UHV Compatible Ultrasonic Motor Operation keeping UHV and Durability Improvement by Modifying Rotor-Stator Interface

To meet with requirement for actuators that are fully nonmagnetic and compatible to ultrabigh vacuum (UHV), the authors have been developing an ultrasonic motor and researching on various phenomena relating to its operation in UHV. This motor is a cylindrical type or mode rotation type ultrasonic motor, on the stator of which is excited two standing waves. The modes are bending modes and sharing the same wavelength but have the perpendicular vibrating directions. At one the shared nodal position, a stainless steeled membrane is fixed to divide the transducer into two sections. The membrane is supporting the transducer and, at the same time, sealing UHV from atmospheric condition completely; one section of the transducer stays inside the chamber and the other stays outside. Owing to this special structure, contamination by gas emission from piezoelectric elements as vibration source and wiring supplying electricity to the elements is completely eliminated. In this paper, operation of the motor keeping UHV in the order of 10(-8)Pa and durability improvement by modifying material and geometry of rotor-stator interface is described. As rotor material, phosphor bronze, copper beryllium and aluminum alloy were tested. Reducing gas emission at the contacting surfaces between stator and rotor, use of these metallic materials decreased gas pressure during motor operation by a factor of 10% to 3x 10(-8)Pa at the minimum. In terms of durability, total life time of operation without replacing friction parts was improved by a factor of 290 reaching 390hours while the longest operation keeping UHV had been only 80minutes in the previous report. In addition, modification of rotor geometry was considered. Trenches were curved on rotor surface in order to enhance debris removal between rotor and stator. Resultantly, significant extension of operation time was observed.