In order to realize the basic task of the automatic control system must be in the process of control of behavior request. On the basic requirement of control system, usually through the system response to a particular input signal to meet. With unit step signal, for example, some characteristics of transient and steady state value. In ensuring the stability under the premise of system dynamic performance and steady-state performance is good, that is:
Dynamic process smoothly (stability);
Quickening the response fast ();
Tracking value more accurate (accuracy).