On A Relaxation of Time-Varying Actuator Placement

TL;DR

This paper studies a relaxed approach to time-varying actuator placement in control systems, providing explicit solutions and showing equivalence with the original problem under certain conditions.

Contribution

It introduces a relaxation allowing continuous actuator activation variables and proves the solutions of the relaxed and original problems coincide under specific conditions.

Findings

Explicit formulas for optimal solutions are derived.

Solutions of relaxed and original problems coincide when input matrix has no zero columns.

The relaxation simplifies actuator placement optimization without losing optimality.

Abstract

We consider the time-varying actuator placement in continuous time, where the goal is to maximize the trace of the controllability Grammian. A natural relaxation of the problem is to allow the binary $\{0,1\}$ variable indicating whether an actuator is used at a given time to take on values in the closed interval $[0,1]$. We show that all optimal solutions of both the original and the relaxed problems can be given via an explicit formula, and that, as long as the input matrix has no zero columns, the solutions sets of the original and relaxed problem coincide.