Design of H2 and H∞ controllers with two degrees of freedom

The tracking behavior of many feedback control systems can be improved by using a compensator with two degrees of freedom which processes the reference signal independently of the plant output signal, e.g., the reference signal can be processed by a static prefilter. This is described in the first part of this paper dealing with LQG/LTR and H₂ controllers. With H_∞ methods, it is easy to design more sophisticated, dynamic feedforward controllers. They can be developed either in a second step, when the loop is already closed by a feedback compensator, or in one step together with the H_∞ feedback part of the compensator. This latter possibility has the advantage that only one observer is needed for the two parts of the controller, thus the number of states of the complete compensator is minimized. An application of these design configurations to a helicopter is shown. Step responses without overshoot are achieved for position control.