Measurement of Holding Force and Transportation Force Acting on Tabular Object in Near-Field Acoustic Levitation

The holding force acting on a levitated object during near-field acoustic levitation has not been statically and directly measured so far. In this study, it was considered to realize such a measurement when a levitated object has a large displacement from the vibration source. In previous studies, under restricted conditions, the holding force has been calculated indirectly by image processing or measured from the balance with gravity by tilting the apparatus. In this article, the force was measured based on the magnetic force (MF) compensation principle. This is the first attempt to measure the force directly and statically under arbitrary conditions. One side of a rectangular rotor, which was placed above the acoustic radiation surface, received the holding force. The other side of the rotor received the MF generated by a solenoid. The holding force was estimated from the electric current flowing in the solenoid when the holding force and the MF were balanced. The holding force acting on the rotor surfaces was affected by the deviation between the rotor and the radiation surface. The holding force increased with increasing vibration amplitude or decreasing air gap between the rotor and the radiation surface. When the vibration sources were opposed, the holding force was affected by their vibration phase difference. The holding force was maximum at the opposite vibration phase and decreased with decreasing vibration phase difference. When the vibration sources were arranged in the transportation direction, the transportation force occurred and increased with increasing vibration amplitude of the destination vibration source. These measurement results agree well with the analysis results.