Closed-loop

A closed-loop controller uses feedback to control states or outputs of a dynamic system. Its name comes from the information path in the system: process inputs (e.g. voltage applied to a motor) have an effect on the process outputs (e.g. velocity or position of the motor), which is measured with sensors and processed by the controller; the result (the control signal) is used as input to the process, closing the loop.

Closed-loop controllers have the following advantages over open-loop controller:

• disturbance rejection (such as unmeasured friction in a motor)
• guarranteed performance even with model uncertainties, when the model structure does not match perfectly the real process and the model parameters are not exact
• unstable processes can be stabilized

To obtain good performance, closed-loop and open-loop are used simultaneously; open-loop improves set-point (the value desired for the output) tracking.

The most popular closed-loop controller architecture, by far, is the PID controller.

See also

• Controller (control theory)
• Open-loop controller
• Process control
• Control theory