Tertiary mirror of thirty meter telescope (TMT) has the tracking and pointing capabilities, and its struc-ture is similar to an alt-az telescope. The cradle serves a similar function to the center block of the track frame, its structure directly affects the performance of the system. M3 prototype as an example,a spliced truss structure for cra-dle is designed to improve stiffness to weight ratio. By using finite element method,the static and dynamic analysis of cradle under different conditions was done according to the working characteristics. The results showed that under the conditions of the gravity vertical along the x、y、z , the maximum deformation of cradle was 0.064mm, 0.015mm, 0.025mm respectively, and the first modal frequency is 74.4Hz. Modal Analysis Instrument was used for modal testing of cradle and the maximum relative error was 10.2％ comparing with the modal analysis results, results showed the structure designed meets the requirement.%三十米望远镜(TMT)三镜系统具有跟踪瞄准功能,其结构类似于地平式望远镜,其镜室托架(cradle)功能相当于望远镜中的四通组件,其结构设计对系统指标有着直接的影响.以三镜缩比系统为例,为了提高cradle组件的结构刚度质量比,根据三镜系统结构特点,提出了桁架拼接式的结构设计,利用有限元方法对其进行了静力分析和模态分析.结果表明,在重力沿x、y、z方向三种工况下,cradle组件最大变形分别为0.064mm、0.015mm、0.025mm,其一阶固有频率为74.4Hz,利用模态分析仪对cradle组件实物进行了模态检测,其结果与模态分析前四阶结果最大相对误差为10.2％,显示所设计结构满足使用需求.
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