A. Talić

Thermal property determination of laminar-flowing fluids utilizing the frequency response of a calorimetric flow sensor

We demonstrate that the thermal conductivity and diffusivity of fluids can be measured independently of their motion state utilizing a micromachined calorimetric sensor. The sensor membrane bears a heating resistor and two symmetrically arranged thermistors. By immersing the sensor into the laminar-flowing sample fluid and applying an AC heating current, the frequency response of the thermistor temperatures can be exploited to evaluate the thermal properties of the fluid. We developed a novel analytical model to describe the conductive transfer in the micromachined sensor as well as the fully conjugated heat transfer in the fluid. The validity of this model was confirmed experimentally by a practical example using nitrogen as fluid flowing through a rectangular flow channel. Based on these results, a thermal parameter extraction procedure can be deduced. Moreover, with known thermal parameters, the arrangement can also be used for flow measurements.

A novel thermal transduction method for sub-mW flow sensors

We present a novel approach to micromachined thermal flow sensors embedding four thin-film germanium thermistors in a silicon-nitride membrane. The appropriately arranged thermistors act as heat sources and as temperature sensors simultaneously. They were connected to form a Wheatstone bridge supplied with a constant electric current. Both, the bridge voltage and the voltage at the bridge supply terminals, are immediately available output signals offering high initial sensitivity or wide measurement range, respectively. The self-heating based flow transduction mechanism combine advantages of the hot-film and the electrocalorimetric flow sensors. With respect to comparable electrocalorimetric flow sensors, the power requirement is reduced by more than an order of magnitude. This feature is beneficial for remote sensing applications and crucial for measuring the flow of fluids that endure only slight temperature elevations.

A Novel calorimetric flow sensor implementation based on thermal sigma-delta modulation

Miniaturized calorimetric flow sensors suffer from a limited measurement range if operated with constant heating power. To overcome this drawback, we utilize a thermal sigma-delta modulator to achieve a constant average excess temperature of the sensor membrane. The duty cycle of the modulator output fully covers the flow information. This novel method exhibits the advantage that the slope of the output characteristic and hence the flow sensitivity can be optimized for a certain flow range by choosing an appropriate amplitude of the heating voltage. Finally, the influence of the clock frequency on the system accuracy was investigated by extensive SPICE modeling of the thermal system.

Novel flow sensors based on a two-state controller scheme

We present a novel control loop scheme for calorimetric flow sensors combining an electronic two-state controller with the electro-thermal transduction system. Such on-off control schemes can be optimized for a specified flow range and by that outperform classical analogue controllers. For a convenient controller design, we developed a comprehensive SPICE model of the thermal system able to fully cover its static and dynamic behavior. The thermal system was characterized experimentally and investigated by finite element analysis software (COMSOL) in order to extract relevant SPICE model parameters. Experiments confirm that this approach enables the implementation of optimized controllers for any calorimetric flow transducer and specific measurement tasks.

Comparison of the Dynamic Response of Calorimetric and Hot-Film Flow Transducers

The dynamic behavior of micromachined calorimetric flow sensors for gaseous fluids was investigated both, experimentally and by computer-numerical simulations. The study covers the influence of geometry parameters on sensitivity and response time as a function of the flow velocity. Using FEM analyzes, the flow dependence of the temperature of the thin-film heat source becomes accessible, which is representative for the response of hot-film transducers. At low flow velocities, calorimetric sensors respond notably faster than comparable hot film sensors to step-like changes of the fluid velocity. For large flow steps, where temperature-controlled operation is mandatory, the hot-film flow sensor promises a slightly faster response.

Novel PCB-based thermal flow sensors for air conditioning systems

This work addresses the development, characterization, and optimization of thermal flow sensors for heating, ventilation, and air conditioning (HVAC) systems. The sensors are based on printed circuit board (PCB) technology in order to obtain flexible, robust, and cost-effective devices. The layout comprises a copper heater meander located in the center of the PCB and two additional meanders placed on either side of the heater, acting as temperature sensors. Two different operating modes were investigated. In the first mode, a constant current applied to the heater leads to flow dependent voltage across the heater which depends on the fluid temperature and saturates at higher flow velocities. In the improved second mode, the difference between the mean temperature near the heater and the mean temperature at the sensors edge is kept constant by an electronic controller. This is a novel approach compared to the common case where the difference between heater and ambient temperature is evaluated. In the proposed quasi-constant temperature mode the heater voltage adjusted by the controller serves as an output signal. It is approximately independent of the ambient temperature and features a good sensitivity over the whole range of interest up to 10 m/s. The sensor development and optimization was supported by FEM simulations.

Miniaturisierte thermische Fluidsensoren – Strömungs- und thermische Leitfähigkeitssensoren

ZusammenfassungSensoren zur Bestimmung der Eigenschaften von Flüssigkeiten und Gasen werden in vielen Bereichen der Mess- und Automatisierungstechnik zur direkten und indirekten Zustandsüberwachung eingesetzt. Dieser Beitrag stellt unterschiedliche Varianten robuster Sensoren zur Bestimmung von Strömungseigenschaften (Geschwindigkeit, Richtung und Strömungsmenge) und zur Vermessung von thermischen Materialparametern (thermische Leitfähigkeit und thermische Diffusivität) von Fluiden vor. Diese Sensoren basieren auf einem thermischen Funktionsprinzip und sind mit Mitteln der Mikrotechnologie kostengünstig herstellbar. Ausgehend von einem einzigen Basisdesign können sie als kalorimetrische Strömungssensoren oder als thermische Leitfähigkeitssensoren ausgeführt und betrieben werden. Im vorliegenden Beitrag werden die theoretischen Grundlagen erläutert und einige Anwendungsbeispiele diskutiert.AbstractSensors for the determination of fluid and gas properties are being employed for direct and indirect condition monitoring in many application areas of instrumentation and automation. This article presents various robust sensors devised to measure flow properties (velocity, direction, and mass flow) as well as thermal properties (thermal conductivity and diffusivity) of fluids. These sensors are based on a thermal operation principle and can be manufactured with cost-effective microtechnology. Starting from one generic design, they can be implemented and operated as calorimetric flow sensors or thermal conductivity sensors. This article explains the theoretical foundations and discusses several application examples.

On-Device Extraction of Thermal Thin-film Properties in Calorimetric Flow Sensors

In diaphragm-based micromachined calorimetric flow sensors, the convective heat transfer through the test fluid competes with the spurious heat shunt induced by the thin-film diaphragm where the heater and the temperature sensors are embedded. Therefore, accurate knowledge of the thermal transport properties (thermal conductivity and diffusivity) and the emissivity of the diaphragm is mandatory for design, simulation, and optimization of such devices. However, these parameters can differ considerably from those stated for bulk material and they are typically dependent on the production process. Commonly used methods for their determination require the fabrication of custom specimens. In order to overcome this serious drawback, we developed a novel technique to extract the thermal thin-film properties directly from measurements carried out on calorimetric flow sensors. Here, the heat transfer frequency response from the heater to the thermistors is measured and compared to a theoretically obtained relationship arising from an extensive two-dimensional analytical model. This model covers the heat generation by the heater, the heat conduction in the diaphragm, the radiation loss at the diaphragms surface, and the heat sink caused by the supporting silicon frame. In this contribution, we report in detail on the measurement setup, the theoretical model for the associated parameter extraction, and the results obtained from measurements on calorimetric flow sensors featuring dielectric thin-film diaphragms made of PECVD Si3N4.

FEM-basierte Analyse eines neuen thermischen Strömungssensors mit drei unterschiedlichen Betriebsmoden

SummaryA novel design of a miniaturised calorimetric flow sensor is presented. The common sensor layout was modified by adding further membrane thermistors. The new configuration features three different operating modes. Their stationary and dynamic properties were analysed and compared to each other by means of FEM-based simulations.ZusammenfassungEin neuer Entwurf eines miniaturisierten kalorimetrischen Strömungssensors wird vorgestellt. Ausgehend von der Standardkonfiguration wurde durch Hinzufügen weiterer Membran-Thermistoren ein neues Sensorlayout entwickelt, das drei unterschiedliche Betriebsmoden erlaubt. Die stationären und dynamischen Eigenschaften dieser Betriebsmoden wurden mittels FEM(Finite-Elemente-Methode)-Simulationen analysiert und miteinander verglichen.