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 anysubset of feedback links of cardinality at most γ. 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 γfeedback links. We first prove that the verification problem is NP-complete even for irreduciblesystems and the design problem is NP-hard even for so-called structurally cyclicsystems. We also show that the design problem is inapproximable to factor (1−o(1))logn,where ndenotes 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 O(log n)-optimal approximation algorithm to solve the design problem for a special feedback structure, so-called back-edgefeedback structure.