Theoretical and Experimental Investigation of Despin Control of Massive Space Debris by Tethered Space Tug
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This doctoral research investigates the problem of despin control of the massive uncooperative rotating target by the tethered space tug in the post-capture phase. Theoretical and experimental studies are conducted to achieve the objective in three parts: dynamics characterization, control strategy design, and experimental validation. The mathematical formulation of the rotating target captured by tethered space tug is modeled in free space to investigate the pure despin motion. It is extended into the central gravitational field to investigate the coupled dynamics between the tethered systems orbital and attitude motion during the despin process. Despin control strategies are proposed with the practical constraints to achieve the purpose of despinning the targets rotation to a permissible level and ensure the systems safety together for orbital maneuvering operation. The advanced nonlinear control techniques are employed to address the tethered space systems underactuation and stability to improve performance. First, a unified control framework of tether tension for a simplified dumbbell model is proposed to precisely control the tether deployment and retrieval. The asymptotic stability of the control framework is proved rigorously. It is improved as a robust sliding mode controller to attenuate the effect of the possible uncertainties and disturbances. Second, a passivity-based nonlinear model predictive control law is designed to handle the constraints on the inputs and states analytically. The tethered systems passivity is revealed and incorporated into nonlinear model predictive control implementation to guarantee the asymptotic stability. Finally, the orbital maneuvering of the rotating target after despun is studied analytically with the non-singularity orbital elements in numerical simulation and validated experimentally in a zero-gravity environment provided by a custom-built spacecraft simulator air-bearing platform.