|Ph.D Student||Dotan Ilssar|
|Subject||Acoustic levitation and Propulsion of Silicon Wafers:|
Accurate Positioning and Traveling Wave
|Department||Department of Mechanical Engineering||Supervisor||Full Professor Bucher Izhak|
|Full Thesis text|
The near-field acoustic levitation phenomenon occurs when a freely suspended planar object is placed in close proximity above a rapidly oscillating driving surface, inside a gaseous medium. As a result, a thin, high-pressured layer of the surrounding gas, commonly referred to as squeeze film, gets trapped in the clearance between the driving surface and the planar object, applying a load carrying force, levitating the latter, above the oscillating driving surface. The near-field acoustic levitation phenomenon originates in the compressibility and the viscosity of the surrounding fluid, allowing to increase the time-averaged pressure in the squeeze film while preventing the fluid from flowing instantly out of the film.
It was formerly shown that when exciting the driving surface in a way that produces a traveling flexural-wave component on this surface, pressure gradients along the traveling component’s direction, are generated in the squeeze film. These pressure gradients produce flow inside the squeeze film, which applies shear forces, propelling the levitated object in the traveling component's direction.
The main goal of this research was to bring the levitation and transportation mechanisms mentioned above, closer to industrial use, by developing manageable, reduced order models and mathematical tools. This dissertation consists of five chapters, dealing with different aspects of the abovementioned mechanisms.
The first two chapters introduce simplified models, describing the governing vertical dynamics of near-field acoustically levitated objects, under a uniform excitation and under excitation with a non-uniform, axisymmetric, standing, flexural wave. These simplified models allow one to relatively easily control the prevailing dynamics of near-field acoustic levitation based systems, using model based control algorithms. Indeed, the more general model, describing the dynamics of the system under a non-uniform excitation, is exploited in order to devise a height dependent, gain-scheduled PID controller, providing a rapid and accurate vertical positioning.
The third chapter deals with simplified modelling, describing the dynamic behavior of a typical near-field acoustic levitation based system that consists of a piezoelectric actuator excited with an oscillatory input voltage, coupled to an acoustically levitated object.
The fourth chapter describes a contactless, ring-type ultrasonic motor, enabling to control both the vertical and the angular positions of a carried object, exploiting the levitation and transportation mechanisms. This chapter covers the physical considerations led to the design of the motor, followed by derivation of a truncated dynamic model, allowing to sense and control the quality of the excitation, which governs the torque applied on the carried object. Furthermore, an approach enabling to optimally control the torque applied by motor, using a single control parameter, is introduced.
Finally, the fifth chapter deals with a reduced order model, capturing the governing dynamics of the contactless motor discussed in the fourth chapter. Namely, this simplified model describes the governing dynamics of both the vertical and the rotational degrees of freedom of the carried object, under excitation with flexural waves of a given amplitude and quality.