Friday, October 18, 2013

What is the logic used in PLC

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The first thing needed for automating any industrial process or machine is to determine the sequence of operation or events which are specific to its operation. This sequence is then organized into a set of logic functions.

Logic functions are of two types:

->Combinatory: Where results will depend on only on the present state of the inputs.
->Sequential: Where results depends on the present and past state of the inputs

Then this Logic scheme is actually turned into a physical system using the essential building blocks of the particular technology selected, i. e. Mechanical, Fluidic, Pneumatic, Electromechanical, Electronics.

There are three basic system options which might be open to a design engineer.

->Relay Logic:
It has for many years been the work horse of most electrical installations.

It was simple for small systems, hence cost benefits due to wide range of available coil voltages.

As the number of relays increases, it requires larger physical area, coupled with costly enclosures, the labour charges, the schematic and connection diagrams, escalates the final cost

->Wired Logic

->Programmable Logic
  • Improved installation time
  • eliminate the need for extensive wiring of timers, relays and other components
  • Improved flexibility
  • enable control system changes simply by reprogramming
  • Much more compact than relay control panels, yet enables complex, high-level control
  • Improved reliability
  • Selection of the most suitable system is largely dependent on the application, availability and acceptability.
Hence programmable logic is used in the controller,it is named as programmable logic controller(PLC).


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