Realization of an automatic, contactless, acoustic levitation motor via degenerate mode excitation and autoresonance

This paper develops analytically and experimentally an acoustic levitation motor which has the ability to automatically levitate and rotate an object in the air without mechanical contact. To realize such a device, a novel combination of two methods is applied simultaneously; (i) Automatic resonance tracking with the Autoresonance algorithm in closed loop, (ii) Production of controlled structural traveling waves in open loop. The purpose of the first method is to achieve near-field acoustic levitation, by exciting an aluminum annulus at resonance, and thus carrying an object of a few kilograms on a layer of compressed air above it. The difficulty in maintaining resonance arises from the very high Q value of the annulus, and therefore operating even slightly off resonance ceases levitation. Compounding this is a natural frequency drift as ambient conditions and loading change. To mitigate this drift, and produce stable levitation automatically, the resonance tracking feedback loop is employed. Simultaneously, the purpose of the second method is to achieve a propulsion torque on the levitated object by controlling acoustic traveling waves in the aluminum annulus to rotate the thin layer of air, and hence the levitated body above it. Even though a single vibration mode can produce only standing waves, an axisymmetric structure such as the annulus possesses two modes per natural frequency, and excitation of a degenerate mode pair can generate a pure travelling wave, a pure standing wave, or a mixture of both. The present paper develops the theory behind the use of the Autoresonance feedback method for achieving constant levitation and propagating travelling waves in co- and counter rotating directions. The result is an acoustic motor where stable levitation is achieved automatically and rotation is controlled via a single parameter.