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.
