Rx Coil Research Articles

Free-Positioning Wireless Power Transfer Using a 3D Transmitting Coil for Portable Devices

The free-positioning wireless power transfer (WPT) system has drawn attention in recent years. Traditionally, a WPT system can transfer energy in one or two directions on the same plane, but it leads the restrictions of angle and axis misalignment between a transmitter and a receiver coil. In this paper, we propose a free-positioning WPT system using a three-dimensional cubic-shaped transmitting coil for portable device charging. A small receiving coil is placed inside the transmitter to achieve the transferred energy through the magnetic resonant coupling. In addition, the equivalent circuit and the mutual inductance between the Tx and Rx coils are analyzed. Finally, a practical experiment is implemented to verify the transfer performance, which can reach up to about 50% power transfer efficiency. The proposed system can charge in spatial freedom.

A Design Technique for Optimizing Resonant Coils and the Energy Transfer of Inductive Links

Power transfer efficiency (PTE) is a key performance parameter in development work on resonant inductive power transfer (IPT) systems. Geometrically optimizing the transmitter (Tx) and receiver (Rx) coil pair is a way of improving the IPT system's efficiency. In this article, a new figure-of-merit (FoM) is proposed to find an optimum coil geometry which maximizes the PTE. The employed FoM parameter, called the “strong coupling factor” (P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">scf</sub> ), is defined such that its value indicates how strongly the Tx and Rx coils are linked together. Considering the IPT application and its physical size constraints, a proper selection method for identifying the numerical value of P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">scf</sub> is essential for optimal coil geometry design. This article presents an iterative algorithm to assist in the selection of the most favorable P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">scf</sub> value which provides maximized PTE for the designed optimum coil geometry. Design examples of two nominal IPT systems at frequencies of 415 and 0.1 MHz are used to investigate the design algorithm. Theoretical calculations show the optimum geometry designed for the IPT system operating at 415 MHz, with coupling coefficient (K) of 0.2, can achieve maximum PTE of 85.70%. Measurements presented from a practical Tx/Rx coil pair in the IPT link operating at 0.1 MHz, with K = 0.05, show a PTE of 83.10% against a calculated PTE of 84.11% validating the design process.

A CMOS closed-loop miniaturized wireless power transfer system for brain implant applications

Near-field inductively coupled wireless power transfer (WPT) system has been extensively utilized for brain implant applications. Still, the efficient and reliable delivery of power is challenging as the received power varies due to different variabilities between the transmitter (TX) and the receiver (RX) coils. A closed-loop adaptive control system utilizing load shift keying, designed in the 0.5 µm standard CMOS process for providing the required power to the implant load compensating for these discrepancies is proposed in this paper. Both the proposed TX and the RX coils are fabricated using FR4 substrate having the dimensions of 10 × 10 mm and 5 × 5 mm, respectively. By changing the supply voltage of the power amplifier, this adaptive closed-loop system regulates the transmitted power to deliver 5.83 mW of power to the load, which is the approximate mid-point of the threshold window. The system achieves power transfer efficiencies of 9% and 8% at 8 mm distance through the air and the tissue media, respectively. Preliminary results show that the miniaturized WPT module with the feedback-loop achieves 8% and 3% of efficiency improvement for 8 mm distance between the TX and the RX coils, compared to the open-loop counterparts.

An Adaptive Current Limiting Controller for a Wireless Power Transmission System Energized by a PV Generator

The use of a wireless power transmission system (WPTS) in modern applications, such as consumer electronics, renewable energy sources (RESs) and electric vehicles (EVs), can significantly increase the safety and convenience of the power supply. However, low efficiency is a major hurdle to the use of a WPTS in these applications. In this article, an adaptive virtual impedance controller (AVIC) is presented to enhance the wireless power transfer (WPT) efficiency of a photovoltaic generator (PVG) to the load. In the proposed controller, a unique method is employed to adaptively estimate the coefficient of coupling and resonant frequency of the WPTS coils as a function of the distance between the coils. Moreover, a modified incremental conductance (IC) based maximum power tracking (MIC-MPPT) technique is presented to operate the PVG at MPPT mode. The proposed MIC-MPPT is tested via a hardware prototype and the controller validation is carried out in the MATLAB/SIMULINK environment under various uncertainties, such as intermittent irradiance, variable load, and the distance between transmitter (Tx) and receiver (Rx) coils. Finally, a comparative analysis between the proposed controller and the conventional non-adaptive and adaptive resonant frequency controller is presented which confirms the superiority of the proposed controller.

A Dual-Band Wireless Power Transfer System With Efficiency-Boosting Converter

A dual-band wireless power transfer (WPT) system is proposed to satisfy multiple distinct-band WPT demands. A power amplifier combined with a dual-band output matching coil, which is relatively compact, is adopted to enable asynchronous two-band power transfer for multiple single-band receivers (RXs). The measurement results revealed that the efficiency reached 71% for both 6.78 and 13.56 MHz when the transmitter (TX) coil and RX coil were in alignment. However, the efficiency of a dual-band WPT system decreases when the spacing between the TX coil and RX coil deviates from its optimal distance. To address this concern, boost converters were used in the RX ends to achieve higher efficiency under coil misalignments. Efficiency improvement of 14% was obtained when the deviation of the RX coil from the alignment center of the TX coil was 5 cm.

Transfer Characteristics of the Nonlinear Parity-Time-Symmetric Wireless Power Transfer System at Detuning

The nonlinear parity-time-symmetric wireless power transfer (NPTS-WPT) system is more robust against transfer distance than the traditional WPT system. Current studies mainly focus on the situation in which the transmitter (Tx) and the receiver (Rx) are completely matched. Our study focuses on the transfer characteristics of the NPTS-WPT system under detuning between the Tx and the Rx. First, the mathematical model of the detuned system is established, and then the model is solved using Shengjin’s formula. Then, the exact analytical solutions for the operating frequency, the amplification factor of the operational amplifier (OP Amp) and the transfer efficiency at detuning are obtained. It was noted, for the first time, that even though the Tx and the Rx were completely matched, a frequency jump could occur when the distance between the Tx and Rx coils slowly changed. Our study found that when the degree of detuning of the system changed, the operating frequency of the system could jump. By investigating the amplification factor of the OP Amp, the reason for the frequency jump when the system was detuned was explained. Our study also revealed that detuning did not imply a decreased transfer efficiency, and the over-detuning can improve the transfer efficiency sometimes. Finally, an experimental system was constructed, and the correctness of the theory was validated using the experimental system.

Design and analysis of coaxial cylindrical WPT coils for two-degree-of-freedom applications

In this work, we propose two-degree-of-freedom coils for a high coupling wireless power transfer (WPT) system to achieve a constant efficiency against rotation and linear movements. The transmitting (Tx) coil and receiving (Rx) coil were wound on a cylindrical joint structure. The Rx coil has a linear motion (Δ) between three assumed positions, as follows. Lower position (P1 ), middle position (P0 ) and an upper position (P2 ). Besides, the Rx coil has an angular movement (α) around its axis between 0° and 360°. The mutual inductance (M) between the Tx and Rx coils is derived and validated by calculations and simulations. Design procedures of the WPT coils and analytical investigations for maximizing the power transfer efficiency are discussed in detail. In addition, simulation results under linear and angular movements are presented. Finally, the WPT coils were fabricated. The experiment results were in good agreement with the calculation and simulation results.

Wireless power transfer-based eddy current non-destructive testing using a flexible printed coil array.

Eddy current testing (ECT) has been employed as a traditional non-destructive testing and evaluation (NDT&E) tool for many years. It has developed from single frequency to multiple frequencies, and eventually to pulsed and swept-frequency excitation. Recent progression of wireless power transfer (WPT) and flexible printed devices open opportunities to address challenges of defect detection and reconstruction under complex geometric situations. In this paper, a transmitter–receiver (Tx–Rx) flexible printed coil (FPC) array that uses the WPT approach featuring dual resonance responses for the first time has been proposed. The dual resonance responses can provide multiple parameters of samples, such as defect characteristics, lift-offs and material properties, while the flexible coil array allows area mapping of complex structures. To validate the proposed approach, experimental investigations of a single excitation coil with multiple receiving coils using the WPT principle were conducted on a curved pipe surface with a natural dent defect. The FPC array has one single excitation coil and 16 receiving (Rx) coils, which are used to measure the dent by using 21 C-scan points on the dedicated dent sample. The experimental data were then used for training and evaluation of dual resonance responses in terms of multiple feature extraction, selection and fusion for quantitative NDE. Four features, which include resonant magnitudes and principal components of the two resonant areas, were investigated for mapping and reconstructing the defective dent through correlation analysis for feature selection and feature fusion by deep learning. It shows that deep learning-based multiple feature fusion has outstanding performance for 3D defect reconstruction of WPT-based FPC-ECT.This article is part of the theme issue ‘Advanced electromagnetic non-destructive evaluation and smart monitoring’.

Inductive wireless power transfer system using multiple coils for rotating propulsive shaft applications

AbstractAn induction wireless power transmission (I‐WPT) system using multiple coils has been proposed to drive a wireless sensor system (WSS) that monitors the status of the propulsion shaft. While maintaining a constant transfer voltage, the minimum number of coils and the minimum air gap between coils were extracted through several experiments. The optimized air gap between transmitter (Tx) and receiver (Rx) coils was 3 mm and the number of the Rx and Tx coils was four and six, respectively. The optimized I‐WPT system was installed on the shaft with a diameter of 20 cm and obtained the stable transferred voltage characteristics and output power of 1.75 W with above 75% efficiency in spite of different rotation speed of the shaft.

Design of a New Hybrid Topology of WPT System to Achieve Load-Independent Constant-Current and Constant-Voltage Output

The usage of the Wireless Power Transfer (WPT) technique for charging lithium-ion (Li-ion) batteries of electric vehicles (EVs) is increasing rapidly. The Li-ion battery requires constant current (CC) followed by constant-voltage (CV) supply for efficient battery charging and to enhance its lifespan. However, it is hard to obtain CC and CV outputs due to the variation of battery resistance during charging. Moreover, a zero-phase angle (ZPA) of the input source is essential to improve the power transfer capability. Therefore, this paper proposes a new hybrid compensation topology to achieve load-independent CC and CV behavior of the WPT system along with ZPA. Two symmetrical coils have been used in the proposed WPT system as transmitter (Tx) and receiver (Rx) coils. A comprehensive mathematical analysis for achieving CC and CV characteristics as well as for power losses is carried out. A new hybrid compensation is designed by combining Series–Series (SS) and Inductor–Capacitor–Capacitor/Series (LCC-S) compensation topologies along with two additional switches. The proposed compensation works on single resonance frequency, and it is simple, reliable, and easy to implement with only one compensation capacitor from the Rx side. Simulations are performed, and a prototype is fabricated to verify the mathematical analysis and simulation results. The overall result shows that the mathematical analysis and simulations comply with the experimental results. Full load efficiencies of 90.33% and 88.91% are achieved in the CC and CV modes, respectively.

Tunable Metamaterial Slab for Efficiency Improvement in Misaligned Wireless Power Transfer

This letter presents a tunable metamaterial (MTM) slab for high-efficiency wireless power transfer (WPT) in misaligned conditions, which operates at 6.78 MHz, an AirFuel Alliance Standard frequency. The tunable MTM slab is located between transmitter (Tx) and receiver (Rx) coils serving as a tunable lens to help focus the magnetic field toward the place where the Rx coil is located. This tunable slab consists of two types of unit cells which can be independently tuned to have proper negative refractive indices. The experimental results show that the tunable MTM slab effectively focuses magnetic fields, thereby improving the power transfer efficiency (PTE) of WPT even in a misaligned condition.

Multi-site benchmarking of clinical 13C RF coils at 3T

A quality assurance protocol for RF coils is proposed, which can be used for volume (Tx/Rx) and surface (Rx) coils. Following this protocol, a benchmarking of seven coils (from three different MR sites) dedicated to 13C MRI at 3T is reported. Coil performance is particularly important for 3T MRI at the 13C frequency, since the coil-to-sample noise ratio is typically high. The coils are evaluated experimentally using the proposed protocol based on MR spectroscopic imaging performed with two different phantoms: one head-shaped, and one with cylindrical shape and nearly twice the volume of the first one. To achieve an unbiased SNR comparison of volume and array coils, coil combination was done using sensitivity profiles extracted from the data. SNR, noise correlation matrices and example g-factor maps are reported. For globally calibrated, equal excitation angles, the measured SNR shows large differences for the volume coils of up to 115% at the phantom center for a head phantom. The arrays show lower differences in superficial SNR. The sample surface depth at which the volume coils outperform the arrays is estimated to 7 cm, and SNR furthest away from the coil surface is 28% lower for the best array compared to the best volume coil. A broad set of coils for 13C at 3T have been benchmarked. The results reported, and the method used to benchmark them, should guide the 13C community to choose the most suitable coil for a given experiment.

Analysis and Performance Enhancement of Wireless Power Transfer Systems With Intended Metallic Objects

While detection of foreign or unintended metallic objects is an important topic for wireless power transfer (WPT), operation of WPT systems with intended metallic objects is largely an unchartered territory. This article presents a methodology for analyzing the effects of intended metallic object on a WPT system through the study of the magnetic field distribution and the use of a reduced-order equivalent model of the systems for comparing the WPT system with and without the metallic object. The methodology is applied to a metallic corona ring of low resistance used with a WPT system. The analysis quantifies how an intended metallic object could change the magnetic coupling between the transmitter and receiver coils, and alter the equivalent resonant tank parameters of the WPT system. Recompensation of the resonant tank can only enhance the power transfer capability but not the energy efficiency. The theory and practical verification of a new performance-enhancement method based on a combination of recompensating the resonant tanks and maximizing the equivalent mutual inductance of the Tx and Rx coils is presented. Such methodology provides a general approach to analyze WPT systems with or without relay resonators in the presence of intended metallic objects.

Position detection and route correction of electric vehicles by dynamic wireless charging

During the dynamic wireless charging of electric vehicles, the offset from the vehicle Rx coil will reduce the mutual inductance of the coupling mechanism, which results in a significant reduction on the transmission efficiency of the system. Now a real-time detection method of electric vehicle position based on double coils is proposed. By analyzing the change trend of the pickup voltage in the detection coil, the correspondence among the lateral deviation distance, the offset angle and the pickup voltage is obtained by difference ratio sum method, the position information of electric vehicle will be worked out. Then the PID with variable structure was deployed algorithm to realize the self-correction of the electric vehicle driving route. The construction of experimental platform verifies the feasibility of the positioning method. The results revealed that when a lateral deviation takes place, this method can guarantee the automatic correction to the path of an electric vehicle during the dynamic wireless charging process. The advantages of this method are high sensitivity, fast correction speed, a simple structure, etc.

Conceptual Design of Isolated Power for Quench Detection System With Highly Insulating Stability Under Super High Field Magnets Using Wireless Power Transfer Technology

The super high field magnets using superconducting wires have been promisingly applied for thermonuclear fusion power generation such as Korea Superconducting Tokamak Advanced Research (KSTAR) and International Thermonuclear experimental reactor (ITER) systems. The quench detection system (QDS) is essential for high reliability in the super high field magnet facilities. Generally, the quench voltage detection system, which uses resistive voltages, has been adopted. Such an ITER magnet, the maximum operation current and voltage of superconducting keeps over 60kA and 50 kV DC. Generally, in the case of medium power, the power transformer has been adopted for QDS as an isolated power due to stable insulating and durable characteristics. However, unfortunately, the magnetic saturation of electric steel for power transformer remained fragile as well as the volume is increased under super high field magnet. From this reason, authors suggest the electron quench detection device (EQDD) module. The main functions of EQDD are to convert high voltage signals of magnet into voltage-drop, and then send into quench monitoring system through optic cable. The EQDD module needs to isolate power system since EQDD module should be operated under high voltage insulation. From this reason, authors apply the wireless power system for EQDD module as a reasonable option instead of power transformer since the wireless power system can transfer power through any non-metallic media between resonance antenna (Tx) and receiver (Rx) coils. In this study, authors describe the possibility of conceptual design for quench detection system with high insulation under super high field magnet using wireless power transfer technology. As the EQDD including isolated wireless power should be electrically shielded, the shielding effects for Tx coil is investigated under different intervals of shielding barrier. Additionally, to expand the delivery distance between Tx and Rx coils, the operation and thermal characteristics for transfer efficiency are investigated under 370 kHz and 750 kHz RF generators, respectively.

Two-Degree-of-Freedom WPT System Using Cylindrical-Joint Structure for Applications With Movable Parts

A new winding structure for wireless power transfer (WPT) systems is proposed. The transmitter (Tx) and receiver (Rx) coils are wound on a semicylindrical and cylindrical structures, respectively. Thus, the Rx coil has two degree-of-freedom (2-DoF) movements. The first is linear ( ${\Delta }$ ) movement along the axial direction of the joint. The second is angular ( ${\alpha }$ ) movement around the axial direction of the joint, while transferring power. Winding methods and design procedures are discussed in detail. A prototype was built to validate the proposed idea. Experiment results have confirmed that the proposed structure achieved a constant power transfer efficiency (PTE) under linear movement. In addition, the Rx coil rotated up to 85° and maintained low variations of the efficiency, where PTE decreased by 4.61% for rotation between 0° and 85°. Moreover, the proposed WPT coils maintained a constant PTE under dynamic conditions. The proposed winding structure could be a good technique for applications that require linear and angular movements.

Modeling and Characterization of Scaling Factor of Flexible Spiral Coils for Wirelessly Powered Wearable Sensors

Wearable sensors are a topic of interest in medical healthcare monitoring due to their compact size and portability. However, providing power to the wearable sensors for continuous health monitoring applications is a great challenge. As the batteries are bulky and require frequent charging, the integration of the wireless power transfer (WPT) module into wearable and implantable sensors is a popular alternative. The flexible sensors benefit by being wirelessly powered, as it not only expands an individual’s range of motion, but also reduces the overall size and the energy needs. This paper presents the design, modeling, and experimental characterization of flexible square-shaped spiral coils with different scaling factors for WPT systems. The effects of coil scaling factor on inductance, capacitance, resistance, and the quality factor (Q-factor) are modeled, simulated, and experimentally validated for the case of flexible planar coils. The proposed analytical modeling is helpful to estimate the coil parameters without using the time-consuming Finite Element Method (FEM) simulation. The analytical modeling is presented in terms of the scaling factor to find the best-optimized coil dimensions with the maximum Q-factor. This paper also presents the effect of skin contact with the flexible coil in terms of the power transfer efficiency (PTE) to validate the suitability as a wearable sensor. The measurement results at 405 MHz show that when in contact with the skin, the 20 mm× 20 mm receiver (RX) coil achieves a 42% efficiency through the air media for a 10 mm distance between the transmitter (TX) and RX coils.

Mutual Inductance Analysis of Rectangular Spiral Coils with Angle and Position Variations for Wireless Power Transfer

For wireless charging, a misalignment between a receiver (Rx) coil in the devices (EV, drone and IMD) and a transmitter (Tx) coil often occurs. In this case, a mutual inductance between the coils changes, and consequently a power transfer efficiency (PTE) of the wireless power transfer (WPT) system also changes. To analyze the WPT system in such a situation, this paper presents an analytical method using Neumann formula to derive mutual inductance considering angular and position variations of two rectangular spiral coils. To verify the proposed method, magnetic resonant WPT systems of 85 kHz and 6.78 MHz are implemented, and the PTE values derived from the analyzed mutual inductance and the measured S-parameters are compared for the cases of angle and position variations. For each case, the two PTE values agree well with each other, and it is verified using the normalized root mean square error (NRMSE) values.

Design of a Cylindrical Winding Structure for Wireless Power Transfer Used in Rotatory Applications

A cylindrical joint structure for wireless power transfer (WPT) systems is proposed. The transmitter (Tx) and receiver (Rx) coils were wound on hemicylindrical and cylindrical structures, respectively. The Rx coil rotates freely around the axial direction of the Tx coil. Different methods of winding the Tx and Rx coils are given and discussed. Electromagnetic fields (EMFs) around the WPT windings should be lower than the limits set by WPT standards. Therefore, the WPT windings were designed to reduce EMF level and maintain constant power-transfer efficiency (PTE). The design procedures of the windings are discussed in detail. EMF analysis was done under different rotation angles (α). The selected design reduced the variation of the mutual inductance (M). As a result, it maintained a constant PTE while rotating the Rx coil between 0° and 85°. Moreover, leakage magnetic fields (LMFs) near the WPT coils of the chosen design were reduced by 63.6% compared with other winding methods that have the same efficiency. Finally, a prototype was built to validate the proposed idea. Experiment results were in good agreement with the simulation results. The WPT system maintained constant efficiency in spite of the rotation of Rx coil, where efficiency dropped by only 2.15% when the Rx coil rotated between 0° and 85°.

Inductive Power Transmission for Wearable Textile Heater using Series-None Topology

In this paper, an inductive-power-transmission (IPT) system for a wearable textile heater is proposed to comfortably provide heating to a user’s body. The conductive thread, which has high electrical resistance, was sewn into a receiver (Rx) coil on clothing to generate high temperature with a low current. The proposed wearable heaters are completely washable thanks to their nonmetallic materials, other than conductive threads in the clothing. We introduced series-none (SN) topology to eliminate a resonant capacitor in the wearable textile heater. A single resonant capacitor in a transmitter (Tx) in SN mode was implemented to resonate both Tx and Rx, resulting in increased power delivered to the load (PDL) while maintaining high-power transfer efficiency (PTE), comparable with conventional series-series (SS) topology. When the supply voltage of the power amplifier was 7 V, while the PTE of the SS and SN modes was 85.2% and 75.8%, respectively, the PDL of the SS and SN modes was 2.74 and 4.6 W, respectively.