| The technique of space-borne optical remote sensing is an indispensable method to ensure national economic construction,an important technical safeguard to national security,and the key to victory in modern warfare.Space-borne optical remote sensing cameras are developing towards the trend of high resolution and large field of view with the further development of satellite remote sensing technology.space-borne off-axis Three Mirror Anastigmatism(TMA)cameras have become one of the most important developing tendency.However,the design and lightweight of the primary mirror of the large off-axis TMA optical system,the flexural support design of the mirror,and the precise mounting and positioning of the primary mirror assembly(PMA)are all technical difficulties in current development.Therefore,it is necessary to research on the lightweight of large-size mirror and its flexural mounting system.The rectangular primary mirror is the most crucial components of the off-axis optical system.The structural stability and reliability of the PMA and the surface figure error(RMS value)of the mirror affect the image quality of the optical system directly;and the lightweighting determines the total mass of the PMA and the stiffness of the main support frame.The mass,surface figure error and first-order natural frequency of the PMA must meet the following requirements: the optimized mass of mirror should be less than 60 kg,the surface figure error under 1G gravity load should be better than/50(=632.8nm)and the first-order natural frequency of the PMA must be more than100 Hz.This paper systematically explores and researches the lightweight of the largesize rectangular primary mirror and its mounting technology of a off-axis TMA optical system.Aiming at the issue of selecting materials and designing configuration of the mirror,the selection principle of material for the mirror has been given interpretation in detail.among the commonly used mirror materials,Reaction Bonded Silicon Carbide(RBSiC)with larger specific stiffness,lower thermal expansion coefficient and higher stability in space environment have been selected as the mirror material.The lightweighting configurations and the lightweighting forms of reinforcing rib structures for the mirror has been studied,then the designing scheme of the large-size rectangular mirror has been determined with the semi-closed backplate,the triangular reinforcing rib structure and the three-point supporting method.The initial structure of the mirror has been optimized based on the Solid Isotropic Material with Penalization Model(SIMP)topology optimization method,and two different design configurations have been obtained with maximizing the stiffness and maximizing the first-order natural frequency of the mirror respectively.In order to cover the defects of these two design configurations,a topology structure for the mirror has been optimized using an integrated optimization design method,then the dimensional optimization of the mirror has been performed,the optimized large-size lightweighting rectangular mirror design has been obtained finally.Aiming at the mounting issue for the large-size rectangular mirror,a bi-axial flexural support has been proposed based on the principle of kinematic equivalent.The flexural parts of the bi-axial flexural support can be equivalent to a flexural short straight Euler beam with one constrained end,the analytical formula for stiffness and its characteristic have been studied,which has provided the theoretical foundation for dimensional optimization and structural parametric calculation of of the flexural supports.In addition,the invar inserts and baseplate of the PMA have been optimized.The mounting position of the flexural supports and the key dimensions of the flexural parts have been optimized.The mounting mechanism of the mirror have been studied and the conditions have been put forward to interprete the optimal mounting.After optimizing the axial mounting position of the mirror,an important conclusion has been drawn: the neutral surface of the large-size rectangular mirror is a curved surface with the center of gravity on it,according to which the flexural supports have been analyzed with different height.The parametric study of the precise mounting angles and the key dimensions of three flexural supports have been optimized.According to the optimized design values,the final optimization design scheme of the PMA have been determined.The static overload analysis of the PMA has been performed,and the stress characteristics of it has been obtained,which can ensure sufficient safety margin.Aiming at the dynamic characteristics of the PMA,dynamic vibration test of the mechanically simulated prototype based on the PMA structure has been conducted.The first-order natural frequency and dynamic response characteristics of the mirror assembly have been obtained through the dynamic environment test.The stiffness and strength of the bi-axial flexural support structure,the accuracy and rationality of finite element analysis under dynamic load have been verified through the experimental test results.The optimized lightweighting design of large-size rectangular mirror and the optimized design of flexural mounting system have been studied systematically in the paper.The large-size lightweighting rectangular mirror is only 54.3kg to reach 86.5%lightweighting ratio after final optimization design.According to the results of finite element analysis,the surface figure error(RMS value)of the PMA under 1G gravity in x and y radial directions are 4.81 nm and 6.09 nm respectively,better than /50(=632.8nm).The first-order natural frequency is 118.22 Hz,which is more than 100 Hz and can satisfy the design requirements.The dynamic tests have shown that the dynamic characteristics of the mirror assembly are good,and the flexural support system is stable and reliable.The relative errors of the first-order natural frequency and engineering analysis in three orthogonal directions are 7.36%,9.22% and 3.16% respectively,which have been verified the rationality and accuracy of the finite element simulation analysis and flexural support modeling theory of the PMA.This research can solve the technical problems of the lightweighting design of large-size ractangular mirror with a length of nearly 2m and its flexural mounting syetem,which can be certain reference for the development of the PMA of space-borne large size off-axis reflective optical system. |
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