Showing posts with label PLC. Show all posts
Showing posts with label PLC. Show all posts

Friday, 18 November 2011

PROGRAMMABLE LOGIC CONTROLER

PLC



Controlled engineering has evolved over a long period of time. in the Past human were the controlling device for a system. More recently electricity has been used for control and the early control is done by the help of electrical relays. The relays allow the supply can switched ON and OFF electrically without out a mechanical switch. Nowadays relays are common to make simple logical control decisions. The development of low cost computes discovers the most recent control system, Programmable Logic Controller (PLC).The advent of PLC began in the 1970’s.Today PLC is most common for controlling processes.


The PLC is used as a standard in the field of automation for control. It replaces earlier relay controls and taken over many additional functions.


The PLC is designed by a computer to operate in industrial environment and arranged with special input, output and a control programming language for use in machine.


The PLC is designed to controlling a system, but it is finding many and more complex applications. The structure of PLC and computer architecture are constructing based on the same principles, so the relay performing not only the relay switching but also counting ,processing ,calculating and comparing of analog signals.


PLC is a microprocessor based computer, which is user friendly and performs control functions of different types and of different levels of complexy. A PC can be used as PLC with input sensing devices such as push button , switches ,.etc; input according some conditions. A program is needed to process Input and the PLC turn ON and OFF load devices by means of Ladder Diagram. It essential to draw a Ladder Diagram of controlling of devices with laptop or PC. The drawing is converted into computer language and runs as user rogram

BLOCK DIAGRAM OF PLC

BLOCK DIAGRAM OF PLC SYSTEM



 
 
 
 
 
 
 
 
 
 
 
 
Simplified Block Diagram of a PLC system. It contain main parts:

  • CPU(Central Processing Unit)
  • Input sensing devices
  • Input interfaces
  • Output interfaces
  • Programmer/Monitor

PLC

LADDER DIAGRAM

Ladder Logic language is a PLC programming language, means by which the user communicate information to PLC. Appearance of the Ladder Logic Language designed as a wired relay system, so it termed as a Graphical Programming Language. It is a symbolic set instruction used to create the controller program (representation of contacts and coils are the basic symbols of the ladder diagram). The corrective arrangement of these symbols made possible to create a desired control logic that is to be entered into the memory of the PLC. Ladder program prepared for a specific function is composed of symbols of contacts and coils and it is also called contact symbology .The Ladder Program controls the output based on input conditions. The control is done according to what is is referred by the ladder rung and the rung is consist of set of input conditions, represented by the contact symbols, at the end of rung output is represented by the coil symbols .Each contacts and coils are represents with an address number and which identifies what is being evaluated and controlled.

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