Chapter 1
Introduction to the
Programmable Logic
Controllers
1.1 Introduction
Of all the devices that are used to
control manufacturing operations,
the Programmable Logic Controller
(PLC) is one of the most important.
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1.1 .1 Generation of PLC
 The first PLC were introduced in the
early 1960s, mainly by the automobile
industry.
 Up until then the automatic control of
manufacturing equipment was achieved
by using hundreds , and even thousands ,
of relays enclosed in metal cabinets.
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1.1 .1 Generation of PLC
 A lot of time, and errors often occurred
when making connections.
 For these reasons, control engineers
developed a computerized programmable
system to replace the relay racks.
 The annual automobile-model changes
required frequent modifications to the
production lines and their associated
relay-control systems.
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1.1 .1 Generation of PLC
 Because the control systems were complex,
the modifications took a lot of time, and
errors often occurred when making
connections.
 For These reasons, control engineers
developed a computerized
programmable system to replace the
relay racks.
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1.1 .1 Generation of PLC
 Because the control systems were complex,
the modifications took a lot of time, and
errors often occurred when making
connections.
 For These reasons, control engineers
developed a computerized
programmable system to replace the
relay racks.
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1.1 .1 Generation of PLC
 Computers that had previously been used to
do accounting jobs were modified to
respond to the needs of industry .
 Little by little, the techniques were
improved and more users of the new
technology were found. however, a full
decade went by before the new concept was
systematically adopted by manufacturers.
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1.1.2 Capacity of Industrial PLCs
 In the beginning, PLC were mainly used to
replace hard-wired physical relays.
 This offered many advantages because the
PLC took up less space than conventional
relay cabinets and consumed much less
energy.
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1.1.2 Capacity of Industrial PLC
 In the beginning, PLC were mainly used to
replace hard-wired physical relays.
 This offered many advantages because the
PLC took up less space than conventional
relay cabinets and consumed much less energy.
 they were programmable and equipped with
LED indicators that made it easy to check the
operation of the control system and to
diagnose problems
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1.1.2 Capacity of Industrial PLC
 In the beginning, PLC were mainly used to
replace hard-wired physical relays.
 This offered many advantages because the
PLC took up less space than conventional
relay cabinets and consumed much less
energy.
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1.1.2 Capacity of Industrial PLC
 Today, thanks to the evolution in
electronics and computer technology, the
performance of PLC is impressive.
While they are still used to replace relays,
PLCs can now perform mathematical
operations and control and regulate
industrial processes.
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1.1.2 Capacity of Industrial PLCs
 For instance, they can regulate
temperature, pressure, flow rates, motor
drives, and so forth.
 PLCs can now communicate with each
other as well as with a central computer.
The latter can collect data, change the
operating parameters, and even modify
the PLC programming.
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1.1.3 Elements of a Control System
A central processing unit (CPU)
Which is a computer that can simulate the
required relay contact and relay coils, as well
as the connections between them.
An input module
which serves as an interface
between the actual control
devices and the CPU.
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1.1.3 Elements of a Control System

An output module, which serves as an
interface between the CPU and the actual
device that are being controlled.

A programming unit consisting of a
keyboard and monitor to program the CPU.
It enables us to select different types of
“relays” and “contacts” that the computer
can simulate, as well as the way the are to be
connected.
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1.1.3 Elements of a Control System
 A power supply that furnishes the
power needed by the CPU, by the
input/output (I/O).
 Modules, and by the programming unit.
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1.2 Functions and Characteristics
 Stepping Sequence Control:
Motor Control Center
Machine Tool Electrical Control
Protection for power supply system
Boiler Safety Protection system
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1.2 Functions and Characteristics
 Timing Control
One equipment acts following
another’s equipment’s action after
certain timing control.
Timer Relay.
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1.2 Functions and Characteristics
 Conditioning Control
All the Other equipment are not
allowed to act during certain
equipment’s action.
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1.2 Functions and Characteristics
 Counting Control
The next action can begin only after
attaining given counting number for a
certain action.
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1.2 Functions and Characteristics

A/D and D/A transformation
A/D: Analog signal to Digital signal;
Input to Computer.
D/A: Digital signal to Analog signal;
Output analog signal to site equipment.
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1.2 Functions and Characteristics
 Data Transaction
Further transaction for the site data
according to the requirements of
computer.
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1.2 Functions and Characteristics

Communication and Network
1) Remote Control;
2) PC Link between the same or different
types of computers.
3) Host Link to Host Computer.
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1.2 Functions and Characteristics
 Spare Control
More than one computers have the
control function in order to improve the
reliabilities of the control system.
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1.2 Functions and Characteristics
 Monitoring and Control Function
Controls are used to the most important
function.
Monitoring is used to identify, remember
or interlock certain parameters.
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1.2 Functions and Characteristics
 Other Functions
1) Indication;
2) Printing;
3) Alarming;
4) Data and programs copying
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1.3 PLC Selection Criteria
PC or PLC?
Number of I/O (memory also)
Type of I/O
Functions (PID, Recipes, Data Storage)
Programming Language
Redundancy
Safety
Standardization
Price (Hardware and Complements)
Reputation
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1.3 PLC Initial System Selection
40% extra space
Fixed vs. Variable
DC vs. AC
Local vs. Multipoint
Special Function
Operator Interface
Documentation
70% input 30% output
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Advantages of DC vs. AC
Smaller Wires
Less Insulation
Smaller Conduit
Smaller Switches
Safety
Smaller Connectors
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1.4 PLC Classification by I/O
Nano
Micro
Small
Medium
Large
Very Large
8-32
MicroLogix
32-250
CompactLogix
250-4k
ControlLogix
2k-16k
Momentum (Mod)
8k-32k
Quantum (Mod)
>32k PLC-5
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Type of PLC I/O
Remote I/O
Networking
Expandability
Specialized I/O
–
–
–
–
–
–
–
High Speed Counters
Motion Control (PID)
Basic
Communications
TTL Digital I/O
RTD
Thermocouple
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