Modal Frequency and Damping Identification of the FAST Cabin-Cable System

Abstract

The Five-Hundred-Meter Aperture Spherical Radio Telescope (FAST) faces challenges in establishing high-precision rigid connections between the receiver and the reflective surface due to its vast spatial span. Innovatively, FAST suspends the feed cabin in mid-air using six supporting cables. The precise positioning of the feed focal point is achieved through the coordinated control of cable extension and retraction, along with the A-B axis and the Stewart platform within the cabin. The cables and the feed cabin form a large parallel mechanism. Since the cables are flexible, and the feed cabin remains at a high altitude during observations, it is inevitably subject to internal and external disturbances. To quickly dissipate these disturbances, the system requires a certain level of damping, which directly affects the pointing and tracking accuracy of FAST. During the 2022–2023 operational period, there were multiple instances where the pulleys of the curtain mechanism on the supporting cables became stuck and were carried to the top of the towers by the cables. This also led to the phenomenon where the pulleys, after being stuck, would rapidly slide down the cables due to accumulation. At such moments, the cabin-cable system would experience instantaneous excitation, causing vibrations. This study uses the intrinsic time-scale decomposition (ITD) method to analyze the inertial navigation data installed in the cabin during these events, identifying modal frequencies and damping ratios. The analysis results show that the lowest primary vibration frequency of the FAST cabin-cable suspension system ranges from approximately 0.12 to 0.2 Hz, with a damping ratio of no less than 0.004. These data indicate that the current structure of FAST has a strong energy dissipation capability, providing important reference points for improving the control accuracy of FAST and for the upgrade of the feed support system.