In any negative feedback system the measured actual control variable xactual is compared with a given reference value xref. Ideally the actual instantaneous value of a control variable should follow the given reference value. However, this is not entirely possible and the quality of a control system depends to a large extent upon the type of controller used. In applications there are linear and nonlinear controllers. Here only the simplest linear controllers will be discussed: the proportional (P) controller and the proportional-integral (PI) controller.