Digital systems
Disturbances and control performance

The main source of error is a presence of various disturbances, acting on a plant. They often appears as random variables, e.g. a sudden weather change, room insolation change, window opening, draught, switching of electrical equipment, arrival or departure of several persons, noise in a temperature measurement or random turbulence in its neighbourhood. Also some regular effects are regarded as being random, because they can hardly be predicted by the controller (e.g. a temperature or flux variance of heating or cooling medium).

There are several criteria to evaluate a control performance. Beside the error in steady state, the behavior of controlled value in time is also important. Each error signal change causes a transient response of the output value, which gets settled to the steady state (theoretically) for an infinitely long period. The control performance is evaluated by the transient response as well. In some application, the shortest settling time is desired, even if causing a large overshoot. Otherwise, no overshoot is required, even if the settling will last longer.