Design And Testing Of A High Precision And High Stiffness Parallel Adjustment Platform In Space Telescope

In a large aperture space optical telescope system,there are strict relative pose requirements between the primary mirror,the secondary mirror and the third mirror.Due to manufacturing and assembly errors,gravity release,temperature change and material property change,the relative pose between these mirrors will change,which will lead to the decline of imaging quality of the telescope.Therefore,it is necessary to actively adjust the relative pose between the primary mirror,the secondary mirror and the third mirror,so as to limit the pose errors within the allowable range.In order to meet the precision adjusting requirement of the pose the third mirror system of space optical telescope,a mechanism with high precision,large load-size ratio,and high stiffness in both the transverse and the vertical directions is studied in this paper.In order to meet the above requirements,a new type of 6-P-RR-R-RR parallel adjustment platform is designed in this paper.Unlike the common 6-UCU and 6-UPS parallel platforms,the offset RR-hinges are used to replace the traditional universal hinges,and the leg drive components are installed on a slope of the lower platform,and the inclination angle between the slope and the horizontal plane is 30 degrees.On the premise of guaranteeing the motion strokes of the platform,the design greatly reduces the height of the whole machine and is conducive to improving the lateral stiffness of the whole machine.At the same time,the design reduces the mass of the moving parts of the whole machine,thereby reducing the harmful disturbance to the platform caused by inertial forces.Compared with the traditional universal hinge,the two rotating axes of the offset RR-hinge do not intersect,and there is an offset distance,which effectively reduces the risk of interference of the hinge in its rotating process,and further enlarges the working space of the whole machine.In addition,offset RR-hinge has the characteristics of large load-carrying capacity,easy processing and adjustment.However,due to the introduction of additional variables into the kinematics model of the 6-DOF parallel platform,the kinematics of the platform becomes more complex.In this study,mathematical models of the inverse and forward kinematics of the parallel platform are established by using joint motion constraint method and D-H parameter method of series mechanism respectively,and then they are solved by Newton-Raphson numerical iteration algorithm.The validity of the two kinds of kinematics modeling methods for kinematics is verified by numerical co-simulations method of MATLAB and ADAMS.The dynamic characteristics of the main parts and the whole machine of the parallel adjustment platform are studied through the finite element analysis method.The natural frequencies and modal modes of the main parts and the whole machine are determined by modal analysis in PATRAN software.The research results of simulation provide the basis for the further structural optimization design.This paper also carries out experimental research on the designed prototype of the parallel adjustment platform.The initial position of the parallel platform is determined by the measurement of multi-degree-of-freedom precision measuring arm.A motion performance test system is constructed using six high precision grating length guages,and then inverse kinematics solutions,motion resolutions and repetitive accuracies of the adjusting steps of the platform are tested using the test system.Motion strokes of the parallel platform in six directions are measured using a laser tracker.In addition,the static stiffness and dynamic stiffness of the parallel platform are also studied by testing.In order to further improve the platform stiffness index,the force transfer chain analysis and finite element simulation analysis of mechanical characteristics of the platform are carried out,and the main factors affecting the platform stiffness index are determined.In this paper,the configuration of the whole mechanism,the layout of RR-R-RR series leg chains,the key parts such as the legs and joints,the upper platform and the oblique sliders are optimized,and also the processing and assembly technique are adjusted.In addition,in order to further improve the accuracy of the platform,the installation datum of joints and legs is set up,and the initial position calibration method of upper platform is improved.The geometric characteristic lines of the lower platform are used as the measurement datum,so that the positions of the oblique sliders which are driving the legs can be accurately determined,and the consistency between the actual initial positon and the theoretical initial position can be ensured.By comparing the testing results of the repetitive positioning accuracy,static stiffness and dynamic stiffness of the parallel adjustment platform before and after the improvement,the rationality and validity of the improving design approaches are verified.The research works carried out in this paper provide a reference for the design and testing methods of high precision high stiffness parallel adjustment mechanism,and also provide technological and practical experiences for the future research work of large aperture space telescope system in China.