Date of Award

Fall 2014

Document Type

Dissertation - Restricted

Degree Name

Doctor of Philosophy (PhD)

Department

Electrical Engineering and Computer Science

First Advisor

Yaz, Edwin E.

Second Advisor

Josse, Fabien

Third Advisor

Burch, Thaddeus J.

Abstract

There has been a growing interest in using SAW devices for sensing various physical parameters such as pressure, acceleration, temperature, and gas. The current research has been undertaken to derive a model of a SAW-based gas flow rate sensor, using the principle of heat transfer with the flow of gas. It consists of a SAW delay line fabricated on a suitable substrate and a thin film heater to heat the SAW device to a suitable temperature above the ambient. The delay line is connected in the feedback loop of an rf-amplifier resulting in a delay line stabilized SAW oscillator. When gas flows over the SAW device, it carries away heat thus lowering the temperature of the substrate. The change in the frequency of the SAW oscillator, which can be accurately measured, is proportional to flow rate of gas. The device is similar in principle to a hot wire or hot-film anemometer. This dissertation is concerned with the quantitative analysis of SAW flow sensor. Theoretical analysis to determine the characteristics of SAW flow sensor for various substrate material and geometries will be presented. From the analyses, a design of an optimal SAW flow sensor will be proposed. Mathematical model(s) will be derived to determine and predict SAW gas flow sensor behavior and characteristics for a wide range of variables and boundary conditions. These characteristics include: * Steady state temperature characteristics * Transient time response characteristics * Frequency versus temperature characteristics * Frequency characteristics versus input power to the integrated thin film heater element * Frequency change versus flow rate at constant current to the thin film heater element * Determine temperature distribution to derive average temperature The model developed in this work enables us to characterize SAW-based sensor by including substrate material properties, dimensional and wide range of operating conditions. It can predict frequency changes, temperature changes, for given values of heater power, under steady state free and forced convection conditions of fluid. It allows time response analyses and thus enables us determine parameters that can be used to obtain an optimal SAW-based sensor designs. Experimental results carried out on preliminary design of SAW flow sensor are presented and result compared with theoretical analyses.

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