Date of Award

Summer 8-23-2019

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Master of Science in Electrical Engineering (MSEE)

Department

Electrical and Computer Engineering

Advisor

Nuri Emanetoglu

Second Committee Member

John Vetelino

Third Committee Member

Mauricio Pereira da Cunha

Abstract

Sensor systems are utilized to provide critical information to an end user which may range from a physician in a heath care facility to a soldier in a battle field environment. The "heart" of the sensor system is the sensing platform, examples of which include semiconductor, piezoelectric and optical devices. The responses of these sensors must be converted into a format that the user can read and interpret. This conversion is achieved through integrating the sensing platform with an electrical interface.

The focus of this thesis is the development of the first electrical interface for Quartz Crystal Microbalance (QCM) sensors in the Lateral Field Excitation (LFE) configuration. Common techniques used for interfacing with thickness field excitation (TFE) QCM devices include impedance-based systems, oscillator systems, and phase-mass based systems. Although oscillators have been successfully designed for TFE QCMs, attempts to develop an oscillator-based interface system for the LFE QCMs operating in air and vacuum media have been unsuccessful. A comparative study of LFE and TFE sensors operating in air and vacuum media was conducted to determine the reason why these interfaces do not work with LFE QCMs. It was concluded that compared to TFE sensors LFE sensors have higher motional resistance, Rm, and narrower separation between the series and parallel resonant frequencies, which inhibited oscillation. To identify an optimum configuration for the 6MHz LFE sensor based on the sensor's impedance response, 45 different configurations for the LFE sensor were fabricated and tested.

Based on the conclusions of the comparative study and further investigation into QCM electrical interfaces, two electrical interface systems were investigated for the chosen LFE: the Balanced Bridge Oscillator (BBO) and the Phase Shift Monitoring system. The BBO, a type of frequency tracking system, was selected as the parallel capacitance seen by the sensor can be compensated for, improving the bandwidth of the sensors impedance response. This circuit can be tuned to match the LFE response, and incorporate automatic gain control. However, The fabricated BBO was unable to achieve a stable oscillation with current LFE devices.

The Phase-Shift Monitoring system, which is based on the Phase-Mass characterization method, utilizes an external signal to excite the sensor, and the change in the phase shift of the sensor is tracked as a load is applied to it. The system outputs two DC signals corresponding to the detected change in phase-shift and signal amplitude. The Phase-Mass Monitoring system was tested using both liquid and solid loading with the LFE sensor, and was able to consistently detect masses in the 10s of micrograms range. When the LFE was loaded with 52μg in air, the system output 7.45mV with a tolerance of ±0.6mV.

The Phase-Shift Monitoring system is the first electrical interface to be successfully integrated with the LFE sensor platform in air and vacuum media, where oscillator-based systems have been unsuccessful. Further work and testing on the system are required to fully characterize the phase-mass relationship of the LFE, as well as developing the system for commercialization.

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