Closed loop cancellations in H infinity control of time-varying systems

The pole/zero cancellation phenomenon is examined in the closed loop system resulting from the solution of the H infinity control problem for time-varying systems. The unobservable and uncontrollable states are identified in the realization of the closed loop system associated with the general Hinfinity control problem. These results are then used, in conjunction with previous results obtained by the authors on the pole/zero cancellation phenomenon in time-varying systems to identify pole/zero cancellations in two specific types of Hinfinity control problems. These results generalize the work of Sefton and Glover for time-invariant systems.