Chapter 1

Introduction to the Programmable Logic

Controllers

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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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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.

1.1.2 Capacity of Industrial PLCs

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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

1.1.2 Capacity of Industrial PLC

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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.

1.1.2 Capacity of Industrial PLC

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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.

1.1.2 Capacity of Industrial PLC

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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 modulewhich 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/OFunctions (PID, Recipes, Data Storage)Programming LanguageRedundancySafetyStandardizationPrice (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 8-32 MicroLogix

Micro 32-250 CompactLogix

Small 250-4k ControlLogix

Medium 2k-16k Momentum (Mod)

Large 8k-32k Quantum (Mod)

Very Large >32kPLC-5

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Type of PLC I/O

Remote I/ONetworkingExpandabilitySpecialized I/O

– High Speed Counters– Motion Control (PID)– Basic– Communications– TTL Digital I/O– RTD– Thermocouple