| With large working range,high dexterity and strong load-carrying capability,space manipulator has been commonly applied to aid or even replace astronauts on completing tasks such as on-orbit assembling and onorbit maintaining.However,the harsh space environment,high frequency and high load level of on-orbit tasks,complex joint structure and other factors make space manipulator vulnerable to the loss of excitation of exciting winding,fracture of drive bearing and the separation of gear engagement.These factors then cause free-swinging joint failure.Free-swinging joint failure has strong negative influence on the reliable execution of on-orbit tasks,which becomes a key constraint of the reliable long-term service of space manipulator.Considering the high security risk,extreme technical difficulty and economical cost of repairing and replacing of the faulty joint conducted by extravehicular operation of astronauts,as well as the urgency of task,it is necessary to conduct research on the treatment of free-swinging joint failure to ensure that the subsequent tasks can be completed.Free-swinging joint failure refers to the scenario that the faulty joint losses its torque output capability and is in free-swinging state.The faulty joint can not be controlled independently and relies on the actuation of remaining healthy joints.As a result,analyzing the coupling relationship between healthy joints(joints without free-swinging failure)and the faulty joint is the crucial premise of determining whether the faulty space manipulator can output expected motion.After failure occurs,the faulty joint degenerates into underactuated unit due to lack of actuation source,causing the faulty manipulator to present high underactuated state,while the degree of freedoms of remaining healthy joints is limited.These two aspects lead to the increase of difficulty of controlling the faulty manipulator.Besides,free-swinging joint failure causes the perturbation or even sever degeneration of the motion capability of the faulty manipulator,making the motion capability of the faulty manipulator be likely to not satisfy the requirement of tasks.This then causes problems such as low load-carrying capability and motion singularity.This thesis is co-supported by the National Natural Science Foundation of China "Motion capability optimization and multi-stage fault tolerant control of space manipulators with free-swinging joint failure"(No.62173044)and BUPT Excellent Ph.D.Students Foundation "Research of fault tolerant control strategy aiming at free-swinging joint failure of highly-underactuated space manipulators"(No.CX2021312).Taking a free-floating base space manipulator as research object and focusing on accomplishment of tasks of space manipulator after free-swinging joint failure occurs,this thesis conducts research on fault tolerant control strategy of space manipulators with freeswinging joint failure,which effectively improves the task execution capability of space manipulator after failure,to continue to carry on subsequent tasks and lengthens the period of on-orbit service of space manipulator.Main research contents are as follows:(1)The motion characteristic of space manipulators with free-swinging joint failure.The kinematic and dynamic coupling relationship between healthy joints and controlled units,including faulty joint,free-floating base and end effector,is derived.And then the kinematic and dynamic controllable degree indexes are constructed to precisely characterize the controllability of healthy joints on controlled units when the space manipulator is under motion state.Through numerical simulation,the influence of single/multiple joint control variable,joint angular velocity and other factors on the controllable degree index is analyzed,to verify the effectiveness of the kinematic and dynamic controllable degree indexes on characterizing the controllability of active joints.(2)The influence of free-swinging joint failure on motion capability of space manipulators with free-swinging joint failure.Based on the free-floating characteristic of base and free-swinging characteristic of the faulty joint,dynamic dexterity indexes are constructed by reconstructing the generalized Jacobian matrix of the kinematic model and the inertia matrix of the dynamic model of the manipulator.Considering the loss of torque of the faulty joint,the dynamic load-carrying capability and end effector operating force is analyzed,and the degree of degeneration of the corresponding indexes are constructed.Finally,simulation experiment is carried out to analyze the influence of different joint configuration and different faulty joint on the motion capability of the space manipulator.(3)Research on trajectory optimization method of space manipulators with free-swinging joint failure.By utilizing high order polynomials containing redundant parameters to characterize trajectory of faulty manipulator,and regarding motion capability,base orientation deflection and energy consumption as optimization object,a trajectory optimization model of the faulty space manipulator is constructed.By converting multi-object optimization problem into single-object optimization problem and utilizing particle swarm optimization algorithm,the optimal motion trajectory of the faulty space manipulator is solved and obtained,improving the capability of carrying on tasks for the faulty manipulator after joint failure occurs.(4)Research on multi-stage fault tolerant control strategy of space manipulators with free-swinging joint failure.Aiming at tasks vulnerable to singularity state due to the lack of controllability caused by the increase of underactuated units,a multi-stage fault tolerant control strategy with switching active joint is proposed,which ensures the controllability of the manipulator system on controlled units and prevent singularity state.Aiming at tasks with fewer input and more output,a fault tolerant control strategy based on top-level planner and bottom-level controller is proposed,which comprehensively optimizes objects in each stage such as base orientation deflection,energy consumption and singularity avoidance with ensuring the controllability of each controlled unit.(5)Research on control method of space manipulators with free-swinging joint failure with uncertainty effecting.Aiming at the effect of parameter uncertainties and external disturbance,close-loop iterative learning control system is designed by introducing feedback assistance strategy.Then interference observer is designed and merged into the close-loop iterative learning control system to realize the accurate estimation and active compensation of nonrepetitive disturbance,ensuring the accurate trajectory tracking of the faulty manipulator.(6)Experiments research on fault tolerant control method of space manipulators with free-swinging joint failure.Experiment system containing manipulator subsystem,motion information measurement subsystem and gravity compensation subsystem is set up.Through designing experiments,the correctness and effectiveness of the proposed method of space manipulators with free-swinging joint failure is verified,containing trajectory optimization method,multi-stage fault tolerant control strategy aiming at singularity avoidance at the midpoint of task,and multi-stage control strategy aiming at task with fewer input and more output. |
