Date of Award

2008

Degree Type

Thesis

Degree Name

Master of Science in Electrical Engineering (MSEE)

Department

Electrical, Computer, and Biomedical Engineering

First Advisor

Otto J. Gregory

Abstract

This thesis describes the design and fabrication of a wireless strain gage (WSG) prototype that utilizes a radio-frequency (RF) transponder for the compressor section of a gas turbine engine. The passive transponder will be printed, welded or deposited directly onto the compressor blades, and thus several key issues have to be addressed in the design of the distributed-element microwave circuit. Some of these issues are the temperature inside the engine, which may vary from 300 °F to 1,600 °F; the large "g" forces experienced by the blades rotating at 12,000 RPM; the RF transponder thickness, which should be below that of the boundary layer thickness (~ 800 microns) so the gas flow path through the engine is not affected. The footprint of the RF transponder circuit should not be larger than a few millimeters in any direction to accurately measure strain The proposed WSG concept employs a capacitive/inductive RF transponder design with a specific resonant frequency, which responds to a short band pulse of energy from a transceiver module, such that the return signal has been modulated by the strain transmitted by the component. The goal is to correlate the frequency shift of the modulated signal to the strain in the substrate. The specifics of the transceiver module are beyond the scope of this research. However, a literature review was conducted to determine some possible technologies and approaches to solve this problem. Specifically, this research explored four different approaches for the design and fabrication of RF transponders; including one based on thin film planar structures; one based on thick film technology; one based on a "free standing" structure with a buckled-beam capacitor and one with an antenna being the actual sensing element. Results from this investigation have shown that the "free standing" structures yielded the largest gage factor, i.e. ~ 1000, compared to the thin-film and thick-film transponders which had gage factors between 11 and 14.

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