Recent development of ultrasonic actuators

A phenomenon of near-field acoustic levitation (NFAL) in air has been successfully applied to non-contact transportation where planar objects of several kilograms in weight are levitated and transported. The characteristics of NFAL in air was again studied, because some peculiar characteristics were found in underwater NFAL in which an attractive acoustic force acts on a planar object under certain conditions, and no attractive acoustic force had been observed in the frequency range of 17 to 30 kHz for the NFAL in air. However, an attractive force was proved to exist also in NFAL in air and the conditions for attractive force were determined. This acoustic attractive force is, however, successfully used for non-contact transportation of plates in liquids as well. In this paper, ultrasonic actuators based on NFAL are described together with the phenomena and the theory