Uniform Magnetic Field Distribution Research Articles

Coupled stack-up volume RF coils for low-field open MR imaging.

Low-field open magnetic resonance imaging (MRI) systems, typically operating at magnetic field strengths below 1 Tesla, has greatly expanded the accessibility of MRI technology to meet a wide range of patient needs. However, the inherent challenges of low-field MRI, such as limited signal-to-noise ratios and limited availability of dedicated radiofrequency (RF) coils, have prompted the need for innovative coil designs that can improve imaging quality and diagnostic capabilities. In response to these challenges, we introduce the coupled stack-up volume coil, a novel RF coil design that addresses the shortcomings of conventional birdcage in the context of low-field open MRI. The proposed coupled stack-up volume coil design utilizes a unique architecture that optimizes both transmit/receive efficiency and RF field homogeneity and offers the advantage of a simple design and construction, making it a practical and feasible solution for low-field MRI applications. This paper presents a comprehensive exploration of the theoretical framework, design considerations, and experimental validation of this innovative coil design. We demonstrate the superior performance of the coupled stack-up volume coil in achieving 47.7% higher transmit/receive efficiency and 68% more uniform magnetic field distribution compared to traditional birdcage coils in electromagnetic simulations. Bench tests results show that the B1 field efficiency of coupled stack-up volume coil is 57.3% higher compared with that of conventional birdcage coil. The proposed coupled stack-up volume coil outperforms the conventional birdcage coil in terms of B1 efficiency, imaging coverage, and low-frequency operation capability. This design provides a robust and simple solution to low-field MR RF coil design.

Design and Parameter Optimization of Double-Mosquito Combination Coils for Enhanced Anti-Misalignment Capability in Inductive Wireless Power Transfer Systems

This paper proposes a novel double-mosquito combination (DMC) coil for inductive wireless power transfer (IPT) systems to improve their anti-misalignment capability. The DMC coil consists of a mosquito coil with single-turn spacing and a tightly wound close-wound coil. By superimposing the magnetic fields generated by both coils, a relatively uniform magnetic field distribution is achieved on the receiving coil plane. This approach addresses the challenges of significant output voltage fluctuations and reduced transmission efficiencies caused by coupling coil misalignments in conventional IPT systems. To further optimize the DMC coil, an interaction law between its parameters and the mutual inductance is established, setting the coil mutual inductance fluctuation rate as the optimization objective, and using the coil turn spacing, number of turns, and outer diameter as constraint conditions. The beetle antennae search algorithm (BAS) is employed to enhance the whale optimization algorithm (WOA), facilitating the adaptive optimization of the coil parameters. An experimental IPT system platform with a 50 mm transmission distance is developed to validate the robust anti-misalignment capability of the proposed coil. The results demonstrate that within a horizontal misalignment range of 50 mm, the system’s output voltage fluctuation rate stays below 7.4%, and the transmission efficiency remains above 83%.

A Cubic Wireless Charging Container System with Highly Uniform Magnetic Field Distribution

A Cubic Wireless Charging Container System with Highly Uniform Magnetic Field Distribution

Deep learning-based field-guided optimization method

TEAM Problem 35, a benchmark problem in the optimization of electrical equipment, is a multi-objective, multi-variable optimization problem involving uniform field optimization. This paper proposes a novel guided optimization algorithm to address this problem and achieve optimal magnetic field regulation. First, the deep learning method is used to predict the magnetic field of the solenoid; second, the generated magnetic field cloud map is screened to extract the magnetic field information of the more optimal solution with uniform magnetic field distribution by calculating the variance of the generated magnetic field map; finally, the magnetic field features are combined with the particle swarm optimization algorithm; specifically, the variance of the calculated magnetic field map is added to the objective function of the optimization algorithm to strengthen the optimization process. This integration improves the convergence speed of the optimization process and ensures the continuity and stability of the obtained Pareto solutions. To optimize the solenoid, this paper considers the uniformity of magnetic field distribution and power loss as the primary optimization objectives. The radius of the coil is chosen as the design variable. The optimization results are compared with those obtained by conventional optimization algorithms, demonstrating superior optimization performance.

Experimental study and characteristic analysis of vacuum arc between cup-type axial magnetic contacts under different diameter-to-gap ratios

Since vacuum circuit breakers gradually advanced to higher voltage levels, axial magnetic field (AMF) contacts have drawn a great deal of attention due to their excellent breaking ability. The cup-type AMF contact is a common kind of AMF contact, which has much potential in contact design and application of high voltage grade systems due to the advantages of strong structural strength, uniform magnetic field distribution, lower resistance, etc. This study analyzes the arcing characteristics of a cup-type AMF contact with a large slotted rotation angle at various diameter-to-gap ratios (DGRs). The arcing process is divided into five stages as follows: initial diffusion, contracting, fully constricted, re-diffusion, and extinguished. Arc self-rotation and anode separation phenomena in the re-diffusion stage appear when the DGR is 58/24. The reasons for these occurrences were discussed and explained with regard to the magnetic field vector's spatial distribution. The duration of each stage and the current threshold of a fully constricted arc will both differ with the change of the DGR. The structural parameters of the fully constricted arc were computed through the method of imaging the luminous intensity distribution after the arc was fully constricted. The source of the change in the arc voltage can be seen in the variation of arc structural parameters, which also reflect anode activity intensity to a certain extent. The transient magnetic field simulation method was used to explain why the arc under the same instantaneous current shows variable morphology at the extinguished stage and contracting stage in one arcing process. The research results presented in the article can be used as a reference for developing high-voltage cup-type AMF contacts.

The magnetic field design of a solenoid for the cold-cathode Penning ion source of a miniature neutron tube

A high-yield and beam-stable neutron tube can be applied in many fields. It is of great significance to the optimal external magnetic field intensity of the cold-cathode Penning ion source (PIS) and precisely controls the movement of deuterium (D), tritium (T) ions and electrons in the source of the neutron tubes. A cold-cathode PIS is designed based on the solenoidal magnetic field to obtain better uniformity of the magnetic field and higher yield of the neutron tube. The degree of magnetic field uniformity among the magnetic block, double magnetic rings and solenoidal ion sources is compared using finite element simulation methods. Using drift diffusion approximation and a magnetic field coupling method, the plasma distribution of hydrogen and the relationship between plasma density and magnetic field intensity at 0.06 Pa pressure and a solenoid magnetic field are obtained. The results show that the solenoidal ion source has the most uniform magnetic field distribution. The optimum magnetic field strength of about 0.1 T is obtained in the ion source at an excitation voltage of 1 V. The maximum average number density of monatomic hydrogen ions (H+) is 1 × 108 m−3, and an ion-beam current of about 14.51 μA is formed under the −5000 V extraction field. The study of the solenoidal magnetic field contributes to the understanding of the particle dynamics within the PIS and provides a reference for the further improvement of the source performance of the neutron tube in the future.

Experimental investigation of metasurface-based resonator for one-to-many wireless power transfer systems in the presence of foreign objects

The wireless charging of multiple devices simultaneously is on a high demand. Metasurface-based resonators with a uniform magnetic field distribution over a large area have been recently proposed as a transmitting resonator for one-to-many WPT systems. However, the problem of the influence of foreign objects on a one-to-many WPT system characteristics has not been deeply studied. Herein, we explore the influence of different foreign objects such as a metal screen, ferrites, coins, and a smartphone on the input impedance of the WPT system utilizing a metasurface-based resonator. We experimentally investigate the impact of foreign objects on the WPT system parameters and define the most destructive foreign objects. To avoid this parasitic impact, we modify the metasurface-based resonator design by shielding the magnetic field. We numerically demonstrate that the modified design does not provide any magnetic field beneath the metasurface-based resonator.

Study of Electromagnetic Characteristics of Brushless Reverse Claw-Pole Electrically Excited Generators for Automobiles

A new type of brushless reverse claw-pole electrically excited generator is proposed for the problems of the low sine degree of the induced electromotive force waveform and the high harmonic content of the output voltage of the conventional electrically excited claw-pole generator. The design uses the equivalent magnetic network method to establish the equivalent magnetic circuit model of the motor and determines the equation for calculating the magnetoresistance of the isosceles trapezoidal variable section claw-pole structure. It derives the mathematical model of the no-load induced electric potential of the generator and completes to optimize the motor parameters with the no-load induced electric potential as the target. This study shows that the brushless reverse claw-pole electrically excited generator has the advantages of uniform magnetic field distribution, high harmonic fundamental amplitude of induced electromotive force and low harmonic content. It can better meet the application requirements compared with the conventional electrically excited claw-pole generator.

A universal UHF near‐field RFID reader antenna with wide bandwidth and adjustable reading area

AbstractA universal ultrahigh‐frequency (UHF) near‐field radio‐frequency identification (RFID) reader antenna with wide bandwidth and adjustable reading area is proposed in this letter. The antenna consists of an inverting power splitter base on double‐sided parallel stripline and adjustable traveling‐wave near‐field radiation units, which consists of two symmetrical meander microstrips and a complementary open resonant ring loaded on the ground plane. A design example with three radiation units is simulated, fabricated, and tested for verification. The result indicates that the proposed antenna has the 10‐dB impedance bandwidth of 128%, which completely covers the universal UHF RFID band from 840 to 960 MHz. The 100% read–write rate is achieved within the read–write distance of 10 mm according to the reading test. The reading area of 731 × 120 mm2 can be obtained for increasing the number of radiation units to 10. Moreover, the antenna features low far‐field gain, low‐cost, strong and uniform magnetic field distribution. Therefore, the proposed antenna and design method have good application prospects in the Internet of Things.

Design of Electrically Large UHF RFID Near-Field Bidirectional Antenna With AMC-Based Unidirectional Improvement

Two novel multisegment loops coupled ultrahigh-frequency (UHF) radio frequency identification near-field (NF) antenna with a uniform and strong magnetic field distribution over an electrically large interrogation zone are designed. Each proposed antenna is composed of eight loops, and the current distribution along the loop is kept in-phase using a segmented line. To further achieve a more uniform magnetic field, the last four loops of the two proposed antennas (ANT 1 and ANT 2) are loaded with parasitic structures. Here, a 100% detection rate is achieved by ANT 1 within an interrogation area of 370×560×105 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> for the bidirectional operation setup. As for ANT2, it is backed by an artificial magnetic conductor to enhance the magnetic field strength and reduce the profile with respect to a configuration with a PEC reflector plane, and it has yielded a 100% detection rate within the interrogation area of 370×560×185 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> for the unidirectional operation setup. The proposed ANT2 can cover the UHF band of China, the Federal Communications Commission band of North America, and European Telecommunication Standards Institute band of Europe.

Design and optimization of a modular wireless power system based on multiple transmitters and multiple receivers architecture

Wireless Power Transfer (WPT) technology has been applied to electric vehicles (EVs), for its convenience, safety, and high reliability. However, for the traditional single-transmitter-single-receiver WPT system, due to its uneven electromagnetic field distribution, the power transfer capability and the anti-offset performance are always insufficient. The multi-transmitter-multi-receiver (MTMR) WPT system based on modular inverters with coils can not only improve the transmission power of the system but also boost the system’s anti-offset ability due to its more uniform magnetic field distribution. In this study, the topology and transmission characteristics of the MTMR WPT system are discussed accordingly: 1) A decoupling method of transmitting coil based on capacitance compensation is proposed, and the particle swarm optimization algorithm is used to derive the optimal design scheme of magnetic coupling coil. 2) A phase-shifting control strategy is proposed to realize the phase synchronization and current sharing control of coil current at each transmitter. Finally, the simulation and experimental results show that the modular wireless power transfer system with multiple transmitters and multiple receivers has strong power transmission capability and anti-offset performance.

A Reticulated Planar Transmitter Using a Three-Dimensional Rotating Magnetic Field for Free-Positioning Omnidirectional Wireless Power Transfer

This article proposes a free-positioning omnidirectional wireless power transfer (WPT) system with a reticulated planar transmitter, which enables multiple receivers to be powered simultaneously in arbitrary positions and orientations. The reticulated planar transmitter is equipped with four interleaved and overlapped meander coils, thus, a three-dimensional (3-D) rotating magnetic field is generated with the designed excitation current. Therefore, only a planar single-coil (such as a loop-type coil) is required to realize free-positioning omnidirectional WPT. According to the theoretical modeling of the magnetic field, the excitation current that can generate a 3-D rotating magnetic field is given. To obtain a uniform and strong magnetic field distribution, the transmitter grid width is optimized. Furthermore, an excitation current modulation strategy based on phase-shift control is presented to regulate the amplitude of the excitation current. The experimental results show that the output power is greater than 13.1 W no matter how the center of the receiver moves or rotates freely within 400 × 400 mm. When five receivers are powered simultaneously in different positions and orientations, 82.5 W total output power and 55.6% efficiency can be achieved. The experimental results demonstrate that free-positioning omnidirectional WPT can be realized with only planar single-coil receivers by using the proposed reticulated planar transmitter and its excitation current modulation strategy.

Deep regression with ensembles enables fast, first-order shimming in low-field NMR

Shimming in the context of nuclear magnetic resonance aims to achieve a uniform magnetic field distribution, as perfect as possible, and is crucial for useful spectroscopy and imaging. Currently, shimming precedes most acquisition procedures in the laboratory, and this mostly semi-automatic procedure often needs to be repeated, which can be cumbersome and time-consuming. The paper investigates the feasibility of completely automating and accelerating the shimming procedure by applying deep learning (DL). We show that DL can relate measured spectral shape to shim current specifications and thus rapidly predict three shim currents simultaneously, given only four input spectra. Due to the lack of accessible data for developing shimming algorithms, we also introduce a database that served as our DL training set, and allows inference of changes to 1H NMR signals depending on shim offsets. In situ experiments of deep regression with ensembles demonstrate a high success rate in spectral quality improvement for random shim distortions over different neural architectures and chemical substances. This paper presents a proof-of-concept that machine learning can simplify and accelerate the shimming problem, either as a stand-alone method, or in combination with traditional shimming methods. Our database and code are publicly available.

A Uniformed DC Magnetic Generator Using Ferromagnetic Slot Cage by Geomagnetism Distortion Cancellation

A widely uniform and highly ambient robust DC magnetic field generator for precise motion sensors using inertial measurement units (IMUs) is newly proposed in this paper. Chronic problems such as low uniformity of magnetic field and geomagnetic disturbance of conventional Helmholtz, Merritt, Rubens, Lee-Whiting, and Tetra coils can be significantly mitigated by the proposed uniform DC magnetic field generator (UFG). Different from the previous air coils, the proposed UFG adopts a core-cage so that a strong and uniform DC magnetic field distribution can be generated inside the cage and that the UFG may be quite magnetically robust to ambient changes such as geomagnetic field and ferromagnetic objects. A dedicated power supply was developed for providing not only a DC current to generate the required magnetic field but also an exponentially decaying AC current to demagnetize the residual magnetic field in the core-cage. Design procedures for the UFG are fully established, considering the manufacturing cost, performance, and mechanical robustness for commercialization. Detailed designs of the UFG for magnetic field generation, enhancing uniformity, geomagnetic cancellation, and demagnetizing of the UFG are provided. Experimental results verified by a 30 cm × 30 cm × 30 cm prototype UFG showed that the geomagnetic field inside the UFG was significantly attenuated by 22 dB while the DC magnetic field uniformity (θ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><i>t</i></sub> < 5.0°) was achieved over 85% of the total volume of 27,000 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> .

Compact Hybrid Metasurface-Inspired Resonator With Uniform Magnetic Field Distribution for Wireless Power Transfer

Compact and safe charging platforms have been one of the main goals in wireless power transfer (WPT) research and development. Here, we propose a compact hybrid metasurface-inspired resonator with a uniform magnetic field distribution for WPT. It consists of two orthogonally oriented layers of parallel printed wires with high-permittivity dielectric pads between them. This design allows us to spatially separate electric and magnetic fields, which provide high WPT efficiency and electromagnetic safety simultaneously. The prototype with 28 cm × 28 cm working area with uniform magnetic field was fabricated and experimentally studied. The measured coefficient of variation of the magnetic field in the working area of the resonator is 6.8%. The maximum achievable WPT efficiency reaches 70% over the working area. The numerically assessed specific absorption rate in human tissues placed on the proposed structure is 11 mW/kg, which is 30 times lower than on a conventional planar spiral coil. Our results pave the way to compact and safe WPT platforms for one-to-many charging.

Design, experiment, and performance analysis of magnetorheological clutch with uniform magnetic field distribution along the radial direction for tension control.

High-precision tension control is very important for industrial production. Benefit from the unique property of magnetorheological (MR) fluid, a MR clutch is a potential device in a tension control system due to its fast response and large output dynamics. In this paper, based on the detailed analysis of axial magnetic induction intensity generated by the coil along the radial distribution, a shear-mode MR clutch with uniform magnetic field distribution along the radial direction is proposed for the tension control system. A theoretical model of torque transmission and a magnetic circuit were derived, and a magnetostatic simulation was carried out to validate the effectiveness of the proposed circuit design. Experimental evaluation was conducted to investigate the static and dynamic transmission torque property. Finally, a proportional-integral-differential controller was designed to regulate the output torque. Experimental testing demonstrated the effectiveness and practical feasibility of the MR clutch in tension control.

Analysis of Litz Wire Losses Using Homogenization-Based FEM

This article proposes a novel analysis method for the losses in a litz wire considering the circulating and eddy currents. In the proposed method, the macroscopic complex permeability is introduced to evaluate the eddy current loss owing to the proximity effect. Homogenization-based finite element analysis using the macroscopic complex permeability can effectively compute the proximity effect loss. The loss owing to the circulating currents is evaluated by solving the corresponding circuit equation. It is shown that the eddy current loss is dominant for the solenoidal and spiral coils without magnetic cores because of the uniform magnetic field distribution along the wire. Moreover, the magnetic reactors in which the circulating current loss cannot be ignored are analyzed using the proposed method. It is shown that the losses computed by the proposed method agree well with the measured results.

Measurement of Stress and Temperature Dependent Vector Magnetic Properties of Electrical Steel Sheet

In addition to the excitation condition, the magnetic properties of electrical steel sheet (ESS) also depend on stress and temperature. This article proposes one systematic measurement method of measuring the vector magnetic properties (VMPs) of ESS. In the proposed method, the coupling effect of temperature and stress are considered. The uniform magnetic field distribution is generated in the measurement area of the specimen using a vertical-type orthogonal double-U shaped yoke. Since the specimen is cross shaped, the mechanical stress can be applied conveniently. The B-probe method and the double compound H-coil method are adopted to measure vector magnetic flux density ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</i> ) and magnetic field intensity ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">H</i> ). Moreover, the vector B-H sensor is assembled into a ceramic frame from within, which can withstand high temperature. A linear actuator can apply tensile or compressive stress along the rolling direction and transverse direction of the specimen. The variable temperature measurement conditions are achieved by a temperature controller and ceramic heating sheets. The stress and temperature dependent VMPs of nongrain oriented ESS and grain-oriented ESS are measured to verify the effectiveness of the proposed measurement method. The change regulation of VMPs with temperature and stress is summarized.

Optimization of a Two-Layer 3D Coil Structure with Uniform Magnetic Field

Conventional magnetically coupled resonant wireless power transfer systems are faced with resonant frequency splitting phenomena and impedance mismatch when a receiving coil is placed at misaligned position. These problems can be avoided by using uniform magnetic field distribution at receiving plane. In this paper, a novel 3D transmitting coil structure with improved uniform magnetic field distribution is proposed based on a developed optimization method. The goal is to maximize the average magnetic field strength and uniform magnetic field section of the receiving plane. Hence, figures of merit (FoM1 and FoM2) are introduced and defined as product of average magnetic field strength and length or surface along which uniform magnetic field is generated, respectively. The validity of the optimization method is verified through laboratory measurements performed on the fabricated coils driven by signal generator at operating frequency of 150 kHz. Depending on the allowed ripple value and predefined coil proportions, the proposed transmitting coil structure gives the uniform magnetic field distribution across 50% to 90% of the receiving plane.

Planar hexagonal broadband near‐field antenna for UHF RFID application

In the design of ultra-high-frequency (UHF) radio frequency identification (RFID) near-field antennas, realising an antenna with electrically large size, wide frequency band, and far read range has always been a challenge. To solve these problems, a planar hexagonal broadband near-field antenna for UHF RFID applications is proposed and investigated. The antenna structure is composed of six dipoles (resembles three sets of double-arrow structures) that are printed on the top and bottom surfaces of an FR4 substrate, forming a dual-layer (inner and outer layers) broken hexagonal loop. The 10 dB return loss bandwidth of the proposed antenna can reach up to 20.8% (856–1055 MHz), thus able to cover the entire European Telecommunication Standards Institute and Federal Communication Commission system operating bands. Since the current distribution along the loop is kept in a single direction and the length of each dipole is ∼1/2 wavelength (λ), strong and uniform z-component magnetic field distribution with an 18 cm read range can be obtained in the near-field zone of the antenna. As the proposed near-field antenna has a planar size of 175 mm × 175 mm, it is suitable for multi-system and wide-range RFID applications.