$\mu$ -Based Theory in Compliant Force Control for Space Docking

In order to simulate the buffering process of the space manipulator docking, a compliant force control system based on the inner position loop of the six degree-of-freedom parallel robot is established. The manipulator docking environment includes the stiffness and damping characteristics of the manipulator, the mass, contact stiffness, and damping characteristics of the docking mechanism, which makes the expected contact force affected by the mass, multiple-stiffness, and multiple-damping. The stiffness and the damping characteristics are time-varying in the docking process, which will have a great influence on the stability of the force control system and the force control results; meanwhile, the low-frequency oscillation element composed of mass, stiffness, and damping in the docking environment not only seriously affects the bandwidth of the control system, but also makes the system characteristics tend to oscillate or even unstable; in addition, the model uncertainty of parallel robot, the measure noise and the process noise in the system will also affect the force control results. Aiming at the complex compliant force control system for simulating the manipulator docking’s buffering process, a robust controller is designed using the $\mu$ synthesis control theory to get a satisfactory system stability and force control characteristics in this paper, in which the system parameter variation, the model change, the external interference, and the system bandwidth are synthetically considered. The simulation and experiment of the compliant force control are carried out using the robust controller and the classical force controller; the results show the effectiveness and superiority of the designed robust controller.

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