PLC (Programmable Logic Controller)

A programmable logic controller is a digital computer used for automation of electromechanical processes, such as control of machinery on factory assembly lines, amusement rides, or light fixtures. PLCs are used in many industries and machines. Unlike general-purpose computers, the PLC is designed for multiple inputs and output arrangements, extended temperature ranges, immunity to electrical noise, and resistance to vibration and impact. Programs to control machine operation are typically stored in battery-backed-up or non-volatile memory. A PLC is an example of a hard real time system since output results must be produced in response to input conditions within a limited time, otherwise unintended operation will result.

                                                            

Programmable logic controllers provide dependable, high speed control and monitoring demanded

by a wide variety of automated applications. Before the automotive industry discovered the advantages of PLC’s, the process of modifying relay circuitry was a headache inducing endeavor. In the past, annual car model changes forced plant engineers to constantly modify production equipment managed by relay circuitry. In some cases, the engineers had to scrap entire relay controlled panels and replace them with completely redesigned systems. Now, PLC’s allow engineers to implement numerous manufacturing changes with relative ease, which reduces changeover costs and downtime.  Prior to PLC’s, cont-actor or relay controls solved many of these control tasks. This is often referred to as hardwired control. Electricians had to design circuit diagrams, specify and install electrical components, and create wiring lists before wiring the components necessary to perform a specific task. Design errors would force the electrician to trace the wires to identify the problem and then reconnect the wires. A change in function or a system expansion required extensive component changes and rewiring. Now, PLC software programming makes wiring changes between devices and relay contacts easier. Although hard wiring is still necessary for connecting field devices, it’s less intensive than before.