Structural Design and Support Characteristics Analysis of Cryogenic Mirror Assembly
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摘要: 红外光机系统在低温环境工作能够抑制背景噪声提高探测灵敏度，也提高了反射镜组件结构设计难度。低温反射镜支撑结构设计主要问题是由反射镜与连接件热线胀系数不同导致的温度变化工况下的面形变化。对工作于240 K的ϕ450 mm反射镜组件进行结构设计，反射镜材料为SiC，连接件材料为殷钢，采用背部中心单点支撑形式与三角形轻量化形式，并设计柔性连接件提高低温面形表现。对主要设计参数进行优化分析，得到各参数对面形的影响曲线。优化后，反射镜光轴方向重力面形为8.585 nm，径向重力面形3.710 nm，240 K低温面形5.086 nm，一阶模态277 Hz，轻量化率89.4%。Abstract: The infrared opto-mechanical system can improve detection sensitivity by working in a cryogenic environment to reduce background radiation, which causes many technical challenges for mirror assembly structure design. In a cryogenic environment, different coefficient of thermal expansion (CTE) of the mirror and the connector cause the surface accuracy change to be the main problem. Design the structure of the ϕ450 mm mirror assembly working at 240 K. The mirror material is SiC, and the connector material is Invar. The support method is rear support in the center. Great flexibility is designed for the connector to improve surface accuracy. Further, the main design parameters are optimized and analyzed. The influence curves on the surface accuracy are obtained. The root mean square (RMS) of gravity along the optical axis is 8.585 nm, the RMS along the radial direction is 3.710 nm, and the RMS is 5.086 nm working at 240 K. The first order frequency is 277 Hz, and the lightweight rate is 89.4%.
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图 6 面形RMS随镜面背部与配合面距离变化趋势
Figure 6. Surface accuracy varying with the distance between face back and mating surface
图 8 面形RMS随镜体厚度变化趋势
Figure 8. Variation trend of surface accuracy varying with height of mirror
表 1 常用反射镜材料的性能和品质因数
Table 1. Performance and quality factors of rational materials for mirror
Material Density ρ/(g/cm3) Young's modulus E/(GPa) Specific stiffness E/ρ(10-6·GNm/g) CTE α(10-6/K) Thermal conductivity λ/[W/(m·K)] Thermal distortion ratio λ/α(106 W/m) Fused silica 2.19 72 32.88 0.50 1.4 2.8 Zerodur 2.53 91 35.97 0.05 1.64 32.8 Al 2.70 68 25.19 22.50 167.00 7.42 Be 1.85 287 155.14 11.40 216.00 18.95 SiC 3.2 400 125.00 2.40 155.00 64.58 Si 2.33 131 56.22 2.60 137.00 52.69 ULE 2.21 67 30.32 0.01 1.31 131 
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表 2 反射镜组件分析结果
Table 2. Analysis results of mirror and flexible connector
nm Connection type RMS-X RMS-Z RMS-T Rigid 1.284 7.027 165.844 Flexible 5.362 9.039 9.869
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表 3 优化参数取值
Table 3. Optimize parameter values
mm Parameters Height of mirror Diameter of hole Hole thickness Distance between face back and mating surface Chamfer height Initial 80 110 10 12 8 Final 80 100 12 10 10
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表 4 优化后反射镜性能
Table 4. Optimized mirror performance
RMS-X/nm RMS-Z/nm RMS-T/nm First order frequency/Hz Light weight rate 3.710 8.585 5.086 277 89.4%
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