Attitude regulation of a free-flying space robot during contact operations

To ensure efficient transportation of large space structures in the future, it has been suggested that they be transported in an unassembled and compact form and then assembled inorbit using space robots. Forces generated during open contact operations have to be taken into careful consideration so that structural components can be successfully acquired and assembled by the robot. Unlike ground-based robots, the free-flying space robot base will deviate from its desired orientation when the spacecraft comes in contact with structural elements. This effect on spacecraft attitude may adversely affect communication, solar energy collection and in extreme cases the stability of the spacecraft. While thrusters can be used to provide regulation torques, in a scenario where the spacecraft is surrounded by structural components that are to be assembled, filing clusters is undesirable. Further, the force impulses generated by thrusters have poor resolution and hence are unsuitable for providing fine regulation. In this paper, the authors utilize reaction wheels (RWs) to provide accurate and smooth control torques without the need to have on-board fuel. By adopting the commonly used canonical-form dynamic model of a space robot as found in the literature, force tracking may be achieved, but a desired orientation of the spacecraft cannot be guaranteed. To solve this problem, the authors build dynamic equations of the space manipulator and the spacecraft base separately to form basis for separate contact-force control and attitude control of the platform. By taking a further step from previous literature which mainly try to maintain contact between the end-effector and the target, hybrid force/motion control is extended to a space robot in this paper in order to track a desired force trajectory and simultaneously maintain a desired relative position between the end-effector and the target to avoid inducing tumbling torques. A recently proposed adaptive variable structure control method is applied to design a robust attitude controller taking into account system uncertainties and external disturbances. A two-link planar space robot model is simulated to track a desired uni-axial contact force and regulate the spacecraft attitude during contact with a free-floating target. The simulation results have demonstrated better robustness and performance of the proposed controller, such as shorter settling time in the presence of disturbance and smaller force tracking error, in comparison to a conventional feedback linearisation attitude controller.