Eddy Current Effects Research Articles

Sensorless parameter estimation of electromagnetic transducer considering eddy currents

This paper presents a method of estimating the parameters of an electromagnetic transducer without sensors. The proposed method utilizes the measured admittance of the electromagnetic transducer, and therefore position, velocity, and/or acceleration sensors are not necessary in this framework. Novel impedance models are proposed based on the basic physical principles of electromagnetics; in particular, the effect of eddy currents has been included in these proposed models. The validity of the proposed estimation method and models was experimentally demonstrated by comparing the parameter estimation and vibration control capabilities of the proposed models with three conventional models.

Connectivity of magnetic induction-based ad hoc networks

Magnetic induction (MI) has been proven to be an efficient wireless communication technique for overcoming the transmission challenges in some very harsh propagation environments, such as underground, underwater, etc. For a random distributed MI ad hoc network in a 3-D space composed of uniform medium, we propose a method for determining the required node density and transmitting power that creates an almost surely fully connected network. For which we involve an MI path-loss model and consider the effect of eddy currents, the effective coverage space and the expected node degree of an MI node are then calculated by a Lambert W-function-based integration. Finally, we propose optimized frequency selection methods for improving the connectivity of MI networks. In addition to an ideal frequency-switching optimization method, we provide for engineering applications a practical frequency-fixed optimization method, which is based on the gradient descent algorithm, where an improved initialization is used to reduce iterations.

A Novel Observer Approach for Self Sensing of Single-Coil Digital Valves

Observer-based self sensing for digital (on–off) single-coil solenoid valves is investigated. Self sensing refers to the case where merely the driving signals used to energize the actuator (voltage and coil current) are available to obtain estimates of both the position and velocity. A novel observer approach for estimating the position and velocity from the driving signals is presented, where the dynamics of the mechanical subsystem can be neglected in the model. Both the effect of eddy currents and saturation effects are taken into account in the observer model. Practical experimental results are shown and the new method is compared with a full-order sliding mode observer.

The Effect of Motion-Induced Eddy Current on Circumferential Magnetization in MFL Testing for a Steel Pipe

Eddy current effect essentially occurs when a relative movement happens between steel pipes and the circumferential magnetizer in high-speed magnetic flux leakage (MFL) testing. To improve the MFL testing for longitudinal defects, the effect of motion-induced eddy current (MIEC) on circumferential magnetization is analyzed. First, in terms of Maxwell equations, the eddy current effect and the MIEC distribution in the steel pipe are analyzed. Then, the finite-element simulations are conducted to investigate the MIEC distribution, the magnetization status change, and the sensitivity difference. With the rotational speed increasing, the MIEC and circumferential magnetic flux density gradually become asymmetrical and non-uniform. The MFL testing signal amplitudes of external and internal defects in the arriving and leaving parts change in different variation trends. Finally, the relevant high-speed MFL testing experiments for steel pipes are performed to confirm the effect of MIEC. The experimental results match well with the theoretical analysis and numerical simulation results.

Intensive Study of Skin Effect in Eddy Current Testing With Pancake Coil

Skin depth is an important parameter that quantifies depth of inspection of eddy current (EC) nondestructive testing. The formula which states that the skin depth is inversely proportional to the square root of the product of frequency, permeability, and conductivity has been widely used for calculating skin depth for many decades. It has been noticed that the true skin depth is less than that obtained by the formula when the EC is excited by the ac current in a pancake coil. In-depth and convincible explanation of this discrepancy has not been published. This paper studies this problem by finite-element analysis and concludes that the difference is due to two reasons. One is that the EC associated with pancake coil diffuses with increasing depth, whereas there is no diffusion effect of EC associated with uniform plane field which is the assumption of the conventional formula. The other reason is that the ECs induced by wire segments symmetric about the coil axis cancel each other and the cancellation becomes more severe with increasing depth due to the diffusion of EC. This paper also investigates the influence of each of coil parameters on skin depth. Then, a new formula for estimating skin depth is proposed.

Eddy-Current-Effect Homogenization of Windings in Harmonic-Balance Finite-Element Models

The eddy current effects in the windings are accounted for by determining a frequency-dependent reluctivity and impedance with a homogenization technique. In this work, these two quantities are further embedded in a harmonic-balance finite element model and applied a saturable inductor with multi-turn coil fed by a pulse-width modulated voltage.

Effect of Eddy Currents Induced in the Thermal Shield During the Quench of the Superconducting Coils for the Super-FRS of the FAIR Project

The Superconducting FRagment Separator (Super-FRS) is a two-stage separator to be built next to the site of GSI, Darmstadt, Germany, as part of Facility for Anti-Proton and Ion Research. Its purpose is to create and separate rare isotope beams and to enable the mass measurement also for very short lived nuclei. Overall, the Super-FRS consists of 24 dipole magnets of so-called superferric type, with superconducting coils but the field shaped by magnetic iron. The coil is trapezoidal-shaped configuration, which is wound with NbTi wires of high Cu/Sc ratio and located inside a cryostat equipped with a thermal shield and cooled with liquid helium; but the dipole will have a warm iron yoke with a wide air trim slot. This paper presents the results of the eddy current calculations in the thermal shield induced during the quench of the magnet. The mechanical behavior of the thermal shield under the Lorentz forces generated by the eddy currents will also be presented.

Analysis of eddy current induced in track on medium-low speed maglev train

Electromagnetic levitation (EMS) maglev train relies on the attraction between the electromagnets and rails which are mounted on the train to achieve suspension. During the movement, the magnetic field generated by the electromagnet will induce the eddy current in the orbit and the eddy current will weaken the suspended magnetic field. Which leads to the attenuation of the levitation force, the increases of suspension current and the degradation the suspension performance. In this paper, the influence of eddy current on the air gap magnetic field is solved by theoretical analysis, and the correction coefficient of air gap magnetic field is fitted according to the finite element data. The levitation force and current are calculated by the modified formula, and the velocity curves of the levitation force and current are obtained. The results show that the eddy current effect increases the load power by 61.9% in the case of heavy loads.

Superb electromagnetic wave-absorbing composites based on large-scale graphene and carbon nanotube films

AbstarctGraphene has sparked extensive research interest for its excellent physical properties and its unique potential for application in absorption of electromagnetic waves. However, the processing of stable large-scale graphene and magnetic particles on a micrometer-thick conductive support is a formidable challenge for achieving high reflection loss and impedance matching between the absorber and free space. Herein, a novel and simple approach for the processing of a CNT film-Fe3O4-large scale graphene composite is studied. The Fe3O4 particles with size in the range of 20–200 nm are uniformly aligned along the axial direction of the CNTs. The composite exhibits exceptionally high wave absorption capacity even at a very low thickness. Minimum reflection loss of −44.7 dB and absorbing bandwidth of 4.7 GHz at −10 dB are achieved in composites with one-layer graphene in six-layer CNT film-Fe3O4 prepared from 0.04 M FeCl3. Microstructural and theoretical studies of the wave-absorbing mechanism reveal a unique Debye dipolar relaxation with an Eddy current effect in the absorbing bandwidth.

Development of a full-ring “add-on PET” prototype: A head coil with DOI-PET detectors for integrated PET/MRI

We developed a full-ring “add-on PET” prototype which is brain-dedicated and consists of a RF-head coil with four-layer depth-of-interaction (DOI) PET detectors for integrated PET/MRI in order to evaluate performance of our previously proposed add-on PET system and to investigate the mutual influences between the individual PET and MRI modalities when they are integrated in simultaneous measurements. In this add-on PET prototype, the DOI detectors are mounted on the head coil and close to the patient head. As a result, higher sensitivity and higher spatial resolution can be achieved for the integrated PET/MRI, compared with conventional whole body PET/MRI systems. In addition, implementation cost can be reduced, tuning of the RF-coil can be optimized and PET and MRI images can be obtained simultaneously in exactly the same positions.Specifically, the full-ring prototype consists of eight DOI-PET detectors and a birdcage type head coil of a 3T MRI. The radius of the PET ring is 123.9mm. The distance from the center to the RF-coil elements is 130.5mm. The scintillator blocks consist of lutetium-yttrium oxyorthosilicate scintillators arranged in 19×6×4 layers with reflectors inserted between them. The size of each crystal element is 2.0mm×2.0mm ×5.0mm.We evaluated performance of the full-ring prototype in simultaneous measurements of the integrated PET/MRI. We obtained spatial resolutions of 2.3mm at the center of the field-of-view (FOV) and lower than 3.5mm in the whole FOV. The energy resolution of 19.4% was obtained for 511keVgamma-rays. In addition, we observed no degradation of PET performance caused by the MRI measurement. The signal-to-noise ratio (SNR) of the MRI image was 209.4 in simultaneous measurements with the PET. The maximum ΔB0 and maximum difference of the secondary magnetic field due to the eddy current effect were smaller than 0.8ppm and ±5.0 μT, respectively. We concluded that sufficient spatial resolution and detector performance of the PET were obtained for the add-on PET prototype even in the simultaneous measurements with the MRI. The influence of the PET detectors on the static magnetic field and the SNR of the MRI images were not problems, although the eddy current effect must be suppressed.

Eddy current analysis and optimization of fast scanning magnet for a proton therapy system

Proton therapy is now recognized as one of the most effective radiation therapy methods for cancers. A proton therapy facility with multiple gantry treatment rooms is under development in HUST (Huazhong University of Science and Technology), which is based on isochronous superconducting cyclotron scheme. In the beam line, the scanning system spreads out the proton beam on the target according to the complex tumour shape by two scanning magnets for horizontal and vertical scanning independently. Since these two magnets are excited by alternating currents and the maximum repetition frequency is up to 100Hz, eddy currents and losses are expected to be significant. Slits are proven to be an effective way to reduce the eddy currents. To evaluate the heat distribution due to eddy losses in the pole end of the scanning magnet, the transient electromagnetic analysis and steady-state thermal analysis are performed. This paper describes design considerations of the scanning system and mainly analyses the eddy current effect of the scanning magnets. Different coil shapes and slit arrangements are simulated and compared to obtain the optimal configuration. The maximum temperatures of two magnets are optimized below 70°C. In addition, the lag effect due to eddy currents is also discussed.

Wireless Charging System Considering Eddy Current in Cardiac Pacemaker Shell: Theoretical Modeling, Experiments, and Safety Simulations

Implantable cardiac pacemakers play a vital role in extending the lives of patients with cardiovascular diseases. The technique of wireless power transfer (WPT) via magnetic coupling resonance (MCR) makes it possible to wirelessly and uninterruptedly supply electricity for pacemakers from external. However, the eddy current in the metallic pacemaker shell seriously impacts the electric energy receiving by receiving coils, making the WPT system operate at low efficiency. To decrease eddy current effect and increase WPT efficiency, the precise implant position of receiving coils is theoretically and experimentally investigated in this paper. The electromagnetic model of pacemaker WPT system is built and the analytical solution of induced voltage across receiving coils is derived, which is verified by experiments. Accordingly, the minimum distance between the receiving coil and the pacemaker shell is reversely calculated for enough induced voltage. Experimental results show that supplied by a power source of 5.66 V/300 kHz, the pacemaker charging system successfully charged Lithium-ion battery from 3.98 (80% residual capacity) to 4.2 V within 30 min by a 0.15 mm thickness receiving coil implanted into 4 mm subcutaneously. On the safety of charging system, electromagnetic and thermal simulation results show that the maximum SAR and temperature rise in tissues are 36.8 W/kg and 0.66 $^\circ$ C, respectively. The results provide a theoretical and practical support for design of a wireless charging system.

Quantitative experiment and analysis of gradient-induced eddy currents on magnetic resonance imaging

Pulsed magnetic field gradients generated by gradient coils are widely used in signal location in magnetic resonance imaging (MRI). However, gradient coils can also induce eddy currents in final magnetic field in the nearby conducting structures which lead to distortion and artifact in images, misguiding clinical diagnosis. We tried in our laboratory to measure the magnetic field of gradient-induced eddy current in 1.5 T superconducting magnetic resonance imaging device; and extracted key parameters including amplitude and time constant of exponential terms according to inductance-resistance series mathematical module. These parameters of both self-induced component and crossing component are useful to design digital filters to implement pulse pre-emphasize to reshape the waveform. A measure device that is a basement equipped with phantoms and receiving coils was designed and placed in the isocenter of the magnetic field. By applying testing sequence, contrast experiments were carried out in a superconducting magnet before and after eddy current compensation. Sets of one dimension signal were obtained as raw data to calculate gradient-induced eddy currents. Curve fitting by least squares method was also done to match inductance-resistance series module. The results also illustrated that pulse pre-emphasize measurement with digital filter was correct and effective in reducing eddy current effect. Pre-emphasize waveform was developed based on system function. The usefulness of pre-emphasize measurement in reducing eddy current was confirmed and the improvement was also presented. All these are valuable for reducing artifact in magnetic resonance imaging device.

Development and implementation of an 84-channel matrix gradient coil.

Design, implement, integrate, and characterize a customized coil system that allows for generating spatial encoding magnetic fields (SEMs) in a highly-flexible fashion. A gradient coil with a high number of individual elements was designed. Dimensions of the coil were chosen to mimic a whole-body gradient system, scaled down to a head insert. Mechanical shape and wire layout of each element were optimized to increase the local gradient strength while minimizing eddy current effects and simultaneously considering manufacturing constraints. Resulting wire layout and mechanical design is presented. A prototype matrix gradient coil with 12 × 7 = 84 elements consisting of two element types was realized and characterized. Measured eddy currents are <1% of the original field. The coil is shown to be capable of creating nonlinear, and linear SEMs. In a DSV of 0.22 m gradient strengths between 24 mT∕m and 78 mT∕m could be realized locally with maximum currents of 150 A. Initial proof-of-concept imaging experiments using linear and nonlinear encoding fields are demonstrated. A shielded matrix gradient coil setup capable of generating encoding fields in a highly-flexible manner was designed and implemented. The presented setup is expected to serve as a basis for validating novel imaging techniques that rely on nonlinear spatial encoding fields. Magn Reson Med 79:1181-1191, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Analysis and Restraining of Eddy Current Damping Effects in Rotary Voice Coil Actuators

Rotary voice coil actuators (RVCAs) have been widely used for precision motion control applications and have drawn more and more attention in recent years. In this paper, the eddy current damping effects of RVCAs of both dual and single magnetic circuit types are studied, the electromotive forces in rotor frame arms of the RVCAs are analyzed and verified by FEM calculation, then corresponding restraining measures are proposed. Especially, influencing factors of magnetic field margin effect and influence of magnetic field structure on the margin effect are analyzed in detail, which are the unique problems in RVCAs. The research results are good references for practical engineering applications of this type of motor.

Microwave Electromagnetic and Absorption Properties of N-Doped Ordered Mesoporous Carbon Decorated with Ferrite Nanoparticles

Lightweight nitrogen-doped ordered mesoporous carbon (NOMC) with high specific surface area and pore volume have been prepared through self-assembly and subsequent heat treatment route. The spherical NOMC particles are decorated with CoFe2O4 nanoparticles via coprecipitation method to enhance their microwave absorption property. The electromagnetic parameters of the NOMC and CoFe2O4/NOMC composites are measured and the microwave reflection loss properties are evaluated in the frequency range of 0.5–18 GHz. The results show that both the real part and imaginary part of permittivity of NOMC totally decline and the real part of permeability increases with the introduction of ferrite. However, the negative values of the imaginary part of the complex permeability appear for the CoFe2O4/NOMC composites, which may be caused by enhanced eddy current effect due to the introduction of ferrite. The reflection loss results exhibit that the CoFe2O4/NOMC composites have excellent microwave absorption performances. The ...

Mechanical-magnetic-electric coupled behaviors for stress-driven Terfenol-D energy harvester

The stress-driven Terfernol-D energy harvester exhibits the nonlinear mechanical-magnetic-electric coupled (MMEC) behaviors and the eddy current effects. To analyze and design the device, it is necessary to establish an accurate model of the device. Based on the effective magnetic field expression, the constitutive equations with eddy currents and variable coefficients, and the dynamic equations, a nonlinear dynamic MMEC model for the device is founded. Comparisons between the measured and calculated results show that the model can describe the nonlinear coupled curves of magnetization versus stress and strain versus stress under different bias fields, and can provide the reasonable data trends of piezomagnetic coefficients, Young’s modulus and relative permeability for Terfenol-D. Moreover, the calculated power results show that the model can determine the optimal bias conditions, optimal resistance, suitable proof mass, suitable slices for the maximum energy extraction of the device under broad stress amplitude and broad frequency.

Calculation of PM Eddy Current Loss in IPM Machine Under PWM VSI Supply With Combined 2-D FE and Analytical Method

A hybrid method combining 2-D finite-element (FE) method and analytical model is proposed for permanent magnet (PM) eddy current loss calculation in interior PM machines under pulsewidth modulation (PWM) voltage source inverter (VSI) supply, in which the PM eddy current reaction effect and PM axial segmentation are taken into consideration. First, the 2-D time-stepping (TS) FE analysis results considering and neglecting PM eddy current under PWM VSI supply are compared so that the cause of PM eddy current loss, the increase of high-order armature current components due to PM eddy current effect, and the decrease of the high-order components in average flux density due to leakage flux are revealed. Second, an analytical model for PM eddy current loss calculation using voltage input is deduced with the average flux density as input and a leakage flux factor is introduced in the model to meet the real case in PM machine. Moreover, the 3-D frequency FE analysis is used to verify the derived model. Third, the ratio between the average flux density and the imposed flux density is defined as a key parameter to reflect the PM eddy current reaction effect quantitatively, for both 2-D and 3-D cases. A functional relationship is established to describe the variation of the average flux density with and without considering the PM eddy current, where the parameter is obtained with nonlinear fitting of the 2-D FEA results. At last, the calculation procedure of the hybrid method is presented and a flux-concentrating field-modulated PM machine is taken as an example for calculation. The results are verified by an FE analysis. The proposed method is more efficient compared with the 3-D TS FE method while maintaining rather good accuracy.

Sizing and Eddy currents in magnetic core nanoparticles: an optical extinction approach.

Optical extinction is a handy and ubiquitous technique that allows us to study colloidal nanoparticles in their native state. The typical analysis of the extinction spectrum can be extended in order to obtain structural information of the sample such as the size distribution of the cores and the thickness of the coating layers. In this work the extinction spectra of Fe3O4, Fe3O4@Au, and Fe3O4@SiO2@Au single and multilayer nanoparticles are obtained by solving full Mie theory with a frequency dependent susceptibility derived from the Gilbert equation and considering the effect of Eddy currents. The results are compared with non-magnetic Mie theory, magnetic dipolar approximation and magnetic Mie theory without Eddy currents. The particle size-wavelength ranges of validity of these different approaches are explored and novel results are obtained for Eddy current effects in optical extinction. These results are used to obtain particle size and shell thickness information from the experimental extinction spectra of Fe3O4 and Fe3O4@Au nanoparticles in good agreement with TEM results, and to predict the plasmon peak parameters for Fe3O4@SiO2@Au three layer nanoparticles.

Study of Induced Eddy Current Effects on Field Homogeneity for Fast Ramped Bending Magnet

An investigation of induced eddy current effects on the field homogeneity in fast ramped bending magnets was presented in this paper. The transverse field homogeneity and integral field homogeneity were calculated based on two-dimensional/three-dimensional finite-element analysis. The results showed that the transverse field homogeneity and integral field homogeneity were less than 0.05% after the field optimization with static analysis. The fast ramped bending magnet is used to deliver a proton beam for a superconducting proton therapy facility. The energy of the proton beam is adjusted by a fast degrader device and it will require all the magnets in the beam line to follow the energy changes of the degrader as fast as possible. During the energy change stage, induced eddy currents will appear in the beam tube. Then, the transient finite-element analysis is presented at different current ramp rates k and beam tube thicknesses b. The induced eddy currents in the beam tube were analyzed to research the eddy current effects on field quality. It shows that the field quality will be influenced more obviously with increased current ramp rate or beam tube thickness. The maximal field decrease in the midplane caused by eddy currents in the beam tube is the main reason for field quality disturbance. The relationship between the field decrease and current ramp rate k can be expressed simply with a mathematical formula. For different situations, the effects caused by eddy currents on field quality may be different. But the maximal field decrease in the center of the magnet is the same.