Generalized Nyquist bands for structured and highly structured uncertainty

Abstract

Additive uncertainty of multivariable systems is often assumed to be either unstructured or structured, depending on whether the size or uncertainty is defined in terms of maximum singular value bounds, or bounds on the moduli of individual elements. In many practical situations, however, more information is available, and in particular the frequency response of the elements of uncertainty often display ‘directionality’. To exploit this ‘directionality’ the present paper adopts elliptical rather than circular bounds on the elements of uncertainty and derives an algorithm that generates exact eigenvalue regions which, in turn, can be deployed in the construction of exact generalized Nyquist bands. With the aid of such bands it is possible to get an exact (non-conservative) assessment of the stability margins and of other aspects of the dynamic performance of a much wider class of uncertain multivariable systems.