Language：English   Publishing type：Research paper (scientific journal)   Publisher：TECH SCIENCE PRESS  

The voxel-based indirect boundary element method (IBEM) combined with the Laplace-kernel fast multipole method (FMM) is capable of analyzing relatively large-scale problems. A typical application of the IBEM is the electric field analysis in virtual-human models such as the model called Duke provided by the foundation for research on information technologies in society (IT'IS Foundation). An important property of voxel-version Duke models is that they have various voxel sizes but the same structural feature. This property is useful for examining the O(N) and O(D-2) dependencies of the calculation times and the amount of memory required by the FMM-IBEM, where N and D are the number of boundary elements and the reciprocal of the voxel-side length, respectively. In this paper, the dependencies were confirmed by analyzing Duke models with voxel-side lengths of 5.0, 2.0, LO, and 0.5 mm. The finest model had 2.2 billion voxels and 61 million square elements. In addition, a technique that improves the convergence performance of the linear equation solver by considering the non-uniqueness of the electric potential is proposed, and its effectiveness is demonstrated.

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：WITPress  

The voxel-based indirect boundary element method (IBEM) using the Laplace-kernel fast multipole method (FMM) is capable of analysing relatively large-scale problems. Furthermore, the voxel-based IBEM is suitable for acceleration using graphics processing units (GPUs). A typical application of the IBEM is the analysis of electrostatic fields for virtual-human anatomical voxel models such as the model called Duke provided by the IT’IS Foundation. An important property of voxel-version Duke models is that they have different voxel sizes but the same structural feature. This property is useful for examining the O(N) and O(D2) dependencies of the calculation times and the amount of memory required by the FMM-IBEM, where N and D are the number of boundary elements and the reciprocal of the voxel side length, respectively. In this study, the O(N) and O(D2) dependencies of the voxel-based GPU-accelerated FMM-IBEM were confirmed by analysing Duke models with voxel side lengths of 5.0, 2.0, 1.0, and 0.5 mm. The finest model comprised 2.2 billion voxels with 61 million square boundary elements, and a linear equation solver on a personal computer with four GPUs required 1,276 s to obtain a solution. In addition, a technique is proposed to improve the convergence performance of the linear equation solver by considering the non-uniqueness of the electric potential, and its effectiveness is demonstrated.

DOI： 10.2495/BE370121

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：WILEY-BLACKWELL  

An indirect boundary element method that is geared to electrostatic field analysis in voxel models is accelerated by graphics processing units (GPUs). The method considers square walls on cubic voxels as boundary surface elements and uses the fast multipole method (FMM) to analyze large-scale models. On the basis of two conventional CPU codes, three GPU codes are programmed in search of higher computing performance. These GPU codes are designed as follows: In GPU code 1, direct and far fields in the FMM are simultaneously calculated on the GPU and the CPU, respectively; in GPU code 2, both fields are calculated on the GPU with a rotation-coaxial translation-rotation decomposition algorithm; and in GPU code 3, both fields are calculated on the GPU with a diagonal translation scheme. The electric fields in human models are generated by applying a 50-Hz magnetic field or by injecting direct-current (DC) current through two electrodes and they were calculated successfully using a personal computer with three GPUs and six CPU cores. An analysis with 3.9million surface elements took 89.4s to solve its governing linear system with double-precision floating-point arithmetic. GPU codes 1, 2, and 3 demonstrated the least memory usage, the greatest speed-up ratio, and the fastest calculation time, respectively. These results show an example of the trade-off relationships of computation performances on a heterogeneous CPU-GPU system. Copyright (c) 2013 John Wiley & Sons, Ltd.

DOI： 10.1002/jnm.1883

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Language：Japanese   Publishing type：Research paper (conference, symposium, etc.)  

EMC-13-10

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Language：Japanese   Publishing type：Research paper (conference, symposium, etc.)  

MBE-13-21

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ACADEMIC PRESS INC ELSEVIER SCIENCE  

In recent years, there has been considerable interest in developing an ultra-low-field magnetic resonance imaging (ULF-MRI) system using an optically pumped atomic magnetometer (OPAM). However, a precise estimation of the signal-to-noise ratio (SNR) of ULF-MRI has not been carried out. Conventionally, to calculate the SNR of an MR image, thermal noise, also called Nyquist noise, has been estimated by considering a resistor that is electrically equivalent to a biological-conductive sample and is connected in series to a pickup coil. However, this method has major limitations in that the receiver has to be a coil and that it cannot be applied directly to a system using OPAM. In this paper, we propose a method to estimate the thermal noise of an MRI system using OPAM. We calculate the thermal noise from the variance of the magnetic sensor output produced by current-dipole moments that simulate thermally fluctuating current sources in a biological sample. We assume that the random magnitude of the current dipole in each volume element of the biological sample is described by the Maxwell-Boltzmann distribution. The sensor output produced by each current-dipole moment is calculated either by an analytical formula or a numerical method based on the boundary element method. We validate the proposed method by comparing our results with those obtained by conventional methods that consider resistors connected in series to a pickup coil using single-layered sphere, multi-layered sphere, and realistic head models. Finally, we apply the proposed method to the ULF-MRI model using OPAM as the receiver with multi-layered sphere and realistic head models and estimate their SNR. (C) 2011 Elsevier Inc. All rights reserved.

DOI： 10.1016/j.jmr.2011.12.014

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PubMed

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：WILEY-BLACKWELL  

Time-harmonic electromagnetic fields in multisphere models were calculated by the method of two scalar potentials proposed by Gumerov. The analyzed models were an eccentric two-sphere model and a ten-sphere model including the brain core and a pair of eyeballs. The applied fields were a plane wave and a dipole magnetic field, and the frequency was 1.8 or 2.0 GHz. The truncation degree p of the multipole and local expansion had values of up to 170 to 200. The peak power density on the x-y plane and the absorbed power in each tissue almost monotonically converged as p increased. These results demonstrate the usefulness of Gumerov's method. (C) 2011 Wiley Periodicals, Inc. Electr Eng Jpn, 178(2): 1-10, 2012; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/eej.21224

DOI： 10.1002/eej.21224

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

using graphics processing units (CPUs) to reduce the time required to calculate a three-dimensional electrostatic field. The BEM is designed to handle cubic voxel models and is specialized to consider square voxel walls as boundary surface elements. The FMM handles the interactions among the surface charge elements and directly outputs surface integrals of the fields over each individual element. The CPU code was originally developed for field analysis in human voxel models derived from anatomical images. FMM processes are programmed using the NVIDIA Compute Unified Device Architecture (CUDA) with double-precision floating-point arithmetic on the basis of a shared pseudocode template. The electric field induced by DC-current application between two electrodes is calculated for two models with 499,629 (model 1) and 1,458,813 (model 2) surface elements. The calculation times were measured with a four-GPU configuration (two NVIDIA GTX295 cards) with four CPU cores (an Intel Core i7-975 processor). The times required by a linear system solver are 31 s and 186 s for models 1 and 2, respectively. The speed-up ratios of the FMM range from 5.9 to 8.2 for model 1 and from 5.0 to 5.6 for model 2. The calculation speed for element-interaction in this BEM analysis was comparable to that of particle-interaction using FMM on a CPU. (C) 2011 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.cpc.2011.01.020

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：WILEY-BLACKWELL  

This paper describes analyses of the electric fields induced in two eccentrically arranged spherical conductors by a uniform ELF magnetic field, utilizing the equivalent multipolar-moment method with re-expansion (RE-EMMM). Although an approximation formula has already been derived by Tarao for this situation, it is limited to a first-order approximation. Here we mathematically derive a general form of the governing linear equations for unknown multipole and local coefficients. We also derive the first-, second-, and third-order electric field formulae by manually solving the linear equations. We confirm that the accuracy of the first-order formula is almost the same as that of Tarao's. The accuracies of the second- and third-order formulae are confirmed to be more precise. 2011 Wiley Periodicals, Inc. Electron Comm Jpn, 94(3): 20-32, 2011; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/ecj.10304

DOI： 10.1002/ecj.10304

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Language：Japanese   Publishing type：Research paper (conference, symposium, etc.)  

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Language：English   Publishing type：Research paper (international conference proceedings)  

In this study, we compared the performances of minimum variance beamformer (MVBF), linearly-constrained (LC)-MVBF, prewhitening beamformer (PWBF), and LC-PWBF with regard to the estimation of signal location in the brain neocortex. Equivalent current dipoles were located in the primary motor cortex in performed numerical simulations by considering applications in brain-computer interfaces based on modulation of μ and β rhythms. It was confirmed that when the signals were highly correlated, the location bias of LC-PWBF was smaller than that of MVBF, LC-MVBF, and PWBF. Obtained results suggest that LC-PWBF is useful for estimating the location of signal sources that are highly correlated and have low signal-to-noise ratio. © 2011 IEEE.

DOI： 10.1109/ICCME.2011.5876810

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：OXFORD UNIV PRESS  

The present study provides an intercomparison of the induced quantities in a human model for uniform magnetic field exposures at extremely low frequency. A total of six research groups have cooperated in this joint intercomparison study. The computational conditions and numeric human phantom including the conductivity of tissue were set identically to focus on the uncertainty in computed fields. Differences in the maximal and 99th percentile value of the in situ electric field were less than 30 and 10 % except for the results of one group. Differences in the current density averaged over 1 cm(2) of the central nerve tissue are 10 % or less except for the results of one group. This comparison suggests that the computational uncertainty of the in situ electric field/current density due to different methods and coding is smaller than that caused by different human phantoms and the conductivitys of tissue, which was reported in a previous study.

DOI： 10.1093/rpd/ncp251

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Language：English   Publishing type：Research paper (international conference proceedings)  

In order to enhance the reliability of electroencephalogram (EEG) signal source analyses, utilizing EEG lead field matrices obtained by field analyses in custom-made real head models is effective technique. Custom-made models are usually constructed from voxel models derived from magnetic resonance (MR) images using a variety of image-processing techniques. We have improved one of the techniques that select threshold levels dividing signals and noises based on the EM (expectation-maximization) algorithm. This technique contributes to rapid and high-quality voxel model acquisition. We demonstrate the following operations: (a) voxel model construction, (b) lead field calculation, and (c) simulation of EEG electrode voltage measurement induced by an equivalent current dipole (ECD), which is set in the primary motor cortex by considering application of brain-machine interfaces. The proposed technique is compared with other techniques based on the voltage differences caused by the constructed model-shape differences. © 2010 IEEE.

DOI： 10.1109/ICCME.2010.5558863

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SCRIPTA TECHNICA-JOHN WILEY & SONS  

This paper describes an application of the equivalent multipole moment method (EMMM) with polar translations to calculation of magnetic fields induced by a current dipole placed in a human head model. Although the EMMM is a conventional Laplacian field solver based on spherical harmonic functions, the polar translations enable it to treat eccentric and exclusive spheres in arbitrary arrangements. The head model is composed of, seven spheres corresponding to skin, two eyeballs, skull, cerebral spinal fluid, gray matter, and while matter. The validity of the calculated magnetic fields and the magnetic flux linkages with a loop coil located near the model is Successfully confirmed by the reciprocity theorem derived by Eaton. (C) 2008 Wiley Pcriodicals, Inc. Electron Comm Jpn, 91(4): 34-44, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecj.10079

DOI： 10.1002/ecj.10079

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SCRIPTA TECHNICA-JOHN WILEY & SONS  

An analytical formula of induced electric field E in a spherical Conductor by an ELF dipole magnetic field Source M = M(parallel to) + M(perpendicular to) is mathematically derived in vector form based on the equivalent mutlipole moment method with reexpansion technique (RE-EMMM), whereM(parallel to) and M(perpendicular to). are parallel and perpendicular components of M, respectively. The validity of the formula is confirmed in the following three ways: (i) the derivation of the formula front the Sarvas equation with the reciprocity theorem derived by Eaton; (ii) the convergence of the formula to that of homogeneous magnetic field when M(parallel to) is located at the infinite distance; (iii) comparison of the analytical Solutions with numerical solutions by RE-EMMM. Furthermore, it formula for the trajectory, which satisfies E = 0, is derived for the field by M(parallel to). (C) 2008 Wiley Periodicals, Inc. Electr Eng Jpn, 166(3): 8-17, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eeej.20739

DOI： 10.1002/eej.20739

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Language：Japanese   Publishing type：Research paper (scientific journal)  

Time harmonic electromagnetic fields in multi-spheres models were calculated by the method of two scalar potentials proposed by Gumerov. The analyzed models were an eccentric two-spheres model and a ten-spheres model including brain core and a pair of eyeballs. The applied fields were a plane wave and a dipole magnetic field, and the frequency was 1.8 or 2.0GHz. The truncation degree p of multipole and local expansion was set to up to 170-200. The peak power density on the x-y plane and the absorbed power in each tissue almost monotonously converged as p increased. It demonstrated the usefulness of the Gumerov's method. © 2009 The Institute of Electrical Engineers of Japan.

DOI： 10.1541/ieejfms.129.168

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Language：Japanese   Publishing type：Research paper (scientific journal)  

Time harmonic electromagnetic fields in multi-spheres models were calculated by the method of two scalar potentials proposed by Gumerov. The analyzed models were an eccentric two-spheres model and a ten-spheres model including brain core and a pair of eyeballs. The applied fields were a plane wave and a dipole magnetic field, and the frequency was 1.8 or 2.0GHz. The truncation degree p of multipole and local expansion was set to up to 170-200. The peak power density on the x-y plane and the absorbed power in each tissue almost monotonously converged as p increased. It demonstrated the usefulness of the Gumerov's method. © 2009 The Institute of Electrical Engineers of Japan.

DOI： 10.1541/ieejfms.129.168

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SCRIPTA TECHNICA-JOHN WILEY & SONS  

This paper presents it fast-multipole surface-charge-simulation method for calculating three-dimensional Laplacian fields in voxel models. This method treats a surface of a voxel that has different inside and Outside conductivities as it surface element of the indirect BEM (boundary element method). The main features of the proposed method are as follows. (1) An O(D-2) performance in the memory capacity and operation cost is provided by applying the diagonal form fast multipole method (FMM), when the number of voxels is about D-3. (2) The boundary matching is imposed by the continuity of the total flux passing through each element, which guarantees the solution globally satisfying Gauss's law; therefore the solution is globally stabilized.
This method is successfully applied to calculate the electric field induced by an applied homogeneous ELF (extremely low frequency) magnetic field in a human head model that has 1 mm x 1 mm x 1 mm voxel size. (C) 2008 Wiley Periodicals, Inc.

DOI： 10.1002/eej.20529

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SCRIPTA TECHNICA-JOHN WILEY & SONS  

We have been studying a multi-point charge measurement method using an electrostatic probe. In this technique, charge densities x must be estimated from the probe Outputs b by an inverse calculation based on an equation Ax = b. The matrix A is obtained by applying a numerical field calculation technique. When the matrix A is in ill-condition, the solution often makes no sense, including extremely large errors. Therefore, we apply the regularized least squares method (RLS) with a penalty term to perform the inverse calculation stably for the ill-conditioned matrix. The penalty term imposes some constraints on the solutions.
In this paper, first, we have analyzed the accuracy of the charge distribution estimated by the inverse calculation. Although the perturbation bound of the solution errors has already been proposed for the least squares method, it has not yet been given for the RLS. We have derived the equations that express the perturbation bound of the solution errors in applying the RLS to evaluate the estimation accuracy. Second, we have applied the above equations to an experimental result for a cylindrical dielectric solid, and estimated the charge distribution represented by 10,140 unknowns. We have utilized an iteration technique and the symmetric configuration of the measured arrangement so as to reduce the amount of operations and memory capacity required for the inverse calculation. (C) 2008 Wiley Periodicals, Inc.

DOI： 10.1002/eej.20412

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Language：English   Publishing type：Research paper (international conference proceedings)  

This paper deals with charging characteristics of the inner surface of an insulating hollow cylinder in vacuum. We conducted measurements of electric field strength near the triple points on cathode by using an electrostatic probe. Also we conducted a computer simulation of charging based on the Secondary Electron Emission Avalanche (SEEA) mechanism. These results are compared with those obtained previously for solid cylinders. As a result, we have clarified that hollow cylinders acquire surface charge which is larger than that of solid cylinders. We have also found that charge controlling effect by roughening the inner surface, which have been proved effective to depress charging on the surface of solid cylinders in our previous studies, is limited for hollow cylinders. © 2008 The Institute of Electrical Engineers of Japan.

DOI： 10.1541/ieejfms.128.606

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Language：English   Publishing type：Research paper (scientific journal)  

This paper describes an accuracy estimation of low frequency electric fields induced in a human head model calculated by the FMM-SCM-V (fast-multipole surface-charge-simulation method for voxel data) by comparing them with the true fields calculated by the EMMM-PT (equivalent multipole moment method with polar-coordinates translations). The head model is composed of seven spheres and the spheres are composed of cubic voxels having 2, 1, 0.5, or 0.34mm side length. A square-coil current (1A, 50Hz) induces eddy field in the model. The number of voxels, the number of elements, and calculation time were 106,575,801, 4,936,874, and 82 minutes, respectively, when 0.34mm size voxel model was analyzed. In this case, the maximum value and the standard deviation of the electric field error vector in gray matter region were less than 19% and 0.89%, respectively. In addition, they were less than 2.9% and 0.33%, respectively, when we intended to estimate spatially averaged field strength in a (0.34xll)3 mm3 volume region. © 2008 The Institute of Electrical Engineers of Japan.

DOI： 10.1541/ieejfms.128.223

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Language：Japanese   Publishing type：Research paper (scientific journal)  

This paper describes analyses of electric fields in eccentric two spherical conductors induced by a uniform ELF magnetic field utilizing the equivalent multipole-moment method with re-expansion technique (RE- EMMM). Although an approximated formula has already been derived by Tarao in this situation, it has been limited to the first order approximation. Here, the authors mathematically derive a general expression of governing linear equation for unknown multipole and local coefficients. Furthermore, we derived the first, second, and third order electric field formulae by manually solving the linear equations. It is confirmed that the accuracy of the first order formula is almost the same to that of Tarao's. On the other hand the accuracies of the second and third order formulae are confirmed to be more precise. © 2008 The Institute of Electrical Engineers of Japan.

DOI： 10.1541/ieejfms.128.710

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Japanese Society for Medical and Biological Engineering  

One of possible problems in fMRI-MEG integrative analysis is mismatches between activated regions detected by fMRI and MEG. These mismatches cause serious degradation of estimation accuracy, especially in the case that fMRI-invisible activities have high temporal correlations to activities detected by fMRI. We proposed a spatial filter which can achieve the accurate reconstruction of MEG source activities even in the case that a priori information of fMRI is insufficient. The proposed filter is based on generalized least square (GLS) estimation method. The GLS method requires to determine the noise covariance matrices, and the proposed filter utilizes measured MEGs for the determination. In the present study, principal component analysis is applied to the measured MEGs to determine the noise covariance matrices. Simulation results with conditions that fMRI-invisible MEG sources are present demonstrated that the proposed filter could reconstruct MEG source activities more accurately than the methods based on both ordinary least square method and minimum variance beamformer. The validities of the proposed method were also discussed, with measured data from the experiment using an apparent motion visual stimulus. The results demonstrated that the proposed method could reconstruct reasonable time courses of activations.

DOI： 10.11239/jsmbe.45.275

CiNii Books

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SCRIPTA TECHNICA-JOHN WILEY & SONS  

The adaptive cross-approximation (ACA) technique is applied to accelerating an inverse-problem solver that estimates charge distribution on a dielectric spacer. The ACA generates an approximated system-matrix that enables us to carry out high-speed inverse calculation. We designed an approximation procedure based on ACA with some additional concepts, that is, (a) partitioning of matrix based on algebraic information, (b) approximation quality control based on matrix norms, and so on. The tested solver (LSQR for regularized least squares) with ACA demonstrates about 10 times faster performance than that without ACA. (c) 2007 Wiley Periodicals, Inc.

DOI： 10.1002/eej.20326

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Language：Japanese   Publisher：The Institute of Electrical Engineers of Japan  

An analytical formula of induced electric field <i>E</i> in a spherical conductor by an ELF dipole magnetic field source <i>M</i> = <i>M</i>_// + <i>M</i>_&perp; is mathematically derived in vector form based on the equivalent multipole moment method with re-expansion technique (RE-EMMM), where <i>M</i>_// and <i>M</i>_&perp; are parallel and perpendicular components of <i>M</i>, respectively. The validity of the formula is confirmed by the following three ways. (i)The derivation of the formula from the Sarvas equation with the reciprocity theorem derived by Eaton. (ii)The convergence of the formula to that of homogeneous magnetic field when <i>M</i>_// is located at the infinite distance. (iii)Comparison of the analytical solutions with numerical solutions by RE-EMMM. Furthermore, an formula for the trajectory, which satisfies <i>E</i> = 0, is derived for the field by <i>M</i>_//.

DOI： 10.1541/ieejfms.127.346

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SCRIPTA TECHNICA-JOHN WILEY & SONS  

This paper presents a generalized equivalent multipole-moment method for calculating three-dimensional Laplacian fields in a multispherical system. Greengard and Rokhlin's M2M, M2L, and L2L formulas enable the multipole-moment method to calculate the fields in general arrangement of multispheres, which involve exclusive and multilayered spherical arrangement. We applied this method to electric field calculation in biological structures induced by ELF magnetic fields. The induced electric fields in a system of three eccentric and exclusive spheres, which models the human head with two eyeballs, are calculated under the application of homogeneous and magnetic-dipole fields. The validity of this method is successfully confirmed by comparing the calculated fields with those by the fast-multipole surface-charge-simulation method. (C) 2006 Wiley Periodicals, Inc.

DOI： 10.1002/eej.20342

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

We have investigated charging and flashover characteristics of a polymeric or glass insulator exposed to ac voltage in vacuum in order to develop compact and reliable high voltage VCBs (Vacuum Circuit Breakers). This paper focuses on charging characteristics of a cylindrical model insulator. The charging of an insulator is investigated using an electrostatic probe that measures the electric field near the triple junction on the grounded electrode. This method allows a time-resolved measurement of the charging process. The insulator was made of borosilicate, fused quartz or polymetyl methacrylate, and was in the shape of a right cylinder with 10 mm in thickness. It has been clarified that the charging is characterized by three sequential states; initiation, quasi-stable and stable states, and that the polarity of the charge is positive for these states irrespective of the voltage phase. The charging characteristics with ac voltage are compared to our previous results with dc voltage excitation. We find that the charge magnitude at the stable state coincides with that obtained by de. The electric field on the grounded electrode, and therefore the charge magnitude, decreases with the surface roughness, and decreases as the insulation strength is increased. A computer simulation has been conducted to investigate the quasi-stable state, which clarifies that the transition in surface charge distribution being synchronous to the voltage phase is responsible for causing the quasi-stable state.

DOI： 10.1109/TDEI.2006.1593395

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J-GLOBAL

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We have studied charging and flashover characteristics of a glass insulator with 0.3 - 50 mm long under DC voltage excitation in vacuum. We conducted flashover tests, charge measurements and calculation of surface charge distribution. The insulator was borosilicate, and was in the shape of a right cylinder. The side surface of the specimen was polished to have an average roughness of sub microns or roughened up to several microns. The charging characteristics of insulators were investigated by using an electrostatic probe, which was embedded in the plane cathode and allowed a time resolved measurement of the charging process. As a result, we have confirmed that roughening the surface mitigates the surface charging and thus improves the insulation strength for a specimen up to 50 mm in length. These results are useful not only for high voltage VCB's but also for other equipment that uses high voltage in vacuum such as particle accelerators.

DOI： 10.1541/ieejfms.126.769

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J-GLOBAL

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Language：English  

DOI： 10.1541/ieejfms.126.355

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Language：Japanese   Publishing type：Research paper (scientific journal)  

Analytical formula of induced electric field at arbitral position in a spherical conductor by an arbitral ELF dipole magnetic field source is mathematically derived in vector form. Its validity is confirmed by comparing the analitical solutions with numerical solutions by the equivalent multipole moment method.

DOI： 10.1541/ieejfms.126.725

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：Electromagnetics Academy  

This paper describes an application of the FMM(fast multipole method (1))-SCM(surface charge simulation method) for voxel data (2) to analyses of current density distribution in a human model induced by 50Hz magnetic fields. The performance of this method is demonstrated by calculating the induced currents in the 'Taro'(3) licensed by NICT (National Institute of Information and Communications Technology). Taro models a Japanese adult male and is composed of 7, 977, 906, 2mm×2mm×2mm voxels. The required calculation time results in about six to seven hours on a 32bit, 3GHz, 2G-RAM, single Pentium IV, Windows PC.

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Language：English   Publishing type：Research paper (scientific journal)  

The adaptive cross-approximation (ACA) technique is applied to accelerating an inverse-problem solver that estimates charge distribution on a dielectric spacer. The ACA generates an approximated system-matrix that enables us to carry out high-speed inverse calculation. We designed an approximation procedure based on ACA with some additional concepts, that is, (a) partitioning of matrix based on algebraic information, (b) approximation quality control based on matrix-norms, and so on. The tested solver (LSQR for regularized least squares) with ACA demonstrates about 10 times faster performance than that by without ACA. © 2005, The Institute of Electrical Engineers of Japan. All rights reserved.

DOI： 10.1541/ieejfms.125.419

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Language：Japanese   Publisher：The Institute of Electrical Engineers of Japan  

This article has no abstract.

DOI： 10.1541/ieejjournal.125.684

CiNii Books

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Other Link： https://jlc.jst.go.jp/DN/JALC/00289534043?from=CiNii

Language：English   Publishing type：Research paper (scientific journal)  

We have been studying a multi-point charge measurement method using an electrostatic probe. In this technique, charge densities x must be estimated from the probe outputs b by an inverse calculation based on an equation Ax = b. The matrix A is obtained by applying a numerical field calculation technique. When the matrix A is in ill-condition, the solution often makes no sense, including extremely large errors. Therefore, we apply the Regularized Least Squares Method (RLS) with a penalty term to perform the inverse calculation stably for the ill-conditioned matrix. The penalty term imposes some constraints on the solutions. In this paper, firstly, we have analyzed the accuracy of the charge distribution estimated by the inverse calculation. Although the perturbation bound of the solution errors has been already proposed for the Least Squares Method, it has not yet been given for the RLS. We have derived the equations that express the perturbation bound of the solution errors in applying the RLS to evaluate the estimation accuracy. Secondly, we have applied the above equations to an experimental result for a cylindrical dielectric solid, and estimated the charge distribution represented by 10140 unknowns. We have utilized an iteration technique and the symmetric configuration of the measured arrangement so as to reduce the amount of operations and memory capacity required for the inverse calculation. © 2005, The Institute of Electrical Engineers of Japan. All rights reserved.

DOI： 10.1541/ieejfms.125.799

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

This paper describes the optimization of a post-type spacer in a gas insulated system. The electric field calculation and the optimization have been performed in three-dimensional conditions. We have utilized the boundary element method to calculate the electric field distributions and the Marquardt method to improve the profile of the spacer so as to minimize the maximal electric field on the spacer surface. The optimization has been performed for spacers with cylindrical column, circular, and elliptical cross-sections, respectively. We have varied additional parameters, such as the permittivity of a spacer and the radius ratio of an inner conductor and a sheath. For elliptical cross-section spacers, the optimization has reduced the maximal value of the total field strength up to 35%, and the maximal value of the tangential field strength up to 56% compared with the respective maximal field without a spacer.

DOI： 10.1109/TDEI.2004.1324345

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Language：Japanese   Publisher：The Institute of Electrical Engineers of Japan  

The fast multipole method (FMM) is an <i>O</i>(<i>N</i>) solver of a full linear system appearing in integral equation methods, for example, surface charge simulation method (SCM) and boundary element method (BEM). An effective preconditioning technique for the FMM, which employs a nested FMM having intentionally deteriorated precision, was proposed and applied to the FMM-SCM and FMM-BEM using the Bi-CGSTAB2. In this paper, this technique called minor iterative preconditioning is applied to the FMM-SCM using the GMRES in order to solve electrostatic field calculation problems. The comparison of CPU times and convergence properties with those by some variants confirms good acceleration performance of the utilized technique.

DOI： 10.1541/ieejfms.124.139

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

The fast multipole method (FMM) is an O(N) solver of a full linear system appearing in integral equation methods. We propose a precondition technique for the FMM using the Bi-CGSTAB2 method, which employs a nested FMM having intentionally deteriorated precision. This enables us to utilize the global information residing in the system matrix.

DOI： 10.1109/TMAG.2003.810335

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SCRIPTA TECHNICA-JOHN WILEY & SONS  

This paper describes triangular surface charge methods (SCM) using fast multipole method (FMM) or tree method (TM) coupled with pseudo-particle method for calculating three-dimensional electric fields. FMM and TM enable us to calculate an N x N matrix and vector product of interacting N bodies by O(N) or O(N log N) times of operations and the same order of memories based on divide-and-conquer algorithm. Application of these techniques to the SCM allows us to utilize a million surface elements on an ordinary personal computer system with I-GB main memory. The calculated results of a benchmark test and two examples, that is, straight and spiral chains of dielectric spheres in a homogeneous field, show the validity and usefulness of the developed SCMs. (C) 2003 Wiley Periodicals, Inc.

DOI： 10.1002/eej.10099

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Language：English   Publishing type：Research paper (scientific journal)  

Charge accumulation on thick solid dielectrics may seriously influence surface discharge and insulation characteristics in SF6, oil, or vacuum, which are the leading insulators for the recent high voltage equipment. Nevertheless, charge measurement technique in such environment has not been fully developed in practical level. One of the authors once proposed a probe method based on multipoint measurement. The procedure is as follows, (i) represent the relationship between charge distribution σ and the multipoint probe-outputs Wp as Aσ = Wp, (ii) calculate the matrix A by a numerical field calculation faithfully simulating the experimental arrangement, and (iii) estimate σ by an inverse calculation of (i) from the measured outputs. In recent papers, we have reported the improved accuracy and stability of (ii) and (iii), respectively, through numerical simulation studies. In this paper, we experimentally measure accumulated charges on a model insulator in order to confirm our numerical improvements. The effects of the following items are investigated, (a) difference of calculation methods of the matrix A, (b) distance between the probe and the insulator surface, (c) length of the guard electrode, and (d) thickness of the insulator. In all cases, we have successfully estimated the charge distribution in practical accuracy. © 2003, The Institute of Electrical Engineers of Japan. All rights reserved.

DOI： 10.1541/ieejfms.123.1051

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

This paper describes three-dimensional electrostatic field calculations in multi-particle systems by a surface charge simulation method (SCM), one of the integral equation methods, sometimes called the indirect boundary element method. The utilized SCM adopts the fast multipole method (FMM), an O(N) algorithm for solving a dense linear system appearing in the SCM, where N is the number of unknowns. The calculated arrangements consist of spherical dielectrics, the radius of which is 1cm, in the form of (a) a string of beads (2 to 12 particles) or (b) a simple three-dimensional lattice (2(3) or 10(3) particles) under a homogeneous applied field. The half-length of the gap between the particles is (i) zero (with contact points) or (ii) 0.01cm to 0.5cm (without contact points). The charge and field distributions on the particles are numerically determined using up to 432000 curved triangular elements and 866000 unknowns in the quadratic basis function.

DOI： 10.1109/ICPADM.2003.1218345

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Language：English   Publishing type：Research paper (scientific journal)  

We propose a curved triangular patch that is suitable for the field calculation by the integral equation methods. The interior control point of this patch is variably represented in the area coordinate as a weighted average of six control points.We can connect the patch to the three surrounding patches with G1 continuity on the sides. The whole surfaces to be treated can be represented as a collection of smoothly connected patches. This patch is applied to the surface charge method with the pseudo-particle fast multipole method. We demonstrate some electrostatic field calculations using several hundreds of thousands of proposed patches. © 2003, The Institute of Electrical Engineers of Japan. All rights reserved.

DOI： 10.1541/ieejfms.123.153

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SCRIPTA TECHNICA-JOHN WILEY & SONS  

This paper describes triangular surface charge methods (SCM) using a curved shape function for calculating electric fields in composite dielectrics. In SCM, the boundary condition on dielectric surface is conventionally represented by the point matching of a normal component of electric flux density (D-n), which we call the D-p method. We have proposed a method based on the continuity equation of electric flux, which is termed the Phi(s) method. The electric flux is calculated by numerically integrating D-n on each corresponding partial area with proper weight functions. When the continuity of the electric potential (V) is also needed, the point matching of V is adopted conventionally (V-p method).
We have applied the same integral procedure as the Phi(s) method to the continuity condition of V by replacing D-n with V for the kernel of the integral equation, and we obtained a new representation of potential condition (V-s method). We have computed the electric field for a spherical dielectric under a uniform field. The calculated results show that the Phi(s) method improves the accuracy of the field by about two orders compared with the D-p method, and that the V-s method gives accuracy comparable to the V-P method. The proposed techniques attain the accuracy of the electric field at the spherical center almost equal to the accuracy of the total surface area of the simulating sphere. (C) 2002 Wiley Periodicals, Inc.

DOI： 10.1002/eej.2017

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

The paper describes the electric field behavior in triple-junction problems for an arrangement with a contact angle between 0 and pi/2. We have fully analyzed the effects of volume and surface conductivity by the results of both applying an analytical solution and numerically calculating electric field distributions. The numerical field calculation is performed by the boundary element method. The analytical solution proposed here agrees well with the numerical calculation results. The presence of volume conductivity usually enhances the field singularity at a contact point, while surface conductivity moderates the field to a uniform distribution near a contact point. (C) 2002 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0304-3886(01)00219-4

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

The paper, describes the electric field behavior near a contact point in various arrangements with a zero contact angle when surface conductivity is present on a solid surface. Electric field distributions are calculated for arrangements with three contact conditions: point, line, and surface contact. We focus on the effect of surface conductivity on the electric field. It was found that the presence of surface, conductivity results in the electric field intensification. Similarly to the effect of volume conductivity, when the surface conductivity is higher than a certain value, a change in the position of the peak electric field takes place. The effect of the surface conductivity is noticeable for as low as 0.8 nS.

DOI： 10.1109/TDEI.2002.1024430

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

The measurement of accumulated surface charge for thick specimens requires multipoint probe outputs to establish the inverse calculation for the determination of an unknown charge distribution. Until now, studies on the various errors associated with the measurement have been conducted only for simplified arrangements mainly in axisymmetric geometry where the charged surface is parallel to the ground. We have numerically analyzed a model measurement set-up more comparable to practical conditions by a highly efficient surface charge method. We have studied the effect of probe position, the induction from charge existing not directly beneath the (probe) sensor and the difference in matrix components computed by two numerical methods. In particular, we have studied the accuracy of the reconstructed charge distributions by numerical simulations of the inverse calculation. It has been shown that the assumed measurement errors make much larger differences in the reconstructed charge distributions, although the influence depends considerably on the assumed charge distribution. Reducing the condition number of the matrix improves the accuracy of the inverse calculation for uniform and linearly changing charge distributions.

DOI： 10.1109/TDEI.2002.1007704

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SCRIPTA TECHNICA-JOHN WILEY & SONS  

We have derived analytical expressions of the potential and electric field generated by a triangular surface charge that has a second- or third-order charge density distribution on each element. These expressions are applicable to the surface charge method (SCM) for calculating electric field in three-dimensional arrangements. In general, the higher-order charge density expression improves the accuracy of the SCM. Moreover, the SCM with analytical expressions needs no troublesome treatment of singular points inevitably involved in the SCM using numerical integration. We have also confirmed the validity of these derived analytical expressions by comparing the calculated results with those by the numerical method. (C) 2002 Wiley Periodicals, Inc.

DOI： 10.1002/eej.1155

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SCRIPTA TECHNICA-JOHN WILEY & SONS  

Many researchers have attempted to clarify the definitions of active power, reactive power, active current, reactive current, etc. for unbalanced and nonsinusoidal three-phase situations. The so-called pq theory has given a new definition of instantaneous reactive power, and it has been discussed and developed by many authors, In this paper, the merits and demerits of the instantaneous reactive power compensator are discussed. It is shown theoretically that applying instantaneous reactive power compensation to unbalanced three-phase systems has a serious disadvantage in that it causes third-order harmonic currents on the source side, which problem cannot be avoided. To overcome this problem the authors propose a new approach, and name it the "quasi-instantaneous" reactive power compensator. It compensates individual-phase reactive currents. The basic principles of the quasi-instantaneous reactive current compensator are discussed in detail, and its validity is confirmed using digital simulation. In particular the authors show that the power factor of each phase becomes unity on the source side, but the source currents remain unbalanced when the proposed method is applied. (C) 2002 Wiley Periodicals, Inc.

DOI： 10.1002/eej.1162

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SCRIPTA TECHNICA-JOHN WILEY & SONS  

We propose a new surface charge method based on the continuity of electric flux passing through each partial area on the dielectric boundary. N partial areas divided on the boundary give the boundary equations for solving N unknown variables representing the surface charge density distribution. The electric flux is numerically calculated by integrating the normal component of electric flux density on each partial area. This method permits us to exclude the singularity of edge parts from the boundary equations because these parts do not contribute to the integration area. In this paper, we apply first-order functions to simulate both triangular surface shape and charge density distribution on its surface as well.
First, we have computed the electric field for a spherical dielectric under a uniform field. The calculated results show that the accuracy of the electric field at the spherical center is almost equal to the accuracy of the total surface area of the polygon which represents the sphere. Furthermore, this method has improved the accuracy of the field by about one order compared with the conventional surface charge methods. Second, we have computed the electric field for a dielectric human model under a uniform field. The calculated results demonstrates that the proposed method works well for a complicated shaped object with a dielectric constant greatly different from that of an ambient medium. (C) 2002 Scripta Technica.

DOI： 10.1002/eej.1133

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

This paper studies the possibility of applying a gas mixture containing c-C4F8 in the gas insulation of power equipment. Environmental aspects such as global warming potential, ozone depletion potential, recycling loss and toxicity are discussed. Insulation characteristics of gas mixtures such as c-C4F8/N-2, c-C4F8/air, and c-C4F8/CO2 are examined experimentally under a quasi-homogeneous or an inhomogeneous electric field condition. Most of the characteristics are compared with those of SF6/N-2, which is now widely studied as the most plausible alternative to pure SF6 for gas insulation. The experimental results, together with the discussion, suggest that gas mixtures containing c-C4F8 are possible substitutes for SF6 and SF6/N2.

DOI： 10.1109/94.971467

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

The electric field behavior, in particular the field intensification at a contact point, is very important in complex dielectric systems with gaseous or vacuum insulation. The paper describes the electric field behavior at and near a contact point in various arrangements with a zero contact angle when volume conductivity is present in the solid dielectric. Contact conditions are separated into line, point, and surface contact. The effect of volume conductivity is investigated analytically, and numerically by using the boundary element method. The electric field behavior near a contact point principally depends on the absolute value of complex relative permittivity, and volume conductivity usually promotes the field intensification. In the arrangements of point contact or line contact, the position of peak electric field shifts from a contact point when the volume conductivity is higher than a certain value, while in the arrangement of surface contact, the position is usually more or less remote from the contact point, whether volume conductivity is present or not.

DOI： 10.1109/94.971448

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SCRIPTA TECHNICA-JOHN WILEY & SONS  

This paper describes a triangular surface charge method (TSCM) called (3,1)-TSCM, which uses curved surface elements for calculating electric fields in composite dielectrics. The boundary element utilizes a cubic shape function with nine degrees of freedom and a linear function for representing the charge density on its surface. Conventional SCMs, including the (3,1)-TSCM, show a very large relative error in the composite dielectrics where the permittivity is much higher in one medium than in the other, A modified method called the beta method can suppress such relative errors, which expresses electric fields by surface charges without subtraction causing large relative errors. We have applied the beta method to the (3,1)-TSCM and calculated electric fields for a spherical dielectric under a uniform field. The calculated results show that the (3,1)-TSCM improves the accuracy of the electric field by more than one order compared with the method using flat surface elements with constant charge density on each element. Furthermore, the beta method completely suppresses the divergence of relative errors even when the ratio of the permittivity of two media reaches 10(10). (C) 2001 Scripts Technica, Electr Eng Jpn, 136(1): 1-8, 2001.

DOI： 10.1002/eej.1044

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：WILEY  

This paper describes the charging phenomena of cylindrical insulators made of PMMA, PTFE, or Al2O3 under ramped and stepped de voltage. It is primarily concerned with the influence of surface roughness on the formation of charging for various insulating spacers. By using an electrostatic probe located on the cathode, we conducted real-time observations of the electric field induced by the surface charge. It was found that the surface roughness positively affects the charging, which becomes more pronounced in insulators polished to a higher degree. Furthermore, polished insulators are characterized by a shorter time lag. These results show the importance of surface treatment for insulating spacers in vacuum. (C) 2001 Scripta Technica.

DOI： 10.1002/eej.1029

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SCRIPTA TECHNICA-JOHN WILEY & SONS  

This paper proposes a technique for calculating electric fields in composite dielectrics by the first-order triangular surface charge method. The proposed technique utilizes two kinds of triangular elements, namely, conforming and nonconforming elements: these are elements whose charge density is continuous and discontinuous between the neighboring elements, respectively. We basically apply nonconforming elements at the edges of a polyhedral shape where the normal component of electric field cannot be fixed. We have computed the electric field for a spherical or a rectangular solid dielectric in the presence of a uniform field. Comparison shows that the proposed first-order method works better than the zeroth-order one, and that, in particular, the combined use of conforming and nonconforming elements produces the highest accuracy. (C) 2001 Scripta Technica, Electr Eng Jpn, 135(1): 8-14, 2001.

DOI： 10.1002/1520-6416(20010415)135:1<8::AID-EEJ2>3.0.CO;2-Y

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SCRIPTA TECHNICA-JOHN WILEY & SONS  

This paper deals with the high-frequency equivalent circuits in an induction motor driven by a PWM inverter. The leakage current flows through stray capacitance among stator windings and iron core (frame) of the induction motor at the switching instants of the inverter transistors. The high-frequency equivalent circuits are derived based on the frequency characteristics of the high-frequency zero-phase impedance and line-to-line impedance of the induction motor. The validity of the derived equivalent circuits is confirmed by simulation and experimental results. (C) 2001 Scripta Technica, Elects Eng Jpn, 135(1): 65-76, 2001.

DOI： 10.1002/1520-6416(20010415)135:1<65::AID-EEJ8>3.0.CO;2-8

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOS PRESS  

A rational quartic triangular Bezier patch enables us to numerically represent an octant of a sphere with neither shape errors nor degeneration of nodes, when it adopts the control points and weights derived by Farin. This patch allows us to avoid undesirable influences caused by the misrepresentation of a spherical shape, if the objects to be treated involve an octant of a sphere in the field analysis by a boundary subdivision method. We have carried out a static field calculation for a spherical dielectric placed in a homogeneous applied field with a surface charge method by introducing a simple method that constructs arbitrary mesh patterns on this patch.

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：WIT PRESS  

Gas insulated systems are widely used nowadays in electric power industry. Practically, solid dielectrics are inevitably required to provide support and separation of a stressed conductor in the systems. The presence of solid dielectrics, called "spacers", results in complex-dielectric field distributions, which cannot be usually analytically calculated.
This paper describes the optimization of a post-type spacer in a gas insulated bus-bar or a transmission line, which contributes a three-dimensional system. We have applied the boundary element method (BEM) for the electric field calculation. As spacers have rounded shapes, calculation models are constructed by second-order isoparametric curved elements. The Marquardt method is an optimization technique to improve the profile (shape) of a spacer from the electric field distribution. Various constraints have been tried, such as maintaining column cross-sections of a spacer circular or elliptical. Spline functions have been applied in some cases in order to maintain smooth surface. The optimization of the spacer has been carried out with respect to two objectives, that are minimizing the tangential electric field strength and the total electric field strength on the spacer surface.

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Language：Japanese  

DOI： 10.1541/ieejfms1990.111.12_1099

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Language：Japanese   Publishing type：Research paper, summary (national, other academic conference)  

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DOI： 10.4018/978-1-4666-2113-8.CH025

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Other Link： http://search.jamas.or.jp/link/ui/2014027769

Language：Japanese   Publishing type：Research paper, summary (national, other academic conference)  

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Language：Japanese   Publisher：The Institute of Electronics, Information and Communication Engineers  

The power increase/decrease of EEG rhythms (event-related synchronization/desynchronization, ERS/ERD) is often used to investigate human brain activities. When subjects execute movements and their imageries, ERS and ERD are observed in the vicinity of the sensorymotor area and the brain activity mostly in the primary motor area is also observed by fMRI. In this study, signal power related to ERS and ERD of μ rhythm (8-13Hz) were estimated by applying prewhitening beamformer with linear constraints to measured EEG and fMRI data during right/left wrist bending movements. It was found that the centers of estimated signal power of ERS and ERD of μ rhythm were located in the ipsilateral and contralateral sensorimotor area, respectively. This demonstrates that prewhitening beamformer with linear constraints is available for signal source estimation of ERS and/or ERD.

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Other Link： http://hdl.handle.net/2433/193892

Language：Japanese   Publisher：The Institute of Electronics, Information and Communication Engineers  

We proposed a signal source estimation method based on simultaneous diagonalization of electroencephalogram (EEG) covariance matrices of two different states such as movement and resting states. We compared performances of the proposed method and conventional minimum variance beamformer them to simulation data in which an increasing signal source of β rhythmic spontaneous EEG activity was located in the primary motor cortex. As a result, it was found that the lower limit of signal-to-noise ratio of the proposed method, in which location bias became lower than 10 mm, was one third of that of minimum variance beamformer. This result indicates that indicates that the proposed method is applicable for signal source estimation that has two different states.

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Other Link： http://hdl.handle.net/2433/193894

Language：Japanese   Publishing type：Research paper, summary (national, other academic conference)  

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J-GLOBAL

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Language：Japanese   Publisher：The Institute of Electronics, Information and Communication Engineers  

We compared source estimation method when equivalent dipole are located in the motor cortex aimed at right/left hand movement or their imagery based brain-computer interface. We have developed a prewhitening EEG beamforming with linear constraint, which is introduced vector beamforming technique and linear constraint that suppress signals in other areas. It was found that the estimated location error by prewhitening beamforming with linear constraint was lower than that by minimum variance beamforming with linear constraint or the beamforming with linear constraint based on generalized least squares even when signal-noise ratio (SNR) is lower than 0.5. This demonstrates that the proposed prewhitening beamforming with linear constraint is useful to estimate the location of signal source even when SNR is low.

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Language：Japanese   Publisher：The Institute of Electronics, Information and Communication Engineers  

For the development of BCIs (Brain-Computer Interfaces) extracting brain information with high accuracy, quantitative evaluation of noises superimposed on signals is necessary. The present study focused on a BCI based on SSSEPs (Steady-State Somatosensory Evoked Potentials) and simulated SSSEPs to investigate ability of the BCI..Sinusoidal source signals with fluctuation and background noises were assumed to be generated from the right and left SI (Primary Somastosensory Cortex). Simulated SSSEPs were obtained from the sinusoidal signals by forward problem in an eccentricity three layered sphere model. Simulated data were made by superimposing noises with different signal-to-noise ratio on the simulated SSSEPs. We proposed a novel discriminant method using spectra of PC (Principal Component) score obtained by PCA (Principal Component Analysis) and compared its performance with that of sLORETA (standardized low-resolution brain electromagnetic tomography) by applying the two methods both to simulated and measured data.

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Language：Japanese   Publisher：The Institute of Electronics, Information and Communication Engineers  

MR images demand a high signal-to-noise ratio (SNR). For estimating a SNR of MR images, calculation of thermal noise, one of the main causes of decline in SNR, is important. Conventionally, Johnson-Nyquist noise from biological objects has been calculated by applying analysis formula to axially symmetric system, or by calculating 3D field numerically when the system is not axially symmetric. In this study, we proposed a new method of calculating thermal noise. We represented biological object with cubic voxels and calculated thermal noise by using Monte-Carlo simulation, assuming each voxel has a current element, current distribution of which is described by Maxwell-Boltzmann distribution. We examined validity of the proposed method by comparing results of conventional and the proposed methods for simple, axial symmetric models. By using the proposed method, we are now able to calculate thermal noise generated from a model of biological object in non-axially symmetric system.

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Other Link： http://search.jamas.or.jp/link/ui/2010320070

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Language：Japanese   Publisher：The Institute of Electronics, Information and Communication Engineers  

Utilization of custom-made real head subjects' models is expected to enhance the reliability of EEG (electroencephalogram) source estimation by calculating EEG lead field matrices. We need to determine the intensity threshold of the MR (magnetic resonance) images to classify the noise and head regions when custom-made real head models are made from MR images. Since it is sometimes difficult to determine the threshold manually, we propose a noise threshold identification procedure using EM (Expectation-Maximization) algorithm. Using the procedure, we could construct head voxel models. In addition, EEG lead field matrices between gray-matter voxels and EEG electrodes were calculated using head voxel models constructed by manual, Otsu's method, and the proposed method. We compared the EEG distributions generated from an equivalent current dipole placed in the primary motor cortex using the three different matrices. The result indicates that there is no big difference among those three EEC distributions.

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Language：Japanese   Publisher：Japan Society for Simmulation Technology  

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Language：Japanese   Publisher：The Institute of Electrical Engineers of Japan  

This paper describes an application of the equivalent multipole moment method (EMMM) with polar translations to calculation of magnetic fields induced by a current dipole placed in a human head model. Although the EMMM is a conventional Laplacian field solver based on spherical harmonic functions, the polar translations enable it to treat eccentric and exclusive spheres in arbitrary arrangements. The head model is composed of seven spheres corresponding to skin, two eyeballs, skull, cerebral spinal fluid, grey matter, and white matter. The validity of the calculated magnetic fields and the magnetic flux linkages with a loop coil locating nearby the model is successfully confirmed by the reciprocity theorem derived by Eaton.

DOI： 10.1541/ieejfms.127.171

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Language：English   Publisher：IEEE  

We have developed an fMRI-MEG integrative neuroimaging method that is capable of analyzing spatiotemporal multiple cortical activities. The method determines the center of gravity in each fMRI activated cluster as the location of the linear constraints. First, activated multiple clusters were determined by statistically analyzing fMRI data. Secondly, using the fMRI activated clusters as spatial constraints, the orientations of equivalent current dipoles (ECDs) placed at the center of gravity in each voxel of all activated clusters were estimated by a procedure maximizing inner product of lead field and measured field vectors. Thirdly, the activities of all ECDs were estimated, and dynamic activities of them were visualized. The linear constraints for the voxels in a cluster are defined to suppress the power from the source at the center of gravity of the other cluster. The spatial distributions and the time courses of activities for all ECDs were successfully reconstructed. We applied the newly developed method to data obtained during an apparent visual motion perception task and could confirm its availability. Although there were individual differences, the present fMRI-MEG integrative neuroimaging method successfully detected dynamic cortical activities in multiple visual areas, such as V1, V2, V5 and IPS.

DOI： 10.1109/ICCME.2007.4381991

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Language：Japanese   Publisher：Japanese Society for Medical and Biological Engineering  

Focusing on the application of linearly-constrained adaptive beamformer techniques to fMRI-MEG integrative analysis, we have examined an approach which determines the center of gravity in each fMRI activated cluster as the location of the linear constraints. First, we simulated MEG data generated from two sources distributed in circular forms placed at visual areas V2 and V5, which are temporally strongly correlated. Two fMRI activated clusters are assumed to cover the two sources. The linear constraints for the voxels in a cluster are defined to suppress the power from the source at the center of gravity of the other cluster. The spatial distributions and the time courses of the two correlated sources at V2 and V5 are successfully reconstructed. We applied the approach to data obtained during an apparent motion perception task and were able to confirm its availability. These results demonstrate that the proposed approach based on linearly-constrained adaptive beamformer techniques is promising as an fMRI-MEG integrative analysis.

DOI： 10.11239/jsmbe.44.722

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DOI： 10.1109/DEIV.2006.357243

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Language：Japanese   Publisher：The Institute of Electrical Engineers of Japan  

This paper presents a generalized equivalent multipole-moment method for calculating three-dimensional Laplacian fields in multi-spherical system. The Greengard &amp; Rokhlin&#039;s M2M, M2L, and L2L formulae enable the multipole-moment method to calculate the fields in general arrangement of multi-spheres, which involve exclusive and multi-layered spherical arrangement. We applied this method to electric field calculation in biological structures induced by ELF magnetic fields. The induced electric fields in a three eccentric and exclusive spheres system, which models human head with two eyeballs, are calculated under the application of homogeneous and magnetic-dipole fields. The validity of this method is successfully confirmed by comparing the calculated fields with those by the fast-multipole surface-charge-simulation method.

DOI： 10.1541/ieejfms.125.533

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DOI： 10.11509/sci.SCI05.0.131.0

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Language：Japanese   Publisher：(公社)日本生体医工学会  

種々の脳機能計測手法の中で,時間分解能に優れる脳磁図(MEG)と,空間分解能に優れる機能的磁気共鳴画像法(fMRI)に注目し,両手法それぞれの長所を活かした時空間的に高精度な統合解析手法の開発を試みた.センサ面における磁気信号分布の内積を評価関数とすることにより単一ダイポールの探索を行い,かつ探索領域としてfMRIの解析結果を利用した.続いて,V1/2およびV5という近接する視覚野領域を賦活する刺激を用いた際の皮質活動に適用し,その有用性について検討した.複数の近接する部位が同時に活動している際においても,各々の独立した部位の活動並びにその時間変動の詳細な解析が可能であることを実験的に検証できた

DOI： 10.11239/jsmbe.43.777

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Other Link： https://search.jamas.or.jp/index.php?module=Default&action=Link&pub_year=2005&ichushi_jid=J03969&link_issn=&doc_id=20060323290055&doc_link_id=10.11239%2Fjsmbe.43.777&url=https%3A%2F%2Fdoi.org%2F10.11239%2Fjsmbe.43.777&type=J-STAGE&icon=https%3A%2F%2Fjk04.jamas.or.jp%2Ficon%2F00007_3.gif

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Language：Japanese   Publisher：The Institute of Electronics, Information and Communication Engineers  

The paper describes the calculation results of induced current in human models due to magnetic field generated from a household appliance. The calculation method is the boundary element method. Human models with five internal organs were constructed from the second order curved elements. The household appliance is a han dryer located in two different positions, which is simulated by a magnetic dipole. In the homogeneous model, maximum current density appears at the body surface near the magnetic dipole. In the models with internal organs, induced current density in the brain substantially varies with the conductivity. However, the effect of organ conductivity is very close to that in the case of a uniform magnetic field.

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Language：Japanese   Publisher：The Institute of Electronics, Information and Communication Engineers  

The paper describes the calculation results of induced current in human models due to a 50-Hz uniform magnetic field. The human models with five internal organs were constructed from the second order curved elements. The calculation method is the boundary element method. In the homogeneous model, the maximum current density appears near the body surface at the front and back for a horizonatal magnetic field and appears at the left and right side of the body for a vertical one. Little effect was found on the induced current of each internal organ for two different posture. We have shown that ratio of the induced current density in an organ to the conductivuty of its surrounding varies proportionally to the ratio of the conductivities in many cases.

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Language：English   Publisher：IEEE  

This paper describes the charging process of a cylindrical insulator made of PMMA or Al2O3 By using an electrostatic probe located inside the insulator on the cathode surface, we have conducted realtime observation of the electric field change due to the surface charging. An axial-symmetrical simulation based on a secondary electron emission avalanche has been performed. The measured results agreed with the simulation concerning the polarity of the accumulated charge as well as the field strength. The simulation also predicts the charging inception at a voltage measured well below the flashover.

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オムニバス講義

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Language：Japanese   Presentation type：Oral presentation (invited, special)  

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Language：Japanese   Presentation type：Oral presentation (invited, special)  

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