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统筹考虑光、机、电、热的相互作用,基于相机所处空间热环境、技术指标、光机结构特点以及设计约束综合考虑。在热设计的过程中,以被动热控为主,主动热控为辅[6];在该热设计基本原则下,相机的整体热控方案如图6所示。
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光阑蒙皮组件包括入光口遮光罩和内部光阑。入光口遮光罩组件在外表面包覆20单元多层隔热组件的基础上,内表面也包覆10单元多层隔热组件,降低了遮光罩加热区的主动加热功耗[7]。选用低反射率涂层绸(αs≥0.90, εH≥0.85)作为内表面多层面膜,避免进入相机内部杂散光增多影响相机的成像质量,如图7所示。
为了最大限度地降低−Y侧两次入射太阳外热流的影响,设计了多层面向导热增强结构,通过在低反射率涂层绸背部内衬柔性碳碳导热膜,在不降低多层法向隔热能力的同时,面向的导热能力提高10倍以上,有效降低了太阳入射热流引起的多层表面温度波动。
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反射镜在轨的温度控制是相机成像成败的关键之一。高分四号相机[8]、某倾斜轨道相机[9-10]等多个型号通过背部设计单层加热罩的方式,控温精度达到±0.5 ℃;针对自由曲面反射镜的高温度敏感性要求,采用分级热控策略对反射镜进行热控(图8),在反射镜背部设置内、外两层辐射加热罩;外侧的辐射热控罩在反射镜支撑板背部,外表面包覆20单元多层隔热组件,内表面喷涂PNC黑漆(εH≥0.95),加热罩内侧和镜体之间形成高发射率空腔,同时热控罩表面粘贴电加热器,反射镜支撑背板的温度可以控制在(20±0.5) ℃;内辐射加热罩设置在反射镜与支撑板之间,内罩外表面包覆5单元多层,同时粘贴高导热率CC材料,增强了加热罩的温度一致性,可以避免单罩下反射镜支撑对反射镜背部的局部遮挡,通过外罩的遮挡,内罩外表面的热扰动大大降低,反射镜控温精度提高到±0.2 ℃。温精度提高到±0.2 ℃。温精度提高到±0.2 ℃。
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在框架主体蒙皮表面包覆多层,有效隔离瞬变的外热流及空间冷黑,减小了热量散失和交变外热流等对其产生的影响,相机内部框架表面喷涂ERB-2B消杂光黑漆(ε≥0.9),以利于其内部温度均匀化,同时进行主动加热分区设计。
框架组件的材料为铸钛合金(ZTC4),材料的导热系数为8.5 W·(m·K)–1,为了增强框架组件的热导性能,在主框架外表面粘贴面向导热系数超过1100 W·(m·K)–1、厚度为0.3 mm的高导热石墨材料(SGF2),如图9所示,处理后的主框架等效导热系数达到112 W·(m·K)–1,能够显著地提高主框架温度均匀性,减少电加热器的使用数量。
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焦面组件包含8片CMOS探测器,单片探测器功耗1.5 W,对应的驱动板功耗1 W,CMOS焦面组件总功耗20 W,每轨工作时间不超过12 min。CMOS器件热量通过设置导热压板和定位块将热量传导至焦面基板,如图10所示,导热路径热阻为1.45 ℃·W–1,可以估算单片CMOS与基板温差不超过3.6 ℃。
焦面基板为铝基复合材料,比热容为820 J·(kg·℃)–1,密度为2700 kg·m–3,质量为4 kg,总热容约3280 J·℃–1。整个焦平面组件与相机腔体内部的辐射换热量可表示为:
$$ \varPhi_{12}=\dfrac{{A}_{1}\left(E_{{b1}}-E_{{b2} }\right)}{{1}/{\varepsilon_{1}}+{{A}_{1}}/{{A}_{2}}\left({1}/{\varepsilon_{2}}-1\right)}=\varepsilon_{{\rm{s}}} \times 5.67\left[\left(\dfrac{T_{1}}{100}\right)^{4}-\left(\dfrac{T_{2}}{100}\right)^{4}\right] $$ (1) $$ \varepsilon_{{\rm{s}}}=\dfrac{1}{{1}/{\varepsilon_{2}}+{A_{1}}/{A_{2}}\left({1}/{\varepsilon_{2}}-1\right)} $$ (2) 式中:εs为系统的发射率;A1、A2和ε1、ε2分别为焦面组件和相机内部腔体的面积及红外发射率。
由于A2比A1大得多,即A1/A2→0,且A1为非凹表面,此时公式(1)可以简化为:
$$ {\varPhi _{12}} = {\varepsilon _1}{A_1}({E_{b1}} - {E_{b2}}) = {\varepsilon _1}{A_1} \times 5.67\left[ {{{\left(\dfrac{{{T_1}}}{{100}}\right)}^4} - {{\left(\dfrac{{{T_2}}}{{100}}\right)}^4}} \right] $$ (3) 其中,ε1取0.7,A1取0.7 m2,当焦面温度T1达到25 ℃,与20 ℃相机内部换热达到20.1 W,可知整个焦面温升不超过5 ℃。
成像电箱功耗100 W,采用石墨烯铝材制成散热结构一体化构件,如图11所示。根据PCB板上大功耗器件的位置,设计对应位置的导热凸台,直接将箱体−Z面设计成散热面;在电箱的±Y面连接两块L型铝板,在铝板表面粘贴OSR作为散热面。
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对相机在轨状态进行热分析仿真,统筹考虑相机与卫星平台的安装点温度、轨道外热流等热边界。仿真分析采用UG12.0/Space thermal 模块。
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根据能量守恒原理,通过空间相机与其所处环境的热交换关系列出相机的热平衡方程组,可以建立热分析计算模型。热平衡方程组如下:
$$ \left\{ \begin{split} & {Q_1} + {Q_2} + {Q_3} = {Q_4} + Q{}_5 \\ & {Q_1} = \left( {{\alpha _{s1}}{\varPhi _{11}}S + {\alpha _{s1}}{\varPhi _{21}}{E_{r1}} + {\varepsilon _{h1}}{\varPhi _{31}}{E_{e1}}} \right){A_1} +\\ & \qquad \left( {{\alpha _{s2}}{\varPhi _{12}}S + {\alpha _{s2}}{\varPhi _{22}}{E_{r2}} + {\varepsilon _{h2}}{\varPhi _{32}}{E_{e2}}} \right){A_2} +\\ & \qquad \left( {{\alpha _{s3}}{\varPhi _{13}}S + {\alpha _{s3}}{\varPhi _{23}}{E_{r3}} + {\varepsilon _{h3}}{\varPhi _{33}}{E_{e3}}} \right){A_3} \\ & {Q_2} = \sum {{H_{ij}}({T_i} - {T_j}) + } \sum {{\varepsilon _{kl}}{\varPhi _{kl}}\sigma ({T_k}^4 - {T_l}^4)} {A_{kl}} \\ & {Q_3} = \sum {{q_i}} \\ & {Q_4} = \sum {\left({m_i}{c_i}\frac{{\partial T}}{{\partial \tau }}\right)} \\ & {Q_5} = {\varepsilon _{h1}}\sigma {T_1}^4{A_1} + {\varepsilon _{h2}}\sigma {T_2}^4{A_2} + {\varepsilon _{h3}}\sigma {T_3}^4{A_3} \\ \end{split} \right. $$ 式中:Q1为相机吸收空间热量;Q2为相机与卫星平台之间的热交换;Q3为相机内部热源;Q4为相机自身产生的能量变化;Q5为相机入光口处对冷黑空间的热辐射能量;αs1、αs2、αs3分别为遮光罩、主镜以及框架表面的太阳吸收系数;εh1、εh2、εh3分别为相机外多层、遮光罩内侧以及主镜表面的红外半球发射率;Φ11、Φ12、Φ13分别为相机外多层、遮光罩内侧以及主镜表面的太阳辐照角系数;Φ21、Φ22、Φ23分别为相机外多层、遮光罩内侧以及主镜表面的地球反照角系数;Φ31、Φ32、Φ33分别为相机外多层、遮光罩内侧以及主镜表面的地球辐照角系数;S为太阳常数,S=1353 W/m2;Er1、Er2、Er3分别为相机外多层、遮光罩内侧以及主镜表面的地球反照热流密度;Ee1、Ee2、Ee3分别为相机外多层、遮光罩内侧以及主镜表面的地球辐照热流密度;A1、A2、A3为分别为相机外多层、遮光罩内侧以及主镜表面的有效换热面积;i、j为存在接触导热关系的组部件;k、l为存在辐射导热关系的组部件;Hij为相机内部组部件;T为光学相机内部组部件的温度;εkl为光学相机内部组部件表面发射率;Φkl为光学相机内部组部件辐射角系数;σ为玻耳兹曼常数,σ=5.67×10−8 W/(m2·K4);Akl为相机内部组部件参与辐射换热的有效面积;qi为相机内部热源;mi为相机内部组部件质量;ci为相机内部组部件热容量。
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有限元建模遵循几何等效和热容等效的原则,基于结构模型进行CAE建模,在热容等效的前提下进行模型简化[11-12],忽略了螺孔、倒角等细小特征,在不影响传热路径的情况下,忽略不关注热分布的零件,采用设置热耦合的方式简化传热路径,正确反映热量的传递,共建立117个热耦合;手动划分7587个节点、7844个单元,有限元模型如图12所示。
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分析工况选取主要考虑相机相对于空间热环境的位置、飞行姿态、内热源、热控涂层退化、卫星安装边界条件5个方面的变化[13-14]。根此设置相机在轨极端低温、极端高温两个工况,如表1所示。
表 1 热分析工况设置
Table 1. Thermal analysis condition setting
Case β angel/(°) Surface coating Operating mode Temperature boundary Cold
Hot16.92
27.15Early
EndIn operation
12 min/orbit−5
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相机反射镜组件和电子学组件瞬态温度变化如图13所示。由分析结果可知,低温工况下主镜、二四镜、三镜的温度波动分别为19.83~20.10 ℃、19.80~20.13 ℃、19.91~20.04 ℃;高温工况下主镜、二四镜、三镜的温度波动分别为19.88~20.10 ℃、19.81~20.14 ℃、20.02~20.20 ℃;反射镜组件温度稳定,高、低温工况下温度波动不超过±0.2 ℃。非摄像期间,CMOS组件温度为19.5~19.8 ℃;摄像期间,CMOS组件温度为19.6~23.6 ℃,CMOS焦面组件温度波动不超过4.5 ℃,成像电箱温度波动不超过8 ℃。
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通过热平衡试验数据与热分析结果对比,能够找出热分析模型建立、材料属性、载荷和边界条件等方面的不足[15-16],可以验证仿真分析的正确性和温度预示的有效性,并对热设计优化提供指导。
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整机热平衡试验在ZM4300 空间模拟器中进行,空间模拟器内压力≤1.3×10−3 Pa,热沉温度(100±5) K。热平衡试验现场如图14所示。
卫星平台、太阳帆板、高分相机均使用模拟件,其中卫星平台安装板、高分相机使用闭环控温提供温度边界。采用多层内表面粘贴加热片的方式模拟相机吸收外热流,使用红外加热笼模拟入光口到达外热流,并在相机入光口布置热流计进行热流密度的监测与闭环控制。
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相机整体温度水平的热分析结果与热试验结果对比见表2。可以看出,试验过程中主镜、二四镜温度不变,三镜温度波动0.23 ℃,热分析与热试验结果对比最大偏差均在5%以内。通过热分析结果与热试验结果对比可见,两者结果吻合较好,验证了热分析结果的有效性。
表 2 热分析结果与热平衡试验结果对比
Table 2. Comparison between thermal analysis and thermal balance test
Position Hot case Cold case Analysis/℃ Thermal balance test/℃ Max deviation Analysis/℃ Thermal balance test/℃ Max deviation Primary mirror 19.82-20.10 20.06 1.20% 19.72-20.10 20.06 1.50% Second and forth mirror 19.77-20.12 19.99 1.10% 19.76-20.12 19.99 1.10% Third mirror 19.90-20.20 20.1 0.99% 19.90-20.04 19.78-20.1 1.43% CMOS 19.92-23.10 20.12-22.82 1.39% 19.32-19.90 18.82-19.26 3.40% Imaging electric box 13.68-20.62 13.78-19.08 4.06% 14.22-15.12 14.88 4.44%
Thermal control design and verification of extra-wide field-of-view camera for GF-6 satellite
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摘要: 甚宽视场相机作为高分六号卫星的核心载荷,具备65.6°视场、862 km超大幅宽和8谱段成像能力。针对其自由曲面离轴四反光学系统的结构特点和任务需求,采用复合型多层隔热组件进行热隔离、高导热率石墨膜进行热疏导及分级热控等措施进行了热控设计,实现了光机结构的精密控温和高热耗/热流密度电子学设备的高效散热,并利用有限元分析软件UG12.0/Space thermal仿真分析了相机高、低温工况下的温度;通过对比热分析、热试验及卫星在轨遥测温度数据,验证了该热控方案的实际效果。在轨遥测数据显示:光机结构在轨温度水平为19.7~20.3 ℃,温度梯度最大不超过0.4 ℃,CMOS焦面组件每轨摄像12 min的情况下,温度波动在19~24 ℃,均满足热控指标要求,遥测数据与热分析及热试验结果偏差小于±0.5 ℃。表明该相机热设计正确可行,热分析及热试验过程合理可靠。Abstract:
Objective The extra-wide field-of-view camera adopts an off-axis four-mirror optical system, and the second and forth mirror adopt an integrated free-form surface structure. A free-form surface is an asymmetric structure that is highly sensitive to thermal changes. Even with a uniform change in bulk temperature, the optical-mechanical structure still undergoes asymmetric geometric changes. In addition, the extra-wide field-of-view camera needs to meet the design requirements of a width of 860 km and a field of view of 65.6°, and the entrance mask adopts a wide special-shaped opening design. The sun shines directly on the inside of the hood for a duration of 5.5 minutes as it enters and exits the Earth's shadow. Both optical and mechanical errors are caused by heat cause line of sight drift (LOS) and wavefront distortion (WFE) in the camera. These issues seriously affect the optical transfer function of the system. Considering its structural characteristics and the thermal control challenges brought about by the large change of heat flow outside the light entrance, targeted thermal control measures must be taken for different parts of the camera to meet the thermal control index requirements of off-axis free-form surface cameras with special-shaped optical apertures in orbit. Methods Six aspects of the camera are analyzed, including its on-orbit state, structural layout, task requirements, orbital environment, technical indicators, and heat sources. The thermal control design is implemented by using composite multi-layer heat insulation components for thermal isolation (Fig.7), graded thermal control for mirrors (Fig.8), and high thermal conductivity graphite film for thermal conduction (Fig.9). This design allows for precise control of the optical-mechanical structure and efficient heat dissipation of high heat consumption/heat flux electronic equipment. The temperature of the camera is simulated and analyzed under high and low temperature conditions using the finite element analysis software UG12.0/Space thermal. The effectiveness of the thermal control scheme is verified through thermal analysis, thermal test, and satellite on-orbit telemetry temperature data. Results and Discussions The transient temperature changes of the camera mirror assembly and electronic components are presented. Under low temperature conditions, the primary mirror exhibits a temperature fluctuation ranging from 19.83 ℃ to 20.10 ℃, while the second and fourth mirrors experience a temperature fluctuation between 19.80 ℃ and 20.13 ℃. The temperature fluctuation of the third mirror falls within the range of 19.91 ℃ to 20.04 ℃ (Fig.13(a)). Similarly, under high temperature conditions, the temperature fluctuation of the primary mirror ranges from 19.88 ℃ to 20.10 ℃, while the second and fourth mirrors exhibit a fluctuation between 19.81 ℃ and 20.14 ℃. The temperature fluctuation of the third mirror ranges from 20.02 ℃ to 20.20 ℃ (Fig.13(b)). It is worth noting that the reflector assembly maintains a stable temperature, with fluctuations not exceeding ±0.2 ℃ under both high and low temperature conditions. During the non-camera period, the CMOS component maintains a temperature range of 19.5 ℃ to 19.8 ℃ (Fig.13(c)). However, during the imaging period, the temperature of the CMOS component varies between 19.6 ℃ and 23.6 ℃. The temperature fluctuation of the CMOS focal plane component does not exceed 4.5 ℃, and the imaging electrical box experiences a temperature fluctuation within 8 ℃ (Fig.13(d)). The overall temperature level of the camera, as determined by thermal analysis, is compared with the thermal test results (Tab.2). The table shows that the temperature of the primary mirror and the second and fourth mirrors remained constant during the test, while the temperature of the third mirror fluctuated by 0.23 ℃. The maximum deviation between the thermal analysis and thermal test results is within 5%. The comparison of the thermal analysis results with the thermal test results confirms the validity of the thermal analysis. Conclusions The deviation between the on-orbit telemetry data and the thermal analysis and thermal test results is within ±0.5 ℃. This indicates that the thermal design of the camera is accurate and feasible, and the thermal analysis and test process are reasonable and reliable. The employed thermal control measures and design methods are suitable for the thermal design of extra-wide space optical camera. -
表 1 热分析工况设置
Table 1. Thermal analysis condition setting
Case β angel/(°) Surface coating Operating mode Temperature boundary Cold
Hot16.92
27.15Early
EndIn operation
12 min/orbit−5
30表 2 热分析结果与热平衡试验结果对比
Table 2. Comparison between thermal analysis and thermal balance test
Position Hot case Cold case Analysis/℃ Thermal balance test/℃ Max deviation Analysis/℃ Thermal balance test/℃ Max deviation Primary mirror 19.82-20.10 20.06 1.20% 19.72-20.10 20.06 1.50% Second and forth mirror 19.77-20.12 19.99 1.10% 19.76-20.12 19.99 1.10% Third mirror 19.90-20.20 20.1 0.99% 19.90-20.04 19.78-20.1 1.43% CMOS 19.92-23.10 20.12-22.82 1.39% 19.32-19.90 18.82-19.26 3.40% Imaging electric box 13.68-20.62 13.78-19.08 4.06% 14.22-15.12 14.88 4.44% -
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