(Programmable Logic Controllers)

PLC

Contents

IntroductionHistorical BackgroundPLC functionPLC vs ComputerBasic components of PLCLadder logic diagramAdvantagesConclusionReferences

Introduction

A digitally operating electronic device which uses a programmable memory for the internal storage of instructions for implementing specific functions such as logic, sequencing, timing, counting, and arithmetic to control, through digital or analog input/output modules, various types of machines or processes.

PLC (Programmable Logic Controller)

Historical Background1968 Programmable concept developed

1969 Hardware CPU controller, with logic instructions.

1974 Use of several (multi) processors within a PLC - timers and counters; arithmetic operations.

1976 Remote input/output systems introduced

1977 Microprocessors - based PLC introduced

1980 Intelligent I/O modules developedEnhanced communications facilitiesEnhanced software features.

1983 Low - cost small PLC’s introduced.

1985 on Networking of all levels of PLC, computer and machine using SCADA software.

Birth of PLCProblem faced before the emergence of PLC:-Lack of flexibility in operation.Problem in Troubleshooting.

PLC Function

A PLC performs the functions of conventional relays, timers, counters.

V/SComputer

PLC

Basic Components There are six basic components in a PLC

system: A PLC processor, or controller I/O (Input /Output) modules Chassis or backplane Power supply Programming software A network interface

Processor or Controller

Stores the control program and data in its memory.

Reads the status of connected input and output devices.

Executes the control program.Commands connected outputs to change

state based on program execution.

Input Modules

Physically connect to field devices • Input modules convert electrical signals

coming from input field devices to electrical signals that the processor can understand.

Input modules interface directly to devices such as switches and temperature sensors.

Output Modules

Output modules interface directly to output devices such as motor starters and lights.

Output modules take digital signals from the Processor and convert them to electrical signals.

Output modules take a signal from processor and convert it to a signal that a field device needs to operate.

All PLCs need some method of communication between the controller, I/O and communications modules.

Chassis/Backplane

Power supply A power supply is needed to provide power to the

PLC and any other modules.

Programming Software Software that runs on a PC is required to configure

and program PLCs.

Network InterfaceMost PLCs have the ability to communicate

with other devices, it lets an operator to enter commands into the PLC, or I/O devices that is located in a remote location from the PLC.

Input contacts(switches, relays))

Output loads(alarms, lights)

Logical inversion(NOT)

Logical AND operation

Logical OR operation

Ladder logic diagram

Advantages More flexibilityLess space, more compactHigher reliability (less mechanical parts)Modular, can be easily connected to

computer systems and to hardware

CONCLUSION

Now a days PLC’s are used in large scale in the industries due to it’s compact size, higher reliability and faster response.

ReferencesWikipedia.Books:- Programmable logic controller (William Bolton) Programmable Logic Controller (Gleen j. sartori)

Thank you…