qWe are interested in the following question: How large D (in the sense of ||D||¥) can be without
destabilizing the feedback system?
•Since the closed-loop poles are
given by det(I-G(s)D)=0 the feedback system becomes
unstable if det(I-G(s)D(s))=0
for some s in the closed right half plane.Now
let a>0 be a sufficiently small number such that the
closed-loop system is stable for all stable ||D||¥< a.Next increase a until amaxso that the closed-loop system becomes unstable. So amaxis the robust stability margin.
qConsider a general feedback interconnection where Dis structured uncertainty block and G(s) is the interconnection