| With the development of space technology,space telescopes are advancing towards high resolution,wide field of view,and high optical performance.Due to the impact and vibration of large space telescope during launch and the temperature change and gravity release after orbit,the relative positions of the primary and secondary mirrors are shifted,which affects the image quality.Therefore,an adjustment mechanism is needed to adjust the relative position of the primary and secondary mirrors.Compared with adjusting the primary mirror,adjusting the secondary mirror requires less power consumption and higher efficiency,so most space telescopes adopt the method of adjusting the secondary mirror to improve the image quality at home and abroad.The secondary lens adjustment mechanism shall have multiple degrees of freedom,shall not block the light path,and shall have high enough accuracy and load-bearing capacity.Therefore,the parallel mechanism becomes the first choice of the secondary mirror adjusting mechanism.In this paper,the secondary lens adjustment mechanism of a 2m magnitude large off-axis three-mirror space camera is studied.Under the working state,the secondary mirror is placed horizontally as an optical axis,so a 6-PUU parallel mechanism of6-DOF is designed as the secondary mirror adjustment mechanism.The secondary mirror adjusting mechanism puts the driving device at the rear end of the mechanism and effectively reduces the weight of the outrigger.In addition,ANSYS software was used to optimize the structure of the adjusting mechanism to minimize the displacement of the gravity direction of the secondary mirror,and the position parameters of the upper and lower 12 Hooke hinges were taken as variables.Through the results of structural optimization design,the overall structure size of the secondary mirror adjustment mechanism is determined,and then the whole machine is designed in detail.The backplane of the secondary mirror is the core part of the secondary mirror component.It not only bears the secondary mirror but also serves as the output end of the adjustment mechanism and plays the role of connecting the preceding and the following.Therefore,this topic will adjust the mechanism of the moving platform directly as the secondary mirror back,and for the secondary mirror back making topology optimization and size optimization design,so that it has sufficient stiffness under the weight of the minimum.The weight of the final secondary mirror backplane is reduced by 75% after optimization,and the total weight of the secondary mirror component is 19.4kg after the design.In this paper,the mathematical models of forwarding and backward solutions of the secondary mirror adjusting mechanism are established by the closed-loop vector method.The inverse kinematics simulation of the adjusting mechanism was carried out by using ADAMS simulation software.In addition,three positions of the moving platform were randomly selected,and the accuracy of forward and inverse kinematics was verified by MATLAB and ADAMS co-simulation.At the same time,the dynamics mathematical model of the secondary mirror adjusting mechanism was established based on the Lagrange method.The thrust of the six actuators acting on the screw nut of the adjusting mechanism was obtained through ADAMS simulation,and the maximum thrust was 51.1N at a given pose.To investigate the mechanical properties of the secondary mirror adjusting mechanism,the finite element model of the secondary mirror adjusting mechanism was established by using Hypermesh software,and the maximum displacement and surface shape RMS values（Root mean square value represents surface shape accuracy）of the secondary mirror component were analyzed under gravity conditions,4℃temperature rise conditions（reference temperature is 20 ℃）and the coupling conditions of gravity and temperature rise of 4℃.According to the static analysis results,the RMS value of the secondary mirror shape is the largest at 7.535 nm under the coupling conditions,which is less than λ/50（λ is 632.8nm）.The displacement in gravity direction（X direction）of the secondary mirror is 9.063μm under gravity conditions and 9.615μm under coupling conditions,both less than 0.02 mm.At the same time,the modal analysis is carried out for the sub-mirror component,and the results show that the first-order natural frequency of the sub-mirror component is115 Hz under constraints,greater than 100 Hz.The above analysis results all meet the requirements of the index and can be used as a reference for the design of similar projects. |
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