15 November 2022, Volume 62 Issue 11

    

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Editorial
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  Innovative operation and maintenance of FAST that looks up at the unknown sky

  Journal of Tsinghua University(Science and Technology). 2022, 62(11): 1739-1740.

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Research Article
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  Repair and maintenance of the FAST equipment

  WU Ruofei, LEI Zheng, YANG Lei, WANG Yong, HUANG Lin, TANG Jiajia, SHEN Ming, GUO Anping, GAN Renbo, SHI Xiaoan, LI Hui, YAO Rui

  Journal of Tsinghua University(Science and Technology). 2022, 62(11): 1741-1750. https://doi.org/10.16511/j.cnki.qhdxxb.2022.26.024

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  The five-hundred-meter aperture spherical radio telescope (FAST) is a key national research facility built by China during the 11th Five-Year Plan period. FAST is the world's largest and most sensitive single-aperture radio telescope and has significantly contributed to astronomical research. The operators seek to guarantee that the FAST facility runs smoothly and efficiently to improve observations and data quality. Focusing on the analysis of the structure and working principle, maintenance system, and maintenance content of the FAST equipment, this study takes the actual FAST engineering maintenance case as the research object and thoroughly analyzes the basic situation and process of engineering maintenance. The findings reveal existing flaws in the current FAST equipment maintenance program, as well as enhanced maintenance procedures for future maintenance work, aiming to improve FAST observation times and service life. This article also provides recommendations for future repair and upkeep.
  
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  Transport robot path planning based on an advantage dueling double deep Q-network

  HE Qijia, WANG Qiming, LI Jiaxuan, WANG Zhengjia, WANG Tong

  Journal of Tsinghua University(Science and Technology). 2022, 62(11): 1751-1757. https://doi.org/10.16511/j.cnki.qhdxxb.2022.26.034

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  An advantage dueling double deep Q-network (AD3QN) algorithm using deep reinforcement learning was developed for the transport robot path planning for the five-hundred-meter aperture spherical radio telescope (FAST) actuator automatic maintenance workshop. The dueling network state value layer is learned in advance so that the state value parameters are initialized according to the environmental state to reduce the steps required to reach the target point the first time. An improved greedy network search algorithm simplifies the environmental exploration and utilization. The action selection strategy avoids local minima in the robot path and improves the algorithm convergence speed. AD3QN provides good dynamic planning and real-time performance and is flexible, robust and accurate. Modeling the actuator actuator maintenance workshop and testing the path planning capability of the network before and after the improvement, simulations show that the time to find the target point the first time is 176% faster with AD3QN than with a general dueling network. This research improves the actuator maintenance efficiency which provides extended observation times.
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  Residual bearing capacity of the steel cables and anchorages of the FAST feed support system

  LI Qingwei, LI Hui, JIANG Peng, YAO Rui, PAN Gaofeng

  Journal of Tsinghua University(Science and Technology). 2022, 62(11): 1758-1763,1779. https://doi.org/10.16511/j.cnki.qhdxxb.2022.26.027

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  The flexible five-hundred-meter aperture spherical radio telescope (FAST) feed support system is a cable-driven parallel manipulator with large spans. The feed cabin is suspended by 6 parallel steel cables and moves slowly over a large range above the reflector through coordinated coiling and uncoiling of the 6 parallel steel cables. The residual bearing capacities of the steel cables and the anchorages that are reaching the design lifespan of 5 years were evaluated using failure tensile tests and tensile fatigue tests for cable and anchorage samples. The tests show that the steel cables and anchorages still have enough residual bearing capacity to continue service after 5 years. This research provides an experimental basis for increasing the service time of steel cables for such monitoring systems.
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  Dynamic modeling of a cable-driven parallel robot with a Stewart platform for use in FAST

  YIN Jianing, JIANG Peng, CHEN Ming, YAO Rui

  Journal of Tsinghua University(Science and Technology). 2022, 62(11): 1764-1771. https://doi.org/10.16511/j.cnki.qhdxxb.2021.26.039

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  Five-hundred-meter aperture spherical radio telescope (FAST) uses a cable-driven parallel robot with a Stewart platform for precision positioning at the millimeter level. This paper presents a dynamic modeling method for this type of rigid-flexible mechanism which accurately models the cable and the Stewart platform and eliminates system vibrations by iterating the Newton-Euler equations of the intermediate structure between the two models. The nature of the coupled system vibrations is also analyzed to explain the impact of the Stewart platform's error compensation on the system motion with the error composition modeled numerically. This research will improve the FAST observation accuracy and provides a theoretical basis for control algorithms for this type of rigid-flexible mechanisms.
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  Dynamic modeling of the FAST cable-driven parallel robots

  ZHAO Yacong, WANG Qiming

  Journal of Tsinghua University(Science and Technology). 2022, 62(11): 1772-1779. https://doi.org/10.16511/j.cnki.qhdxxb.2022.26.022

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  The feed support system of the five-hundred-meter aperture spherical radio telescope (FAST) is a kind of giant cable-driven parallel robot that uses flexible cables to move the platform in the working space. Thus, the system combines variable length flexible cables and rigid body motion. This study analyzed the cable-driven parallel robot. The variable length cable elements were analyzed based on the absolute node coordinate method, the dynamic equations of the cable elements were then derived using the virtual work principle. Then, the dynamics of the rigid moving platform were analyzed, the dynamics equations of the cable-driven parallel robot with constraints between the cables and the moving platform were derived. Thus, the model included the degrees of freedom of the flexible cables and the platform. MATLAB was used for the simulations. The results compare well with measured data to verify the model feasibility.
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  RFI intelligent monitoring techniques for FAST

  ZHANG Haiyan, HU Hongliang, WANG Yu, JIANG Huajing, GAN Hengqian, HU Hao, HUANG Shijie

  Journal of Tsinghua University(Science and Technology). 2022, 62(11): 1780-1788. https://doi.org/10.16511/j.cnki.qhdxxb.2022.26.030

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  Radio frequency interference (RFI) intelligent monitoring techniques are being developed for the electromagnetic environment of the five-hundred-meter aperture spherical radio telescope (FAST), including interference source classification and identification and high-precision localization. According to the characteristics of FAST sites, the interference detection and identification uses multi-site cooperative spectrum sensing and signal identification based on deep neural networks. The interference source localization uses time difference of arrival (TDOA) for accurate localization with low signal-to-noise ratios. The time difference between signals arriving at different receivers is estimated using the generalized mutual correlation method. This intelligent monitoring research lays the groundwork for the FAST radio interference intelligent monitoring system as well as the FAST spectrum management and electromagnetic environmental protection work.
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  A high voltage power EMI filter based on the ceramic capacitor

  GAN Hengqian, SUN Jinghai, JIANG Peng, SI Keke

  Journal of Tsinghua University(Science and Technology). 2022, 62(11): 1789-1795. https://doi.org/10.16511/j.cnki.qhdxxb.2022.26.035

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  Radio astronomical observations are very sensitive to electromagnetic interference (EMI), so the EMI from the radio telescope's own electrical equipment must be strictly shielded. The five-hundred-meter aperture spherical radio telescope (FAST) power supply system containing 6 power stations of 10 kV whose shielding effectiveness need to be 90 dB at least. A high voltage, high current EMI filter was built to protect the FAST equipment from the equipment EMI. The EMI filter uses a high voltage ceramic capacitor that can work at 10 kV and 100 A with an insertion loss of more than 90 dB. The core components of the EMI filter are the feed-through ceramic capacitors and an inductor having a group of magnetic rings. Its enclosure is made of 12-cm diameter stainless steel tube that has advantages such as stable performance, simple structure, safe and secure and light weight. Passing the strictly test, the EMI filter was installed on the power station to provide the required shielding effectiveness of 100 dB that meet the FAST electromagnetic environment protection criterion.
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  Prognostics health management system for FAST actuators

  LEI Zheng, JIANG Peng, WANG Qiming

  Journal of Tsinghua University(Science and Technology). 2022, 62(11): 1796-1802. https://doi.org/10.16511/j.cnki.qhdxxb.2022.26.028

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  The observation of FAST is beset by the long fault maintenance cycle of actuators. To solve this problem, a prognostics health management (PHM) system is designed. First, the health status of the actuator is evaluated using real-time operational data. Second, based on the evaluation results, the causes of actuator fault can be identified, and the residual life of actuator can be predicted. Finally, the optimal maintenance strategy is provided. The repair efficiency can be increased via the fault diagnosis system. Moreover, the residual life of the actuator parts can be predicted using the remaining life prediction system. The maintenance mode could also be improved from passive to active, and condition-based maintenance can be realized to reduce the mean time to repair and improve the reliability of the actuator. The PHM system could be used as guidance for the problem of the actuator of FAST. Additionally, it could be used as a reference for similar system maintenance research.
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  Improved multi-beam unit layout and orthomode transducers for FAST

  YU Jinglong, LIU Hongfei, GAN Hengqian, CAO Yang

  Journal of Tsinghua University(Science and Technology). 2022, 62(11): 1803-1808. https://doi.org/10.16511/j.cnki.qhdxxb.2022.26.025

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  Five-hundred-meter aperture spherical radio telescope (FAST) is a multi-disciplinary basic research platform that is used for a wide range of astronomy research. The L band 19 beam receiver is mainly used for large sky surveys and pulsar searches. The receiver plays an important role in the discovery of new pulsars and the study of fast radio burst (FRB). The design of the multi-beam feed front-end was analyzed to improve the multi-beam feed system layout and the orthogonal mode coupler. This paper describes the improvements and the corresponding test results. These design methods provide design and technical guidelines for the design and upgrading of large aperture antenna feed systems.
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  Structural response of the FAST cable-net at large zenith angles

  ZHANG Ningyuan, LUO Bin, SHEN Yuzhou, JIANG Peng, LI Hui, LI Qingwei

  Journal of Tsinghua University(Science and Technology). 2022, 62(11): 1809-1815,1822. https://doi.org/10.16511/j.cnki.qhdxxb.2022.26.026

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  The five-hundred-meter aperture spherical radio telescope (FAST) is supported by a cable-net structure with active shape changing of the reflecting surface realized by hydraulic actuators controlling the cables. The strength and fatigue life of the cable-net have not been verified for large zenith angles and large source changes. The cable-net analysis model was modified based on current monitoring data to analyze the structural response of the cable-net for shape-changing operations at large zenith angles. A modified focal ratio and shape-changing strategy were developed to reduce excess stresses and stress amplitude at large zenith angles. The structural response analysis were carried out during the source-changing process when changing the zenith angle. The results show that the total transition zone always involves the 150 cables in the outermost ring for large zenith angles, with this result not being affected by temperature. The use of modified focal ratios can significantly reduce the number of excess stress conditions. A safe shape-changing strategy ensures that the surface cable stresses and stress amplitudes remain within the stress limits while still maintaining the surface accuracy. Source changing from one large zenith to another large zenith angle is the most unfavorable condition.
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  Quasi-real-time evaluation system for the FAST cable network operations

  ZHANG Ningyuan, LUO Bin, SHEN Yuzhou, JIANG Peng, LI Hui, LI Qingwei

  Journal of Tsinghua University(Science and Technology). 2022, 62(11): 1816-1822. https://doi.org/10.16511/j.cnki.qhdxxb.2022.26.023

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  The reflecting surface of the five-hundred-meter aperture spherical radio telescope (FAST) is supported on a cable-net structure with the active shape changing of the reflecting surface relying on hydraulic actuators to adjust the tie-down cable lengths. The hydraulic actuator linkage control is then a complex multiple degree-of-freedom coupled system. In this paper, a quasi-real-time evaluation system for the FAST cable-net operation is developed. According to the FAST operating characteristics, the system was developed using MATLAB and ANSYS based on mechanical simulation technology. The system included a model processing module, pre-operation analysis module, and quasi-real-time tracking analysis module, which can calculate the face cable stresses, face cable stress amplitudes, tie-down cable forces, and the nodal radial displacement deviations during the cable network operation. The system can also provide monitoring and early fault warnings. The system effectiveness was verified by installing the system on the FAST facility with tests for actual working conditions. The system simplifies the operations and analyses, improves the analysis efficiency, reduces the safety risk in case of actuator failure, and ensures safe operation of the FAST cable network.
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  Analyses and improvement method of sliding failures and structural damage of the triangular reflector elements in FAST

  LI Hui, SONG Liqiang, YANG Qingge, LI Qingwei

  Journal of Tsinghua University(Science and Technology). 2022, 62(11): 1823-1832. https://doi.org/10.16511/j.cnki.qhdxxb.2022.26.033

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  This paper describes the configuration and operating principle of the five-hundred-meter aperture spherical radio telescope (FAST) reflector elements with sliding failures and the resulting structural damage analyzed using mechanical modelling of the triangular reflector elements. Design improvements were then developed to avoid such failures and damages, including new arrangements of the adaptive connecting mechanisms, element failure identification methods and upgrade of the adaptive connecting mechanisms. The analysis model helps maintenance staff find the main reasons for triangular element failures and predict possible failure modes and damaged element positions to develop improved designs. The results enable the staff to more quickly locate abnormal reflector elements using routine visual inspection and prevent the worse failures to improve equipment maintenance and enable future upgrades.
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  Mass evaluation algorithm and maintenance key technology of reflector unit about FAST

  SONG Liqiang, LI Hui, YANG Lei, YANG Qingge

  Journal of Tsinghua University(Science and Technology). 2022, 62(11): 1833-1838. https://doi.org/10.16511/j.cnki.qhdxxb.2022.26.036

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  Reflector unit of five-hundred-meter aperture spherical radio telescope (FAST) has many characteristics, such as many types, high precision, large number of components, complex structure and difficult to maintain. In order to solve the technical problems of reflector unit quality evaluation and efficient maintenance, this paper introduced the reflector unit component parameter factor, created the reflector unit pace mathematical model, developed the reflector unit mass evaluation algorithm, and obtained the element mass of all reflectors. The results show that there are 4 300 pieces of 187 types of triangular reflector units, the mass distribution range is 427.34~480.69 kg, and the sum of the mass is 1 952.67 t. There are 150 quadrilateral reflector units of 15 types, the mass distribution range is 379.74~974.58 kg, and the sum of the masses is 99.32 t. An innovative cable maintenance system of 500 m full span with full coverage of reflector has been developed. The key technical problems of complex boundary and efficient maintenance of reflector have been solved. High efficiency maintenance work such as replacement of damaged units, target update and node disk rust removal and coating has been completed. This achievement has great significance for evaluating the total mass of FAST reflector units, improving the reflector maintenance technology and methods, and ensuring its normal operation.