Non-radiative Wireless Power Transfer Research Articles

A Non-Radiative Wireless Power Transfer System for Medical Implant Deployment: An Experimental Validation

A Non-Radiative Wireless Power Transfer System for Medical Implant Deployment: An Experimental Validation

Rotation manipulation of high-order PT-symmetry for robust wireless power transfer

Magnetic resonance coupling plays an important role in the non-radiative wireless power transfer without cables. However, the basic near-field coupling also has some key limitations on the resonance wireless power transfer (WPT). On the one hand, when the transfer distance is short, the eigenfrequencies of the WPT system will split because of the strong near-field coupling between the resonance transmitter and receiver. On the other hand, although the working frequency is fixed for the long-distance case, the transfer efficiency will be significantly dropped for the weak coupling strength. Therefore, a long-captivated problem of the resonance WPT systems is that it is difficult to balance the fixed working frequency and high efficiency of the devices. In this work, we put forward the composite transmitter with a pair of resonance coils of different size. By assisting the rotational degrees of freedom in the composite transmitter, a high-order parity-time-symmetric non-Hermitian system can be established. Especially, in contrast to the conventional resonance WPT scheme, the robust WPT can maintain fixed working frequency and efficiency over a varying transfer distance in the optimized WPT system with composite transmitter, since this eigenfrequency is insensitive to the operational conditions. Our findings demonstrate that the composite transmitter possesses better performance for WPT than its single counterpart, offering the opportunities for engineering novel designer electromagnetic transfer states with unique robustness.

Frequency-Stable Robust Wireless Power Transfer Based on High-Order Pseudo-Hermitian Physics.

Nonradiative wireless power transfer (WPT) technology has made considerable progress with the application of the parity-time (PT) symmetry concept. In this Letter, we extend the standard second-order PT-symmetric Hamiltonian to a high-order symmetric tridiagonal pseudo-Hermitian Hamiltonian, relaxing the limitation of multisource/multiload systems based on non-Hermitian physics. We propose a three-mode pseudo-Hermitian dual-transmitter-single-receiver circuit and demonstrate that robust efficiency and stable frequency WPT can be attained despite the absence of PT symmetry. In addition, no active tuning is required when the coupling coefficient between the intermediate transmitter and the receiver is changed. The application of pseudo-Hermitian theory to classical circuit systems opens up an avenue for expanding the application of coupled multicoil systems.

Perfect-Lens Theory Enables Metasurface Reflectors for Subwavelength Focusing

Breaking the so-called diffraction limit on the resolution of optical devices and achieving subwavelength focusing requires tailoring the evanescent spectrum of wave fields. There are several possible approaches, all of which have limitations, such as the generation of strong additional scattering, limited focusing power, issues at the implementation step, and the need for a drain at the focal point. This paper presents a feasible strategy based on the concepts of the perfect lens and power flow-conformal metasurfaces. Desired fields for subwavelength focusing are integrated using double-negative media and then the surface profile of a focusing reflector is designed to be tangential to the desired power flow, so that the metasurface can be modeled as a local impedance boundary, and can be easily implemented using passive and lossless elements. Full-wave simulations demonstrate that an example reactive metasurface is able to break the diffraction limit and provide near-field focusing with subwavelength hotspot size. We expect that the outcome will find applications in antennas, beam-shaping devices, nonradiative wireless power transfer systems, microscopy, and lithography.

Demonstration of topological wireless power transfer

Recent advances in non-radiative wireless power transfer (WPT) technique essentially relying on magnetic resonance and near-field coupling have successfully enabled a wide range of applications. However, WPT systems based on double resonators are severely limited to short- or mid-range distance, due to the deteriorating efficiency and power with long transfer distance. WPT systems based on multi-relay resonators can overcome this problem, which, however, suffer from sensitivity to perturbations and fabrication imperfections. Here, we experimentally demonstrate a concept of topological wireless power transfer (TWPT), where energy is transferred efficiently via the near-field coupling between two topological edge states localized at the ends of a one-dimensional radiowave topological insulator. Such a TWPT system can be modelled as a parity-time-symmetric Su-Schrieffer-Heeger (SSH) chain with complex boundary potentials. Besides, the coil configurations are judiciously designed, which significantly suppress the unwanted cross-couplings between nonadjacent coils that could break the chiral symmetry of the SSH chain. By tuning the inter- and intra-cell coupling strengths, we theoretically and experimentally demonstrate high energy transfer efficiency near the exceptional point of the topological edge states, even in the presence of disorder. The combination of topological metamaterials, non-Hermitian physics, and WPT techniques could promise a variety of robust, efficient WPT applications over long distances in electronics, transportation, and industry.

Stacked-Coil Technology for Compensation of Lateral Misalignment in Nonradiative Wireless Power Transfer Systems

A stacked-coil technology is demonstrated that realizes size-adjustable coil systems to compensate the notorious problem of lateral misalignment in nonradiative wireless power transfer (WPT) systems. The proposed system has a simple structure that consists of multiple layers of coils with various sizes. Depending on the misalignment, the coil pair that provides the best transfer efficiency is switched on. Thus, not only the transfer null is removed, but also a very high transfer efficiency is maintained even with extreme misalignment. Experimental results at 6.78 MHz show the effectiveness of the stacked coil for both short- and mid-range wireless power transfer systems. The transfer efficiency maintains abovementioned 80% up to 87% and 65% lateral misalignment when the separation between Tx and Rx coils are 10% and 50%, respectively, relative to the largest dimension of the coil. The three-layer short-range system successfully removes the transfer null to maintain a minimum transfer efficiency of 39.7% up to 126% misalignment. The two-layer mid-range system maintains transfer efficiency abovementioned 50% up to 103% misalignment. Furthermore, the performance is virtually the same even in an asymmetric WPT system, i.e., when the technique is applied to only one of the two coils. A detailed design procedure is also provided.

Improvement of Magnetic Field for Near-Field WPT System Using Two Concentric Open-Loop Spiral Resonators

This letter introduces two concentric open-loop spiral resonators (OLSRs) that are used to improve magnetic field for nonradiative wireless power transfer (WPT) systems. OLSRs are fed through metal–insulator–metal (MIM) capacitive coupling using a 50- $\Omega $ microstrip transmission line. First, a single OLSR is designed and implemented for WPT, then two OLSRs are used instead of a single OLSR to emphasize the surface current on the spiral resonators. Therefore, it helps to intensify the electromagnetic field in order to get a high transmission distance or higher efficiency. The proposed WPT system operates at 438.5 MHz with a measured power transfer efficiency (PTE) of 70.8% at a transmission distance of 31 mm and a design area of 576 mm2. An equivalent circuit of the proposed WPT system is presented as a heuristic approach to show the electrical behavior of the WPT system.

Design and analysis of 2-coil wireless power transfer (WPT) using magnetic coupling technique

2-coil non-radiative wireless power transfer (WPT) is studied to find the coil diameter ratio to effective distance of power transfer efficiency (PTE). Single circular coil and spiral coil are designed and simulated using CST software to compare the result of coil diameter versus effective distance of PTE by using S<sub>21</sub> value. Accordingly, the quality factor (Q) of both coils are presented as Q factor is one of the parameter that affect the performance of WPT system. The result is promising as the effective distance is more than the coil diameter with (PTE) more than 50% using spiral coil as compare to single coil design.

Dynamic Capabilities of Multi-MHz Inductive Power Transfer Systems Demonstrated With Batteryless Drones

This paper presents the design of a multi-MHz inductive power transfer (IPT) system showcasing lightweight and energy-efficient solutions for non-radiative wireless power transfer. A proof of concept is developed by powering a drone without a battery that can hover freely in proximity to an IPT transmitter. The most challenging aspect, addressed here for the first time, is the complete system-level design to efficiently provide uninterrupted power flow while allowing for variable power demand and highly variable coupling factor. The proposed solution includes the design of lightweight air-core coils that can achieve sufficient coupling without degrading the aerodynamics of the drone, and the design of newly developed resonant power converters at both ends of the system. At the transmitting-end, a load-independent Class EF inverter, which can drive a transmitting-coil with constant current amplitude and achieves zero-voltage switching for the entire range of operation, was developed; and at the receiving-end, a hybrid Class E rectifier, which allows tuning for large changes in coupling and power demand, was used. For the demo, the range of motion of the drone was limited by a 7.5 cm nylon string tether, connected between the center of the transmitting-coil and the bottom of the drone. The design of the IPT system, including all the power conversion stages and the IPT link, is explained in detail. The results on performance and specific practical considerations required for the physical implementation are provided. An average end-to-end efficiency of 60% was achieved for a coupling range of 23%-5.8%. Relevant simulations concerning human exposure to electromagnetic fields are also included to assure that the demo is safe, according to the relevant guidelines. This paper is accompanied by a video featuring the proposed IPT system.

Circuit Analysis of Achievable Transmission Efficiency in an Overcoupled Region for Wireless Power Transfer Systems

On nonradiative wireless power transfer (WPT) using a pair of magnetically coupled resonators, we investigate the achievable transmission efficiency for a different set of resonators. Using a simplified equivalent circuit model, we analyze the phenomenon of frequency splitting mathematically in the overcoupled region ( $k > k_{\rm {ref}}$ ). Even though the transmission efficiency tends to gradually decrease as the distance between the two resonators gets closer and closer in the overcoupled region, the transmission efficiency can be improved by tracking one of two separated frequency modes. Based on this observation, we prove mathematically that the achievable transmission efficiency at $k_{\rm {ref}}$ is maintained almost constantly even when $k > k_{\rm {ref}}$ if the identical resonators are used. In order to validate the accuracy of the analysis, we design and fabricate two types of magnetically coupled resonators: one is the set of identical resonators and the other is the set of nonidentical resonators. Through experiments in various environments, we show that the analytical results are in good agreement with the measured ones. Our work can provide an insight into design resonators for short-range WPT systems, in order to ensure better and stable performance.

Analysis of coupling between magnetic dipoles enhanced by metasurfaces for wireless power transfer efficiency improvement

In this paper, we investigate the possibility of improving efficiency in non-radiative wireless power transfer (WPT) using metasurfaces embedded between two current varying coils and present a complete theoretical analysis of this system. We use a point-dipole approximation to calculate the fields of the coils. Based on this method, we obtain closed-form and analytical expressions which would provide basic insights into the possibility of efficiency improvement with metasurface. In our analysis, we use the equivalent two sided surface impedance model to analyze the metasurface and to show for which equivalent surface impedance the WPT efficiency will be maximized at the design frequency. Then, to validate our theory, we perform a full-wave simulation for analyzing a practical WPT system, including two circular loop antennas at 13.56 MHz. We then design a metasurface composed of single-sided CLSRRs to achieve a magnetic lensing based on the calculated equivalent surface impedance. The analytical results and full-wave simulations indicated non-radiative WPT efficiency improvement due to amplifying the near evanescent field which can be achieved through inserting the proposed metasurface.

Power Transfer Efficiency Analysis of Intermediate-Resonator for Wireless Power Transfer

We investigate the effect of intermediate resonators (i.e., intermediate receiver (i-Rx) or relay) on the power transfer efficiency (PTE) for nonradiative wireless power transfer (WPT) with transmitter (Tx), intermediate-resonators, and end receiver (e-Rx). Specifically, we consider WPT systems with two different types of an intermediate resonator: 1) WPT relay systems, where the relay has no load resistor and just forwards the power from the Tx to the e-Rx, and 2) WPT i-Rx systems, where both i-Rx and e-Rx have a load resistor each and the power transmitted by the Tx is consumed at each Rx. Using an equivalent circuit model, we derive a closed-form solution for representing the optimal coupling coefficients between the Tx and the intermediate resonator for a given placement of the intermediate resonator and the e-Rx, i.e., $k_{12,\rm {opt}}$ for WPT i-Rx systems and $k_{1r,\rm {opt}}$ for WPT relay systems, respectively. The analytical result indicates that the quality factors of resonators have a great effect on determining their optimal positions. We also provide performance comparisons between the considered WPT systems. From the result, it is observed that $k_{12,\rm {opt}}$ is always larger than $k_{1r,\rm {opt}}$ , which indicates that the optimal position of the Tx is closer to the i-Rx rather than the relay. Moreover, in this case, WPT i-Rx systems can attain a higher PTE than WPT relay systems. Performing experiments under a variety of scenarios, we verify that the analytical results are in concordance with the measured ones.

Robust wireless power transfer using a nonlinear parity-time-symmetric circuit.

Considerable progress in wireless power transfer has been made in the realm of non-radiative transfer, which employs magnetic-field coupling in the near field. A combination of circuit resonance and impedance transformation is often used to help to achieve efficient transfer of power over a predetermined distance of about the size of the resonators. The development of non-radiative wireless power transfer has paved the way towards real-world applications such as wireless powering of implantable medical devices and wireless charging of stationary electric vehicles. However, it remains a fundamental challenge to create a wireless power transfer system in which the transfer efficiency is robust against the variation of operating conditions. Here we propose theoretically and demonstrate experimentally that a parity-time-symmetric circuit incorporating a nonlinear gain saturation element provides robust wireless power transfer. Our results show that the transfer efficiency remains near unity over a distance variation of approximately one metre, without the need for any tuning. This is in contrast with conventional methods where high transfer efficiency can only be maintained by constantly tuning the frequency or the internal coupling parameters as the transfer distance or the relative orientation of the source and receiver units is varied. The use of a nonlinear parity-time-symmetric circuit should enable robust wireless power transfer to moving devices or vehicles.

Stability Improvement of Transmission Efficiency Based on a Relay Resonator in a Wireless Power Transfer System

We investigate the stability of the transmission efficiency (TE) in nonradiative wireless power transfer (WPT) using a relay resonator. An equivalent circuit model is used to show mathematically that two suppositions are satisfied in an overcoupled region: first, the TE of relay-based WPT systems is always larger than that of conventional two-coil-based WPT systems, and second, as the coupling coefficient increases, the TE of two-coil-based WPT systems decreases severely, while the TE of relay-based WPT systems increases slightly. Consequently, we surmise that an intermediate relay can be used to achieve higher TE and stability, compared with conventional two-coil-based WPT systems. We verify that our analytical results are in good agreement with the experimental ones.

Effect of Quality Factor on Determining the Optimal Position of a Transmitter in Wireless Power Transfer Using a Relay

In this letter, the optimal placement of a transmitter is investigated under a given arrangement of a relay and a receiver for nonradiative wireless power transfer. Specifically, a closed-form equation is derived for the optimal coupling coefficient between the transmitter and the relay using an equivalent circuit model. Based on this theoretical analysis, it is found that the quality factor of the transmitter has a great effect on determining its optimal placement. In particular, the transmission efficiency can be improved by locating the resonator with a smaller quality factor (e.g., transmitter or receiver) closer to the relay. In order to support the validity of the analysis, the resonators, including the relay, receiver, and three different types of transmitters, are experimentally designed and fabricated. It is shown that our analytical results are well matched with the measured ones in various settings.

A Computational Study on the Magnetic Resonance Coupling Technique for Wireless Power Transfer

Non-radiative wireless power transfer (WPT) system using magnetic resonance coupling (MRC) technique has recently been a topic of discussion among researchers. This technique discussed more scenarios in mid-range field of wireless power transmission reflected to the distance and efficiency. The WPT system efficiency varies when the coupling distance between two coils involved changes. This could lead to a decisive issue of high efficient power transfer. This paper presents case studies on the relationship of operating range with the efficiency of the MRC technique. Demonstrative WPT system operates at two different frequencies are projected in order to verify performance. The resonance frequencies used are less than 100MHz within range of 10cm to 20cm.

Past, Present and Future Trends of Non-Radiative Wireless Power Transfer

Past, Present and Future Trends of Non-Radiative Wireless Power Transfer

Wireless power transfer between one transmitter and two receivers: optimal analytical solution

This paper focuses on non-radiative wireless power transfer implemented by means of a resonant magnetic coupling. The case of one transmitter and two receivers is considered and a rigorous analytical procedure is developed demonstrating that maximum power transfer or maximum efficiency can be achieved by appropriately selecting the load values. Both cases of coupled and uncoupled receivers are solved; closed formulas are derived for the optimal loads, which maximize either power or efficiency. It is shown that the resistances that realize maximum power transfer are always greater than the resistances that realize maximum efficiency. According to this observation, an optimal range of operation for the load resistances is also determined. Furthermore, it is demonstrated that in the case where the receivers are coupled the introduction of appropriate compensating reactances allows retrieving the same results corresponding to the uncoupled case both for powers and efficiency. Theoretical data are validated by comparisons with numerical results.

Magnetic Resonance for Wireless Power Transfer [A Look Back

Magnetic resonance has been a cornerstone of nonradiative wireless power transfer (WPT) since the late 19th century. However, some researchers have the misconception that magnetic resonance for WPT was developed recently. This article traces some early work of Tesla and other researchers related to the use of magnetic resonance in WPT. Included are some examples of magnetic resonance-based WPT projects conducted by researchers in the biomedical and power electronics communities over the last few decades. Two principles used in WPT are reiterated in this article, and their advantages and disadvantages are addressed. Some issues that may affect future trends of short- and midrange applications are discussed.

A Method of Using Nonidentical Resonant Coils for Frequency Splitting Elimination in Wireless Power Transfer

In this paper, an efficient method is proposed to eliminate frequency splitting in nonradiative wireless power transfer via magnetic resonance coupling. In this method, two nonidentical resonant coils (NIRCs) are used as wireless power transmitter and receiver, respectively. According to the elliptic integral term in the analytical expression, the pole of the mutual inductance function with respect to transfer distance can be eliminated by using the two NIRCs, and hence overcoupling between transmitter and receiver with close transfer distance is avoided. Therefore, frequency splitting caused by overcoupling can be suppressed and stable output power can be achieved. The NIRCs are analytically calculated, numerically simulated and finally, fabricated and tested to verify the theory. All the calculated and experimental results show that frequency splitting is completely eliminated and uniform voltage across the load is achieved. Furthermore, lateral misalignment between the NIRCs barely introduces frequency splitting, and the suppression level of frequency splitting can also be controlled freely.