How Does a Programmable Logic Controller Work?

by Isaiah David

Programmable Logic Controller Overview

A programmable logic controller (PLC) is a highly specialized computer used for manufacturing, packaging, and other industrial situations. Programmable logic controllers are designed to be able to take information from a variety of different sensors and use it to control a variety of different machines. PLCs need to be able to accurately control a process in real time, so they are very fast. When a PLC receives an instruction, it responds instantly, issuing controls to its motors.


PLCs receive information from a variety of different sensors. Many of the sensors are actually simple switches. For example, if a PLC in a furniture plant had to move a board down a conveyor belt into a certain position, it might have a pressure switch at the end of the conveyor belt. When the board reached that position it would push the switch, letting the PLC know that it was in position. Other types of sensors a PLC might use are pressure gauges which measure force, accelerometers to measure motion and light-activated switches. PLCs can also use more complex inputs such as cameras or microphones designed to recognize certain patterns.


Programmable logic controllers can be used to control almost any automatic process. A PLC can turn on and off the power to lights and other electronic equipment, for example. In industry, however, PLCs are usually used to drive machines. A plant might use a PLC to pour molten metal, to move a saw, to attach rivets to a car frame, or to open and close a valve. A PLC will usually do a variety of simple tasks in a specific order. For example, a PLC used in wood work might first make rectangular cuts in a board, then make curved or angled cuts for decoration, then sand the piece and finally spray paint it. This could involve several machines connected together by a conveyor belt and all controlled by the same PLC.

About the Author

Isaiah David is a freelance writer and musician living in Portland, Ore. He has over five years experience as a professional writer and has been published on various online outlets. He holds a degree in creative writing from the University of Michigan.

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