A COMPARATIVE-STUDY OF BEARING DESIGNS AND OPERATIONAL ENVIRONMENTS FOR HARMONIC SIDE-DRIVE MICROMOTORS

This paper reports the development of flange bearing harmonic side-drive (hereafter referred to as wobble) micromotors and the results from a comparative study of the operation of flange and center-pin bearing micromotors in different gaseous environments. Our flange and center-pin wobble polysilicon micromotors have been operated with excitations as low as 30 V across 1.5 mum rotor/stator gaps. Since the flange in the flange bearing design is located at a smaller radius than the bushing in the center-pin bearing design, changes in the frictional parameters of the flange/bushing contact affect the performance of flange bearing micromotors less than that of the center-pin bearing micromotors. As a result, flange bearing wobble micromotors show superior performance in gaseous environments over their center-pin counterparts. They can be operated in room air for longer periods and are not significantly affected by the operating environment or by extended storage between operations. The coefficient of friction, mu(pp), for polysilicon on polysilicon is observed to change with the operational environment; mu(pp) is 0.38 for operation in nitrogen and argon, 0.48 for operation in oxygen, and 0.54 for operation in room air. A small increase in the flange/bushing normal contact force is observed in room air as compared to nitrogen.