Eddy Current Effects Research Articles

Effects of inner materials on the sensitivity and phase depth of wireless inductive pressure sensors for monitoring intraocular pressure

In this research, we developed wireless, inductive, pressure sensors with high sensitivity and investigated the effects of the inner materials (copper or ferrite) on the performance of the sensors. The proposed sensor is comprised of two parts, i.e., the top and the bottom parts. The top part includes a micro coil and a capacitor for the wireless transfer of data, and the bottom part includes the inner materials and a thick or thin flexible membrane to induce changes in the inductance. An anchor is used to assemble the top and bottom parts. The behavior of the sensor with copper was based on the eddy current effect, and, as the pressure increased, its resonance frequency increased, while its phase depth decreased exponentially. The principle of the sensor with ferrite was related to the effective permeability between a ferrite and a coil, and its response was the opposite of that with copper, i.e., as the pressure increased, the resonance frequency decreased linearly, and the phase depth increased linearly. These different operational mechanisms can be explained by the changes in the equations of inductance presented in this paper. After characterizing four different types of inductive pressure sensors in ambient air, one type of inductive pressure sensor was used to monitor the intraocular pressure (IOP) of a rabbit's eye as a biomedical application. The results showed that, in the animal tests, the measured responsivity and sensitivity were 16.7 kHz/mmHg and 1340 ppm/mmHg, respectively. These data indicate that the proposed sensor is a good candidate for monitoring IOP.

Estimation of the Number of Compartments Associated With the Apparent Diffusion Coefficient in MRI: The Theoretical and Experimental Investigations

The goal of the present study was to estimate the number of compartments and the mean apparent diffusion coefficient (ADC) value with the use of the DWI signal curve. A useful new mathematic model that includes internal correlation among subcompartments with a distinct number of compartments was proposed. The DWI signal was simulated to estimate the approximate association between the number of subcompartments and the molecular density, with density corresponding to the ratio of the ADC values of the compartments, as determined using the Monte Carlo method. Various factors, such as energy depletion, temperature, intracellular water accumulation, changes in the tortuosity of the extracellular diffusion paths, and changes in cell membrane permeability, have all been implicated as factors contributing to changes in the ADC of water (ADCw); therefore, one may consider them as pseudocompartments in the new model proposed in this study. The lower the coefficient is, the lower the contribution of the compartment to the net signal will be. The results of the simulation indicate that when the number of compartments increases, the signal will become significantly lower, because the gradient factor (i.e., the b value) will increase. In other words, the signal curve is approximately linear at all b values when the number of compartments in which the tissues have been severely damaged is low; however, when the number of compartments is high, the curve will become constant at high b values, and the perfusion parameters will prevail on the diffusion parameters at low b values. Therefore, normal tissues will be investigated when the number of compartments and the ADC values are high and the b values are low, whereas damaged tissues will be evaluated when the number of compartments and the ADC values are low and the b values are high. The present study investigates damaged tissues at high b values for which the effect of eddy currents will also be compensated. These b values will probably be used in functional MRI.

Hybrid-Space SENSE Reconstruction for Simultaneous Multi-Slice MRI.

Simultaneous Multi-Slice (SMS) magnetic resonance imaging (MRI) is a rapidly evolving technique for increasing imaging speed. Controlled aliasing techniques utilize periodic undersampling patterns to help mitigate the loss in signal-to-noise ratio (SNR) in SMS MRI. To evaluate the performance of different undersampling patterns, a quantitative description of the image SNR loss is needed. Additionally, eddy current effects in echo planar imaging (EPI) lead to slice-specific Nyquist ghosting artifacts. These artifacts cannot be accurately corrected for each individual slice before or after slice-unaliasing. In this work, we propose a hybrid-space sensitivity encoding (SENSE) reconstruction framework for SMS MRI by adopting a three-dimensional representation of the SMS acquisition. Analytical SNR loss maps are derived for SMS acquisitions with arbitrary phase encoding undersampling patterns. Moreover, we propose a matrix-decoding correction method that corrects the slice-specific Nyquist ghosting artifacts in SMS EPI acquisitions. Brain images demonstrate that the proposed hybrid-space SENSE reconstruction generates images with comparable quality to commonly used split-slice-generalized autocalibrating partially parallel acquisition reconstruction. The analytical SNR loss maps agree with those calculated by a Monte Carlo based method, but require less computation time for high quality maps. The analytical maps enable a fair comparison between the performances of coherent and incoherent SMS undersampling patterns. Phantom and brain SMS EPI images show that the matrix-decoding method performs better than the single-slice and slice-averaged Nyquist ghosting correction methods under the hybrid-space SENSE reconstruction framework.

New Electromagnetic Actuator for On-Orbit Inspection

Induction couplers are a new technology for actively controlling the relative position and orientation between an inspection spacecraft and a target without mechanical contact. To the authors’ knowledge, no other contactless actuators exploit eddy-current effects to produce force and torque relative to a conductive target. Instead, other actuators rely on magnetic dipole interactions or cooperation from the target. A model for dynamic induction-coupler systems based on previous fast eddy-current force models is presented; design requirements through a case study of an inspection mission relevant to the International Space Station are outlined; and the feasibility of flight applications is established. This model has two restrictions; it assumes the gap between the inspector and the surface is already fixed, and it holds only near locally planar surfaces. Experiments show that induction couplers in normal operating conditions can generate forces of 5 mN with a specific force of for spacecraft actuation. Using these experimental values, a simulation shows that an inspection vehicle could traverse a flat surface the size of the Destiny International Space Station module in 2 h.

A novel sensing chip with dual-coil inductance for determining raw milk quality

This paper presents a novel inspection method that uses dual-coil inductance to evaluate the quality of raw milk. The device principally contains a silicon-based chip with two co-planar coils, which is fabricated using a simple microelectroplating process. When the coils are entirely immersed in raw milk and a specific alternating electrical signal is applied to one of the coils, the other coil induces a phase-shifted electrical signal because of the effect of eddy currents. Using the phase variation between both coils, the quality of the raw milk, in terms of the somatic cell count (SCC), can be evaluated, because the milk from a cow that suffers from mastitis is a better conductor of electric current. In this study, the characteristics of the magnetic inductance for a specified coil design and its sensing performance are simulated and evaluated using COMSOL software and by experiment. The experimental results show that higher SSCs cause larger phase shifts. This shift increases as the excitation frequency is increased. For a typical fabrication result, using a sensing coil with 25 loops, the measurement sensitivity in terms of SCC is 3°/log(SCC) at an excitation frequency of 9MHz. The proposed dual-coil chip reduces size and cost and has a rapid response, which allows efficient quality inspection of raw milk.

Al-based magnetic composites produced by accumulative roll bonding (ARB)

Abstract Accumulative roll bonding (ARB) was used to fabricate Al/Ni (with two initial Ni thicknesses) and Al/Ni/Fe 3 O 4 composites. During ARB, Ni layers necked and fractured owing to the difference in flow properties of Al and Ni. In addition, large and elongated clusters of Fe 3 O 4 particles were broken into smaller ones and distributed gradually in Al matrix. After the fourth and sixth cycle, weak bonding and several pores were observed at particle/matrix interface while the bonding quality increased after the eighth cycle. Results show that after eight cycles, an Al-based composite with a satisfactorily magnetic behavior was produced. The Al/Ni (0.5) composite exhibited the highest tensile strength (182 MPa) with the highest value of saturation magnetization (13.47 emu/g). The relative permeability decreased with increasing frequency for a given number of ARB cycles. The Fe 3 O 4 particles caused the saturation magnetization to decrease and the critical frequency to increase due to the eddy current effects.

Influences of an Aluminum Covering Layer on the Performance of Cross-Like Hall Devices

This work studies the effects of an aluminum covering on the performance of cross-like Hall devices. Four different Hall sensor structures of various sizes were designed and fabricated. The sensitivity and offset of the Hall sensors, two key points impacting their performance, were characterized using a self-built measurement system. The work analyzes the influences of the aluminum covering on those two aspects of the performance. The aluminum layer covering mainly leads to an eddy-current effect in an unstable magnetic field and an additional depletion region above the active region. Those two points have influences on the sensitivity and the offset voltage, respectively. The analysis guides the designer whether to choose covering with an aluminum layer the active region of the Hall sensor as a method to reduce the flicker noise and to improve the stability of the Hall sensor. Because Hall devices, as a reference element, always suffer from a large dispersion, improving their stability is a crucial issue.

A novel method for overcoming the impact of spatially varying ambient magnetic fields on a DC magnetic field-based tracking system

A magnetic indoor local positioning system enables positioning in harsh environments due to the advantageous characteristics of magnetic signals such as penetrating of various obstacles and robustness against fading effects. Nevertheless, the indoor tracking remains a challenge, due to the non-stationary property of the ambient magnetic fields, signal transients and eddy current effects. In this paper, we propose a method, which is based on zero velocity detection, in order to overcome the mentioned constraints by the pedestrian tracking.

Effect of Perpendicular Anisotropy and Eddy Currents on the Microwave Performance of Single-Layer and Multi-Layer Permalloy Films

In this letter, microwave and magnetostatic measurements of single- and multilayer Permalloy films are made to find physical reasons causing the decrease of the microwave permeability of the films with increase in film thickness. The permeability is measured in the frequency range of 0.1–10 GHz by a coaxial technique. From the measured permeability, a contribution to the effective field from the magnetoelastic effect is determined. The decrease of microwave permeability in thick films is shown to arise mainly due to the perpendicular magnetic anisotropy rather than from the effect of eddy currents, as is frequently suggested. It is also shown that both these effects are reduced in multilayer films comprising alternating magnetic and dielectric layers. Therefore, such films are promising materials for developing magnets that possess high permeability at high frequencies.

Gravity Probe B spacecraft description

The Gravity Probe B spacecraft, developed, integrated, and tested by Lockheed Missiles & Space Company and later Lockheed Martin Corporation, consisted of structures, mechanisms, command and data handling, attitude and translation control, electrical power, thermal control, flight software, and communications. When integrated with the payload elements, the integrated system became the space vehicle. Key requirements shaping the design of the spacecraft were: (1) the tight mission timeline (17 months, 9 days of on-orbit operation), (2) precise attitude and translational control, (3) thermal protection of science hardware, (4) minimizing aerodynamic, magnetic, and eddy current effects, and (5) the need to provide a robust, low risk spacecraft. The spacecraft met all mission requirements, as demonstrated by dewar lifetime meeting specification, positive power and thermal margins, precision attitude control and drag-free performance, reliable communications, and the collection of more than 97% of the available science data.

Dynamic Nonlinear Model With Eddy Current Effect for Stress-Driven Galfenol Energy Harvester

A dynamic nonlinear model with eddy current effect has been developed to predict the dynamic responses of the output current, the induced voltage, the electrical power, and the displacement of a Galfenol energy harvester subjected simultaneously to stress and bias magnetic field. This model is obtained by the constitutive equations with eddy current effect, the established electromechanical model and the Armstrong model. The calculated magnetic induction curves of a polycrystalline Galfenol are compared with the measured curves. The effects of the varied operating conditions on the performance of the Galfenol energy harvester are predicated and analyzed.

Numerical and Experimental Characterization of Thread-Type Magnetic Core With Low Eddy Current Loss

This paper proposes a thread-type magnetic core as a new structured magnetic core made of iron threads and a surrounding insulator. The thread-type core has the advantages of lower eddy current loss, simple production, and low cost. The eddy current loss of the thread-type core is compared with that of bulk- and lamination-type cores. The losses are numerically analyzed using the finite-element method and are measured with an experimental test. The effect of eddy current on copper loss is also addressed. The numerical and experimental results show the usefulness of the thread-type core. Further numerical results show the size effect of the thread core, whereby the smaller size thread results in less eddy current loss.

Comparison of three formulations for eddy-current and skin and proximity problems in a meander coil electromagnetic acoustic transducer

Three differential equations based on different definitions of current density are compared. Formulation I is based on an incomplete equation for total current density (TCD). Formulations II and III are based on incomplete and complete equations for source current density (SCD), respectively. Using the weak form of Finite Element Method (FEM), the three formulations were applied in a meander coil Electromagnetic Acoustic Transducer (EMAT) example to solve magnetic vector potential (MVP). The FEM results from frequency domain and time domain models are in excellent agreement with previously published works. Results show that the errors for Formulations I and II vary with coil dimensions, coil spacing, lift-off distance and external excitation frequency, for the existence of eddy-current and skin and proximity effects. And the current distribution across the coil conductors also follows the same trend. It is better to choose Formulation I instead of Formulation III to solve MVP when the coil height or width are less than twice the skin depth, due to the low cost and high efficiency of Formulation I.

Saturation of attenuation length of spin waves in thick permalloy films

The permalloy (Py) thickness dependence of the magnetostatic spin wave (MSSW) propagation was investigated. Large-group velocity is realized for thick Py films and the MSSW can propagate more than 150 µm. However, attenuation length hardly changes for samples with a Py thickness of more than 100 nm, despite the increasing group velocity with increasing thickness. The eddy current effect decreases the wave channel thickness and it could cause the damping enhancement due to intralayer spin pumping in thick Py films.

Duality-Based Transformer Model Including Eddy Current Effects in the Windings

This paper presents a general method for building equivalent electric circuits of power transformers, including eddy current effects in windings and core. A high-frequency equivalent dual model for single- and three-phase transformers with two multilayer windings is derived from the application of the principle of duality. The model is built from elements available in circuit simulation programs, such as Electromagnetic Transients Program (EMTP)–Alternative Transients Program, EMTP-RV, PSCAD, and PSpice. The parameters of the frequency-dependent leakage inductance and winding resistance are computed with analytical formulae obtained from the solution of Maxwell’s equations that are based on the geometrical dimensions and material information. Ideal transformers are utilized to isolate the electric components (winding resistors and capacitors) from the magnetic components (inductors). The physically correct connection points for electric and magnetic components are clearly identified. The proposed methodology is successfully validated versus finite- element simulations and laboratory measurements.

Calculation of Leakage Inductance for High-Frequency Transformers

Frequency-dependent leakage inductance is often observed. The high-frequency eddy current effects cause a reduction in leakage inductance. The proximity effect between adjacent layers is responsible for the reduction of leakage inductance. This paper gives a detailed analysis of high-frequency leakage inductance and proposes an accurate prediction methodology. High-frequency leakage inductances in several interleaved winding configurations are also discussed. Interleaved winding configurations actually give a smaller degree of reduction of leakage induction at high frequency. Finite-element analysis simulation and measurement validate the models.

Making Use of the Eddy Currents Effect in the Examination of the Tribological Characteristics of Strip Rolling Mill Rolls

Making Use of the Eddy Currents Effect in the Examination of the Tribological Characteristics of Strip Rolling Mill Rolls

Analytical modeling of linear oscillating motor with a mixed method considering saturation effect

Linear oscillating motor receives much attention recently due to direct and efficient linear motion output. In order to achieve high thrust and eliminate eddy current effect, circumferential silicon steel stack is usually utilized in stator as a universal topology design. However, circumferential arrangement of silicon steel introduces 3D magnetic circuit, which complicates the design process and cannot be solved using conventional modeling method. In addition, it leads to severe magnetic saturation effect and must be taken into accounts in the analytical model for accurate results. In this paper, a novel mixed analytical modeling method for linear oscillating motor with silicon steel stator is proposed. Magnetic potential vector (MPV) and 3D equivalent magnetic circuit (3D-EMC) methods are combined together in this mixed model. Magnetic potential vector method sets up the coarse flux density distribution, while 3D equivalent magnetic circuit method is used for refinement considering saturation. Moreover, the performance between radial and parallel magnetized structure is compared based on this model. For validation, FEM and experimental results are compared with the proposed model and proves its effectiveness and accuracy, which can be used in further motor structure optimization. It may also act as a universal guidance to other PM motor design.

Compressive Pressure and Noncontact Gap Measurements Based on 2-D Coil Covered by Polydimethylsiloxane Film

To realize the compressive pressure measurement and the noncontact gap measurement in the small interlayer structure of industrial equipment, a compressive pressure and noncontact gap sensing element based on eddy-current effect is designed and fabricated by covering a polydimethylsiloxane film on the 2-D coil. The impedance of the 2-D coil in the sensing element is used to reflect the compressive pressure and the noncontact gap. There exists a critical impedance, which can be used to judge whether the sensing element contacts the metal object or not. At the noncontact stage, the impedance amplitude and the impedance angle of the 2-D coil increase with the increase of the gap between the sensing element and the object made of aluminum. At the contact stage, the impedance amplitude and impedance angle decrease with the increase of the compressive pressure. The experimental results verify the feasibility of using the 2-D coil covered with polydimethylsiloxane film to measure the compressive pressure and the noncontact gap.

Dynamic multi-coil tailored excitation for transmit B1 correction at 7 Tesla.

Tailored excitation (TEx) based on interspersing multiple radio frequency pulses with linear gradient and higher-order shim pulses can be used to obtain uniform flip angle in the presence of large radio frequency transmission (B 1+) inhomogeneity. Here, an implementation of dynamic, multislice tailored excitation using the recently developed multi-coil nonlinear shim hardware (MC-DTEx) is reported. MC-DTEx was developed and tested both in a phantom and in vivo at 7 T, and its efficacy was quantitatively assessed. Predicted outcomes of MC-DTEx and DTEx based on spherical harmonic shims (SH-DTEx) were also compared. For a planned 30 ° flip angle, in a phantom, the standard deviation in excitation improved from 28% (regular excitation) to 12% with MC-DTEx. The SD in in vivo excitation improved from 22 to 12%. The improvements achieved with experimental MC-DTEx closely matched the theoretical predictions. Simulations further showed that MC-DTEx outperforms SH-DTEx for both scenarios. Successful implementation of multislice MC-DTEx is presented and is shown to be capable of homogenizing excitation over more than twofold B 1+ variations. Its benefits over SH-DTEx are also demonstrated. A distinct advantage of MC hardware over SH shim hardware is the absence of significant eddy current effects, which allows for a straightforward, multislice implementation of MC-DTEx. Magn Reson Med 76:83-93, 2016. © 2015 Wiley Periodicals, Inc.