Acoustic levitation of objects larger than wavelength using a circular orbital single ultrasound focus

Acoustic levitation techniques using ultrasound differ in the method used based on whether the levitation object and the levitation distance are larger or smaller than about a wavelength. This paper proposes a method for real-time control of the levitation position of a lightweight object of a size larger than the wavelength, which is levitated to a position farther from the transducer than the wavelength. This paper uses a single-focus sound field moving in a circular orbit to form a time-averaged bowl-shaped pressure field and traps an object at its center. The proposed method can levitate an object about 10 times larger than the wavelength in a feedforward manner to a distance of about 40–80 times the wavelength without prior calculation. In this paper, the basic characteristics and limitations of the levitation method are verified through experiments and simulations. The results showed that acoustic levitation of a hemispherical shell-shaped object with a diameter of 98 mm and mass of 0.608 g can be realized in a workspace as wide as 20 cm2 horizontally and 40 cm2 vertically using a phased array with an aperture of 384 mm × 454 mm and containing 1494 transducers driven at 40 kHz.