Symmetric Mechanical Plate Resonators for Fluid Sensing

In this contribution, we present a symmetric arrangement of electromagnetically actuated, resonating plates for viscosity and density measurement. Regarding the oscillation mode, the design resembles a tuning fork and is based on previously introduced resonating plate designs. It exhibits similar sensitivity to viscosity as previously characterized single plate resonators, while having two shear modes at different frequencies, that can be both actuated with the same setup allowing a multi-frequency analysis of viscoelastic liquids. We investigate the effects of the actuation method on the sensors performance by actuating with two different magnetic field configurations and by trying different configurations of the actuation and readout circuit, from which we show that with a particular configuration, Q-factors of up to 5000 can be reached for the symmetric shear mode. We then proceed to suggesting a simplified lumped mechanical oscillator model explaining the dependence of the resonator's Q-factor operated in air on induced eddy currents damping. Measurement results are presented which show the general dependence of the new design's actuated modes, the anti-symmetric and symmetric mode, on viscosity and density. We apply two different models to our data: a newly developed generalized model and a simplified version of it which better describes the observed relation of our measurements to the square root of the product viscosity density. We then proceed to perform an estimated error analysis on viscosity, density, and the square root of their products based on the above mentioned applied models. From this error analysis several conclusions are drawn, mainly that the symmetric mode is more accurate than the anti-symmetric one, the sensor is generally more accurate for lower viscosity liquids, and lastly that the square root of the viscosity density product is a more suited value for the description of the sensor's behavior than the viscosity or the density alone.