Block diagram of a simple negative feedback control system.  The box P represents the Plant, the system to be controlled.  Its output y(t) is continuously compared to a reference signal r(t)  representing the desired output.  The difference between the two, called the error signal e(t) is used by a Controller device (C)  to generate a control signal u(t) to change the behavior of the plant.
This file was derived from: Simple feedback control loop.png:
Simple_feedback_control_loop.png: en:User:Ap
derivative work: Chetvorno