Robustness of Velocity Feedback Controllers for Flexible Spacecraft

The robustness properties of collocated velocity feedback controllers used for damping enhancement in large, flexible space structures are investigated. It is proved that the closed-loop system using such controllers is asymptotically stable in the large when: 1) unmodeled linear time-invariant dynamics (such as sensors/actuators) are present, provided that the phase angle of such dynamics is between -90° and 90°, 2) time-varying or invariant nonlinearities lying in the first and the third quadrant (i. e., belonging to the (0, ¿) sector) are present, or 3) sector-type nonlinearities following first-order stable dynamics are present in the feedback loop.