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26 July 2004 Resonance tracking in nonlinear system: application to a squeeze-film levitation device
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This paper presents a method to change the excitation frequency such that a piezoelectric actuator keeps the levitation height of a floating object suspended on a layer of air at its maximal possible height, despite changes to the system. An ultrasonic actuator capable of injecting ultrasonic vibrations to an air layer is being using as a demonstrator in this work. The high frequency oscillations and the compressibility of air generate an average pressure higher than the ambient one thus causing a flat object to float. Environmental changes and temperature effects render this system inefficient, unless the excitation frequency is constantly varied to maintain the best possible efficiency of energy transfer. An adaptive method with which an optimal excitation frequency is generated is shown to yields the best mechanical efficiency and it thus essential when the available input power is restricted. The proposed method uses a minimal amount of frequency dither to revive an identification process that is otherwise singular. With this identified model, the algorithm seeks the best momentary excitation frequency. The algorithm is validated in a simulation and on a dedicated experimental apparatus.
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Izhak Bucher and Ran Gabay "Resonance tracking in nonlinear system: application to a squeeze-film levitation device", Proc. SPIE 5383, Smart Structures and Materials 2004: Modeling, Signal Processing, and Control, (26 July 2004);

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