Magneto-inductive Passive Relaying in Arbitrarily Arranged Networks

We consider a wireless sensor network that uses inductive near-field coupling for wireless powering or communication, or for both. The severely limited range of an inductively coupled source-destination pair can be improved using resonant relay devices, which are purely passive in nature. Utilization of such magneto-inductive relays has only been studied for regular network topologies, allowing simplified assumptions on the mutual antenna couplings. In this work we present an analysis of magneto-inductive passive relaying in arbitrarily arranged networks. We find that the resulting channel has characteristics similar to multipath fading: the channel power gain is governed by a non-coherent sum of phasors, resulting in increased frequency selectivity. We propose and study two strategies to increase the channel power gain of random relay networks: i) deactivation of individual relays by open-circuit switching and ii) frequency tuning. The presented results show that both methods improve the utilization of available passive relays, leading to reliable and significant performance gains.Comment: 6 pages, 9 figures. To be presented at the IEEE International Conference on Communications (ICC), Paris, France, May 201

Beamforming for Magnetic Induction based Wireless Power Transfer Systems with Multiple Receivers

Magnetic induction (MI) based communication and power transfer systems have gained an increased attention in the recent years. Typical applications for these systems lie in the area of wireless charging, near-field communication, and wireless sensor networks. For an optimal system performance, the power efficiency needs to be maximized. Typically, this optimization refers to the impedance matching and tracking of the split-frequencies. However, an important role of magnitude and phase of the input signal has been mostly overlooked. Especially for the wireless power transfer systems with multiple transmitter coils, the optimization of the transmit signals can dramatically improve the power efficiency. In this work, we propose an iterative algorithm for the optimization of the transmit signals for a transmitter with three orthogonal coils and multiple single coil receivers. The proposed scheme significantly outperforms the traditional baseline algorithms in terms of power efficiency.Comment: This paper has been accepted for presentation at IEEE GLOBECOM 2015. It has 7 pages and 5 figure

Insights into dynamic tuning of magnetic-resonant wireless power transfer receivers based on switch-mode gyrators

Magnetic-resonant wireless power transfer (WPT) has become a reliable contactless source of power for a wide range of applications. WPT spans different power levels ranging from low-power implantable devices up to high-power electric vehicles (EV) battery charging. The transmission range and efficiency of WPT have been reasonably enhanced by resonating the transmitter and receiver coils at a common frequency. Nevertheless, matching between resonance in the transmitter and receiver is quite cumbersome, particularly in single-transmitter multi-receiver systems. The resonance frequency in transmitter and receiver tank circuits has to be perfectly matched, otherwise power transfer capability is greatly degraded. This paper discusses the mistuning effect of parallel-compensated receivers, and thereof a novel dynamic frequency tuning method and related circuit topology and control is proposed and characterized in the system application. The proposed method is based on the concept of switch-mode gyrator emulating variable lossless inductors oriented to enable self-tunability in WPT receiversPeer ReviewedPostprint (published version

Master of Science

thesisThis thesis discusses the design, modeling, and experimental validation of an inductively coupled wireless power transfer (WPT) system to power a micro aerial vehicle (MAV) without an onboard power source. MAVs are limited in utility by flight times ranging from 5 to 30 minutes. Using WPT for MAVs, in general, extends flight time and can eliminate the need for batteries. In this paper, a resonant inductive power transfer system (RIPT), consisting of a transmit (Tx) coil on a fixed surface and a receive (Rx) coil attached to the MAV, is presented, and a circuit is described. The RIPT system design is modeled to determine a suitable geometry for the coils, and the model validated experimentally. It is found that for the MAV used in this work, a suitable geometry of coils is a 19cm diameter planar spiral Tx coil made with 14 AWG copper wire, seven turns, and 5cm pitch paired with an Rx coil made of 16-20AWG wire, 13cm-20cm diameter, 1mm pitch, and one to two turns. A demonstration of an MAV being powered 11cm above the Tx coil with the WPT system in a laboratory setting is presented. The MAV consumes approximately 12 Watts. The overall power efficiency of the RIPT system from RF power source output to MAV motors is approximately 32%

Magneto-Inductive Powering and Uplink of In-Body Microsensors: Feasibility and High-Density Effects

This paper studies magnetic induction for wireless powering and the data uplink of microsensors, in particular for future medical in-body applications. We consider an external massive coil array as power source (1 W) and data sink. For sensor devices at 12 cm distance from the array, e.g. beneath the human skin, we compute a minimum coil size of 150 um assuming 50 nW required chip activation power and operation at 750 MHz. A 275 um coil at the sensor allows for 1 Mbit/s uplink rate. Moreover, we study resonant sensor nodes in dense swarms, a key aspect of envisioned biomedical applications. In particular, we investigate the occurring passive relaying effect and cooperative transmit beamforming in the uplink. We show that the frequency- and location-dependent signal fluctuations in such swarms allow for significant performance gains when utilized with adaptive matching, spectrally-aware signaling and node cooperation. The work is based on a general magneto-inductive MIMO system model, which is introduced first.Comment: 6 pages, to appear at IEEE WCNC 2019. This work has been submitted to the IEEE for possible publication. Copyright may be transferred without notice, after which this version may no longer be accessibl

Network Methods for Analysis and Design of Resonant Wireless Power Transfer Systems

In this chapter we illustrate networks methods for the analysis an design of Wireless Power Transfer (WPT) systems. We begin with an introduction which compares the alternatives available for transfering electromagnetic power. In particular, we illustrate the advantages and disadvantages of the various possibilities: transmission lines, antennas, and mid-range reactive field couplings. Then, in the introduction, we also illustrate practical applications for WPT and discuss relevant papers published so far. In the second section, after introducing a basic structure for realizing WPT (see Fig.1), we discuss the relevant theory for WPT by considering a very simple network which, nevertheless, contains all the relevant phenomenology. We derive formulas for maximizing the efficiency of power transfer and we show the necessity of introducing matching networks. Several possible realizations of matching networks are then illustrated. In the next section we introduce appropriate methods, based on the ABCD matrix, for the narrow-band analysis of WPT systems including matching networks. An example of such a network is reported in Fig. 2. A section will be devoted to the input and output coupling design where we will provide new formulas for the design of the matching networks. In particular we show that, for a given type of resonators with a given quality factor Q and a given value of the coupling between the two resonators, we can find the optimal coupling coefficients which maximize the efficiency. An example of the results achievable when optimizing the input/output coupling is reported in Fig. 3. Having derived a procedure for attaining maximum efficiency, it is also possible to establish the theoretical limits that can be achieved for a given value of coupling and for specified values of the resonators Q. A section will be also devoted to the case of multiple transmitting and multiple receiving resonators. For this arrangement, which has practical relevance and is illustrated in Fig 5, we also introduce a rigorous general network model for its analysis. Several different types of resonators will be investigated and compared. Closed form formulas relevant to the resonators' design will be introduced and also fullwave analysis of resonators well be exploited. Theoretical results will be compared with measured ones and measurement methods will be discussed. One of the problems of WPT, i.e. the frequency shift occurring when resonators are placed at different distances, will be discussed and the solution will be outlined. This is very important in practice because allows to realize systems without the need of complex sources or difficult tracking mechanisms. Finally, we will also illustrate how to analyze, both in frequency and time domain, the network representations used for WPT

Exploitation of MOSFET based AC switches in capacitive impedance matching networks in inductive wireless power transfer systems

This paper investigates the exploitation of MOSFET based AC switches in capacitive impedance matching networks (IMN) for inductive wireless power transfer (WPT). The IMN optimum capacitance has been chosen for a 200 kHz resonant frequency. The activation of tuning capacitor on the tuning branch is achieved with the use of MOSFET AC switches in order for the WPT system to achieve maximum power transfer efficiency. The MOSFET AC switch is modelled as an internal parasitic resistor and capacitor connected in parallel. A WPT analytical model is developed to study the effects of the MOSFET’s parasitic elements on the WPT system’s efficiency and is verified experimentally. Various MOSFET switches and relays have been implemented as the IMN switching elements and compared when tested under the same conditions. It is concluded that MOSFET AC switches which have low on-resistances and small parasitic capacitances are desirable as they have a smaller impact on the efficiency. Additionally, parasitic capacitances of MOSFET AC switches need careful consideration for different resonant frequencies as they can affect the overall IMN tuning capacitance especially when they are turned off. Comparing to commonly used relay switches, MOSFET based AC switches have similar switching performance but are significantly smaller in size

Impact of Coil Misalignment in Data Transmission over the Inductive Link of an EV Wireless Charger

The penetration rate of electric vehicles (EVs) will experience a relative increment in the future, so easy to use ways to recharge will be demanded. In this sense, wireless charging represents a safe charging method that minimises user intervention. In a similar way to conductive charge, wireless charging requires some information exchange between the charger primary side and secondary side (battery) for safety and operational reasons. Thus, wireless chargers depend on a communication system for their controlled and correct operation. This paper analysed the communication performance of a wireless EV charger in which the communiction device is part of the wireless power transfer system. Particularly, this work studies how the communication system reacts to power coil displacements, which commonly occur in their conventional performance. The results show that the compensation topology selected to ensure the resonant operation clearly impacts on the communication performance. In particular, the theoretical model and the simulation results demonstrate that the asymmetrical compensation topologies are more stable in terms of the wireless communication channel capacity

Radio frequency energy harvesting for autonomous systems

A thesis submitted to the University of Bedfordshire in partial fulfilment of the requirements for the degree of Doctor of PhilosophyRadio Frequency Energy Harvesting (RFEH) is a technology which enables wireless power delivery to multiple devices from a single energy source. The main components of this technology are the antenna and the rectifying circuitry that converts the RF signal into DC power. The devices which are using Radio Frequency (RF) power may be integrated into Wireless Sensor Networks (WSN), Radio Frequency Identification (RFID), biomedical implants, Internet of Things (IoT), Unmanned Aerial Vehicles (UAVs), smart meters, telemetry systems and may even be used to charge mobile phones. Aside from autonomous systems such as WSNs and RFID, the multi-billion portable electronics market – from GSM phones to MP3 players – would be an attractive application for RF energy harvesting if the power requirements are met. To investigate the potential for ambient RFEH, several RF site surveys were conducted around London. Using the results from these surveys, various harvesters were designed and tested for different frequency bands from the RF sources with the highest power density within the Medium Wave (MW), ultra- and super-high (UHF and SHF) frequency spectrum. Prototypes were fabricated and tested for each of the bands and proved that a large urban area around Brookmans park radio centre is suitable location for harvesting ambient RF energy. Although the RFEH offers very good efficiency performance, if a single antenna is considered, the maximum power delivered is generally not enough to power all the elements of an autonomous system. In this thesis we present techniques for optimising the power efficiency of the RFEH device under demanding conditions such as ultra-low power densities, arbitrary polarisation and diverse load impedances. Subsequently, an energy harvesting ferrite rod rectenna is designed to power up a wireless sensor and its transmitter, generating dedicated Medium Wave (MW) signals in an indoor environment. Harvested power management, application scenarios and practical results are also presented