Wireless Power Transfer Coils Research Articles

Cylindrical Transmitting Coil for Two-Dimensional Omnidirectional Wireless Power Transfer

Megahertz-range wireless power transfer has become a promising approach for increasing spatial freedom of charging. This article proposes a cylindrical-shaped coil, which can produce the homogeneous magnetic field in a plane. The coil consists of two helical windings, which are wound to guide the current in opposite directions. A single power source is used to excite the transmitter without any current amplitude or phase control circuits. Furthermore, since the parasitic capacitance of the coil is not negligible due to the complexity of the coil shape, we develop a general equivalent model with parasitic capacitance for the analysis of complex coils. The system efficiency of the proposed omnidirectional wireless power transfer device is validated by a practical experiment. The measured dc-to-dc efficiency of approximately 72.4% and the load power of 13 W are demonstrated for the proposed wireless charging system at 6.78-MHz operating frequency. Finally, we verify that the electromagnetic exposure satisfies the safety regulations.

Modified tri-axial square Helmholtz coil for omnidirectional wireless power transfer

Modified tri-axial square Helmholtz coil for omnidirectional wireless power transfer

The Design and Optimization of Ground-Side Coils for Dynamic Wireless Power Transfer Considering Coupling Variations

Dynamic wireless power transfer (DWPT) has attracted widespread attention for its charging flexibility; short-segmented DWPT systems are more suitable for EV charging scenarios because of their higher charging efficiency and lower electromagnetic radiation, compared to long-track DWPT systems. For short-segmented DWPT systems, the structural design of the ground-side coil affects the coupling characteristics of the system, while simultaneously the electric vehicle driving speed and coil arrangement also cause coupling variations, and this will inevitably have an impact on the system’s performance. Therefore, this paper demonstrates the coupler design of a short-segmented system for electric vehicles, focusing on the optimization of ground-side coil. The coupling variations causing by driving speed of EV and coil arrangement are taken into account. Considering the tradeoffs and restrictions, a multi-objective optimization process of coils in DWPT systems is proposed based on the Pareto optimizing method, with three objectives: transfer power, high efficiency and low cost. A reasonable optimal solution is selected from the Pareto front to verify the optimizing method through a constructed prototype.

Birdcage Transmitter for Wireless Power Transfer

With the proliferation of Internet of Things (IoT) technology, the number of devices and gadgets used per person is ever-increasing. There is an indispensable need for the extension of wireless power transfer (WPT) to charge multiple devices simultaneously without compromising the charging efficiency. To accommodate multiple units (tens) of wireless devices, planar transmitter coils are constrained by space and their efficiency is sensitive to the variation in receiver position and orientation. Hence in this letter, we adapt the concept of volume birdcage coil from magnetic resonance imaging (MRI) and design a cylindrical transmitter coil for WPT with a diameter of 500 mm and a length of 300 mm, called birdcage WPT charging bin, to efficiently charge multiple wireless devices in the bin at various positions and orientations. The designed birdcage WPT charging bin supplies a homogeneous and circularly polarized magnetic field which leads to a stable efficiency for the receivers at different locations and orientations around the bin axis in the bin. The bin was built and measurement was conducted for validating the performance.

A Three-Coil Wireless Power Transfer System using Self-Resonant Open-Bifilar Coils

A common trend in the field of wireless power transfer consists in the use of more than two coils in order to extend the link distance in terms of efficiency, in which the 3-coil system is of great interest due to its high efficiency. In these circuits, usually, the preferred method of compensation is the series (capacitor and inductor in series), mainly due to its simplicity. Another promising topic is the use of coils tuned at their self-resonant frequency, mainly because it could avoid the use of external capacitors to achieve the resonant state in the involved circuits. However a multiple-coil self-resonant frequency with series compensation system was not yet presented in literature. In this paper, using open-bifilar coils, we present a self-resonant 3-coil wireless power transfer system that behaves as common series compensated circuit. Practical results confirm the validity of proposed solution, in a proof of concept methodology, that is, without any optimization strategy applied. For this sake, three different distances between transmitter and relay coil are adopted, while a 10 Ω resistor is used as load, allowing a maximum efficiency of 47% at a distance of 8 cm between relay and load coil (coils diameter are 15 cm).

Frequency splitting suppression in wireless power transfer using hemispherical spiral coils

In wireless power transfer (WPT) systems, frequency splitting phenomena always occur when the transfer distance is too small to obtain the maximum output efficiency in the working frequency. In this paper, a hemispherical spiral coil structure is proposed to suppress frequency splitting phenomena by containing the coupling the system without changing the original system size and transfer distance. Meanwhile, compared with the planar system, the hemispherical system can maintain high power and transfer efficiency and has better angular misalignment tolerance. A WPT system consisting of a hemispherical spiral coil and a planar spiral coil resonating at 110 kHz called the single hemispherical spiral coil WPT (SHWPT) system and another with two hemispherical spiral coils called the double hemispheric spiral coils WPT (DHWPT) system are suggested. Analytic models of these two structures are established to analyze the influence of the coil height on the frequency splitting. The critical coil height is closely related to the critical coupling that provides a reference for the design of the coil. Finally, the experimental results show that the hemispherical coil structure can suppress the frequency splitting to ensure the transmission stability of the WPT system. In the SHWPT system with a 1 cm inner diameter and a 13 cm outer diameter, the critical height of the coil is 5.2 cm, which can suppress frequency splitting when the transmission distance is 0 cm. In the DHWPT system, the critical height of the coil is 4.3 cm.

Coil material and magnetic shielding methods for efficient wireless power transfer system for biomedical implant application

In this paper, a passive shielding method is presented to minimize the magnetic field around the transmitting (Tx) and receiving (Rx) coil of wireless power transfer (WPT) system for biomedical implant application. Different conductive materials for coil construction, as well as dielectric materials for shielding, are being considered to improve WPT performance. Several cases of WPT coils have been examined using the simulation approach at 800 kHz and 6.78 MHz in the existence of a pacemaker casing with and without a magnetic shield. According to simulated results, it is found that high conductivity coils and high permeability shield materials are excellent materials for increasing the WPT system's efficiency. The results of the simulation method and physical measurement in Lab are validated using an analytical method. The efficiency obtained by using simulation method is very close to that obtained using the analytical method. The voltage gain obtained using an analytical and a simulation method at 800 kHz and 6.78 MHz is very close to the actual measurement in the lab. It is found that the WPT efficiency at 6.78 MHz is higher as compare to 800 kHz.

Decoupled-Double D Coils Based Dual-Resonating-Frequency Compensation Topology for Wireless Power Transfer

In this article, a dual-resonating-frequency (DRF) compensation topology is adopted and implemented for exciting the decoupled-double D (DDD) transmitting coils in the dual-frequency wireless power transfer (WPT) systems. Concretely speaking, two double D (DD) coils are located orthogonally and compactly to obtain the effect of magnetically decoupled in both primary and pickup sides. Meanwhile, by adopting the proposed DRF compensation, two arbitrarily predefined resonating frequencies can be realized, thus forming a dual-channel transmission mechanism. Besides, the power distribution of two different frequencies can be randomly regulated based on the users’ requirement by adopting the proposed frequency overlay modulation (FOM) excitation scheme. Combining with the merits of dual-frequency simultaneous transmission, no cross-coupling among pickup coils, and arbitrarily power distribution, the proposed WPT systems can satisfy customers’ various demands and boost transmission efficiency. Theoretical analysis, simulation, and experimentation are given to verify the feasibility of the proposed WPT systems.

Geometrical parameter optimization of planner square-shaped printed spiral coil for efficient wireless power transfer system to biomedical implant application

In this paper, the wireless recharging of pacemaker's battery for biomedical implant is considered. Planner square-shaped printed spiral coils (PSCs) are selected as primary (Tx) and secondary (Rx) coil for wireless power transfer (WPT) system. FEM based simulation method is used to optimize the geometrical parameters of square-shaped PSCs. The receiving coil is embedded inside the biomedical implant. The coil track width (w), pitch(s), thickness (t), number of turns(N1,N2) and outer diameter (Dout) of square-shape PCS's are the key parameters for higher coupling coefficient and quality factors. Using magnetic resonance coupling (MRC) based on Series-Parallel (SP) topology; experiments are carried out in an analytical, simulation, and physical environment for four operating frequencies: 300 kHz, 800 kHz, 6.78 MHz, and 13.56 MHz. Efficiency obtained by analytical and simulation method is higher with frequency. The optimum frequency for recharging the pacemaker battery is 6.78 MHz, which provides an efficiency of 85.51% at 100Ω load and is very close to the maximum theoretical efficiency. Voltage gain obtained by physical method is in close approximation with analytical and simulation methods.

A Study of Magnetic Coupling Characteristics of Dual Receiver Coil for Dynamic Wireless Power Transfer

For the problem of fluctuation of transmission power and load voltage between magnetically coupled mechanisms in the dynamic wireless power transfer (DWPT) system of the electric vehicles (EV), a magnetically coupled mechanism with dual receiving coils (R-C) of electric energy on the receiving side is proposed in this paper. The transmitting coils (T-C) are connected in parallel in a multi-stage manner flat on the road, and when the EV drives above a section of T-C, this T-C is energized to charge the EV. In the paper, the theoretical model of the circuit and the theoretical model of the magnetic circuit of the dual R-C magnetic coupling mechanism are first established. The feasibility of this structure is verified through the derivation and calculation of the working principle of the system. Finally, the simulation and experimental results verify the effectiveness of the coupling mechanism. The experimental results further demonstrate that the magnetically coupled structure with dual R-C can effectively reduce the power fluctuation during the T-C switching and reduce the laying of T-C to reduce the cost.

非接触給電用多層コイルのトポロジー最適化の基礎検討

Wireless power transfer (WPT), as a new charging method for electrical vehicle (EV), is anticipated for its higher safety and convenience compared to wiring charging system. Meanwhile, the performances of WPT, such as transfer efficiency and leakage magnetic field, are sensitive to the position misalignment between transmitting and receiving coils. Therefore, enhancing the robustness to misalignment is important WPT. In this work, stream function method, which is widely used for design of MRI gradient coil, is applied to designing WPT coil. Normalized Gaussian network (NGnet) is adopted for representation of the stream function. A transmitter consisting of three-layers coils is considered and the coil shapes of each layer is optimized respectively, with respect to the output power and leakage flux density simultaneously, with MO-CMA-ES as the optimization algorithm.

A study on transmission coil parameters of wireless power transfer for electric vehicles

Electric vehicles (EVs) are becoming more popular as people become more concerned about global issues, such as fossil fuel depletion and global warming, which cause severe climate change. Wired charging infrastructure is inefficient because it requires the construction of one charging station for each electric vehicle. As a result, wireless power transfer via magnetic coupling, which is small, compact, and may be placed underground, is a promising technology for the future of charging electric vehicles. One of the disadvantages of wireless power transfer is that efficiency drops rapidly as air gaps grow larger, and it is particularly sensitive to other electrical characteristics such receiver unit capacitance. The purpose of this paper is to investigate the coil parameter, more specifically the outer diameter of wireless power transfer coil effects on the wireless power transfer efficiency at various air gaps and receiver capacitance values for EV applications. The simulations show that a larger outer diameter coil has a better power transfer efficiency at larger air gaps and a more stable range.

Efficient Wireless Drone Charging Pad for Any Landing Position and Orientation

A wireless charging pad for drones based on resonant magnetic technology to recharge the internal battery is presented. The goal of the study was to design a robust, reliable and efficient charging station where a drone can land to automatically recharge its battery. The components of the wireless power transfer (WPT) system on board the drone must be compact and light in order not to alter the payload of the drone. In this study, the non-planar receiving coil of the WPT system is integrated into the drone’s landing gear while the transmitting pad is designed to be efficient for any landing point and orientation of the drone in the charging pad area. To meet these requirements, power transmission is accomplished by an array of planar coils integrated into the ground base station. The configuration of the WPT coil system, including a three-dimensional receiving coil and a multicoil transmitter, is deeply analyzed to evaluate the performance of the WPT, considering potential lateral misalignment and rotation of the receiving coil due to imprecise drone landing. According to the proposed configuration, the battery of a light drone (2 kg in weight and 0.5 kg in payload) is recharged in less than an hour, with an efficiency always greater than 75%.

Analysis and Modeling of a Wireless Power Transmission System for Multiple Receivers

In this article the model for the study of wireless power transfer to several receivers is presented. The problem of determining the ideal loads to obtain the maximum transfer of energy in a connection between a transmitter and several receivers was considered and solved by applying the maximum power transfer theorem for N ports. This condition is derived from the circuit analysis and discussed together with the respective efficiency of the Wireless Power Transfer (WPT) system (). In sequence, it is shown that a WPT system with multiple coils without loss of energy in the two communication circuits presents, from the maximum transfer of power and efficiency point of view, performance similar to that of a 2 coil WPT system. Simple analytical expressions for the load impedances were validated through experiments and circuit simulations. These expressions are also valid in the case of mutual coupling between receivers.

A Numerical Method to Reduce the Stray Magnetic Field Around the Asymmetrical Wireless Power Transfer Coils for Electric Vehicle Charging

A Numerical Method to Reduce the Stray Magnetic Field Around the Asymmetrical Wireless Power Transfer Coils for Electric Vehicle Charging

Miniature Coil for Wireless Power and Data Transfer through Aluminum.

This paper presents the design and development of miniature coils for wireless power and data transfer through metal. Our coil has a total size of 15 mm × 13 mm × 6 mm. Experimental results demonstrate that we can harvest 440 mW through a 1 mm-thick aluminum plate. Aluminum and stainless-steel barriers of different thicknesses were used to characterize coil performance. Using a pair of the designed coils, we have developed a through-metal communication system to successfully transfer data through a 1 mm-thick aluminum plate. A maximum data rate of 100 bps was achieved using only harvested power. To the best of our knowledge, this is the first report that demonstrates power and data transfer through aluminum using miniature coils.

Influence of Posture and Coil Position on the Safety of a WPT System While Recharging a Compact EV

In this study, the human exposure to the magnetic field emitted by a wireless power transfer (WPT) system during the static recharging operations of a compact electric vehicle (EV) is evaluated. Specifically, the influence of the posture of realistic anatomical models, both in standing and lying positions, either inside or outside the EV, is considered. Aligned and misaligned coil configurations of the WPT system placed both in the rear and front position of the car floor are considered as well. Compliance with safety standards and guidelines has proven that reference levels are exceeded in the extreme case of a person lying on the floor with a hand close to the WPT coils, whereas the system is always compliant with the basic restrictions, at least for the considered scenarios.

Control of WPT Transmitter Coils for Power Distribution to Two Receiver Coils without Feedback

This paper proposes the algorithm to control the current ratio of the transmitting (Tx) coils for proper power distribution to the two receiving (Rx) coils in wireless power transfer (WPT) system. The proposed algorithm assumes that each Rx coil appears at different times to consider the situation where multiple users request power transmission as practically as possible. That is, suppose the second Rx coil enters the charging space later than the first Rx coil. When each coil enters the charging space, only the Tx coil is used to obtain the value required for calculation. Using the obtained result, the optimized Tx coil current is calculated by the proposed algorithm and proper power distribution to both Rx coils is achieved. Three Tx coils and two Rx coils are constructed using the ANSYS MAXWELL simulation tool. As a result of applying the proposed algorithm, it was confirmed that a similar level of power was transmitted between 40∼60%, respectively. The sum of the power transmitted to the two Rx coils also appeared as more than 75%.

Development of dynamic wireless power transfer coils for 3rd generation wireless in‐wheel motor

AbstractShort cruise range is a major issues in electric vehicles, and dynamic wireless power transfer (WPT) has been proposed to solve this problem. In this study, a new dynamic power transfer system is introduced, called the hird generation wireless in‐wheel motor, that integrates the drive circuit and motor. Additionally, a methodology of the transmitter and receiver coil design that can calculate inductance and resistance of the rectangular spiral coils in a fast manner is proposed. It considers the theoretical efficiency of coils, current of coils, and voltage of resonance capacitor as constraints. Inductance calculation models only use coordinates of coil edge and the distance between coil and ferrite core. The coils designed by the proposed method were manufactured and evaluated using an LCR meter. The calculation error of the self‐inductance and mutual inductance was found to be ± 9%. With the transmitter coil and receiver coil, an output and DC to DC efficiency of is 18 kW and 95.2%, respectively, were obtained by actual WPT test.

Development of in‐wheel receiver coil for dynamic wireless power transfer

AbstractShort cruise range is one of the major limitations of electric vehicles. To address this problem, dynamic wireless power transfer has been proposed. However, the eddy current loss in foreign matter is one of the greatest disadvantages of dynamic wireless power transfer. Eddy current loss decreases efficiency of wireless power transfer and increases temperature of the foreign matter, which causes burning of the combustible materials around the foreign matter in the worst case. Herein, we propose a wireless power transfer system named in‐wheel coil wherein the magnetic flus in concentrated in the tires and wheels. In addition, we propose suitable materials for the tires and the wheels of the system, to reduce the eddy current loss, are proposed using actual measurement data.