Digital systems
Feedback

Principal schematic of feedback control system is shown on the Figure above. The input of the whole system is a desired value (w) and an output is the actual value (y). The subtraction element evaluates an error e = w – y, what is input to a controller R. The controller processes the error and outputs a control variable (u), which acts via actuators to the controlled system (plant), S. The controller tries to minimize the error, for the actual value y to approach the desired w.

In practice, often a discontinuous controllers are used, e.g. a two step (thermostat), three or more step, various variants of PID controllers (PI, P, PII2). There are also several types of much more complex controllers.