J. Toledo

Piezoelectric MEMS resonators for density and viscosity sensing in engine oil with diesel fuel

This work demonstrates the potential of AlN-based resonators as on-line sensors for monitoring lubricant oil dilution with diesel. Two devices are compared, one actuated in the first extensional in-plane mode and the other in an out-of-plane mode (14-mode). Both devices are designed to feature high quality factor in liquid, and allow discriminating variations in density or viscosity in the medium. Sensor resolutions for these two variables are compared in lubricant oil SAE 2.5W, and the device with the best result (14-mode) was also tested in continuous-flow measurements, showing a resolution of 0.5 ppm of diesel contamination in this oil.

Out-of-plane piezoelectric microresonator and oscillator circuit for monitoring engine oil contamination with diesel

Real-time monitoring of the physical properties of liquids is an important subject in the automotive industry. Contamination of lubricating oil by diesel soot has a significant impact on engine wear. Resonant microstructures are regarded to be a precise and compact solution for tracking the viscosity and density of lubricant oils. Since the measurement of pure shear forces do not allow an independent determination of the density and viscosity, two out-of-plane modes for the monitoring of oil dilution with diesel have been selected. The first one (12-mode) is working at 51 kHz and the second mode (14-mode) at 340 kHz. Two parameters were measured: the quality factor and the resonance frequency from which the viscosity and density of the fluids under test can be determined, requiring only a small amount of test liquid. A PLL-based oscillator circuit was implemented based on each resonator. Our results demonstrate the performance of the resonator in oils with viscosity up to 90 mPa·s. The quality factor measured at 25°C was 7 for the 12-mode and 19 for the 14-mode. A better resolution in density and viscosity was obtained for the 14-mode, showing a resolution of 3.92·10-5 g/ml for the density and 1.27·10-1 mPa·s for the viscosity, in pure lubricant oil SAE 0W30. An alternative tracking system, based on a discrete oscillator circuit, was tested with the same resonator, showing a comparable stability and supporting our approach.

Piezoelectric MEMS resonators for monitoring grape must fermentation

The traditional procedure followed by winemakers for monitoring grape must fermentation is not automated, has not enough accuracy or has only been tested in discrete must samples. In order to contribute to the automation and improvement of the wine fermentation process, we have designed an AlN-based piezoelectric microresonator, serving as a density sensor and being excited in the 4th-order roof tile-shaped vibration mode. Furthermore, conditioning circuits were designed to convert the one-port impedance of the resonator into a resonant two-port transfer function. This allowed us to design a Phase Locked Loop-based oscillator circuit, implemented with a commercial lock-in amplifier with an oscillation frequency determined by the vibrating mode. We were capable of measuring the fermentation kinetics by both tracking the resonance frequency and by determining the quality factor measurements of the microresonator. Moreover, the resonator was calibrated with an artificial model solution of grape must and then applied for the monitoring of real grape must fermentation. Our results demonstrate the high potential of MEMS resonators to detect the decrease in sugar and the increase in ethanol concentrations during the grape must fermentation with a resolution of 100 μg/ml and a sensitivity of 0.16 Hz/μg/ml as upper limits.

Piezoelectric resonators and oscillator circuit based on higher-order out-of-plane modes for density-viscosity measurements of liquids

We report the use of two AlN-based piezoelectric microresonators for the monitoring of density and viscosity of liquids and its application to detect lubricant oil dilution with diesel fuel. Two devices designed to resonate in the 4th-order roof tile-shaped vibration mode, but with two different anchor schemes, were fabricated and characterized. Interface circuits were designed to convert the one-port impedance into a resonant two-port transfer function. This allowed us to implement a phase locked loop (PLL)-based oscillator circuit based on the resonators, the interface circuit and a commercial lock-in amplifier. Our results demonstrate the performance of the resonators in fluids having viscosities up to 500 mPa s. The performance of the sensors in terms of sensitivity and resolution are compared for both anchor configurations.

Modelling and characterization of the roof tile-shaped modes of AlN-based cantilever resonators in liquid media

In this work, roof tile-shaped modes of MEMS (micro electro-mechanical systems) cantilever resonators with various geometries and mode orders are analysed. These modes can be efficiently excited by a thin piezoelectric film and a properly designed top electrode. The electrical and optical characterization of the resonators are performed in liquid media and the device performance is evaluated in terms of quality factor, resonant frequency and motional conductance. A quality factor as high as 165 was measured in isopropanol for a cantilever oscillating in the seventh order roof tile-shaped mode at 2 MHz. To support the results of the experimental characterization, a 2D finite element method simulation model is presented and studied. An analytical model for the estimation of the motional conductance was also developed and validated with the experimental measurements.

Characterization of oscillator circuits for monitoring the density-viscosity of liquids by means of piezoelectric MEMS microresonators

Real-time monitoring of the physical properties of liquids, such as lubricants, is a very important issue for the automotive industry. For example, contamination of lubricating oil by diesel soot has a significant impact on engine wear. Resonant microstructures are regarded as a precise and compact solution for tracking the viscosity and density of lubricant oils. In this work, we report a piezoelectric resonator, designed to resonate with the 4th order out-of-plane modal vibration, 15-mode, and the interface circuit and calibration process for the monitoring of oil dilution with diesel fuel. In order to determine the resonance parameters of interest, i.e. resonant frequency and quality factor, an interface circuit was implemented and included within a closed-loop scheme. Two types of oscillator circuits were tested, a Phase-Locked Loop based on instrumentation, and a more compact version based on discrete electronics, showing similar resolution. Another objective of this work is the assessment of a calibration method for piezoelectric MEMS resonators in simultaneous density and viscosity sensing. An advanced calibration model, based on a Taylor series of the hydrodynamic function, was established as a suitable method for determining the density and viscosity with the lowest calibration error. Our results demonstrate the performance of the resonator in different oil samples with viscosities up to 90 mPa•s. At the highest value, the quality factor measured at 25°C was around 22. The best resolution obtained was 2.4•10-6 g/ml for the density and 2.7•10-3 mPa•s for the viscosity, in pure lubricant oil SAE 0W30 at 90°C. Furthermore, the estimated density and viscosity values with the MEMS resonator were compared to those obtained with a commercial density-viscosity meter, reaching a mean calibration error in the best scenario of around 0.08% for the density and 3.8% for the viscosity.

Potential of Piezoelectric MEMS Resonators for Grape Must Fermentation Monitoring

In this study grape must fermentation is monitored using a self-actuating/self-sensing piezoelectric micro-electromechanical system (MEMS) resonator. The sensor element is excited in an advanced roof tile-shaped vibration mode, which ensures high Q-factors in liquids (i.e., Q ~100 in isopropanol), precise resonance frequency analysis, and a fast measurement procedure. Two sets of artificial model solutions are prepared, representing an ordinary and a stuck/sluggish wine fermentation process. The precision and reusability of the sensor are shown using repetitive measurements (10 times), resulting in standard deviations of the measured resonance frequencies of ~0.1%, Q-factor of ~11%, and an electrical conductance peak height of ~12%, respectively. With the applied evaluation procedure, moderate standard deviations of ~1.1% with respect to density values are achieved. Based on these results, the presented sensor concept is capable to distinguish between ordinary and stuck wine fermentation, where the evolution of the wine density associated with the decrease in sugar and the increase in ethanol concentrations during fermentation processes causes a steady increase in the resonance frequency for an ordinary fermentation. Finally, the first test measurements in real grape must are presented, showing a similar trend in the resonance frequency compared to the results of an artificial solutions, thus proving that the presented sensor concept is a reliable and reusable platform for grape must fermentation monitoring.

Fluid-structure interaction modelling of the roof tile-shaped modes in piezoelectric plate microresonators

In this paper, the fluid-structure interaction in cantilever-type devices vibrating in the first and higher roof tile-shaped modes is studied. These modes can be most efficiently excited by a thin piezoelectric film on top of the structure in combination with a tailored electrode design. The electrical and optical characterization of the different devices and modes is carried out in liquid media and then the performance of the resonators is evaluated in terms of quality factor and resonant frequency. The effect of the fluid on the in-liquid response is studied using analytical and finite element method models. For the latter, a fully coupled fluid-structure interaction model is developed and compared to a simpler model, in which no coupling feedback from the fluid to the structure is taken into account. The results show that, despite the substantially larger computational effort, the consideration of the fluid-structure coupling is absolutely necessary to explain the experimental results for higher order modes.