Development of MEMS Piezoelectric Vibration Energy Harvesters with Wafer-Level Integrated Tungsten Proof-Mass for Ultra Low Power Autonomous Wireless Sensors

La génération d’énergie localisée et à petite échelle, par transformation de l’énergie vibratoire disponible dans l’environnement, est une solution attrayante pour améliorer l’autonomie de certains noeuds de capteurs sans-fil pour l’Internet des objets (IoT). Grâce à des microdispositifs inertiels résonants piézoélectriques, il est possible de transformer l’énergie mécanique en électricité. Cette thèse présente une étude exhaustive de cette technologie et propose un procédé pour fabriquer des microgénérateurs MEMS offrant des performances surpassant l’état de l’art. On présente d’abord une revue complète des limites physiques et technologiques pour identifier le meilleur chemin d’amélioration. En évaluant les approches proposées dans la littérature (géométrie, architecture, matériaux, circuits, etc.), nous suggérons des métriques pour comparer l’état de l’art. Ces analyses démontrent que la limite fondamentale est l’énergie absorbée par le dispositif, car plusieurs des solutions existantes répondent déjà aux autres limites. Pour un générateur linéaire résonant, l’absorption d’énergie dépend donc des vibrations disponibles, mais aussi de la masse du dispositif et de son facteur de qualité. Pour orienter la conception de prototypes, nous avons réalisé une étude sur le potentiel des capteurs autonomes dans une automobile. Nous avons évalué une liste des capteurs présents sur un véhicule pour leur compatibilité avec cette technologie. Nos mesures de vibrations sur un véhicule en marche aux emplacements retenus révèlent que l’énergie disponible pour un dispositif linéaire résonant MEMS se situe entre 30 à 150 Hz. Celui-ci pourrait produire autour de 1 à 10 μW par gramme. Pour limiter la taille d’un générateur MEMS pouvant produire 10 μW, il faut une densité supérieure à celle du silicium, ce qui motive l’intégration du tungstène. L’effet du tungstène sur la sensibilité du dispositif est évident, mais nous démontrons également que l’usage de ce matériau permet de réduire l’impact de l’amortissement fluidique sur le facteur de qualité mécanique Qm. En fait, lorsque l’amortissement fluidique domine, ce changement peut améliorer Qm d’un ordre de grandeur, passant de 103 à 104 dans l’air ambiant. Par conséquent, le rendement du dispositif est amélioré sans utiliser un boîtier sous vide. Nous proposons ensuite un procédé de fabrication qui intègre au niveau de la tranche des masses de tungstène de 500 μm d’épais. Ce procédé utilise des approches de collage de tranches et de gravure humide du métal en deux étapes. Nous présentons chaque bloc de fabrication réalisé pour démontrer la faisabilité du procédé, lequel a permis de fabriquer plusieurs prototypes. Ces dispositifs ont été testés en laboratoire, certains démontrant des performances records en terme de densité de puissance normalisée. Notre meilleur design se démarque par une métrique de 2.5 mW-s-1/(mm3(m/s2)2), soit le meilleur résultat répertorié dans l’état de l’art. Avec un volume de 3.5 mm3, il opère à 552.7 Hz et produit 2.7 μW à 1.6 V RMS à partir d’une accélération de 1 m/s2. Ces résultats démontrent que l’intégration du tungstène dans les microgénérateurs MEMS est très avantageuse et permet de s’approcher davantage des requis des applications réelles.Small scale and localized power generation, using vibration energy harvesting, is considered as an attractive solution to enhance the autonomy of some wireless sensor nodes used in the Internet of Things (IoT). Conversion of the ambient mechanical energy into electricity is most often done through inertial resonant piezoelectric microdevices. This thesis presents an extensive study of this technology and proposes a process to fabricate MEMS microgenerators with record performances compared to the state of the art. We first present a complete review of the physical and technological limits of this technology to asses the best path of improvement. Reported approaches (geometries, architectures, materials, circuits) are evaluated and figures of merit are proposed to compare the state of the art. These analyses show that the fundamental limit is the absorbed energy, as most proposals to date partially address the other limits. The absorbed energy depends on the level of vibrations available, but also on the mass of the device and its quality factor for a linear resonant generator. To guide design of prototypes, we conducted a study on the potential of autonomous sensors in vehicles. A survey of sensors present on a car was realized to estimate their compatibility with energy harvesting technologies. Vibration measurements done on a running vehicle at relevant locations showed that the energy available for MEMS devices is mostly located in a frequency range of 30 to 150 Hz and could generate power in the range of 1-10 μW per gram from a linear resonator. To limit the size of a MEMS generator capable of producing 10 μW, a higher mass density compared to silicon is needed, which motivates the development of a process that incorporates tungsten. Although the effect of tungsten on the device sensitivity is well known, we also demonstrate that it reduces the impact of the fluidic damping on the mechanical quality factor Qm. If fluidic damping is dominant, switching to tungsten can improve Qm by an order of magnitude, going from 103 to 104 in ambient air. As a result, the device efficiency is improved despite the lack of a vacuum package. We then propose a fabrication process flow to integrate 500 μm thick tungsten masses at the wafer level. This process combines wafer bonding with a 2-step wet metal etching approach. We present each of the fabrication nodes realized to demonstrate the feasibility of the process, which led to the fabrication of several prototypes. These devices are tested in the lab, with some designs demonstrating record breaking performances in term of normalized power density. Our best design is noteworthy for its figure of merit that is around 2.5 mW-s-1/(mm3(m/s2)2), which is the best reported in the state of the art. With a volume of 3.5 mm3, it operates at 552.7 Hz and produces 2.7 μW at 1.6 V RMS from an acceleration of 1 m/s2. These results therefore show that tungsten integration in MEMS microgenerators is very advantageous, allowing to reduce the gap with needs of current applications

Advanced Energy Harvesting Technologies

Energy harvesting is the conversion of unused or wasted energy in the ambient environment into useful electrical energy. It can be used to power small electronic systems such as wireless sensors and is beginning to enable the widespread and maintenance-free deployment of Internet of Things (IoT) technology. This Special Issue is a collection of the latest developments in both fundamental research and system-level integration. This Special Issue features two review papers, covering two of the hottest research topics in the area of energy harvesting: 3D-printed energy harvesting and triboelectric nanogenerators (TENGs). These papers provide a comprehensive survey of their respective research area, highlight the advantages of the technologies and point out challenges in future development. They are must-read papers for those who are active in these areas. This Special Issue also includes ten research papers covering a wide range of energy-harvesting techniques, including electromagnetic and piezoelectric wideband vibration, wind, current-carrying conductors, thermoelectric and solar energy harvesting, etc. Not only are the foundations of these novel energy-harvesting techniques investigated, but the numerical models, power-conditioning circuitry and real-world applications of these novel energy harvesting techniques are also presented

A critical analysis of research potential, challenges and future directives in industrial wireless sensor networks

In recent years, Industrial Wireless Sensor Networks (IWSNs) have emerged as an important research theme with applications spanning a wide range of industries including automation, monitoring, process control, feedback systems and automotive. Wide scope of IWSNs applications ranging from small production units, large oil and gas industries to nuclear fission control, enables a fast-paced research in this field. Though IWSNs offer advantages of low cost, flexibility, scalability, self-healing, easy deployment and reformation, yet they pose certain limitations on available potential and introduce challenges on multiple fronts due to their susceptibility to highly complex and uncertain industrial environments. In this paper a detailed discussion on design objectives, challenges and solutions, for IWSNs, are presented. A careful evaluation of industrial systems, deadlines and possible hazards in industrial atmosphere are discussed. The paper also presents a thorough review of the existing standards and industrial protocols and gives a critical evaluation of potential of these standards and protocols along with a detailed discussion on available hardware platforms, specific industrial energy harvesting techniques and their capabilities. The paper lists main service providers for IWSNs solutions and gives insight of future trends and research gaps in the field of IWSNs

Pulse mode of operation : a new booster of TEG, improving power up to X2.7 : to better fit IoT requirements

Internet of Things (IoT) is becoming the new driver for semiconductor industry and the largest electronic market ever seen. The number of IoT nodes is already many times larger than the human population and is continuously growing. It is thus mandatory that IoT nodes become self-supplying with energy harvested from environment since periodic exchange of batteries in such a huge number of units (often located in inaccessible places e.g. industrial environment or elements of constructions) is impractical and soon will be simply impossible. Photovoltaic generators may easily harvest energy where light is available, but the IoT nodes often work in dark, hidden locations where the only available energy sources are heat losses. There, ThermoElectric Generators (TEGs) could be the best candidate, if not that if we speak of exploiting heat losses it often means very low temperature differences. This means conditions where TEGs power production drops down dramatically. In this paper we put forward a new idea of TEG's pulse operation that boosts the power production up to X2.7. This extends the domain of applicability of TEGs to lower temperature differences, where conventional TEGs are out of the game. Next, we show that the improvement X2.7 maintains also at larger temperature differences that presents obvious advantages

Wideband vibration energy harvesting using electromagnetic transduction for powering internet of things

The ‘Internet of Things-(IoT)’ envisions a world scattered with physical sensors that collect and transmit data about almost anything and thereby enabling intelligent decision-making for a smart environment. While technological advancements have reduced the power consumption of such devices significantly, the problem of perpetual energy supply beyond the limited capability of batteries is a bottleneck to this vision which is yet to be resolved. This issue has surged the research to investigate the prospect of harvesting the energy out of ambient mechanical vibrations. However, limited applications of conventional resonant devices under most practical environments involving frequency varying inputs, has gushed the research on wideband transducers recently. To facilitate multi-frequency operation at low-frequency regime, design innovations of the Silicon-onInsulator based MEMS suspension systems are performed through multi-modal activation. For continuous bandwidth widening, the benefits of using nonlinear stiffness in the system dynamics are investigated. By topologically varying the spring architectures, dramatically improved operational bandwidth with large power-density is obtained, which is benchmarked using a novel figure-of-merit. However, the fundamental phenomenon of multi-stability limits many nonlinear oscillator based applications including energy harvesting. To address this, an electrical control mechanism is introduced which dramatically improves the energy conversion efficiency over a wide bandwidth in a frequencyamplitude varying environment using only a small energy budget. The underlying effects are independent of the device-scale and the transduction methods, and are explained using a modified Duffing oscillator model. One of the key requirements for fully integrated electromagnetic transducers is the CMOS compatible batch-fabrication of permanent magnets with large energy-product. In the final module of the works, nano-structured CoPtP hard-magnetic material with large coercivity is developed at room-temperature using a current modulated electro-deposition technique. The demagnetization fields of the magnetic structures are minimized through optimized micro-patterns which enable the full integration of high performance electromagnetic energy harvesters

Vibration-based condition monitoring of wind turbine blades

Significant advances in wind turbine technology have increased the need for maintenance through condition monitoring. Indeed condition monitoring techniques exist and are deployed on wind turbines across Europe and America but are limited in scope. The sensors and monitoring devices used can be very expensive to deploy, further increasing costs within the wind industry. The work outlined in this thesis primarily investigates potential low-cost alternatives in the laboratory environment using vibration-based and modal testing techniques that could be used to monitor the condition of wind turbine blades. The main contributions of this thesis are: (1) the review of vibration-based condition monitoring for changing natural frequency identification; (2) the application of low-cost piezoelectric sounders with proof mass for sensing and measuring vibrations which provide information on structural health; (3) the application of low-cost miniature Micro-Electro-Mechanical Systems (MEMS) accelerometers for detecting and measuring defects in micro wind turbine blades in laboratory experiments; (4) development of an in-service calibration technique for arbitrarily positioned MEMS accelerometers on a medium-sized wind turbine blade. This allowed for easier aligning of coordinate systems and setting the accelerometer calibration values using samples taken over a period of time; (5) laboratory validation of low-cost modal analysis techniques on a medium-sized wind turbine blade; (6) mimicked ice-loading and laboratory measurement of vibration characteristics using MEMS accelerometers on a real wind turbine blade and (7) conceptualisation and systems design of a novel embedded monitoring system that can be installed at manufacture, is self-powered, has signal processing capability and can operate remotely. By applying the conclusions of this work, which demonstrates that low-cost consumer electronics specifically MEMS accelerometers can measure the vibration characteristics of wind turbine blades, the implementation and deployment of these devices can contribute towards reducing the rising costs of condition monitoring within the wind industry

Power Generation by Resonant Self-Actuation

Die Forschung im Bereich der Mikro-Energiegewinnung wurde durch den Bedarf an au-tarken sowie stabilen Energiequellen für vernetzte und drahtlose Sensoren vorangetrieben. Abwärme, insbesondere bei Temperaturen unter 200 °C, stellt eine vielversprechende, aber mit den derzeitigen Umwandlungstechnologien schwer zu gewinnende Energiequelle dar. Der Fortschritt von thermomagnetischen Generatoren (TMGs) mit hoher Leistung wurde durch den Mangel an Weiterentwicklungen von thermomagnetischen Materialien behindert. Diese Arbeit stützt sich auf frühere Forschungsarbeiten zu TMGs im kleinen Maßstab. Die Hauptziele sind: • Entwicklung eines LEM-Modells (Lumped Element Model) zur Simulation des TMG, um die Leistung zu analysieren und zu optimieren. • Nutzung von LEM und Experimenten, um die Auswirkungen verschiedener De-signparameter zu verstehen. • Die Hochskalierung des Volumens des aktiven Materials eines TMG, um die absolute Ausgangsleistung eines einzelnen Generators zu erhöhen. • Die Hochskalierung des TMG durch Parallelbetrieb mehrerer TMGs zur Vergrößerung der lateralen Größe. • Erweiterung des Betriebsbereichs der Wärmequelle auf Temperaturen nahe der Raumtemperatur, ohne die resonante Selbstaktivierung zu verlieren. Zunächst werden mittels experimenteller Messungen und LEM-Simulationen TMGs, die auf verschiedenen Materialien wie dem Ni-Mn-Ga Heusler-Legierungsfilm, Gadolinium und La-Fe-Si-H basieren, grundsätzlich erforscht. Die Auswirkung verschiedener Designparameter auf die Leistung des TMGs wird untersucht. Dabei beschreiben LEM-Simulationen die gekoppelten dynamischen Eigenschaften von TMGs, die Filme aus magnetischen Formgedächtnislegierungen (MSMA) verwenden. Die TMG nutzen Selbstaktivierung, indem ein temperaturabhängige Magnetisierungsänderungen und einen schnellen Wärmetransfer durch thermomagnetische Dünnschichten ausgenutzt wird. Detaillierte LEM-Simulationen zeigen die Temperaturänderung, die Magnetfeldänderung und die daraus resultierende Magnetisierung der TM-Filme über Zeit und Position. Opti-male Bedingungen für eine resonante Selbstaktivierung werden durch sorgfältiges Design der TMG-Parameter erreicht, was zu einer kontinuierlichen, ungedämpften Oszillation des TMG-Ausleger führt. In dieser Arbeit werden verschiedene Design-Parameter erörtert, die sich auf die resonante Selbstaktivierung im Falle von Ni-Mn-Ga-Dünnschichten auswirken, wobei die Bedeutung der Feinabstimmung jedes Parameters für eine maximale Ausgangsleistung hervorgehoben wird. Die Auswirkungen von Faktoren wie Magnet, Spulenwindungen, Auslegersteifigkeit, Lastwiderstand (RL), Curie-Temperatur (Tc), Wärmeübergangskoeffizient (hf) und Wärmewiderstand (Rb) werden untersucht, um ihren Einfluss auf die TMG-Leistung zu verstehen. LEM-Simulationen zeigen kritische Werte für hf und Rb, die eine stabile Energieerzeugung mit signifikantem Hub und Frequenz ermöglichen, was zu einer deutlichen Steigerung der elektrischen Leistung führt. Die Hochskalierung des TMG mit Ni-Mn-Ga-Dünnschicht zeigt gegensätzliche Auswir-kungen auf die Leistungsabgabe und die Grundfläche, wobei eine verbesserte elektrische Leistung pro Grundfläche durch eine Erhöhung der Schichtdicke von 5 auf 40 µm erreicht wird. Bei einer Temperaturänderung von nur 3 °C und einer Frequenz von 146 Hz wer-den Werte von 50 µW/cm2 erreicht. Die parallelen Architekturen sind entscheidend für die Erzeugung ausreichender Energie für die direkte Anwendung. Die thermische Kreuz-kopplung beeinträchtigt die dynamische Leistung und die Leistungsabgabe von parallel betriebenen TMGs. Thermische Effekte machen sich vor allem bei geringen Abständen zwischen den Bauelementen und hohen Temperaturen der Wärmequelle bemerkbar, wobei jedoch keine magnetischen oder mechanischen Wechselwirkungen zwischen den parallel arbeitenden TMGs beobachtet werden. Bei Verwendung von Gadolinium als aktiver TM-Schicht ist ein Betrieb bei einer niedrigen Wärmequellentemperatur (Tsource) von 40 °C möglich. Der TMG kann bei dieser Tsource eine Leistung von 1,3 µW bei einer Frequenz von 54 Hz erzeugen, was einer Ausgangs-leistung von 10 µW/cm2 pro Fläche entspricht. Bei einer Tsource von 65 °C steigt dieser Wert sprunghaft auf 24 µW/cm2 bei einer Frequenz von 117 Hz an. Obwohl für eine opti-male Leistung eine Tamb von 11 °C erforderlich ist, kann das Bauelement die resonante Selbstaktivierung bis zu einer Umgebungstemperatur (Tamb) von 19 °C aufrechterhalten und dabei immer noch 8,7 µW/cm2 Leistung bei einer Tsource von 50 °C erzeugen. Außer-dem werden die scharfen Grenzen der Betriebstemperaturen in Bezug auf Tsource und Tamb untersucht und vorgestellt. Ein TMG, bei den hydrierten La-Fe-Si-Legierungen als aktiven TM-Film verwendet, kann 38 µW/cm2 aus einer Tsource von 90°C erzeugen, während es mit einer Frequenz von 137 Hz arbeitet

Wireless sensor system for infrastructure health monitoring

In this thesis, radio frequency identification (RFID)-based wireless sensor system for infrastructure health monitoring (IHM) is designed and developed. It includes mountable semi-passive tag antenna integrated sensors capable of measuring critical responses of infrastructure such as dynamic acceleration and strain. Furthermore, the system is capable of measuring structural displacement. One of the most important parts of this system is the relatively small, tunable, construction material mountable RFID tag antenna. The tag antenna is electronically integrated with the sensors. Leading to the process of developing tag antenna integrated sensors having satisfactory wireless performance (sensitivity and read range) when mounted on concrete and metal structural members, the electromagnetic performance of the tag antenna is analyzed and optimized using both numerical and experimental procedures. Subsequently, it is shown that both the simulation and the experimental measurement results are in good agreement. The semi-passive RFID-based system is implemented in a wireless IHM system with multiple sensor points to measure dynamic acceleration and strain. The developed system can determine the natural frequencies of infrastructure and identify any state changes of infrastructure by measuring natural frequency shifts. Enhancement of the spectral bandwidth of the system has been performed under the constraints of the RFID hardware. The influence of the orientation and shape of the structural members on wireless power flow in the vicinity of those members is also investigated with the RFID reader-tag antenna system in both simulation and experiments. The antenna system simulations with a full-scale structural member have shown that both the orientation and the shape of the structural member influence the wireless power flow towards and in the vicinity of the member, respectively. The measurement results of the conducted laboratory experiments using the RFID antenna system in passive mode have shown good agreement with simulation results. Furthermore, the system’s ability to measure structural displacement is also investigated by conducting phase angle of arrival measurements. It is shown that the system in its passive mode is capable of measuring small structural displacements within a short wireless distance. The benchmarking of the developed system with independent, commercial, wired and wireless measurement systems has confirmed the ability of the RFID-based system to measure dynamic acceleration and strain. Furthermore, it has confirmed the system’s ability to determine the natural frequency of an infrastructure accurately. Therefore, the developed system with wireless sensors that do not consume battery power in data transmission and with the capability of dynamic response measurement is highly applicable in IHM