Ultrasonic motor with stator projections and at least two concentric rings of electrodes

An ultrasonic motor that inputs two ac voltages having a 90.degree. phase difference to cause rotation comprising:

• a rotation shaft;
• rotor means for rotating around said rotation shaft;
• and
• vibrating stator means for generating an elastic traveling wave so that frictional forces cause rotation of said rotor means, said vibrating stator means including:
• a disk shaped piezoelectric element attached at its center to said rotation shaft,
• a disk shaped elastic member attached at one side to one side of said disk shaped piezoelectric element shaft,
• a plurality of projections, said plurality being a multiple of four times said wavelength of said elastic travelling wave, attached to another side of said disk shaped elastic member in a circular configuration at regular intervals that correspond to loops of flexural vibrations and positioned in the radial direction to avoid loops of standing waves that produce rotation in a circumferential direction, said plurality of projections touching said rotor means, and
• a plurality of small driving electrodes attached to another side of said piezoelectric element and disposed in at least two concentric electrode groups, each small driving electrode being one-half wavelength of said elastic travelling wave in the circumferential direction, one of said electrode groups being shifted in the circumferential direction one-quarter wavelength of said elastic traveling wave with respect to the other electrode group in such a manner that a radial boundary of each small electrode within one electrode group corresponds to a center of a circumferential boundary of a small driving electrode that is part of the other electrode group and in which mechanical impedances seen from said two electrode groups are made equal using predetermined radial widths, each electrode group inputting one of said ac voltages to produce vibration modes of second order or higher in radial direction and flexural vibration modes of third order or higher in circumferential direction in which standing waves of two flexural vibrations cause said elastic traveling wave and result in vibration of said disk shaped elastic member and rotation of said rotor means.