A 4 mm Micro Servo Control System in Fiber Positioner

As multi-object spectrographs (MOSs) continue to evolve, a notable trend has emerged—the increasing accommodation of fiber positioners on ever-more compact focal planes. This progression has seen the traditional stepper motors being supplanted by more space-efficient miniature hollow-cup motors. A significant challenge faced in the employment of these 4 mm diameter motors is the absence of compatible angle sensors, resulting in reliance on open-loop control methods for positioning. Addressing this challenge, this paper introduces a novel miniature angle sensor designed specifically for 4 mm hollow-cup motors, and presents a newly formulated closed-loop control scheme, which leverages this sensor to achieve accurate positioning. This marks the first implementation of an angle closed-loop control system within a 4 mm miniature hollow-cup motor used in MOS fiber positioners. Experimental evidence suggests that this sensored closed-loop mode substantially improves upon the energy efficiency and precision of fiber positioner placement, compared with traditional open-loop stepper control methods. Furthermore, the integration of these microsensors mitigates collision risks during the concurrent operation of fiber positioners by deactivating the motor power supply to prevent potential damage to the system.