Research On Hardpoint Positioning Mechanism Of Pneumatic Active Supporting For SiC Primary Mirror

With the increasing aperture of the primary mirror for the telescope,the primary mirror support system not only needs to actively control the surface precision of the primary mirror,but also adjusts the position of the primary mirror in real time to accurately align the optical system according to the changes in the actual observation process of the telescope.The active support system of the primary mirror of the large aperture telescope can be divided into two parts,support system and positioning system.The support mechanism is used to ensure the surface precision of the primary mirror shape.The positioning mechanism is used to constrain the six degrees of freedom of the primary mirror and ensure the accuracy of the primary mirror position.The rigidity of the positioning mechanism largely determines the natural frequency of the primary mirror support system.According to this characteristic,it can be called as hardpoint.Hardpoint is the execution unit of the primary mirror position adjustment.In order to ensure the high-precision position adjustment,it is necessary to optimize the performance parameters of the hexapod hardpoint positioning mechanism.The accurate and reasonable design of the hardpoint positioning mechanism is an important technology that needs to overcome in the development of large aperture telescopes.As the high-precision requirements of the telescope position adjustment,the primary mirror adopts an active support system which is combined by the hexapod hardpoint positioning mechanism and the pneumatic force actuator,which can realize the real-time adjustment of the primary mirror position to ensure the surface accuracy.A set of hexapod hardpoint positioning mechanism was designed and tested for the performance requirements of the SiC primary mirror with a position accuracy of ±0.02 mm and a natural frequency of the support system of 15 Hz or more.Firstly,this paper introduced the principle and classification of hardpoint in detail,and pointed out the application of different hardpoints in the primary mirror support system of telescopes at home and abroad.Secondly,a model of hardpoints was established,and kinematics and dynamics analysis were carried out.The degree of freedom of hardpoints was analyzed from the perspective of kinematic constraints.Then,the finite element method is applied and the relevant parameters of the hardpoints positioning mechanism are analyzed by means of finite element software.Combined with the analysis of positioning accuracy and positioning stiffness,the specific structural design of the hardpoint positioning mechanism was carried out.The key components have been selected and designed,mainly including position actuator,flexible supports,axial Breakaway mechanism,load cell and torque sensor.Finally,a set of test pieces of hardpoint positioning mechanism was manufactured.The test platform was built with the existing conditions of the laboratory to test the performances of key components,including minimum resolution and repetitive positioning accuracy test,axial stiffness of flexible supports and angle stiffness test.The hexapod hardpoint positioning mechanism designed in this paper is equally distributed at 1345 mm of the radius on the circumference in the primary mirror backface.The axial stiffness of a single hardpoint reaches 15000 N/mm,and the minimum resolution of the output is 1.05 μm.The primary mirror is realized the precise positioning with a position accuracy of ±0.02 mm,and ensured that the natural frequency of the primary mirror support system is greater than or equal to 15 Hz at the same time,meeting the design requirements.