Design And Research Of High-precision Hexapod Platform For Secondary Mirror Adjustment Mechanism Of Large Telescope

Large telescopes are an important tool for mankind to explore the mysteries of the universe and understand the unknown world in outer space.They are also an important symbol to measure the scientific and technological strength and economic strength of the country.In large telescopes,the position and orientation of primary and secondary mirrors greatly affects the imaging quality of the telescope.However,due to temperature,air pressure,vibration and other factors,the relative pose of the primary and secondary mirrors cannot be guaranteed to remain unchanged.Therefore,it is necessary to design an adjustment mechanism to adjust the position and pose of the primary and secondary mirrors.In practical research,considering the size,mass and energy consumption of large telescopes,the secondary mirror is usually selected as the object of adjustment.Therefore,the research on the related technology of the secondary adjustment mechanism is one of the important topics in the development of large telescopes.Relying on the large telescope research project of the Academy of Space Technology,this paper has carried out the research on the related technology of the secondary mirror adjustment mechanism of the large telescope.In this paper,a 6-RRRPRR parallel 6-DOF Hexapod platform is designed as a secondary mirror adjusting mechanism according to the working requirements of high precision and high stiffness.First,according to the kinematic characteristics of the axis offset Hooke joint structure,a kinematics model based on the space circle auxiliary model is established;then,an inverse kinematics solution algorithm based on the shortest distance of the space circle was proposed,and a compensation scheme is proposed for the derivative motion of the screw nut pair.Then,the Brent root finding algorithm is introduced and applied to the inverse kinematics solving algorithm.Finally,the forward kinematics solution algorithm of Hexapod platform based on Newton-Raphson algorithm is established,which serves as the feedback mechanism of the inverse kinematics algorithm.Considering the working mode of the secondary mirror adjusting mechanism that needs to move in multiple coordinate system,this paper designs and develops a parallel six-degree-offreedom Hexapod platform coordinate system for the secondary mirror adjustment mechanism.The kinematic form of the platform was summarized,and the concept of tool/working coordinate system was proposed,and the control algorithm program of the platform running in the tool/working coordinate system and the platform kinematics control algorithm under the custom rotation center were designed.Finally,the simulation experiment verifies the accuracy and practicability of the coordinate system control algorithm.After theoretical research and kinematics control algorithm compilation,the driving leg layout design,component selection,electronic control system design and other work are completed.Finally,the high-precision Hexapod platform experimental prototype for the adjustment mechanism of the secondary mirror of large telescope is built.The characteristics of the experimental prototype are: the two ends of the driving legs adopt an improved axial offset Hooke hinge structure,which not only increases the workspace of the platform,but also improves the control precision of the platform.Due to the requirement of micron and arc second repeat positioning accuracy of the submirror adjusting mechanism,kinematics simulation and forward kinematics verification of the proposed inverse kinematics control algorithm are carried out in this paper,which proves the accuracy of the proposed algorithm.At the same time,the existing inverse kinematics solution method based on Newton-Raphson algorithm and the proposed inverse kinematics solution method were compared to prove the efficiency of the proposed inverse kinematics solution method of 6-RRRPRR type Hexapod platform based on the spatial circle distance solution.Finally,the repeated positioning accuracy test is carried out on the experimental prototype,and it is proved that the Hexapod platform designed in this paper fully meets the operating conditions of the secondary mirror adjusting mechanism.