“…In particular, the development of Z?_ theory and structured singular value computation algorithms has greatly simplified the robust stability, performance analysis and controller design (see Doyle, 1982;Packard and Doyle, 1988;Doyle et al, 1989Doyle et al, , 1991Krause ef al., 1989;and references therein). For systems with time-varying uncertainties, some new results regarding the system robust stability have also been developed using the notion of quadratic stability (see Boyd andYang, 1989, Khargonekar et al, 1990;Doyle, 1990, Packard er al., 1991;Becker and Packard, 1991). However, the robust performance problem for systems with time-varying uncertainties has not been sufficiently explored.…”