# Development of a Pre-Evaluation and Health Monitoring System for FAST Cable-Net Structure
## 1 Introduction
The FAST cable net needs to be deformed frequently when undertaking long-term and arduous observation tasks, which makes the cable net bear cyclically changing loads for long periods, which will eventually lead to the fatigue and failure of cables. The FAST reflector realizes displacement drive by controlling the actuators to stretch or relax the pull-down cables. The displacement work is essentially a long-term fatigue load. Therefore, the fatigue life of the cable net under long-term displacement is a critical factor in determining whether FAST can be successfully constructed. According to calculations, the maximum fatigue stress amplitude of the FAST cables is between 500 and 525 MPa, while standard cables generally do not exceed 300 MPa. FAST researchers invented a high-strength OVM.ST-type high-stress-amplitude cable system that meets the requirements of more than 30 years of durability, after quite a few years of indignation [10; 11]. Even so, the frequent variation of angles and calibres in daily tasks could still lead to the actual stress amplitude deviating from the designed stress amplitude, which is more likely to cause cables to be inconsistent with the designed fatigue condition. Moreover, once cables fail due to fatigue, they may cause damage to the reflector and actuators in the adjacent area, which could ultimately cause casualties. Furthermore, it is difficult for the cables to accurately reach the predetermined positions under the influence of fatigue failure; thus, the corresponding celestial-observation efficiency could be reduced. In fact, the FAST cable net has nearly 10,000 cables with different areas and lengths, and these cables are all customized, so it is difficult to replace damaged cables at any time. Due to the great difficulty of producing these cables, once the cables are damaged, it may take quite a long time for them to be replaced. Consequently, it is imperative to realize in advance which cables will have fatigue failure and when. Over 500 sensors are installed on FAST at present, which still seems a little scarce in the face of around 10,000 cables supporting FAST. It is hard to effectively evaluate the safety of cable-net structures only by arranging these sensors [12]. Because there are many problems, such as the temperature difference between day and night and potential actuator failure during the operation of FAST, these factors will be coupled with one another in the process of cable-net displacement. At the same time, they present considerable randomness, which could significantly affect control accuracy and even the safety of the reflector. Therefore, realizing assessment of control accuracy and fault warning of the reflector is a significant problem faced by the telescope. Accordingly, the development of an efficient and stable FAST cable-net operation and maintenance technology is an urgent problem to be solved. [PERSON] [13] proposed a method of using hydraulic-actuator data to determine whether there are structural safety issues. Still, it is difficult to determine the magnitude of the impact on structural accuracy. [PERSON] [14] put forward an approach of installing a large number of fibre Bragg grating sensors on the FAST cable net. The surface strain of the cable head is tested to indirectly obtain the cable-force value and adjust the model periodically. However, this method is inefficient and cannot meet the immediate requirements of reflector warnings. [PERSON] [15] established an early-warning platform, which could provide timely feedback on the failure of the FAST active reflector. Still, it was unable to determine the gap between the actual stress change of the cable net and the theoretical stress spectrum, which would lead to fatigue risk for the cables in the cable net. The safety early-warning system is an integral part of FAST health monitoring. It is an essential guarantee for the safe operation of the structure. The work efficiency of the traditional manual inspection method can no longer meet the requirements of the structural-safety early warning.