Based on the application characteristics of flexible unit and the structural characteristics of the baffle components of the high-accuracy star sensor，leaf spring structure is selected as the body of flexible supporting structure for the baffle. The main effects on bending rigidity are analyzed，which are made by length，width and thickness size parameters of the supporting structure. And the main variables are identified and the optimization design is carried out. The simulation analysis verifies that the design result meets the requirement on stiffness and strength，and the deformation of the flange structure with flexible supporting design is 2.14% of that with rigid design. The result shows that the design of flexible supporting structure for the baffle is reasonable and valid. It can satisfy the spatial application demand，and be used as an important method for the design optimization of thermal stability of the high-accuracy star sensor.
High precision star sensors (abbreviated as star sensors) are one of the key components for spacecraft attitude measurement, and currently have the highest accuracy in spacecraft attitude measurement. They play an important role in spacecraft attitude and orbit control systems. With the rapid development of space remote sensing technology, the requirements for attitude measurement accuracy of star sensors are gradually increasing. Star sensors are installed outside the spacecraft cabin and are affected by the alternating thermal environment in space during their operation in orbit, It is prone to structural deformation and increases measurement error
Thermal stability is one of the important factors affecting the low-frequency error of star sensors. Improving the thermal stability of star sensors can effectively reduce the thermal drift of the optical axis of the star sensor, thereby improving its measurement accuracy. In the design of star sensors, thermal noise is an important indicator for selecting image detectors, and the non thermal design of optical systems is the focus of optical mechanical structure design, The software also incorporates algorithms such as thermal compensation to reduce low-frequency errors caused by thermal environments
To further reduce the impact of space thermal environment on star sensors, star sensors usually use thermal control design to provide precise temperature control for the flange structure, making the temperature fluctuation of the flange structure relatively small and its own thermal stability good. Star sensor light shields are generally designed without thermal control, and their inner walls are directly exposed to the space environment, receiving radiation from 4K deep cold background and stars such as the sun. The temperature fluctuation is large and the thermal stability is relatively poor, The structure is prone to deformation. The light shield is generally installed above the flange structure, and its deformation will be rigidly transmitted to the flange structure, causing deformation of the flange structure and affecting measurement accuracy. If a flexible connection is used between the light shield and the flange structure, the impact of the light shield deformation on the deformation of the flange structure can be reduced
This article proposes to design a flexible support structure to replace the rigid connection between the shield and the flange structure for high-precision star sensors used in high-resolution space optical remote sensing satellites, in order to reduce the impact of shield deformation on the flange structure and achieve optimal design of the overall structure thermal stability. The size parameters of the flexible support structure are determined through optimization analysis, and the effectiveness and reliability of the design are verified through simulation
Flexible support design can selectively release the degrees of freedom of the supported object, reduce the stress and deformation of the supported object, and is commonly used in high-precision optical component positioning design and high-precision mechanical component positioning control design
The structural form of flexible support is determined by the degree of freedom design of the supported object, with various structural forms. Commonly used flexible structural units include flexible beams, leaf springs, notched leaf springs, folded leaf springs, and bipods, as shown in Figure 1. To achieve optimal design, flexible support structures are usually composed of multiple flexible units. The flexible rod constrains the translational degrees of freedom of the supported object along the rod direction, Usually, rectangular or circular notches are added to the rod to increase its flexibility in other degrees of freedom. The leaf spring constrains the two translational degrees of freedom and one rotational degree of freedom of the supported object in the leaf spring plane; The cut leaf spring constrains a translational degree of freedom of the supported object in the leaf spring plane; The folding leaf spring constrains the translational freedom of the supported object along the folding edge. The bipod is in a cross or triangular shape, with adjustable length. Generally, flexible hinges are designed at the root and intersection points, and the free state of the supported object is determined by the hinge design
Fig.1 Diagram of flexible structure elements
The installation method of flexible support is divided into edge tangential installation and bottom installation, and the specific installation method is determined by the structural form of the supported object. When designing flexible support, the following principles should be followed: 1) The support must act on the supported object with a small additional force to ensure minimal impact on the supported object; 2) The support system should have sufficient stiffness to meet the positioning requirements of the supported object; 3) Design space is one of the important factors in determining the form of supporting structures; 4) It is necessary to consider the stability and creep effects of the material to ensure that the supported object remains stable over time; 5) The smaller the size and quality of the support structure, the better; 6) Realize performance optimization while considering cost constraints
The flange structure is the main structure of the star sensor and serves as the external installation interface for the star sensor. The light shield is a thin-walled cylindrical structure with a wall thickness of 1 mm, located above the flange structure, as shown in Figure 2. The bottom of the light shield can be connected to the flange structure through the design of the light shield flange, and the connection method is rigid connection, As shown in Figure 3, a rigid connection will transfer the deformation of the hood in the thermal environment to the flange structure. The use of a flexible support structure can reduce the impact of hood deformation on the flange structure. From Figure 2, it can be seen that the design space of the flexible support structure is limited, only the space range between the outer wall of the hood and the outer edge of the flange structure
Fig.2 Relative position between baffle and flange
Fig.3 Rigid connection between baffle and flange
Due to the need for sufficient space in the height direction in the design of the flexible beam structure, as well as sufficient space in the width and height directions in the design of the bipod structure, the star sensor configuration adopts a leaf spring structure suitable for narrow spaces. The area above the installation hole of the flange structure is the operating space, and no other structures are allowed to be installed. Therefore, the installation space of the leaf spring structure is the middle area of the four sides of the flange structure. Based on spatial characteristics and symmetrical design principles, Using a four leaf spring structure as the support structure for the light shield, as shown in Figures 4-5
Fig.4 Top view of flexible support structure of baffle
Fig.5 Side view of flexible support structure of baffle
The material of the light shield and flange structure is generally aluminum alloy material. To reduce the heat transmitted by the light shield to the flange structure, the flexible support structure connecting the light shield and flange structure usually uses materials with relatively small thermal conductivity. At the same time, the flexible support structure should have good mechanical properties and meet the requirements of the space environment. Titanium alloy material has high strength and low thermal conductivity characteristics, which is suitable for high-strength insulation design, Therefore, titanium alloy is selected as the flexible support structure material
The key point of flexible support design is stiffness. If the stiffness is too large, the deformation of the light shield will have an impact on the flange structure, which cannot achieve the expected effect; Just getting too small, the mechanical performance of the star sensor does not meet the requirements of the space environment. This contradiction can be effectively solved by optimizing the flexible support parameters. Based on the sensitivity of structural characteristics and size parameters to design indicators, a preliminary plan is determined, and the parameters are optimized on this plan to obtain the optimal plan
After the initial determination of the parameters of the flexible support structure, the flexible support is further optimized using finite element method. In the optimization design, t is taken as the main optimization variable. The constraint condition is that the fundamental frequency of the hood component is not less than 200 Hz. In this case, the flange structure deformation is minimized, and the strength of the component meets the requirement of a safety margin greater than zero in the spatial mechanical environment
During the analysis process, the installation holes of the flange structure are constrained by 6 degrees of freedom, and the two ends of the flexible support structure are fixedly connected to the light shield and flange structure. The analysis working condition is that the temperature of the light shield decreases from high temperature 40 ℃ to low temperature -70 ℃, and the temperature of the flange structure is controlled by 20 ℃. The finite element model of the component is shown in Figure 8
Through simulation analysis, the flange deformation size and component fundamental frequency corresponding to different size t values were obtained. Table 2 shows that the design stiffness of the flexible support structure of the light shield meets the requirements and can effectively reduce the impact of the light shield deformation on the flange structure; The design strength meets the requirements, and there is no risk of plastic deformation and damage in the spatial thermal environment, with a design safety margin
Tab. 2 Summary of the simulation results
Research has shown that the flexible support design of the sunshade is an important method to improve the thermal stability of star sensors, which is of great significance.
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