| Space optical remote sensing has become an important field of domestic strategic development.In order to satisfy user's demand for high resolution and large field of view,the aperture of key optical components and the supporting structure in space cameras have become increasingly large and complex,resulting in the contradiction between the increasing weight of whole payload and the limited carrying capacity of rocket.Under such a background,it is of great significance for the development of space optics to study how to further improve the lightweight ratio of supporting structure,so as to reduce the overall weight in space cameras.The application of new structural materials is an important way to improve the lightweight ratio of space payloads.Carbon fiber honeycomb sandwich structure is a special structural material with low surface density,high specific stiffness and good anti-fatigue characteristics,which is regarded as an ideal material for the lightweight design of space cameras.Carbon fiber honeycomb sandwich structure has become one of the indispensable materials in aerospace engineering because of its excellent mechanical properties.Structures of this sort have been applied to many space-based optical instruments abroad,however,its application on space cameras is still in the exploration stage in China,and there is a lack of relevant scientific research practice in this field.Therefore,this paper focuses on the opto-mechanical structure design of space camera based on carbon fiber honeycomb sandwich structure,and a series of related research work has been carried out.In order to adapt to the analysis of space camera,a new honeycomb sandwich structure modeling method based on orthogonal anisotropic solid elements is proposed to replace the two-dimensional modeling method,which is commonly used in engineering analysis.In the new method,honeycomb core is equivalent to the‘interlayer',and solid elements and shell elements are used to represent honeycomb core and panels respectively,so that the stress distribution and local deformation inside honeycomb sandwich structure can be demonstrated.Combining with the mechanical properties of honeycomb,9 independent elastic parameters were derived to describe the equivalent physical properties of the interlayer.Through the numerical comparison with precise model in an example and the modal tapping tests on typical payload mounting panels made of honeycomb sandwich structure,it is shown that the modeling method can fully reflect the mechanical characteristics of honeycomb sandwich structure and its calculation accuracy can meet the requirements of engineering analysis.In order to further improve the lightweight ratio and improve the working performance,carbon fiber honeycomb sandwich structure was used to design the main bearing plate for a certain type of coaxial space camera in this paper.Considering the opto-mechanical structure characteristics of the space camera,an aluminum honeycomb main bearing plate was designed according to the traditional sandwich structure of CFRP panels with aluminum honeycomb core,as well as an all-CFRP main bearing plate based on the concept of all-CFRP honeycomb sandwich structure.M40J epoxy prepreg was used for all the carbon fiber composites in both sandwich structures,and its layering sequence was all carried out in accordance with[0°/45°/90°/-45°]_S.Strengthening measures were introduced into the honeycomb sandwich structure to improve the overall stiffness and local bonding strength.Structure configurations such as the main direction of honeycomb,the ply stacking sequence of CFRP panels and the specification of honeycomb core are discussed and determined.By means of adding aluminum reinforcement ring and improving the structure of penetrating inserts,the overall stiffness and local bonding strength of the plate are greatly improved.According to the load condition and distribution characteristics of the interfaces,as well as the requirements of later grinding and assembly,a variety of inserts are designed.Using the newly proposed modeling method,the whole camera model of each scheme was established,then the influence of honeycomb main direction,core specifications and key sandwich structure parameters on the mechanical characteristics of the main bearing plate was analyzed and discussed,and the influence of the arrangement of bottom supporting points on the working performance of the coaxial camera was also studied.By comparing the two schemes,it was pointed out that the all-CFRP main bearing plate has significant advantages in working performance,lightweight ratio and processing reliability,which is more suitable for the design of opto-mechanical structure with high precision and high stability.Further optimization design was carried out for the all-CFRP main bearing plate scheme.Based on the principle of simplifying structure and improving reliability,two types of compact inserts suitable for sandwich structure are proposed,namely the independent inserts and the patching inserts.The parametric finite element model of the all-CFRP main bearing plate was built,and the integrated optimization model of this scheme was established based on the Isight software environment.The sensitivity analysis was carried out,and the influence of the important structural parameters of honeycomb sandwich on the main precision performance of camera was studied.In the integrated optimization,the optimal combination of structural parameters in the all-CFRP main bearing plate was determined by MIGA algorithm,taking the weight minimization as the objective and the main performance parameters of camera as the constraints.In the optimized scheme,the circular main bearing plate with the diameter of 484mm is 42mm high,the thinnest part of the whole sandwich structure is the honeycomb wall,which is only 0.64mm thick,the upper and lower panels are 1.92mm and 1.28mm thick respectively,and the design weight is only 1.98kg.Through the comprehensive simulations of the space camera,the maximum relative deviation and tilt between the primary and secondary mirrors are only 3.719?m and 3.38",which fully meets the optical tolerance requirements,and the first order frequency of the whole camera reaches 114.4Hz,the amplifications of sine and random vibration are moderate,indicating that the camera has excellent adaptability to the space environment and dynamic characteristics.For the all-CFRP main bearing plate scheme,focusing on the connection strength and structural reliability,a unique manufacturing process of the all-CFRP honeycomb sandwich structure was developed,and the complete forming course was described in detail,and the technical difficulties and relevant manufacturing details were also discussed.An all-CFRP main bearing plate was successfully manufactured,its measured weight is 2.01kg,which is about 67.6%lighter than the original main bearing plate scheme made of aluminum-based silicon carbide.A thermal cycling test was carried out on the plate,and the flatness of these main mounting surfaces was found to be maintained in good condition,using a coordinate measuring instrument.A prototype camera was built with this all-CFRP main bearing plate specimen as the core component.A series of environment tests and corresponding measurements were implemented:a modal tapping test was also carried out on the main bearing plate to measure its modal information;by monitoring the angle change between the reference prisms in the overturn test,the deformation of the prototype under gravity was tested;duing the vibration test of the prototype,the fundamental frequency and the acceleration response of certain key positions were measured,and the angles between reference prisms were measured again;with the real high precision primary mirror connected to the plate,the shape of the mirror was detected with the interferometer,etc.Under the gravity condition of assembly,the relative angle between the primary and secondary mirrors of the prototype is 2.88",while the angle between the primary mirror and the main bearing plate is only 0.36";the measured fundamental frequency of the prototype is 110.74Hz,the acceleration amplification ratios of the key components are moderate,and the angle changes between the main reference prisms are all within 3"after vibration tests;with the main mirror installed onto the main bearing plate,the RMS value of its surface shape is better than 0.02?.Test results show that the all-CFRP main bearing plate scheme not only has good structural stability,but also has appropriate dynamic and static stiffness,which can adapt to the optical system assembly and meet the dynamic requirements for remote sensors.The above work accurately demonstrates the mechanical characteristics of the all-CFRP main bearing plate and the working performance of the space camera.The research of this paper also fully verifies the feasibility of the technical route of all-CFRP main bearing plate,as well as the rationality and correctness of the structure design proposed in this paper.The pioneering work done in this paper provides important guidance and reference significance for the application of carbon fiber honeycomb sandwich structure in space cameras and the optimization of similar structures in the future. |
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