Dissertation Defense

Design and Development of Ultrasonic Jet Array (UJA) for Micro Propulsion Applications

Seow Yuen Yee

High-speed air micro-jets can be generated using an electrostatically actuated ultrasonic jet array (UJA) for eventual use in a variety of applications including micro propulsion and chip cooling. Of these applications, the most challenging is building a flying micro-platform that can generate sufficient thrust to overcome gravity. Such a platform can be used to support emergency search and rescue operations or for environmental monitoring.This thesis work presents the development and optimization of a UJA through acoustic streaming generated by a forced Helmholtz resonator. Navier-Stokes equations are used to model the high-frequency and large-gap diaphragm actuator when actuated with a trapezoidal waveform at voltages beyond pull-in. It is hypothesized that the high rise and fall time of the trapezoidal waveform and maximum volume displacement when actuated beyond pull-in will provide for additional momentum and larger response of the diaphragm, leading to higher thrust. An optimized design based on these equations was developed, and device and structural parameters were identified.A new, simple, and versatile fabrication technology was developed to produce high-frequency (> 90 kHz), large-gap (~ 10 Â µm) electrostatic diaphragm actuators with high yield and reliable actuation (> 229 billion actuation cycles). These actuators utilize a new type of electrode, filleted electrode, which was fabricated using a photoresist solvent reflow process. With the new design, these actuators can be reliably actuated for > 56 days, or > 229 billion cycles, without any diaphragm breakage. The fabricated UJA is compact and 5x lighter weight than previous work.
Finally, a pendulum test was set up and a thrust of ~ 46 Â µN with a thrust-per-weight ratio of 0.043 is measured, which is a 4.3x improvement. Although the thrust-per-weight ratio is far from one, we are one step closer to the goal of flight. The resonance frequency of the cavity has to be closer to the diaphragm's mechanical resonance frequency, and damping needs to be better understood and reduced to enable efficient streaming of the air micro-jets.

Sponsored by

Khalil Najafi