Research On Space Manipulator On-orbit Servicing System And Operation Strategy

With the development of China’s Manned Space Engineering to a higher stage,higher requirements are put forward for space manipulation capability.In order to reduce the daily workload of astronauts,reduce the risk of extravehicular activities,and enhance the ability of space manipulation,all major space powers have increased their efforts to support the research of on-orbit servicing technology of space manipulators,and applied it to the on-orbit assembly and maintenance technology of spacecrafts.The space manipulator has many error sources and strong randomness during the working period of orbit.For the operation task with high precision requirement,it is necessary to study fine operation control strategy to improve the task accuracy of the system.For the contact operation task,it is necessary to study the compliance control strategy,which can not only complete the operation task,but also ensure the safety of the manipulator and the object to be operated.This thesis is derived from the Space Laboratory Mission of China’s Manned Space Engineering,it mainly studies the control strategy of space manipulator on-orbit operation,especially the control strategy of on-orbit servicing.The main research contents focus on the manipulator on-orbit servicing system construction,joint friction on-orbit identification,the research on fine operation control strategy of manipulator on-orbit servicing,the research on the compliance control strategy of manipulator on-orbit servicing and experimental verification on-orbit.This paper firstly developed the space manipulator on-orbit servicing system.A manipulator based on modular joints has been developed with position sensing and force sensing.In order to meet the diverse requirements of on-orbit servicing tasks,HIT/DLR II Hand was selected as the end-effector of the manipulator.A hand-eye camera and a global camera were selected as external position sensing sensors for the system.A simulate on-orbit replaceable unit(ORU)is designed which integrates typical mechanical,electrical and thermal control maintenance elements.Based on the system scheme,the kinematics and dynamic models of the manipulator and ground verification system are established.The experiment of joint friction identification was designed and completed,the on-orbit characteristics of joint friction were determined,and the differences between on-orbit and ground friction were compared.This part of the work laid the mathematical foundation for the subsequent control strategy research.For the on-orbit fine operation task,two control strategies are proposed to improve the operational precision of the manipulator system.Aiming at the task of direct operation of the manipulator,a calibration strategy based on manipulator off-line planning trajectory compensated with hand-eye camera measurements is proposed.Through the ground test,it is verified that this strategy can successfully use one target to complete the on-orbit servicing task,which greatly reduces the requirement of calibration resources and simplifies the control strategy,avoids the influence of gravity difference between on-orbit and ground,on-orbit installation error,and meets the high precision requirement of operational task.Aiming at the task of manipulator using tools to operate,a visual servoing control strategy is proposed.The strategy is adopted power tool position and posture measuring from global cameras closing loop in manipulator control system,after astronauts calibrating the exact operating position and posture of power tool.A visual servoing controller is designed,and the convergence proof and stability analysis of the control algorithm is given.It avoids the random error caused by power tool grabbing of manipulator,and improves the accuracy of the system operation.Using astronauts’ power tools can complete the operation of manipulator,simplifies the design difficulty for operating tool,greatly simplifies the system supporting and investment.For the constrained space contact operation control problem,through the task analysis of the screwing bolts,it is determined the control target is the manipulator can not introduce the position and orientation errors during the free space and contact space switching process,and the force of the screwing process does not exceed 40 Newtons.Based on this,a Cartesian space impedance control strategy based on friction compensation and joint torque feedback is designed to improve the static precision of the impedance control in the free motion space and the response rate of the external force in the contact space.The stability of the system is proved,and the impedance parameters are determined according to the task indexes.The simulation results show that the control strategy and control parameters can satisfy the screw task.Finally,the control algorithm and theory are verified by on-orbit experiments of screwing the connector,screwing the bolts and astronaut-manipulator handshaking,and the validity and correctness of the control strategy and algorithm are proved.