Development of a novel radial-torsional hollow ultrasonic motor and contact interface coating test

A novel miniature radial-torsional hollow ultrasonic motor is proposed in this work, which is mainly composed of a base, a hollow rotor, a stator. One piece of piezoelectric wafer bonded on the bottom face of the stator is used to excite the radial in-plane vibration mode of the outer ring, which is converted into the revolved motion of the inner ring through the connection beams. The revolved motion of the inner ring can drive the rotor pressed on the inner ring to rotate through friction. The finite-element method was performed to verify the working principle of the motor and optimize the key dimensions of the motor. One prototype motor with the appearance size of Φ35 mm × 12 mm and weight of 20.1 g is fabricated and characterized. In addition, Si₃N₄ thin film is coated on the contact interface of the stator by the magnetron sputtering coating machine to improve the output performance of the ultrasonic motor. The experiment results show that the locked-rotor torque of the motor with coated stator can reach 0.42 mN m, which is 55.56 % higher than that of the uncoated motor.