Computation of the nonlinear Nyquist robust stability margin for structured uncertainties

The critical direction theory for analyzing the robustness of uncertain feedback systems is extended to include nonlinear elements in the closed loop, making the approach applicable to a wider class of systems. A redefinition of the critical perturbation radius is introduced, preserving key properties of the original theory. Necessary and sufficient conditions for robust stability are developed, and the case of real affine parametric uncertainties is treated in detail. An example is presented to illustrate the theory.