On-Orbit Calibration Of Kinematic Parameters Of Space Manipulator Based On Visual Measurement

With the development of space technology,space manipulator has been widely used in the fields of load capture and delivery,on-orbit assembly,on-orbit maintenance and so on,and the absolute positioning accuracy of the manipulator is required to be higher and higher.The environment change and vibration impact of space manipulator during launching,the deformation of rod after long-time on-orbit service,the change of tightness between joints and the on-orbit replacement of joints will cause the decline in positioning accuracy.It is very important to calibrate the kinematic parameters of the space manipulator on orbit and improve the positioning accuracy of end effector of the manipulator.In this paper,a 7-DOF space manipulator is taken as the research object,and the kinematics model of the manipulator is established by using MDH method.In order to solve the problem that the kinematics parameters will change suddenly when there are small errors in the two adjacent parallel joints of the manipulator,the parameter β is introduced to modify the kinematics model,The kinematic parameter error model of 7-DOF space manipulator is established.Based on error modeling,a method of on-orbit kinematic calibration of space manipulator based on vision measurement is proposed.The robot end effector with camera and matching target are used to measure the pose of end effector on orbit.The least square method is used to identify the kinematic parameter error,and the direct compensation method is used to compensate the error.In order to study the influence of redundant parameters on the accuracy in calibration,based on the characteristics of identification Jacobian matrix,the discrimination method of redundant parameters in the calibration of space manipulator is analyzed.The effect of calibration on kinematic parameters before and after eliminating redundant parameters is analyzed,and the conclusion that the calibration a ccuracy is higher after removing redundant parameters is obtained.In order to optimize the measurement configuration of the manipu lator,balance the calibration accuracy and efficiency,based on the singular value decomposition of the identification Jacobian matrix,the observability index is introduced as the evaluation index of the optimal measurement configuration,and the optimiz ation strategy of the measurement configuration of the manipulator is proposed.The variation of the observability index and the pose error at the end effector of the manipulator with the number of pose measurements is analyzed,and the minimum number of pose measurements is determined.The optimization method of pose measurement based on particle swarm optimization algorithm is studied,and the on-orbit calibration of space manipulator is simulated.It is concluded that the kinematic parameter calibration algorithm can effectively identify the parameter error and improve the positioning accuracy of end effector of the manipulator.Moreover,when the number of measurement pose is the same,the accuracy in calibration of optimal configuration is higher.Based on the microgravity simulation system,the experimental platform is built,and the on-orbit calibration of space manipulator kinematics parameters is verified by the ground experiment.After calibration,the average position error of the terminal is reduced by 39.23% and the average attitude error is reduced by36.97%.It is concluded that the on-orbit calibration of space manipulator based on vision measurement can effectively reduce the pose error of end effector of the manipulator.