# Development of a Pre-Evaluation and Health Monitoring System for FAST Cable-Net Structure
## 3 Establishment of the FAST MST Model  
### Method of Obtaining Data of the FAST Cable Net  
When establishing the pre-evaluation and health monitoring system, the first and most critical work is to establish an MST model consistent with the actual FAST cable-net structure. There are various sensors and other measuring instruments installed on all parts of the FAST cable net. For example, magnetostrictive sensors were set up on every actuator to obtain the strokes of the equipment; magnetic flux sensors for measuring cable force were installed on 150 edge main cables; the coordinates of lug-plate pinholes can be measured and recorded automatically by assigning reflected targets of the laser measurement to them; and so on [17]. The measuring equipment makes it possible to establish the MST model of the FAST cable net quickly and accurately. ANSYS Parametric Design Language (APDL) is an advanced application of ANSYS that is widely used in engineering projects and has a powerful programming capability. It is beneficial to use APDL in establishing the MST model. The complete ANSYS model of the cable net, including cables, beams, and columns, is shown in Figure 3.  
The BEAM 44 element is a beam element that can withstand tension, compression, torsion, and bending, so we use it to simulate ring beams and pillar components. The LINK 180 element is a tension and compression element in the direction of the rod axis, which cannot bear the bending moment. It is used to simulate all the cables in the model. Because the cables are not compressed, they are set to be tension-only. The main subject of the MST analysis is the main cables and pull-down cables. While the requirements for the speed of analysis are significant, each cable is meshed into one element. The weight of the reflector panel is transformed to vertical loads in intersections of pull-down cables and main cables. As mentioned above, MST data collection is based on the dynamic changes of previous and present data. In other words, when structural forces change, they need to be collected and transmitted to the model. According to the differing speeds of data change, data can be divided into two categories: regularly measured data and real-time collected data. The data measured regularly will not change in a short time period, so these data are usually updated to the MST model uniformly after the scheduled regular measurement. In contrast, the data collected in real time may change at any time, which has a more significant impact on the overall stress level of the FAST cable net and belongs to the type of data that is of crucial concern. Therefore, updating the real-time collected data in time is essential. FAST adopts active reflector technology to observe celestial bodies by controlling the movement of the cable net to form a paraboloid, while the cable net shifts by means of changing the stroke of actuators. Therefore, ineffective actuators lead to loss of the  
Figure 3: ANSYS model of FAST cable net. working ability of the pull-down cables connected to them and cause the stress level of the corresponding main cables to be abnormal. Moreover, since FAST is located in a mountainous area, the surrounding environment has a large temperature difference with the massive volume of FAST, so the effect of temperature difference on the stress in the whole structure cannot be neglected. Hence, the number of ineffective actuators, the current strokes, and the ambient temperature in the mountains are the most important data need to be obtained during the daily work of FAST. Such data must be updated into the MST model without delay. Real-time collected data are obtained through communication between MATLAB's OLE for process control module and a programmable logic controller. The data transmission time per cycle is generally 0.3\(\sim\)0.4 s. Once the new data are transferred to the computer, they will be stored in the designated folder in the form of Excel, and the data from the previous round will be replaced.