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J. Toledo, T. Manzaneque, V Ruiz-Díez, F. Jimenez-Molinos, M. Kucera, G. Pfusterschmied, E Wistrela, U. Schmid, J.L. Sànchez-Rojas:
"Comparison of in‑plane and out‑of‑plane piezoelectric microresonators for real‑time monitoring of engine oil contamination with diesel";
Microsystem Technologies - Micro- and Nanosystems - Information Storage and Processing Systems, 22 (2016), 1781 - 1790.

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
two different resonators for the monitoring of oil dilution
with diesel fuel; one device was designed to vibrate in outof-
plane modes (12-mode or 14-mode), while the other
micro-plate was actuated in the first extensional in-plane
mode. To determine from the measurements the resonance
parameters of interest (resonance 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. A model with fitting
parameters was validated allowing for the determination
of the viscosity and density of the fluids under test,
for which only a small amount of test liquid, in the range
of 0.5 ml, was required. Our results demonstrate the performance
of the resonators in oils with viscosity up to
90 mPa s. For such viscosity, the quality factor measured
at 25 °C was 7 for the 12-mode, 19 for the 14-mode and 16 for the extensional mode. The best resolution for both fluid
material parameters was obtained in the 14-mode, showing
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. Finally,
the resonator with the best result (14-mode) was also
tested in continuous-flow measurements, showing a resolution
of 0.5 ppm of diesel contamination in a pure lubricant
oil SAE 2.5 W.

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
two different resonators for the monitoring of oil dilution
with diesel fuel; one device was designed to vibrate in outof-
plane modes (12-mode or 14-mode), while the other
micro-plate was actuated in the first extensional in-plane
mode. To determine from the measurements the resonance
parameters of interest (resonance 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. A model with fitting
parameters was validated allowing for the determination
of the viscosity and density of the fluids under test,
for which only a small amount of test liquid, in the range
of 0.5 ml, was required. Our results demonstrate the performance
of the resonators in oils with viscosity up to
90 mPa s. For such viscosity, the quality factor measured
at 25 °C was 7 for the 12-mode, 19 for the 14-mode and 16 for the extensional mode. The best resolution for both fluid
material parameters was obtained in the 14-mode, showing
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. Finally,
the resonator with the best result (14-mode) was also
tested in continuous-flow measurements, showing a resolution
of 0.5 ppm of diesel contamination in a pure lubricant
oil SAE 2.5 W.