On the Separation of Two Degree of Freedom Controller and Its Application to H∞ Control for Systems with Time Delay

An advantage of the two degree of freedom control scheme lies in the fact that the feedback controller and the feedforward controller can be designed separately. The former is designed to meet feedback properties such as sensitivity, robust stability and disturbance rejection, while the latter is designed to improve tracking performances. However, it is not completely clear whether the designs of feedback controller and feedforward controller can be done independently. The two kinds of specifications may interfere each other. In this note, a design scheme of the two degree of freedom control is established that guarantees the independence of each controller. This result is applied to the tracking H ∞ control for systems with time delay based on previous researches [1][2]. It is shown that the problem is reduced to two independent mixed sensitivity problems with time delay.