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978-3-8439-1563-2, Reihe Elektronik
Microcontroller-compatible Stabilization and Signal Processing of TDLS Gas Sensor Systems for ISM Applications
152 Seiten, Dissertation Universität Erlangen-Nürnberg (2014), Softcover, A5
The focus of this thesis is about the development of signal processing methods for extending the application fields of compact, laserspectroscopic gas sensors. It includes techniques for the self-calibration and the self-monitoring of the sensor system as well as for an optimized evaluation under certain conditions.
Due to a manufactural variance of the diode laser characteristics, the calibration of the sensor system is required. For wavelength tuning, the laser chip is mounted on a Peltier element which exhibits a different control behavior depending on the setup of the laser. In order to ensure a stable operation of the sensor, the control parameters are determined by means of recording and evaluating the step response. A state of the art VCSEL (vertical-cavity surfaces-emitting laser) is able to reach a double-digit number of absorption lines, of which not all lines are suitable for gas measurement. Overlapping lines or lines which highly depend on the ambient conditions as well as lines of other gases must not be used spuriously for the measuremen. A new algorithm based on comparing spectral features, identifies the absorption lines and so the wavelength of the laser. After these calibrations steps, a permanent monitoring of the sensor system is of high importance in order to detect possible laser failures and to react on them early. Different failures are analyzed and techniques for detection are presented.
The second part of this thesis focuses on the development of optimized evaluation techniques for certain application fields. At first, a well-known problem of wavelength modulation spectroscopy is analyzed. The imprecise evaluation of higher absorbances is effectively solved by a mathematical extension and the results are presented by means of humidity measurement. This optimized evaluation enables also the handling of larger signals in order to increase the dynamical range of the sensor. The reduction of the absorption cell size and the consequential appearance of optical interferences is the focus of the last section in this part. A novel approach for reducing the influence of such interferences is developed. The results of this technique are presented by means of humidity measurement as well as the measurement of the carbon dioxide value in exhaled air (capnography).