An ultrasonic motor using a metal-ceramic composite actuator generating torsional displacement

An ultrasonic motor composed of three components; a stator, a rotor and a ball bearing is proposed. The stator is made of an active piezoelectric ring, poled in the thickness direction and bonded with two windmill-like slotted metal endcaps. The stator is excited through its two electrodes, generating ultrasonic vibration in the radial direction at a radial mode resonance frequency. The two identical metal endcaps on both sides of the ring, bonded after shifting 45 degrees with each others, transfers the radial vibration into longitudinal and tangential vibrations. The combination of these vibrations (longitudinal and tangential) generates rotation due to frictional interaction between the center part of the top endcap and the periphery of the rotor. A prototype motor with 11.0 mm diameter was fabricated and its transient characteristics were measured. The maximum torque of 1.36 mNm was obtained at a speed of 480 rpm.