Stability analysis of near-field acoustic levitation considering misalignment and inclination

The near-field acoustic levitation (NFAL) technique is attracting increasing attention because of its compact design and environmental friendliness. An intriguing aspect of NFAL is the restoring performance when the levitated object is misaligned with the radiator. While numerous studies have proposed models to predict this performance, they typically consider only the misalignment of the levitated object and disregard its inclination. Consequently, this paper presents a numerical model that accounts for both misalignment and inclination to investigate the stability of NFAL systems. First, the Reynolds equation, which describes the variation in pressure distribution, is introduced. Due to the misalignment, a hypothetical reflector with a groove is applied to conveniently express the film thickness. The Reynolds equation is then solved using the eight-point discrete method in conjunction with the boundary equations, and spline interpolation is used to determine the inclination angle. This approach yields the relationship between the restoring force and the eccentricity, which is validated experimentally. Furthermore, parametric studies reveal that increasing the radiator vibration amplitude or the reflector weight enhances the stability. It is also observed that air provides higher stability compared to hydrogen, and a flexible radiator exhibits higher stability compared to a rigid radiator. With decreasing film thickness (e.g., by reducing the vibration amplitude or increasing the weight of the reflector), the stability in the inclination direction is improved at the cost of increased probability of levitation failure.