A micro electromagnetic generator for vibration energy harvesting

Vibration energy harvesting is receiving a considerable amount of interest as a means for powering wireless sensor nodes. This paper presents a small (component volume 0.1 cm3, practical volume 0.15 cm3) electromagnetic generator utilizing discrete components and optimized for a low ambient vibration level based upon real application data. The generator uses four magnets arranged on an etched cantilever with a wound coil located within the moving magnetic field. Magnet size and coil properties were optimized, with the final device producing 46 µW in a resistive load of 4 k? from just 0.59 m s-2 acceleration levels at its resonant frequency of 52 Hz. A voltage of 428 mVrms was obtained from the generator with a 2300 turn coil which has proved sufficient for subsequent rectification and voltage step-up circuitry. The generator delivers 30% of the power supplied from the environment to useful electrical power in the load. This generator compares very favourably with other demonstrated examples in the literature, both in terms of normalized power density and efficiency

3D Energy Harvester Evaluation

This paper discusses the characterization and evaluation of an MEMS based electrostatic generator, a part of the power supply unit of the self-powered microsystem[1,2,3]. The designed generator is based on electrostatic converter and uses the principle of conversion of non-electric energy into electrical energy by periodical modification of gap between electrodes of a capacitor [4]. The structure is designed and modeled as three-dimensional silicon based MEMS. Innovative approach involving the achievement of very low resonant frequency of the structure (about 100Hz) by usage of modified long cantilever spring design, minimum area of the chip, 3D work mode, the ability to be tuned to reach desired parameters, proves promising directions of possible further development

Effect of charged line defects on conductivity in graphene: Numerical Kubo and analytical Boltzmann approaches

Charge carrier transport in single-layer graphene with one-dimensional charged defects is studied theoretically. Extended charged defects, considered an important factor for mobility degradation in chemically-vapor-deposited graphene, are described by a self-consistent Thomas-Fermi potential. A numerical study of electronic transport is performed by means of a time-dependent real-space Kubo approach in honeycomb lattices containing millions of carbon atoms, capturing the linear response of realistic size systems in the highly disordered regime. Our numerical calculations are complemented with a kinetic transport theory describing charge transport in the weak scattering limit. The semiclassical transport lifetimes are obtained by computing scattered amplitudes within the second Born approximation. The transport electron-hole asymmetry found in the semiclassical approach is consistent with the Kubo calculations. In the strong scattering regime, the conductivity is found to be a sublinear function of electronic density and weakly dependent on the Thomas-Fermi screening wavelength. We attribute this atypical behavior to the extended nature of one-dimensional charged defects. Our results are consistent with recent experimental reports.Comment: 15 pages, 9 figure

Intrinsic Geometries and Properties of Piezo-MEMS Power Harvesters with Tip Mass Offset using New Electromechanical Finite Element Vibration Analysis

Autonomous self-powered wireless sensor devices are inevitable future technology that will potentially become ubiquitous in many sectors such as industry, intelligent infrastructure and biomedical devices. This has spurred a great attention from researchers to develop self-sustained power harvesting devices. For this paper, we present a new numerical technique for modelling the MEMS power harvesters using parametric design optimisation and physical properties for various piezoelectric materials. This technique enables the prediction of optimal power harvesting responses that can be used to identify the performance of piezoelectric materials and particular piezoelectric geometry where this technique can alleviate tedious analytical methods for analysing parametric design optimisation and can assist for analysing piezo-MEMS system response before conducting the micro-fabrication process

Piezoelectric and Magnetoelectric Thick Films for Fabricating Power Sources in Wireless Sensor Nodes

In this manuscript, we review the progress made in the synthesis of thick film-based piezoelectric and magnetoelectric structures for harvesting energy from mechanical vibrations and magnetic field. Piezoelectric compositions in the system Pb(Zr,Ti)O3–Pb(Zn1/3Nb2/3)O3 (PZNT) have shown promise for providing enhanced efficiency due to higher energy density and thus form the base of transducers designed for capturing the mechanical energy. Laminate structures of PZNT with magnetostrictive ferrite materials provide large magnitudes of magnetoelectric coupling and are being targeted to capture the stray magnetic field energy. We analyze the models used to predict the performance of the energy harvesters and present a full system description

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

Fabrication and characterization of free-standing thick-film piezoelectric cantilevers for energy harvesting

Research into energy harvesting from ambient vibration sources has attracted great interest over the last few years, largely as a result of advances in the areas of wireless technology and low power electronics. One of the mechanisms for converting mechanical vibration to electrical energy is the use of piezoelectric materials, typically operating as a cantilever in a bending mode, which generate a voltage across the electrodes when they are stressed. Typically, the piezoelectric materials are deposited on a non-electro-active substrate and are physically clamped at one end to a rigid base. The presence of the substrate does not contribute directly to the electrical output, but merely serves as a mechanical supporting platform, which can pose difficulties for integration with other microelectronic devices. The aim of this paper is to describe a novel thick-film free-standing cantilever structure that does not use a supporting platform and has the advantage of minimising the movement constraints on the piezoelectric material, thereby maximising the electrical output power. Two configurations of composite cantilever structure were investigated; unimorph and multimorph. A unimorph consists of a pair of silver/palladium (Ag/Pd) electrodes sandwiching a laminar layer of lead zirconate titanate (PZT). A multimorph is an extended version of the unimorph with two pairs of Ag/Pd electrodes and three laminar sections of PZT