Feedback Robustness In Structured Closed-Loop System

This paper addresses the robustness of large-scale closed-loop structured systems in the sense of arbitrary pole placement when subject to failure of feedback links. Given a structured system with input, output, and feedback matrices, we first aim to verify whether the closed-loop structured system is robust to the simultaneous failure of any subset of feedback links of cardinality at most gamma. Subsequently, we address the associated design problem in which given a structured system with input and output matrices, we need to design a sparsest feedback matrix that ensures the robustness of the resulting closed-loop structured system to simultaneous failure of at most any gamma feedback links. We first prove that the verification problem is NP-complete even for irreducible systems and the design problem is NP-hard even for so-called structurally cyclic systems. We also show that the design problem is inapproximable to factor (1 - o(1)) log n, where n denotes the system dimension. Then we propose algorithms to solve both the problems: a pseudo-polynomial algorithm to address the verification problem of irreducible systems and a polynomial-time 0(log n)-optimal approximation algorithm to solve the design problem for a special feedback structure, so-called back edge feedback structure. (C) 2020 European Control Association. Published by Elsevier Ltd. All rights reserved.