Any control system in which the output has an effect upon the input quantity. To improve the performance of a system with respect to maintaining the output quantity as close to the desired quantity as possible, a person is replaced by a mechanical, electrical, or other form of comparison unit. The comparison between the reference input and the feedback signals results in an actuating signal that is the difference between these two quantities (see diagram). The actuating signal maintains the output at the desired value.

Closed-loop control system