC: Excited Closed Position.
• EO: Excited Open Position.

EC
(Excited Closed)
NO
(Normally Open)
EO
(Excited Open)
NC
(Normally Closed)

Addressing of Input & Output
• Input: I:1/0, I:1/1, ……………., I:1/15
• Output: O:2/0, O:2/1, …………, O:2/15
• In Siemens Input is denoted by ‘I’ & Output is denoted by ‘Q’.

Problem Statement- 1
• Develop a LAD programe such that the lamp L1 should glow when the
switch S1 is made ON and the lamp gets OFF when switch S1 is made OFF

Problem Statement- 2
• Develop a LAD programe such that the Lamp L2 should not glow when
switch S2 is made ON Lamp should glow when switch S2 is made OFF.

Problem Statement-3
• The lamp L3 should glow when both the switch S1 & S2 are ON. If any one is
OFF then L3 will not glow. Draw LAD logic.

Problem Statement-4
• The lamp L4 should glow when any one of the switch S1 or S2 is ON. Draw
the LAD logic.

Problem Statement-5
• The lamp should glow when either the switch (S1 & S2) or S3 is ON. Draw
LAD logic.

Problem Statement-6
• Develop Ladder Logic for a car door/seat belt safety system. When the car
door is open, or the seat belt is not done up, the ignition power must not be
applied. If all is safe then the key will start the engine.

Relay
• It was invented by Joseph Henry in 1835.
• A relay is an electrical switch that opens and closes contacts under control of
another electrical circuit.
• The switch is operated by an electromagnet (Solenoid) to open or close one or many
sets of contact points.
• A relay is able to control an output circuit of higher power than the input circuit
which is at lower power.

Lever
(Magnetic Material)
Solenoid
Coil
Spring
NC Contact
NO Contact
Common
Contact

Operation of Relay
• When a current flows through the coil, the resulting magnetic field attracts an
armature that is mechanically linked to a moving contact.
• The moving contact is supported by a spring, which keeps the common contact
normally connected to the NC and when the coil get activated and a magnetic
field is produced, the moving contact get attracted.
• Resulting a closed path between NO contact and Common contact.
• The movement either makes or breaks a connection with a fixed contact.

Inside a Relay Magnetic force deflects
the lever downward
when the coil is energizedSpring force restores
the lever when the
coil is not energized
Connections to coil
Normally open
contact
Normally closed
contact
NO
NCCommon contact

Magnetic force deflects
the lever downward
when the coil is energizedSpring force restores
the lever when the
coil is not energized
Connections to coil
Normally open
contact
Normally closed
contact
NO
NCCommon contact
Inside a Relay

Problem Statement
• When a switch S1 is pressed, then it activated a relay. After activating the
relay a bulb B1 gets on, which was previously off. Another bulb B2 gets off
which was previously on.

MEMORY
• It is a internal device of PLC and can be used as output.
• Memories are generally used for providing internal switching in program.
• Memory is denoted by ‘B3’.
• Range of memory is 0/0 to 31/7.

PROBLEM STATEMENT
• In a one way tunnel there are three proximity sensors S1, S2 & S3. When the
car passes sensor S1 the bulb B1 glows. When the car passes sensor S2 the
bulb B1 gets OFF & the bulb B2 glows. When the car finally passes the
sensor S3 the bulb B2 gets OFF.

TIMER
• Basically Timer is a device which counts time and provides a delay in a
circuit.
• Timer is addressed with ‘T4’.
• Timer range is 0 to 255.
• There are 3 types of timer in PLC- 1) On Delay Timer
2) Off Delay Timer
3) Retentive On Delay Timer

ON DELAY TIMER
• Initially Contact Part of this timer remains in NO position.
• After getting power supply its started achieving its preset value.
• After achieving its preset value the Contact of the Timer becomes EC.

PROBLEM STATEMENT
• A lamp will be on 10 sec after pressing a switch. If switch will be off then
timer will reset automatically.

OFF DELAY TIMER
• When We provide power supply Off Delay Timer remains in on
position.
• When we off the power supply its start achieving preset value.
• After achieving the preset value timer becomes off.

PROBLEM STATEMENT
• A lamp should be get off 5 sec after pressing a switch.

Retentive ON DELAY TIMER
• Same as ON DELAY TIMER.
• The only difference is when power is interrupted its retain its accumulated value,
when power is again on then its starts counting from its accumulated value.
• To reset this timer we use ‘Reset’.

COUNTER
• Basically Counter is a device which counts no of events.
• Counter is denoted by ‘C5’.
• Counter range is 0 to 255.
• To reset the counter we use ‘RESET’.
• There are Two types of counter in PLC- 1) UP COUNTER
2) DOWN COUNTER

UP COUNTER
• UP COUNTER counts up.
• Initially up counter remains in off position.
• When count reaches counter becomes on.

PROBLEM STATEMENT
• There is a conveyor belt. After passing 5 elements a indicating lamp will glow.
A switch is used to reset the counter.

DOWN COUNTER
• DOWN COUNTER counts down.
• Initially down counter remains in on position.
• After count reaches counter becomes off.

PROBLEM STATEMENT
• A lamp L1 is on. After 5 men enter a room the lamp should go off. A switch
is used to reset the counter. Use Down Counter.

PROBLEM STATEMENT
• A lamp L1 will glow after pressing a switch S1 10 times. Another lamp L2 is already
lit. It will be off when switch S2 is pressed 5 times after L1 glows. A reset switch S3
is used to reset both the counters.

PLC

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