This document provides an in-depth overview of programmable logic controllers (PLCs), detailing their design, functions, and components such as the processor unit, memory, power supply, input/output interfaces, and communication interfaces. It explains the programming methods, particularly ladder programming, and includes examples of control systems that can be implemented using PLCs for various applications like automated drilling, traffic light control, and water level management. The advantages of PLCs over traditional control systems, including flexibility and ease of programming, are also highlighted.

1. CHAPTER FIVE
5. Programmable
Logic Controllers (PLCs)
Prepared by: Daniel .K 2016EC
Design of Electromechanical System I

2. ❑ A programmable logic controller (PLC) is a special form of microprocessor-based controller
❑ PLCs are similar to computers but whereas computers are optimized for calculation and
display tasks, PLCs are optimized for control tasks and the industrial environment.
❑ It uses a programmable memory to store instructions and to implement functions such as
logic, sequencing, timing, counting and arithmetic in order to control machines and processes.
❑ The term logic is used because programming is primarily concerned with implementing logic
and switching operations,
e.g.The program may be in the form of:-
if A or B occurs switch on C,
if A and B occurs switch on D.
❑ The operator then enters a sequence of instructions, i.e. a program, into the memory of
the PLC.The controller then monitors the inputs and outputs according to this program
and carries out the control rules for which it has been programmed.
❑ Introduction

3. ❑ By changing the instructions in the program we can use the same microprocessor system
to control a wide variety of situations.
❑ In the ‘traditional’ form of control system, the rules governing the control system and when
actions are initiated are determined by the wiring.
• When the rules used for the control actions are changed, the wiring has to be changed.
❑ So in PLC or microprocessor control system, instead of hardwiring each control circuit for
each control situation we can use the same basic system for all situations if we use a
microprocessor-based system and write a program to instruct the microprocessor how to
react to each input signal from, say, switches and give the required outputs to, say, motors
and valves.
Conti…..

4. ❑ What type of task (function) might a control system have?
❖ It might be required
✓ To control a logic, sequence of events, timing, counting
and arithmetic or ON-OFF state.
✓ To maintain some variable constant , motion
✓ To follow some prescribed change.
❑ PLC initially designed to replace relay logic boards
o Sequence device actuation
o Coordinate activities
• Accepts input from a series of switches
• Sends output to devices or relays
Conti…..

5. For example,
❑ The control system for an automatic drilling machine
as shown in figure might be required to
• start lowering the drill when the workpiece is in
position,
• start drilling when the drill reaches the workpiece,
stop drilling when the drill has produced the
required depth of hole,
• retract the drill and then switch off and wait for the
next workpiece to be put in position before repeating
the operation

6. ❑ PLCs have the great advantage that:-
✓ There is no need to rewire.
✓ It is a flexible, cost effective,
✓ Are easily programmed and have an easily understood
programming language
✓ used with a wide range of control systems by resetting the
programed instructions.
✓ It is Rugged and designed to withstand vibrations, temperature,
humidity and noise.
✓ Have interfacing for inputs and outputs already inside the
controller.

7. ❑ Parts of PLC
❑ Typically a PLC system has the basic functional components of
✓ Rack or chassis
✓ processor unit,
✓ memory,
✓ power supply unit,
✓ input/output interface section,
✓ communications interface and
✓ the programming device.

8. ✓ is the unit containing the microprocessor and
✓ this interprets the input signals and carries out the control actions, according to
the program stored in its memory,
✓ communicating the decisions as action signals to the outputs.
2. The processor unit or central processing unit (CPU)
1. Rack or chassis
✓ In all PLC systems, the PLC rack or chassis forms the most important module and
acts as a backbone to the system.
✓ PLCs are available in different shapes and sizes.
✓ It distributes the power supply evenly. It houses different input-output modules.
✓ It acts as a communication link between the CPU and different modules.
✓ When more complex control systems are involved, it requires larger PLC racks.
✓ All I/O modules will be residing inside this rack/chassis.

9. ✓ It is where the program is stored that is to be used for the control actions to be
exercised by the microprocessor and data stored from the input for processing
and for the output for outputting.
4. The memory unit
3. The programming device
✓ It is used to enter the required program into the
memory of the processor.
✓ The program is developed in the device and then
transferred to the memory unit of the PLC.
2. The power supply unit
✓ This module is used to provide the required power to the whole PLC system.
✓ It converts the available AC power to DC power which is required by the
CPU and I/O module.
✓ It generally works on a 24V DC supply.
✓ Few PLC uses an isolated power supply

10. ✓ is used to receive and transmit data on communication networks from or to
other remote .
✓ It is concerned with such actions as device verification, data acquisition,
synchronization between user applications and connection management.
6. The communications interface
✓ are where the processor receives information from
external devices and communicates information to
external devices.
✓ Input devices, e.g. Switches or other sensors such as
temperature sensors, flow sensors, proximity sensor,
and output devices in the system being controlled,
e.g. motors, solenoid valves, etc., are connected to
the PLC.
5. The input and output sections

11. ❑ Ladder programming
• A very commonly used method of programming PLCs is based on the use of
Ladder programming or (ladder diagrams).
• A ladder diagram (also called contact symbology) is a means of graphically
representing the logic required in a relay logic system.
• Writing a program is then equivalent to drawing a switching circuit.
❑ In drawing a ladder diagram, certain conventions are adopted:
▪ The vertical lines represent the power rails between which
circuits are connected.
▪ Each rung defines one operation in the control process.
▪ A ladder diagram is read from left to right and from top to
bottom,
▪ The end rung indicated by a block with the word END or RET for
return, since the program promptly returns to its beginning.

12. ▪ Each rung must start with an input or inputs and must end with at least
one output.
▪ A particular device can appear in more than one rung of a ladder.
▪ The inputs and outputs are all identified by their addresses, the notation
used depending on the PLC manufacturer.
❑ To illustrate the drawing of the rung of a ladder
diagram, consider a situation that input is the
switch and the output the motor.
✓ In drawing ladder diagrams the names of the
associated variable or addresses of each element
are appended to its symbol, see fig c.
Fig. Siemens notation a, b, c.
Conti….

13. ❑ Logic functions
❑ Logic states:-
ON :TRUE, contact closure, energize, etc.
OFF: FALSE, contact open , de-energize, etc.
❖ Do not confuse the internal relay and program with the external
switch and relay.
✓ Internal symbols are used for programming.
✓ External devices provide actual interface.
❖ AND & OR LOGIC
Fig. AND gate Fig. OR gate
❑ Logic gate representation using ladder diagram

14. ❖ NAND & NOR gate
Fig. NAND gate Fig. NOR gate
❖ Not gate
Fig. Not gate
Conti….

15. ❑ Latching
o There are often situations where it is necessary to hold an output energized,
even when the input ceases.
o The term latch circuit is used for the circuit used to carry out such an
operation.
o It is a self-maintaining circuit in that, after being energized, it maintains that
state until another input is received.
Fig. Latched circuit
Example.
❑ Thus, once the motor energize, even if the input A
opens, the circuit will still maintain the output
energized.
▪ The only way to release the output is by operating the
normally closed contact B.

16. ❖ COUNTER
❑ A counter is a PLC instruction that either increments (counts up) or decrements
(counts down) an integer number value when prompted by the transition of a bit
from 0 to 1 (“false” to “true”).
❑ A counter is set to some preset number value and, when this value of input pulses
has been received, it will operate its contacts. Thus normally open contacts would be
closed, normally closed contacts opened.
❑ Counter instructions come in three basic types: up counters, down counters, and.
up/down counters
❑ Example:- This might be used where items have to be counted as they pass along a
conveyor belt, or the number of revolutions of a shaft, or perhaps the number of
people passing through a door.

17. ❑ Up-Counters:
❑ Up counter is an increment counter which means it counts “up” with
each off-to-on transition input to its “CU” input.
❑ Up-counters count from zero up to the preset value, i.e. events are
added until the number reaches the preset value.When the counter
reaches the set value, its contacts change state.

18. ❑ Whenever there is a true value entry in the CU input, the counter will turn on
and CV will display the count.
❑ This counter output must be activated whenever the current value is equal to
or greater than the pre-set value (Q is active if CV ≥ PV).
❑ The counter output instruction will increase by 1 each time the counted
event occurs
Conti….

19. Example program for UP counter

20. ➢ Operating pushbutton PB1 provides the off-to-on transition
pulses that are counted by the counter.
➢ The pre-set value of the counter is set to 7.
➢ Each false-to-true transition of rung 1 increases the counter’s
accumulated value by 1.
➢ After 7 pulses, or counts, when the pre-set counter value equals
the accumulated counter value, output DN is energized.
❑ Working principle of the above up counter ladder program

21. ➢ As a result, rung 2 becomes true and energizes output O:2/0 to
switch the red pilot light ON.
➢ At the same time, rung 3 becomes false and de-energizes output
O:2/1 to switch the green pilot light off.
➢ The counter is reset by closing pushbutton PB2, which makes
rung 4 true and resets the accumulated count to zero.
➢ Counting can resume when rung 4 goes false again
Conti….

22. ❑ Down Counter:
❑ Down-counters count down from the preset value to zero, i.e. events are subtracted
from the set value.When the counter reaches the zero value, its contacts change
state.
❑ The down-counter instruction will count down or decrease by 1 each time the
counted event occurs.
❑ Each time the countdown event occurs, the accumulated value decreases.
❑ Normally, the down counter is used together with the ascending counter to form
an up / down counter

23. Conti….
Fig. Down counter:

24. Up/down counting
❑ It is possible to program up- and down-counters together.
❑ Consider the task of counting products as they enter a conveyor line and as they
leave it, or perhaps cars as they enter a multi-storage parking lot and as they
leave it.
❑ When an item enters it gives a pulse on input In 1.This increases the count by
one.Thus each item entering increases the accumulated count by 1.
❑ When an item leaves it gives an input to In 2.This reduces the number by 1.
Thus each item leaving reduces the accumulated count by 1.
❑ When the accumulated value reaches the preset value, the output Out 1 is
switched on.

25. Conti….

26. Example for up-Down counter:
❑ One application for an up/down-counter is to keep count of the cars that enter
and leave a parking garage. A typical PLC program that could be used to
implement this.

27. ➢ As a car enters, the enter switch triggers the up counter output instruction and
increments the accumulated count by 1.
➢ As a car leaves, the exit switch triggers the down counter output instruction and
decrements the accumulated count by 1.
➢ Because both the up- and down-counters have the same address, C5:1, the
accumulated value will be the same in both instructions as well as the pre-set.
➢ Whenever the accumulated value of 150 equals the pre-set value of 150, the
counter output is energized by the done bit to light up the Lot Full sign.
➢ A reset button has been provided to reset the accumulated count
❑ The operation of the program can be summarized as follows:

28. ❑ A PLC timer is a program that controls and operates a device for a set period.
❑ We can use the timer to do any certain activities for a fixed period or to build
up a time-based action.
❑ The timer action is used to provide programming logic and to decide when
to switch on and off the circuit
❑ There are a number of different forms of timers that can be found with PLCs:
ON Delay Timer , OFF Delay Timer , and Retentive Timer
❑ TIMER

29. ❑ ON-Delay Timer :- Which means delayed on i.e. the timer will not update the contact
until the pre-set time has passed
✓ As you can see in the diagram, the input supply is delivered, but there is no output until the
predetermined time(pre-set time) has passed .

30. ❑ Off delay Timer:- means that delayed off , even if the timers input power is turned off, the timer
continues to give contact to the exiting circuit.
✓ As shown in the diagram, the input signal is turned off, but the contact remains closed.

31. ❑ Retentive Timer: Retain accumulated value through power loss, processor mode
change, or rung state going from true to false.
• Use a retentive timer to track the running time of a motor for maintenance purpose.
Each time the motor is turned off, the timer will remember the motor elapsed
running time.The next time the motor is turned on, the time will increase from there.
• To reset this time, use a reset instruction.

32. ✓ Consider the task of obtaining cyclic movement of a piston in a cylinder. Figure
shows the valve and piston arrangement that might be used, write a possible
ladder program that operate the two solenoid sequentially.
A. Control of Cyclic movement of a piston
❑ Project examples
Fig. valve and piston arrangement

33. Fig. Ladder logic diagram for valve and piston control.
Conti….

34. ▪ Working principle
✓ When the start contact X400 are closed, Timer one T450 start timing and
there is an outputY431(solenoid B)(i.e. the cylinder retract).
✓ After 10sec that the Timer T450 start timing and solenoid B energized, all
normally closed T450 open so that solenoid B de-energized, solenoid A
energized (i.e. the cylinder extract) and also Timer two T451 start timing.
✓ After 10sec that T451 start timing and solenoid A energized, normally closed
contact of T451 open that cause the timer T450 de-energize and energize
Y431 solenoid(i.e. the cylinder retract),
✓ At the same time thatY431 energize, the latch contact y431 cause the Timer 1
T450 to start timing .
✓ Thus the sequence repeat it self.

35. B. Traffic light control
❑ Suppose we have three light Red, Green, and Yellow.Write a possible ladder
program that first Red light on for 5sec, thenYellow for 2sec, then green for
2sec and then repeat the operation

36. Fig. Ladder logic diagram for traffic light control.
Conti….

37. Working principle
• When start button pressed(closed), memory bit M0 energized, then Timer 1
(T1) start timing and also green light on for 2secon.
• After 2sec timer 1 relay energized and make normally open timer one
contact closed & normally closed one open, and then Timer two(T2) start
timing and also read light ON for 5sec .
• After 5sec , normally closed timer two contact open and normally opened
one closed , and so timer three(T3) start timing,Yellow light ON for 2sec.
• After 2sec, normally opened timer three(T3) contact closed and normally
closed one open, and then Timer one(T1) start timing and also green light
ON for 2sec.
• Then the sequence repeat itself.
• If stop button pressed, the process end.

38. C. Water level control.
❑ As shown in the figure we have a water thank, we
need to control the water level in the thank, write
a possible ladder program that fill the water to
the thank, if the level is low and discharge the
water, if the level is high.
# Quiz

39. Fig. Ladder logic diagram for water level control.
Conti….

40. Working principle.
• When the start button(X2) is pressed(closed), the internal relay(M1)
energized and the inlet valve start filling(i.e.Y0 is open) until the water
level reach the higher level sensor(Xo).
• When the water level exceed the high level sensor(i.e. Xo energized ), it
cause to close the inlet valve(Y0) (i.e. water doesn’t flow in) .
• When thee water level is high, both high level sensor(X0) and low level
sensor(X1) energized, those cause the discharge valve(Y1) open(i.e. water
start flow out).
• After the water level being below the lower level sensor(X1), both sensor de-
energized, it causes the discharge valve(Y1) close(i.e. water doesn’t flow
out) and also the inlet valve(Y0) open.
• Those the sequence repat itself.
• If the stop button(X3) pressed(opened), the prosses stop.