Pointing control and vibration suppression of a slewing flexible frame

An analytical and experimental control system design for a slewing frame containing active members is presented. This testbed is a model for the slewing of dynamically complex structures such as solar arrays and space trusses. Two of the frame's passive members are integral in suppressing vibrations because the torsional vibrations are difficult to control with the slewing actuator. The design is performed with a finite element based model that contains significant error, therefore, both control loops must have performance and stability robustness. Suppression of the torsional motion is accomplished with an active member in a positive position feedback (PPF) loop designed with frequency domain and Nyquist techniques. The pointing controller is designed using mu synthesis to maximize the robustness of the control law. This compensator slews the structure with a two second settling time, 7% overshoot, and less than a 2% steady state error due to static friction. With the supplementary PPF control loop closed, the settling time of the torsional vibrations is reduced from over 30 s to approximately 6 s. Thus, vibration suppression and satisfactory step response is obtained using robust control laws in multiple feedback loops.