JP2002257914A - Active magnetic shield device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To realize a high-performance device using an open loop control in an active magnetic shield technique jointly used for improving the performance of a magnetic shield room. SOLUTION: This active magnetic shield device has a shield room, a magnetic shielding coil means disposed near the shield room, and a flux gate magnetic flux meter means disposed remote from the shield room. According to the measurement output of the flux gate magnetic flux meter means, the generated magnetic field of the magnetic shielding coil means is controlled in open-loop.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement in shield characteristics of a magnetically shielded room always used in a magnetoencephalography measurement system for measuring a weak brain magnetic field. Related to the shield device.

The present invention is applicable not only to magnetoencephalograms but also to magnetically shielded rooms for measuring biomagnetic fields such as magnetocardiograms. Further, the present invention can be applied to a simple magnetic shield room for an electron beam exposure apparatus or an MRI apparatus (nuclear magnetic resonance imaging diagnostic apparatus).

[0003]

2. Description of the Related Art FIG. 4 is a conceptual diagram of a magnetoencephalography measuring system, in which 1 is a magnetic shield room with a part cut away,
2 is a table installed in the magnetically shielded room 1;
Is a dewar, and 4 is a sensor unit using SQUID. Reference numeral 5 denotes an electronics unit installed outside the magnetic shield room 1.

Conventionally, in a biomagnetic measurement system using SQUID, in order to measure an extremely weak signal, environmental magnetic noise is reduced in the magnetically shielded room 1 and a magnetic pickup is provided from a distance using a differential pickup coil called a gradiometer. Magnetic fluctuation noise is reduced.

The shape of the pickup coil includes a primary differential gradiometer and a secondary differential gradiometer according to the component of the magnetic field to be reduced, and there is also a so-called magnetometer having no effect of reducing environmental magnetic field fluctuation. Generally, first-order differential gradiometers are most often used because of their trade-offs between effect and size.

In this pickup coil, the zero-order component of the distant magnetic field fluctuation is canceled by the differential coil, but the magnetic field distribution generated from a magnetic field source close to the sensing coil (ie, neural activity of a living body) has a primary or higher order component. Because it has, it can be measured.

FIG. 5 is a plan view of the magnetically shielded room 1 and includes two layers of permalloy walls 1a and 1a having spaces.
b and a structure in which a high conductivity layer 1c such as aluminum or copper is inserted between the permalloy layers 1a and 1b for the purpose of increasing the magnetic shielding effect by using an eddy current and for high-frequency magnetic shielding.

On the other hand, an active magnetic shield method as shown in FIG. 6 has been proposed to reduce the cost of the magnetic shield as well as the shield by the magnetic shield room (Japanese Patent Laid-Open No. 4-357481). This is composed of a SQUID magnetometer 6 and a pair of noise canceling Helmholtz coils 7, 7 'as minimum units.
The feedback current I output from the QUD magnetometer 6 is supplied to the Helmholtz coils 7, 7 ', and the magnetic flux detection coil 6a of the SQUID magnetometer 6 is provided in the center space 8 of these coils.
Are arranged to make the space 8 a magnetic noise elimination space.

[0009]

First, problems in the structure of the permalloy-based magnetic shield room shown in FIG. 5 will be described. First, low-frequency magnetic fluctuation is reduced by surrounding a region to be magnetically shielded with a soft magnetic material having a high magnetic permeability such as Permalloy.

[0010] Permalloy is mainly composed of nickel and iron,
It is expensive because it requires heat treatment and the amount of demand is small. Further, since the weight is extremely large, a thin material is combined to form a multilayer (1 in the example of FIG. 5).
a, 1b) to increase the cost-effectiveness ratio.

However, since permalloy alone cannot provide a sufficient effect, shielding by eddy current in the high conductivity layer 1c is also used. The shielding performance due to the eddy current effect in this high conductivity layer is proportional to the thickness and frequency of the layer. However, since this cannot be so thick due to restrictions due to weight and cost, at low frequencies of several Hz or less, Has almost no effect as shown in FIG.

As shown in FIG. 7, a magnetic field Hi in a rectangular box having a thickness t, a width w, and a height h surrounded by a metallic wall having a conductivity σ has a uniform external AC magnetic field Ho. In the case of (ω), the shielding effect due to the eddy current effect is expressed by the following equation. Hi = Ho (ω) / (1 + jωτ) τ = μoσwt / [2 (w + h)] ω: angular frequency, μo: vacuum permeability According to the above equation, in the case of aluminum, σ = 0.36 × 10 ⁸
When Ω− ¹ m ⁻¹ , w = h = 2000 mm, and t = 3 mm, the cutoff frequency is about 2 Hz, and it can be seen that there is a shielding effect above this frequency.

If the thickness is increased, the cutoff frequency is decreased, but the thickness cannot be so large because the weight is increased and the cost is increased. In addition, the environmental magnetic fluctuation varies from -2 to
Because of the −3 power amplitude characteristic, the magnetic shielding performance is not as good as low frequency, and the measurement band of the biomagnetism measurement is about 0.1 Hz to 100 Hz, so that the shielding is not completely inside the magnetic shield room. No magnetic noise penetrates and its effects are inevitable.

FIG. 8 is a characteristic diagram showing a reduction effect obtained by comparing the magnetic noise characteristic A outside the magnetic shield room corresponding to the frequency with the magnetic noise characteristic B inside the magnetic shield room. As is apparent from this characteristic, the shielding effect is not exhibited in the frequency band of 1 Hz or less.

On the other hand, there is the following problem. In other words, permalloy, which is the main magnetic shield material, has a finite magnetic permeability, so that even if the environmental magnetic field fluctuation is spatially uniform outside the shield room (that is, there is no gradient and the order is zero), the magnetic permeability is limited. In the shield room, a spatial harmonic is generated, and not only the 0th-order component whose amplitude is attenuated by the shield factor, but also the 1st-order and 2nd-order magnetic noise components are additionally generated.

This means that, in other words, the distribution in the magnetically shielded room is not uniform (ie, the first and second spatial harmonics are generated). The amplitudes of these harmonic components are smaller than the zeroth order, and decrease rapidly as the order increases. However, the first-order differential gradiometer can remove the zeroth-order component but removes the first-order and higher harmonic components. It is not possible.

In the case of a gradiometer, the coil balance varies depending on the dimensional accuracy and the presence or absence of a nearby superconductor. When expressed as a canceling rate of a differential coil, it is about 0.1% to several%, and is not necessarily a zero order. Neither can the components be completely removed.

Next, problems and problems of the active magnetic shield method described with reference to FIG. 6 will be described. First, the control sensor must operate under extremely large environmental noise. However, SQUID magnetometers generally have extremely high sensitivity to electromagnetic fields from DC to high frequency. Alternatively, an electromagnetic shield must be provided for the SQUID magnetometer itself, which deteriorates controllability and narrows the dynamic range.

In this case, since the cancel magnetic field includes thermal noise generated from the shield, there is a drawback that the baseline of the noise magnetic field is rather raised.
Further, this technique teaches only cancellation of an environmental magnetic field by a Helmholtz coil, and does not assume a system including a magnetically shielded room.

When applying this method in combination with a magnetically shielded room, the effect of phase rotation on the wall surface of the magnetically shielded room must be considered. In other words, in order to ensure stability, the dynamic range of the control system must be reduced to generate a feedback magnetic field in accordance with the control means (electronic circuit) for feedback, realizing shielding to a weak signal level. It is difficult to do.

On the other hand, the problem of spatial harmonics generated by the Helmholtz coil itself has not been solved. Even if the zero-order component is removed, higher harmonics will be generated as a by-product.Therefore, except for the vicinity of the canceling sensor, if it is a little away, the environmental noise will be extremely large, and a higher-order harmonic magnetic field will be generated. Cause it to occur. This causes the same problem even when correction is performed by a higher-order sensor.

Further, when performing three-axis control, crosstalk between the coils makes controllability unstable. When this is used together with a magnetically shielded room, the effect of phase rotation is added, so that the control performance further deteriorates. .

These problems can be summarized as follows: (1) It is difficult to achieve a sufficient level of an environmental magnetic field for biomagnetism measurement using a Helmholtz coil alone. Therefore, an active magnetic shield must be provided in combination with the magnetic shield room. (2) In this case, in the method of FIG. 6 in which the feedback system is configured by receiving the magnetic fluctuation signal from the sensor in the shield room, the phase rotation occurs on the wall surface of the shield room, so that the cancel magnetic field cannot be generated in a wide band. (3) Crosstalk is likely to occur with a 3-axis sensor,
Difficult to control. (4) The Helmholtz coil generates a new distortion magnetic field (spatial harmonic component).

An object of the present invention is to provide a magnetic shield device having a novel configuration which solves the above-mentioned problems of the conventional method in an active magnetic shield method used in combination to improve the performance of the magnetic shield room.

[0025]

In order to achieve the above object, the present invention is characterized in that a shield room and a magnetic shield coil means disposed near the shield room are provided. Having fluxgate magnetometer means disposed far from the shield room,
The magnetic field generated by the magnetic shield coil means is adjusted in an open loop based on the measurement output of the fluxgate magnetometer means.

A second feature of the present invention is that the shield coil means is provided in a plurality of stages, and the measurement output of the same fluxgate magnetometer is distributed to each of the fluxgate magnetometer means.

A third feature of the present invention is that the magnetic shield coil means shields three axes of the shield room.

A feature of the present invention resides in that a low-pass filter adapted to the frequency characteristics of the eddy current shield in the magnetic shield room is provided between the measurement output of the fluxgate magnetometer means and the coil means for magnetic shield. The point is that it is inserted into the signal path to suppress overcompensation.

A fifth feature of the present invention resides in that the fluxgate magnetometer is housed in an electromagnetic shield housing adapted to the frequency characteristics of an eddy current shield in the magnetic shield room.

A feature of the present invention is that a feedback coil means for offset compensation is provided inside the electromagnetic shield housing of the fluxgate magnetometer.

A feature of the present invention resides in that offset adjusting means by geomagnetism superimposed on the measurement output of the fluxgate magnetometer means is provided outside the fluxgate magnetometer means.

A feature of the present invention resides in that a reference signal of a commercial frequency is superimposed on a measurement output of the fluxgate magnetometer means via a phase adjusting means and supplied to the magnetic shield coil means. .

[0033]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows an embodiment of a magnetic shield device according to the active shield technique to which the present invention is applied. In this embodiment, for ease of understanding, only one axis is described. However, when three axes are installed at the same time, a similar configuration may be added for each axis.

Since the fluctuation component of the external environment magnetic field usually has its origin at a distant place, it may be considered to be uniform near the magnetically shielded room 1. Therefore, to avoid being affected by the magnetically shielded room 1 itself, the location P
If the fluxgate magnetometer 9 is installed in the device, it can be used as a signal source for the active shield.

Reference numerals 10 and 10 'denote a pair of Helmholtz coils which form a magnetic shield coil means disposed with the shield room 1 interposed therebetween. The sensitivity direction Q of the fluxgate magnetometer 9 and the direction of the magnetic field generation of the Helmholtz coils 10 and 10 'are matched to generate an active magnetic field for cancellation. By making the Helmholtz coil sufficiently larger than the magnetic shield room 1, magnetic coupling with the magnetic shield room can be suppressed.

Since the measurement output signal m of the fluxgate magnetometer 9 has a DC offset due to the terrestrial magnetism DC component, it is necessary to remove the offset in order to control the fluctuation. The offset adjuster 11 provided outside the fluxgate magnetometer 9 has this function, and can be realized by a high-pass filter using a DC servo or a simple DC adder / subtractor.

This offset removing function is performed by the magnetic flux meter 9 itself as shown in FIG.
It can also be realized by applying C feedback, and any of these known methods can be selected. In the case of a DC servo or a filter, it is sufficient that the cutoff point is located sufficiently lower than the control frequency band.

The magnetic shield room 1 has a high conductivity layer 1c.
Since the eddy current shield is used in combination, the shield ratio is improved from a certain frequency, and high frequency cutoff characteristics are exhibited. Therefore,
If the measurement output signal m from the fluxgate magnetometer 9 shows a characteristic in which the amplitude and phase are flat at the frequency, overcompensation occurs on the Helmholtz coils 10 and 10 '.

The twelve low-pass filters compensate for this. By providing the cut-off frequency with a first-order characteristic that matches the eddy current shielding characteristic of the magnetically shielded room 1, the low-pass and high-pass filters can be used. Flat shield characteristics can be obtained.

The high conductivity layer 1c and the permalloy layers 1a, 1
Since the cut-off point is different in b, two poles including the permalloy layer may be inserted at a different frequency. The output of the low-pass filter 12 is adjusted by a gain adjuster 13 to a signal n having an amplitude necessary for cancellation, and guided to adders 14 and 14 ′.

On the other hand, as a typical example of the near magnetic field, there is a noise magnetic field mixed by a commercial power supply. This usually indicates a different value on each surface of the magnetically shielded room 1. To compensate for this, it is necessary to generate a cancel magnetic field independently on each surface.

Reference numeral 15 denotes a reference commercial AC power supply, which extracts only necessary signals from the waveform through a band-pass filter 16 having a commercial frequency as a center, and outputs gain signals from the gain adjusters 17 and 1.
At 7 ', the amplitude is adjusted to the amplitude required for cancellation. 18, 18
The phase adjuster ′ is for adjusting the cancel magnetic field to the phase of the magnetic field around the Helmholtz coils 10, 10 ′ rotated under the influence of the eddy current shield.

The outputs r and r 'of these phase adjusters 18 and 18' are led to adders 14 and 14 ', respectively, and mixed with the output n of the gain adjuster 13, so that the Helmholtz coil 1
0, 10 'are supplied to the coil drivers 19, 19'. (0000) FIG. 2 shows a configuration for enclosing the fluxgate magnetometer 9 in an electromagnetic shield housing 20 so as to have the same characteristics as the eddy current shield in the magnetic shield room. Reference numeral 21 denotes a feedback coil wound outside the main body of the fluxgate magnetometer in the electromagnetic shield housing 20, whereby the offset of the fluxgate magnetometer can be magnetically corrected by itself. (000
0) When the offset is electrically canceled by the offset adjuster 10 provided externally as in the configuration of FIG. 1, this feedback coil configuration is not particularly necessary. According to the (0000) fluxgate magnetometer 9, 3
When the magnetic flux measurement on the axis is measured simultaneously by one housing, the coil is wound in each axis direction. Further, when the fluxgate magnetometer having the configuration shown in FIG. 2 is used, the correction by the low-pass filter 12 shown in FIG. 1 can be omitted.

FIG. 3 shows another embodiment of the present invention, and shows only one axis of a configuration example of a multiple active shield apparatus utilizing mutual incoherence in the case of open loop control. Here 1
In the case where the present invention is applied to a two-layer permalloy seed room 1 composed of a and 1b, the high conductivity layer 1c is omitted.

The outermost Helmholtz coils 22, 22 '
Is similar to the Helmholtz coils 10, 10 'in FIG.
It is arranged near the outside of the shield room 1. The Helmholtz coils 22, 22 'are magnetic shield coils of an intermediate layer, and are disposed between the permalloy layers 1a, 1b. Numerals 23 and 23 'denote Helmholtz coils of the innermost layer, which are arranged close to the permalloy layer 1b in the shield room 1.

A block 25 is a simplified diagram of a coil driver of the Helmholtz coil having a plurality of stages, and similarly to the configuration of FIG. 1, a measurement output signal of the fluxgate magnetometer 9 and a commercial AC signal for reference. A signal from the power supply 15 is input, appropriate signal processing is performed, and an optimal drive signal is supplied to the Helmholtz coil.

In the signal processing of the coil driver 25, it is assumed that the polarity of the Helmholtz coil of the intermediate layer and the innermost layer can be individually adjusted for each coil constituting the Helmholtz coil in terms of phase and gain. Further, the signal is attenuated toward the inner layer to generate an active shield magnetic field. It is assumed that the phase is delayed for the inner layer. Note that a configuration in which the Helmholtz coil of either the intermediate layer or the innermost layer is omitted may be employed.

In the embodiment shown in FIG. 3, since the shield is multiplexed by each Helmholtz coil, the leaked magnetic field can be finely adjusted and canceled. Generally, a magnetic gradient is generated inside the shield room due to a disturbance magnetic field. However, by independently supplying a drive signal to the Helmholtz coil inside the shield room, the gradient can be easily corrected.

The original disturbance magnetic field is a zero-order component measured by the fluxgate magnetometer 9, and the internal magnetic field is spatially modulated in the magnetic shield room. Therefore, the signal from the fluxgate magnetometer is adjusted. By doing so, it is possible to generate a gradient magnetic field for canceling the internal magnetic field.

[0050]

In general, a magnetic shield room is composed of a layer of high magnetic permeability such as permalloy and a layer of high electrical conductivity, and the shield principle is different. Qualitatively, the former provides a shielding effect by providing a magnetic path around the space where you want to shield and the magnetic flux lines detour, whereas the latter cancels the magnetic flux lines that are going to penetrate due to the repulsive magnetic field due to the eddy current. Shield.

Therefore, the eddy current shield is based on a principle similar to an active shield in which a current flows through a coil to cancel the current. However, the eddy current has frequency characteristics, and the higher the frequency is, the higher the shielding effect is, but conversely, there is almost no effect at low frequencies where the environmental magnetic noise is large. If the coil current is set so as to perform active shielding at a low frequency, a shielding effect due to an eddy current appears at high frequencies, resulting in overcompensation, and conversely, a magnetic field due to the coil current enters the shield room.

In the present invention, since a low-pass filter matching the frequency characteristics of the eddy current shield is inserted, a compensation current according to the frequency flows in both the low band and the high band.
There is no risk of overcompensation.

On the other hand, a commercial power supply is an extremely large proximity noise, and suppressing this is extremely effective in securing a signal dynamic range of a device used in a magnetically shielded room. Therefore, in the present invention, the commercial power source is used as a signal source and a phase adjuster is inserted so that a signal synchronized in phase with the commercial power source is superimposed as a cancel magnetic field.

The band-pass filter prevents unnecessary signals from being superimposed. Usually, since the fundamental frequency and the amplitude of the third harmonic are large, both circuits can be handled by providing a parallel circuit for each frequency and synthesizing signals by an adder.
Although not shown in the drawing, the same effect can be obtained by storing a noise pattern separately in a storage device, adjusting the phase using a commercial power supply as a trigger source, and superimposing the noise pattern on an adder.

In the present invention, since the fluxgate magnetometer, which is a sensor, is sufficiently separated from the compensation coil and the magnetic shield room because of the open-loop configuration, there is no possibility that the coil current will interfere with the sensor. Therefore, even when the three axes are controlled, the control does not become unstable due to mutual interference. Also,
When the fordback loop is configured, the frequency characteristics of the magnetically shielded room limit the control performance. However, this is not the case with this method, and a wide band shield can be achieved by the frequency characteristics of the sensor.

In the present invention, since the signal in the magnetically shielded room is not used as a reference, there is no effect when noise processing is performed based on a signal from a reference signal detecting sensor such as a magnetoencephalograph. Similarly, even when using a method in which the active shield is configured by a feedback loop with a reference signal detection sensor such as a magnetoencephalograph, since the signal source is separate, linear addition can be performed and this method can be used together And higher shielding performance can be realized.

According to the present invention, the shielding by permalloy can be reduced because the magnetic shielding characteristics are improved. Where the environmental magnetic fluctuation is relatively small, the permalloy layer can be made only the innermost layer, which is advantageous in terms of space saving, weight reduction, and cost reduction.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing an embodiment of an active magnetic shield device to which the present invention is applied.

FIG. 2 is a configuration diagram of a fluxgate magnetometer used in the apparatus of the present invention.

FIG. 3 is a configuration diagram showing another embodiment of the active magnetic shield device to which the present invention is applied.

FIG. 4 is a conceptual diagram of a magnetoencephalography measurement system.

FIG. 5 is a plan view of the magnetic shield room 1.

FIG. 6 is a configuration diagram illustrating an example of a conventional active magnetic shield device.

FIG. 7 is a perspective view showing dimensions of each part of the magnetic shield room 1.

FIG. 8 shows a magnetic noise characteristic A outside the magnetically shielded room and a magnetic noise characteristic B inside the magnetically shielded room corresponding to the frequency.
FIG. 9 is a characteristic diagram showing a reduction effect in comparison with FIG.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Shield room 9 Flux gate magnetometer 10, 10 'Helmholtz coil 11 Offset adjuster 12 Low-pass filter 13 Gain adjuster 14, 14' Adder 15 Commercial AC power supply 16 Band-pass filter 17, 17 'Gain adjuster 18, 18 'phase adjuster 19, 19' coil driver

Claims (8)

[The claims] A shielded room; a magnetic shield coil disposed in the vicinity of the shielded room; and a fluxgate magnetometer disposed far from the shielded room. An active magnetic shield device wherein the magnetic field generated by the magnetic shield coil means is adjusted in an open loop based on a measurement output. 2. The apparatus according to claim 1, wherein said shielding coil means is provided in a plurality of stages.
An active magnetic shield device wherein the measurement output of the same fluxgate magnetometer is distributed to each of the fluxgate magnetometer means. 3. The active magnetic shield device according to claim 1, wherein said magnetic shield coil means shields three axes of said shield room. 4. A low-pass filter adapted to the frequency characteristics of the eddy current shield in the magnetic shield room is inserted into a signal path between the measurement output of the flux gate magnetometer means and the coil means for magnetic shield, and 4. The active magnetic shield device according to claim 1, wherein compensation is suppressed. 5. The active device according to claim 1, wherein said fluxgate magnetometer is housed in an electromagnetic shield housing adapted to frequency characteristics of an eddy current shield of said magnetic shield room. Magnetic shield device. 6. The active magnetic shield device according to claim 5, wherein a feedback coil means for offset compensation is provided inside the electromagnetic shield housing of the fluxgate magnetometer. 7. The flux gate magnetometer according to claim 1, further comprising: an offset adjuster for terrestrial magnetism superimposed on a measurement output of the fluxgate magnetometer, provided outside the fluxgate magnetometer. Active magnetic shield device. 8. The magnetic shield coil means according to claim 1, wherein a reference signal of a commercial frequency is superimposed on a measurement output of said flux gate magnetometer means via a phase adjusting means and supplied to said magnetic shield coil means. The active magnetic shield device according to any one of the above.