What is a plc used for?

Answer 1

Plc's are used for a variety of applications. Namely automation and machine industries as it is cost effective, time effective and a much more reliable source of "computer" than the traditional relay controllers. They are capable of withstanding harsh environments such as heat, cold, moist areas. Programmable Logic Controllers are obviously programmable to carry out certain program executions using 'ladder logic' in order to activate certain outputs when the condition are met with the inputs.

Added

'Programmed Logic Controllers' keep the cost of cabling down and simplify interlocks and logic control.

A complex control panel, can control a complex system and the only things connecting them, will be a power supply line and a small data cable.

Heavy duty power cables can be kept short and local to the machinery being controlled. The switches and indicator lamps only deal with minute control signals. The function is passed to a local PLC receiver via a small data cable.

Instead of a complex system of relays and wiring, interlocks are controlled by a computer program, which enables changes to be made without re-wiring.

For example, certain pump motors can only be started when the correct valves are open. This can be dealt with by a program instead of physical wiring.

Answer 2

PLCs control machine actions are based on what events are or are not happening. Logic based on Boolean Algebra. The Programmable Logic Controller "watches" switches and internal relays and turns on and off devices like valves and motors. It scans constantly through its' program and makes logical decisions about the system. There are timers and counters to enable it to repeat functions correctly over and over. It can be hooked to a computer and be its' muscle.