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Parallel observation of a large number of astronomical spectra in the night sky has become an urgent need with the increasing development of astronomy. We are currently entering a new era of ultra-large-scale fiber optic spectroscopic telescopes. To improve observation quality, many major telescope projects around the world are making their focal planes smaller and smaller. At the same time, to ensure observation efficiency and parallel observation of more target celestial bodies, the spatial density of fiber positioners used to guide fiber optic reception of celestial spectra on multi-spectral telescope focal planes is also increasing. Multi-target fiber optic spectroscopic telescopes require the use of more numerous and smaller fiber positioners, which inevitably necessitates smaller mechanical structures, as well as the development of miniaturized motors used to drive rotational axis. To adapt to the trend of miniaturization and efficient driving of fiber positioner on the focal plane, many fiber positioner design schemes in focal plane systems are beginning to consider using smaller hollow cup Permanent Magnet Brushless motors as a replacement for the widely implemented Faulhaber micro stepper motors as a driving source for rotational movement. However, for a micro brushless motor with a Φ 4mm, there is a lack of original encoder solutions that are compatible with it and can be used for fiber positioner at the very small sizes needed, and motor suppliers have no corresponding plans in this field. Researchers have developed a micro magnetic encoder solution at the end of the sensorless Φ 4mm micro hollow cup motor to achieve closed-loop field-oriented control (FOC) of the motor. The solution uses a diametrically magnetized permanent magnet with a diameter of only 2mm and a height of 1.5 mm and a diameter 4mm miniature printed circuit board (PCB), which is equipped with a 2mm × 2mm magnetic encoder chip for detecting the phase of the airgap magnetic field of the magnet. When the rotor of the motor drives the magnet on the shaft to rotate, the magnetic encoder chip can detect the phase change of the magnetic field in real-time rotor angle position provided by the magnetic encoder chip and generates an excitation magnetic field that is 90° ahead of the rotor magnetic field in the motor stator coil in a timely manner. This measure can significantly improve the energy utilization efficiency of the motor in the fiber positioning process on the focal plane of a multi-spectral telescope and reduce the additional thermal impact caused by the driving system. Compared with the existing open-loop driving method for micro motors, the introduction of a micro position sensor can also respond promptly and power off in case of unexpected motor blockage, reducing damage to individual mechanical components caused by blockage. In addition, the closed-loop system of the micro motor can further improve the fiber positioning accuracy of the focal plane system, solve the step loss problem caused by open-loop driving motors, and realize the world's smallest micro-servo control system. This paper focuses on the development ideas, methods, and technical implementation of the motor magnetic encoder system in miniature fiber positioners, and successfully assembles and applies the magnetic encoder system in the center shaft mechanism of the miniature fiber positioner in the new generation of the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST).
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