Glossary · Control theory and process control
Control loop
Also known as: Feedback loop, Closed control loop
German: Regelkreis
In control engineering, a control loop is the closed signal path formed by the controlled system, the measuring device, the controller and the actuator, in which the measured value is fed back and compared with the setpoint.
- Control theory
In one sentence
A control loop is the closed path of sensor, controller, actuator and process in which the measurement is fed back and compared with the setpoint.
Example
The level control loop of a buffer tank consists of the level transmitter, the PID block in the PLC, the outlet control valve and the tank itself.
How it applies
- Engineering: Every element of the loop contributes dynamics: sensor filtering, bus Latency, controller cycle time, actuator speed and the Controlled system itself. Loop performance is limited by the slowest element.
- Maintenance: Loops degrade silently through sticky valves, sensor drift or changed process conditions. Loop performance monitoring tools compare actual behavior with expected behavior.
- Documentation: A loop diagram or loop sheet shows all elements with their tags, signal ranges and wiring. Keep it in sync with the P&ID and the controller configuration; it is a key document for commissioning and troubleshooting.
Control loop vs. closed-loop control
Closed-loop control is the principle of correcting an output from measured feedback. A control loop is one concrete instance of that principle, with specific devices and tags. A plant has one control concept but hundreds of loops.