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FAST主动反射面支承结构理论与试验研究
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摘要
反射面的面积是衡量射电望远镜性能的主要指标之一,只有拥有更大的接受面积才能观测到更多、更远的天体。自重和风荷载的作用会引起反射面的形状变化,从而降低反射面的形状拟合精度,影响望远镜电性能,如何更有效支承反射面一直是建造更大口径射电望远镜的瓶颈。1994年中国天文学家提出了利用贵州喀斯特洼地,建造500m口径具有主动反射面的大型射电望远镜FAST(Five-hundred-meter Aperture Spherical Telescope)的计划,建成之后将是世界上正在建造及计划中的口径最大、最具威力的单口径射电望远镜。但是FAST的口径大、地貌复杂、精度高及反射面的主动性等特点同时也对其反射面支承结构提出了更高的要求。在对FAST反射面支承结构的前期研究中还未找到合适(或完善)的反射面支承结构形式。鉴于此,本文结合FAST的功能要求和特点,对采用柔性的拉索支承反射面的方案进行了系统研究。
     提出了采用“整体索网结构+背架结构”共同支承FAST反射面的总体方案,对索网网格的划分方式、单元网格的尺寸、及下拉索的布置方式分别进行了研究,得到了整体索网结构的具体结构形式及关键参数。
     从不同角度对FAST反射面整体索网结构进行了系统的研究,首先,对整体索网结构初始形态分析方法进行了研究,并结合索网网格划分特性提出了初始形态应力优化方法,研究了不同背架结构自重对整体索网结构各项性能指标的影响;其次,对索网结构工作态变位(望远镜的寻源和跟踪)模拟方法进行了研究,并对索网结构的变位响应规律进行了大量的统计分析,得到了比较实用的FAST整体索网结构变位响应统计方法,在此基础上对工作态反射面的变位策略进行优选,得到较合理的变位模式;第三,对索网结构在不同荷载态的受力性能进行了分析,并针对反射面的工作原理,提出了减小由风荷载引起的反射面变形的方法,同时结合FAST反射面的工作原理,提出了温度补偿方法以减小温度荷载对结构的响应;最后,在对索网结构的三种状态(基准态、工作态和荷载态)分别研究的基础上,对FAST索网结构的设计方法进行了研究,得到简单实用的设计方法,并编制了相应的计算程序模块。
     结合FAST对反射面的高精度要求,提出了FAST反射面背架结构保形设计方法;对FAST反射面背架结构进行了选型及优化研究,提出了空间网格结构、弦支结构、混合结构等多种背架结构可能的结构形式,并对每种结构形式进行了详细的参数分析,得到了其各项性能指标随不同参数的变化规律,最后提出了几种较优的结构形式,为今后的背架结构试验和设计提供了详细的资料。同时对背架结构与整体索网结构的连接节点形式进行了详细的探讨,提出了两种可能的构造形式:滑动式节点和机械式节点。
     为了验证FAST各项子课题所取得的系列理论成果,中国科学院国家天文台在密云观测站建立了一口径为30m的缩尺试验模型,同时该模型也将作为一科学观测仪器投入使用。本文从FAST 30m模型反射面支承结构的设计、建造及试验等方面展开了研究:对模型的关键部件分别进行了试验研究;对整体索网结构的基准态成形数值模拟方法进行了研究,并在此基础上提出了较优的基准态成形方案,同时结合实际基准态成形过程提出了相应的数字模型修正方法,使其更加接近实际模型;在基准态成形过程中,对每一个中间环节均采取了相应保证结构整体成形精度的措施,最终取得了超过预期的效果。通过模型试验初步验证了整体索网这种新颖的结构形式作为FAST反射面支承结构的可行性,推动了FAST项目的顺利进展,为FAST原型的建造提供了宝贵的经验和第一手资料。
Area of the reflector is one of the main indexes to evaluate the performance of a telescope. A large area is indispensable to observe more celestial bodies in a further distance. On the other hand, the shape of a reflector is sometimes affected by its self-weight or the wind load, which definitely reduces its shape accuracy, and consequently makes the performance weakened. From this point of view, how to support the reflector efficiently is always a key problem on constructing new larger- aperture telescopes. In 1994, Chinese astronomers proposed to construct a five-hundred-meter aperture spherical telescope with an active reflector, named FAST, making use of Karst terrain in Guizhou, China, which is expected to be the largest and most effective radio telescope in the world. However, the proper supporting structures of reflector were not achieved for FAST in previous research, due to the great difficulties brought by the big size of the telescope, the complexity of Karst terrain and the activation of the reflector. In order to solve this problem, a cable-net with steel sub-structures is proposed and studied as the supporting structure of FAST in this paper.
     First of all, the general plan of the cable-net and steel sub-structures is discussed. The patterns and sizes of cable mesh, as well as locations of control cables are investigated respectively to find out the best set of some key parameters.
     Then some researches are focused on the cable-net: (1) the analysis methods of initial stress state of cable-net are studied and an optimization method for initial stress is developed based on the characteristics of the cable mesh. In addition, the effects of sub-structures’self-weight on the cable-net are illustrated; (2) the active shape-changing of reflector (for searching and tracking) is simulated numerically. And the regularity of response of cable-net caused by the shape-changing is analyzed sufficiently. Consequently, a practical statistical method is developed for the analysis of response caused by shape-changing. Based on the work above, the strategy of shape-changing of working reflector is optimized; (3) the mechanical characteristics of cable-net are discussed under different loads. And the methods of reducing wind-induced deformation are given. A temperature compensation method is also adopted to reduce the effect of temperature variation; (4) through the analysis of three states of cable-net (initial state, working state and load state), a practical and simple design method and concerned program for the cable-net of FAST are developed.
     Further more, the researches on sub-structures are carried out in detail. A design method for maintaining the shape of sub-structures is presented, which can accommodate the high accuracy demand. And the sub-structure of reflector is designed and optimized. Three kinds of structures, including lattice structures, suspended structures and hybrid structures, are proposed as sub-structures. In order to get effective sub-structures, they are compared through parametric analysis, which also provides detailed information for future experiments and designing. Meanwhile, the joints which are used to attach sub-structures to the cable-net are designed in detail and two types of feasible joints are given: sliding joints and mechanical joints.
     To verify the research achievements on FAST, a 30m-diameter model of FAST is constructed in Miyun Astronomic Observation Station by National Astronomical Observatory of China (NAOC), which will be applied as an actual astronomic observation device. Several researches on this model are presented: crucial components of the model are studied and tested; numerical simulation method for forming of initial state is investigated. As a result, an optimal forming scheme is attained. Moreover, the modification method is applied to make the numerical model closer to the actual model; effective measures are taken in each step of forming to get the desired accuracy. By the tests on the 30m-diameter model, the feasibility of the cable-net as a supporting structure of FAST reflector is verified, and the whole project of FAST is promoted. At the same time, some valuable experiences and credible information for constructing FAST in real site are accumulated during this process.
引文
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