Les matériaux à gradient d'indice de réfraction (GRIN) présentent des propriétés mécaniques variant en temps ou/et en espace. Ils ont été testés pour des applications prometteuses dans de nombreuses applications d'ingénierie, comme pour le contrôle santé structurale ou la surveillance de structure (SHM), le contrôle des vibrations et bruit, la récupération d'énergie, etc. D'un autre côté, les matériaux piézoélectriques offrent la possibilité de réaliser des cellules composites dont les propriétés mécaniques peuvent être contrôlées en ligne. Motivé par ces deux approches, cette thèse étudie la mise en œuvre de structures GRIN adaptatifs pour le contrôle des ondes élastiques. Deux types de structures GRIN adaptatifs sont étudiés dans ce travail. Le premier exemple concerne la mise en œuvre d'une lentille piézoélectrique dans une plaque. Il est composé de patchs piézoélectriques shuntés, collés périodiquement en surface du guide d'ondes. Les circuits de shunt utilisés permettent d'émuler une capacité négative (NC). En accordant les valeurs de NC on peut ajuster l'indices de réfraction du milieu à l'intérieur de la lentille piézoélectrique et pour satisfaire une fonction sécante hyperbolique. Les résultats numériques montrent que les lentilles piézoélectriques peuvent alors focaliser les ondes de flexion de la plaque sur les points focaux. La lentille piézoélectrique est efficace dans une grande bande de fréquences et efficace dans une grande plage de fonctionnement. Ainsi elle peut focaliser des ondes sur différent points par simple ajustement des valeurs de NC réalisés par le circuit. Cette focalisation adaptative la rend très intéressante pour de nombreuses applications comme la récupération d'énergie ou le SHM. La mise en œuvre de ces techniques pour la récupération d'énergie est discutée dans cette thèse. Le second exemple concerne l'étude d'une structure dont les propriétés mécaniques sont contrôlées en temps et en espace. En particulier, une modulation périodique permet de créer une onde artificielle se propageant dans la structure. L'interaction avec des ondes mécaniques entraîne une rupture de réciprocité visible dans un diagramme de bande non symétrique. De nombreux phénomènes inhabituels sont observés dans ce type de structures variables : fractionnement des fréquences, conversion d'ondes et transmission unidirectionnelles. Deux types de conversion fréquentielle sont démontrés et expliqués. Le premier est induit par la transmission d'énergie entre les différents modes Bloch et le second type est dû à la diffusion de Bragg dans les structures modulées. La transmission unidirectionnelle des ondes pourrait être exploitée pour réaliser des diodes dans des systèmes infinis ou semi-infinis. Cependant, la transmission unidirectionnelle n'existe pas dans les systèmes finis en raison des phénomènes de conversion de fréquence. / GRadient INdex (GRIN) media are those whose properties smoothly vary in space or/and time. They have shown promising effects in many engineering applications, such as Structural Health Monitoring (SHM), vibration and noise control, energy harvesting, etc. On the other hand, piezoelectric materials provide the possibility to build unit cells, whose mechanical properties can be controlled on-line. Motivated by these two facts, adaptive GRIN structures, which can be realized using shunted piezoelectric materials, are explored in this dissertation to control guided elastic waves. Two types of adaptive GRIN structures are studied in this work. The first type is a piezo-lens. It is composed of shunted piezoelectric patches bonded on the surfaces of plates. To control the mechanical properties of the piezoelectric composite, the piezoelectric patches are shunted with Negative Capacitance (NC). By tuning the shunting NC values, refractive indexes inside the piezo-lens are designed to satisfy a hyperbolic secant function in space. Numerical results show that the piezo-lens can focus waves by smoothly bending them toward the designated focal point. The piezo-lens is effective in a large frequency band and is efficient in many different working conditions. Also the same piezo-lens can focus waves at different locations by tuning the shunting NC values. The focusing effect and tunable feature of piezo-lens make it useful in many applications like energy harvesting and SHM. The former application is fully discussed in this thesis. The focusing effect at the focal point results in a known point with high energy density, therefore harvesting at the focal point can yield more energy. Besides, the tunable ability makes the harvesting system adaptive to environment changes. The second type is the time-space modulated structure. Its properties are modulated periodically both in time and space. Particularly, the modulation works like a traveling wave in the structure. Due to the time-varying feature, time-space modulated structures break the reciprocity theorem, i.e., the wave propagation in them is nonreciprocal. Many unusual phenomena are observed during the interaction between waves and time-space modulated structures: frequency splitting, frequency conversion and one-way wave transmission. Two types of frequency conversion are demonstrated and explained. The first type is caused by energy transmission between different orders Bloch modes. The second type is due to the Bragg scattering effect inside the modulated structures. The one-way wave transmission could be exploited to realize one-way energy insulation in equivalent infinite or semi-inffnite systems. However, the one-way energy insulation fails in finite systems due to the frequency conversion phenomenon.

<p align="justify">Many different active control techniques can be used to control the vibrations of a mechanical structure: they however require at least a sensitive signal amplifier (for the sensor), a power amplifier (for the actuator) and an analog or digital filter (for the controller). The use of all these electronic devices may be impractical in many applications and has motivated the use of the so-called shunt circuits, in which an electrical circuit is directly connected to a transducer embedded in the structure. The transducer acts as an energy converter: it transforms mechanical (vibrational) energy into electrical energy, which is in turn dissipated in the shunt circuit. No separate sensor is required, and only one, generally simple electronic circuit is used. The stability of the shunted structure is guaranteed if the electric circuit is passive, i.e. if it is made of passive components such as resistors and inductors.</p><p><p><p align="justify">This thesis compares the performances of the electric shunt circuits with those of classical active control systems. It successively considers the use of piezoelectric transducers and that of electromagnetic (moving-coil) transducers.</p><p><p><p align="justify">In a first part, the different damping techniques are applied on a benchmark truss structure equipped with a piezoelectric stack transducer. A unified formulation is found and experimentally verified for an active control law, the Integral Force Feedback (IFF), and for various passive shunt circuits (resistive and resistive-inductive). The use of an active shunt, namely the negative capacitance, is also investigated in detail. Two different implementations are discussed: they are shown to have very different stability limits and performances.</p><p><p><p align="justify">In a second part, vibration isolation with electromagnetic (moving-coil) transducers is introduced. The effects of an inductive-resistive shunt circuit are studied in detail; an equivalent mechanical representation is found. The performances are compared with that of resonant shunts and with that of active isolation with IFF. Next, the construction of a six-axis isolator based on a Stewart Platform is presented: the key parameters and the main limitations of the system are highlighted.</p> / Doctorat en Sciences de l'ingénieur / info:eu-repo/semantics/nonPublished

Recently, the proposal to use voltage amplification from ferroelectric negative capacitance (NC) to reduce the power dissipation in nanoelectronic devices has attracted significant attention. Homogeneous Landau theory predicts, that by connecting a ferroelectric in series with a dielectric capacitor, a hysteresis-free NC state can be stabilized in the ferroelectric below a critical film thickness. However, there is a strong discrepancy between experimental results and the current theory. Here, we present a comprehensive revision of the theory of NC stabilization with respect to scaling of material and device dimensions based on multi-domain Ginzburg–Landau theory. It is shown that the use of a metal layer in between the ferroelectric and the dielectric will inherently destabilize NC due to domain formation. However, even without this metal layer, domain formation can reduce the critical ferroelectric thickness considerably, limiting not only the range of NC stabilization, but also the maximum amplification attainable. To overcome these obstacles, the downscaling of lateral device dimensions is proposed as a way to prevent domain formation and to enhance the voltage amplification due to NC. These insights will be crucial for future NC device design and scaling towards nanoscale dimensions.

Frequency reference oscillator is a critical component of modern radio transceivers. Currently, most reference oscillators are based on low-frequency quartz crystals that are inherently bulky and incompatible with standard micro-fabrication processes. Moreover, their frequency limitation (<200MHz) requires large up-conversion ratio in multigigahertz frequency synthesizers, which in turn, degrades the phase-noise. Recent advances in MEMS technology have made realization of high-frequency on-chip low phase-noise MEMS oscillators possible. Although significant research has been directed toward replacing quartz crystal oscillators with integrated micromechanical oscillators, their phase-noise performance is not well modeled. In addition, little attention has been paid to developing electronic frequency tuning techniques to compensate for temperature/process variation and improve the absolute frequency accuracy. The objective of this dissertation was to realize high-frequency temperature-compensated high-frequency (>100MHz) micromechanical oscillators and study their phase-noise performance. To this end, low-power low-noise CMOS transimpedance amplifiers (TIA) that employ novel gain and bandwidth enhancement techniques are interfaced with high frequency (>100MHz) micromechanical resonators. The oscillation frequency is varied by a tuning network that uses frequency tuning enhancement techniques to increase the tuning range with minimal effect on the phase-noise performance. Taking advantage of extended frequency tuning range, and on-chip temperature-compensation circuitry is embedded with the sustaining circuitry to electronically temperature-compensate the oscillator. Finally, detailed study of the phase-noise in micromechanical oscillators is performed and analytical phase-noise models are derived.

This master’s thesis is dealing with possibilities of measurement of inductance by using of accurate capacitance bridge AH 2500E. In metrology there are no appropriate devices available for accurate measurement of inductance. It can be realized by four methods. The newest one is based on T elements, it is not used for this purpose yet and it is practically investigated in CMI Brno. For its introduction to practice it is neccessary to complete computer simulations and analysis of influences affecting measurement. It is also inevitable to examine repeatability of measurement, define methodology of measurement and potential corrections of measured values.