Doctor of Philosophy

dissertationMagnetic fields are permeable to the biological tissues and can induce electric field in the conductive structures. Some medical devices take advantage of this ability to transfer energy from the source to the receiving site without direct contact. Prosthetic devices such as retinal implants use time-varying magnetic field to achieve wireless power transfer to the implanted magnetic coil. However, devices such as magnetic stimulators use the induction principle to create an electric field at the stimulation site. Efficiency of these devices is primarily dependent on the design of the magnetic coils. Therefore, in this work, we designed and validated efficient magnetic coils for wireless power transfer to implanted devices and magnetic stimulation of the peripheral nerves. Typical wireless power transfer (WPT) systems uses two-coil based design to achieve contactless power transfer to the implanted electronics. These systems achieve low power transfer efficiency (< 30%) and frequency bandwidth. Moreover, efficient wireless system requires high coupling and load variation tolerance during device operation. To design an electromagnetic safe WPT system, the power absorbed by the tissue and radiated field due to the proximal magnetic coils needs to be minimized. In this work, we proposed a multi-coil power transfer system which solves some of the current challenges. The proposed multi-coil WPT system achieves more than twice the power transfer efficiency, controllable voltage gain, wider frequency bandwidth, higher tolerance to coupling and load variations, lower absorbed power in the tissue and lower radiated field from the magnetic coil than a comparable two-coil system. In this work, we have developed analytic models of the multi-coil WPT system and validated the accuracy of the solutions using experiments. Magnetic coils play an important role in controlling the distribution of induced electric field inside the nerve during magnetic stimulation. In the past, homogeneous models were used to estimate the field profile inside conductive tissue due to the time varying current in the magnetic coil. Moreover, the effect of the surrounding media and stimulation mechanisms was understudied, which limits the optimization accuracy of the magnetic coils. In this work, we developed anatomically correct tissue models to study the effect of tissue heterogeneity and the surrounding media on the induced electric field. We also developed an optimization algorithm for designing energy efficient cm-size magnetic coils, that were then used for ex-vivo magnetic stimulation of the frog's sciatic nerve

Doctor of Philosophy

dissertationMore than a century ago, in his labs in Colorado Springs and New York, Nikola Tesla started experimenting with wireless power transfer (WPT). His ideas were ahead of his time, but they fell into obscurity shortly after his death. Nowadays, WPT is no longer thought of as science fiction: neural prostheses, wearables, cellphones, and even electric vehicles can be powered through WPT. In its most common implementation, WPT leverages the magnetic coupling between resonant transmitter and receiver coils to exchange energy. Considerable work is devoted to the design and optimization of WPT antennas; efficiently transmitting the required amount of power can only be accomplished when the coil coupling is in the right range. In this work, we explore the use of spatial filters in WPT systems. Spatial filters are capable of controlling the harmonic content of an incident wave: subwavelength focal spots, perfect lensing, and diffractionless beams are some of their uses. In the first part of this dissertation, the focus is on the analysis and design of a compact negative permeability metamaterial slab. Compared to other works in the literature, this slab is an extremely small fraction of the wavelength, and works at a low operating frequency. Analysis and experimental validation demonstrate that the resulting metamaterial sample can be used in a 2-coil WPT system to achieve large range and efficiency enhancements. In the second part of this dissertation, the analysis and implementation of holographic screens is presented. A method to reduce the fabrication complexity of the desired holographic screen while maintaining the fidelity of the prescribed field distribution is presented. We demonstrate our method through the analysis, design, and experimental validation of a nondiffractive beam launcher with a Bessel field distribution. Finally, we utilize the analysis and methods presented in this work to design an antenna capable of prescribing a uniform field distribution. Because of this property, this WPT antenna is capable of transmitting near constant power with near constant efficiency to the target receiver load, without the need of an adaptive compensation system

Optimum Modelling Of Flux-pipe Resonant Coils For Static And Dynamic Bidirectional Wireless Power Transfer System Applicable To Electric Vehicles

Wireless power transfer (WPT) technology enables the transfer of electrical power from the electric grid to the electric vehicles across an airgap using electromagnetic fields with the help of wireless battery chargers. WPT technology addresses most problems associated with the “plug-in” method of charging EVs like vandalization, system power losses, and safety problems due to hanging cables and opened electrical contact in addition to the flexibility of charging electric vehicles while in a static or dynamic mode of operation. Significant research has been undertaken over the years in the development of efficient WPT topologies applicable to electric vehicles. A preliminary review of these revealed that the ferrite core WPT is a promising and efficient method of charging electric vehicles. The charging method is suitable for wireless charging of electric vehicles because of its low cost, high efficiency and high power output. This research proposed the use of the flux-pipe model as a suitable ferrite core, magnetic resonance coupled-based WPT system for the charging of the electric vehicle. The traditional flux-pipe model has some specific benefits which include high coupling coefficient, high misalignment tolerance and high efficiencies under misalignment conditions. However, it has a major drawback of low power output due to the generation of an equal amount of useful and non-useful fluxes. A set of governing equations guiding the performance output of a WPT system was presented. It was identified that the losses in the WPT system can be minimized by reducing the value of the maximum magnetic flux density while the power output and efficiency can be increased by increasing the value of the coupling factor and quality factor. Based on these findings, 3-D finite element modelling was employed for the optimal design and analysis of a typical flux-pipe model for higher coupling strength, high power output and low losses. The magnetic coupling performance of flux-pipe resonant coils was enhanced with an increased number of turns along the core length relative to increasing the width of each coil turns along the coil width. The high power transfer and efficiency was attained by splitting of the coil windings into two in order to reduce intrinsic coil resistances; copper sheet was employed as a shielding material in order to reduce the eddy current losses and finally, an air gap was introduced in the ferrite core in order to reduce the core losses and invariably increased the amount of excitation current required to drive the core into saturation. The proposed optimization methodology results in the creation of two models for application in static and dynamic charging operations respectively. From the simulation results presented, the model designed for static charging operations can transfer up to 11 kW of power across the airgap at a coil-to-coil efficiency of 99.12% while the model design for dynamic charging of electric vehicles can transfer up to 13 kW of power across the airgap at a coil-to-coil efficiency of 98.64% without exceeding the average limit specified for the exposure of human body to electromagnetic fields

Wireless Power Transfer For Biomedical Applications

In this research wireless power transfer using near-field inductive coupling is studied and investigated. The focus is on delivering power to implantable biomedical devices. The objective of this research is to optimize the size and performance of the implanted wireless biomedical sensors by: (1) proposing a hybrid multiband communication system for implantable devices that combines wireless communication link and power transfer, and (2) optimizing the wireless power delivery system. Wireless data and power links are necessary for many implanted biomedical devices such as biosensors, neural recording and stimulation devices, and drug delivery and monitoring systems. The contributions from this research work are summarized as follows: 1. Development of a combination of inductive power transfer and antenna system. 2. Design and optimization of novel microstrip antenna that may resonate at different ultra-high frequency bands including 415 MHz, 905 MHz, and 1300MHz. These antennas may be used to transfer power through radiation or send/receive data. 3. Design of high-frequency coil (13.56 MHz) to transfer power and optimization of the parameters for best efficiency. 4. Study of the performance of the hybrid antenna/coil system at various depths inside a body tissue model. 5. Minimizing the coupling effect between the coil and the antenna through addressed by optimizing their dimensions. 6. Study of the effects of lateral and angular misalignment on a hybrid compact system consisting of coil and antenna, as well as design and optimize the coilâs geometry which can provide maximum power efficiency under misalignment conditions. 7. Address the effects of receiver bending of a hybrid power transfer and communication system on the communication link budget and the transmitted power. 8. Study the wireless power transfer safety and security systems

Design of Wireless Power Transfer and Data Telemetry System for Biomedical Applications

With the advancement of biomedical instrumentation technologies sensor based remote healthcare monitoring system is gaining more attention day by day. In this system wearable and implantable sensors are placed outside or inside of the human body. Certain sensors are needed to be placed inside the human body to acquire the information on the vital physiological phenomena such as glucose, lactate, pH, oxygen, etc. These implantable sensors have associated circuits for sensor signal processing and data transmission. Powering the circuit is always a crucial design issue. Batteries cannot be used in implantable sensors which can come in contact with the blood resulting in serious health risks. An alternate approach is to supply power wirelessly for tether-less and battery- less operation of the circuits.Inductive power transfer is the most common method of wireless power transfer to the implantable sensors. For good inductive coupling, the inductors should have high inductance and high quality factor. But the physical dimensions of the implanted inductors cannot be large due to a number of biomedical constraints. Therefore, there is a need for small sized and high inductance, high quality factor inductors for implantable sensor applications. In this work, design of a multi-spiral solenoidal printed circuit board (PCB) inductor for biomedical application is presented. The targeted frequency for power transfer is 13.56 MHz which is within the license-free industrial, scientific and medical (ISM) band. A figure of merit based optimization technique has been utilized to optimize the PCB inductors. Similar principal is applied to design on-chip inductor which could be a potential solution for further miniaturization of the implantable system. For layered human tissue the optimum frequency of power transfer is 1 GHz for smaller coil size. For this reason, design and optimization of multi-spiral solenoidal integrated inductors for 1 GHz frequency is proposed. Finally, it is demonstrated that the proposed inductors exhibit a better overall performance in comparison with the conventional inductors for biomedical applications

AC loss modelling in high temperature superconducting coils for EV wireless power transfer application

With the rapid development of high temperature superconducting (HTS) technology, new second generation (2G) HTS materials are promising to replace the traditional materials in the power transmission industry. Since the 2010s, the topic of using HTS materials in electric vehicles (EVs) wireless power transfer (WPT) systems has received widespread attention from research groups around the world. When cooled down to a particular cryogenic temperature, superconductivity offers near-zero resistance to electric current. Meanwhile, using superconducting material in different power applications can improve the overall effciencies. In the WPT system, HTS coils could achieve high quality factors and power densities compared to traditional resonant coils such as copper and litz wire coils, which lead to a decreasing in coil size and weight. However, the optimal working frequency for the EVs WPT is signi cantly higher in terms of kilohertz level. HTS coils working under high frequency transport current could generate AC losses, thus increasing the cooling power and reducing the overall effciency of the WPT system. This thesis aims to study the AC loss characteristics and coil design optimisations for HTS coils working at EVs WPT frequency range when subjected to various input conditions. At the outset of this thesis, the first chapter reviews the EVs WPT systems and its current research combining the HTS technology. Subsequently, a review of the superconducting theory and the AC loss fundamental are provided in the second chapter. Following the literature review, the fi nite element modelling method for the HTS coated conductor (CC) simulated in this thesis is presented. The fi nite element modelling method used in this thesis is developed based on a set of equations referred to as the H formulation to solve the magnetic fi eld component. To perform a thorough investigation of the AC loss characteristics and the electromagnetic properties in the HTS coils at EVs WPT frequency range, the multi-layer numerical model, which considers all layers within the HTS CC is constructed in this thesis. As a starting point, the two-dimensional (2D) HTS tape model is built. The single tape model is then extended to the 2D axisymmetric HTS coil model. The simulation results from the HTS tape and coil models are validated by analytical calculations and the published measurement data from the literature at both low and high frequency ranges. Subsequently, the model-building process of three commonly used HTS coil con figurations and the simulation results concerning the transport loss, magnetisation loss and total loss under high frequency transport current, external magnetic field and combined are presented and discussed in detail. In particular, the weak turn analysis within different coil con figurations and the transition frequency study under these conditions are investigated. Consequently, an efficiency model is also proposed to evaluate the performance of three HTS coil confi gurations under WPT conditions, followed by a discussion regarding three coil optimisation designs. Finally, two AC loss mitigation methods are studied and analysed. The AC loss characteristics and the turn loss distributions for three HTS coils at different inter-turn spacings and tape widths are simulated and discussed. Further, the impact of these two loss reduction methods on the performance of the WPT system is estimated and evaluated based on the efficiency model. The outcome of this research can be used to provide information for designing HTS coils with lower AC losses for EVs WPT applications

A High Frequency Wireless Power Transfer System for Electric Vehicle Charging Using Multi-layer Non-uniform Self-resonant Coil

Wireless EV (Electric Vehicle) charging is an emerging technology with rapid development in the past decade. Compared to wired EV chargers, wireless power transfer (WPT) enables safe and unobtrusive charging for EVs. This work proposes high frequency wireless charging using a self-resonant (SR) coil at several megahertz. A multi-layer self-resonant coil structure is proposed, allowing high quality factor coils to be fabricated from layers of inexpensive copper foil and dielectric film. Additionally, the self-resonant coil utilizes its interlayer capacitance for resonance, eliminating the external compensation capacitor and shrinking the overall volume of passive component to increase the power density. Comparing to other self-resonant coils in the literature, it exhibits the characteristics of achieving high quality factor and high inductance simultaneously. Prototype coils with 200 mm radius are fabricated and tested, achieving quality factor over 450 at 3 MHz. The fabricated air-core coil structure is low-cost and lightweight, with 200 mm radius, 3 mm thickness and only 2 oz copper traces. The power stages, including GaN (Gallium Nitride) transistor based inverter and SiC (Silicon Carbide) diode based rectifier, are designed with emphasis on reduction of PCB (Printed Circuit Board) layout parasitics. Experimental tests show 95.2% dc-dc efficiency with 6.6 kW power transferred across a 100 mm coil-to-coil distance. The power density is 52.5 kW/m2, without need for any external compensation components. This work validates the concept of high frequency compact WPT system for EV. Practical shielding design is proposed for the WPT system with self-resonant coils, considering the high frequency parallel resonance effect. Complete coil pads are fabricated and assembled, incorporating the ferrite cores, PTFE (Polytetrafluoroethylene) spacer, and aluminum plate. The system is validated with shielded SR coils, achieving 92.3% DC-DC efficiency and 7.1 kW/dm3 volumetric power density. This work demonstrates the first 6.6-kW WPT system using compact self-resonant coils with practical shielding implementation. The concept of proposed multi-layer self-resonant coil is extended to other possible structures. Different multi-layer self-resonant coil structures are compared and analyzed, giving design guidelines for their capabilities at different system operating frequencies

Emerging Works on Wireless Inductive Power Transfer: AUV Charging from Constant Current Distribution and Analysis of Controls in EV Dynamic Charging

Wireless power transfer through inductive coupling, termed as inductive power transfer (IPT), is one of the important technologies in power electronics that enable transfer of power between entities without physical connections. While it has seen significant growth in the areas such as electric vehicle charging, phone charging and biomedical implants, its emerging applications include charging of autonomous underwater vehicles (AUVs) and dynamic charging of electric vehicles from the roadway. This dissertation addresses a few key challenges in these areas of IPT applications, paving the way for future developments. For the WPT for AUV, the recently developing sea-bed installed marine systems are targeted, which typically gets power from on-shore sources through constant dc low-current distribution. As the present underwater IPT topologies are not suitable for such applications, this dissertation proposes underwater IPT topologies to interface directly with such constant current distribution and provide a constant voltage output supply to the on-board systems inside the AUVs. The considerations for seawater losses and the small-signal models for control purposes are also addressed. Analysis and experimental results are provided for validations of the analytical designs and models. In the area of electric vehicle dynamic wireless power transfer (EV DWPT), the comparison of control performances of different coupler, compensation topologies and control implementations are addressed. The effect of communication latency on control bandwidth are also addressed. The outcomes are presented through analysis and simulations, and based on that future research recommendations are made to pave way for future commercial developments of well regulated and interoperable EV DWPT systems

Cost Effective, Highly Efficient Wireless Power Transfer Systems for EV Battery Charging

The impact of changing inner diameter of wireless power transfer (WPT) coils on coupling coefficient is studied. It is demonstrated that at a certain outer and inner coil diameter, turn space variation has minor effect on the coupling coefficient. Next, two compensation networks, namely primary LCC and secondary LCC, which offer load-independent voltage transfer ratio and zero voltage switching for WPT, are presented. For both compensation networks, the condition for having zero voltage switching operation are derived. In addition, load-independent voltage transfer ratio (VTR) frequencies are obtained and VTR at each frequency is derived. Then, required equations for calculation of WPT system efficiency based on its equivalent circuit are presented. Eventually, by defining a time-weighted transfer average efficiency (TWTAE), and based on measured values of resistance and inductance of a WPT prototype and experimental charging curve of a Li-ion battery, a design procedure for both compensation networks is proposed. The proposed design leads to high TWTAE as well as low material usage. Simulation and experimental results verify the superiority of proposed coil and compensation design compared to conventional one