Robust Transducer–Reflector Distance Control in Acoustic Levitation

Abstract

Acoustic levitation enables contactless manipulation for applications in containerless processing in chemistry and materials science. Single-transducer levitators, consisting of an ultrasonic transducer and a reflector, offer a minimal hardware configuration but require precise control of the transducer–reflector distance $H$ . We propose a distance stabilization system based on a motorized linear stage and a precision balance that measures the acoustic radiation force (ARF) as a time-averaged, surface-integrated force magnitude on the reflector. Because ARF versus $H$ exhibits extremely narrow resonance peaks, micrometer-scale deviations markedly reduce force and destabilize levitation. In practice, high-power transducers are actively kept at resonance using resonance frequency tracking (RFT). As the tracked frequency changes, the wavelength changes as well and the optimal distance $H$ shifts. We introduce a two-stage control strategy in order to robustly stabilize $H$ . First, a physics-informed, one-sided adaptive hill climb algorithm locates the cavity resonance. Second, a physics-informed extremum-seeking controller maintains the experimentally determined resonance distance despite drift and disturbances, using only the absolute ARF feedback. Long-term validation at node index $n$   $\text {=}$  5 confirmed that ARF remained within the 95% tolerance bound for two hours, ensuring stable levitation. Performance was limited by the balance, mainly due to its internal averaging time. This occasionally caused convergence to local maxima or temporary loss of track during object insertion. The strategy presented enables robust, long-duration operation of acoustic levitators.