Thesis onProgrammable Logic Controller (PLC)

Introduction

Programming is instructing a computer to do

something with the help of a programming

language. A programming language is an artificial

language designed to communicate instructions to

a machine. Programming languages can be used

to create programs that control the behavior of a

machine.

What is PLC ?

A programmable logic controller is a specialized

computer used to control machines process.

It uses a programmable memory to store

instructions and specific functions that include

ON\OFF functions, timing, counting,

sequencing, arithmetic and data handling

It can be programmed, controlled and

operated by a person unskilled in operating

(programming) computers.

Aim and Objectives of PLC

Its purpose is to monitor crucial process

parameters and adjust process operation

accordingly.

Eliminates much of the hard wiring.

It is reprogrammable, reusable and reliable.

Can be connected with plant computer systems

easily.

History

The first PLC systems evolved from conventional

computers in the late 1960s and early 1970s.

In 1990’s different ways of programming PLCs was

introduced and in some cases it took the place of PCs.

The first PLC systems evolved from conventional

computers in the late 1960s and early 1970s.

In the 1980's the lack of standardization were

improved and size of PLCs was also reduced.

In 1990’s different ways of programming PLCs was

introduced and in some cases it took the place of

pc’s.

How PLC works

Receives information's through different sensors.

Sequentially executes programs according to

the information from input sensors.

Gives the solution of problems and sends

appropriate commands as output.

Main Parts of PLC

Input Module

Output Module

Processor

Memory

Power Supply

Programming Device

Diagram of PLC

Programming The PLC

It is mostly based on ladder logic. The sequential

approaches are:

Entry of the ladder logic diagram

Low-level computer-type languages

High-level computer type languages

Functional blocks

Sequential function chart

Concept of Ladder Logic

It is a programming language that represents a program by a graphical diagram.

Programs in this language resemble ladders, with two vertical rails and a series of horizontal rugs between them.

Symbols of ladder logic

-( )- A regular coil,

-(\)- A “ not” coil,

-[ ]- A regular contact,

-[ ]- A not contact

PLC Processor Scan

Three major tasks in a PLC processor scan are

executed in the following order :

Read the physical inputs

Scan the ladder logic program

Write the physical outputs

Application of PLC

Some remarkable sectors using PLC application:

In Traffic light Signal system

In automatic transfer switch

In Nano technology and robotics

In industrial manufacturing

In medical instruments and technologies

In machining, packaging, material handling,

automated assembly etc.

PLC

Designed for extreme

industrial environments

Integrated command

interpreter (proprietary)

No secondary memory

available

Optimized for Single

task

Computer

Designed mainly for data processing and calculation

Optimized for speed

Lost of secondary memory available

Multitasking capability

PLC Vs. Computer

Advantages

Flexibility

Large Quantities of Contacts

Lower Cost

Visual Observation

Reliability and Maintainability

Disadvantages

Single-function applications do not pay to use a

PLC that includes multiple programming

capabilities.

Their operational sequence is seldom or never

changed.

if the power fails, the system stops.

Future of PLC

Continuation of new product developments and

making integration of PLCs with other control.

PLC technology include features such as GUIs

and more human-oriented machine interfaces.

New PLC instructions are developed to add

intelligence to a controller.

Concept of the flexible manufacturing system

(FMS) will determine the control philosophy of the

future.

Conclusion

PLC applications are typically highly customized

systems. PLC is low compared to the cost of a specific

custom-built controller design. In general, PLCs are

more robust and require less maintenance, thus

making the things they control work better, in spite of

the environment. Overall, PLCs appear to be an

excellent solution for many different problems.

THE END