PLC

What is a PLC?

A PLC (programmable logic controller) is an industrial computer that controls machines and processes with a program. Push buttons, sensors and limit switches are wired to its inputs; contactors, valves and lamps to its outputs. The control logic that would be wired with contactors and relays is written as a program in the PLC.

What does a PLC do and where is it used?

PLCs are used in factory automation, packaging and filling machines, conveyor lines, pump stations, water treatment plants, lifts, buildings and almost every modern machine. Because it runs on a program, changes do not require rewiring, and it easily handles timing, counting, calculation and communication.

PLC structure and programming

A PLC has a power supply, a processor (CPU), digital and analog inputs, and relay or transistor outputs. Compact PLCs have all of these in one housing; modular PLCs add separate modules. PLCs are usually programmed in ladder logic, which looks like a control schematic and is easy to learn for anyone who knows control circuits.

What to watch out for

The voltage and type of the inputs and outputs (24 V DC, relay output and so on) must suit the devices connected. Safety functions such as emergency stop are not left to software alone; they are also done in hardware. Output contacts have limited current; large loads are switched through contactors.

Terminals

PLC

Supply
Supply for buttons and sensors (DC)
Input common, connected to M
Inputs
Relay output common
Outputs (relay contacts)
Network terminal: for the operator panel (HMI)

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