Research On Manipulator For On-Orbit Servicing Tasks Of Spacecraft And Its Ground Test

Currently, on-orbit servicing tasks of spacecraft are completed mainly by human astronauts, but complicated space environment makes them extremely expensive and risky. The manipulator for these tasks, also called on-orbit servicing arm, is no doubt the best choice to assist or replace astronauts, and in ternational space community has accumulated rich experience in theoretical research and engineering applications. However, most on-orbit servicing tasks’ ground verifications or space tests are only limited to demonstrating projects of free space movement, but fine manipulation experiments for contiguous engineering tasks haven’t been carried out yet, or have just been postponed. Supported by the manned space flight project from General Armament Department, a manipulator system for contiguous on-orbit servicing tasks of spacecraft is designed, and ground test is carried out based on the established control strategies. This paper mainly includes manipulator system’s design and analysis, on-orbit servicing arm’s mathematical modeling and joint parameters’ identification, research on position control and Cartesian impedance control, experimental verification of different control modes, etc.Firstly, based on on-orbit servicing tasks and system design requirements, the on-orbit servicing arm system is designed. Through a comprehensive analysis of different schemes, the body configuration of six degrees of freedom and ―elbow unbiased with spherical wrist‖ is determined. By means of local modularization idea, the drive transmission system adopts a magnet synchronous motor with small permanent and a harmonic reducer with large reduction gear ratio. Considering system operation’s compliance requirement, a kind of joint torque sensor based on the principle of resistance strain is designed, whose design indicators are v erified by the finite element analysis and calibration experiment. In order to obtain accurate joint position, the information fusion algorithm involving the resolver and optical encoder is studied and applied. Besides, the joint’s bearing components and humanoid arm links are designed, whose strength and stiffness are verified through statics analyses. In order to satisfy the joints’ assembly requirements, the joint resistance torque test is carried out.Secondly, the on-orbit servicing arm’s mathematical model is established, and the key dynamics parameters are identified. Considering the Jacobian singularity in inverse kinematics planning, singularity avoidance strategy is formulated. By improving Paul quartic polynomial planning algorithm, the problem that planned trajectory doesn’t pass through the desired path points is solved. Because there exists flexibility in joints, stiffness identification is carried out by means of mechanical impact method. By studying the Lu Gre friction model, a simplified linear model is adopted and similar recognition result is obtained. According to the relationship between joint velocity and control input, the step response analysis of first-order system is carried out. From the above recognition results, the motor torque coefficient identification is directly completed, avoiding the complicated experiment.Thirdly, the position control algorithm fit for on-orbit servicing tasks is studied, which is verified through the simulation and experiment. In order to satisfy tracking requirement of a continuous time-varying trajectory, the computed torque control strategy is formulated, which is verified through the simulation in Simulink. In order to verify the tracking performance of a trajectory in position control, the tracking experiment of a sinusoidal trajectory is conducted. After that, the repeat positioning accuracy is tested by a API laser tracker, which obviously meets the design indicator.Finally, in order to meet the operational requirements of flexibility, the Cartesian impedance control algorithm is studied and the effectiveness of the algorithm and on-orbit servicing tasks is experimentally verified. The Cartesian impedance control strategy based on the joint torque sensor is formulated, and online gravity compensation based on the motor position is achieved. After that, stability of the control system is proved. Tracking performance of a trajectory in impedance control is verified through the tracking experiment of a sinusoidal trajectory, which shows that Cartesian position error is related to the speed of trajectory planning. For the operation tasks of screwing a electrical connector, screwing a screw and handshaking, system performance in three control modes, namely impedance control, directional impedance control and zero force control, is verified. In order to ensure the safety of operation, dynamic collision experiment with a kind of rigid environment is arranged. The result shows that the system can steadily switch from free space motion to the rigid body’s surface, which can protect both the system itself and the operation objective.