A new method is proposed for computing the robust stability margin for a single-input single-output systems subject to real interval uncertainties that appear multiaffinely in the system transfer function. The approach utilizes the recently developed critical direction theory to study the robust stability problem directly in the Nyquist plane, and permits the calculation of the robust stability margin using a bisection search. A further benefit of the method is that a reduced range of frequencies yielding the robustness margin can be readily identified. An algorithm is presented which utilizes both of these properties, resulting in a significant reduction in computational burden.
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