Plc Basics

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PLC BasicsMuhammad Atta ul GhaffarSenior Electrical Engineer

CENTURY PAPER & BOARD MILLSPAKISTAN

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Programmable Logic ControllerA digital electronic device that uses a programmable

memory to store instructions and to implement specific functions such as logic, sequence, timing, counting and arithmetic to control machines and process. “

A PLC is a computer designed to work in an industrial environment.

It uses a programmable memory to store the instructions and specific functions that include On/Off control, timing counting, sequencing, arithmetic and data handling.

PLCs are equipped with special input/output interfaces .

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PLC System

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What is Control?“ CONTROL is the process in a system in

which one or several input variables influence other variables “

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A Simple Overview of Control System

CONTROL

SYSTEM

PLANT

INFORMATION

COMMANDSACTUATORS

SENSORS

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Types of Control System

CONTROLSYSTEM

OPEN-LOOPCONTROL SYSTEM

CLOSED-LOOPCONTROL SYSTEM

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Open Loop Control SystemIn an open loop system, the controller is told

what the desired process output is, and the controller sends a command to the actuator to try and achieve this. The process is shown below as a block diagram:

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Closed Loop Control SystemSystems that utilize feedback are called

closed-loop control systems. The feedback is used to make decisions about changes to the control signal that drives the plant. An open-loop control system doesn't have or doesn't use feedback.

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PLC and Conventional Control SystemThe essential difference between

programmable control and traditional control technology may be summed up as follows:

The functions are no longer determined by the wiring, but rather by the program

Programming is simplified to enable symbols familiar to the control engineer to be used (contacts or logic graphic symbols)

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Hard Wire & PLC Wiring diagramL

N

S1

S2

K1

S1 S2

PLC

K1

Hardwire PLC

0 V

24 VDC

K1

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ComparisonHardwired control systems

The functions are determined by the physical wiring.

Changing the function means changing the wiring

Can be contact-making type (relays, contactors) or electronic type (logic circuits)

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ComparisonProgrammable control system

The functions are determined by a program stored in the memory.

The control functions can be changed simply by changing the program.

Consist of a control device, to which all the sensors and actuators are connected.

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History of PLCDuring the late 1960s, General Motors (USA) was

interested in the computer application to replace the hardwire systems.

Bedford Associates (Modicon) and Allen Bradley responded to General Motors.

The name given was “Programmable Controllers” or PC.

Programmable Logic Controller or PLC was a registered trademark of the Allen Bradley.

Later, PC was used for “Personal Computer” and to avoid confusion PLC for “Programmable Controller” and PC for a personal computer

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PLC History

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PLC History

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PLC Timeline

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Advantages of PLC compared to HardwireImplementing changes and correcting errors

Pilot run - trial / test run

Visual observation - online monitoring

Speed of operation

Reliability

Documentation

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Basic Structure of PLCPOWERSUPPLY

CENTRALPROCESSINGUNIT (CPU)

INPUTMODULES OUTPUT

MODULES

MEMORY(EPROM/RAM)

PG/PC

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PLC Inputs & Outputs (I/Os)

PLC

USERPROGRAM

(LOGIC)

USERPROGRAM

(LOGIC)

InputDevices

OutputDevices

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Input/output ModulesDigital input modules adapt digital signals e.g. from

proximity sensors

Digital output modules convert the internal signal level of PLC into digital process signals e.g. relays

Analog input modules adapt analog process signals e.g. from transducers

Analog output modules convert internal digital values of the PLC to analog process signals e.g. temperature controller

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Central Processing Unit (CPU)What is a CPU?

The “brain” of a PLC

Controlled by a program called the executive or operating system (OS)

The executive is a collection of supervisory programs permanently stored in memory

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CPUFour basic types of CPU operations:

Input and output operation

Arithmetic and logic

Reading or changing contents of memory locations

Jump operations

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CPU

PROCESSOR

ACCUMULATOR

TIMERS, COUNTERS,

Memory

PII

INTERNALPROGRAMMEMORY

(RAM)

PIQ

SERIALINTERFACE

MEMORYSUBMODULE

(EPROM/EEPROM/

RAM)

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CPUThe CPU reads in input signal states, processes the

control program and controls the outputs.

The CPU provides internal Memory, timers and counters.

Restart procedure can be preset and errors can be diagnosed using the CPU’s LEDs.

The overall Reset on the CPU is used to delete the contents of the RAM.

A PG or a Memory sub-module is used to transfer the control program to the CPU.

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Power Supply ModuleThe power supply module supplies the

operational voltage for the PLC and provides backup for the RAM with a battery

Backup battery

The backup battery maintains the program and data when the PLC is switch off

The backup battery has a service life of approximately 2 years

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How does a Programmable Controller Works?

PowerSupply

ProgramMemory

ProcessorInput modules

Output modules

24 VDC

GND

Sensors

Actuators / Annunciators

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