Large-scale spectroscopic surveys are revolutionizing our comprehension of the universe. By analyzing the spectra emitted or absorbed by celestial bodies, scientists can obtain important information such as their properties, composition and distribution. The key for obtaining numerous spectra within a reasonable time is the use of fiber positioner arrays. To guarantee no blind area, positioners are hexagonally distributed with overlapped patrol areas, introducing issues such as fiber assignment and collision avoidance. These issues are heavily affected by the precision of positioners. However, errors are inevitable. The machining of components and the engagement of gears may result in discrepancies between the actual and theoretical data regarding arm length and rotation angles. The central shaft deviation caused by assembly will result in the arm being unable to reach the expected position after rotation. The lack of precision in controlling the gear reduction ratio can impact results as well. The research focus of the paper is to explore the impact of various factors on system accuracy through simulations. The kinematic model of theta-phi positioners has been established based on geometry and Denavit-Hartenberg (D-H) method. The factors are analyzed and their impacts are simulated through the method of controlled variables. Through error analysis, the influence of various factors can be reduced by adjusting parameters before manufacturing the positioners, or to screen for positioners that require repair during maintenance. This study provides a reference for the design, calibration, and control of positioners.
Reducing the pitch between fiber positioners increases the number of positioners at the focal plate, improving the efficiency of sky surveys. However, the collision probabilities between positioners will also increase, leading to path planning challenge. To solve this problem, a new R- θ fiber positioner with the double composite parallelogram mechanism is proposed. It adopts the micro displacement system containing a piezoelectric ceramic actuator and a flexible hinge mechanism. To guarantee the precise linear movement of R mechanism in a small space, a parameterized virtual prototype of the robot is constructed and optimized. Finally, the performance of the manufactured R mechanism is verified by experiment. Results show that the newly designed R- θ fiber positioner can meet the requirements of accuracy.
The key to solving numerous crucial astronomical and astrophysics issues lies in massive spectroscopic surveys mainly conducted at multi-object spectroscopy (MOS) facilities on large-aperture telescopes such as Large Sky Area Multi-Object Fiber Spectroscopy Telescope (LAMOST). The efficiency of these surveys is greatly influenced by the fast and accurate movement of fiber positioners. However, due to the overlapped patrol discs and unexpected mechanical failure, mutual interference and collisions between positioners may occur. One existing solution is based on Markov chains and greedy selection method for navigation, which has high time complexity and results in oscillations of fiber positioners. Through analysis and research on collision types and trajectories, a fast collision-free path planning method for fiber positioners based on the RRT algorithm and B-spline curvefitting is proposed. The proposed algorithm can generate continuous and smooth feasible paths when collisions happen. Simulation experiments show that the proposed method can meet the requirements of fast, safe, and collision-free path planning for fiber positioners, help to improve the efficiency of massive spectroscopic surveys. This method can be adopted by not only LAMOST but also other projects such as Dark Energy Spectroscopic Instrument(DESI) and MOS.
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