Principal gains and principal phases in the analysis of linear multivariable feedback systems

The concepts of principal gain and principal phase are introduced for linear multivariable systems, and their use in the analysis of feedback behavior is demonstrated. A sufficient Nyquist-type stability criterion is presented in terms of these quantities and is used to characterize the robustness of the closed-loop stability property when the system model is subjected to a linear perturbation (either multiplicative or additive) at any point in the feedback configuration. The results presented are less conservative than those obtained via the small gain theorem.