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Thesis (Ph. D.)--Virginia Commonwealth University, 2009. / Prepared for: Dept. of Mechanical Engineering. Title from title-page of electronic thesis. Bibliography: leaves 122-145.

Smart things, spaces, and structures are created by embedding computation into them. Embedded computers sense, compute, and communicate at the edge, closer to the physical rather than the cyber world. Not any computer can be embedded, because many deployment settings demand small size, long lifetime, and robustness to a harsh environment. .

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The aim of the work described in this thesis is firstly to improve the collection of vibration energy for piezoelectric cantilever harvesters, by a mechanical technique, so that the devices can harvest energy over a wider bandwidth. Secondly to investigate a new circuit topology for achieving complex conjugate load matching to the piezoelectric harvester. The thesis has been divided into two parts - the mechanical approach and the electrical approach. For the mechanical approach, a novel multiresonant beam, comprising piezoelectric fiber composites on a clamped-clamped beam and side mounted cantilevers, was proposed. The side cantilevers are tuned by tip masses to be resonant at different frequencies. A Rayleigh-Ritz model was developed to predict the vibration response of the proposed model multiresonant beam. This model showed that the bandwidth of the multiresonant beam was increased over that of a single cantilever harvester. A multiresonant beam for energy harvesting was experimentally tested and compared with a single cantilever energy harvester. The transmissibility and voltage responses were investigated, the beam showed a wide frequency response between 14.5Hz and 31Hz, whereas the single cantilever only showed one resonant frequency. Therefore the multiresonant beam system is feasible for wide band energy harvesting. For the electrical approach, the task was to investigate complex conjugate impedance matching for the piezoelectric energy harvesters, so that the output impedance from the piezoelectric harvester can be reduced, and maximum energy extracted from the device with a possibility of frequency tuning. A new amplified inductor circuit was proposed to enable the capacitive output impedance of the piezoelectric device to be cancelled. Experimental and software simulations are provided to verify the theoretical predictions. A prototype amplified inductor circuit was simulated and tested. The results showed that a variable effective inductance was achieved. However the circuit is lossy due to imperfections within the system, and needs further work to eliminate these imperfections.

Ambient energy harvesting has attracted significant attention over the last years for applications such as wireless sensors, implantable devices, health monitoring systems, and wearable devices. The methods of vibration-to-electric energy conversion can be included in the following categories: electromagnetic, electrostatic, and piezoelectric. Among various techniques of vibration-based energy harvesting, piezoelectric transduction method has received the most attention due to the large power density of the piezoelectric material and its simple architectures. In contrast to electromagnetic energy harvesting, the output voltage of a piezoelectric energy harvester is high, which can charge a storage component such as a battery. Compared to electrostatic energy harvester, the piezoelectric energy harvester does not require an external voltage supply. Also, piezoelectric harvesters can be manufactured in micro-scale, where they show better performance compared to other energy harvesters, owing to the well-established thick-film and thin-film fabrication techniques. The main drawback of the linear piezoelectric harvesters is that they only retrieve energy efficiently when they are excited at their resonance frequencies, which are usually high, while they are less efficient when the excitation frequency is distributed over a broad spectrum or is dominant at low frequencies. High-frequency vibrations can be found in machinery and vehicles could be used as the energy source but, most of the vibration energy harvesters are targeting at low-frequency vibration sources which are more achievable in the natural environment. One way to overcome this limitation is by using the frequency-up-conversion technology via impacts, where the source of the impacts can be one or two stoppers or more massive beams. The impact makes the piezoelectric beam oscillate in its resonance frequency and brings nonlinear behavior into the system.

Au cours de la dernière décennie, nous avons observé à une croissance rapide du nombre d’appareils de communication, et cette tendance devrait se poursuivre tant que les technologies essentielles telles que des objets connectes façonnent l’avenir de technologies d’information et de communication. Cette croissance a entraîné une augmentation considérable de la demande d’énergie, donc l’empreinte carbone de l’écosystème des ICT ne peut plus être ignorée. De plus, dans les systèmes de communication traditionnelle alimentés par batterie, où l’infrastructure énergétique n’est pas disponible après le déploiement, énergie limitée dans la batterie devient le goulot d’étranglement car elle détermine le lifetime de réseau. Alimenter appareil de communication avec des sources d’énergie ambiante, grâce à technologie de récupération l’énergie, non seulement réduit l’empreinte carbone du secteur de ICT mais aussi augmente l’autonomie des réseaux de communication que dépend de la batterie. Un appareil de récupération d’énergie peut piéger l’énergie de l’entourant environnement (sources typiques sont l’énergie solaire, le vent, les vibrations, thermique, etc.). Cependant, variabilité dans temps de l’énergie ambiant modifie la conception de stratégies de communication très différente des systèmes traditionnels. En dehors de la récupération d’énergie, un débit plus élevé peut être obtenu dans un système sans fil en concevant des systèmes de transmission basé sur des informations de canal de propagation. Comme les techniques d’adaptation de canal exigent d’avoir une certaine connaissance de l’état du canal sans fil envoyé au émetteur, l’augmentation du débit vient a un coût de l’estimation de l’information de canal qui consomment des ressources dans un système de communication, particulièrement, l’énergie. En outre, lorsque l’objectif dans un système de communication est à envoyer des informations sur la source à une destination avec au minimum distorsion erreur, des stratégies de transmission et de compression a être conçu sur la base à la fois sur la variable temps des conditions de canal et la statistiques de la source. Cette thèse porte sur la conception de stratégies de transmission prenant en compte le coût de l’obtention des informations d’état de canal (CSI) à l’émetteur, et les statistiques de sources variables dans le temps lorsque la communication dispositifs reposent sur l’énergie récoltée (donc variant dans le temps) des fournitures. / Over the last decade, we have witnessed a rapid growth in the number of communication devices, and this trend is expected to continue as the key technologies such as Internet of Things (IoT), wearable devices, are shaping the future of information and communication technology (ICT) industry. This growth has resulted in a tremendous increase in the energy demand, and hence the carbon footprint of the ICT ecosystem can no longer be ignored. Additionally, in traditional battery powered communication systems where energy infrastructure is not available after deployment, the limited available energy in the battery becomes the bottleneck as it determines the network lifetime. Powering up nodes with ambient energy sources, thanks to the energy harvesting technology, not only reduces the carbon footprint of ICT sector but also increases the autonomy of battery powered communication networks. An energy harvesting node can scavenge energy from the surrounding environment (typical sources are solar, wind, vibration, thermal, etc.). However, time varying nature of the ambient energy makes the design of communication strategies quite different from the traditional communication systems. Besides energy harvesting, higher throughput can be obtained in a wireless communication system by designing transmission schemes on the basis of propagation channel information. As channel adaptation techniques require to have some knowledge of the wireless channel conditions feedbackto the transmitter, the gain in throughput comes at the cost of pilot-based training and feedback which consume resources in a communication system, especially, energy. In addition when the goal in a communication system is to send information about the source to a destination such that mean squared error distortion is minimized, transmission and compression strategies hasto be designed based on both the time varying channel conditions and the source statistics. This dissertation focuses on the design of transmission strategies taking into account the cost of obtaining the channel state information (CSI) at the transmitter, and time varying source statistics when the communication nodes rely on harvested energy (hence time-varying energy) supplies.

Presently, wireless sensor nodes are widely used and the lifetime of the system is becoming the biggest problem with using this technology. As more and more low power products have been used in WSN, energy harvesting technologies, based on their own characteristics, attract more and more attention in this area. But in order to design high energy efficiency, low cost and nearly perpetual lifetime micro energy harvesting system is still challenging. This thesis proposes a new way, by applying three factors of the system, which are the energy generation, the energy consumption and the power management strategy, into a theoretical model, to optimally design a highly efficient micro energy harvesting system in a real environment. In order to achieve this goal, three aspects of contributions, which are theoretically analysis an energy harvesting system, practically enhancing the system efficiency, and real system implementation, have been made. For the theoretically analysis, the generic architecture and the system design procedure have been proposed to guide system design. Based on the proposed system architecture, the theoretical analytical models of solar and thermal energy harvesting systems have been developed to evaluate the performance of the system before it being designed and implemented. Based on the model's findings, two approaches (MPPT based power conversion circuit and the power management subsystem) have been considered to practically increase the system efficiency. As this research has been funded by the two public projects, two energy harvesting systems (solar and thermal) powered wireless sensor nodes have been developed and implemented in the real environments based on the proposed work, although other energy sources are given passing treatment. The experimental results show that the two systems have been efficiently designed with the optimization of the system parameters by using the simulation model. The further experimental results, tested in the real environments, show that both systems can have nearly perpetual lifetime with high energy efficiency.

Vibration energy harvesting has been widely investigated by academia and industry in the past decade with focus on developing distributed power sources. One of the prime goals of energy harvesters is to provide power to wireless sensors allowing for the placement of these sensors in the remote and inaccessible areas where battery is not an option. Electromechanical modeling approaches have been developed for enhancing the mechanical to electrical conversion efficiencies utilizing electromagnetic, piezoelectric, and magnetostrictive mechanisms. Models based upon the constitutive equations for these three conversion mechanisms, supported by extensive experimental results available in literature, suggest that power requirement through energy harvesters can be met only when the total volume is in the range of 1-100 cm3. There exists a critical volume of 0.5 cm³ at which above which the electromagnetic mechanism exhibits higher power density as compared to the other mechanisms. Therefore, in this thesis electromagnetic energy conversion was adopted to develop high power energy harvesters. We also present a novel vibration energy harvesting method which rivals the power density and bandwidth of the traditional methods. The overarching theme throughout the design process was selecting the structure and fabrication methodology that facilitates the transition of the technology. The experimental models were characterized at accelerations and frequencies typically found in the environmental vibration sources. The thesis provides in-depth the design, modeling, and characterization of a vibration energy harvester which creates relative motion differently than the conventional harvesters. Conventional designs rely on amplifying the original source displacement operating at the resonance condition. In the harvester design proposed in this thesis, the relative motion is created by cancelling the vibration at one location and transferring the source vibration directly to another location by combining a vibration isolator with a vibration absorber. In this novel configuration, termed as Direct Vibration Harvester (DVH), the energy is harvested directly from the vibrating source mass rather than a vibrating seismic mass attached to the source increasing the harvesting bandwidth and power density. Four bar magnet and magnetic levitation architectures were modified and modeled to reach closer to the theoretical maximum power densities. Extensive FEM was utilized to understand the performance limitations of the existing structures and the results from this analysis paved the pathway towards the development of the DVH. �A comparative analysis of the performance of the DVH with the traditional harvesting methods in terms of normalized power output and bandwidth was conducted. Performance improvements of DVH required development of the high efficiency rotational generators as linear to rotational conversion occurs in the DVH. The optimized rotational generator was modeled and all the predicted performance metrics were validated through experiments. The generator was applied towards the fabrication of DVH and also in a micro windmill. The power density of the micro windmill was found to be better than all the other results reported in literature. Extensive fluid and structural modeling was conducted to tailor the performance of the micro windmill in the desired wind speed range. Combined, this thesis provides significant advancement on many fronts. It pushes the magnetic levitation and four-bar mechanism harvester systems to their theoretical limits. It demonstrates a novel direct vibration harvester that has the possibility of surpassing the power density and bandwidth of all the known vibration harvester with large magnitude of output power. It provides a design process for an efficient small scale electromagnetic generator that can form for the backbone of many rotational and linear harvesters. This generator was used to develop the world's highest power density micro windmill in the small wind speed range. / Ph. D.

Du, Xiaona. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2011. / Includes bibliographical references (leaves 65-74). / Abstracts in English and Chinese. / ABSTRACT --- p.i / 摘要 --- p.iii / ACKNOWLEDGEMENTS --- p.iii / TABLE OF CONTENTS --- p.iv / LIST OF FIGURES --- p.xi / LIST OF TABLES --- p.ix / Chapter CHAPTER ONE --- INTRODUCTION --- p.1 / Chapter 1.1 --- Background --- p.2 / Chapter 1.1.1 --- Development of portable devices --- p.2 / Chapter 1.1.2 --- Energy harvesting --- p.2 / Chapter 1.1.3 --- Piezoelectric energy harvesting --- p.5 / Chapter 1.1.4 --- Impact based piezoelectric energy harvester --- p.10 / Chapter 1.1.5 --- Operation of piezoelectric materials --- p.14 / Chapter 1.2 --- Research Objective --- p.16 / Chapter 1.3 --- Thesis Organization --- p.19 / Chapter CHAPTER TWO --- DESIGN AND MODELING OF AN EXTENDED ROTARY ENERGY HARVESTER --- p.20 / Chapter 2.1 --- Design Considerations of an Extended Rotary Energy Harvester --- p.20 / Chapter 2.2 --- Models of Rotary Energy Harvesters --- p.24 / Chapter 2.3 --- Simulation Results --- p.33 / Chapter 2.4 --- Chapter Summary --- p.37 / Chapter CHAPTER THREE --- "PROTOTYPE, TESTING AND OUTPUT POWER OF EXTENDED ROTARY ENERGY HARVESTER" --- p.38 / Chapter 3.1 --- Prototype and Experiment --- p.38 / Chapter 3.2 --- Output Power --- p.44 / Chapter 3.2.1 --- Maximum tip displacement on output power --- p.44 / Chapter 3.2.2 --- Rotational frequency on output power --- p.47 / Chapter 3.3 --- Chapter Summary --- p.50 / Chapter CHAPTER FOUR --- COMPARISON BETWEEN E-REH AND REH --- p.51 / Chapter 4.1 --- Force on Output Power --- p.52 / Chapter 4.2 --- Rotational Frequency on Output Power --- p.54 / Chapter 4.3 --- Comparison on Design Space --- p.59 / Chapter 4.4 --- Chapter Summary --- p.62 / Chapter CHAPTER FIVE --- CONCLUSION AND FUTURE WORK --- p.63 / Chapter 5.1 --- Conclusion --- p.63 / Chapter 5.2 --- Future Work --- p.64 / BIBLIOGRAPHY --- p.65