Integral methods for analysis and design of low-frequency conductive shields

Canova, Aldo; Gruosso, Giambattista; Repetto, Maurizio

doi:10.1109/tmag.2003.812718

Cited by 41 publications

(20 citation statements)

References 14 publications

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“…The third comparison, referred to U-shaped shields at the frequency of 50 Hz, is again with the results presented in Reference [2] based on FEM-BEM but also with the ones presented in Reference [6] based on a circuital approach (multiconductor model, MCM).…”

Section: Comparison With Numerical Methodsmentioning

confidence: 80%

“…The validation based on other numerical methods consists in comparisons with results published in technical literature [2,6,15,16] that are based on different techniques.…”

Section: Generalmentioning

confidence: 99%

“…We can find different approaches: finite elements method (FEM) [1], hybrid finite elements method/boundary elements method (FEM-BEM) [2,3], analytical and semi-analytical techniques (that can be applied only in cases of particular geometries and materials) [4,5] and integral equations (IE) method [6,7]. It is worth to mention also certain methods based on a circuital approach; see References [8,6].…”

Section: Introductionmentioning

confidence: 99%

“…It is worth to mention also certain methods based on a circuital approach; see References [8,6]. As far as the IE method is concerned, we remark that, in the just mentioned works, only conductive shields are considered; here we want to overcome such a limitation by proposing an Integral Method that is able to deal also with magnetic materials provided that they can be considered linear.…”

Section: Introductionmentioning

confidence: 99%

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Integral equations method for the analysis and design of ELF conductive and magnetic shields

Lucca

2008

Int Trans Elec Energy Syst

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SUMMARYThe need to shield ELF (extremely low frequency) magnetic fields generated by power and industrial installations arises for two main reasons: the first one is a typical EMC (electromagnetic compatibility) problem related to possible interference among electric and electronic devices while the second one is related to potential health effects due to magnetic field exposure. Different techniques exist to reduce the magnetic field mainly depending on the kind of source considered; here we treat only the shielding action offered by long and metallic structures like slabs, sheets, and foils with respect to the field produced by long sources like power cables, bus-bars, and wires. In particular, we focus our attention on magnetic metallic shields where the shielding action is due to both 'magnetic flux shunting' and 'eddy current cancellation' mechanisms. From the design point of view, it is useful to have at disposal a calculation tool able to predict the magnetic field reduction, in a given region of space, as a function of the shield characteristics (geometry, material) and source characteristics (conductors disposition, distance from the shield). The typical calculation technique used in this kind of problem is based on the finite elements method (FEM) that is broadly described in the technical literature. Here, we want to propose a different approach that is based on integral equations (IE) and is able to model shields made by materials like iron, steel, and mumetal having both conductive and magnetic properties; nevertheless, as special cases, the algorithm is also able to deal with purely conductive shields like copper and aluminium, (frequently used in applications) where only the eddy current shielding action exists, and with purely magnetic shields, under the action of a d.c. source, where only the magnetic flux shunting effect is present. The main hypothesis that is necessary to adopt in the model is the linear behaviour of the magnetic material; such hypothesis is certainly reasonable in many real situations where the material is far from saturation.

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Section: Comparison With Numerical Methodsmentioning

confidence: 80%

“…The validation based on other numerical methods consists in comparisons with results published in technical literature [2,6,15,16] that are based on different techniques.…”

Section: Generalmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Integral equations method for the analysis and design of ELF conductive and magnetic shields

Lucca

2008

Int Trans Elec Energy Syst

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“…Finite element (FEM) and finite difference time domain (FDTD) methods have been developed to calculate eddy currents at the cost of massive computer calculations and inaccurate handling of complex boundary conditions [63,96]. The boundary element method (BEM) discretises active parts only while avoiding simulating the air [97]. The network methods based on a semi-analytical solution of the diffusion equation of the eddy currents in thick cylinders and the multi-layer integral method (MIM) have been employed to simulate eddy current phenomena in irregular geometries induced by gradient coils of arbitrary geometry [77,98].…”

Section: Eddy Currentsmentioning

confidence: 99%

Gradient coil design and intra-coil eddy currents in MRI systems

Tang¹

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The work in this thesis is primarily concerned with the enhancement of gradient coil performance in magnetic resonance imaging (MRI). In MRI, gradient coils are used to produce gradient fields to spatially encode magnetic resonance signals. However, rapid switching on and off of the gradient coils induces fields and eddy currents in the human tissues and surrounding conducting structures. This thesis will provide novel solutions for the design of gradient coils and the reduction of eddy currents.Gradient switching induces electric fields in human tissue that can cause safety problems, in particular peripheral nerve stimulation (PNS), which limits the gradient performance such as the slew rate and maximum gradient strength. One approach to solve this problem is to use local insertable gradient coils to achieve high gradient performance over localised regions of interest (ROI) yet not trigger PNS. Head only coils or head and neck coils are by far the most commonly used local coils. However, due to the constraints of the human head and shoulder, the head gradient coils are usually designed in an asymmetric configuration, with the region-of-uniformity (ROU) close to the patient end of the coil. This asymmetric configuration leads to technical difficulties in maintaining a high gradient performance for the head coil inserts given the very limited space available. Therefore, in this thesis, a practical configuration of an insertable asymmetric gradient head coil that offers improved performance was proposed. In the proposed design, at the patient end, the primary and secondary coils were connected using an additional radial surface, thus allowing the coil conductors distributed on the flange to ensure an improvement in the coil performance. At the service end, the primary and shielding coils were not connected, to permit access to shim trays, cooling system piping, cabling, and so on. It was found that with a similar field quality in the ROI, the proposed coil pattern improved construction characteristics (open service end, well-distributed wire pattern) and offers a better coil performance (lower inductance, higher efficiency etc.) than conventional head coil configurations.Another obstacle to the advancement of MRI is the eddy currents induced in the surrounding conducting structures including the gradient coils themselves. The eddy currents induced in the surrounding conductors depend on the geometry of the conductor and the excitation waveform. These alternating fields caused by the switching gradient coils change the spatial profile of the current density within the coil tracks and surrounding coils with the applied frequencies of the input waveform and by their proximity to other conductors. Therefore, in this thesis, inductive coupling between coil tracks and gradient coils themselves was first investigated and then solutions for mitigation of the eddy current effects were provided. IIFirst, the inductive coupling between coil tracks was studied, considering skin and proximity effects. In this investiga...

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