Experimental verification and characterization of sub-harmonic traveling wave on an ultrasonic micromotor

In this paper, we studied the nonlinear subharmonic traveling resonant modes which generate the rotational motion on the surface-macromachined ultrasonic micromotors. Actuated by a die-attached PZT plate, when the resonant frequency of the one of the stator´s vibration mode is close to PZT plate´s thickness mode resonance (~4.2MHz), rotating wave is generated on the stator. Driven at 3.78MHz, the rotating wave on the stator with 820μm diameter is optically detected and spatially characterized with an optical microscope. Together with laser interferometer measurements, two subharmonic modes are obtained, (1, 13) traveling mode at 1.78MHz and (0, 20) standing mode at 2MHz. The experimental results are further verified with finite element simulation. The traveling wave is acoustically coupled to the rotor without sliding to generate motor rotation. To study motor rotation control, signal bursts with various number of cycles are applied to the motor, and rotation rate of 2 degree per thousand cycles with angular control of as little as 0.1 degree is achieved.