Five-hundred-meter Aperture Spherical Radio Telescope Research Articles

Seismic Responses of the Supporting Structure of FAST under Multiple-Support Excitations

Seismic ground motions are spatial incoherent partly due to uncertainty in fault mechanism and wave propagation. The spatial coherency and its effect can be important and need to be investigated for multiple-support structures. In this study, an investigation of seismic responses of a five-hundred-meter aperture spherical telescope (FAST) is investigated. The structure is a cable net structure with multiple supports. For the structural analysis, parameters of an existing coherency model for excitations along the same orientation but considering the multi-component excitations and spatially separated locations are developed using the records from Taiwan. This coherency model together with adopted target power spectral density function and phase difference spectrum model are used to simulate multi-component seismic ground motions at 338 grid points covering the FAST. The simulated records are used as the basis to interpolate the time history of the ground motions at 2395 supports for the considered structure. The time-history responses of the structure under incoherent multiple-support excitations are evaluated and compared to that under uniform excitations. The results show that the influence of coherency on the supporting structure of the FAST in three excitation directions differs. The analysis results show that as compared to uniform excitations, the consideration of the seismic coherency in seismic excitations leads to, on average, 87% of increase in stress for 94% of the main cables, and 22% of increase in stress for 44% of the tie-down cables. It was also noted that the maximum tensile stress of the cable by considering the coherency is 835 MPa while that under uniform excitations is 671 MPa. In both cases, they are smaller than the characteristic tensile strength of the cables used for design and construction of the FAST.

光纤Bragg光栅应变计在500 m口径球面射电望远镜工程索力监测中的应用

To monitor the cable force of the cable-net supporting structure of the Five-hundred-meter Aperture Spherical radio Telescope(FAST)and to avoid the effect of traditional measuring methods on the cable-net supporting structure and the electromagnetic interference,this paper presents a method to obtain the cable force values indirectly.The method uses Fiber Bragg Grating(FBG)sensors to monitor the cable connector strain to measure the cable force indirectly.In the measurement,a FBG strain gauge was installed at the linear strain region of the cable connector axially by using a dedicated welded base,by which the adverse effect of welding directly on the performance of cable connector structure was avoided.A temperature compensation method was used to compensate the effect of temperature changes on the FBG stain gauge measurement.The correlation coefficients were acquired by the calibration of the cable prestressing process.The cable force monitored then was calculated fromthe calibrated relation in actual engineering applications.Test results indicate that the fitting linearity of the cable force and the cable connector strain is up to 0.98 and the Root Mean Square(RMS)error in practical measurement is 1.38 t.The relative error is better than 3% within the range of the cable force for the FAST.The method is characterized by no electromagnetic interference,simple wiring,good reliability and long duration.It has been successfully applied to the cable force monitoring of 316 typical cables of the supporting cable-net in the FAST project.

Research on the dynamic coupling of the rigid-flexible manipulator

Because of its large dimension, a rigid-flexible manipulator was designed to serve as the feed support system in a Five-hundred-meter Aperture Spherical radio Telescope (FAST). The rigid-flexible manipulator is composed of a flexible cable-driven parallel manipulator and a rigid Stewart platform. Motion of the Stewart platform may induce vibration of the cable-driven parallel manipulator due to reaction forces. The goal of this study is to investigate the dynamic coupling of the rigid-flexible manipulator. The “Virtual Stewart platform” was introduced to obtain a homogenous matrix to describe how much base motion is produced by a given Stewart platform motion. On this basis, an index was proposed to characterize the dynamic coupling of the rigid-flexible manipulator. Based on the proposed index, the factors influencing the dynamic coupling are investigated. The proposed index can be considered as a performance index in design and control of the system. Influence of the dynamic coupling on the feedback control of the rigid-flexible manipulator was also discussed. The simulation results showed that the dynamic performance of the rigid-flexible manipulator is strongly determined by the proposed index.

Pulsar–black hole binaries: prospects for new gravity tests with future radio telescopes

The anticipated discovery of a pulsar in orbit with a black hole is expected to provide a unique laboratory for black hole physics and gravity. In this context, the next generation of radio telescopes, like the Five-hundred-meter Aperture Spherical radio Telescope (FAST) and the Square Kilometre Array (SKA), with their unprecedented sensitivity, will play a key role. In this paper, we investigate the capability of future radio telescopes to probe the space–time of a black hole and test gravity theories by timing a pulsar orbiting a stellar-mass black hole (SBH). Based on mock data simulations, we show that a few years of timing observations of a sufficiently compact pulsar–SBH (PSR–SBH) system with future radio telescopes would allow precise measurements of the black hole mass and spin. A measurement precision of 1 per cent can be expected for the spin. Measuring the quadrupole moment of the black hole, needed to test general relativity's (GR's) no-hair theorem, requires extreme system configurations with compact orbits and a large SBH mass. Additionally, we show that a PSR–SBH system can lead to greatly improved constraints on alternative gravity theories even if they predict black holes (practically) identical to GR's. This is demonstrated for a specific class of scalar–tensor theories. Finally, we investigate the requirements for searching for PSR–SBH systems. It is shown that the high sensitivity of the next generation of radio telescopes is key for discovering compact PSR–SBH systems, as it will allow for sufficiently short survey integration times.

Progress of FAST in science, technique and instrument

High-sensitive and large aperture telescope is the key equipment for radio observation. Five-hundred-meter Aperture Spherical radio Telescope (FAST) is a Chinese mega-science project to build the largest single dish radio telescope in the world. FAST will enable astronomers to jump-start many science goals, for example, surveying the neutral hydrogen in the Milky Way and other galaxies, detecting faint pulsars, hearing the possible signals from other civilizations, etc. The paper aims to overview the latest progress of FAST including science, technique and equipment.

The initial value problem for the mathematical model of a multiple-input multiple-output flexible parallel mechanism

The cable-net support structure of the Five-hundred-meter Aperture Spherical Telescope (FAST) is a multiple-input multiple-output (MIMO) flexible parallel mechanism. The mathematical model of the cable-net structure is set up with the catenary equation, which produces a large number of non-linear equations. Finding the initial value plays an important role in the working state of the FAST. In this paper, a novel method is proposed to solve the equations of the model. This method uses the solution of the (n−1)-th layer cable-net as the initial value of the n-th layer cable-net. The n-layer cable-net equations are then solved simultaneously and the tension of each cable and the coordinate of each node in the cable-net can be obtained. With the introduction of the symmetry of cable-net, the number of unknown parameters in the equations is reduced to about one-tenth of the whole cable-net. The initial value of the cable-net support structure of the FAST is provided to show the validity of the method. The results of this study are of practical value for the FAST's new type of flexible cable driving mechanism design and control method.

Numerical Investigation on Fatigue Problem of Cable Net Structure of Five-Hundred-Meter Aperture Spherical Radio Telescope

The reflector of Five-hundred-meter Aperture Spherical radio Telescope (FAST) is supported by cable-net structure, so that to enable its reflector surface to form a paraboloid from a sphere in real time through active control. However, such form-changing operation would lead to about 500MPa stress range for the cable-net structure. Such stress range is nearly twice as much of that defined by the relative standard. So the cable-net structure is the most critical and expendable part of FAST reflector system. So, the service life of FAST would directly depend on the residual fatigue life of its cable-net structure. The present paper would make an effort to find a more appropriate deformation strategy to decrease the stress amplitude of the cable during form-changing operation.

Study on the working state of the five hundred-meter aperture spherical telescope using step-wise assignment method

The working state of the five hundred-meter aperture spherical telescope (FAST) is solved using the step-wise assignment method. In this paper, the mathematical model of the cable-net support structure of the FAST is set up by the catenary equation. There are a large number of nonlinear equations and unknown parameters of the model. The nonlinear equations are solved by using the step-wise assignment method. The method is using the analytical solutions of the cable-net equations of one working state as the initial value for the next working state, from which the analytical solutions of the nonlinear equations of the cable-net for each working state of the FAST and the tension and length of each driving cable can be obtained. The suggested algorithm is quite practically well suited to study the working state of the cable-net structures of the FAST. Also, the working state analysis result of the cable-net support structure of a reduced model of the cable-net structure reflector for the FAST is given to verify the reliability of the method. In order to show the validity of the method, comparisons with another algorithm to set the initial value are presented. This method has an important guiding significance to the further study on the control of the new type of flexible cable driving mechanism, especially the FAST.

Accuracy synthesis of a multi-level hybrid positioning mechanism for the feed support system in FAST

Abstract This study examines the terminal accuracy synthesis of a multi-level hybrid positioning mechanism for the feed support system in a Five-hundred-meter Aperture Spherical Radio Telescope (FAST) project, specifically considering three types of error influence factors such as time-varying barycenter, geometric error, and structural deformation. An error model caused by a time-varying barycenter and that for the entire mechanism are first established, and the physical meaning of the latter is further explained. The three factors that influence terminal accuracy are then analyzed. An efficient approach for acquiring the terminal error boundary over the entire workspace is proposed on the basis of the vector set theory and a linear algebra method. After employing an error compensation method for the first-level mechanism through control strategy, terminal accuracy synthesis is conducted by using a nonlinear optimization algorithm. Both constraints for accuracy and weight are satisfied after introducing measurement systems. Moreover, the proposed method of accuracy analysis and synthesis can be applied to other multi-level hybrid mechanisms.

Self-Excited Vibration Analysis for the Feed Support System in FAST

China is currently building the largest single dish radio telescope in the world, which is called the Five-hundred meter Aperture Spherical radio Telescope (FAST). The feed support system in the FAST is composed of a cable-driven parallel manipulator, an A-B rotator, and a Stewart platform. Since the stiffness of the cable-driven parallel manipulator is low, the feed support system is prone to vibrate under the action of the Stewart platform. The main purpose of this paper is to study the self-excited vibration of the feed support system. Self-excited vibration involves the natural frequencies of the system and the resultant forces produced by the motion of the Stewart platform. This paper linearizes the dynamic equations of the system at an operating point and determines the configuration-dependent natural frequencies in the given workspace. This paper obtains the resultant forces from the legs due to the motion of the Stewart platform by using the numerical method. According to the natural frequencies and the resultant forces, the condition of self-excited vibration is given and verified by simulations. In order to verify the linearization method, an experimental platform of a cable-driven parallel manipulator is set up. The experimental results match well with the theoretical arithmetic. This paper provides a reference point for further studies on vibration suppression in the FAST.

Research on Catenary Characteristics of FAST Tie-Down Cable and its Effect on Actuator and Joint

To study catenary characteristics of FAST (Five-hundred-meter Aperture Spherical radio Telescope) tie-down cable, this paper establishes tie-down cable equilibrium differential equation and cable length formula based on catenary theory, analyzes numerical simulation of multi-segment bar element method, and designs an experimental scheme. Taking 1×7 Ø12.7steel cable and Ø10 CFRP (Carbon Fiber Reinforced Polymer) cable for example, we can obtain three results respectively through manners mentioned above, and the agreement among them is excellent. Meanwhile, difference between results and elastic deformation of no-weight cable is not so big that catenary of two cables can be negligible. Further research shows that cable length is more sensitive to catenary than horizontal angle; cable longer deformation will benefit actuator to meet position precision effortlessly, however, stroke and velocity of actuator will increase and initial tension displacement of tie-down cable decrease correspondingly, this alteration should be compensated during reflector surface measuring and controlling; due to chord-tangent angle and tangential force is less than 1.28°and 241.4N, catenary effect on joint pose is weaker; generally speaking, CFRP cable has less catenary problem than steel cable, better mechanics characteristics, but bigger stroke and velocity for actuator and smaller initial tension displacement for tie-down cable. The research will provide references for design of tie-down cable, actuator, joint, and reflector measurement and control.

Real-Time Filter Method Based on the Structural Frequency of FAST Cabin-Cable System

Due to great flexibility, low damping, and variable structure in the cabin-cable system of five hundred meter Aperture Spherical Radio Telescope (FAST), a real-time digital low-pass filter based on the analysis of frequency is presented in this paper. Firstly, by the Lomb-Scargle theorem, it can obtain the fundamental frequency of cabin-cable system. Then, using the obtained frequency, a digital low-pass filter is designed to filter the measured data. After being filtered, the measured data are used for coarse control. Finally, the results of the experiments on the FAST 5 m model show that calculating the fundamental frequency is accurate and the filter is effective.

The Effects of Structural Parameter Variation on Cable Force of Fast Cable-Net Structure

Five-hundred-meter aperture spherical radio telescope (FAST) is supported by a cable-net structure, which enables its surface to form a real-time paraboloid by active control. FAST project is currently in the construction and implementation stage. However, there are always a considerable amount of errors that existed in practice which may result in the deviation of the structure from its ideal model or design. Therefore, structural parameter sensitivity analysis was discussed, which is indispensable. However, such deformation operation would lead to about 500 MPa of fatigue stress variation amplitude in the cable-net structure. Optimized deformation strategy is proposed to release the fatigue stress of the cable-net structure, which would be of advantage to improve the reliability of the cable-net structure. In the paper, the variation ranges of structural parameters were rationally determined. Based on local sensitivity analysis and global sensitivity analysis method, finite element model was used to study the effect of different structural parameters on the static behavior. It can be concluded that the effect of several key design parameters such as the cutting length and the elastic modulus of cable on the cable force is significant. The global sensitivity analysis indicates that the cable force range of the cable-net is −19% to 27%.

Study on Telescope Gain Affected by a Multilevel Hybrid Mechanism in FAST

This paper focuses on the telescope gain affected by a multilevel hybrid mechanism for the feed positioning in the five-hundred-meter aperture spherical radio telescope (FAST) project, which is based on the positioning accuracy analysis of the mechanism. First, error model for the whole mechanism is established and its physical meaning is clearly explained. Then two kinds of error sources are mainly considered: geometric errors and structural deformations. The positioning error over the mechanism's workspace is described by an efficient and intuitive approach. As the feed position error will lower the telescope gain, this influence is analyzed in detail. In the end, it is concluded that the design of the mechanism can meet the requirement of the telescope performance.

Pose Planning for the Feed Support System of FAST

A six-cable driven parallel manipulator and an A-B rotator in the feed support system of the Five-hundred-meter Aperture Spherical radio Telescope (FAST) are adopted for realizing the position and pose of nine feeds. The six-cable driven parallel manipulator is a flexible mechanism, which may not be stably controlled due to a small cable tension. The A-B rotator is a rigid mechanism, and its stability and accuracy can be improved by small pose angle. Based on the different characteristics, a pose planning function is presented. The optimization target of the pose planning function is to get the smallest pose angle of the A-B rotator, and the constraint condition can reflect the controllability of the six-cable driven parallel manipulator. Then, the pose planning realization process of the feed support system is proposed. Based on the pose planning method, optimized pose angles of the feed support system for the nine feeds are obtained, which suggests that the pose angle of the six-cable driven parallel manipulator changes from 0° to 14° and the pose angle of the A-B rotator changes from 0° to 26.4°.

Trajectory generation and tracking control of a multi-level hybrid support manipulator in FAST

The Feed Support System (FSS) of Five-hundred-meter Aperture Spherical radio Telescope (FAST) is a multi-level redundant support manipulator, which consists of a cable driven parallel manipulator, an A–B rotation mechanism and a Gough–Stewart platform. In this article, we report our work on FAST in the following aspects: first, kinematic model and trajectory generation strategy of the FSS are established. Second, considering preventing the pseudo-drag problem of flexible cable and realizing the accurate pose control for the cable driven parallel manipulator, the hybrid position/force control is implemented and validated. Then, the prediction control is adopted in the Gough–Stewart platform to improve the terminal accuracy. Finally, with the 1:15 similarity model of FSS, experiments are carried out to prove the control accuracy of the cable driven parallel manipulator, showing that the terminal error is within 10mm and the cable tension is kept in the given range. Further experiments on tracking control of the entire FSS illustrate terminal tracking accuracy of the astronomical observation is less than 2mm, meeting the design requirement. Trajectory generation and tracking control given in this paper lay the foundation for the FAST prototype.

A Study on Generic Models of Control Systems of Large Astronomical Telescopes

The control system is an essential and important part of an astronomical telescope. For large telescopes, an efficient control system is vital for successful observations. In this paper, an exhaustive study of generic models of control systems for large telescopes and a wide range of their common requirements are presented. This paper also discusses a basic hierarchical structure, workflow model, and telescope control model based on object-oriented analysis, as well as the main data flow model of a general purpose telescope. After analysis of these models, a layered and orthogonal architecture, which has a wide range of adaptability, is proposed. The design of a concrete architecture based on the message bus, which is used in the LAMOST (Large Sky Area Multi-Object Fiber Spectroscopy Telescope) project and FAST (The Five-Hundred-Meter Aperture Spherical Radio Telescope) project, is discussed. The design of the control systems proposed in this paper can serve as a useful reference in the construction of large telescopes.

Study of central control system for FAST

FAST(The Five-Hundred-Meter Aperture Spherical Radio Telescope) is an under-building radio telescope which will be the largest single dish in the world. Through the study of the central control system, in accordance with the actual operation of the telescope and observation process, this article introduces the physical models for engineers and observers, the central control system architecture design, basic support modules and the necessary interfaces. We simulated observation control process and telescopes monitoring and control process, and took Active Reflector System as a subsystem example to complete the control system design and implementation using EPICS (Experimental Physics and Industrial Control System). The Central control system, with active reflector systems, feed supporting system has been taken to an integration test at Miyun model. In the case of the normal operation of the various sub-systems of the Miyun model by the central control system, a coordinated control is achieved.

Flexible Parallel Robot and its Experiment Models Used in FAST

FAST (Five-hundred-meter Aperture Spherical Telescope) will become the largest single dish ever built in the world. The feasibility of FAST concept, including observatory location, active reflector and feed support has been proven by theoretical analysis and experiments. Results of study indicate that this project is ready to move on to preliminary design. In order to verify the design idea and the key technologies of FAST, the 5-meter experiment model, the 50-m experiment mode and the active reflector model have been designed and constructed. The experiment shows that the feasibility of FAST is been proven by theoretical analysis and experiments.

Measurement scheme and simulation for the main reflector of FAST

FAST (five-hundred-meter aperture spherical radio telescope) is a radio telescope being built in a karst depression in Guizhou Province of China, which will be the largest single dish radio telescope in the world [1]. The reflector of the telescope is composed of over 4000 panels, and each panel could adjust its position according to observation requirements. During observations, panels in the illuminated area could form a paraboloid to correct spherical aberration [2]. Therefore, accurate measurement of the positions of panels is crucial for the operation of the telescope. In this paper, we introduce the measurement scheme for the reflector of FAST, and simulate its accuracy using direct linear transform, Gauss–Newton algorithm, Levenberg–Marquardt algorithm and an algorithm referred to as multi-point algorithm. Advantages and disadvantages of using these four methods are compared for analysis at different locations of the panels on the reflector, and suggestions are given in choosing algorithms in implementation.