Electromagnetic-Acoustic Resonators for Remote, Multi-Mode Solid and Liquid Phase Sensing

Electromagnetic-acoustic excitation of resonator sensors has been established as a viable alternative to piezoelectric or capacitive transduction with unique advantages. The non-contact principle is based on eddy current induction, Lorentz force generation, and movement induction. Depending on geometric and electrical parameters, several distinct resonant modes of vibration can be utilized. The acoustic behavior is reflected in the electrical impedance of the primary excitation setup. Changes to the acoustic load in contact with the resonator, such as added mass or viscous liquids, result in characteristic resonance frequency shifts and changes in the damping behavior. Similar to other acoustic sensors, these changes are directly related to physical properties, such as mass, density, viscosity, or liquid volume, and thus allow for remote, non-invasive characterization of liquid and solid analytes. Simulations and equivalent circuit models are presented to better describe the physical mechanisms involved and as a means to relate measurement data to the physical properties. Several prototype resonators and possible applications are theoretically and experimentally characterized, ranging from mass microbalance and viscosity sensors, to liquid level measurements and multi-mode resonator arrays.