Robust stability against structured and unstructured perturbations

The problem of unstructured perturbations is considered when the linear system is also subject to parameter perturbations or uncertainties which are not necessarily small. This motivates the consideration of a family of interval plants, i.e. plants with transfer function coefficients varying in prescribed ranges. For such families of systems subject to prescribed ranges of parameter perturbations precise and nonconservative bounds on the level of unstructured perturbations can be tolerated. Auxiliary results on passivity and strict positive realness that are of interest in their own right are also developed