Digital systems
Numerical control

Let us describe the process of a feedback control (see Fig. 5.2d). The controller R evaluates the controlled actual output value (y), measured at the controlled plant (S), and compares it with the desired input value (w). Resulting difference is an error signal (e) (equal to subtraction e = w – y), which is converted to a control variable (u) by the controller. Controller actuates the plant by the control variable to eliminate, or at least to minimize the error. The controlled value (y) can be e.g. a temperature (in a room or inside a process), position or velocity (of moving mechanical parts, lifts, and vehicles), liquid level, pressure, flux, humidity etc.

The objective of feedback control is to regulate or track the desired value behavior in time, with minimal costs and risk. The control can be regarded as a continuous process of error compensation. The error can be caused by desired input value change (e.g. a change of desired temperature in a room by resident's wish, by given time schedule or by detection of person presence).