A novel bonded-type 3-degree-of-freedom ultrasonic motor: design, simulation, and experimental investigation

Most of 3-degree-of-freedom (3-DOF) ultrasonic motors generally have the problems of low positioning accuracy, difficulty to apply preload, and excessive longitudinal dimension, leading to the limitation in industrial application. To address these issues, a novel bonded-type 3-DOF ultrasonic motor, consists of a convex cylindrical stator, a pre-tightening force structure, and a hemispherical rotor, is proposed in this study. A quadratically polarized toroidal piezoelectric ceramic is bonded to the upper surface of the stator. Three vibration modes of the stator are excited by applying different excitation signals on the piezoelectric ceramic. By coupling any two of the three vibration modes, the elliptical motion of the driving feet of the convex cylindrical stator around the x, y, and z axes are excited, respectively. A novel pre-tightening force application method is proposed to adjust the contact between the stator and rotor. Under the action of friction, the elliptical motion of the driving feet in three directions drives the rotor to rotate around three axes, respectively. The vibration modes of the stator are first simulated by using the finite element method. In addition, the optimal structural dimensions of the stator and the elliptical motion trajectories of the driving feet are calculated. Finally, a prototype of the proposed 3-DOF ultrasonic motor is fabricated and experimentally investigated. The experimental results show that the no-load rotary velocities of the motor prototype in the three rotation directions are 25 r min⁻¹, 22 r min⁻¹, and 380 r min⁻¹, respectively, its start/stop response time are 4.1 ms/3.9 ms, 6.3 ms/7.5 ms, and 6.1 ms/7.9 ms, respectively, and its angular displacement resolutions are 4.3 μrad, 5.1 μrad, and 10.2 μrad, respectively, when the excitation voltage is 250 V_PP. The proposed motor holds the advantages of compact structure, adjustable preload, and high positioning accuracy, and presents a broad application prospect in the fields of robot joint technology, optical tracking system, and laser communication.