Monitoring Physical Fluid Properties Using a Piezoelectric Tuning Fork Resonant Sensor
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For fluid analysis applications, such as oil condition monitoring, fuel quality, or gas concentration measurements, resonant sensors deliver an outstanding performance when signal processing is optimized and the fluid-mechanical model of the electromechanical resonator is suitable and accurate for the particular resonator. By combining recent advancements, significant improvements in accuracy, measurement speed, dynamic range, and suppression of cross-sensitivities could be achieved. These features enable the development of new solutions for a variety of measurement issues in industry and bio technology. In this contribution the performance of a highly universal evaluation system is demonstrated using a commercially available quartz crystal tuning fork resonator as sensing element for liquid viscosity and mass density. The obtained results are quantified with respect to an accurate lab bench viscosity and mass density meter. A significant advantage of this system is that it operates reliably and accurately even for strongly damped resonators. Therefore, the sensor elements can be used in a larger viscosity range than with alternative evaluation methods.
KeywordsFuel analysis Oil condition monitoring High accuracy Micro acoustic liquid sensor Low cross-sensitivity Mass density Viscosity
Überwachung physikalischer Fluidparameter mit einer piezoelektrischen Stimmgabel als resonantem Sensor
Die Bestimmung von Fluidparametern, beispielsweise in der Ölzustandsüberwachung, zur Messung der Treibstoffqualität oder von Gaskonzentrationen ist ein Anwendungsgebiet, in dem resonante Sensoren vielfältige Vorteile aufweisen. In den letzten Jahren konnten durch verschiedene Entwicklungen im Zusammenhang mit der Auswertung solcher Sensoren signifikante Verbesserungen in Messgeschwindigkeit und Messgenauigkeit und eine deutliche Reduktion von potentiellen Querempfindlichkeiten erzielt werden. Besonders bei der Auswertung stark gedämpfter Resonatoren, wie bei der Messung von Flüssigkeitseigenschaften, ergeben sich dadurch neue Anwendungsfelder für diese Sensorfamilie. In diesem Beitrag wird die Leistungsfähigkeit dieser Technologie am Beispiel eines miniaturisierten Quarz-Resonators gezeigt und mit einem hochwertigen kommerziellen Analysegerät verglichen.
SchlüsselwörterResonante Sensoren Zustandsüberwachung Akustische Sensoren Massendichte Viskosität
Resonant sensors can be used in a wide range of applications, e.g. as microbalances, chemical sensors in liquid and gaseous environments, and for physical property sensing of liquid and viscoelastic media . Sensor elements with direct linear relation between the measured quantity and a processable output signal are desired in measurement practice but are often not available, especially when high accuracy and suppression of cross influences are required. For a viscosity and mass density sensor, the utilization of a measurement principle which evaluates the frequency response of an electroacoustic resonator in contact with the fluid under test is advantageous. The frequency response of such a resonant sensor is related to the fluid properties by a nonlinear function but is also affected by several other – in most cases spurious – influences .
For alternative sensor concepts, e.g. using Lorentz-force excitation and inductive readout , similar extensions to a resonant circuit are required to model inductive crosstalk or wire resistances, for instance. It can be shown that these sensors can be described by circuits similar to the BVD model , and thus the approach described here is also applicable to a wider class of sensors. For the sake of convenience, this contribution focuses on piezoelectric resonators as an illustrative example.
2 Readout of Resonant Sensors
The use of oscillator circuits detuned by the measurement parameters is one of the most common approaches for the readout of resonant sensors and is particularly suitable for sensors with high quality factors or when the damping of the resonance remains constant. This is the case for, e.g. quartz crystal microbalances (QCM) monitoring the deposed metal layer in vapor deposition lines. One of the major advantages of oscillators is that this approach allows for very cost-efficient implementations yielding a frequency as output parameter which, in principle, can be measured very accurately.
For operating conditions where increased damping of the resonator is expected, e.g. when electromechanical resonators are exposed to liquid environments, additional effort has to be made to separate the behavior of the motional branch from parasitic effects like the parallel capacitance and the fluid’s permittivity and conductivity (Fig. 1).
Several variations of locked loop circuits were reported to consider these influences (e.g. ,), but, with this approach, spurious influences that are changing during operation can be addressed only to a very limited extent (e.g. compensation of changes in parallel capacitance as shown in ). With increasing demands for measurement accuracy or with higher damping of the resonator (viscous liquids), the only practicable solution is to record the behavior of the resonator in vicinity of the resonance and to shift the extraction of desired parameters to a post processing step in the digital domain. In this context several dedicated analyzer systems based on measurement methods like those implemented in gain-phase/network/impedance analyzers were reported (e.g. ,).
In our previous work, we addressed numerous details in order to improve the performance of these compact analyzer systems. This concerns data acquisition concepts for minimal signal processing effort , approaches to reduce parasitic signal components , numerical methods for separating motional from parasitic behavior , and improvements in sensor modeling .
The latest development in this respect is a highly universal evaluation system for interfacing resonant sensors, which utilizes and combines various approaches that are required or simply beneficial for a high performance measurement system. This system is developed by the university spin-off Micro Resonant Technologies and tested in collaboration with the Institute for Microelectronics and Microsensors at the Johannes Kepler University Linz.
3 Tuning Fork Resonator as Sensor for Viscosity and Density of Fluids
Various publications address the use of tuning fork resonators for determination of physical fluid properties such as viscosity and mass density (e.g. ,). In contrast to resonators with dominant shear oscillation (like torsional resonators or thickness shear mode QCR), the tuning fork resonator allows better separation of mass density and viscosity ,,.
4 Experimental Setup
Reference measurements for comparison purposes as well as for calibration of the resonator model were performed with a Stabinger Viscometer (SVM3000), which nominally provides an accuracy of ±0.35% for viscosity and ±0.5 kg/m³ for mass density.
For demonstration purposes, two experiments are shown below: one to determine the achievable accuracy when monitoring liquid properties of fuels (Diesel) and a second one on the condition monitoring of engine oil, where the dilution with fuel is a severe issue for engine manufactures. Previous results of similar experiments were reported in  and .
5.1 Fuel Analysis
From the acquired sequences, mean values and standard deviations were derived and compared to the reference measurements. The standard deviations of frequency and quality factor and subsequently of viscosity and density results are in close agreement with the theoretically predicted error propagation .
Measured values for viscosity and mass density compared to reference data (obtained with an Anton Paar SVM3000)
tuning fork sensor
RT5 silicone oil
5.2 Fuel Dilution of Engine Oil
The results show that a change of fuel content in the range of 0.1 % can be reliably determined within seconds even at elevated temperatures as expected in a running engine. In order to obtain a high absolute accuracy of the diesel content ratio, the implementation of a mixing model for the specific oil is recommended.
A viscosity and density measurement system based on a resonant sensor has been presented. For demonstration purposes, the system was evaluated using a commercial off-the-shelf resonator as sensor. The signal processor based system for evaluating the resonant behavior of the sensor can be used with virtually any kind of resonator from small quartz crystal tuning forks up to rugged resonators made from stainless steel.
The results obtained with this setup have been compared to an accurate top-grade lab bench viscosity and density meter and show outstanding trueness and precision at a significantly higher measurement speed, ideally suited for a large range of applications, such as online process monitoring, condition monitoring of lubricants, low fluid volume measurements, hand-held devices, laboratory use, and more.
This work has been supported by Micro Resonant Technologies and the Linz Center of Mechatronics (LCM) in the framework of the Austrian COMET-K2.
Open access funding provided by Johannes Kepler University Linz.
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