Modeling and characterization of piezoelectric transducers by means of scattering parameters. Part II: Application on colloidal suspension monitoring

This paper illustrates the application of the S-parameter concept for the analysis of the response of a piezoelectric sensor exploited for the discrimination of bentonite colloids of interest in construction engineering. The device is a millimeter-sized cantilever beam fully immersed in the suspension under examination. The beam is energized by a sweeping sinusoidal voltage: the electrical admittance is measured and the corresponding S-parameter is calculated. A procedure for the extraction of the lumped element circuital model is presented and the identification process clearly improves when the S-parameter response is used. Dealing with variable loading conditions, any change on density and/or viscosity of the interacting fluid determines a well-behaved variation of the frequency response associated with the principal modes of mechanical vibration. Inspection of response variations, especially in the S-parameter Smith chart and in the neighborhoods of resonance, allows the discrimination of different loading mixtures. Experimentation was carried out with 2.5%, 5.0%, 7.5% and 10.0% bentonite mixtures and the sensor has been able to fully discriminate between these different bentonite colloidal suspensions.