Programmable Logic Controller (PLC)
A programmable logic controller (PLC) is a ruggedised industrial computer that cyclically executes a stored control program to monitor inputs and control outputs in machines and processes. It comprises a CPU, power supply and digital and analogue input/output modules, either in a compact unit or a modular rack, with remote I/O connected over fieldbus or industrial Ethernet. In each scan cycle it reads inputs, executes the program logic and updates outputs. PLCs are used for machine automation, pump stations, conveyors, packaging lines and process skids. Selection parameters include I/O count and type (for example 24 V DC or 4–20 mA), memory, scan time, communication protocols such as Modbus, PROFINET or EtherNet/IP, redundancy and environmental ratings. Programming languages are standardised in IEC 61131-3, and safety PLCs are certified to IEC 61508.
Examples
- Pump station control with level and pressure interlocks
- Packaging line sequencing
- Redundant PLC on a process skid