Wireless power transmission: R&D activities within Europe

Wireless power transmission (WPT) is an emerging technology that is gaining increased visibility in recent years. Efficient WPT circuits, systems and strategies can address a large group of applications spanning from batteryless systems, battery-free sensors, passive RF identification, near-field communications, and many others. WPT is a fundamental enabling technology of the Internet of Things concept, as well as machine-to-machine communications, since it minimizes the use of batteries and eliminates wired power connections. WPT technology brings together RF and dc circuit and system designers with different backgrounds on circuit design, novel materials and applications, and regulatory issues, forming a cross disciplinary team in order to achieve an efficient transmission of power over the air interface. This paper aims to present WPT technology in an integrated way, addressing state-of-the-art and challenges, and to discuss future R&D perspectives summarizing recent activities in Europe.The work of N. Borges Carvalho and A. J. S. Soares Boaventura was supported by the Portuguese Foundation for Science and Technology (FCT) under Project CREATION EXCL/EEI-TEL/0067/2012 and Doctoral Scholarship SFRH/BD/80615/2011. The work of H. Rogier was supported by BELSPO through the IAP Phase VII BESTCOM project and the Fund for Scientific Research-Flanders (FWO-V). The work of A. Georgiadis and A. Collado was supported by the European Union (EU) under Marie Curie FP7-PEOPLE-2009-IAPP 251557 and the Spanish Ministry of Economy and Competitiveness Project TEC 2012-39143. The work of J. A. García and M. N. Ruíz was supported by the Spanish Ministries MICINN and MINECO under FEDER co-funded Project TEC2011-29126-C03-01 and Project CSD2008-00068. The work of J. Kracek and M. Mazanek was supported in part by the Czech Ministry of Education Youth and Sports under Project OC09075–Novel Emerging Wireless Systems

Multi-frequency microwave energy harvesting receivers: theory and applications

Mención Internacional en el título de doctorEmissions across the electromagnetic spectrum are not only used for communications, but they can also be used for powering electronic devices. This resource has been made more and more abundant in the last years thanks to the recent deployment of 4G and 5G, and the popularization of broadband wireless networks such as WiFi, including traditional services such as TV and radio broadcasting. In order to take advantage of the energy (currently wasted), rectennas (a rectifier integrated with an antenna) are used. This thesis has the objective of studying these rectifying elements, to reduce or eliminate the use of batteries that are employed in millions of low-power devices and sensor networks planned for deployment in the near future. To do this, a self-supply system in situ is required. This could be achieved with photovoltaics or piezoelectrics, but they require the presence of light or vibration. However, the electromagnetic energy produced by mobile communications, TV base stations and radar is noticeable inside a large coverage area, 24 hours a day. This includes difficult access areas where it is nearly impossible to provide appropriate maintenance to replace batteries. As explained through the thesis, energy harvesting applications have a severe limitation on the available levels of power density to scavenge, constraining the RF-DC power conversion effciencies. Therefore, the amount of DC power to feed a sensor is limited and some techniques must be applied to improve the performance. This thesis proposes an alternative for improving the RF-DC power conversion efficiency based on the multiple-tone scenario (the electromagnetic spectrum). Previous studies have been published about an empirical improvement in the power efficiency when working with high Peak to Average Power Ratio (PAPR) multiple-tone signals, compared to a CW signal with the same average power, although the theoretical proof was not accurate enough. A mathematical model that predicts the expected DC current of the diode when excited with multiple tones is proposed along the thesis, having good agreement with simulations and measurements, demonstrating the good performance of the theoretical model. With this mathematical approach, convergence problems in simulation software can be avoided. This document comprises six chapters and it is organized as follows: In the first chapter a brief introduction on the evolution of wireless power transfer is presented, including all the different approaches that compose it, emphasizing the far-filed non-directional powering or harvesting, which is the topic of this thesis. In addition, an analysis of the state of the art is presented with the most signifficant values of conversion effciency, as well as the main characteristics of various designs. In the second chapter, the performance of the diode is explored theoretically. For very low incident power densities (those present in the environment), the diode works in a non-linear region, where a power effciency improvement is obtained when using high PAPR multiple-tone signal instead of a single tone with the same average power. This fact has been empirically tested but an accurate theoretical model has not been accomplished. Therefore, this chapter deals with this issue, showing a novel mathematical analysis of the diode operation in that region for multiple input tones, varying their relative amplitude and frequency. In Chapter 3, the theoretical analysis is compared with simulations and experiments for multiple input tones with a large resulting PAPR using three different rectifier circuits. To properly compare the results, it is necessary to use an accurate Spice diode model (including parasitics) and an appropriate measurement setup. Otherwise, results will differ due to an inadequate characterization of the non-linear device. This chapter addresses those issues. The analysis shows that the relative frequency and amplitude of multiple simultaneous signals impacts the amount of efficiency improvement. Once the recti er element is studied, Chapter 4 deals with the antenna design, which is part of the rectenna deployment. It is seen that different design criteria must be used when working with a WPT directive beaming application or a non-directive harvesting one, as happens in this thesis. The integration between the antenna and the recti er is analyzed, showing possible alternatives. Finally, a rectenna design is built and tested through indoor and outdoor measurements. An analysis of the electromagnetic spectrum is included to demonstrate the feasibility of the rectenna model. In Chapter 5 a wearable rectenna application is shown, with a broadband 2 to 5 GHz rectenna array, implemented on a cotton shirt. This application allows to collect enough energy to power energy-efficient devices. Different rectenna array sizes were tested at different power densities. The single element is a self-complementary tightly-coupled bow-tie. Simulations and measurements were performed over a phantom and over body tissues taking into account the electrical properties of the torso. The thickness of each layer was varied analyzing its influence in the antenna performance, to check what happens under different body compositions (people with more adipose tissue or on the contrary more brous). Finally, Chapter 6 collects the conclusions of the work shown in this thesis and ideas for future work. Some ideas are proposed about Chapter 2 to reduce the error of the mathematical approach when working in the non-linear region. Also, some possible improvements to the printed antenna of Chapter 5 are included such as adding a dual linear polarization.Las emisiones a lo largo de todo el espectro electromagnético no sólo se pueden utilizar para las comunicaciones, sino que también pueden emplearse para la alimentación de dispositivos electr onicos. Este recurso se ha hecho cada vez más abundante en los ultimos años gracias a los recientes despliegues en telefonía móvil de 4G y 5G y a la popularización de las redes inalámbricas de banda ancha (WiFi), sin olvidar las comunicaciones de difusión ya existentes como la radio o televisión. Para poder aprovechar este recurso (actualmente desaprovechado), se utilizan las llamadas rectenas, que son antenas con un elemento rectificador integrado. Esta tesis tiene por objetivo el estudio de estos elementos rectificadores, para desarrollar aplicaciones capaces de reducir o eliminar el uso de baterías en los millones de dispositivos y redes de sensores de bajo consumo existentes hoy día, mediante el autoabastecimiento de energía. Este proceso podría llevarse a cabo con paneles fotovoltaicos o sistemas piezoeléctricos, pero estos requieren de la presencia continua de la fuente que los origina (vibraciones, horas de sol). Sin embargo, la energía electromagnética producida por las estaciones base, de telefonía o televisión, está presente bajo su zona de cobertura las 24 horas del día, lo cual incluye zonas de difícil acceso, en las que es complicado el recambio o mantenimiento de las baterías. Además, estas emisiones tienen como principal limitación la baja densidad de potencia, obteniéndose valores de eficiencia de conversión RF-DC muy bajos. Esto conlleva que los valores de corriente DC para alimentar al sensor sean muy pequeños, de nA o uA, y por tanto, deben emplearse técnicas para la mejora del rendimiento. Esta tesis propone una alternativa para mejorar la eficiencia de conversión, basada en la probada mejora de eficiencia cuando se trabaja con señales con un Peak to Average Power Ratio (PAPR) grande. Esto se da en escenarios multitonales como puede ser el espectro electromagnético. Esta mejora no ha sido abordada teóricamente con resultados precisos en trabajos previos, por lo que en esta tesis se desarrolla un modelo matemático que predice la componente DC de la corriente del diodo, cuando se excita con múltiples tonos. Los resultados obtenidos han sido validados en el laboratorio, demostrándose la mejora en la eficiencia de conversión y el buen comportamiento del modelo teórico. De esta forma, se pueden agilizar los cálculos cuando no se tiene un software de simulación disponible, o cuando este arroja problemas de convergencia. Esta tesis consta de seis capítulos y está organizada de la siguiente manera: En el primer capítulo se expone una breve introducción sobre la evolución de la transferencia inalámbrica de potencia y sobre las diferentes tecnologías que la componen, haciéndose hincapié en la transferencia de potencia no directiva en campo lejano, puesto que se corresponde a la recolección de la energía electromagnética ambiental. Además, se incluye un análisis del estado del arte con los valores más significativos de eficiencia de conversión, así como las principales características de varios diseños (como por ejemplo la potencia o las bandas de trabajo empleadas). En el segundo capítulo se explora el comportamiento del diodo desde el punto de vista matemático. Bajo densidades de potencia pequeñas, como las presentes en este entorno, el diodo opera en su región no lineal, produciendo un incremento de eficiencia cuando se trabaja con señales con gran PAPR, respecto a un tono con la misma potencia media. Este hecho ha sido probado empíricamente pero ningún modelo teórico preciso ha sido realizado. En este capítulo se incluye un novedoso análisis matemático del funcionamiento del diodo en esa región para múltiples tonos de entrada, variando la amplitud y frecuencia de estos. En el capítulo 3 se muestra la comparativa entre el modelo teórico, las simulaciones y las medidas en el laboratorio, usando múltiples tonos entrada en tres rectificadores. Para comparar adecuadamente todos los resultados, es necesario utilizar un modelo Spice del diodo preciso (incluyendo los parásitos del encapsulado) y un correcto setup de medida. De lo contrario, existiría un error en los resultados debido a una caracterización inadecuada del dispositivo no lineal. Este capítulo aborda esos problemas. El análisis muestra que la frecuencia y amplitud relativa de múltiples señales simultáneas afectan a la eficiencia. Una vez estudiado el rectificador, el capítulo 4 de la tesis aborda el diseño de la antena. Para ello, se analizan los diferentes criterios de diseño que deben emplearse cuando se trabaja con una transmisión de potencia inalámbrica directiva o no directiva, como es en caso bajo estudio, así como las técnicas de integración entre rectificador y antena. Para concluir, se diseña y mide una rectena tanto en laboratorio como en espacio abierto, usando la energía ambiental, previamente caracterizada con medidas espectrales. Los resultados demuestran que es posible recolectar y rectificar la energía ambiental. En el capítulo 5 se muestra una posible aplicación al integrarse una rectena impresa en una camiseta para alimentar sensores biológicos o \wearable". Se trata de un diseño de banda ancha que opera en el rango de 2 a 5 GHz, que permite recolectar suficiente energía para alimentar sensores de bajo consumo. Se analiza el funcionamiento de dos tamaños distintos de arrays con diferentes densidades de potencia. Al ser un diseño \wearable", la aplicación ha sido diseñada y probada sobre un maniquí y un cuerpo humano, analizándose el comportamiento de la antena impresa sobre distintas composiciones corporales (personas con más tejido adiposo o por el contrario más fibrosas). Finalmente, el capítulo 6 recopila las conclusiones del trabajo que se muestra en esta tesis e ideas para trabajos futuros, proponiéndose desde enfoques para reducir más el error en la aproximación del comportamiento no lineal del diodo en el capítulo 2, a posibles mejoras en la antena impresa del capítulo 5, incluyendo la doble polarización lineal.Programa de Doctorado en Multimedia y Comunicaciones por la Universidad Carlos III de Madrid y la Universidad Rey Juan CarlosPresidente: Carlos Martín Pascual.- Secretario: Simon Jacques Hemour.- Vocal: Nuno Miguel G. Borges De Carvalh

Co-Design Strategies for Energy-Efficient UWB and UHF Wireless Systems

This paper reviews the most recent methods, combining nonlinear harmonic-balance-based analysis with electromagnetic (EM) simulation, for optimizing, at the circuit level, modern radiative RF/microwave systems. In order to maximize the system efficiency, each subsystem must be designed layoutwise, accounting for the presence of the others, that is, accounting for its actual terminations, rather than the ideal ones (50 Ω). In this way, the twofold goal of minimizing size and losses of the system is obtained by reducing intersystem matching networks. Indeed, terminations are complex, frequency-dispersive, and variable with the signal level, if active operations are concerned, and are responsible for performance degradation if not properly optimized. This approach is nowadays necessary, given the ever increased spread of pervasively distributed RF microsystems adopting miniaturized antennas, such as radio frequency identification (RFID) or wireless sensor networks, that must be low-cost, low-profile, low-power, and must simultaneously perform localization, identification, and sensing. For the design of a transmitter and a receiver connected with the respective antennas, suitable figures of merit are considered, encompassing radiation and nonlinear performance. Recent representative low-profile realizations, adopting ultra-wideband (UWB) excitations are used to highlight the benefit of the proposed nonlinear/EM approach for next generation energy autonomous microsystem, such as UWB-RFID tags

IoT応用向け環境RF信号RFエネルギーハーベスティングシステムの研究

電気通信大学201

Capture opportuniste d'énergie micro-onde pour l'autonomie des objets communicants

In recent years, the decrease of electronic components consumption has led to the development of wireless devices. An interesting application concerns Internet of Things (IoT) and Wireless Sensor Networks (WSNs). Sensors are used in various scenarios such as intelligent monitoring systems for office, home automation, medical or military applications. Today these sensors are powered by batteries. Despite significant progress, batteries still have a limited lifetime and their replacement is often complicated. This explains the motivation to find alternative ways to power these objects. A promising method consists in harvesting energy from the ambient environment of the sensor nodes. For instance, thermal gradients, mechanical vibrations, light or electromagnetic waves could serve such a purpose. This thesis has been funded by PRACOM and has been held in this context. The thesis aim is to contribute to the development of innovative solutions to design self-powered sensor networks particularly by exploiting the energy of Radio Frequency (RF) waves. These sensors are located in ambient environment, i.e. an environment for which the RF power densities are not controlled and generally low. First of all, different sources and energy harvesting techniques have been investigated. This study helps to position the RF energy harvesting to other systems such as the mechanical, thermal, chemical, photovoltaic energy. After having demonstrated the added value of harvesting energy over a wide frequency band, a statistical study has been conducted to evaluate the RF power density in urban environment and countryside. The sensor environment involves choices for the chosen architecture, such as the addition of an energy storage cell or the use of a dc-dc converter. Several multi-band RF harvesters have been designed and tested in various environments. They show the feasibility of powering small sensors. The issue of autonomous sensors worn by the person is also addressed. The study highlights how the presence of the human body has an impact on RF harvester performance. Several solutions are proposed such as the improvement of the impedance matching network or the use of a high impedance surface.Les réseaux de capteurs sans fil (WSN: Wireless Sensor Network), l'Internet des objets (IOT), profitent des progrès récents en terme de consommation énergétique pour concevoir des entités de contrôle intelligentes. Les batteries ou piles ont permis le développement de ces systèmes en les rendant autonomes. Néanmoins, cette méthode d'alimentation est inadaptée pour les applications modernes. Une solution alternative pour alimenter ces capteurs est d'utiliser l'énergie disponible dans leur environnement, comme par exemple les gradients thermiques, les vibrations mécaniques, ou encore les ondes lumineuses ou Radio-Fréquences. C'est dans ce contexte que s'est déroulé ce travail de thèse financé par PRACOM. Cette thèse propose de contribuer au developpement de solutions innovantes visant à rendre autonome en énergie un réseau de capteurs en exploitant notamment l'énergie des ondes Radio-Fréquences (RF). Ces capteurs sont placés en environnement ambiant, c'est-à-dire dans un environnement pour lequel les densités de puissances incidentes ne sont pas maîtrisées et sont généralement faibles. Tout d'abord différentes sources et techniques de récupération d'énergie ont été étudiées comme l'énergie mécanique, thermique, chimique et celle des ondes lumineuses et Radio-Fréquences. Cette étude a permis de positionner les systèmes de récupération d'énergie des ondes Radio Fréquences par rapport aux autres systèmes. Après avoir démontré l'intérêt de collecter l'énergie sur une large bande de fréquence, une étude statistique a été menée sur l'évaluation de la densité de puissance RF présente dans un environnement urbain et à la campagne. L'environnement du capteur implique des choix pour l'architecture choisie, comme par exemple l'ajout de cellule de stockage d'énergie ou encore l'utilisation d'un convertisseur dc-dc. Plusieurs récupérateurs d'énergie RF multibandes ont été conçus et testés dans divers environnements. Ceux-ci montrent la faisabilité d'alimenter des petits capteurs en extérieur. La problématique des capteurs autonomes en énergie portés par la personne est également abordée. Il s'agit de voir comment la présence du corps humain a un retentissement sur les performances du système de récupération d'énergie RF. Plusieurs solutions sont proposées comme l'amélioration du réseau d'adaptation d'impédances du récupérateur d'énergie RF, ou encore l'utilisation d'une surface à hautes impédances

Rectenna circuits for RF energy harvesting in miniature DBS devices.

 Development of an optimum rectenna for radio frequency energy harvesting in miniature head-mountable deep brain stimulation (DBS) devices. The designed miniature rectenna can operate a DBS device without battery for murine preclinical research. The battery-less operation of the device eliminates battery related difficulties

Electromagnetic Energy Harvesting

The advent of rectenna systems has opened the door for many applications, with the main goal of harvesting electromagnetic waves. Many designs have been presented to provide a high conversion efficiency. Some works have been focused on the antenna part, where a high-efficient electromagnetic wave to AC conversion can be achieved. Other works focused on the rectification part, where a high-efficient AC to DC conversion is desired. Even though each part can separately achieve a high conversion efficiency, connecting both parts can dramatically deteriorate the achieved efficiency. In general, increasing the accuracy of components' characterization leads to a high-performance design. Since the main component in the rectifying circuit is the diode, an accurate model for a nonlinear system is required for efficient use of computer-aided design (CAD) technology. Currently, many models of nonlinear devices consist of basic electric elements such as capacitors, inductors, and resistors. However, building such models needs complex measurement procedures to extract the required parameters precisely. Furthermore, it may take a year to develop and validate an accurate model. Considering challenges associated with traditional approaches of modeling, an accurate and relatively easy method based on X-Parameters to model nonlinear devices has been presented. X-Parameters can be measured at different frequencies and at different power levels and stored in one file. The availability of all information in one file enables us to characterize the systems that operate under unstable conditions, such as the rectenna circuits that are used for energy harvesting. To accurately simulate a rectenna circuit, we presented a simulation methodology that calls for modeling the antenna part as a circuit that is fully independent in its characteristics from the remaining parts of the rectenna system. This can be accomplished by using Thevenin’s theorem. According to Thevenin’s theorem, a linear circuit can be replaced with an equivalent circuit consisting of an independent voltage source in series with an impedance. The Thevenin’s equivalent circuit provides all the required information about the antenna. It is important to emphasize that the equivalent circuit of the antenna obtained using Thevenin theory is not affected by the rectenna circuit that connects the antenna to the load. Antennas that can be used both as flat and as curved (conformal antennas) are becoming increasingly attractive antennas because of their wide range of applications. The microstrip patch antenna is one of the most widely used antennas, however, it has a high sensitivity to bending. An antenna with a lower sensitivity to bending is required to avoid the effect of bending on antennas. The metasurface antenna is one of the best candidates to overcome the problem of the antenna's sensitivity to bending. To construct a metasurface, an ensemble of periodic resonators is structured to operate as an effective electromagnetic radiator. In this thesis, we explored the impact of bending on metasurface antennas and microstrip patch antenna arrays