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为研究机构是否满足设计要求,利用仿真软件对其进行有限元仿真分析。
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首先对其进行模态分析[14],仿真结果如图4所示,得到其一阶模态为1157.4 Hz,模态频率较高,六阶模态频率如表1所示。进行模态仿真分析能够在设计控制系统时避开相应模态频段,防止共振。
表 1 6阶模态频率
Table 1. Sixth order modal frequency
Modal order Modal frequency/Hz Modal shape 1 1157.4 Rotating around Y 2 3178.6 None 3 3483.3 None 4 3532.5 None 5 5553.1 None 6 6855.7 None -
对柔性铰链进行应力分析。柔性铰链选用钛合金(TC4)材料,仿真结果如图5所示,偏转角度为
$ {\text{779 μrad}} $ 时最大应力为$ {\text{62}}{\text{.7 MPa}} $ ,满足钛合金材料应力特性要求。 -
给予压电陶瓷不同驱动位移,得到不同偏转角度,绘制如图6所示的曲线图,可以看出其斜率约为
$ {\text{48}}{\text{.7}} \;{{{\text{μrad}}} / {{\text{μm}}}} $ ,位移为$ {\text{16 μm}} $ 时,得到的偏转角度为$ {\text{779 μrad}} $ ,满足$ {\text{500 μrad}} $ 设计目标。 -
根据设计方案研制一套超前瞄准机构,主要特性如表2所示。对该超前瞄准机构进行一系列测试实验。
表 2 整体设计的主要特性
Table 2. Main features of the overall design
Design features Values Volume $ {\text{56 mm}} \times {\text{35 mm}} \times {\text{32 mm}} $ Material TC4; AL6061; K9 Quality About $ {\text{150 g}} $ Mirror size ${ {\varPhi 10 \; {\rm{mm} } } }$ Surface accuracy ${\lambda}/\text{30}\left(\text{RMS}; {\lambda=632}\text{.8 nm}\right)$ -
首先,对偏转角度和偏转精度进行了验证。在常温常压相对湿度60%的环境下,利用自准直仪对机构的偏转特性进行验证,测试原理如图7所示。超前瞄准机构采用压电陶瓷自闭环控制,自准直仪(ELCOMAT-3000)的可读精度设置为
$ {\text{0}}{\text{.1 μrad}} $ 。测得其偏转范围大于$ {\text{500 μrad}} $ ,可达到约$ {\text{709}}{\text{.4 μrad}} $ (压电陶瓷驱动位移$ {\text{16 μm}} $ )。偏转范围与仿真分析结果相差约$ {\text{70 μrad}} $ ,相对误差约为9.8%,这与零件加工精度、装配误差等有关。在自闭环控制下,对该机构在不同驱动位移下 (0~16 μm)的偏转角度进行了线性拟合,如图8(a)、(b)所示,得到最大残差为$ {\text{0}}{\text{.44 μrad}} $ ,满足设计初期目标偏转精度$ {\text{2 μrad}} $ 。 -
光程差(Optical Path Difference,OPD)是指激光经过超前瞄准机构前后,由于机构本身的抖动引起的光程变化误差[8,15]。利用激光干涉原理搭建了光程差测试装置,再通过相位计读出相位信息[16],解算出光程差,如图9所示。测试实验使用的相位计相位分辨率为
$ {\text{0}}{\text{.5 μrad}} $ ,经过理论计算,得到理论探测光程分辨率约为$ {\text{0}}{\text{.04 pm}} $ (激光波长$ {{1\;064 \; {\rm{nm}}}} $ )能够用于探测皮米级光程差。因为超前瞄准机构选择驱动位移为$ {\text{8 μm}} $ 时为动态偏转零点来实现一维双向偏转,所以分别选择驱动位移为${\text{0 μm}}$ 、$ {\text{8 μm}} $ 和$ {\text{16 μm}} $ 时,测试其光程差。为确保光斑位置与反射镜面中心轴位置重合,将机构安装在位移调整台(LY60-C/L/R,位移精度$ {\text{10 μm}} $ )上进行调节。在常温(24 ℃)真空环境下(小于$ {\text{50 Pa}} $ ),输入驱动位移为0 μm、8 μm和16 μm测试了超前瞄准机构的光程差,最后测得的数据经过处理得到结果如图10(a)~(c)所示。从图中可以看出,得到的光程差测试结果相似,当频率在1~10 Hz时的光程差小于${\text{10}} \;{{{\text{pm}}} /{\sqrt {{\text{Hz}}} }}$ 。而当频率在1 mHz~1 Hz时的光程差大于${\text{10}} \;{{{\text{pm}}}/{\sqrt {{\text{Hz}}} }}$ ,这主要与测试环境中温度变化等影响有关[8]。后续针对这些因素,采取诸如主动精密温控等措施进一步优化测试方案。因为环境温度变化缓慢,所以温度变化会在低频区域对光程差测试结果产生影响。针对温度变化对光程差测试实验带来的影响,对不同温度下镜面中心点法向位移进行仿真分析,结果如图11所示。从图中斜率可以得到温度变化对位移影响明显,为
$ 3.5\times {10}^{5} \; \text{pm}/ $ ℃,要想达到皮秒级测试精度,需要控制环境温度波动在$ {10}^{-6} $ ℃量级。
Development and test of the Point Ahead Angle Mechanism for space gravitational wave detection
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摘要: 探测低频引力波需要脱离地缘噪声干扰,在空间搭建激光干涉引力波探测装置。太极、LISA、天琴等空间引力波探测任务,计划在几十万到几百万公里量级的臂长上实现皮米级的位移测量精度,以满足引力波探测的要求。在探测任务中,考虑轨道季节性变化和星间激光传输时间等因素,发射光束需要一个超前角度,确保远端望远镜能够接收到光束,从而完成星间激光干涉。针对发射光束需要超前角度的需求,设计并研制了一款用于激光干涉链路中提供超前角度的光束指向机构,即超前瞄准机构。该机构基于将偏转轴配置在反射镜面上的设计理念,采用柔性铰链和杠杆配合的结构形式,利用压电陶瓷自闭环进行驱动控制,实现光束一维高精度偏转。对该机构进行仿真分析,验证其力学特性以及偏转范围。对所研制的机构进行了一系列实验测试,结果表明,该机构偏转范围可达到
$ 709.4 $ μrad,偏转精度可达到$ {\text{0}}{\text{.44 }} $ μrad,机构偏转引起的光程差优于${\text{10}}\;{{{\text{pm}}}/{\sqrt {{\text{Hz}}} }}\;$ (1~10 Hz)。从而验证了该机构设计的可行性,为实现光束超稳高精度偏转提供一定的参考。Abstract:Objective To detect low-frequency gravitational waves, it is necessary to eliminate the interference of geo-noise and build a laser interference gravitational wave detection device in space. Taiji, LISA, Tianqin and other space gravitational wave detection missions have been planning to achieve pm-sensitivity on the arm length of several million kilometers to meet the requirements of gravitational wave detection. Because of orbit evolution and time delay in the interferometer arms, the direction of transmitted laser beam changes, consequently, a remote telescope cannot receive the laser beam to complete the inter-satellite laser interference. Aiming at the need for the point ahead angle of the emission beam, a beam pointing mechanism that provides the point ahead angle in the laser interference link is designed and developed for the space gravitational wave detection device, called the Point Ahead Angle Mechanism. Methods Based on the design concept of aligning the rotary axis on the mirror surface, the Point Ahead Angle Mechanism employs the structural form of flexible hinges and lever (Fig.2), and the control scheme of piezoelectric ceramic self-closing loops to achieve one-dimensional high-precision beam rotation (Fig.3). Mechanical properties are verified by the simulation analysis (Fig.4-5). Rotary range of the mechanism is verified by the simulation analysis (Fig.6). Under the condition of normal temperature and pressure with a relative humidity of 60%, the rotary characteristic test is carried out by using an autocollimator (Fig.7). And under the conditions of normal temperature (24 ℃) and vacuum environment (less than 50 Pa), a special interferometer is built to test the optical path difference (Fig.9). Results and Discussions A series of experiments are conducted on the mechanism, and the results show that the rotary range of the mechanism is ${\rm{709}}{{.4\; \text{μ} {\rm{rad}}}} $ , rotary accuracy is${\rm{0.44}}{{\; \text{μ} {\rm{rad}}}} $ , and the results meet the requirements (Fig.8). The optical path differences are better than$10\; \mathrm{pm} / \sqrt{\mathrm{Hz}}$ when the frequency is between 1 Hz and 10 Hz, and the results meet the requirement (Fig.10). But when the frequency was between 1 mHz and 1 Hz, the optical path differences are greater than$10 \;\mathrm{pm} / \sqrt{\mathrm{Hz}}$ . After simulation analysis, they are mainly related to the influence of temperature changes in the experimental environment (Fig.11). This is also the direction of further research. In short, it is proven that the principal design of the mechanism is feasible, and it is a reasonable reference for achieving ultra-stable and high-precision beam rotation.Conclusions In this study, the Point Ahead Angle Mechanism for space gravitational wave detection is designed and developed, and the corresponding index tests are completed, which verify the rationality of the mechanism design. The mechanism is a one-dimensional and two-way rotation, the maximum rotary range can reach about 709.4 μrad, and the rotary accuracy can reach about 0.44 μrad, all of which meet the expected design requirements. When the frequency is between 1 Hz and 10 Hz, the optical path difference caused by the mechanism is better than $10\; \mathrm{pm} / \sqrt{\mathrm{Hz}} $ , and when the frequency is between 1 mHz and 1 Hz , the optical path difference is greater than$10\; \mathrm{pm} / \sqrt{\mathrm{Hz}} $ . The optical path difference of the Point Ahead Angle Mechanism developed in this paper still has a gap with the foreign advanced level and design requirements, and the mechanism needs to be optimized. At the same time, the influence of temperature on the optical path difference test should be considered in further research. -
表 1 6阶模态频率
Table 1. Sixth order modal frequency
Modal order Modal frequency/Hz Modal shape 1 1157.4 Rotating around Y 2 3178.6 None 3 3483.3 None 4 3532.5 None 5 5553.1 None 6 6855.7 None 表 2 整体设计的主要特性
Table 2. Main features of the overall design
Design features Values Volume $ {\text{56 mm}} \times {\text{35 mm}} \times {\text{32 mm}} $ Material TC4; AL6061; K9 Quality About $ {\text{150 g}} $ Mirror size ${ {\varPhi 10 \; {\rm{mm} } } }$ Surface accuracy ${\lambda}/\text{30}\left(\text{RMS}; {\lambda=632}\text{.8 nm}\right)$ -
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