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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.
1

A study on the optimization of design of high frequency immersion probes

Hosseini, S. M. R. January 1989 (has links)
No description available.
2

Perfectly Matched Layer (PML) for Finite Difference Time Domain (FDTD) Computations in Piezoelectric Crystals

Chagla, Farid 08 1900 (has links)
The Finite-Difference Time-Domain (FDTD) method has become a very powerful tool for the analysis of propagating electromagnetic waves. It involves the discretization of Maxwell's equations in both time and space that leads to a numerical solution of the wave propagation problem in the time domain. The technique's main benefits are that it permits the description of wave propagation in non-uniform media, it can easily accommodate a wide range of boundary conditions, and it can be used to model nonlinear effects as well as the wave behaviour near localized structures or material defects. In this study, we extend this technique to mechanical wave propagation in piezoelectric crystals. It is observed to give large reflection artefacts generated by the computational boundaries which interfere with the desired wave propagation. To solve this problem, the renowned absorbing boundary condition called perfectly matched layer (PML) is used. PML was first introduced in 1994 for electromagnetic wave propagation. Our research has further developed this idea for acoustic wave propagation in piezoelectric crystals. The need to improve the large reflection artefacts by introducing a finite thickness PML has reduced acoustic wave reflection occurring due to practical errors to less than 0.5 %. However, it is found that PML can generate numerical instabilities in the calculation of acoustic fields in piezoelectric crystals. Theses observations are also discussed in this report. / Thesis / Master of Applied Science (MASc)
3

Systèmes de récupération d'énergie vibratoire large bande / Wideband mechanical energy harvester system

Ahmed-Seddik, Bouhadjar 04 October 2012 (has links)
Dans ce travail de thèse nous nous sommes intéressés principalement à la récupération de l'énergie mécanique et plus particulièrement l'énergie vibratoire. Cette technologie repose sur l'utilisation des transducteurs résonants, ces dispositifs permettent d'amplifier l'amplitude de vibration et donc de stocker d'avantage d'énergie mécanique dans le convertisseur à la résonance. La quantité de l'énergie en sortie du convertisseur chute lorsque la fréquence de vibration n'est plus égale à la fréquence de résonance, il est donc nécessaire d'assurer un asservissement de la fréquence de résonance de la structure de récupération d'énergie vibratoire sur la fréquence de vibration, si possible sur tout le spectre fréquentiel que couvre la source de vibration. L'objectif de la thèse est de proposer des solutions, à basse consommation, permettant d'assurer un ajustement dynamique en temps réel de la fréquence de résonance en fonction de la fréquence de vibration. Les travaux de cette thèse s'articulent autour de trois solutions : 1) Ajustement de la fréquence de résonance par application d'un champ électrique dans un matériau piézoélectrique 2) Ajustement de la fréquence de résonance par adaptation de la charge électrique d'un matériau piézoélectrique 3) Amplification du mouvement vibratoire par technique de rebond Une modélisation et optimisation à la fois de la plage de fréquence de fonctionnement et de la conversion mécano-électrique ont été réalisées. Trois structures ont été développées et testées et permettent de valider chacune des trois approches. Enfin, une électronique très basse consommation a été mise au point pour asservir en temps réel la fréquence de résonance sur la fréquence de la source de vibration et optimiser le taux d'énergie électrique extraite du système (pour maintenir un facteur de qualité de la structure optimum). / The work of this thesis is focused on the mechanical energy harvesting. This technology is generally based on the use of resonant transducers. Such systems work efficiently when their resonant frequency is equal to the vibration one. Otherwise, the output power from the harvester drops dramatically. Hence, it's necessary to ensure a continuous control of the resonant frequency of the harvester in order to avoid a possible shift between the resonant frequency and the vibration one, and doing this over the frequency spectrum covered by the vibration source. The main goal of this thesis is to develop new efficient solutions able to control in real time and tune the resonant frequency, these solutions should be low power consumption. During this thesis, three solutions have been developed: 1) adjustement of the resonant frequency by applying an electric field on the piezoelectric material; 2) adjustement of the resonant by adapting the electrical load; 3) the amplification of the structure relative displacement using a rebound technique. Modelling and optimization of both the frequency adjustment techniques and the mechanical-to-electrical conversion were performed. Three structures have been developed, tested and used to validate the three approaches. Finally, a very low power consumption electronic has been developed for a real time control of the resonant frequency, by regarding the vibration frequency, and also to optimize the extracted electrical energy from the harvester by maintaining an optimum quality factor.

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