Andreas Schütze

Identification of organic solvents by a virtual multisensor system with hierarchical classification

To improve the selectivity of semiconductor gas sensors, temperature modulation is often used. We present a study showing the potential of this technique with information gained comparable to multisensor systems. A dynamic operating mode coupled with a low-complexity evaluation strategy allows the identification of six organic solvent vapors over a wide concentration range (2-200 ppm) with a single sensor, for example, for leak detection systems. The system features low false alarm rates; in addition, interference by other gases, such as CO or NO/sub 2/, can be suppressed. For even higher identification power, switching on-line between different temperature cycles was studied, which provides better information for critical decisions.

Thermal Stabilization of NiTiCuV Shape Memory Alloys: Observations During Elastocaloric Training

The paper presents novel findings observed during the training process of superelastic, elastocalorically optimized Ni–Ti-based shape memory alloys (SMA). NiTiCuV alloys exhibit large latent heats and a small mechanical hysteresis, which may potentially lead to the development of efficient solid-state-based cooling processes. The paper starts with a brief introduction to the underlying principles of elastocaloric cooling, illustrating the effect by means of a typical thermodynamic cycle. It proceeds with the description of a custom-built testing platform that allows observation of temperature profiles and heat transfer between SMA and heat source/sink during high-loading-rate tensile tests. Similar to other SMA applications, a training process is necessary in order to guarantee stable performance. This well-known mechanical stabilization affects the stress–strain hysteresis and the cycle-dependent evolution of differential scanning calorimetry results. In addition, it can be shown here that the training is also accompanied by a cycle-dependent evolution of temperature profiles on the surface of an SMA ribbon. The applied training leads to local temperature peaks with intensity, number, and distribution of the temperature fronts showing a cycle dependency. The homogeneity of the elastocaloric effect has a significant influence on the efficiency of elastocaloric cooling process and is a key aspect of the specific material characterization.

Increasing the selectivity of Pt-gate SiC field effect gas sensors by dynamic temperature modulation

Based on a diode coupled silicon carbide field effect transistor (FET) with platinum as catalytic gate material, the influence of dynamic temperature modulation on the selectivity of gas analysis sensors FETs has been investigated. This operating mode, studied intensively for semiconductor gas sensors, has only recently been applied to FETs. A suitable temperature cycle for detection of typical exhaust gases (CO, NO, C3H6 , H2, NH3) was developed and combined with appropriate signal processing. The sensor data were evaluated using multivariate statistics, e.g., linear discriminant analysis. Measurements have proven that typical exhaust gases can be discriminated in backgrounds with 0, 10, and 20% oxygen. Furthermore, we are able to quantify the mentioned gases and to determine unknown concentrations based on training data. Very low levels of relative humidity below a few percent influence the sensor response considerably but for higher levels the cross interference of humidity is negligible. In addition, experiments regarding stability and reproducibility were performed.

Fire detection in coal mines based on semiconductor gas sensors

Purpose – Due to the environmental conditions, the detection and identification of hazardous situations in coal mines is a challenge. The purpose of this research is to focus on the underground fire detection, especially of smoldering fires, which are characterized by the outgassing of CO and C2H4.Design/methodology/approach – The study developed a system based on a single semiconductor gas sensor and sensors for relative humidity and temperature. With a high‐dynamic‐range hardware control and data acquisition platform a commercial semiconductor gas sensor is operated with an application‐optimized temperature cycle to improve stability and selectivity.Findings – A hierarchical evaluation strategy not only allows identification of smoldering fires signified by CO and C2H4 with a ratio of 100:1, but is also suitable for separating or quantifying typical interfering compounds such as CH4, CO, NOX or H2 thus helping to avoid costly false alarms. After promising laboratory pre‐tests, a system was built for fie...

Routes for GMR-Sensor Design in Non-Destructive Testing.

GMR sensors are widely used in many industrial segments such as information technology, automotive, automation and production, and safety applications. Each area requires an adaption of the sensor arrangement in terms of size adaption and alignment with respect to the field source involved. This paper deals with an analysis of geometric sensor parameters and the arrangement of GMR sensors providing a design roadmap for non-destructive testing (NDT) applications. For this purpose we use an analytical model simulating the magnetic flux leakage (MFL) distribution of surface breaking defects and investigate the flux leakage signal as a function of various sensor parameters. Our calculations show both the influence of sensor length and height and that when detecting the magnetic flux leakage of μm sized defects a gradiometer base line of 250 μm leads to a signal strength loss of less than 10% in comparison with a magnetometer response. To validate the simulation results we finally performed measurements with a GMR magnetometer sensor on a test plate with artificial μm-range cracks. The differences between simulation and measurement are below 6%. We report on the routes for a GMR gradiometer design as a basis for the fabrication of NDT-adapted sensor arrays. The results are also helpful for the use of GMR in other application when it comes to measure positions, lengths, angles or electrical currents.

Optimierung des temperaturzyklischen Betriebs von Halbleitergassensoren

Zusammenfassung Wir stellen ein Verfahren zur Optimierung des temperaturzyklischen Betriebs (TCO, temperature cycled operation) im Hinblick auf Sensorsignal, Sensitivität und Selektivität vor. Dieses basiert auf der Grundlage eines Sensormodells unter der Annahme, dass die Leitwertänderung durch die Besetzungsänderung mit ionosorbierten Sauerstoff auf dem Sensor hervorgerufen wird. Das Verfahren wurde mit einem SnO2-Sensor (AS-MLV, ams Sensor Solutions Germany GmbH, Reutlingen) auf einem Membransubstrat getestet, welches aufgrund der geringen thermischen Masse eine schnelle Temperaturänderung zulässt. Das Optimierungsverfahren kann sehr hohe Sensorsignale (GGas/GLuft − 1) erzielen, beispielsweise ein Sensorsignal von ca. 8000 bei 1 ppm ethanolhaltiger synthetischer Luft, die das isotherme Sensorsignal um den Faktor 800 übertrifft. Der Zusammenhang zwischen dem Sensorsignal und der Gaskonzentration kann für die meisten Zeitpunkte im TCO durch eine Potenzfunktion mit Exponenten von 0,5 bis 4 beschrieben werden. Es zeigt sich, dass direkt nach einem Temperaturabfall das Modell so stark vereinfacht werden kann, dass nur noch ein freier Modellparameter bleibt. Die Geschwindigkeit der Relaxation bei reduzierendem Gasangebot kann durch Ratenkonstanten beschrieben werden, die in guter Näherung proportional zur Gaskonzentration sind. Die Temperaturcharakteristik der Ratenkonstanten variiert für verschiedene Gase unabhängig von der Konzentration. Daher kann auch die Selektivität optimiert werden, wie am Beispiel von ethanol- und benzolhaltiger synthetischer Luft gezeigt wird.

A self-monitoring and self-diagnosis strategy for semiconductor gas sensor systems

We developed a self-diagnostic strategy for intelligent gas measurement systems based on a single semiconductor gas sensor especially for safety and security applications. Selectivity is improved via temperature cycling and/or electrical impedance spectroscopy (EIS). Comparing the redundant data obtained from both methods or from a separate sensor, i.e. a humidity sensor, allows detection of a system malfunction due to sensor damage, caused i.e. by sensor poisoning. If different results (gas type or alarm situation) are obtained a warning (pre-alarm) is given and a self-diagnosis routine is started to identify the cause, i.e. by using another T-cycle or a special signal evaluation. This self-diagnostic strategy improves the robustness of a sensor system and helps to avoid costly false alarms or downtimes. Finally, the acceptance of semiconductor gas sensor systems for safety and security applications will be enhanced with this strategy.

A hierarchical strategy for under-ground early fire detection based on a T-cycled semiconductor gas sensor

Based on a single semiconductor gas sensor device operated in a T-cycled mode we developed a hierarchical identification strategy for the use in under-ground early fire detection systems, i.e. in coal mines. This strategy not only provides the identification of the alarm scenario to be detected (CO and ethene in a specific ratio) but is also suitable for identifying or quantifying typical interfering compounds (such as relative humidity, methane, CO, NOx or hydrogen) thus helping to avoid costly false alarms. Based on a high-dynamic-range hardware control and data acquisition platform, a commercial semiconductor gas sensor operated with an application-optimized temperature cycle and the evaluation strategy presented here our development provides classification reproducibility as well as robustness allowing the realization of a novel, cost-efficient and fast under-ground early fire detection system.

On the widths of the hysteresis of mechanically and thermally induced martensitic transformations in Ni–Ti-based shape memory alloys

Abstract It is well known that a good crystallographic compatibility between austenite and martensite in Ni–Ti-based shape memory alloys results in narrow thermal hystereses (e.g. Ball and James, Arch. Ration. Mech. Anal., 1987). The present work suggests that a good crystallographic fit is moreover associated with a small mechanical hysteresis width, observed during a forward and reverse stress-induced transformation. Furthermore, shape memory alloys with a good crystallographic fit show smaller transformation strains. The results obtained in the present study suggest that these correlations are generic and apply to binary Ni–Ti (with varying Ni contents) and quaternary Ni–Ti–Cu–X (X = Cr, Fe, V) alloys. For binary Ni–Ti, it was observed that Ni-rich compositions (good lattice fit) show a lower accummulation of irreversible strains during pseudoelastic cycling.

Versatile infrared gas measurement system with tunable microstructured Fabry-Pérot Filter

Based on the combination of non-dispersive infrared (NDIR) transmission and photoacoustic absorption spectroscopy (PAS) measurements, the detection of multiple gases and their mixtures was investigated using an integrated, microstructured Fabry-Pérot Filter (FPF). The proposed versatile system can be utilized as IR spectrometer for a wide range of gas concentrations using IR transmission spectroscopy (IRTS) measurements for high concentrations and PAS for low concentrations. The system is based on low cost components: a thermopile detector for the IRTS signal, condenser microphones for the PAS signal and a microstructured IR source. The source is modulated electrically with a frequency in the range of 1 to 10 Hz, i.e. using a nonresonant operating mode of the gas chamber. The investigated, commercially available FPF allows the analysis of gases with gas specific absorption bands between 3.9 and 5.0 μm. Our IRTS system, therefore, can be adapted for many application scenarios in which two or more gases have to be detected over a wide concentration range. One example is the isotope specific detection of carbon dioxide (CO2) for the urea breath test.