Programmable Logic Controller

What is a Programmable Logic Controller (PLC)?

A programmable logic controller is a digital computer used for the automation of electromechanical processes. Some of these processes include the control of machinery on factory assembly lines, lighting fixtures, and amusement rides. Programmable logic controllers are used in many different machines (semiconductor and packaging machines) and industries. They are very different to general-purpose computers, as it is designed for immunity to electrical noise, extended temperature ranges, multiple input and output arrangements, as well as a resistance to impact and vibration. A programmable logic controller is a real time system, which means that output results are produced in response to input conditions within a certain time period, otherwise unintended operation will take place.

Features of a Programmable Logic Controller:

Two distinctive features are:

  • that they can withstand severe conditions which include cold, heat, moisture, and dust, and
  • they have the facility for extensive input/output (I/O) arrangements.
These arrangements connect the PLC to actuators and sensors, which makes it possible to read limit switches, analog process variables (like pressure and temperature), as well as the positions of complex positioning systems. Some programmable logic controllers make use of machine vision to record readings. On the actuator side, PLC's operate analog outputs, solenoids, magnetic relays, hydraulic or pneumatic cylinders, or electric motors. While most input/output arrangements are built into the controller, some have external I/O modules attached to a computer network which plugs into the device.

Programming of a Programmable Logic Controller

Programs are written on a personal computer using a special application. It is then downloaded through either a direct-connection cable or over a network to the PLC. The program is then stored in the programmable logic controller either in battery-backed-up RAM or some other non-volatile flash memory. It is interesting to note that a single controller can be programmed in order to replace thousands of relays.

Even though the fundamental concepts of programmable logic controller programming is the same for all manufacturers, differences in instruction sets, memory organization, and I/O addressing are responsible for the fact that it's programs will never be perfectly interchangeable between the different makers. It might even happen that different models within the same product line of a certain manufacturer might not be directly compatible.


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