Density-viscosity sensor based on piezoelectric MEMS resonator and oscillator circuit

There are many examples of physical sensing in liquid employing resonators, especially involving TSM quartz crystals. Nevertheless, to determine density and viscosity independently, out-of-plane vibration modes are required, what typically results in low quality factors. Non-fundamental out-of-plane vibration modes have not been deeply studied for this application. Here we demonstrate the benefit of the 2nd-order mode of a microplate. In addition, the device needs to be electronically interfaced in order to measure its resonant characteristics that are ultimately translated into density and viscosity. For this, we have implemented an oscillator circuit with discrete components, which supposes a novelty regarding piezoelectric microstructures immersed in liquid. As a result, we present for the first time a complete density-viscosity sensor system based on a MEMS resonator embedded in low-cost circuitry. The sensitivities involved are evaluated, along with the resolutions for density and viscosity, below 4.09·10^-6 g/ml and 2.07·10^-6 mPa·s respectively.