PLC ( Programable Logic Controllers)

KHADIJAH MUSHTAQ (CIIT/FA14-BEE-048/ATK)HIKMATULLAH (CIIT/FA1X-BEE-0XX/ATK)TALHA NADEEM (CIIT/FA14-BEE-007/ATK)

Topical Layout• Types and Purpose of Control

• Evolution of logic controller devices

• Leading PLC manufacturers around the world

• PLC architecture

• PLC operation

• Criteria for PLC selection

• Advantages of PLC

Types of Control• Temporal -- control based in time

• State -- control based in state level

• Hybrid – both temporal and state

Purpose of Control

• On-Off Control• Sequential Control• Feedback Control• Motion Control

Evolution of logic controller devices• Relay controllers

• Semi-conductor logic controllers

• Programmable Logic Controllers (PLC)

RELAY CONTROLLERS

• Large dimensions

• Slow

• Noisy

• Requires a lot of energy to work

• Sensible for dirt

SEMI-CONDUCTOR LOGIC CONTROLLERS

• Fast

• Small

• Reduced energy

• Silent

• Hard to adapt to another controlling task

PROGRAMMABLE LOGIC CONTROLLERS (PLC)• All advantages of the semi-

conductor logic controllers

• Easy to adapt (re-programming)

• Real-time working

• Minimal maintenance

• Low costs

PLC Vs. Microcontrollers

• Dissimilar to Microcontrollers:• Intended for Industrial Applications• I/O Designed to interface with Control Relays• Emphasis on Maximum Reliability

Areas of application• Water handling

• Agriculture

• Traffic control

• Food industry

• Manufacturing / Machining

• Mining

Leading PLC manufacturers (US)• Allen Bradley

• Gould Modicon

• Texas Instruments

• General Electric

• Westinghouse

• Cutter Hammer

• Square D

Leading PLC manufacturers (EU)• Siemens

• Klockner & Mouller

• Festo

• Telemechanique

Leading PLC manufacturers (Japan)• Toshiba

• Omron

• Fanuc

• Mitsubishi

Criteria for PLC selection• Number of logical inputs and outputs.

• Memory

• Number of special I/O modules

• Scan Time

• Communications

• Software

PLC architecture

Program memory

Data memory

Processor

Communicationunit

Input module

Programming unit

Output module

Systemmemory

Processor• The main function of the microprocessor is to analyze data coming

from field sensors through input modules, make decisions based on the user’s defined control program and return signal back through output modules to the field devices.

• Field sensors: switches, flow, level, pressure, temp. transmitters, etc. Field output devices: motors, valves, solenoids, lamps, or audible devices.

Memory• System memory

• ROM (Read Only Memory) • PROM (Programmable Read Only Memory) • EPROM (Erasable Programmable Read Only Memory)• EEPROM (Electric Erasable Programmable ROM)

• Program/Data memory• RAM (Random Access Memory)

Input/Output modules• The I/O interface section of a PLC connects it to external field

devices.

• The main purpose of the I/O interface is to condition the various signals received from or sent to the external input and output devices.

• Input modules converts signals from discrete or analog input devices to logic levels acceptable to PLC’s processor.

• Output modules converts signal from the processor to levels capable of driving the connected discrete or analog output devices.

Input Modules

DC INPUT MODULESTANDARD DIGITAL INPUT S IGNALS: 24V

AC INPUT MODULESTAN DARD A NA LOG I NPUT S IGN A LS : 0 -10V

Current limiting resistor

Opto-isolator

Buffer, filter,

hysteresiscircuits

DC input Processor

Used to drop the

voltage to logic level

Prevents voltage transients from damagigng the processor.

Helps to reduce the effects of electrical noise.

Rectifier, Resistor Network

Opto-isolator

Buffer, filter,

hysteresiscircuits

AC input Processor

Converts the AC input to DC and drops the voltage

to logic level

Prevents voltage transients from damagigng the processor.

Helps to redice the effects of electrical noise.

Output module

Standard Analog Output signals: 4-20mA; 0-5V; 0-10V

TTL circuits

Opto-isolator

AplifierProcessor Output

Communication unit

Manages the communication between the PLC and• the controlled process,• an other PLC,• a PC,• different peripheries.

Used ports:• Serial (RS 232, RS 422, RS 485)• Ethernet

PLC Operation Scheme

Power-on the PLC

Clear the output table

Input links Input table update

PLC program

Instruction 1Instruction 2…Instruction n

Output table update

Output links

…

…

Input signals

Output signals

Advantages of PLC• The less cable the less cost.• Higher reliability and performance.• Smaller installation and service costs.• The ability of teleprogramming and telemonitoring.• Changing resident PLC programs - uploading/downloading from a

supervisory controller (Laptop or desktop computer). • Forcing I/O points and memory elements from a remote terminal.

Reference• Skvarenina/DeWitt, Electrical Power and Controls, Pearson-Prentice

Hall, 2004

“Don’t Make Excuses,Make Improvements.”- Tyra Banks

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