Magnetic Dipole Source Research Articles

A New Iterative Algorithm for Magnetic Motion Tracking

Motion analysis is of great interest to a variety of applications, such as virtual and augmented reality and medical diagnostics. Hand movement tracking systems, in particular, are used as a human–machine interface. In most cases, these systems are based on optical or acceleration/angular speed sensors. These technologies are already well researched and used in commercial systems. In special applications, it can be advantageous to use magnetic sensors to supplement an existing system or even replace the existing sensors. The core of a motion tracking system is a localization unit. The relatively complex localization algorithms present a problem in magnetic systems, leading to a relatively large computational complexity. In this paper, a new approach for pose estimation of a kinematic chain is presented. The new algorithm is based on spatially rotating magnetic dipole sources. A spatial feature is extracted from the sensor signal, the dipole direction in which the maximum magnitude value is detected at the sensor. This is introduced as the “maximum vector”. A relationship between this feature, the location vector (pointing from the magnetic source to the sensor position) and the sensor orientation is derived and subsequently exploited. By modelling the hand as a kinematic chain, the posture of the chain can be described in two ways: the knowledge about the magnetic correlations and the structure of the kinematic chain. Both are bundled in an iterative algorithm with very low complexity. The algorithm was implemented in a real-time framework and evaluated in a simulation and first laboratory tests. In tests without movement, it could be shown that there was no significant deviation between the simulated and estimated poses. In tests with periodic movements, an error in the range of 1° was found. Of particular interest here is the required computing power. This was evaluated in terms of the required computing operations and the required computing time. Initial analyses have shown that a computing time of 3 μs per joint is required on a personal computer. Lastly, the first laboratory tests basically prove the functionality of the proposed methodology.

Achieving Ideal Magnetic Light Emission with Electric-Type Emitters.

Optical magnetic dipole (MD) emission predominantly relies on emitters with significant MD transitions, which, however, rarely exist in nature. Here, we propose a strategy to transform electric dipole (ED) emission to a magnetic one by elegantly coupling an ED emitter to a silicon nanoparticle exhibiting a strong MD resonance. This emission mode transformation enables an artificially ideal magnetic dipole source with an MD purity factor of up to 99%. The far-field emission patterns of such artificial MD sources were experimentally measured, which unambiguously resolved their magnetic-type emission origin. This study opens the path to achieving ideal magnetic dipole emission with nonmagnetic emitters, largely extending the availability of magnetic light emitters conventionally limited by nature. Beyond the fundamental significance in science, we anticipate that this study will also facilitate the development of magnetic optical nanosource and enable potential photonic applications relying on magnetic light emission.

A Revisit of Electromagnetic Wave Scattering by a Metal Isotropic Body in a Lossless Environment with Magnetic Sensor Excitation.

This paper investigates the electromagnetic fields being scattered by a metal spherical object in a vacuum environment, providing a numerical implementation of the obtained analytical results. A time-harmonic magnetic dipole source, far enough, emits the incident field at low frequencies, oriented arbitrarily in the three-dimensional space. The aim is to find a detailed solution to the scattering problem at spherical coordinates, which is useful for data inversion. Based on the theory of low frequencies, the Maxwell-type problem is transformed into Laplace's or Poisson's interconnected equations, accompanied by the proper boundary conditions on the perfectly conducting sphere and the radiation conditions at infinity, which are solved gradually. Broadly, the static and the first three dynamic terms are sufficient, while the terms of a higher order are negligible, which is confirmed by the field graphical representation.

High-Resolution Detection, Localization, and Classification of Multiple Magnetic Dipole Sources

High-Resolution Detection, Localization, and Classification of Multiple Magnetic Dipole Sources

Electromagnetic Detection System with Magnetic Dipole Source for Near-Surface Detection.

This paper proposes a nondestructive, separate transmitter-receiver (TX-RX) electromagnetic measurement system for near-surface detection. Different from the traditional dual-coil integrated design, the proposed transient electromagnetic (TEM) system performs shallow subsurface detection using independent TX coil and movable RX coils. This configuration requires a large primary field so that the far-away secondary field is able to generate reliably induced voltages. To achieve this goal, a bipolar current-pulsed power supply (BCPPS) with a late resonant charging strategy is designed to produce a sufficiently large magnetic moment for the exciting coil with low source interference. The magnetic dipole source (MDS) with a large proportion of weight is separated from the field observation device and does not need to be dragged or transported during the detection process. This setup lowers the weight of the scanning device to 3 kg and greatly improves the measurement efficiency. The results of the laboratory test verify the effectiveness of the separate MDS and RX module system. Field experimental detection further demonstrates that the proposed system can realize highly efficient and shallow surface detection within a 200 m range of the MDS device.

Multi-wavelength unidirectional forward scattering properties of the arrow-shaped galliumphosphide nanoantenna.

An arrow-shaped gallium phosphide nanoantenna exhibits both near-field electric field enhancement and far-field unidirectional scattering, and the interference conditions involve electric and magnetic quadrupoles as well as toroidal dipoles. By using long-wavelength approximation and exact multipole decomposition, the interference conditions required for far-field unidirectional transverse light scattering and backward near-zero scattering at multiple wavelengths are determined. The near-field properties are excellent, as exemplified by large Purcell factors of 4.5×109 for electric dipole source excitation, 464.68 for magnetic dipole source excitation, and 700V/m for the field enhancement factor. The degree of enhancement of unidirectional scattering is affected by structural parameters such as the angle and thickness of the nanoantenna. The arrow-shaped nanoantenna is an efficient platform to enhance the electric field and achieve high directionality of light scattering. Moreover, the nanostructure enables flexible manipulation of light waves and materials, giving rise to superior near-field and far-field performances, which are of great importance pertaining to the practicability and application potential of optical antennas in applications such as spectroscopy, sensing, displays, and optoelectronic devices.

Progressive Expansion Sampling of Quasi-Static Magnetic Fields in Unconfined Regions

We present a fast and accurate measurement technique for quasi-static magnetic fields by employing a progressive sampling method in an unconfined input space. The proposed machine learning algorithm is tested against uniform sampling on printed circuit board test structures and a buck converter. We prove allocation of multiple, separated regions with predefined lateral field limits at MHz frequencies. The feasibility of equivalent magnetic dipole source modeling based on a small number of samples is demonstrated. Compared to uniform testing, progressive expansion sampling identifies contours of given field limits in less than 3% of the reference measurement time.

Electromagnetic radiation from a spherical static current source coupled to harmonic axion field

The electromagnetic fields generated from a static current source on a spherical surface are calculated in the framework of axion electrodynamics to first order in the coupling parameter. Comparisons of the results are made with reference to various results obtained in conventional Maxwell electrodynamics, as well as previous results obtained for point magnetic dipole source coupled to harmonic axion fields. Distinct features from the results so obtained are highlighted for possible experimental probing of the axions via electromagnetic interactions. In particular, electromagnetic radiation from sources with strong magnetic field is studied which may enable the detection of a cosmic axion field from its interaction with objects like neutron stars.

A 3D frequency-domain electromagnetic solver employing a high order compact finite-difference scheme

Three-dimensional (3D) electromagnetic (EM) modeling is an important tool for geophysical applications. In EM induction methods the modified Helmholtz equation is often used to describe scattered or residual electric fields in three dimensions. Throughout this paper, a high order compact finite difference scheme for the solution of that equation for vertical magnetic dipole source (VMD) is presented. The approximation of the residual electric field intensity using a fourth order compact finite difference (FD) discretizer is achieved with the solution of a block linear system, the coefficient matrices are large and sparse with a particular structure, implying the application of a matrix-free Krylov subspace method for an efficient numerical solution. The proposed solver is being examined using a number of test problems in uniform and non-uniform grid spacing where numerical and analytical solutions for the homogeneous half-space cases are being compared.

Transient Electromagnetic Smoke Ring Due to a Grounded-Wire Source

The concept of a “smoke ring” in electromagnetic fields can be used to describe the spatial distribution and diffusion of electromagnetic fields with either frequency or time. This provides an intuitive basis for the qualitative interpretation of artificial-source electromagnetic measurement results. The existing discussions regarding this field of study have mainly focused on the smoke rings of a magnetic dipole or loop source. In the present work, smoke rings of other commonly used grounded-wire sources in the time domain are investigated. The results show that the smoke ring of grounded-wire sources is more complex than that of magnetic sources. A grounded-wire source can generate an induced current in both horizontal and vertical directions. The horizontal induced current includes positive and negative parts. Both parts of the horizontal induced current maximum diffuse downward perpendicular to the surface, but the diffusion velocity of the negative part is faster, approximately 5.6 times that of the positive part. The vertical induced current maximum diffuses at 45° from the grounded surface with a velocity approximately 5 times that of the positive horizontal induced current. The smoke ring of the horizontal induced current closely resembles a basin, while that of the vertical induced current is more similar to two separate cylinders. Finally, the imaging depth based on horizontal and vertical induced currents is obtained by the statistical relationship between the induced current maximums and the diffusion depth, which are equal to 1.4 times and 1.25 times the diffusion depth, respectively.

3D finite-difference transient electromagnetic modeling with a whole-space initial field

A 3D finite-difference time-domain transient electromagnetic forward-modeling method with a whole-space initial field is proposed to improve forward efficiency and flexibility. The open-source software WFTEM3D is developed based on this method with two language versions: a FORTRAN code and a MATLAB code. First, the scheme calculates the whole-space initial field excited by magnetic dipole sources at an initial time after the current is switched off. Then, the scheme steps Maxwell’s equations in time using a staggered grid and a modified DuFort-Frankel method. Multiple magnetic dipole sources are superimposed to achieve the modeling for large loop configurations. The air is included into the grids and the Dirichlet boundary condition is used on the outer boundaries. The forward-modeling method has three advantages. First, the method can be used to simulate the whole-space model and the half-space model. Second, big mesh growth factors of 1.5–3 are allowed, which significantly improves the computational efficiency and facilitates the simulation of the multiscale model. Third, the approximate initial field can be calculated using a conductivity independent of the model, which facilitates the simulation of the nonflat topography model. These advantages are tested by examples. The model of a conductive brick in a half-space and the model of a complex conductor at a vertical contact are simulated. The run times are 0.48 and 9.72 s, respectively, on an ordinary computer with an Intel Core i7-6700 CPU, and the solutions are consistent with that presented in the literature. Examples of a multiscale model with three conductive bricks, a valley model, and a hill model are simulated, which further verify these advantages.

Cobalt Ferrite/Polyetherimide Composites as Thermally Stable Materials for Electromagnetic Interference Shielding Uses.

The progress of the automated industry has introduced many benefits in our daily life, but it also produces undesired electromagnetic interference (EMI) that distresses the end-users and functionality of electronic devices. This article develops new composites based on a polyetherimide (PEI) matrix and cobalt ferrite (CoFe2O4) nanofiller (10-50 wt%) by mixing inorganic phase in the poly(amic acid) solution, followed by film casting and controlled heating, to acquire the corresponding imide structure. The composites were designed to contain both electric and magnetic dipole sources by including highly polarizable groups (phenyls, ethers, -CN) in the PEI structure and by loading this matrix with magnetic nanoparticles, respectively. The films exhibited high thermal stability, having the temperature at which decomposition begins in the interval of 450-487 °C. Magnetic analyses indicated a saturation magnetization, coercitive force, and magnetic remanence of 27.9 emu g-1, 705 Oe, and 9.57 emu g-1, respectively, for the PEI/CoFe2O4 50 wt%. Electrical measurements evidenced an increase in the conductivity from 4.42 10-9 S/cm for the neat PEI to 1.70 10-8 S/cm for PEI/CoFe2O4 50 wt% at 1 MHz. The subglass γ- and β-relaxations, primary relaxation, and conductivity relaxation were also examined depending on the nanofiller content. These novel composites are investigated from the point of view of their EMI shielding properties, showing that they are capable of attenuating the electric and magnetic parts of electromagnetic waves.

Resonant channeling of light near metal surface by passive and active silicon nanoparticles

We investigate the optical response of passive (homogeneous) and active (doped by a magnetic-dipole source) silicon nanospheres placed above the gold substrate supporting excitation and propagation of surface plasmon polaritons (SPPs). In the case of the passive system, the influence of the particle electric and magnetic dipole resonances on the channeling of scattered light into the two channels: light-into-light and light-into-SPPs, is studied around the telecommunication wavelength range. Scattering efficiencies of both channels are analytically and numerically calculated and compared for different gaps between the nanosphere and gold substrate. Conditions, under which the resonant distribution of energy between SPPs and light channels can be approximately equal to each other, are revealed. In the case of an active system, associated with a magnetic dipole source embedded in a silicon resonator, two separate emission channels into light and into SPPs are considered as well. The radiation efficiency in each channel as a function of the gap is examined and compared. In particular, it is revealed the conditions when the efficiency of resonant light energy propagation along the two channels is practically equal to each other, and the Purcell factor achieves the value almost two times higher than for the case without the substrate. The obtained results elucidate the features introduced by the Mie dipole resonances of silicon nanospheres to control the light scattering (radiation) and the SPP excitation in dielectric/metal interface systems. The results obtained are perspectives for the development of new compact photonic and plasmonic devices operating at standard telecommunication wavelength ranges.

Analysis by Systemic Approach of Magnetic Dipole Source Location Performances by Using an IoT Software Gradiometer Head

Based on the development of numerous commercial cheap or low noise magnetometers, some publications feature systems of magnetic anomaly detection. They are meanly based on matrix inversion gradients and are built with multi-magnetometer single head in order to derive spatially the sensed magnetic field. Meanwhile, these examples often suffer from a lack of true performance analysis, based on a &#x201C;systemic approach.&#x201D; As an example, they take account partially of the main sensor, mechanical or conditioning electronic characteristics. In that way, we propose a theoretical modeling of 1D and 2D magnetic head detection capabilities that could be extended to 3D. We show that the distance of detection could be quantified with the help of three main parameters: the baseline, the sensor mismatch in terms of sensitivity (V/T) or misalignment, and a defined signal-to-noise ratio of the measurement (<inline-formula> <tex-math notation="LaTeX">$\textit {SNR}_{m}$ </tex-math></inline-formula>). With a reasonable built system (sensitivity disparity of 0.6 &#x0025;, misalignment lower than some (&#x00B0;) and <inline-formula> <tex-math notation="LaTeX">$\textit {SNR}_{m}$ </tex-math></inline-formula> of 120 dB), a magnetic dipole could be localized and quantified, from <inline-formula> <tex-math notation="LaTeX">$\times1$ </tex-math></inline-formula> up to <inline-formula> <tex-math notation="LaTeX">$\times20$ </tex-math></inline-formula> gradiometer base line with a relative error lower than 30 &#x0025;, and from <inline-formula> <tex-math notation="LaTeX">$\times3$ </tex-math></inline-formula> up to <inline-formula> <tex-math notation="LaTeX">$\times50$ </tex-math></inline-formula> with a <inline-formula> <tex-math notation="LaTeX">$\textit {SNR}_{m}$ </tex-math></inline-formula> of 160 dB, as an example. Notice that the highest distance of detection is mainly bounded by the <inline-formula> <tex-math notation="LaTeX">$\textit {SNR}_{m}$ </tex-math></inline-formula> value and the intrinsic head spatial transfer function. At a close distance, we have enlarged significantly the surface of the source localization, by using optimization algorithms when classical gradiometric method appears non-functional. To exemplify this systemic approach, we have built and characterized a setup. It is based on the IoT principles, where acquired data are pushed over-the-air through a gateway. Finally, they are post-processed in quasi-real time with a laptop.

Long-Distance Crosswell EM Logging of Copper Ore Using Borehole-Surface Current Injection in Slim Holes

Crosswell electromagnetic (EM) methods are widely used in subsurface geophysical prospecting because they can achieve more effective long-distance detection than single-well methods. However, a large-diameter borehole is required to increase the magnetic moment of the magnetic dipole source. For the long-distance detection of copper ores, which is usually performed in slim holes, we present a borehole-surface current-injection-based crosswell EM logging method. Considering the cost of deploying casing, we inject a low-frequency AC directly into the ground, and converging current is formed around low-resistance anomalies in the formation. Then, the distribution of the anomalies can be inferred by detecting the low-frequency alternating magnetic field of the converging current in the receiver well. Moreover, to further improve the detection performance, we design a placement scheme for the grounding electrode for multi-anomaly crosswell detection based on the Gauss&#x2013;Newton inversion algorithm, where the EM responses for different grounding electrode locations are analyzed. Field experiments are conducted using two slim open holes spaced approximately 1000 m apart for the detection of two copper ores. Through the processing and interpretation of measured EM signals, the conductivity imaging results of the crosswell EM method indicate that the measured distribution of anomalies is consistent with prior knowledge obtained from numerous single-well loggings, demonstrating the feasibility of the proposed application for long-distance crosswell EM logging in slim open holes.

Imaging subsurface orebodies with airborne electromagnetic data using a recurrent neural network

Conventional interpretation of airborne electromagnetic data has been conducted by solving the inverse problem. However, with recent advances in machine learning (ML) techniques, a 1D deep neural network inversion that predicts a 1D resistivity model using multifrequency vertical magnetic fields and sensor height information at one location has been applied. Nevertheless, since the final interpretation of this 1D approach relies on connecting 1D resistivity models, 1D ML interpretation has low accuracy for the estimation of an isolated anomaly, as in conventional 1D inversion. Thus, we have developed a 2D interpretation technique that can overcome the limitations of 1D interpretation and consider spatial continuity by using a recurrent neural network (RNN). We generate various 2D resistivity models, calculate the ratio of primary and induced secondary magnetic fields of the vertical direction in ppm scale using a vertical magnetic dipole source, and then train the RNN using the resistivity models and the corresponding electromagnetic (EM) responses. To verify the validity of 2D RNN inversion, we apply the trained RNN to synthetic and field data. Through application of the field data, we demonstrate that the design of the training data set is crucial to improve prediction performance in a 2D RNN inversion. In addition, we investigate changes in the RNN inversion results of field data dependent on the data preprocessing. We demonstrate that using two types of data, logarithmic transformed data and linear-scale data, having different patterns of input information can enhance the prediction performance of the EM inversion results.

Localization of Alternating Magnetic Dipole in the Near-Field Zone with Single-Component Magnetometers

Tri-axis magnetometers are widely used to measure magnetic field in engineering of the magnetic localization technology. However, the magnetic field measurement precision is influenced by the nonorthogonal error of tri-axis magnetometers. A locating model of the alternating magnetic dipole in the near-field zone with single-component magnetometers was proposed in this paper. Using the vertical component of the low-frequency magnetic field acquired by at least six single-component magnetometers, the localization of an alternating magnetic dipole could be attributed to the solution for a class of nonlinear unconstrained optimization problem. In order to calculate the locating information of alternating magnetic dipole, a hybrid algorithm combining the Gauss–Newton algorithm and genetic algorithm was applied. The theoretical simulation and field experiment for the localization of alternating magnetic dipole source were carried out, respectively. The positioning result is stable and reliable, indicating that the locating model has better performance and could meet the requirements of actual positioning.

Numerical Modeling of Nondestructive Testing of Various Conductive Objects inside Metal Enclosures Using ELF/VLF Magnetic Fields

Nondestructive evaluation of various conductive objects through metal enclosures is investigated by using ELF/VLF magnetic induction fields in detailed simulations. ELF/VLF magnetic fields (&lt;30 kHz) have a unique ability to penetrate highly conductive or permeable shields. Using a magnetic dipole source antenna, objects hidden inside a metal enclosure are imaged via examining distortions to the field outside the enclosure. The field distortion is parametrically studied by varying the size, conductivity, and permeability of the hidden objects. Furthermore, the importance of the conductivity of the enclosure itself is investigated using both low (106 S/m) and high (108 S/m) conductivity metallic shields. It is shown that the responses are quite sensitive to the object and shield parameters; both qualitative and quantitative properties of the field distortions are described in detail. The simulation results suggest that properties of hidden conductive or permeable objects, over a relatively wide range of parameters (both geometry and material), can be inferred nondestructively using ELF/VLF magnetic induction fields.

The magneto-seismic method in geoscience

Key concepts and potential applications associated with a phenomenon hitherto unexplored by the geoscience community, which we have named the magneto-seismic effect, are introduced. The method is based on the simple principle that when an electric charge moves in the presence of an external magnetic field, the charge carrier experiences an instantaneous force, which is equal to the vector cross product of the current it carries and the magnetic field that is present. This “Lorentz force” can create both compressional and shear sound waves in electrical conductors by passing an alternating current through them via an electromagnetic source. In laboratory settings, this magneto-seismic effect can produce readily detectable rock frame displacements. This opens up the possibility of developing new experimental methods to interrogate elastic and poroelastic response of rocks in a broad frequency range from subhertz to megahertz, potentially closing the frequency gap between traditional ultrasonic characterization and properties of interest in the seismic frequency band. In exploration settings, electric current dipole/bipole sources, or novel rotating magnetic dipole sources, can be used to generate electric currents at depth. These currents produce seismic waves at interfaces (or boundaries) where conductivity abruptly changes. The amplitude and propagation directions of these generated seismic waves depend on the relative orientation of the interfaces (or boundaries) with respect to the earth's magnetic field. These seismic waves can then be recorded by receivers at the surface and, in principle, might be processed to yield a resistivity map with seismic resolution. It is shown that processing to obtain a signal from deep targets is significantly limited by seismic background noise. However, an acceptable signal-to-noise ratio might be achieved for shallower targets. The difference between the magneto-seismic response and the previously well-studied electro-seismic response will be discussed.

Two-point magnetic field positioning algorithm based on rotating magnetic dipole

The detection and localization of magnetic targets based on magnetic field signals have become a research hotspot in recent years. In this study, a magnetic field model is established for the rotating propeller, which is regarded as a magnetic dipole. Moreover, a positioning algorithm of two-point shaft-rate magnetic field is proposed according to the rotating magnetic dipole. In addition, the positioning accuracy of the algorithm and the influence of some factors such as uneven magnetic distribution of magnetic dipole and measurement noise on the accuracy are analyzed. The simulation results indicate that the positioning accuracy increases with the increase in the circular trajectory radius of magnetic moment when there is an uneven magnetic distribution. The error caused by measurement noise increases with the increase in the distance between the measurement point and magnetic dipole source. Finally, the feasibility of the algorithm is verified through experiments.