A robot finger joint driven by hybrid multi-DOF piezoelectric ultrasonic motor

Ultrasonic motor is one of the most promising candidates for arranging the joints of robot arms or fingers. In this work, a novel robot finger joint based on a hybrid multi-DOF piezoelectric ultrasonic motor is designed and fabricated. The motor's stator is assembled by 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 composed by two PZT rings and four-divided bronze electrodes can excite the bending vibration in two orthogonal directions under phased voltages. The combination of the longitudinal and bending vibrations enable a ball rotor rotate in three directions. In our designed model, a stainless steel layer is adhered on the stator's surface to improve the rotation torque, and the maximum torque about 23.5 mNm is obtained. An orthogonal bearing holding mechanism 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.