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Abstract
Free-Flying Space Robots (FFSRs) have the potential to assemble large space structures in orbit autonomously or telerobotically instead of time-consuming, risky and expensive astronaut Extra Vehicular Activities (EVA). However, dynamic coupling between the space manipulator and the spacecraft base can introduce modelling and control problems distinguished from fix-base robots. In this thesis, systematic modelling and control approaches for an FFSR are presented. Before proceeding to a complex on-orbit assembly case where FFSRs are used, a simple on-orbit assembly case, i.e. a deploying spacecraft is first analyzed. The subsequent chapters then investigate modeling, motion control and force control of an FFSR.
A robust controller is developed for a deploying spacecraft based on the twisting algorithm to control its attitude despite the substantial inertia change caused by structural reconfiguration. The controller delivers smooth control torques which are perfectly practical for the control of Reaction Wheels (RWs) and is able to steer the satellite to the desired orientation with reduced settling times.
In the on-orbit assembly case where FFSRs are used, a comprehensive dynamic model for a reaction-wheel actuated FFSR is first presented. The reformulated model incorporates the contribution of reaction-wheel momentum to the entire system. Based on the decoupled form of the model, two types of robust controllers are developed to implement coordinated control of both the space manipulator and the spacecraft in the presence of system uncertainties. The control methodologies can be applied for both the approaching phase and post-capture phase. It is shown that the controllers successfully achieve motion control for each sub-channel of the system, including the attitude states and manipulator motion states.
To implement target capture, a new control-oriented model structure for an FFSR is proposed. The developed model allows simultaneous end-effector motion/force control and active base attitude control. Hybrid motion and force control method is extended to enforce the FFSR to track a desired trajectory of contact force which incorporates the consistent motion between FFSR’s end-effector and the floating target. Meanwhile, attitude control of the spacecraft is achieved by taking the constraint forces from the articulated joint as disturbances.
All the control approaches are verified through numerical simulations in each corresponding chapter.