# Development of a Pre-Evaluation and Health Monitoring System for FAST Cable-Net Structure
## 1 Introduction  
The Five-hundred-meter Aperture Spherical radio Telescope (FAST) belongs to China's national \"Eleventh Five-Year Plan\" major infrastructure construction projects and is now the largest single-aperture radio telescope in the world [1]. The construction of FAST was completed in 2016 and has been officially put into use since 2020. Its reflector is approximately 30 football fields in size, and its sensitivity is more than 2.5 times that of the world's second largest telescope. Its advanced design ideas, coupled with the advantage of its unique site, ensures that FAST will be in the world's leading position for the next 30 years [2]. Active displacement is the most prominent feature of the FAST reflector. FAST uses the overall cable-net structure as the supporting structure of the reflector (see Figure 1). The overall cable-net structure comprises a main cable net, pull-down cables, and a peripheral-ring support structure [3]. The 6670 main cables are woven into a spherical surface in a grid manner, forming 2225 nodes. Each node connects a pull-down cable. The lower ends of these pull-down cables are combined with ground actuators to control the displacement of the entire cable net. The peripheral ring supporting the structure includes a ring beam and steel pillars, and the inner side of the ring beam is connected to the main cables by slug-plate joints. The inner diameter of the ring beam is 500.8 m, while the total weight is 5350 tons, and it is composed of a ring truss with dimensions of 11 m \(\times\) 5.5 m [4]. The ringbeam is located on sliding bearings and sustained by 50 steel columns of differing heights. Displacement of the cable net is realized by the linkage control of 2225 actuators, which is essentially a multi-degree-of-freedom, complex coupled control system [5]. Different from cable-net structures in the general architectural field, FAST has a particular use function as an astronomical radio telescope. Playing the most crucial role in FAST displacement, the cable net has an ultralong span and flexible shape. Its active reflector state includes two main parts: (1) The basic spherical state. The supporting structure of the FAST active reflector is subject to its own weight (including the weight of the back frame structure and the cable-net structure) and pre-stressing force. The main cable-net nodes are in the equilibrium state on the designated reference sphere, which is the basic spherical state. (2) Active displacement working state. In order to meet the needs of astronomers for observing celestial bodies in different directions and distances, FAST needs to form paraboloids of various calibres at particular angles, and this function is achieved by the actuators dragging the pull-down cables (See Figure 2) [6,7]. Figure 1: Sketch of the FAST cable-net structure. Figure 2: Schematic diagram of the FAST reflector. 1: focusing surface; 2: paraboloid form; 3: basic spherical surface; 4: reflector panel; 5: main cable; 6: cable-net node; 7: pull-down cable; 8: hydraulic actuator; 9: anchor block. FAST faced various disadvantages and risks during its design period. Although detailed analyses were carried out at that time, they were based on the initial design model. FAST has a design life of at least 30 years, which means that the design model will necessarily change during actual operation [8]. For example, (1) FAST uses giant, natural karst depressions on the earth as the telescope site, which are easy to collapse and erode, resulting in insufficient foundations. (2) FAST is located in a humid and rainy area. Chronic rainfall and stagnant water can easily cause uneven structural forces. (3) Certain actuators are prone to failure once under high-intensity observation, which could cause the actual force state to be inconsistent with the theoretical state. Even in the most unfavourable case, the cables connected to these ineffective actuators will bear loads that exceed their ultimate bearing capacity and eventually lead to the striking of nearby main cables [9].