微型星敏感器热稳定性设计与试验验证

Design and Experimental Verification of Thermal Stability forMicro Star Sensors

新一代商业遥感卫星为了实现高精度地形测绘、厘米级地表形变检测,分辨率需要优于0.5 m,但热控条件不如大卫星充裕,微型星敏感器的安装面温度存在较大的变化浮动,引起光轴指向漂移,影响卫星姿态控制精度和高分辨率指标。为此,首先提出了一种热稳定性试验方法,模拟在轨环境,测试微型星敏感器的光轴指向变化,随后根据试验数据分析和仿真结果,对产品的结构和材料进行针对性的优化设计,最后对改进的产品进行热稳定性仿真和试验验证。结果表明,热稳定性试验方法可以有效测量星敏感器受到热应力变形产生的光轴指向偏移量,材料的优化可以提高整机结构强度、减小热变形,结构对称设计可提高光轴指向X、Y两个方向的平衡性,最终使得光轴指向偏移从1.0394″/℃减小到0.1695″/℃。遮光罩独立安装并隔热的设计可以有效改善遮光罩受热形变对光轴指向的影响,光轴指向偏移从0.2403″/℃减小到0.0054″/℃。

Objective The resolution of the new generation of commercial remote sensing satellites should be better than 0.5 m for highprecision topographic mapping and centimeter-scale surface deformation detection, but the thermal control conditions are not as abundant as those of the satellite, and the temperature of the mounting surface of the micro star sensor fluctuates greatly, which leads to the drift of the optical axis and affects the attitude control precision of the satellite as well as the high-resolution index. Star sensors work in the harsh environment with strong radiations, bearing periodic heating and cooling from the solar radiation and the space heat sink, experiencing a variety of thermodynamic environment drastic changes, so that the temperature of star sensors fluctuates periodically. The fluctuation of the ambient temperature leads to the fluctuation of the temperature of the baffle of the star sensor up to 80 ℃ and the fluctuation of the mounting surface up to 40 ℃. The temperature distribution in the star sensor is not uniform, resulting in thermal stress and thermal deformation, which leads to wavefront distortion of the optical system and image blurring, thus affecting the attitude measurement accuracy. In order to ensure the normal operation of the star sensor in orbit, it is urgent to study the temperature distribution characteristics of the star sensor and the mechanism influencing the measurement accuracy of the star sensor,with the operating environment in orbit and the structural characteristics of the star sensor considered. The influence of temperature change on optical axis pointing offset of star sensor is studied quantitatively by thermal stability test. It provides an important theoretical basis and technical method for the scheme design, index demonstration, performance evaluation and improvement of star sensor system. In a word, it is of great significance and practical value to study the thermal stability test method and corresponding improvement and optimization of star sensor.Methods Firstly, a set of thermal stability testing equipment is constructed and thermal stability tests are conducted on the current star sensor, as shown in Fig. 1. In response to the high accuracy of thermal stability test and high sensitivity to the surrounding environment, a marble optical platform is used on the isolation foundation to ensure the isolation of star sensors, static multi star simulators, and installation brackets from the surrounding environment. Besides, molecular pumps and flexible pipelines are used to prevent vibration transmission during vacuum operations.Secondly, we conduct thermal stability simulation on the star sensor, simplify the structural model of the star sensor, remove small features and holes, refine the grid of the circuit box area locally, and establish a thermal simulation model. The meshing of the model and coordinate system definition are shown in Fig. 4. Under the given thermal boundary conditions(installation surface temperature control of 10-30 ℃, mainly achieved through precision temperature control modules on the inner side of the bracket, and temperature control of 25-45 ℃ for the baffle, mainly achieved through an outer ring of heating plates), a micro star sensor model is used to complete the thermal simulation according to the simulation input conditions. The simulated temperature field is mapped to the finite element model for structural thermal deformation simulation. The change in the optical axis is calculated based on the tilt result data of the optical axis.Based on the simulation results and the thermal conductivity and expansion coefficients of the material, as shown in Table 2, the structure and material are optimized. Silicon carbide is selected and the structure is designed symmetrically. In order to avoid the baffle being exposed to sunlight and heat being transmitted to the star sensor circuit box, the installation method of the baffle is improved and changed from the original contact type installation to an isolated type installation(Fig. 6). The baffle is fixed to the installation surface through a bracket and an insulation washer.Finally, thermal stability tests are conducted on the improved star sensor design to verify the simulation results.Results and Discussions In this paper, we give a thermal stability design and experimental verification method for micro star sensors.Most previous literature has modeled the optical mechanical system of star sensors and conducted simulation analysis by setting temperature, structure, and other parameters. A few have described the thermal stability test methods of star sensors. However, due to the complexity of the optical mechanical system of the star sensor and the influence of solar radiation changes on the space orbit,simulation analysis is difficult to fully reflect the actual situation in orbit. The reported thermal stability test method is only a test of existing products and cannot improve the thermal stability of the star sensor. Instead, we propose a thermal stability test method to simulate the in-orbit environment and test the changes in the optical axis direction of the micro star sensor. Then, based on the analysis of experimental data and simulation results, targeted optimization design is carried out for the structure and materials of the product. Finally, thermal stability simulation and experimental verification are carried out on the improved product. The results indicate that the thermal stability test method can effectively measure the optical axis pointing offset caused by thermal stress deformation of the star sensor. Material optimization can improve the overall structural strength and reduce thermal deformation, and structural symmetry design can improve the balance between the optical axis pointing in both directions, ultimately reducing the optical axis pointing offset from 1.0394″/℃ to 0.1695″/℃. The design of independent installation and insulation of the baffle can effectively reduce the impact of thermal deformation of the baffle on the optical axis direction, reducing the optical axis pointing offset from 0.2403″/℃ to 0.0054″/℃.Conclusions In this paper, we simulate the in-orbit environment and conduct thermal stability tests on existing micro star sensors based on the thermal stability requirements and harsh thermal control conditions of the new generation of commercial remote sensing satellites. Based on the test results and simulation results, the optical mechanical structure of the star sensor is symmetrically balanced, the main frame material is improved, the baffle is installed independently, and insulation is optimized to improve the thermal stability ability. The optimized design of the micro star sensor increases the optical axis pointing accuracy. Thus, it can adapt to the harsh thermal control conditions of commercial remote sensing satellites and ensure the stability of star sensor optical axis pointing under a wide range of temperature fluctuation. The thermal stability test scheme, simulation design scheme, optical mechanical structure optimization design scheme, and measured data mentioned in this paper can provide some reference for other space situational awareness sensors.