Abstract

Ultrasonic motors are a new type of actuators that use mechanical vibrations of its stator to drive the rotor through frictional forces between the interface of the stator and rotor. Ultrasonic motors have recently been attracting considerable attention in industrial applications where high precision, fast response and high torque at low speed are desirable. In contrast to traditional electromagnetic motors, ultrasonic motors provide larger torque to volume ratio, silent operation, and compact design. Furthermore, its operation is not affected by electromagnetic fields, and the motor shaft remains braked when the power supply is removed. The commercial development of ultrasonic motors, however, has been limited primarily due to lack of complete models for the purpose of control. Consequently, the development of an accurate model based on the physical structure is extremely vital for controllers development. The present dissertation proposes a comprehensive model for the purpose of predicting the motor performance. A lumped-mass model of the stator is proposed to consider the coupling effects on the stator based on the flexibility influence coefficients, which are estimated from a finite element method. Moreover, the stator natural frequencies are obtained using modal analysis. Nonlinear tangential interface forces between the rotor and stator are incorporated into the driving torque function. A rotor model based on the Stribeck effect, Coulomb and viscous friction is also incorporated to describe the friction torque associated with the motor response. An extensive series of experiments have been designed together with a state-of-the-art test bench built at the laboratory of Concordia University. The validity of the proposed model was examined by comparing the model results with the measured data under different excitation conditions. The lumped-mass model is able to reproduce reasonably well the stator displacements and the excited modes when it is driven at different frequencies as a result of the stiffness coupling. Several rotor model parameters are identified by using the experimental data and a weighted error minimization function. From the results obtained in the present study, it is concluded that the proposed USM model can reproduce reasonably well the motor behavior to different excitations inputs such as step, ramp and harmonic excitations as well as the torque-speed characteristics. It is important to note that the developed model shows torque-speed hysteresis behavior, which has not been reported in literature. To experimentally confirm this new phenomenon, extensive experimental tests have been conducted to identify the torque-speed characteristics on the ultrasonic motor. It has been observed from experiments a clear torque-speed hysteresis in the ultrasonic motor attributed mainly to the friction drive principle at the contact interface. This behavior, however, has not been addressed previously. It is suggested that the lack of an appropriate instrumentation together with inadequate models have made very difficult to describe this phenomenon. On the other hand, the torque-speed hysteresis exhibits a high asymmetric behavior at low speed as a result of the friction torque produced by the Stribeck effect