Piezoelectric MEMS resonator-based oscillator for density and viscosity sensing

The resonant characteristics of a mechanical resonator immersed in liquid provide valuable parameters for density and viscosity sensing. In particular, the combination of microresonators with electronic circuits, for tracking their natural frequency and quality factor, has great interest as low cost and size solution. In this work, we focus on an oscillator featuring an in-liquid piezoelectric microplate resonator as the frequency-selective element. Specifically, by selecting the second-order bending mode in the length-direction, a reliable oscillator operation, for a range of liquid properties, was achieved at moderate frequency. Besides, the out-of-plane vibration allowed for both density and viscosity to be deduced separately. The influence of parasitic capacitances in the device response has been identified as the major difficulty for the realization of the oscillator circuit. To minimize this effect, a compensation method based on a non-released reference device and an instrumentation amplifier was implemented, which resulted in a clear resonance with low baseline and high phase step. In a first test with the resonator immersed in isopropanol, the circuit generated a stable wave with an Allan deviation of 2.32·10−7, improving by one order of magnitude the only published value, to the authors knowledge, for a comparable device in liquid. An alternative tracking system, based on digital phase-locking benchtop instrumentation, was tested with the same resonator, showing a comparable stability (2.35·10−7) and supporting our approach. Finally, the operation of the system as a sensor was demonstrated. After a calibration process, the density and viscosity of eight test liquids could be compared to measurements with a commercial instrument, showing differences lower than 0.4% in density and 8% in viscosity. The minimum detectable changes were also evaluated, being 4.09·10−6 g/ml for the density and 2.07·10−3 mPa s for the viscosity, both for a viscosity of 7.36 mPa s at 10 samples per second.