A method of control, used for many different types of control system, in which a portion of the output derived from a system is fed back to the input circuit in order to control the output signal in a desired manner (see diagram).

The external signal applied to the loop is the loop input signal; the controlled signal that is output by the loop is the loop output signal. The portion of the signal fed back to the input circuit is the loop feedback signal. The feedback signal is mixed with the input signal and produces a loop actuating signal, which is used to produce the controlled output. The transmission path between the loop input and the loop output is the through path; the path between the loop actuating signal and the loop output is the forward path.

The forward transfer function is the mathematical relationship between the actuating signal and the controlled loop output. The value of the loop actuating signal is determined by the loop error, which is the difference in value between the actual output and the desired value. The actuating transfer function is the relationship between the loop input signal and the loop actuating signal. The difference transfer function is the relationship between the loop error and the loop input signal. The loop feedback signal and the loop input signal are mixed in a suitable manner to maintain the desired transfer functions. In many systems the loop error is used as the loop actuating signal.

Feedback control loop

http://www.electronics-tutorials.ws/systems/feedback-systems.html A tutorial on feedback systems