PLC (programmable logic controller)

A PLC (programmable logic controller) is an industrial computer that controls the operation of machinery and installations.

The PLC is the “brain” of an installation. It works in a continuous cycle:

1. it reads signals from sensors, such as temperature, pressure, tank level or valve position,
2. it runs the program written into it by the automation engineer and, on that basis, decides what needs to be done,
3. it sends commands to devices, such as open a valve, start a pump or change the agitator speed.

This cycle repeats very quickly, many times per second, so the controller responds to changes in the installation almost instantly.

PLCs are built to operate in harsh industrial conditions: they withstand temperature fluctuations, vibration, moisture and electrical interference. They are usually mounted in control cabinets. The PLC works together with operator panels (HMIs) and supervisory systems (SCADA), through which people can see how the installation is running and issue commands.

Example: In a dog food plant, a PLC runs the sauce preparation process. It reads the water level in the tank from a sensor and closes the inlet valve when the level reaches the setpoint. It then starts the agitator, opens the steam valve and monitors the temperature. When the sauce reaches 85°C, the controller starts timing the mixing, and once that time has elapsed, it starts cooling and the pump that transfers the sauce onward.