A.O. Niedermayer

Miniaturized sensors for the viscosity and density of liquids-performance and issues

This paper reviews our recent work on vibrating sensors for the physical properties of fluids, particularly viscosity and density. Several device designs and the associated properties, specifically with respect to the sensed rheological domain and the onset of non-Newtonian behavior, are discussed.

Methods for the robust measurement of the resonant frequency and quality factor of significantly damped resonating devices

For many applications, resonating sensors can be designed which exhibit excellent sensitivity of the resonant parameters (resonant frequency and quality factor) to the wanted physical parameters. When the resonant parameters are to be derived from measured data, the utilized signal processing algorithm significantly affects the precision of the obtained results, in particular, when the resonance is impaired with spurious contributions. In particular, for systems with low Q-factors (e.g., electromechanical resonators in viscous liquids with Q < 100), the measurement precision suffers from various unwanted spectral components induced by parasitic effects of the resonator and measurement errors. In order to separate the adverse effects from the ideal second-order characteristics, three related methods for estimating the parameters of second-order resonant systems are introduced in this paper extending established methods. To this end, the spectrum is separated into a component induced by a second-order resonant system and an unknown background spectrum.

Resonance parameter estimation from spectral data: Cramér–Rao lower bound and stable algorithms with application to liquid sensors

A recently introduced method for robust determination of the parameters of strongly damped resonances is evaluated in terms of achievable accuracy. The method extracts and analyzes the locus of the resonant subsystem of noisy recorded complex spectra, such that the interfering influences of many environmental factors are eliminated. Estimator performance is compared to the absolute lower limit determining the Cramer–Rao lower bound (CRLB) for the variance of the estimated parameters. A generic model that is suitable for representation of a large class of sensors is used and analyzed. It is shown that the proposed robust method converges to the CRLB for low measurement noise.

Double membrane sensors for liquid viscosity and mass density facilitating measurements in a large frequency range

A resonating double membrane based sensor for viscosity and mass density facilitating measurements at different frequencies and two adjustable modes of vibration is presented. The sensor is designed to be suitable, e.g., for low cost handheld devices with inline capabilities and disposable sensor elements. In the sensor, a sample liquid is subjected to time harmonic shear stress induced by two opposed vibrating polymer membranes, placed in an external static magnetic field. These membranes carry two conductive paths each. The first path is used to actuate the membranes by means of Lorentz forces while the second acts as a pick-up coil providing an induced voltage representing the movement of the membrane. From the measured frequency response the liquids viscosity and mass density can be deduced. This double membrane based setup allows to examine the test liquid at adjustable frequencies in the low kilohertz range from 1 kHz to 15 kHz. Two different designs are presented and the results obtained by experimental investigation are compared to previous theoretical findings.

Reactance-locked loop for driving resonant sensors

Quartz tuning fork resonators are suitable sensors for the measurement of density and viscosity of a surrounding gas. For the continuous measurement with these sensors, a digital reactance-locked loop is proposed in this contribution. Furthermore, insight on the theoretical background of density and viscosity measurement with tuning fork resonators is given and the application of the system for simultaneous density and viscosity measurement in gases with a tuning fork resonator is presented.

Viscosity measurement cell utilizing electrodynamic-acoustic resonator sensors: Issues and improvements

Miniaturized viscosity sensors operating at frequencies in the KHz-range offer portability and results comparable to existing lab viscometers. In this paper we will briefly describe a viscosity measurement cell utilizing electrodynamic-acoustic resonator sensors and provide a modified design with interchangeable resonator cards, which allow for ease of maintenance of the device and provides higher measurement repeatability. Some basic experiments made with the new resonator cards are presented, which show the clear dependence of the resonance frequency and Q factor on viscosity. In addition, an approach is taken to optimize the magnetic circuit for the measurement cell using finite element simulations and experiments. A temperature dependence comparison is made between straight beam resonators and S shaped (Meander) beam resonators the result of which is the adoption of the S shaped beam design for further investigation. Finally the data processing approach using a curve fitting function designed for low quality factors is briefly described.

Smart analog compensation of spurious signals for a fully differential interface for resonating sensors

Resonating sensors are used for measurement tasks in a wide range of applications. Depending on sensor type and operating conditions, the signals of these sensors are often afflicted with unwanted components induced by parasitic sensor effects or the implemented sensor interface. Especially when using digitizing interface circuits it can be advantageous concerning the utilizable resolution of the system to compensate these parasitic signals before the analog to digital conversion. In this contribution, a method of analog compensation for parasitic signal components is presented, that is implemented in an largely digital impedance analyzer circuit for resonating sensors.

Ein präziser Impedanzanalysator für resonante SensorenA Precise Impedance Analyser for Resonating Sensors

Zusammenfassung Präzise Impedanzanalysatoren werden in jüngerer Vergangenheit vermehrt zur Auswertung resonanter Sensoren eingesetzt. In diesem Beitrag wird dazu ein Schaltungskonzept vorgestellt und der geplante Einsatz, ein Viskositätssensor basierend auf einem Quarz-Schwinger zur Überwachung eines Zeolith-Kristallisationsexperimentes auf der Internationalen Raumstation (ISS), präsentiert.