Coil Efficiency Research Articles

Use of the Perturbing Sphere Method for the Estimation of Radiofrequency Coils' Efficiency in Magnetic Resonance Applications: Experience from an Electromagnetic Laboratory.

Radiofrequency (RF) transmitter and receiver coils are employed in in magnetic resonance (MR) applications to, respectively, excite the nuclei in the object to be imaged and to pick up the signals emitted by the nuclei with a high signal-to-noise ratio (SNR). The ability to obtain high-quality images and spectra in MR strongly depends on the RF coil's efficiency. Local coil efficiency can be estimated with magnetic field mapping methods evaluated at a fixed point in space. Different methods have been described in the literature, divided into electromagnetic bench tests and MR techniques. In this paper, we review our experience in designing and testing RF coils for MR in our electromagnetic laboratory with the use of the perturbing sphere method, which permits coil efficiency and magnetic field mapping to be estimated with great accuracy and in a short space of time, which is useful for periodic coil quality control checks. The method's accuracy has been verified with simulations and workbench tests performed on RF coils with different surfaces and of different volumes. Furthermore, all the precautions taken to improve the measurement sensitivity are also included in this review, in addition to the various applications of the method that have been described over the last twenty years of research in our electromagnetic laboratory.

Topology Optimization-Based Design of Magnet System in Unilateral NMR Logging Sensor Considering Coil Efficiency

Topology Optimization-Based Design of Magnet System in Unilateral NMR Logging Sensor Considering Coil Efficiency

Evaluating a Controlled Electromagnetic Launcher for Safe Remote Drug Delivery

Biologists and veterinarians rely on dart projectors to inject animals with drugs, take biopsies from specimens, or inject tracking chips. Firearms, air guns, and other launchers are limited in their ability to precisely control the kinetic energy of a projectile, which can injure the animal if too high. In order to improve the safety of remote drug delivery, a lidar-modulated electromagnetic launcher and a soft drug delivery dart were prototyped. A single-stage revolver coilgun and soft dart were designed and tested at distances up to 8 m. With a coil efficiency of 2.25%, the launcher could consistently deliver a projectile at a controlled kinetic energy of 1.00 ± 0.006 J and an uncontrolled kinetic energy of 2.66 ± 0.076 J. Although modifications to charging time, sensors, and electronics could improve performance, our launcher performed at the required level at the necessary distances. The precision achieved with commercial components enables many other applications, from law enforcement to manufacturing.

Analysis and Design of Configurable Rectifier With Compensated Near-Zero Impedance Angle for Megahertz Wireless Power Transfer

Adapting to wide load range variations is always one of the main goals pursued by megahertz (MHz) wireless power transfer (WPT) systems. For this, the nonlinear phenomenon of the rectifier has become a problem that must be solved. Compared with the method of optimizing the efficiency around the maximum power point in the traditional design, this paper proposes a global load optimization method. Meanwhile, a novel configurable compensated near-zero impedance angle rectifier (NIAR) is proposed to take full advantage of the global optimization method. The NIAR not only realizes the compression of a wide range of impedance angles, but also solves the classic problems of Class E rectifiers such as large stress, voltage asymmetry, and low real part of input impedance. The configurable design allows the system to adjust the rectifier unit according to the output index, thereby fully improving the coil efficiency, and providing a solution for the application of high-voltage and high-power MHz WPT systems in the future. To verify the effectiveness of the proposed NIAR and design method, a 60 W prototype was built, and a traditional full-wave rectifier was used for comparison. The results show that the proposed NIAR system exhibits better characteristics in terms of system efficiency, voltage stress, and symmetry.

Magnetic coupling resonant ultra‐high voltage electric vehicle wireless charging system based on parity‐time symmetry

SummaryIn recent years, the electric vehicle (EV) industry has developed rapidly, and research on wireless charging systems for the EV is very meaningful. This paper proposes a magnetic coupling resonant ultra‐high voltage EV wireless charging system based on parity‐time symmetry to meet the need of high transmission efficiency and high transmission power for the EV wireless charging system. Through the application of magnetic coupling resonance and parity‐time symmetry, the charging system can maintain constant transmission efficiency within a larger spatial range, which greatly reduces the high‐precision requirements for coil alignment. Through the proposed intelligent coil alignment system, the alignment degree of the power transmission coil, user experience, and energy transmission efficiency can be improved further, promoting the intelligence of the charging system. At the same time, an ultra‐high voltage booster module is added to the primary side of the system to provide high‐power input for the EV charging system. Simulation analysis and experiments verify the rationality and practicability of the charging system. Finally, the practical significance of the proposed system applied to power grid inspection work is discussed.

The Minimum Admissible Detuning Efficiency of MRI Receive-Only Surface Coils.

The minimum admissible detuning efficiency (DE) of a receive coil is an essential parameter for coil designers. A receive coil with inefficient detuning leads to inhomogeneous B1 during excitation. Previously proposed criteria for quantifying the DE rely on indirect measurements and are difficult to implement. To present an alternative method to quantify the DE of receive-only surface coils. Theoretical study supported by simulations and phantom experiments. Uniform spherical (100 mm diameter) and cylindrical (66 mm diameter) phantoms. Dual repetition time B1 mapping sequence at 1.5T, and Bloch-Siegert shift B1 mapping sequence at 3.0T. One non-planar (80 × 43 mm2) and two planar (40 and 57 mm diameter) surface coils were built. Theoretical analysis was performed to determine the minimum DE required to avoid B1 distortions. Experimental B1 maps were acquired for the non-planar and planar surface coils at both 1.5T and 3.0T and visually compared with simulated B1 maps to assess the validity of the theoretical analysis. None. Based on the theoretical analysis, the proposed minimum admissible DE, defined as DEthr = 20 Log (Q) + 13 dB, depended only on the quality factor (Q) of the coil and was independent of coil area and field strength. Simulations and phantom experiments showed that when the DE was higher than this minimum threshold level, the B1 field generated by the transmission coil was not modified by the receive coil. The proposed criterion for assessing the DE is simple to measure, and does not depend on the area of the coil or on the magnetic field strength, up to 3T. Experimental and simulated B1 maps confirmed that detuning efficiencies above the theoretically derived minimal admissible DE resulted in a non-distorted B1 field. 2 TECHNICAL EFFICACY: Stage 1.

Novel cellular coil design for improved temperature uniformity in inductive heating of carbon fibre composites

Electromagnetic induction has great potential for energy efficient manufacturing of polymer composites. Carbon fibre can be directly heated inductively, achieving rapid, localised, and volumetric heating. The main obstacle to wider adaption of induction in composites processing is the non-uniformity of temperature distribution in-plane and through thickness of composite laminates. This study puts forward a new cellular architecture of the coil and examines the feasibility of addressing the problem of field uniformity. The geometrical parameters of the coil were examined in concurrent modelling and experimental work, aiming at reducing the effect of field cancellation in neighbouring cells and maximising both the coil efficiency and the field uniformity. The paper demonstrates good potential of the new coil designed with significantly improved uniformity of temperature field compared to traditional commercially available coils. This result has been further optimised with parametric finite element analysis to reduce the standard deviation in surface temperature by a further 37%.

High inductance magnetic-core coils have enhanced efficiency in inducing suprathreshold motor response in rats

Objective. Transcranial magnetic stimulation (TMS) coil design involves a tradeoff among multiple parameters, including magnetic flux density (B), inductance (L), induced electric (E) field, focality, penetration depth, coil heating, etc. Magnetic materials with high permeability have been suggested to enhance coil efficiency. However, the introduction of magnetic core invariably increases coil inductance compared to its air-core counterpart, which in turn weakens the E field. Our lab previously reported a rodent-specific TMS coil with silicon steel magnetic core, achieving 2 mm focality. This study aims to better understand the tradeoffs among B, L, and E in the presence of magnetic core. Approach. The magnetic core initially operates within the linear range, transitioning to the nonlinear range when it begins to saturate at high current levels and reverts to the linear range as coil current approaches zero; both linear and nonlinear analyses were performed. Linear analysis assumes a weak current condition when magnetic core is not saturated; a monophasic TMS circuit was employed for this purpose. Nonlinear analysis assumes a strong current condition with varying degrees of core saturation. Main results. Results reveal that, the secondary E field generated by the silicon steel core substantially changed the dynamics during TMS pulse. Linear and nonlinear analyses revealed that higher inductance coils produced stronger peak E fields and longer E field waveforms. On a macroscopic scale, the effects of these two factors on neuronal activation could be conceptually explained through a one-time-constant linear membrane model. Four coils with different B, L, and E characteristics were designed and constructed. Both E field mapping and experiments on awake rats confirmed that inductance could be much higher than previously anticipated, provided that magnetic material possesses a high saturation threshold. Significance. Our results highlight the novel potentials of magnetic core in TMS coil designs, especially for small animals.

Coil Design Method Considering Power, Efficiency, and Magnetic Field Strength of Coreless Coils Using Numerical Analysis in Wireless Power Transfer

Wireless power transfer to electric vehicles while driving is a technology that is currently attracting attention as it has the potential to solve the problems faced by EVs that will become popular worldwide in the future. In the case of dynamic wireless power transfer (DWPT) to electric vehicles, it is necessary to transmit the required power with high efficiency and ensure safety. While there are multiple safety requirements, there are also regulatory values for the magnetic field strength emitted from the coil, which must be satisfied before it can be implemented in society. This research aims to speed up the design of coils by using only numerical analysis, in contrast to conventional coil design using electromagnetic field analysis. In the future construction of wireless power transfer systems, it is of great significance to discuss the design using only numerical analysis, as there are many design parameters. Experiments were carried out and compared with the numerical analysis and found to be in agreement. A power transmission of 20 kW converted value was carried out to verify the effect of the size, pitch, and number of turns of the transmission coils on the efficiency and magnetic field strength. The effect of input voltage was also discussed by simulation, and the optimum coil was derived. As a result, when the coil size was 800 × 600 mm, the number of turns was 46, the pitch was 6.1 mm and the input voltage was 1400 V, the analysis showed that a power transmission of 20 kW with an efficiency of 99.3% and a far magnetic field leakage strength of 81.6 dBμA/m could be achieved. Even with a coreless coil, the magnetic field strength could be below the regulation value. © 2023 Institute of Electrical Engineer of Japan and Wiley Periodicals LLC.

Low-cost inductively coupled stacked wireless RF coil for MRI at 3 T.

Inductively coupled RF coils are an inexpensive and simple method to realize wireless RF coils in MRI. They are low cost and can greatly ease the MR scan setup and improve patient comfort, since they do not require bulky components such as cables, baluns, preamplifiers, and connectors. Previous works have typically used single-layer loops as wireless coils. In this work, we present a novel wireless coil, where two loops are stacked and decoupled with a shared capacitor. We found that such a stacked structure could increase the coil efficiency and SNR. Compared with the single-layer wireless coil, both electromagnetic simulation and MR experiment results demonstrate that the stacked wireless coil has a considerable SNR improvement of approximately 35%.

Systematic Dimensional Analysis of the Scaling Relationship for Gradient and Shim Coil Design Parameters.

To demonstrate systematic, linear algebra-based, dimensional analysis to derive a scaling relationship among the design parameters of MRI gradient and harmonic shim coils. The dimensions of five physical quantities relevant for gradient coil design (inductance, gradient amplitude, inner diameter [ ], current, and the permeability of free space) were decomposed into fundamental units, and their exponents were arranged into a dimensional matrix. The resulting set of homogenous equations was solved using standard linear algebraic methods. Inclusion of the number of turns as an additional unit yielded a 5 × 5 dimensional matrix with a unique, nontrivial solution. The analysis was extended to harmonic shim coils. The gradient coil scaling relationship was compared with data from 24 published gradient coil sets. Only when the unit of turns was included did the linear algebra-based analysis uniquely produce the known scaling relationship that gradient inductance is proportional to gradient efficiency squared times . By applying the same methodology to an lth order shim coil, a novel result is obtained: Shim inductance is proportional to its efficiency squared times . The predicted power-law relationship between inductance-normalized gradient efficiency and the diameter accounted for > 92% of the efficiency variation of the surveyed gradient coils. A dimensionless parameter is proposed as an intrinsic figure-of-merit of gradient coil efficiency. Systematic application of linear algebra-based dimensional analysis can provide new insight in gradient and shim coil design by revealing fundamental scaling relations and helping to guide the design and comparison of coils with different diameters.

Lateral and angular misalignments of coil in wireless power transfer system

Wireless power transfer requires the transmission and reception of energy through coils arranged coaxially and the direction does not change to ensure maximum transmission efficiency and power. However, in actual working environments, coils have lateral and angular misalignments, thereby reducing the transmission performance. In this study, we investigate the transmission efficiency of coils and the mutual inductance of the lateral and angular misalignments theoretically. Coils with lateral and angular misalignments are modeled with the Maxwell simulation software. Finally, a 3D test experimental platform is built. The experimental results show that the use of a magnetic isolation material on the coil can effectively improve the electromagnetic field intensity of the system. The coupling coefficient of the two coils is increased by more than 15%. At a coupling distance of 5 mm, the coupling mechanism with ferrite is approximately 10% more efficient than that without ferrite. Furthermore, it shows that when the misalignment is large, the magnetic isolation material can significantly improve the efficiency of the wireless charging system.

2-D Thin Coil Designs of IPT for Wireless Charging of Automated Guided Vehicles

As the automated guided vehicles (AGVs) play an important role in automated logistics systems, the wireless power transfer (WPT) technologies become a viable solution to provide the AGV with the automatic power charging service. In order to install the WPT systems to the small confined space in the AGV, a 2-D thin coil design is necessary for the receiver (Rx) system installed in the AGV. To greatly reduce the thickness of the Rx coil, including resonant capacitors, an ac/dc rectifier, and dc-link capacitors, an 2-D coil design for a minimum core loss is newly proposed in this article. Based on a finite-element-method (FEM) 3-D simulation analysis, various coil structures have been comparatively evaluated to find the most appropriate core structure. Furthermore, to obtain the highest coil efficiency at nominal operating condition, the appropriate number of turns design is established. Thus, according to the coil design guideline of the proposed inductive power transfer (IPT) system, the appropriate number of turns for transmitter (Tx) and Rx coils <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$N_{\mathrm {1,op}}$ </tex-math></inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$N_{\mathrm {2,op}}$ </tex-math></inline-formula> can be appropriately selected for the high efficient operation of the IPT coils. The 2.5-kW prototypes of the IPT systems for wireless charging of AGV were fabricated and verified by simulations and experiments. The results showed that the total thickness of the coil sets including all the components is 32.0 mm for the Rx coil. The measured maximum coil efficiency for <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$P_{L} =2.5$ </tex-math></inline-formula> kW at 40-mm coil distance was 98.9% by the proposed IPT coil design procedure. The measured temperatures at the ferrite core and coil for the Rx coil were managed below 53.3 °C and 59.3 °C during 2.5-kW wireless charging, respectively, under a 10-min charging and 20-min AGV operation cycle, which provides the thermal-stable operation of the proposed IPT systems.

Coil Efficiency for Inductive Peripheral Nerve Stimulation.

Magnetic stimulation of peripheral nerves is evoked by electric field gradients caused by high-intensity, pulsed magnetic fields created from a coil. Currents required for stimulation are very high, therefore devices are large, expensive, and often too complex for many applications like rehabilitation therapy. For repetitive stimulation, coil heating due to power loss poses a further limitation. The geometry of the magnetic coil determines field depth and focality, making it the most important factor that determines the current required for neuronal excitation. However, the comparison between different coil geometries is difficult and depends on the specific application. Especially the distance between nerve and coil plays a crucial role. In this investigation, the electric field distribution of 14 different coil geometries was calculated for a typical peripheral nerve stimulation with a 27 mm distance between axon and coil. Coil parameters like field strength and focality were determined with electromagnetic field simulations. In a second analysis, the activating function along the axon was calculated, which quantifies the efficiency of neuronal stimulation. Moreover, coil designs were evaluated concerning power efficacy based on ohmic losses. Our results indicate that power efficacy of magnetic neurostimulation can be improved significantly by up to 40% with optimized coil designs.

Finite Element Analysis-Aided Optimization of Rectangular Coil Assemblies Applied in Electric Vehicle Inductive Chargers

Energy efficiency and leakage magnetic field (LMF) are two important issues in electric vehicle inductive chargers. In this work, the maximum achievable coil efficiency and the corresponding LMF strength are formulated as functions of system parameters, and figures of merits (FOM) are proposed for assessing the efficiency and LMF performance of the coil assembly pair. The impacts of the coil assemblies’ geometric parameters on both FOMs are examined with the aid of finite element analysis (FEA), and measures to improve the FOMs are suggested based on FEA results. A manual optimization process is conducted on a coil assembly pair. Compared with the initial design, the optimized one results in a higher DC-to-DC efficiency and lower LMF strength while consuming less copper. The performance improvement is verified by FEA results and experimental data measured on an 85 kHz electric vehicle inductive charger prototype. The key measures for coil assembly optimization are summarized.

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.

A 31-channel integrated "AC/DC" B0 shim and radiofrequency receive array coil for improved 7T MRI.

To test an integrated "AC/DC" array approach at 7T, where B0 inhomogeneity poses an obstacle for functional imaging, diffusion-weighted MRI, MR spectroscopy, and other applications. A close-fitting 7T 31-channel (31-ch) brain array was constructed and tested using combined Rx and ΔB0 shim channels driven by a set of rapidly switchable current amplifiers. The coil was compared to a shape-matched 31-ch reference receive-only array for RF safety, signal-to-noise ratio (SNR), and inter-element noise correlation. We characterize the coil array's ability to provide global and dynamic (slice-optimized) shimming using ΔB0 field maps and echo planar imaging (EPI) acquisitions. The SNR and average noise correlation were similar to the 31-ch reference array. Global and slice-optimized shimming provide 11% and 40% improvements respectively compared to baseline second-order spherical harmonic shimming. Birdcage transmit coil efficiency was similar for the reference and AC/DC array setups. Adding ΔB0 shim capability to a 31-ch 7T receive array can significantly boost 7T brain B0 homogeneity without sacrificing the array's rdiofrequency performance, potentially improving ultra-high field neuroimaging applications that are vulnerable to off-resonance effects.

Metamaterial-Inspired Radiofrequency (RF) Shield With Reduced Specific Absorption Rate (SAR) and Improved Transmit Efficiency for UHF MRI.

To prevent the interferences between radiofrequency (RF) coils and other components in the magnetic resonance imaging (MRI) system such as gradient coils, it is essential to place an RF shield between the RF coils and gradient coils. However, the induced currents on conventional RF shields have negative influences on the RF coil performance. To reduce these influences, metamaterial absorbers (MA), a class of metamaterials exhibiting nearly unity absorption rate for the incident electromagnetic fields, can be employed for the design of a novel RF shield. However, the adoption of metamaterials in MRI systems is usually problematic because of the bulkiness of the metamaterial structure. In this work, capacitors and metallic interconnectors are used to miniaturize the MA so that the unit MA cell can operate at the Larmor frequencies of 7T and 9.4T MRI and stay compact. This MA-RF shield is used to improve the transmit efficiency of RF surface coils and reduce the specific absorption rate (SAR) in the region of interest (ROI). It is successfully demonstrated by simulations and experiments that, compared with conventional RF shield structure, the transmit efficiency can be enhanced by more than 32% and the peak SAR value can be reduced by 22% using the MA-RF shield. Moreover, it is observed that the transmit field penetration is improved when the surface coil is used with the MA-RF shield. This proof-of-concept study suggests a new practical way for the utilization of metamaterials in ultra-high field MRI applications.

Coil characteristic analysis and efficiency optimisation of wireless power transmission system

How to charge more conveniently is a challenging problem, which affects people's daily life from two aspects of distance and portability. Considering the coil characteristics and coil size, the tra...

Coil characteristic analysis and efficiency optimisation of wireless power transmission system

Coil characteristic analysis and efficiency optimisation of wireless power transmission system