Robustness of discrete periodically time varying control under different model perturbations

This paper presents a quantitative analysis on the robustness of discrete linear periodically time varying (LPTV) control under different perturbations in comparison with linear time invariant control. For unstructured perturbations, it is shown that the system stability margin is definitely deteriorated by LPTV control. For structured perturbations, the paper clarifies the mechanism of LPTV control in stability margin improvement and the side effect of this improvement deterioration of the stability margin for unstructured perturbations. In a unified frequency domain framework, the paper exposes the relevance and confliction of the previous results on the robustness of LPTV control, and therefore provides a deeper insight into the problem.