Gain perturbation tolerance in H^∞ state feedback control

The gain perturbation tolerance property of H^∞-optimal state feedback (HISF) systems is considered. The following two problems are discussed: (1) for what gain perturbations would the perturbed closed-loop system remain stable? (2) for what gain perturbations would the perturbed closed-loop transfer function satisfy the specified H^∞-norm performance? In each case we establish general sufficient conditions in the frequency domain. In particular, under a certain orthogonality condition, the HISF systems are shown to possess an infinite gain increase tolerance for stability. The result obtained also provides insight into the gain reduction tolerance problem. Furthermore, we show that, under certain conditions, the specified H^∞-norm performance of the HISF systems is preserved even with an infinite gain increase. Concerning closed-loop performance robustness against gain perturbations, we see remarkable differences between the case where controlled outputs include a direct transmission of controls and the case where they do not.