The Stability And Height Research In Near Field Acoustic Levitation

Near Field Acoustic Levitation (NFAL) is a phenomenon of realizing non-contact clamping and transportation through the air layers formed between the sound source and plane levitation solids. NFAL has broad application prospects in the non-contact gripping field of high-precision machining, since it has many advantages over other ways, such as the large levitation force, no electromagnetic interference, no demands for the levitated materials. The restoring force and the levitation force are the main influential factors in the practical clamping effect. However, the mechanism of restoring force in NFAL is still not clear. Simultaneously, there is still a big deviation on the levitation force between the calculation in traditional methods and the practice. The error was considered by neglecting the change of acoustic radiation force due to levitation solids. In this paper, the restoring force and the levitation force of NFAL are studied from both theoretical and experimental respects. The main research contents are as follows:To clarify the mechanism of acoustic levitation, the properties of near field and far field have been analyzed based on sound propagation theory. The levitation force has been presented based on the analytical solution of Rayleigh equation and Reynolds equation respectively. In addition, the acoustic pressure distribution has been obtained by the numerical solution of Reynolds equation.For improving levitation stability, the acoustic pressure distribution and motion direction of the air layers have been presented when the levitation disk deviates from the geometric center of the horn, through the comprehensive analysis from the motion state of levitation disk and the levitation force. Furthermore, a method of improving the stability on NFAL has been summarized based on the theory of viscous shear stress of air. Finally, two horns with flat and spherical end face were designed by finite element method (FEM) respectively. In order to verify correctness of the above design, the simulation of acoustic field has been implemented on BEM software. The simulation result shows a good agreement with the design target.A device for measuring the motion state of levitation disk has been set up by the means of the image processing of Matlab. The experiment results show the correction of the above method on improving the stability on NFAL. An inclination angle measuring apparatus has been built through the point laser generator, based on the analysis of geometric connection between inclination angle and displacement of light spot. Eventually, the stability of the acoustic levitation is characterized by the motion cycle and inclination angle. The experiment results showed that the horn with spherical end face had a stronger stability on NFAL.An experimental study on sound reflection coefficient affecting on the levitation height on NFAL was detected by laser displacement sensor. Experiment results show that levitation height increase with the sound reflection coefficient of the levitation disks when they are of the same quality. Furthermore, a revised Rayleigh equation on NFAL has been presented from the energy perspective. Compared with the experimental results, the result calculated by the revised one has a better accuracy than that by the traditional Rayleigh equation.