PLC in Automation

A
Presentation on
PLC in Automation
Under the Module IV
of
Mechatronics
(T.E. Mechanical SEM-VI)
By Mr. Harshal Chaudhari

Contents:
1. Understanding PLC
i. Differencing MC and PLC
ii. Introduction to PLC*
2. Basic PLC Structure at glance
3. I/O Processing
4. Programming Fundamental- Ladder Logic
diagram*
5. PLC for industrial process control*
6. Selection of PLC
Note: (*) Titles will be explained on green board only

 PLC is a special microcontroller designed for
industrial application. It is for controlling machinery
or processes. A microcontroller is a
microprocessor that can be used for any type
of application. The basic difference between PLC
and microcontrollers higher speed, performance,
and reliability.
 PLC works with power devices and Microcontroller
works with electronic devices i.e. PLC works with
relays, while Microcontroller works with transistors
(even it may work with electronic relays)
Understanding the PLC
(1/3)
MAHARSHI PARSHURAM COLLEGE OF ENGINEERING, VELNESHWAR
Mechanical Engineering

 Usually the Microcontroller doesn’t work as a
stand alone controller but it came as a part of
electronic circuit or device, as NOT ROBUST in
nature.
 While the PLC is a stand alone controlling device
that can be programmed for any process.
 Number of I/O ports in PLC is more than
microcontroller.
 A microcontroller is a chip.
PLC = number of chips.
(2/3)

 Watchdog for the PLC is faster than MC , means
scan cycle time of PLC is more than MC.
 PLC can communicate on different protocols ,
MC can not support all protocols.
(3/3)

 A programmable logic controller (PLC) is a digital
electronic device that 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 and has
been specifically designed to make programming
easy.
 The term logic is used because the programming is
primarily concerned with implementing logic and
switching operations.
Introduction to PLC

 PLCs are similar to computers but have certain
features which are specific to their use as controllers.
These are:
1 They are rugged and designed to withstand vibrations,
temperature, humidity and noise.
2 The interfacing for inputs and outputs is inside the
controller.
3 They are easily programmed and have an easily
understood programming language. Programming is
Introduction to PLC

Contents:
1. Understanding PLC
i. Differencing MC and PLC
ii. Introduction to PLC
2. Basic PLC Structure at glance
3. I/O Processing
4. Programming Fundamental- Ladder Logic
diagram
5. PLC for industrial process control
6. Selection of PLC

Basic PLC Structure at Glance
Input 5V, 24V, 110V
and 240V
Output 5V

DIGITAL SIGNALS
DIGITALSIGNALS
DIGITAL SIGNALS
DIGITAL SIGNALS
Data bus: sending data between the
constituent elements,
Address bus: send the addresses of
locations for accessing stored data,
Control bus: for signals relating to internal
control actions.
System bus: used for communications
between the input/output ports and the

The CPU
Main part is microprocessor. CPUs have the
following:
ALU responsible for data manipulation and
carrying out arithmetic operations of addition
and subtraction and logic operations of AND,
OR, NOT, and EXCLUSIVE-OR. Memory,
termed registers, located within the
microprocessor and used to store information
involved in program execution. A control unit

Clock:
Clock that has a frequency of
typically between 1 and 8 MHz.
This frequency determines the
operating speed of the PLC and
provides the timing and
synchronization for all elements in

System read-only-memory (ROM):
Gives permanent storage for the
operating system and fixed data used by
the CPU.
Random-access memory (RAM): Used
for the user’s program. Random-access
memory (RAM) is used for data. This is
where information is stored on the status
of input and output devices and the

Data RAM:
Sometimes referred to as a data
table or register table. Part of this
memory, that is, a block of
addresses, will be set aside for
input and output addresses and
the states of those inputs and
outputs.

Input/Output Unit:
The I/O unit provides the interface
between the system and the outside
world, allowing for connections to be
made through I/O channels to input
devices such as sensors and output
devices such as motors and solenoids. It
is also through the I/O unit that programs
are entered from a program panel. Every

With the relay type,
 Able to switch currents of the order of a few
amperes in an external circuit.
 Not only allows small currents to switch
much larger currents but also isolates the
PLC from the external circuit.
 Relays are, however, relatively slow to
operate. Relay outputs are suitable for AC
and DC switching.
 Withstand high surge currents and voltage

With transistor type,
 Uses a transistor to switch current through
the external circuit.
 This gives a considerably faster switching
action. Strictly for DC switching and is
destroyed by over current and high
reverse voltage.
 For protection, either a fuse or built-in
electronic protection is used.

3. I/O Processing
 The process of updating the input or outputs with
variations caused at respective position, demanding
the change.
 I/O are interfacing between the system and outside
world
 And hence invoke the need of processing the I/O
every cycle.
 So, lets understand the process of I/O processing:
1. Scan the inputs associated with one rung of the

3. I/O Processing
1. Scan the inputs associated with one rung of the
ladder program.
2. Solve the logic operation involving those inputs.
3. Set/reset the outputs for that rung.
4. Move on to the next rung and repeat operations
1,2,3.

3. I/O Processing
 Scan the inputs associated with one rung of the
ladder program.
1. Solve the logic operation involving those inputs.
1,2,3.
R1

3. I/O Processing
ladder program.
 Solve the logic operation involving those inputs.
1,2,3.
R1

3. I/O Processing
ladder program.
 Set/reset the outputs for that rung.
1,2,3.
R1

3. I/O Processing
ladder program.
 Set/reset the outputs for that rung.
 Move on to the next rung and repeat operations
1,2,3.
R1

3. I/O Processing
 Each input point is examined individually and its effect on the
program determined.
 There will be a built-in delay, typically about 3 ms, when each
input is examined in order to ensure that only valid input
signals are read by the microprocessor.
 This delay enables the microprocessor to avoid counting an
input signal twice, or more frequently, if there is contact
bounce at a switch.
 A number of inputs may have to be scanned, each with a 3
ms delay, before the program has the instruction for a logic
operation to be executed and an output to occur.
 The outputs are latched so that they retain their status until
the next updating.
A. CONTINUOUS UPDATING

3. I/O Processing
 Because, with continuous updating, there has to be a 3 ms
delay on each input, the time taken to examine several
hundred I/O points can become comparatively long.
 To allow a more rapid execution of a program, a specific area
of RAM is used as a buffer store between the control logic
and the I/O unit.
 Each I/O has an address in this memory. At the start of each
program cycle the CPU scans all the inputs and copies their
status into the I/O addresses.
 As the program is executed the stored input data is read,
from RAM and the logic operations carried out. The resulting
output signals are stored in the reserved I/O section of RAM.
 At the end of each program cycle all the outputs are
transferred from RAM to the output channels.
B. Mass I/O Copying:

6. Selection of PLC
a. System (task) requirements
b. Application requirements.
c. What input/output capacity is required?
d. What type of inputs/outputs are required?
e. What size of memory is required?
f. What speed is required of the CPU?
g. Electrical requirements.
h. Speed of operation.
i. Communication requirements.
j. Software.
k. Operator interface.
l. Physical environments

System requirements
a. The starting point in determining any solution must be to understand what is
to be achieved.
b. The program design starts with breaking down the task into a number of
simple understandable elements, each of which can be easily described.
Application requirements
a. Input and output device requirements. After determining the operation of the
system, the next step is to determine what input and output devices the system
requires.
b. List the function required and identify a specific type of device.
c. The need for special operations in addition to discrete (On/Off) logic.
d. List the advanced functions required beside simple discrete logic.
Electrical Requirements
a. The electrical requirements for inputs, outputs, and system power; When
determining the electrical requirements of a system, consider three items:
b. Incoming power (power for the control system);
c. Input device voltage; and
d. Output voltage and current.
6. Selection of PLC

Speed of Operation
How fast the control system must operate (speed of operation).
When determining speed of operation, consider these points:
– How fast does the process occur or machine operate?
– Are there “time critical” operations or events that must be detected?
Communication
a. If the application requires sharing data outside the process, i.e.
communication.
b. Communication involves sharing application data or status with another
electronic device, such as a computer or a monitor in an operator’s station.
c. Communication can take place locally through a twisted-pair wire, or
remotely via telephone or radio modem.
Operator Interface
a. If the system needs operator control or interaction. In order to convey
information about machine or process status, or to allow an operator to input data,
many applications require operator interfaces.
b. Traditional operator interfaces include pushbuttons, pilot lights and LED
6. Selection of PLC

c. Electronic operator interface devices display messages about machine status in
descriptive text, display part count and track alarms. Also, they can be used for
data input.
Physical Environment
The physical environment in which the control system will be located. Consider the
environment where the control system will be located. In harsh environments, house
the control system in an appropriate IP-rated enclosure. Remember to consider
accessibility for maintenance, troubleshooting or reprogramming.
6. Selection of PLC

References
[1] W. Bolton, reference book on “Meacatronics”, Fifth Edition, Pearson.
[2] W. Bolton, reference book on “Programmable Logic Controllers”, fourth
edition, Newnes is an imprint of Elsevier

PLC in Automation

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