Research On The Redundant Manipulator And Its Compliance Control For ORU Replacement

With the development of space technology, how to ensure the spacecraft lasting stability in the complex space environment in operations has become an urgent problem in the field of space technology. Development of on-orbit maintenance technology will provide logistical support for spacecraft. At present, there is still a technological gap between our country and developed countries, and developments of on-orbit maintenance technology have important practical significance for high-tech development of China’s aerospace technology. In this paper, based on the background of the National Defense Science and Industry Bureau civilian space program “Research on XXX testing system”, the principle prototype of a small manipulator for ORU(Orbital Replacement Unit) replacement task was developed, and key technologies including inverse kinematics and motion optimization of manipulators and operating strategy for on-orbit maintenance were carried out. Also, ground experiments of ORU replacements were completed.From the perspective of optimization design of manipulator configuration, typical manipulator configurations for ORU replacement were studied, and the reasonable manipulator configuration and geometry parameters were determined. According to the mission requirements, nine different manipulator c onfigurations were proposed, and the operational capacity, flexibility, folding capacity, structural simplicity and kinematic calculation, etc. were comprehensive analized, and the SRS configuration with non-offset elbow joint was selected as the manipulator configuration. And the optimal size of the manipulator was determined through theoretical analysis and optimization calculation; In order to theoretically explain the inverse kinematics complexity of different configurations, the inverse coordinate method, virtual reconstruction method and parametric arm angle method were proposed for closed form solutions of three kinds of manipulators respectively, including the manipulator(Configuration-I) with a spherical base joint and the non spherical wrist, the manipulator(Configuration-II) with Pieper configuration or Duffy configuration and SRS humanoid manipulator(Configuration-III). These three methods established the unified framework of corresponding manipulators for closed-form solutions, and simplifies computations of inverse kinematics; Compared with reverse coordinate method, the application range of virtual reconstruction method more widely. In addition, algebraic method was used to determine the effective range of arm angle parameter for the 7-DOF manipulator with joint limit constraints, which laid a foundation for the motion optimizations.From the perspective of software and hardware realization of manipulator system, this paper analyzed the characteristics of manipulators according to ORU replacements, and established the hierarchical system framework includes the seven degrees of freedom manipulator and the end effector. Based on the local modular design, three different sizes of rotary joints were developed with force/position sensing, overload protection, brake function and high integration; In addition, the end effector with capacity of target docking/release and position tolerance was developed. Based on this, the use of distributed control and internal cabling enabled the removable electrical connection between joints, and between manipulator and the end effector, and realized large motion range, high reliability and load weight ratio and the target tolerance operational capacity to meet the grounding operational requirements.From the perspective of motion optimization of redundant manipulators, this paper has studied the motion optimization methods of redundant manipulator with motion constraints. The self-optimization method was proposed based on the planar redundant robot model, and motion optimizations are performed during robot moves to its equilibrium state under the action of the virtual torque. On this basis, motion optimizations of joint limits avoidance for redundant manipulators were realized using the self-optimization method. With respect to the problem of joint trajectory planning of redundant robotmanipulator, the one dimensional optimization model based on arm angle parameters was established, and global optimization of joint trajectory during task operations was achieved fast and efficiently using asexual reproduction optimization method. For the pose less constrained operational tasks, an improved gradient projection method was proposed by decomposing of the task space into two orthogonal subspaces, and multi-objective optimization of the manipulator including both instrinsic redundancy and functional redundancy was achieved effectively.From the perspective of the implementation of ORU replacements, operation strategy and compliance control methods of the redundant manipulator system were studied. According to the characteristics of ORU replacement task, the operation process is divided into four different stages, and the corresponding operation strategy of manipulator system was established; Based on the hybrid impedance control and inverse kinematics of two times calculation method, the modefied hybrid impedance control model was established, achieving multi-objective optimization of redundant manipulator and impedance control or force control in different tasks space; In the force tracking direction, performance of force tracking was improved using the desired trajectory planning method based on coordinated motion relationship. Finally, the ground experiments for ORU replacement was demonstrated, and the validity of impedance control and operation strategy of the manipulator with inaccurate environmental information was verified.