Accuracy Design And Kinematic Calibration Of Parallel Adjustment Mechanism In Large Space Telescope

In a large multi-functional space telescope,the passive support frame hardly achieve the support stiffness requirements of the optical system.Due to the influence of launch vibration environment,on orbit gravity unloading,high and low temperature vacuum environment,the relative position deviation of each optical element has a serious impact on the imaging quality.At the same time,in order to meet the multi-functional requirements of civil,commercial,military and other fields,the module switching mechanism is needed to switch the optical path to different back-end modules.For the requirements of on orbit precise position and attitude adjustment of optical components and switching optical path of each module components of large space telescope,this paper focuses on the precision design and kinematics calibration technology of the 6-RRRPRR parallel kinematic mechanism.It aims to achieve fast,stable,multi-mode and high-resolution imaging of large space telescope.Owing to the offset RR hinge is easier to process and assemble,and the offset dimension makes it more rigid and larger rotation angle.However,compared with the accuracy design and kinematic calibration of the traditional 6-SPS,6-UCU and 6-UPS parallel kinematic mechanisms,the offset RR joint introduces the offset parameters of the hinge axis into the kinematic model,thus increasing the number of motion parameters to be identified.In this paper,the forward and inverse kinematics of the 6-RRRPRR parallel kinematic mechanism are solved by Newton-Raphson numerical iterative algorithm.On this basis,the workspace analysis is completed according to the design constraints such as driving leg length and hinge rotation angle,it lays a foundation for selection of calibration measurement position and attitude.Precision design is a prior method to solve the end precision of parallel adjustment mechanism.In order to fully study the influence of design parameters of offset RR hinge on the end pose precision.Based on the D-H parameter method,this paper establishes the kinematic model of the parallel adjusting mechanism with all geometric parameters,including base,lower hinge axis at the lower hinge point,upper hinge axis at the lower hinge point,leg moving amplitude,leg rotating amplitude,lower hinge axis at the upper hinge point,upper hinge axis at the upper hinge point and moving platform,and completes the flexibility of each parameter to the end precision of the parallel adjusting mechanism within the space of the moving platform Sensitivity analysis.On this basis,the configuration optimization index is established,and the main structural parameters of the parallel adjustment mechanism are optimized by multi-objective function,which lays a theoretical foundation for the kinematic calibration of the parallel adjustment mechanism to determine the type of structural parameters to be identified.In terms of geometric parameter identification and error compensation,in order to verify the influence of offset parameter error on the calibration accuracy,the kinematic calibration results considering hinge offset error and neglecting hinge offset error are compared.The results show that considering hinge offset error can significantly improve the kinematic calibration accuracy.At the same time,an optimal pose selection algorithm is proposed to determine the optimal measurement configuration.The algorithm can select the smallest number and the best combination of the measurement configuration in the spare set of calibration measurement positions and poses.In order to verify the accuracy of the optimal measurement configuration,the random selection of the calibration measurement position and pose is compared with the kinematic calibration simulation of the optimal measurement configuration.The end-effector accuracy is significantly improved in the same order of magnitude due to kinematic calibration applied to 256 random measurement and 29 optimal measurement configuration respectively.Kinematic calibration experiment is a posteriori method to solve the end-effector accuracy of parallel adjustment mechanism.In order to improve the reliability of kinematic calibration test results,the resolution,stroke and repeatability of the end-effect of the moving platform need to be tested.After ensuring the high resolution and repeatability of the end-effect,the kinematic calibration experiment of the 6-RRRPRR parallel adjustment mechanism is carried out by using the complete error model and the optimal measurement configuration proposed above.Considering the adjustment accuracy of the parallel adjustment mechanism,an AT901-LR laser tracker is used to measure position and orientation of the moving platform in the process of kinematic calibration.The results of kinematic calibration show that the maximum position error is reduced to 1.2e-2 mm,the maximum attitude angle error is reduced to 0.0051°,and the position and orientation accuracy of the end-effector of the moving platform is significantly improved in the global workspace.This research topic has laid a foundation for the research of on orbit precise active compensation technology of large space telescope in China,and has important theoretical and practical significance for the further development of this technology.