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

Abstract

High-speed air micro-jets can be generated using an array of electrostatically-actuated Helmholtz resonators that form an 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. This thesis work presents the development and optimization of an UJA which builds on previous work in forming micro-fabricated air micro-jets. The UJA consists of high-frequency and large-deflection actuators that enclose acoustic cavities formed under them. Navier-Stokes equations are used to study the high-frequency and large-gap diaphragm actuator when actuated with a trapezoidal waveform at voltages beyond pull-in. 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 µm) electrostatic diaphragm actuators with high yield and reliable actuation (> 229 billion actuation cycles). These actuators utilize a new type of electrode (a filleted electrode), which was fabricated using a photoresist solvent reflow process. The fabricated UJA is compact with a footprint of 0.9 x 0.9 cm2 and is 5x lighter in weight than previous work. The thrust generated by the UJA is measured through a pendulum test setup where the UJA was suspended at the end of a 50 cm long wire. A thrust of ~ 46 µN with a thrust-per-weight ratio of 0.043 is measured, which is a 4.3x improvement over previous work. The applied voltage is ± 330 V and the power consumption is < 10 mW per actuator. The UJA thrust and performance could be further increased by: (1) matching the acoustic resonance of the cavity to the diaphragm’s mechanical resonance, (2) reducing damping, (3) measuring and characterizing the air micro-jets in more detail, and (4) improving the analytical and simulation models to match experimental results.