Further order reduction of a singular H∞ controller

A new class of reduced-order controllers is obtained for the H_∞ problem. The reduced-order controller does not compromise the performance attained by the full-order controller. An algorithm for deriving the reduced-order H_∞ controller is presented in the case of continuous time. The reduction in order is related to unstable invariant zeros of the subsystem from disturbance inputs to measurement outputs. In the case where the subsystem has no infinite zeros, the resulting order of the H_∞ controller is lower than that of the existing reduced-order H_∞ controller designs which are based on reduced-order observer design. Furthermore, the mechanism of the controller order reduction is analyzed on the basis of the two-Riccati equation approach