Rotation phase analysis of surface particle motion of coiled waveguide caused by flexural ultrasound wave

We have developed a traveling-wave-type miniature ultrasonic motor using a helical coiled waveguide as a stator. In this motor, the elliptical motion of the surface particle due to the flexural ultrasonic waves rotates the rotor, which can be placed adjacently inside or outside the stator, via the frictional force. In this motor, the rotation direction of the outer rotor is reverse to that of the inner rotor for a low frequency of the flexural wave. To clarify this phenomenon, we have confirmed that the rotation direction of the surface particle at the outer surface is same as that at the inner surface. In this study, we examine the rotation phase at the both surfaces.