The Application of Permanent Magnet Synchronous Motor with Small Electrical Time Constant in Fiber Positioner

Abstract

With the development of cutting-edge multi-object spectrographs, fiber positioners located in the focal plane are being scaled down in size, and miniature hollow-cup Permanent Magnet motors are now being considered as a suitable replacement for Faulhaber Precistep stepper motors. However, the small electrical time constant of such coreless motors poses a challenge, as the problem of severe commutation torque ripple in a fiber positioner running a position loop has been tricky. To overcome this challenge, it is advised to increase the Pulse Width Modulation (PWM) frequency as much as possible to mitigate the effects of the current fluctuation. This must be done while ensuring adequate resolution of the PWM generator. By employing a voltage open-loop field-oriented control based on a modulation frequency of 1 MHz, the drive current only costs 25 mA under a 3.3 V power supply. The sine degree of phase current is immaculate, and the repeat positioning accuracy can reach 2 μm. Moreover, it is possible to further shrink the bill of devices and the layout area of the Printed Circuit Board, especially in size-sensitive applications. This device has been developed under the new generation of The Large Sky Area Multi-Object Fiber Spectroscopic Telescope.