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A new type of contactless motor has been developed and tested using ultrasonic near-field acoustic levitation and structural traveling waves to levitate and induce a torque on the rotor. The component of interest is the stator which consists of a flat vibrating ring excited by three evenly spaced piezoelectric actuators. The stator performs two simultaneous functions; 1) To levitate the rotor, thus acting as a contactless bearing. 2) To induce airflow between itself and the rotor to apply a torque on the rotor. The first function is accomplished by exciting a high-amplitude vibration mode of the stator at exactly its natural frequency. In this way, the air pressure between the stator and rotor becomes significantly higher than the ambient pressure, and levitation will occur. Autoresonance (AR) is the feedback algorithm implemented on an FPGA used to track the natural frequency. The second function, to apply a torque, is realized by inducing traveling vibrational waves through the stator. This is possible if the mode shape of the corresponding resonant frequency is known. By accomplishing both tasks, a few kilograms can be levitated and precisely rotated without physical contact. The direction and strength of the torque applied by the stator can be easily controlled. Experiments have shown that when levitated, the rotor can be modeled as a mass with extremely low damping. Closed loop control of angular position can be accomplished with an encoder and simple PID. A possible application could be precise positioning of a silicon wafer in a metrology lab.

AR is a nonlinear feedback control method used to continuously excite a system at its natural frequency. AR is accomplished by rotating the Nyquist Plot of the open loop system such that its natural frequency crosses the negative real axis. In practice, this rotation is performed by digitally integrating a vibration sensor signal. After integration, the signal is passed through a digital relay to scale it to a constant amplitude. This is the signal that is sent to the actuators. Though less popular than the Phase Locked Loop (PLL), AR has the following advantages which make it suitable for this application: i) No tuning of a PID is required. ii) Resonance locking is very fast.

Due to the cylindrical symmetry of the stator, the modes of interest appear in pairs. In theory, these modes have identical temporal response, but are shifted in space by a quarter wavelength such that the nodes of one overlaps the antinodes of the other. If both modes are excited with identical amplitude a pure traveling wave can be excited in the stator. The quality (torque strength) and direction of the traveling waves can be controlled by manipulating the phases of the actuators with respect to each other.

Presented here is a method for inducing AR to lock onto a specific mode of the stator, even though many are present. Additionally, a model of the rotor will be presented based on perturbations from the stator. Finally, experimental results from closed loop position control will be presented and analyzed.