Coaxial Coils Research Articles

A Three‐Phase Omnidirectional Wireless Charger Based on Segmented Arc‐Shaped Coupler With High Anti‐Rotation Offset Feature for AUVs

ABSTRACTWhen the autonomous underwater vehicle (AUV) is charged by wireless charging technology (WCT), the coupler will be misalignment due to the impact of marine environment factors, which can lead to the weak stability of the system output. Besides, the electromagnetic field generated by the coaxial coil is concentrated in the center of the AUV hull, and the strong electromagnetic field may interfere with the normal operation of the power electronic equipment inside the AUVs. To address the above problems, this paper proposes a novel coupler with high anti‐offset and suppression of the central magnetic field of the AUVs. The coupler is composed of a three‐phase segmented arc‐shaped transmitter coils and a cylindrical receiver coil. The coupler can implement the suppression of the AUV central magnetic field and the promotion of anti‐rotation offset feature under the omnidirectional uniform magnetic field control method. To better adapt the proposed coupler and meet the battery charging requirements, a three‐phase wireless charging topology with a relay is designed, which can acquire the smooth transition between the two charging modes without constructing the wireless communication between two sides and detecting the state of charge (SOC). Finally, to verify the feasibility of the proposed scheme, a 1‐kW AUV wireless charging prototype is built. The experimental results indicate that the maximum output power fluctuation rate of the system is 1.1% in the case of the full‐angle rotation offset.

An 8Tx/32Rx head-neck coil at 7T by combining 2Tx/32Rx Nova coil with 6Tx shielded coaxial cable elements.

Standard head coils used at 7T MRI suffer from high signal loss at lower brain regions and neck. This study aimed to increase the field of view (FOV) of a birdcage coil to image the lower brain regions and neck with a straightforward approach of using add-on transmit shielded coaxial cable coil (SCC) elements. A new head-neck coil was modeled as a combination of the 2Tx/32Rx Nova head coil and 6Tx SCC elements. The add-on transmit SCC elements have been produced. The full wave electromagnetic simulations were performed to analyze the coil geometry and estimate the local specific absorption ratio (SAR). The field maps and structural images were acquired in a phantom and in vivo on a 7T scanner. The computed SAR histogram revealed a peak of 4.08 W/kg. The simulated and measured maps are in good agreement. The manufactured coil's S-parameters are below 10 dB. The field measurements on a subject presented the increase in the FOV. The T1-weighted structural images of three subjects acquired with the head-neck coil showed increased coverage compared to the head coil only. Combining the 2Tx/32Rx Nova head coil and 6Tx SCC elements allowed imaging of the whole brain with an increased FOV down to the C4 spine. The coil stayed fully functional when different subjects were scanned. We conclude that the SCC transmit-only coils are a robust adjunct to conventional coil designs and can meaningfully enhance and expand their field of view.

Introduction of a novel longitudinal-flexural-torsional actuator using a magnetostrictive rod

Abstract This paper presents a novel bulk magnetostrictive actuator, utilizing a rod made of Permendur alloy. This actuator demonstrates versatility in accommodating longitudinal, torsional, and flexural deformations. Longitudinal deformation is induced by applying magnetic fields along the material’s axis, exploiting the Joule effect. The axial magnetic field is generated by a coaxial coil surrounding the material. Torsional deformation is achieved by passing current through the magnetostrictive material and applying circumferential magnetic field, following the Wiedeman effect. To induce flexural deformation, a magnetic field is applied at the material’s surface, facilitated by a magnetic coupler. Actuation is executed using DC magnetic fields, granting the actuator control over four degrees of freedom. Numerical analysis conducted utilizing COMSOL software, showed the combined modes and their mutual influences. The actuator’s displacement range reaches a maximum of 12 µms longitudinally, 7 µms flexurally, and 0.15 degrees torsionally. It also exhibits the capability of simultaneous excitation in two or three different modes. The experiments and studies conducted show the potential application of this innovative actuator in precise positioning systems such as microscopes and optical systems.

Assemblies of Coaxial Pick-Up Coils as Generic Inductive Sensors of Magnetic Flux: Mathematical Modeling of Zero, First and Second Derivative Configurations.

Coils are one of the basic elements employed in devices. They are versatile, in terms of both design and manufacturing, according to the desired inductive specifications. An important characteristic of coils is their bidirectional action; they can both produce and sense magnetic fields. Referring to sensing, coils have the unique property to inductively translate the temporal variation of magnetic flux into an AC voltage signal. Due to this property, they are massively used in many areas of science and engineering; among other disciplines, coils are employed in physics/materials science, geophysics, industry, aerospace and healthcare. Here, we present detailed and exact mathematical modeling of the sensing ability of the three most basic scalar assemblies of coaxial pick-up coils (PUCs): in the so-called zero derivative configuration (ZDC), having a single PUC; the first derivative configuration (FDC), having two PUCs; and second derivative configuration (SDC), having four PUCs. These three basic assemblies are mathematically modeled for a reference case of physics; we tackle the AC voltage signal, VAC (t), induced at the output of the PUCs by the temporal variation of the magnetic flux, Φ(t), originating from the time-varying moment, m(t), of an ideal magnetic dipole. Detailed and exact mathematical modeling, with only minor assumptions/approximations, enabled us to obtain the so-called sensing function, FSF, for all three cases: ZDC, FDC and SDC. By definition, the sensing function, FSF, quantifies the ability of an assembly of PUCs to translate the time-varying moment, m(t), into an AC signal, VAC (t). Importantly, the FSF is obtained in a closed-form expression for all three cases, ZDC, FDC and SDC, that depends on the realistic, macroscopic characteristics of each PUC (i.e., number of turns, length, inner and outer radius) and of the entire assembly in general (i.e., relative position of PUCs). The mathematical methodology presented here is complete and flexible so that it can be easily utilized in many disciplines of science and engineering.

Detecting and Estimating Local Corrosion Damages in Long-Service Aircraft Structures by the Eddy Current Method with Double-Differential Probes

ABSTRACT Monitoring corrosion in aircraft structures through nondestructive testing is crucial for maintaining long-term aircraft serviceability. Corrosion monitoring is particularly challenging when corrosion damage is situated on internal surfaces of multilayer aircraft structures. The eddy current method is one of the most promising techniques for detecting and measuring such subsurface corrosion damage without direct contact or disassembly. However, due to their low sensitivity traditional eddy current probes with coaxial coils are not well suited for detecting corrosion damages of the local type, such as pitting or corrosion pits, in multilayer aircraft structures. This study tested the use of low-frequency eddy current probes of the double-differential type, characterized by 8 and 10 mm operational diameters, in detecting and measuring hidden corrosion damages of this local type. Such corrosion damages were simulated by means of flat-bottomed drilled holes of differing diameters and depths (or different diameters and residual thicknesses of the inspected sheet in the damaged area). The signals from the eddy current probes were evaluated in the complex plane using a universal eddy current flaw detector. The correlations between the amplitude and phase of the eddy current signal and depth of location of the local corrosion damages were analyzed. Results indicate that it is possible to estimate the residual thickness of the skin in locally corroded areas by measuring the eddy current signal phase, independently of the local corrosion damage diameter (size), providing useful information for residual service life determination.

AC Magnetic Susceptibility: Mathematical Modeling and Experimental Realization on Poly-Crystalline and Single-Crystalline High-Tc Superconductors YBa2Cu3O7-δ and Bi2-xPbxSr2Ca2Cu3O10+y.

The multifaceted inductive technique of AC magnetic susceptibility (ACMS) provides versatile and reliable means for the investigation of the respective properties of magnetic and superconducting materials. Here, we explore, both mathematically and experimentally, the ACMS set-up, based on four coaxial pick-up coils assembled in the second-derivative configuration, when employed in the investigation of differently shaped superconducting specimens of poly-crystalline YBa2Cu3O7-δ and Bi2-xPbxSr2Ca2Cu3O10+y and single-crystalline YBa2Cu3O7-δ. Through the mathematical modeling of both the ACMS set-up and of linearly responding superconducting specimens, we obtain a closed-form relation for the DC voltage output signal. The latter is translated directly to the so-called extrinsic ACMS of the studied specimen. By taking into account the specific characteristics of the studied high-Tc specimens (such as the shape and dimensions for the demagnetizing effect, porosity for the estimation of the superconducting volume fraction, etc.), we eventually draw the truly intrinsic ACMS of the parent material. Importantly, this is carried out without the need for any calibration specimen. The comparison of the mathematical modeling with the experimental data of the aforementioned superconducting specimens evidences fair agreement.

Sensitivity Analysis and Comparative Study for Different Detection Modes of Logging-While-Drilling Ultradeep Azimuthal Electromagnetic Tools

Summary The logging-while-drilling (LWD) ultradeep azimuthal electromagnetic tool plays a pivotal role in real-time drilling optimization operations. Established tool designs include arrays of coaxial and tilted coils that, during drilling operations, can be processed to a multicomponent magnetic induction data. These data can then be combined into different detection modes, which accentuate sensitivity to particular geological features. Leveraging the established coil design and definitions of various detection modes for an electromagnetic look-ahead (EMLA) tool, this study undertakes a comprehensive exploration of the disparities in detection performance and characterization of subsurface parameters. Through sensitivity analysis, the varying degrees of sensitivity exhibited by these detection modes concerning parameters such as subsurface formation resistivity, formation inclination, and electrical anisotropy have been investigated. The ensuing conclusions derived from an in-depth analysis are as follows: Detection Mode I exhibits remarkable prowess in delineating subsurface boundaries. Optimal exploration distances can be achieved through the judicious selection of source-receiver distances and frequencies. Detection Mode II displays heightened sensitivity to wellbore inclination and anisotropy, effectively elucidating subsurface resistivity anisotropy. This sensitivity is particularly pronounced at wellbore inclinations approaching 60°. Detection Mode III, while lacking directional capability, nonetheless furnishes fundamental insights into subsurface resistivity. Detection Mode IV demonstrates exceptional sensitivity to electrical anisotropy, particularly at higher wellbore inclinations, manifesting a conspicuous response to subsurface resistivity anisotropy. In summary, the diverse detection modes within the realm of ultradeep azimuthal electromagnetic technology each offer distinctive attributes, facilitating optimal mode selection to attain superior outcomes as per specific requisites. This research contributes significantly to an enhanced comprehension of the performance and applicability of the ultradeep azimuthal electromagnetic tool in the field of optimal drilling.

Numerical and experimental investigation of a high-static-low-dynamic stiffness vibration isolator with tunable stiffness and damping

Nonlinear vibration isolators that take advantage of negative stiffness elements can simultaneously meet the requirements of acceptable static deflection and frequency band of isolation and are thus superior to their linear counterparts. Incorporating electromagnetic elements into the vibration isolator is a common approach to provide the required negative stiffness. Electromagnetic mechanisms are low-cost, tuneable, and operate without friction. Also, they can extract or dampen the energy of the host system through electromechanical coupling. Therefore, electromechanical elements are ideal choices for designing multi-purpose vibration control mechanisms. In this study, three coils surrounded by arrays of permanent magnets are considered. When the system vibrates, their magnetic fields interact with that of a moving magnet attached to a rod supporting an isolated mass. The electromagnetic properties of the proposed system are simulated using the finite element method (FEM) and experimental measurements are used to validate the numerical simulations. Different scenarios for a system consisting of three coaxial coils are considered and compared, including powering the coils, open circuit coils, short circuit coils and coils shunted with an external circuit. Features of each case are discussed and general guidelines for designing a multipurpose vibration controller are given.

Evaluation of coaxial dipole antennas as transceiver elements of human head array for ultra-high field MRI at 9.4T.

The aim of this work is to evaluate a new eight-channel transceiver (TxRx) coaxial dipole array for imaging of the human head at 9.4T developed to improve specific absorption rate (SAR) performance, and provide for a more compact and robust alternative to the state-of-the art dipole arrays. First, the geometry of a single coaxial element was optimized to minimize peak SAR and sensitivity to the load variation. Next, a multi-tissue voxel model was used to numerically simulate a TxRx array coil that consisted of eight coaxial dipoles with the optimal configuration. Finally, we compared the developed array to other human head dipole arrays. Results of numerical simulations were verified on a bench and in the scanner including in vivo measurements on a healthy volunteer. The developed eight-element coaxial dipole TxRx array coil showed up to 1.1times higher SAR-efficiency than a similar in geometry folded-end and fractionated dipole array while maintaining whole brain coverage and low sensitivity of the resonance frequency to variation in the head size. As a proof of concept, we developed and constructed a prototype of a 9.4T (400 MHz) human head array consisting of eight TxRx coaxial dipoles. The developed array improved SAR-efficiency and provided for a more compact and robust alternative to the folded-end dipole design. To the best of our knowledge, this is the first example of using coaxial dipoles for human head MRI at ultra-high field.

A Rigorous Explicit Expression for the Mutual Inductance of Two Co-Axial Thin-Wire Coil Antennas Placed above a Layered Ground

This paper presents a quasi-analytical method that allows the derivation of a rigorous series-form representation for the mutual inductance of two co-axial coil antennas located above an arbitrarily layered earth structure. Starting from Biot–Savart law, which gives the integral representation for the primary vector potential generated by the source coil, the potential reflected by the layered ground is derived, and the resulting total vector potential is then integrated along the external circumference of the receiving coil to give the mutual inductance of the two antennas. The obtained representation for the flux is then evaluated analytically through the usage of the Gegenbauer addition theorem once an accurate, rational approximation is used in place of the factor of the integrand that exhibits branch cuts. It is shown how the resulting explicit solution exhibits the same degree of accuracy as purely numerical approaches like the finite-difference time-domain (FDTD) method and conventional numerical quadrature schemes, while it is less time-demanding than the latter methods.

Impedance Variation in a Coaxial Coil Encircling a Metal Tube Adapter.

The impedance change in an induction coil surrounding a metal tube adapter is investigated using the truncated region eigenfunction expansion (TREE) method. The conventional TREE method is inapplicable to this problem as a consequence of the numerical overflow of the eigenfunctions of the air-metal multi-subdomain regions. The difficulty is surmounted by a normalization procedure for the numerical eigenfunctions obtained from the 1D finite element method (FEM). An efficient algorithm is devised by the Clenshaw-Curtis quadrature rule for integrals involving the numerical eigenfunctions. The numerical results of the TREE and FEM simulation coincide very well in all cases, and the efficiency of the proposed method is also confirmed.

Investigation of the discharging performance of an ice bank with a falling film flow around a melting surface

For the first time, the technical characteristics of an ice bank with a falling film flow around melting ice on the surface of coaxial tube coils in the temperature range of 20÷60 °C and mass flow rates of 0,25÷1,0 kg/s of water at the apparatus inlet are studied. The discharging rate is determined depending on the mass flow to wetted perimeter ratio, the temperature of the water at the apparatus inlet and the discharging duration. The film flow allows obtaining water at the apparatus outlet with a temperature of only 0,5÷1 °C higher than the melting point of ice. Keywords: ice bank, ice, film flow, melting, peak load, refrigeration galinagoncharova@mail.ru, korolev.vnihi@mail.ru

A 50 kA Superconducting Transformer for the Upcoming High-Field High-Current Testing Station at the BNL

Brookhaven national laboratory is upgrading its exist-ing user facility to support R&D for High Energy Physics (HEP) and Fusion Energy Sciences (FES). The goal of this upgrade is to host the testing of the superconducting cables, conductors, joints and insert coils under high magnetic field (10 T), with currents up to 50 kA (present limit 20 kA) and at temperatures varying from 4 K to 40 K. To provide a current up to 50 kA to the sample, a superconducting core-less transformer is under construction. The superconducting transformer would additionally reduce the heat loss at the current leads at high currents. The SC transformer consists of two coaxial coils. The primary coil is inside the secondary coil. The primary coil consists of 3048 turns of rectangular wires (1.91 mm × 1.27 mm), wound in 24 layers. The secondary coil consists of 13 turns of the secondary cable (18.35 mm × 5.96 mm), wound in one layer. The parameters of the transformer are chosen to satisfy the space re-strictions while using the available conductor. This paper presents the design of the 50-kA transformer.

Vector Potential Coupling From Outside the Loop Through the Superconducting Shield

We tested whether the Maxwell-Lodge effect, a classical version of the Aharonov-Bohm effect, occurs even when electromagnetic shielding is provided by a superconducting tube. A long, thin superconducting solenoid coil was confined inside a lead tube so that there was no flux leakage from the coil ends. The double semi-circular vector potential coils ensure that 1) there is no local magnetic field at the location of the copper wire in the secondary coil and 2) there is no magnetic flux inside the loop of the secondary coil. Three different materials with different permeability were used for the core. A coaxial wire secondary coil shielded by a lead tube was placed orthogonally to the center of the semicircle of the primary coil. The voltage generated in the secondary coil at 4.2 K was only reduced to about 1/10 of 300 K. This coupling between the primary and secondary coils is due to the vector potential, since it cannot be completely shielded even with a superconducting shield.

Performance Improvement of Different Resonant Inductive Couplers Used for Wireless Power Transfer

This paper experimentally investigates the performance of spiral coil configurations commonly used in the magnetic resonant wireless power transfer (WPT) inductive couplers. Three different designs are illustrated, namely, single-core, twin-core “bifilar”, and single-core coaxial cable spiral coils. The major concern is the improvement of both the voltage gain and transmitted power efficiency at different distances between the transmitter and receiver coils. Moreover, the performance analysis of the experimental setup is compared with the results of three-dimensional frequency-domain extensive finite element (FE) simulations of COMSOL Multiphysics package, through radio frequency modelling of coupled electromagnetic waves and electric circuits. Such FE simulations help in realizing the relative effect of inherent parameters that are hardly to be investigated through experimental measurement without disturbing the WPT system. Additionally, PSPICE is used to facilitate the performance investigation of such magnetic resonant WPT couplers. Results reveal that adding the ferrite sheets to single-core spiral coils helps in improving the WPT efficiency. Shorted bifilar wound flat spiral coils show higher WPT efficiency at short coil separation due to their intrinsic self-capacitance and high self-inductance. Self-resonating “capacitorless”, single-core coaxial cable wound flat spiral coils show an enhanced performance improvement when compared with the other two designs.

Development of a Radially Coupled Wireless Charging System for Torpedo-Shaped Autonomous Underwater Vehicles

Spiral coaxial coils are widely used in wireless charging systems for autonomous underwater vehicles (AUVs). However, these coils can generate axial electromagnetic interference that may adversely affect the electronic components contained within the AUV. In order to overcome this issue, this paper introduced a pair of radially coupled coils which implement distributed ferrite cores. The mathematical model of the curly coils was derived, and its geometry parameters were optimized through the use of genetic algorithms. ANSYS Maxwell was used to analyze and optimize the layout of the ferrite cores. A prototype of the AUV wireless charging system was presented, demonstrating a maximum efficiency of 94% at 2.2 kW in salt water. The rotation adaptivity of the system was also tested, revealing stable output performance within the possible roll-angle variations of the AUV.

Optimization Design Method of Spherical Magnetic Field Generation Coil Based on Differential Evolution Algorithm

In a coil magnetometer, the size and uniformity of the bias magnetic field generated by the Helmholtz coil directly determine the accuracy of the solution of the geomagnetic direction. The design of traditional spherical coils relies heavily on the manual experience or mathematical derivation, making it difficult to obtain optimal parameters or requiring larger spherical coils. To address the problem, first, a coaxial symmetrical spherical coil model that improves space utilization was established. Second, an optimal design method for the spherical magnetic field generation coil based on a differential evolution algorithm was proposed. Third, the optimal bias magnetic field was obtained without increasing the volume of the coil. The verification results showed that the magnetic non-uniformity and magnetic gradient of the bias field generated by the optimized coil were reduced by 63.2% and 82.8%, respectively.

Enhancing hydraulic stability in once-through steam generators of small nuclear power plants: A novel approach using throttling baffles and feed pipes.

Given the increasing global interest in small nuclear power plants (SNPPs), the proposed research aims to investigate the design of once-through steam generators (OTSGs) used in SNPPs, with a particular focus on ensuring hydraulic stability in the heat exchange tubes. The study will examine the theoretical issues of ensuring stability and propose a new approach that includes the installation of throttling baffles and feed pipes to achieve hydraulic stability in OTSGs. The research will use a 45 MW capacity OTSG as an example, based on the design of the ship nuclear power plant KLT-40S, whose heat exchange surface (HES) is a set of coaxial cylindrical coils, similar to 60% of SNPPs with pressurized water reactors (PWRs). The rest of the SGs have either straight or U-shaped vertical heat exchange tubes (HET). The study will also explore the results of calculating the additional hydraulic resistance in front of the heat exchange tubes and propose a new method for clarifying the existing inequality used to ensure stability. Ultimately, the goal of this research is to provide a novel design for OTSGs based on throttling baffles and feed pipes to ensure hydraulic stability in SNPPs and provide new insights and methods for research in this field.

A modular system of flexible receive-only coil arrays for 3 T Magnetic Resonance Imaging

Flexible form-fitting radiofrequency coils provide high signal-to-noise ratio (SNR) for magnetic resonance imaging (MRI), and in array configuration large anatomical areas of interest can be covered. We propose a modular system - “ModFlex”- of flexible lightweight 4-channel coaxial coil arrays for 3 T MRI. We investigated the performance difference between commercial reference coils and 8- and 16-channel ModFlex receive-only array systems. In vivo, six anatomical targets in four regions of interest – the neck, the ankle, the spine and the hip – were imaged with the novel coil array system. The versatility of ModFlex and the robustness of the coil characteristics for different use cases is demonstrated. We measured an SNR gain for 4 out of 6 and similar SNR for 2 out of 6 anatomical target regions as compared to commercial reference coils. Parallel imaging capabilities are comparable to standard coils in hip and neck imaging, but ModFlex outperforms standard coils in ankle and spine imaging. High SNR combined with high acceleration possibilities enables faster imaging workflows and/or high-resolution MR acquisitions. The coil’s versatility is beneficial for use cases with varying subject sizes and could improve patient comfort.

Mutual Inductance Calculation of Circular Coils for an Arbitrary Position With a Finite Magnetic Core in Wireless Power Transfer Systems

Mutual inductance can be increased, and magnetic leakage can be reduced by adding finite magnetic core materials to the coil structure. However, the problem of the calculation of the mutual inductance between two arbitrarily positioned circular coils with a finite magnetic core has not been resolved. In this article, an analytical formula for calculating the mutual inductance between two circular coaxial coils on a finite magnetic core is obtained based on regional hierarchical expansion analysis (RHEA), the Maxwell equation, the Bessel function, and the boundary conditions for electromagnetic fields. The mutual inductance between two arbitrarily positioned coils is obtained by using the analytical geometry method. A series of experiments are carried out to verify the analytic calculations. The experimental results for each misalignment case are compared with the calculated results and simulated results. The theoretical results and simulated results are in good agreement with the experimental data, which verifies the validity of the proposed method.