A rotational traveling wave based levitation device - Modeling, design,   and control
  Described is a device acting on an acoustically levitated object by manipulating the pressure and flow of a thin layer of air such that its rotation can be precisely controlled without mechanical contact. Virtual work analysis assists in simplifying the multi-actuator control problem into a problem governed by a controllable parameter. Actuation is done with a vibrating ring capable of producing ultrasonic standing and traveling waves, creating the acoustic excitation that affects the pressure in a thin, intermediate layer of gas. A distinctive vibration pattern is required to generate the temporal and spatial pressure field of the squeezed air layer that gives rise to both acoustic levitation force and rotational torque. Described are the physical and design development stages leading to an optimized structure, all followed by verifying and dynamics-calibration experiments. Moreover, by precisely controlling the ratio of standing and traveling waves in a closed-loop, one can affect the shear forces applied by the squeezed air layer, thus creating a non-contacting manipulation mechanism. An over-actuated set-up is converted via an algebraic transformation, into a simplified single control-parameter problem. The transformation ties the standing waves ratio, and hence the rotational torque, to the amplitudes and phases of the actuators. This arrangement leads to an effective closed loop methodology that was implemented experimentally showing good performance and exhibiting rapid angular positioning. 
Ran Gabai, Dotan Ilssar, Ran Shaham, Nadav Cohen, Izhak Bucher
physics.class-ph