Thesis (Ph. D.)--University of Texas at Austin, 2001. / Vita. Includes bibliographical references. Available also from UMI/Dissertation Abstracts International.

Applications of laser absorption spectroscopy for trace gas detection are many and diverse, ranging from the environmental and atmospheric to the medical and industrial. The aim of creating a spectrometer which combines high sensitivities and selectivities (in order to measure small amounts of absorbers or species that are only weakly absorbing, in a complex background matrix) with a wide spectral coverage (to allow broadband absorbers or multi-component samples to be studied) can be realised by implementing three separate concepts: the exploitation of the strong, fundamental transitions of the mid-infrared; the use of sensitive spectroscopic techniques; and the selection of a widely tunable laser source. In this thesis, these ideas are investigated individually and in combination in order to achieve such a goal. Laser spectroscopic techniques based on optical cavities are used to build a high resolution spectrometer covering a large spectral range capable of selectively detecting low levels of gaseous compounds of interest, especially those of medical or environmental significance. Work in both the near- and mid-infrared is presented, including much of the initial, developmental work which was conducted in the former region. The thesis begins with an overview of both narrowband and broadband near-infrared radiation sources, with a particular emphasis on commonly available diode lasers (DLs). A novel laser source, the digital supermode distributed Bragg reector (DS-DBR) laser, is introduced as a useful laser source for spectroscopy, combining the usual benefits of telecom DLs with a wide tunability (1563 – 1613 nm). The laser can be operated in an internal or external ramping mode, allowing the output wavelength to be scanned or stepped across a desired region. The observation of mode-hopping during the application of the scanning methodology is examined and rationalised. The ability of the DS-DBR laser to perform high resolution spectroscopy over its entire spectral coverage is demonstrated by recording spectra of carbon dioxide (CO₂) over this range, covering transitions from two of the four Fermi resonance components of the 3ν₁ + ν₃ combination band. The results of conducting wavelength modulation spectroscopy on CO₂ are also reported. A system developed for performing cavity ring-down spectroscopy (CRDS), capable of the real-time retrieval of ring-down times (RDTs), is presented and discussed. The outcomes of initial tests performed with a conventional DL at 1557 nm, to study a calibrated mixture of CO₂ in air at various pressures, are given. In addition, the results of combining this system with the DS-DBR laser are discussed. The bandwidth of the DS-DBR laser was found to be larger than that of a standard DFB DL, resulting in the presence of noisy cavity modes. Despite this, the acquisition of reproducible RDTs is demonstrated, with single wavelength studies of an evacuated cavity at 1605.5 nm yielding a RDT of 24.54 ± 0.04 µs and Allan variance calculations signalling an attainable minimum detectable absorption coefficient, α_min, of 2.8 x 10^-10 cm^-1 over 20 s. The ability to perform CRDS across the whole DSDBR laser wavelength range without the need for cavity re-alignment is illustrated, and studies conducted on CO₂ in air, calibrated mixtures and breath are reported. Investigations are also described into the accurate determination of the ¹³C/¹²C ratio in exhaled CO₂ undertaken using CRDS and cavity enhanced absorption spectroscopy (CEAS) on CO₂ isotopologues, an approach which can be utilised as a diagnostic aid in determining Helicobacter pylori infection. The focus of the thesis then moves to the mid-infrared, to describe quasi phase matching difference frequency generation (QPM-DFG) and its use to generate laser light at 3 µm by optically mixing near-infrared DLs. The theory behind this non-linear optical interaction is outlined, and the construction of a free-space QPM-DFG system using periodically poled lithium niobate is detailed and characterised. This DL-based QPM-DFG arrangement has been coupled with the CRDS system developed to create a mid-infrared CRD spectrometer. The results of single wavelength studies indicate RDTs of ~ 6 µs and an achievable αmin of 2.9 x 10^-9 cm^-1 over 44 s for an evacuated cavity. Spectroscopic investigations carried out on methane (CH₄), acetone and deuterium are documented; for the latter species, Dicke narrowing of the electric quadrupole ν(1←0) Q(2) transition at 2987.29 cm^-1 is observed and the integrated absorption cross-section for the same transition measured as 2.29 ± 0.03 x 10^-27 cm²cm^-1molec^-1. The results of modifications made to the system, namely the use of a more powerful Nd:YAG laser as the pump radiation source, as well as a faster detector combined with a variable amplifier, are presented; these include the observation of an improved optimal α_min of 6.4 x 10^-10 cm^-1 over 151 s for an empty cavity. Finally, work utilising the DS-DBR laser as one of the near-infrared sources for the QPM-DFG set-up is presented. This configuration generates radiation covering a wide mid-infrared range (3130 – 3330 nm) and has been used to perform direct absorption and wavelength modulation spectroscopy on ro-vibrational transitions within the fundamental ν₃ (F₂) band of CH₄. The spectrum of methanethiol (CH₃SH) over this region has also been investigated, with preliminary studies identifying a feature at 3040 cm^-1 as a potential indicator for monitoring this biomarker in breath. The results of coupling this mid-infrared radiation with an optical cavity to perform CEAS combined with phase sensitive detection are subsequently reported. Studies were conducted on calibrated CH₄ mixtures and ambient air to examine two transitions of the fundamental ν₃ (F₂) band of CH₄ in order to characterise the system: effective path lengths of ~ 700 m and α_min of 6.2 x 10^-8 cm^-1 over 8 s were found. The ^RQ₄ CH₃SH absorption feature at 3040 cm^-1 was also further studied with this system using prepared samples of CH₃SH in N₂ at different concentrations, yielding a CH₃SH detection limit of 2.4 ppm at 19 Torr. The potential of such a cavity-based, DS-DBR sourced, QPM-DFG mid-infrared spectrometer for trace gas sensing having thus been demonstrated, possible improvements that could be implemented to increase the sensitivity of the system are then discussed.

This thesis addresses the problem of target positioning and localization using Radio Frequency (RF) based measurements and using a variety of modulation including Time of Arrival (ToA), Phase of arrival (PoA) and Received Strength Indicator of RF signals (RSSI). Starting from finding the planar coordinates of a device from a collection of ranging measurements using weighted least square (WLS) methods, we explore the dependency of the solution uncertainty from the geometric configuration of anchors and then develop solutions that compensate for the effects of geometry and reduce the positioning uncertainty to a value close to the Cramer–Rao Lower Bound (CRLB), a measure which is then used in the proceeding chapters for developing optimal anchor configurations for positioning problem with guaranteed estimation uncertainties. The findings in the positioning part are also used to address the limitations of initializing Ultra-Wideband (UWB) anchors through a random trajectory. This is done by studying the dual of the positioning problem addressed in the first part, that is incorporating CRLB as a measure of optimality to design a trajectory that minimizes the uncertainty of anchor initialization. We finally close the positioning part of the thesis by studying the range and bearing measurements provided by radar sensors for people tracking and positioning in indoor environments. Taking into account the target dynamics, in the second part of the thesis we present observabilty analysis and localization for non-holonomic robots, using a combination of onboard sensors and range-only anchors. By using a discrete-time formulation of the system’s kinematics, we identify the geometric conditions that make the system globally observable and thereby derive the observability-based filter (ObF) to outperform the limitations of the classic Bayesian filters. We then use the implications of this analysis to design an active control and optimal path-planning strategy with guaranteed maximum observability. We close this part of the thesis by investigating localization in presence of intermittent measurements and discuss how the observability of a trajectory can be quantified by the condition number of the system matrix, a subject related to the maneuvers executed by the robot and to the sampling time used to collect the measurements. Eventually, in the last part of this thesis, we address the localization in presence of offset and ambiguities in measurements. First, we show that, while using range-only measurements corrupted with offset, the trajectories can be observed and the offset can be estimated in a finite number of steps. Next, we present an approach to resolve the ambiguity of rang-only measurements obtained from RSSI values at the Ultra-High Frequency (UHF) band by proposing an optimization algorithm that merges RFID and odometry data to reconstruct the entire robot trajectory. Finally, we present a solution to resolve the ambiguity of the RFID signal phase and reconstruct the robot trajectory through sensor fusion and using UHF-RFID passive tags.

This research study presents a new vibration-based non-destructive global structural damage identification and condition monitoring technique that can be used for detection, localization and quantification of damage. A two-stage damage identification process that combines non-model based and model-based damage identification approaches is proposed to overcome the main difficulties associated with the solution of structural damage identification problems. In the first stage, performance assessment of various response parameters obtained from the time-domain, frequency-domain and spectral-domain analysis is conducted using a non model-based damage detection and localization approach. In addition, vibration response parameters that are sensitive to local and global damage and that possess strong physical relationships with key structural dynamic properties are identified. Moreover, in order to overcome the difficulties associated with damage identification in the presence of structural nonlinearity and response nonstationarity, a wavelet transform based damage-sensitive parameter is presented for detection and localization of damage in the space domain. The level of sensitivity and effectiveness of these parameters for detection and localization of damage are demonstrated using various numerical experimental data determined from one-dimensional and two-dimensional plate-like structures.

[pt] A resposta dinâmica de sistemas estruturais não lineares tem sido um item de grande interesse nas pesquisas em engenharia civil. Problemas onde há interação base flexível-estrutura são de grande importância na análise estrutural, já que a maioria das estruturas civis é apoiada sobre sistemas flexíveis (solo ou sistemas de apoio com dissipação de energia). Nesta área, o estudo de sistemas submetidos a ações sísmicas é um tópico relevante, já que estas solicitações têm um grande conteúdo de frequências, o que pode influenciar consideravelmente as respostas da estrutura. Neste contexto, o conhecimento da resposta em frequência de estruturas não lineares sob uma excitação de base é uma ferramenta útil para avaliar os potenciais efeitos de ações sísmicas sobre estes sistemas. Na presente tese é desenvolvida uma metodologia de análise não linear dinâmica de sistemas estruturais reticulados sob excitações de base, considerando não linearidade geométrica e apoios flexíveis, representados por molas unidimensionais, com comportamento elasto-plástico. Através de uma análise paramétrica é avaliada a variabilidade das respostas de sistemas esbeltos submetidos a ações sísmicas reais, sismos artificiais, assim como ações sísmicas sucessivas. O problema no espaço é resolvido pelo método dos elementos finitos. Para a análise em frequência, é apresentada uma metodologia baseada no método do balanço harmônico e no método de Galerkin, juntamente com técnicas de continuação para a obtenção das curvas de ressonância não lineares. O problema no tempo é abordado através da integração das equações de movimento pelos métodos de Runge-Kutta e Newmark, associado ao método de Newton-Raphson. / [en] The dynamic response of nonlinear structures has been a topic of interest in civil engineering research. Problems in which base-structure interaction is present have a great importance in structural analysis, since most structures rests on flexibel systems (soil or supports with dissipation). In this research area, the study of structures under the action of seismic loads represent a relevant topic, since this kind of excitations may excite several vibration modes and thus influence strongly the dynamic response. In this context, the prediction of the nonlinear structural behavior in frequency domain of structures under base excitation is a useful resource to assess the potential effects of sismic loads on these systems. In this thesis, a methodology for nonlinear dynamic analysis of plane frame structures under base excitation is presented considering geometric nonlinearity and elastic supports represented by elasto-plastic unidimensional springs. Trough a parametric analysis, the variability of the dynamic responses of slender structural systems under the actions of real earthquakes, synthetics earthquakes, as well as the action of multiple earthquakes is assessed. The structural systems here analyzed are discretized in space using a nonlinear finite element formulation. For the response in frequency domain, a scheme based on the Balance Harmonic Method and the Galerkin method, in conjunction with continuation methods, is formulated to obtain the nonlinear resonance curves. The nonlinear dynamic response in the time domain is calculated by direct integration of the equations of motion. For this, the Runge-Kutta method and the Newmark method in association with the iterative Newton-Raphson scheme are employed.

This thesis examines three decomposition techniques and their usability for economic and financial time series. The stock index DAX30 and the exchange rate from British pound to US dollar are used as representative economic time series. Additionally, autoregressive and conditional heteroscedastic simulations are analysed as benchmark processes to the real data. Discrete wavelet transform (DWT) uses wavelike functions to adapt the behaviour of time series on different time scales. The second method is the singular spectral analysis (SSA), which is applied to extract influential reconstructed modes. As a third algorithm, empirical mode decomposition (END) leads to intrinsic mode functions, who reflect the short and long term fluctuations of the time series. Some problems arise in the decomposition process, such as bleeding at the DWT method or mode mixing of multiple EMD mode functions. Conclusions to evaluate the predictability of the time series are drawn based on entropy - and recurrence - analysis. The cyclic behaviour of the decompositions is examined via the coefficient of variation, based on the instantaneous frequency. The results show rising predictability, especially on higher decomposition levels. The instantaneous frequency measure leads to low values for regular oscillatory cycles, irregular behaviour results in a high variation coefficient. The singular spectral analysis show frequency - stable cycles in the reconstructed modes, but represents the influences of the original time series worse than the other two methods, which show on the contrary very little frequency - stability in the extracted details.:1. Einleitung 2. Datengrundlage 2.1. Auswahl und Besonderheiten ökonomischer Zeitreihen 2.2. Simulationsstudie mittels AR-Prozessen 2.3. Simulationsstudie mittels GARCH-Prozessen 3. Zerlegung mittels modernen Techniken der Zeitreihenanalyse 3.1. Diskrete Wavelet Transformation 3.2. Singulärsystemanalyse 3.3. Empirische Modenzerlegung 4. Bewertung der Vorhersagbarkeit 4.1. Entropien als Maß der Kurzzeit-Vorhersagbarkeit 4.2. Rekurrenzanalyse 4.3. Frequenzstabilität der Zerlegung 5. Durchführung und Interpretation der Ergebnisse 5.1. Visuelle Interpretation der Zerlegungen 5.2. Beurteilung mittels Charakteristika 6. Fazit