Matthew J. Chabalko

Experimental demonstration of the equivalence of inductive and strongly coupled magnetic resonance wireless power transfer

In this work, we show experimentally that wireless power transfer (WPT) using strongly coupled magnetic resonance (SCMR) and traditional induction are equivalent. We demonstrate that for a given coil separation, and to within 4%, strongly coupled magnetic resonance and traditional induction produce the same theoretical efficiency of wireless power transfer versus distance. Moreover, we show that the difference between traditional induction and strongly coupled magnetic resonance is in the implementation of the impedance matching network where strongly coupled magnetic resonance uses the mini-loop impedance match. The mini-loop impedance mach provides a low-loss, high-ratio impedance transformation that makes it desirable for longer distance wireless power transfer, where large impedance transformations are needed to maximize power transfer.

Long Range Passive UHF RFID System Using HVAC Ducts

In this paper, the use of hollow metal heating, ventilating, and air-conditioning (HVAC) ducts as a potential communication channel between passive ultrahigh-frequency (UHF) radio-frequency identification (RFID) readers and tags is studied. HVAC ducts behave as electromagnetic waveguides with much lower signal attenuation compared to free-space propagation. This low-loss electromagnetic environment allows one to greatly increase the communication range of passive UHF RFID systems and build, for example, a long range passive sensor network spanning an entire infrastructure such as a large building. In this work, it is shown both theoretically and experimentally that the read range of passive UHF RFID systems can be increased by multiple times compared to operation in a free-space environment.

On the efficient wireless power transfer in resonant multi-receiver systems

In this paper we present an analysis of resonant wireless power transfer in systems with multiple receivers. We show that maximum power transfer can be achieved when the source is impedance matched to the set of receivers, i.e. matched to their equivalent impedance as seen by the source. The interaction of the receivers, or coupled modes, simply represent an interdependence of impedances that can be modeled and impedance matched. We explore three methods to achieve impedance matching: frequency tuning, impedance transformation and resonant tuning and show that the later two can achieve the maximum theoretical power transfer for a wide range of coupling between receivers.

Tri-Loop Impedance and Frequency Matching With High-

Resonant wireless power transfer (WPT) using magneto quasi-static fields is an attractive means for delivering power in many applications. Central to the efficient delivery of power in these systems is the use of high-ratio impedance transformers to impedance-match the load to the source via the WPT network. Mini-loop transformers, which use a resonant tank for impedance matching, have become popular as they provide high-ratio impedance matching and low loss. Key to the low loss is the use of high- Q resonators. There are two challenges with this approach. The first is impedance matching the resonators to source/load impedances, and the second is adjusting the center frequency of resonance to exactly match the operating frequency, which can be very difficult with helical coils that use their parasitic capacitance for resonance. In this letter, we introduce a tri-coil impedance-matching network for WPT that provides accurate impedance matching and precise frequency tuning for high- Q coils to overcome these limitations.

Q

Maximum power transfer and maximum efficiency are two important design constraints in wireless power transfer applications. Several works have investigated the proper load and impedance match conditions to optimize either efficiency or power transfer. In this paper we show that the optimal load for maximum power transfer and maximum efficiency is the same (a conjugate matched load) when the source resistance is zero. This is important, as many WPT systems have a relatively low, unknown source impedance. Since the optimal load for both efficiency and power is the same as the source impedance approaches zero, the designer can use a bi-conjugate load for a near optimal design for both maximum power and efficiency. As the source impedance becomes significant, the bi-conjugate matched system provides higher power, but at the expense of lower efficiency. Maximum efficiency is achieved with a non-bi-conjugate load, when the source impedance is non-negligible.

Resonators in Wireless Power Transfer

Resonantly coupled wireless power transfer (RWPT) has become a popular means to deliver energy without direct contact between the source and load. One challenging application is nonstationary loads; those that move spatially in time. Such loads change the coupling between the source and load and with it the efficiency and maximum power transfer possible. One emerging application is in the automotive environment, where nonstationary loads such as powered seats and doors exist. Moreover, the automotive environment is particularly challenging due to the presence of metallic objects and the safety requirements of the passengers. In this work, we examine RWPT for nonstationary loads and present a design methodology for optimal efficiency and power transfer and show an RWPT of 70 W across a 24 cm distance in an automotive environment. We also examine the impact of the metallic environment and show how its effects can be mitigated. Finally, we examine the field intensity during RWPT and examine the safety of the passengers. We show that 70 W can be transmitted within 10 cm of a passenger while operating below safety limits.

Optimization of WPT efficiency using a conjugate load in non-impedance matched systems.

In this paper, we present a time reversal SAR (TR-SAR) algorithm to improve the target images and reduce the ghost images by utilizing different polarizations in an environment filled with trees.

Resonantly Coupled Wireless Power Transfer for Non-Stationary Loads With Application in Automotive Environments

In this work, we report on the experimental demonstration of magnetoquasistatic volume wave resonances in a 2-dimensional near field metamaterial (MM). Previous works have described only theoretically the magnetostatic waves in near field MMs and have reported peaks and valleys in the mutual coupling of MM enhanced wireless power transfer that they have attributed to Fabry-Perot resonances, however, neither has been experimentally measured nor characterized. We report on the direct magnetic field measurement of magnetostatic volume waves in a 2D near-field MM and show that the periodic peaks and valleys in mutual coupling observed previously are indeed due to a Fabry-Perot oscillation. In addition, we show that these resonances can be predicted from experimentally extracted permeability and the dimensions of the system.

Polarization Sensitive time reversal SAR imaging in an environment filled with trees

Existing methods for permeability measurement of metamaterials have been limited to far-field applications, where the fields interacting with the metamaterial are plane waves. In near-field applications, such as wireless power transfer, the previous plane-wave approach is not viable. In this work, we present a simple and accurate method for extracting the complex permeability of a near-field metamaterial.

Experimental characterization of Fabry-Perot resonances of magnetostatic volume waves in near-field metamaterials

In this work we examine the effects of near field metamaterials (NF-MM) on the quality factor (Q) of an excitation (i.e. source) coil. We show that when the permeability becomes negative, magneto-static resonances (MR) occur that are coupled strongly to the excitation coil and increase the loss in the coil. Our simulation and experimental results show that Q can be degraded by a factor of 4 or more due to the MR induced loss in an excitation coil.