Ichiro Sasada

Orthogonal fluxgate mechanism operated with dc biased excitation

A mode of operation is presented for an orthogonal fluxgate built with a thin magnetic wire. By adding a proper dc bias to the wire excitation, the new mode is easily established. In this case, the fundamental component of the induced voltage at the sensing coil (secondary voltage) is made sensitive to the axial magnetic field, compared to the second harmonic in a conventional orthogonal fluxgate. The operating principle is explained using a magnetization rotation model. A method is proposed to cancel the offset that is inevitable when the magnetic anisotropy is present in a magnetic wire at an angle to its circumference. Experimental results are shown for a sensor head consisting of a 2-cm-long Co-based amorphous wire 120 μm in diameter with a 220-turn sensing coil. The sensitivity obtained is higher than that obtained using a conventional type of the orthogonal fluxgate built with the same sensor head. It is also demonstrated that the proposed method for canceling the offset works well.

Symmetric response obtained with an orthogonal fluxgate operating in fundamental mode

Summary form only given. Magnetic field sensors are important for building automated functions in production lines, intelligent transportation systems, security systems and so on. The orthogonal fluxgate mechanism provides miniaturized low-cost magnetic sensors which can be incorporated in a compact electronic systems. The author proposed a new operating mode, fundamental mode, for the orthogonal fluxgate in which greater linear response and higher sensitivity can be obtained with an as-cast amorphous wire core compared to the conventional orthogonal fluxgate mechanism where the second harmonic is a measure of applied magnetic field. When the magnetic anisotropy is present, however, in an amorphous wire at off circumferential direction, the offset may arise and the input-output response curve becomes asymmetric with respect to the origin. In this paper, a method is presented to solve this problem.

Design of a large-scale vertical open-structure cylindrical shield employing magnetic shaking

The shield developed consists of four concentric magnetic shells positioned on the outer surfaces of paper pipes of ∼2.7 m length, ∼1 cm thickness, and with outer diameters of 67, 72, 82.2, and 97.4 cm, respectively. The first (innermost) shell is a Permalloy shell of 2.1 mm thickness and 1.8 m length. The second, third, and fourth shells are made of ∼50 mm wide, ∼22 μm thick Metglas 2705M amorphous ribbons. The second shell, which is a 2.2 m long helical structure, consists of 48 layers of Metglas ribbon divided into four equal sections by ∼1 cm thick flexible Styrofoam sheets. The third shell, 2.43 m in length, and fourth shell, 2.7 m in length, consist of 26 and 30 layers, respectively. A thin polyethylene film is tightly wound on each section of the second shell as well as on the third and fourth shells. It increases the friction between the Metglas ribbons and prevents them from sliding down; there is no foreign material in between the layers of the ribbon. All shells are enclosed by toroidal coils w...

A new estimation of the axial shielding factors for multishell cylindrical shields

A new, more accurate formula describing double and multiple-shell axial magnetic shielding is obtained as a result of numerical verification of existing estimations. A standard ANSYS® software package was used. Parameters of the numerical model are as follows. Two concentric, closed cylinders of equal thickness and constant permeability are considered. The thickness-to-diameter ratio of the outer cylinder is t/D2=1/100, its length-to-diameter ratio varies as L2/D2=3, 4, and 5, the ratio of the cylinders’ outer diameters varies as D1/D2=0.5, 0.6, 0.7, 0.8, and 0.9, a range of the ratio of the cylinders’ lengths is L1/L2=0.1–0.9, and a range of the relative permeability is μu2003=103–106. A significant disagreement between the existing estimations and between each of them and the numerical model is found. One of the examined algorithms is modified to improve its precision. A remarkable improvement in the accuracy of the new algorithm compared to both existing methods is achieved. On a basis of the new algorithm, a new formula describing multishell axial magnetic shielding is suggested.A new, more accurate formula describing double and multiple-shell axial magnetic shielding is obtained as a result of numerical verification of existing estimations. A standard ANSYS® software package was used. Parameters of the numerical model are as follows. Two concentric, closed cylinders of equal thickness and constant permeability are considered. The thickness-to-diameter ratio of the outer cylinder is t/D2=1/100, its length-to-diameter ratio varies as L2/D2=3, 4, and 5, the ratio of the cylinders’ outer diameters varies as D1/D2=0.5, 0.6, 0.7, 0.8, and 0.9, a range of the ratio of the cylinders’ lengths is L1/L2=0.1–0.9, and a range of the relative permeability is μu2003=103–106. A significant disagreement between the existing estimations and between each of them and the numerical model is found. One of the examined algorithms is modified to improve its precision. A remarkable improvement in the accuracy of the new algorithm compared to both existing methods is achieved. On a basis of the new algorithm...

Spontaneous magnetoencephalography alpha rhythm measurement in a cylindrical magnetic shield employing magnetic shaking

This article shows a demonstration of magnetoencephalography measurement in a cylindrical magnetic shield made of cobalt-based amorphous tape with magnetic shaking. The noise levels of the first-order superconducting quantum interference device gradiometers that operated in the shield were reduced to as low as 40u2009fT/√Hz at 10 Hz by surrounding it with a simple rf shield made of conductive cloth. We observed spontaneous alpha rhythms from a human brain in this shielding system. Alpha rhythms and their suppression caused by opening the eyes were clearly found, which was also confirmed by electroencephalography measurement from the same volunteer under similar conditions.

Effect of magnetic anisotropy on magnetic shaking

The effect of magnetic shaking on both the transverse and axial shielding factors (TSF and ASF) is investigated using open-ended cylindrical shields made of Metglas 2705M amorphous ribbons. Shaking enhancement is found to be strongly dependent on the orientation of the magnetic anisotropy of the shielding material, that is, the anisotropy axis should be aligned along the corresponding shielding direction to achieve a greater enhancement of the TSF or ASF. Magnetic shaking provides an ∼40-fold increase in the TSF and only an ∼twofold increase in the ASF for a shield consisting of a helical structure of the ribbons. The situation is almost completely different if the ribbons’ structure is axial: ∼twofold increase in the TSF and ∼20-fold increase in the ASF. The shaking field intensity (∼320 mOe at 1 kHz) for axial shielding is found to be about 10 times larger than that for transverse shielding. Experiments with a three-shell axial structure shield show an ∼350-fold increase in the ASF (∼40u200a000, which is one order larger than that of similar conventional shields). The TSF of this shield is, however, about one tenth of its ASF. Reorientation of the ribbons in the innermost shell, from an axial structure to a helical one, increases the total TSF (∼50u200a000) while still maintaining a large ASF (∼20u200a000). Hence, combining shells of helical and axial structures and having a proper distribution of the shielding material between them may allow the construction of an open shield with a large (>20u200a000) total ASF and TSF.

Experimental correction of the axial shielding equation

Summary form only given. Analytical descriptions of axial shielding for cylindrical shields (neglecting the effect of the openings or caps) is based on the assumption that the field reduction inside the shield is due to the demagnetizing field within the equivalent ellipsoid: S/sub ax/ = 1 + N/sub ell/ /spl mu//sub av/ = 1 + 4 N/sub ell/ /spl mu/t/D ( equation 1), where S/sub ax/ is the axial shielding factor, N/sub ell/ is the demagnetizing factor of the equivalent ellipsoid, /spl mu//sub av/ is the average permeability of the shield (the permeability that is averaged over the shields cross-sectional area, /spl mu//sub av/ = 4/spl mu/t/D), /spl mu/ is the permeability and t/D is the thickness-to-diameter ratio of the shield. Equation (1) shows some discrepancy; it seems more reasonable to replace the N/sub ell/ in (1) by the demagnetizing factor of the equivalent rod, N/sub rod/, because the shields analyzed have the same outer surface as the rod does. In order to support the above idea experimentally, we built and investigated ten cylindrical shields with L/D ratios from 1 to 8.3. The results obtained show that the employment of the N/sub rod/ in (1) provides much better agreement between the experimental and analytical results.

High performance bench-top cylindrical magnetic shield with magnetic shaking enhancement

A high-performance bench-top cylindrical magnetic shield made of amorphous ribbons is presented. It has double active shells made of Metglas 2705M amorphous ribbon to which magnetic shaking enhancement is applied. Passive shells made of Metglas 2714A amorphous ribbons were installed to reduce the leakage field of magnetic shaking. With total weight 1.3 kg of the amorphous ribbons, sufficiently large shielding performance was measured against 10-/spl mu/T, 10-Hz uniform magnetic field. The value of the axial shielding factor (ASF) was 700 and transverse shielding factor (TSF) was more than 20,000. The residual dc fields were less than 20 nT in the axial direction and 5 nT in the radial direction under the worst condition, where the shield axis is oriented along the horizontal component (/spl sim/ 32 /spl mu/T) of the Earths magnetic field.

Self-compensation of the residual field gradient in double-shell open-ended cylindrical axial magnetic shields

An important and not-obvious demonstration made in this work is that replacing a closed double-shell axial cylindrical shield with a similar but open-ended shield can lead to an increasing in not only the axial shielding factor, but in the residual uniformity as well. It is shown that a self-compensation of the residual field gradient and a significant improvement of the total residual field uniformity are potentially possible by combining two open-ended cylindrical shells having different length-to-diameter ratios and providing opposite signs and appropriate magnitudes of the residual field gradients. A numerical example confirms the applicability of the method. It is shown that an /spl sim/6-fold reduction of the residual field gradient along the axial direction and an /spl sim/2-fold reduction of that along the radial direction over a relatively wide area inside a double-shell cylindrical axial shield are possible if its ends are open.

Noise characteristics of Co-based amorphous tapes to be applied as magnetic shielding shell for magnetic shaking

In order to seek a magnetic shell material of low magnetic noise under magnetic shaking, fluctuations of the magnetization under the 1 kHz excitation are characterized and compared to the Barkhausen noise (BHN) under the sine-current excitation of 2 Hz. Two Co-based amorphous magnetic tapes, Metglas 2705M and 2714A, are examined. Measurements were carried out in a cylindrical magnetic shield with two identical toroidal samples to cancel the fundamental component and harmonics of the excitation voltage. Induced voltages in the frequency range less than 500 Hz were considered as fluctuation noise. A good correlation between fluctuation noise and BHN was confirmed for both samples. Metglas 2705M has high shaking efficiency but higher noise than Metglas 2714A.