An approach for improving the quality of bounds on the structured singular value in the case of mixed real and complex uncertainty models is described. The proposed approach uses constrained non-linear optimization to improve the lower bound on μ generated by standard commercially available algorithms. A similar approach is used to compute more reliably bounds on the peak value of μ for problems that give rise to sharp peaks in the μ plot as a function of frequency. Both approaches are shown to be simple and computationally efficient to implement, and their practical usefulness is demonstrated on the problem of checking a stability margin clearance criterion for a V/STOL (vertical/short take-off and landing) aircraft flight control law.
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