A weighted mixed-sensitivity H/sub /spl infin//-control design for irrational transfer matrices

Approximate solutions to a weighted mixed-sensitivity H/sub /spl infin//-control problem for an irrational transfer matrix are obtained by solving the same problem for a reduced-order (rational) transfer matrix. Upper and lower bounds for the sensitivity are obtained in terms of the sensitivity for the reduced-order model and the approximation error. Moreover, rational suboptimal controllers for the irrational transfer matrix are constructed which achieve a sensitivity close to the minimum.