Robust stability of spherical plants with unstructured uncertainty

This paper studies the problem of robust stability of a feedback system where the plant is affected by parametric and unstructured perturbations. Parametric perturbations are modelled by locating plant coefficients in an l/sub 2/ ball, while unstructured perturbations are represented via coprime factor perturbations bounded in an H/sub /spl infin// norm. A solution technique for the exact determination of the parametric and unstructured stability margins is proposed. Simple formulae for upper and lower bounds for these margins are also derived.