A variable reluctance stepping microdynamometer

Abstract

A magnetic variable reluctance stepping micromotor fabricated using LIGA processing is described which is integrated with an external planar coupled gear train and electromagnetic brake. Variable mechanical loading of the micromotor through the electrically controlled brake allows dynamometry measurements to be performed. All components are fabricated from electroplated 78 Permalloy. A planar gear coupling is achieved by using involute gear teeth as the salient elements in the rotor. The rotor used for testing has a pitch radius near 500 μm with 180 μm thickness and is complemented with a stator which provides for a step rotation of 2.4°. Coil windings are implemented with assembled spring clip coil forms with 400 turns of 50 AWG magnet wire. Improved coil efficiency is responsible for decreasing the minimum power required for rotor rotation to 52 microWatts. Maximum speeds near 8000 rpm are achieved although stepping action is affected by stepping instabilities which are a function of excitation magnitude and frequency. Calibration of the electromagnetic brake is accomplished with static beam bending measurements and indicates a maximum peripheral rotor output force of 0.6 milliNewton corresponding to an output torque near 0.3 microNewton-meter. Dynamometry measurements indicate a maximum mechanical power output of 20 microWatts.