Friday, October 18, 2013

PLC vs Relay logic circuits

Listed below the comparison of PLC vs Relay logic circuits.
  • Relays usually used for protection applications (Example: over-current , under-voltage ,Etc). But PLC devices are used for control applications (Example:load shedding) and it needs timer circuit.
  •  Modern numerical relays have advance microprocessors but PLC is depend on simple processor that can't support the new requirements of systems.
  • PLC system is more expensive for a simple circuit than Relay system. However, they can both be used for the same application and they both require an analog timer and a timer socket.
  • PLC are used for industrial automation which can be programmed easily but relays as a sequence logic as per process and  hence relays are used for isolation purposes.
  • Wiring ,maintenance modification are difficult with relay but in PLC,user can easily modify the logic to add number of input and outputs.
  • PLC is the much less complex than large relay based system.
  • PLC is the very flexible due to network architecture compare to relay panel.
  • PLC is the virtually self-documenting compare to relay panel.
  • PLC is the very space efficient and less manpower required to install compare to relay panel.
  • PLC is the upfront material costs are similar, but savings on installation, maintenance, and troubleshooting increase rapidly as system size increases compare to relay panel.
  • PLC`s reliability in industry and lab experience demonstrate PLCs comparable to relay system.
  • PLCs huge knowledge base in industry compares to relay panel
  • PLCs training and operating is very easy to handle its hardware and programming is also easy to learning
  • In the PLC have no complicated wiring compare to relay panel.

Relay Logic CircuitPLC
It is an operating on analog system.t is an operating on digital system.
The maximum time any change in input is reflected in any output is t1.The maximum time any change in input is reflected in any output is t1+t2+…+ Tn.
Since this is made from analogue components. It is possible to replace. A logic section without stopping execution of other logic sections if wired correctly.This is made with a digital micro-controller. The micro-controller must be halted to replace a logic section. All other logic sections will stop operation
Since parallel execution of logic sections, all outputs are a function of one set of inputs.Since serial execution of logic sections, all outputs may not be a function of one set of inputs.

What is the logic used in PLC

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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