On Polynomially Solvable Constrained Input Selections for Fixed and Switched Linear Structured Systems

TL;DR

This paper identifies new polynomial-time solvable cases for optimal input selection in structured systems by leveraging total unimodularity, expanding the class of problems that can be efficiently solved.

Contribution

It introduces a restricted total unimodularity notion to characterize polynomial solvability for input selection problems in fixed and switched systems, covering new cases like the source-SCC separated input constraint.

Findings

Problems are solvable via LP relaxation under certain regularizations.

The characterization depends only on the incidence matrix between inputs and source SCCs.

New polynomially solvable cases, including the joint SSSI constraint, are identified.

Abstract

This paper investigates two related optimal input selection problems for fixed (non-switched) and switched structured systems. More precisely, we consider selecting the minimum cost of inputs from a prior set of inputs, and selecting the inputs of the smallest possible cost with a bound on their cardinality, all to ensure system structural controllability. Those problems have attracted much attention recently; unfortunately, they are NP-hard in general. In this paper, it is found that, if the input structure satisfies certain `regularizations', which are characterized by the proposed restricted total unimodulairty notion, those problems can be solvable in polynomial time via linear programming (LP) relaxations. Particularly, the obtained characterizations depend only on the incidence matrix relating the inputs and the source strongly connected components (SCC) of the system structure, irrespective of how the inputs actuate states within the same SCC. They cover all the currently known polynomially solvable cases (such as the dedicated input case), and contain many new cases unexploited in the past, among which the source-SCC separated input (SSSI) constraint is highlighted. Further, for switched systems, the obtained polynomially solvable condition (namely the joint SSSI constraint) does not require each of the subsystems to satisfy the SSSI constraint. We achieve these by first formulating those problems as equivalent integer linear programmings (ILPs), and then proving total unimodularity of the corresponding constraint matrices. This property allows us to solve those ILPs efficiently via LP-relaxation. We also discuss solutions obtained via LP-relaxation and LP-rounding in the general case. Several examples are given to illustrate the obtained theoretical results.