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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
261

Transport-reaction Modeling of the Impedance Response of a Fuel Cell

Coignet, Philippe 26 May 2004 (has links)
Electrochemical impedance spectroscopy (EIS) is a technique consisting of the application of a small perturbing current or voltage to an electrochemical system and measuring the response of the system. The response of the system can be described through the notion of impedance, Z, which is defined as the transfer function between the voltage and the current signal. By describing the impedance, one can gain insight into the interpretation of EIS experiments for the measurement of fundamental physical properties (eg diffusion coefficients). The impedance responses of electrochemical systems have been described in the past as an arrangement of ideal equivalent-circuit elements. Simple lumped-parameter circuits and more complex finite-transmission-line circuits have been used in the past, but the disadvantage of this approach is the difficulty in interpreting the equivalent-circuit parameters in terms of fundamental properties. It is then interesting to determine impedance by describing mathematically the fundamental physical processes that govern the response of the system. By describing and predicting analytically the impedance response induced by the perturbing current signal, one can: (i) gain considerable insight into the electrochemical process of interest, (ii) make explicit use of the modeling approach to address operational issues such as process design optimization, monitoring, diagnostics and control, and (iii) offer an interpretation to carefully designed EIS experiments for the measurement of fundamental physical properties such as diffusion coefficients or surface of active catalyst.
262

Estimation of Impedance About the

Krishna, Karthik 25 April 2005 (has links)
In performing manual tasks, muscles are voluntarily contracted in order to produce force and orient the limb in the desired direction. Many occupational tasks are associated with frequent musculoskeletal disorders. In tasks involving skilful manipulation, very frequently the forces are focused on the upper limb and neck. Upper extremity cumulative trauma disorders are among the more common worker related injuries. These muscle disorders may be related to repetitive exertions, excessive muscle loads and extreme postures. One of the major challenges is to quantify the muscle load and researchers have tried various measures to quantify muscle load. Joint mechanical impedance can be a robust method to quantify muscle load. Joint mechanical impedance characterizes the dynamic torque-angle relationship of the joint. Joint impedance has been measured by earlier researchers, for limited tasks, by imparting force (or angle) perturbations on the joint and relating resultant angular (or force) changes. The joint impedance gives a quantitative measure related to muscle co-contraction level. Measurement of the mechanical impedance at the workplace may provide useful information relevant to the understanding of upper limb disorders. Electromyogram (EMG) is the electrical activity of the muscle. Usually, an estimate of the EMG amplitude is obtained from the raw waveform recorded from the surface of the skin. EMG amplitude estimates can be used to non-invasively estimate torque about joints. Presently, there exists no means by which mechanical impedance can be estimated non-invasively (i.e., without external perturbations). Therefore, we proposed the use of EMG to noninvasively estimate the joint mechanical impedance. Our objective in this project was to determine the extent to which surface EMG can be used to estimate mechanical impedance. Simulation studies were first performed to understand the extent to which this tool could be useful and to determine methods to be used for the experiment. The simulations were followed by evaluating and estimating mechanical impedance using data collected from one experimental subject. Simulations helped to devise processing techniques for the measured signals and also to determine the length of data to be collected. Low pass filters for derivatives (used in the development of impedance estimates) were designed. Subtracting out a polynomial was the best approach to attenuate a low frequency drift (artifact) that occurs in torque measurements. Thirty seconds of data provided impedance estimates with a relative error of 5% when EMG amplitude estimates with SNR of 15 were used. Experimental data from constant-posture, slowly force-varying background torque level showed that the elbow joint system behaved like a second order linear system between 2 Hz and 10 Hz. Co-contraction by subjects during experiments caused impedance estimates to be unexpectedly high even at low background torque. Further experiments would need to be conducted with the subjects being instructed to avoid co-contraction.
263

Electrical Characterization of a Textile Sensor for Moisture Detection

Swaminathan, Arun, Babar Khan, Muhammad January 2011 (has links)
Electrical impedance is the frequency domain ratio of the voltage to the current. Electrical impedance extends the concept of resistance to AC circuits, describing not only the relative amplitudes of the voltage and current, but also the relative phases. Many new generation impedance measuring instruments measure the real and imaginary part of the impedance vector.Textile sensors are becoming an emerging field in industry. The possibilities that this technology holds seem almost limitless. Currently, textiles are being developed for many applications and markets, including biomedical sensing, wearable computing, large area sensors and large area actuating devices.A novel concept for a textile sensor for detection of moisture surroundings The sensor has been theoretically analysis. The results of the developed sensor shows less resistance drop characteristics against sweat.
264

Estudo das propriedades dielétricas e de bloqueio das monocamadas automontadas /

Benites, Tiago Azevedo. January 2013 (has links)
Orientador: Paulo Roberto Bueno / Coorientador: Márcio de Sousa Góes / Banca: Antonio Pupim / Banca: Ernesto Chavez / Resumo: O estudo das propriedades dielétricas das monocamadas automontadas de alcanotióis sobre o ouro (SAMs) é importante no campo da biotecnologia, já que essas camadas proporcionam uma estrutura fundamental na construção dos eletrodos quimicamente modificados, com sua principal aplicação na confecção dos biossensores. A análise das propriedades de bloqueio pelas SAMs é frequentemente feita por meio de técnicas eletroquímicas e de resposta em frequência, pois são extremamente eficazes no entendimento das características físico-químicas interfaciais de modificação desses filmes, além de apresentarem praticidade e velocidade de realização. Uma forma bastante apropriada de analisar a capacidade dielétrica das SAMs é quantificando a velocidade de transferência eletrônica frente a uma espécie eletroativa. Nesse trabalho foram caracterizadas monocamadas de diferentes espessuras em um meio com espécie eletroativa em concentrações variadas, a partir do cálculo da constante de transferência heterogênea de elétrons, feitas pelas metodologias propostas por diferentes autores e por uma metodologia inédita que será detalhada no corpo do trabalho / Abstract: The study of the dielectric properties of self-assembled monolayers of alkanethiols on gold (SAMs) is of paramount importance to biotechnology, as these layers provide a fundamental structure in the construction of chemically modified electrodes, with its main application in fabrication of biosensors. The analysis of the blocking properties of SAMs is often done by electroanalytical techniques and frequency response, as they are extremely effective in understanding the physico-chemical interfacial modification of these films, and offer convenience and speed of execution. A very appropriate manner of evaluating the blocking ability of SAMs is quantifying dielectric constant and the heterogeneous electron transfer rate across from one electroactive species in solution. In this study monolayers of different thicknesses were characterized in a medium with electroactive species in different concentrations to calculate of the heterogeneous electron transfer constant by classical methods which were compared with a new methodology detailed in the present work / Mestre
265

Avaliação experimental de um sistema não linear para extração de energia através de vibrações utilizando material piezelétrico /

Carvalho, Felipe Carmo. January 2014 (has links)
Orientador: Vicente Lopes Junior / Co-orientador: Samuel da Silva / Banca: Joao Antonio Pereira / Banca: Milton Dias Junior / Resumo: Afim de suprir a necessidade por fontes alternativas de energia para a alimentação de sistemas microeletrônicos autônomos, os estudos têm se voltado para a utilização de materiais piezelétricos para a transformação de energia derivadas de vibrações mecânicas em energia elétrica. Como a potência fornecida pelas vigas com materiais piezelétricos é pequena e, geralmente, insuficientes para alimentar dispositivos eletrônico de forma autônoma, técnicas de extração, conversão e armazenamento de energia são utilizadas para otimizar a energia final acumulada. Neste sentido, este trabalho tem por objetivo a otimização de um sistema extrator de energia através de vibrações utilizando material piezelétrico, maximizando a potência extraída e melhorando o rendimento do sistema. Para isto, foram analisadas condições de linearidade e não linearidade no sistema e os respectivos circuitos conversores foram projetados e testados experimentalmente. A melhor configuração eletromecânica para extração de energia, o sistema será utilizada para alimentar um sistema autônomo. Foram projetados dois sistemas de monitoramentos com consumo de energia diferentes, para avaliar as limitações e potencialidades da proposta de sistema para extração de fontes alternativas de energia elétrica / Abstract: In order to meet the need for alternative energy sources to power autonomous microelectronic systems, studies have focused on the use of piezoelectric materials to transform energy derived from mechanical vibrations into electrical energy. Once the power delivered by the cantilever beams with piezoelectric materials are insufficient to power electronic devices autonomously, extraction techniques, conversion and energy storage was used to optimize the final accumulated energy. Considering that, this work aims at the optimization of an extractor system energy through vibrations using piezoelectric material, maximizing the power extracted and improving system performance. To this end, conditions of linearity and non-linearity in the mechanical system, as well as designed and tested converter circuits were analyzed. The best setup for electromechanical energy extraction, was used to feed an autonomous system. Two different power consumption-monitoring systems were designed to assess the limitation and potentialities of the proposed alternative for energy harvesting / Mestre
266

Estudo comparativo de técnicas de medição e aquisição de sinais de transdutores piezelétricos para detecção de dano baseada na impedância eletromecânica /

Budoya, Danilo Ecidir. January 2018 (has links)
Orientador: Fabricio Guimarães Baptista / Banca: Fernando de Souza Campos / Banca: Mário Anderson de Oliveira / Resumo: Sistemas de monitoramento de integridade estrutural (SHM - Structural Health Monitoring) são científica e economicamente relevantes como métodos de detecção de danos estruturais em diversos tipos de estruturas, aumentando assim a segurança e reduzindo os custos de manutenção. Entre os vários princípios de detecção de danos, o método da impedância eletromecânica (E/M) baseia-se na medição da impedância elétrica do transdutor piezelétrico fixado à estrutura monitorada. Aqui, a exatidão e precisão do sistema de medição são fundamentais para o diagnóstico correto da estrutura. Portanto, essa dissertação apresenta uma análise comparativa de duas técnicas de medição de impedância para detecção de danos que são tipicamente utilizadas em analisadores de impedância comerciais e em outros sistemas de medição alternativos: medições em estado transitório utilizando um sinal de excitação de varredura e medições em estado estacionário utilizando um sinal senoidal puro para cada frequência de excitação. Os testes foram realizados com cargas resistivas e capacitivas de valores nominais 100 Ω e 10 nF, respectivamente, e com um transdutor piezelétrico fixado em uma barra de alumínio que representa uma estrutura monitorada. As duas técnicas foram comparadas com base na exatidão, precisão, sensibilidade à danos e tempo necessário para as medições. Os resultados destacam as características importantes de cada técnica, as quais devem ser consideradas para o desenvolvimento de sistemas de SHM basea... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: Structural health monitoring (SHM) systems are scientifically and economically relevant as methods of detecting structural damage to various types of structures, thus increasing safety and reducing maintenance costs. Among the various principles of damage detection, the electromechanical impedance (EMI) method is based on the electrical impedance measurement of piezoelectric transducers attached to the monitored structure. Here, the accuracy and precision of the measurement system are fundamental for the correct diagnosis of the structure. Therefore, this dissertation presents a comparative analysis of two impedance measurement techniques for damage detection that are typically used in commercial impedance analyzers and other alternative measurement systems: transient-state measurements using a sweep excitation signal and steady-state measurements using a pure sinusoidal signal for each excitation frequency. Tests were performed with resistive and capacitive loads with nominal values of 100 Ω e 10 nF, respectively, and a piezoelectric transducer fixed to an aluminum bar representing a monitored structure. The two techniques were compared based on the accuracy, precision, sensibility to damage and time required for the measurements. The results highlight the important features of each technique, which should be considered for the development of impedance-based SHM systems and the correct diagnosis of monitored structures. / Mestre
267

Identification of electrolytic cell parameters using a self-tuning predictor.

Cohen, Frederick L January 1978 (has links)
Thesis. 1978. M.S.--Massachusetts Institute of Technology. Dept. of Electrical Engineering and Computer Science. / Includes bibliographical references. / M.S.
268

Control of penetration in gas-tungsten-arc welding : a puddle impedance approach

Zacksenhouse, Miriam January 1982 (has links)
Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 1982. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND ENGINEERING / Includes bibliographical references. / by Miriam Zacksenhouse. / M.S.
269

Acoustic-reflex effects on the input admittance and transfer characteristics of the human middle-ear.

Rabinowitz, William Mark January 1977 (has links)
Includes bibliographies. / Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 1977. / Microfiche copy available in the Institute Archives and Barker Engineering Library. / by William Mark Rabinowitz. / Ph.D.
270

Peripheral mechanical loading and the mechanism of abnormal intention tremor

Adelstein, Bernard D January 1981 (has links)
Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 1981. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND ENGINEERING. / Includes bibliographical references. / by Bernard Dov Adelstein. / M.S.

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