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相机与成像性能和稳定性相关的两个主要指标包括:系统波前误差是光学系统的实际波面与理想波面之间的偏差,反映了光学系统的成像质量;视轴稳定性误差是外界载荷作用下物体在光学系统像面上的成像位置变化,反映了系统指向精度的稳定性[20]。在设计过程中,为了使光机结构能够在满足这两项相机性能要求的同时,实现最优的结构尺寸参数设计,从而具有较高的轻量化率,使用了光机集成分析与优化技术进行了优化设计。
将光机结构的关键尺寸参数作为优化设计变量,将质量、视轴稳定性误差和系统波前误差最小作为目标。与此同时,为避免火箭发射过程中产生共振现象对结构造成破坏,约束结构的一阶固有频率不小于100 Hz。有限元模型如图5所示。建立次镜主支撑结构和主反射镜的集成优化数学模型表示为:
式中:Li为次镜主支撑结构和主反射镜的关键尺寸参数,设置为优化的设计变量;L1~L4分别代表反射镜背板加强筋厚度、支撑孔壁厚、加强筋间距和边缘材料切除高度;L5和L6代表主支撑结构的支撑杆厚度和底部安装面厚度;f1为结构第一阶固有频率,f1≥100 Hz;MASS、LOS和WFE分别为结构质量、相机的视轴稳定性误差和系统波前误差,设置为尺寸优化模型的目标。
此外,由于主背板处于相机的最底部,主要对主反射镜的面形误差以及整机的固有频率具有较大影响,因此,首先开展了主背板以主镜面形误差为主要指标的优化设计。主镜的面形误差需要考虑加工和检测两种状态,在优化模型中将平行光轴方向面形误差作为约束,约束其不大于5 nm;将垂直光轴方向面形误差最小设为目标,尽量提高其面形稳定性,则背板的优化模型为:
式中:Pi为背板的结构尺寸参数;P1~P3分别代表了背板的厚度、高度、加强筋厚度,并且每个尺寸应控制在一定的尺寸范围内
$\underline {{P_i}} \leqslant {P_i} \leqslant {\overline P _i}$ 进行优化;RMSX和RMSZ分别代表主反射镜在水平和竖直重力作用下的面形畸变的RMS值。根据材料的制造工艺性和设计经验制定边界约束条件:1.5≤P1≤4.5,7.0≤P2≤20.0,2.5≤P3≤7.5。 -
采用适用多目标、复杂目标函数、高度非线性和非连续设计空间的第二代非序列排序遗传算法(NSGA-Ⅱ)对参数进行优化。通过多学科软件集成,调用NSGA-Ⅱ算法迭代寻优,求得尺寸参数多目标优化的最优解。主镜和主支撑结构主要尺寸优化结果如表1所示,主反射镜的质量由1.241 kg减小到0.561 kg,次镜主支撑结构的质量由1.109 kg减小到0.577 kg。主背板在满足面形误差设计要求的前提下,背板的厚度、高度及加强筋厚度初始尺寸分别为3、13.5、5 mm,经优化后尺寸分别为2.0、9.6、2.2 mm,质量由1.054 kg减小到0.591 kg。含主镜组件、次镜组件、遮光罩组件、校正镜组件、次镜主支撑结构及主背板在内的光机结构总重仅3.03 kg。
Parameters L1 L2 L3 L4 L5 L6 Initial values 3.0 5.0 30.0 15.0 5.0 5.0 Optimized values 2.3 2.9 32.5 18.6 3.1 3.3 Table 1. Integrated optimization results of the primary mirror and main support structure(Unit: mm)
为了对设计结果进行验证,首先对整机进行静力学分析,在重力、均匀温升4 ℃以及安装不平度0.01 mm耦合工况下,主、次反射镜面形及偏心与倾斜如表2所示,其中偏心和倾斜均以主镜为基准。
Mirror Displacement RMS /nm Decenter/μm Tilt /(″) X Y X Y Primary 1.35 − − − − Secondary 1.03 7.22 4.13 14.87 2.19 Table 2. FEA results of coupling conditions
对优化后的相机进行约束模态分析,前两阶振型云图如图6所示,一阶模态为123.6 Hz,满足相机指标要求。
Optomechanical structure design and experiment of high-resolution video camera for micro-nano satellite(Invited)
doi: 10.3788/IRLA20210477
- Received Date: 2021-07-13
- Rev Recd Date: 2021-08-15
- Publish Date: 2021-10-20
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Key words:
- camera structure /
- spaceborne video camera /
- optomechanical integrated optimization /
- micro-nano remote sensing satellite /
- mechanical performance
Abstract: The design, assembly and experiment of the optomechanical system of a compact spaceborne video camera developed for the 20 kg micro-nano optical remote sensing satellite were introduced, and the integrated optimization method was also proposed. The camera was a Cassegrain optical system including two mirrors and one corrector assembly. In order to obtain the best thermal stability, the mirrors were made of silicon carbide. Firstly, based on the task and overall design of 20 kg micro-nano video satellite, the requirements of video camera were proposed; Then, the optical and optomechanical structure system of the video camera were introduced respectively; In order to further improve the lightweight rate, while meeting the requirements of optical performance, the optomechanical integration optimization method was used to the lightweight design. After optimization, the mass of the optomechanical system was only 3.03 kg, the weight of the whole camera was only 4.76 kg, and the 1st mode was larger than 120 Hz; Finally, the assembly and ground mechanical experiments of the camera were summarized. The results showed that the camera had very dynamic properties and stability.