Define Programmable Logic Controller (PLC) and its function?

A programmable logic controller (PLC) is essentially a user friendly micro-processor based microcomputer, consisting of hardware and software, designed to control the operation of Industrial equipment and processes. An important advantage of the PLC is that it can be easily programmed and reprogrammed. PLC has tremendous impact on Industrial control and instrumentation due to its high reliability and flexibility at the design and implementation stages.

The decreasing cost of microprocessor with increasing facilities in them is acting as catalyst in their widening scope of applications. In recent years, PLC are being used in place of electromechanical relays or cam operated logic controllers to control fluid power systems. Modern day PLCs are developed into a sophisticated and highly versatile control system component capable of performing complex mathematics functions and operate at fast microprocessor speeds. Some leading PLC manufacturers are ABB, Allen Bradley, Honeywell, Siemens, GE Fanuc, Mitsubishi, Modicon, Omron etc.

PLC – Defined

PLC can be defined as digital electronic device that uses a programmable memory to store
instructions and to implement functions such as logic, sequencing, counting, timing and
arithmetic in order to control machine, processes and instrumentation

PLC is user –friendly digital computer used for making logic decisions and providing output. It
consists of solid state digital elements and is a replacement for hard-wired electro-mechanical
relays to control pneumatic systems.

The term „programmable logic controller‟ is defined as follows by IEC 1131 ( PLC standard)
part 1

“A digitally operating electronic system, designed for use in an industrial environment, which
uses a programmable memory for the internal storage of user- oriented instructions for
implementing specific functions such as logic , sequencing , timing , counting and arithmetic, to
control through digital or analog inputs and outputs, various types of machines or processes.
Both the PC and its associated peripherals are designed so that they can easily integrated into an
industrial control system and easily used in all their intended function”

PLC is quite similar to digital computers. They also have certain features which are specific to
logic controllers. They are

  1. PLC are rugged and designed to withstand vibrations, temperature , humidity and noise
  2. The interfacing for input and output is part of the controller
  3. They are easily programmable and primarily use logic and switching functions