Two-joint robot finger design based on multi-degree-of-freedom ultrasonic motors

Abstract

Ultrasonic motor is one of the most promising candidates for arranging the joints of robot arms or fingers. In this paper, we design and fabricate a robot finger involving two joints with four degree-of-freedom (DOF) based on two hybrid multi-DOF piezoelectric ultrasonic motors. The motor’s stator consists of a multi-layered piezoelectric longitudinal vibrator and a sandwich bending vibrator. The multi-layered vibrator can produce large longitudinal vibration velocity in the z direction at low driving voltage. The sandwich vibrator is composed of two PZT rings and four-divided bronze electrodes, and is driven by phased voltages to excite the bending vibrations in two orthogonal directions. The synthesis of an elliptical motion with the longitudinal and bending vibrations can make a ball rotor rotate in three directions. Two multi-DOF motors are designed and installed in the prototype two-joint-finger. The frequency of the upper motor is 33 kHz and that for the lower motor is 22.8 kHz. In our model, a stainless steel layer is adhered on the stator’s surface to improve the rotation torque, and the maximum torque of 23.5 mNm was obtained for the lower motor. Holding mechanism using a bearing-Teflon disk is introduced to hold the ball rotor without preventing the multi-DOF motions of the robot finger joint. The motions of the finger are demonstrated and evaluated experimentally.