Controllability measure for disturbance rejection capabilities of control systems with undamped flexible structures

This paper introduces a controllability measure for quantitatively evaluating the disturbance rejection capabilities of control systems with undamped flexible structures. The measure is derived by obtaining the steady-state solution of the degree of disturbance rejection capability (DoDR), a Gramian-based measure used to assess controllability under external disturbances. To address the issue of Gramian matrices diverging over time in undamped systems, we have developed and proven several theorems related to Gramian matrices in undamped systems. The resulting solution, derived using these theorems is represented in a closed-form and expressed in terms of the modal matrix, input matrix, disturbance matrix, and disturbance covariance matrix. Since the derived solution does not require solving Lyapunov equations, which is typically required in most Gramian-based measures, it enables efficient computations, even for high- dimensional systems. Numerical examples confirm that the proposed measure serves as an exact DoDR solution for undamped systems, preserving the previously established physical meaning of DoDR. Control simulations further validate its accuracy in predicting disturbance rejection performance, highlighting its value in actuator allocation.