JP3837741B2 - Magnetic shield device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an improvement in shielding characteristics of a magnetic shield room that is always used in a magnetoencephalographic measurement system that measures a weak cerebral magnetic field, and relates to an active magnetic shield device that reduces an environmental magnetic field by electrical active correction. The present invention can be applied not only to the magnetoencephalogram but also to a magnetic shield room for measuring a biomagnetic field such as a magnetocardiogram. Furthermore, the present invention can be applied to a simple magnetic shield room for an electron beam exposure apparatus or a high-resolution MRI apparatus (nuclear magnetic resonance imaging apparatus).
[0002]
[Prior art]
A biomagnetic measurement system using a SQUID (Superconducting Quantum Interference Device) in the prior art measures an extremely weak signal and reduces environmental magnetic noise in a magnetic shield room and is called a gradiometer. Using a differential pickup coil, magnetic fluctuation noise originating in the distance is reduced.
[0003]
SQUID is a high-sensitivity magnetic sensor that can detect a magnetic field of 1 / 100,000 or less of the geomagnetism. It utilizes the superconducting quantization phenomenon and has a sensitivity of 3 digits or more compared to a conventional magnetic sensor. It can be operated at liquid helium temperature (4.2K) or liquid nitrogen temperature (77.3K) depending on the material used.
[0004]
The shape of the pickup coil includes a first-order differential gradiometer and a second-order differential gradiometer depending on the component of the magnetic field to be reduced, and there is also a so-called magnetometer that has no effect of reducing environmental magnetic field fluctuations. In general, the first derivative gradiometer is most often used because of the balance between effect and size. In this pickup coil, the zero-order component of the far field fluctuation is canceled by the differential coil, but the magnetic field distribution generated from the magnetic field source close to the sensing coil (ie, the nerve activity of the living body) has a first-order component or more. It can be measured.
[0005]
FIG. 13 is a perspective view of the magnetic shield room, and FIG. 14 is a sectional view thereof. 13 and 14, the magnetic shield room 11 has an inner wall 12 and an outer wall 13 made of two layers of permalloy with a space, and aims at increasing the magnetic shielding effect by utilizing eddy current and high-frequency magnetic shielding. A high conductivity layer 14 such as aluminum or copper is inserted between the inner wall 12 and the outer wall 13 of the permalloy layer, and an entrance / exit 15 for entering / exiting the room is provided, and a SQUID 16 is provided inside. And the measurement table 17 is installed.
On the other hand, in order to reduce the cost of not only the shield by the magnetic shield room 11 but also the magnetic shield, the active magnetic shield methods described in the following references 1 to 3 have been proposed.
This is to reduce the environmental magnetic field around the sensor by generating a canceling magnetic field by placing an environmental magnetic field measuring sensor in a Helmholtz coil as a canceling coil.
[0006]
(1) Reference 1: Tomita et al. The 15th Annual Meeting of the Biomagnetic Society of Japan vol. 13 No1, 2000 p172-173
(2) Reference 2: Kato et al. 15th Annual Meeting of the Biomagnetic Society of Japan vol. 13 No1, 2000 p170-171
(3) Reference 3: Kurisaki Research Institute of Electronic Science, Hokkaido University “Electronic Science Research” p18-23, 2000.
[0007]
First, the problem of the above-described permalloy-based magnetic shield room 11 and the problem of the active magnetic shield method will be described below.
[0008]
First, the problems of the permalloy-based magnetic shield room 11 will be described.
It is a well-known and general technique that low-frequency magnetic fluctuations are reduced by surrounding a region to be magnetically shielded with a soft magnetic material having a high magnetic permeability typified by Permalloy.
[0009]
However, permalloy is mainly composed of nickel and iron, and is expensive because it has to undergo heat treatment and the demand is small. In addition, since the weight is extremely large, the ratio of effect to cost is increased by combining thin materials into a multilayer (in the above example, two layers). However, since permalloy alone does not provide a sufficient effect and has a high frequency shield, a shield by eddy currents in the high conductivity layer 14 is also used.
[0010]
The shielding performance due to the eddy current effect in the high conductivity layer 14 is proportional to the thickness and frequency of the layer, but this is also limited by weight and cost and cannot be so thick, so at a low frequency of several Hz or less. As shown below, there is almost no effect.
[0011]
As shown in FIG. 15, a magnetic field Hi in a rectangular box 21 having a thickness t, a width w, and a height h surrounded by a metallic wall having conductivity σ is a uniform external AC magnetic field. When Ho (ω), the shielding effect by the eddy current effect is expressed by the following equation.
Hi = Ho (ω) / (1 + jωτ)
τ = μoσwht / [2 (w + h)]
ω: angular frequency, μo: vacuum permeability
[0012]
According to the above formula, for aluminum, σ = 0.36 × 10 ⁸ Ω ^―1 m ^-1 When w = h = 2000 mm and t = 3 mm, about 2 Hz is the cutoff frequency, and it can be seen that there is a shielding effect above this frequency.
[0013]
If the thickness t is increased, the cut-off frequency is lowered. However, since the weight is increased and the cost is increased, the thickness cannot be increased so much.
[0014]
Also, as shown in FIG. 16, the magnetic field noise inside and outside the magnetic shield room 11, that is, the environmental magnetic fluctuation is an amplitude characteristic of -2 to -3 power of the frequency, and the magnetic shielding performance is not as good as the low frequency. In addition, since the measurement band of biomagnetic measurement is about 0.1 Hz to 100 Hz, it can be understood that magnetic noise that cannot be shielded enters the magnetic shield room 11 and its influence cannot be avoided.
[0015]
FIGS. 17 to 19 are for supplementing the above description with schematic views of the cross section of the magnetic shield room 11 and the two-dimensional magnetic field line distribution outside thereof. FIG. 17 and FIG. 18 show the case where the inner wall 12 and the outer wall 13 which are mainly permalloy layers contribute to the shield in the low frequency region. Since the lines of magnetic force concentrate on the wall surface of the magnetic shield room 11 parallel to the uniform environmental magnetic field distribution, the magnetic flux density near the wall surface is increased.
[0016]
On the other hand, focusing only on the effect of the high conductivity layer 14 that is a layer between the inner wall 12 and the outer wall 13 of permalloy, as shown in FIG. 18, the shielding effect by the eddy current 23 at a relatively high frequency of several Hz or more. Comes out. This grows near the center. As indicated above, this effect increases as the frequency increases.
As a result, as shown in FIG. 19, as the frequency increases, the internal magnetic field is increasingly eliminated by the effects of both magnetic shielding and eddy current shielding by permalloy.
[0017]
Next, problems and problems of the active magnetic shield method described in Reference Documents 1 to 3 will be described.
[0018]
(1) The method of Reference Document 1 uses a SQUID magnetometer installed in the magnetic shield room 11 as a sensor, generates a canceling magnetic field in a feedback coil around the magnetic shield room 11, and enters the magnetic shield room 11. This is to reduce the magnetic field.
[0019]
If the feedback gain is sufficiently large, it can be reduced to approximately 1 / (gain) in the vicinity of the SQUID magnetometer. However, when applied in combination with the magnetic shield room 11, the signal rotated in phase due to the eddy current shielding effect on the wall surface (inner wall 12 and outer wall 13) of the magnetic shield room 11 enters as described above. The stability of the feedback system must be ensured, and the frequency limit that can be canceled by itself is determined. In a normal magnetic shield room for biomagnetism measurement, the cutoff point of eddy current shielding is several Hz, so that the vicinity of this frequency becomes feedback gain = 1 and the canceling effect does not work. Also, the gain cannot be increased to ensure stability.
[0020]
(2) The method of Reference 2 is a feedback coil installed independently on each surface by attaching a fluxgate magnetometer so as to have sensitivity in a direction perpendicular to the center of each surface of the outer wall 13 of the magnetic shield room 11. In other words, a canceling magnetic field is generated to enhance the shielding effect.
[0021]
However, since the fluxgate magnetometer is necessary for each surface, there is a problem that it cannot be installed in the existing magnetic shield room 11 later. For example, it is difficult to install on the bottom surface.
On the other hand, since the environmental magnetic field is converged on the permalloy wall surfaces (inner wall 12 and outer wall 13) of the magnetic shield room 11, the magnetic field strength is large in the vicinity of the wall surface and conversely small in the center portion of the wall surface. Therefore, when a cancel magnetic field is generated based on a magnetic signal detected from the fluxgate magnetometer, even if the cancel magnetic field is canceled at a position where the fluxgate magnetometer is present, a sufficient cancel magnetic field cannot be obtained at the end face of the wall.
[0022]
In this method, since the magnetic field proportional to the magnetic flux incident on each surface is independently detected, gradient control is possible. However, with this method, it is difficult to accurately measure and control the magnetic flux passing through the permalloy inner wall 12 and outer wall 13 constituting the magnetic path in the plane direction. In addition, in this detection method, if there is a magnetic field generation source or a magnetic material in the vicinity of the fluxgate magnetometer, a canceling magnetic field is generated for a value different from the magnetic field applied to the entire magnetic shield room 11, so that compensation can be performed accurately. There is a disadvantage that it can not.
[0023]
Furthermore, a high-sensitivity fluxgate magnetometer is required for active control of the magnetic shield room for biomagnetism measurement, but since it is generally expensive, it is not economical to use all six surfaces.
[0024]
(3) The method shown in p21 of Reference Document 3 and FIG. 4 is a method in which an induction coil is wound around a magnetic shield room 11 and a voltage induced in the coil due to environmental magnetic field fluctuation is used as a feedback signal.
[0025]
Since the electromotive force induced in the coil is proportional to the area, sufficient sensitivity can be obtained compared to the fluxgate magnetometer with a number of turns of several tens of turns on the surface of the size of the magnetic shield room 11. In addition, since the magnetic shield room 11 has a magnetic field convergence effect, the sensitivity is high. Furthermore, it has sufficient frequency characteristics to constitute a feedback system.
[0026]
Since the voltage induced in the induction coil corresponds to the total magnetic flux interlinked with the magnetic shield room 11 (on the induction coil surface), distortion of the detected magnetic field due to a nearby magnetic field generation source or magnetic material as in the sensor of Reference 2. There is an advantage that there are few influences. Further, it can be configured at a low cost. That is, the above-mentioned drawbacks (1) and (2) References 1 and 2 are overcome.
[0027]
On the other hand, the position of the coil taught in this reference 3 is the center of the magnetic shield room 11 because the SQUID 16 for biomagnetic measurement (see FIG. 13) is often placed in the center. It can be interpreted that the feedback system acts so that the sense coil surface has a minimum magnetic field.
[0028]
Since it is the space where the SQUID magnetometer is installed that wants to control the magnetic field, high control performance can be expected if a magnetic field detection sensor is placed in a portion most correlated with the magnetic noise in this space. The SQUID magnetometer of Reference 1 is used in consideration of this point. The installation of the sense coil in the center as taught in Reference 3 suggests that the most correlated signal is detected when the magnetoencephalograph installation space and the coil installation position of the magnetic shield room 11 are on the same plane or close to each other. ing.
[0029]
In general, the magnetic flux passing through the wall surface (inner wall 12 and outer wall 13) of the magnetic shield room 11 has few end faces along the magnetic field direction and the largest central portion. FIG. 20 shows this state, and the magnetic flux lines are converged along the wall from the top to the bottom, and the magnetic flux density is maximized at the central portion. Therefore, if a magnetic field detection coil is provided at the end of the magnetic shield room 11, for example, in the vicinity of the feedback coil, the magnetic field at the center cannot be fully compensated. Therefore, there is a restriction that the magnetic field detection coil must be provided in the vicinity of the same plane as the SQUID 16 (see FIGS. 13 and 14) in the magnetic shield room 11.
[0030]
However, since the magnetic shield room 11 has the entrance / exit 15 (see FIGS. 13 and 14), the arrangement of the magnetic flux detection coils is inconvenient. Moreover, since the magnetic shield room 11 has several tons, it is difficult to lay coil wiring under the floor of the existing magnetic shield room 11. Furthermore, it is not possible to detect and compensate for magnetic gradients only with the taught method.
[0031]
In the above, three examples of the general magnetic shield room 11 and the active magnetic shield system that compensates for this characteristic have been described. Putting these together,
1) The high permeability layer which is the inner wall 12 and the outer wall 13 formed of permalloy in the magnetic shield room 11 exhibits a shielding performance in a low range, and the principle is that the magnetic flux is converged on a high permeability plate, By reducing the number of magnetic flux lines, the magnetic flux density in the shielded space is reduced.
2) The high conductivity layer 14 formed between the inner wall 12 and the outer wall 13 reduces magnetic flux density in the shielded space by eliminating magnetic flux intrusion due to the eddy current shielding effect in a relatively high frequency range.
3) An active magnetic shield is a magnetic sensor that directly or indirectly measures the magnetic field inside or outside the magnetic shield room 11 and generates a repulsive magnetic field (cancellation magnetic field) in the Helmholtz coil provided around the magnetic shield room 11. This compensates for the reduction in the characteristic 2) in the low frequency range.
[0032]
That is, the active magnetic shield can be considered that the eddy current shield of the high conductivity layer 14 is electrically compensating for the characteristics in the low frequency region.
[0033]
[Problems to be solved by the invention]
However, since the active magnetic shield described in the prior art is compensated by the Helmholtz coil, the internal magnetic field is distorted in a saddle shape, which causes a new magnetic gradient.
[0034]
In general, the magnetic field gradient inside the magnetic shield room has a large gradient in the low range due to the influence of the high permeability material, but the gradient of the internal leakage magnetic field is extremely small in the high range. This is presumably due to the fact that eddy currents generated in the high frequency region spontaneously distribute and flow through the high conductivity layer so as to make the internal magnetic field uniform. The reason can be considered that currents flowing in the same direction have the property of repelling each other.
[0035]
On the other hand, it can be said that the Helmholtz coil for active magnetic shield has a different result from the magnetic field distribution due to the eddy current because the current flows in a concentrated manner.
An infinite length solenoid coil is known as a method for generating a uniform magnetic field distribution, but in reality, only a finite length solenoid coil can be applied to a magnetic shield room, and the internal magnetic field is still distorted.
Therefore, even if it is detected by any measurement method, it is impossible to finally generate a uniform magnetic field, so it is difficult to obtain a high magnetic shield effect on the low frequency side in a large space, and its performance is limited. There is a problem that it ends up.
[0036]
Therefore, the present invention has been made to improve the essential drawbacks of the active shield device combined with the magnetic shield room, and suppresses the generation of magnetic field gradient due to the Helmholtz coil and obtains high shielding performance from low frequency to high frequency. Therefore, it aims to realize a lightweight, low-cost, ultra-high performance active magnetic shield device.
[0037]
[Means for Solving the Problems]
In order to achieve the above object, a magnetic shield device according to the present invention is configured as follows.
[0038]
(1) Represented by permalloy that is formed in a sealed room and includes at least one high-conductivity layer formed of a high-conductivity member including at least aluminum or copper, and does not include the high-conductivity member. A magnetic shield room having a wall made of a high magnetic permeability layer formed of a high magnetic permeability member, and a magnetic shield means for performing shielding by an eddy current shielding effect due to conductivity of the high magnetic permeability layer of the wall. The magnetic shield device comprises a current that is arranged on the wall and generates a magnetic field distribution similar to a repulsive magnetic field generated by the eddy current distribution formed by the high conductivity layer forming the wall. A magnetic shield device comprising a coil for flowing.
(2) In the magnetic shield device according to (1), the coils are arranged around the wall by gathering one-turn coils into a solenoid shape, and non-uniform current in each coil of the one-turn coils A magnetic shield device characterized in that an electrode for supplying power is connected.
(3) In the magnetic shield device according to (2), the electrodes are connected such that the current value at the solenoid end of the coil formed by collecting the one-turn coils into a solenoid is larger than the current distribution in the solenoid body. A magnetic shield device characterized by that.
(4) In the magnetic shield device according to (2), the one-turn coil is concentrically formed on the surface covering the magnetic field generating surface, separately from the coil arranged to collect the one-turn coil into a solenoid and arranged around the wall. A magnetic shield device characterized in that a distribution current similar to an eddy current is caused to flow in the coil.
(5) In the magnetic shield device according to (1), the magnetic shield means measures the magnetic field at one end of the wall with an induction coil, and negatively feeds back the measured signal to control the current flowing through the coil. A magnetic shield device comprising means.
(6) In the magnetic shield device according to (2), each of the solenoid-like elements including the one-turn coil assembly is obtained by measuring the input magnetic field of each end face of the wall with an induction coil and negatively feeding back the measured signal. A magnetic shield device, wherein a magnetic field gradient is corrected by providing a component corresponding to the magnetic field strength of each of the end faces by controlling a distributed current flowing through the coil.
(7) The magnetic shield device according to (1), wherein a magnetometer is installed outside the magnetic shield room, a magnetic field is detected by the installed magnetometer, and open loop compensation is performed.
(8) The magnetic shield device according to (1), wherein a SQUID magnetometer is installed inside the magnetic shield room, and a feedback system is configured so that closed loop compensation is performed by the installed SQUID magnetometer. Shield device.
(9) In the magnetic shield device according to (2), the coil formed by gathering the one-turn coils into a solenoid is characterized in that a thin wire made of a vertical and horizontal mesh-like conductive material is a sheet coil. Magnetic shield device.
(10) In the magnetic shield device according to (2), the coil formed by collecting the one-turn coils into a solenoid shape is provided with a slit in the conductive film so as to form a current distribution in one direction. Magnetic shield device.
(11) The magnetic shield device according to (1), wherein a conductive connection is made at an entrance / exit portion provided in the magnetic shield room.
(12) In the magnetic shield device according to (1), the high-conductivity layer is thinned to such an extent that eddy currents can be generated, or an eddy current shield is formed by a cross-shaped mesh having conductivity in directions orthogonal to each other. A magnetic shield device characterized in that a switching point between an active shield and a passive shield is provided on the high frequency side by combining a layer and the coil.
[0039]
In this way, by winding a one-turn coil assembly around the entire magnetic shield room or incorporating it into a panel in advance to form a finite-length solenoid coil, a current distribution similar to or simulating an eddy current shield can be artificially created. It is possible to generate a uniform magnetic field by generating a passive eddy current shield, which reduces the eddy current for generating a repulsive magnetic field with a decrease in frequency. Thus, a current for generating a repulsive magnetic field can be actively passed through each coil, so that even a thin conductive layer can be covered from a low range to a wide range.
[0040]
DETAILED DESCRIPTION OF THE INVENTION
Next, an embodiment of a magnetic shield device according to the present invention will be described with reference to the drawings. In addition, the same code | symbol is attached | subjected and demonstrated to the same thing as what was demonstrated by the prior art.
[0041]
The electromagnetic shield device according to the present invention is a magnetic material having a wall including at least one high conductivity layer formed of a high conductivity member in a sealed room as shown in FIGS. 13 and 14 described in the prior art. Active magnetic shielding means that also serves as a shield by an eddy current shielding effect on a shield room and a wall made of a high magnetic permeability layer formed of a high magnetic permeability member without including a high electric conductivity layer formed of a high electric conductivity member. Prepare. The active magnetic shield means includes an active magnetic shield type coil that can electrically compensate and improve the magnetic shield performance based on the eddy current shielding effect.
[0042]
This active magnetic shield type coil is made of a sheet-like conductive material around the outer wall surface of the magnetic shield room, a coil made of a conductive material knitted into an orthogonal mesh, or a conductive warp yarn made of non-conductive material. Weft In addition, the electrodes of these coils are devised so that a current can be applied so that the magnetic field in the coil is uniformly generated, and the magnetic field gradient generated in the magnetic shield room is reduced. It can be suppressed.
[0043]
Various embodiments for realizing the active magnetic shield type coil will be described below.
[0044]
As shown in FIGS. 1 and 2, the coil of the first specific example is a coil 33 formed by circulating a sheet of conductive material around the outer wall 12 of the magnetic shield room 11. Generate a magnetic field in the direction.
[0045]
The coil 33 may be a conductive film or conductive material knitted in a mesh shape, or a conductive material provided with slits to give conductivity anisotropy as a whole, and has a structure of a one-turn coil assembly. ing. Further, the coil 33 may be bonded to the inner wall 12 or the outer wall 13 of the high conductivity layer 14 or the high magnetic permeability layer of the magnetic shield room 11, or may be disposed on the outermost layer of the outer wall 13 as shown in FIG. Also good.
Further, although not shown, this coil 33 is connected to the electrode at a rounded end so that a current distribution is given to the coil corresponding to each position of the Z coordinate in the figure. .
[0046]
In this way, an assembly of one-turn coils is wound around the entire magnetic shield room or previously incorporated into a panel to form a finite-length solenoid coil, and a current distribution that simulates or simulates an eddy current shield is generated artificially, Generate a uniform magnetic field. In this way, the passive eddy current shield reduces the eddy current for generating a repulsive magnetic field as the frequency decreases, but the sensor measures the magnetic field and actively sends current to each coil to simulate pseudo eddy currents. If it produces | generates, even a thin conductive layer can cover from a low range to a wide area.
[0047]
As shown in FIG. 3, the coil of the second specific example is a sheet formed by knitting a conductive material in a mesh shape. The respective insulating materials knitted into a mesh shape are covered with each other, and the contacting wires are insulated from each other.
This sheet is made a round along the periphery of the outer wall 12 of the magnetic shield room 11, and an electrode is connected to the end. The conductive material may be only in one direction or in two orthogonal directions.
[0048]
FIG. 4 is a connection example of the end of the coil 33 that has been rotated once. First and second electrodes 34 and 35 having low electrical conductivity are in surface contact with the end portions of the sheet-like coil 33 extending around the outer wall 13, and the first and second electrodes 34, The first and second lead wires 36 and 37 are connected to an external coil driver (not shown).
When a current is passed through the first and second electrodes 34 and 35, the coil 33 close to the first and second electrodes 34 and 35 has a low resistance, and the far portion has a high resistance. I | That is, the current density is large at the end of the solenoidal coil and small at the center. Since the internal magnetic field distribution is an integral value of the magnetic field generated from each coil, the distribution in the Z-axis direction is | B |.
Conversely, a large number of connection points to the electrodes may be provided to adjust the current distribution for each point so that the magnetic field distribution is uniform.
[0049]
Thus, unlike the magnetic field generated by the Helmholtz coil, the current distribution is close to that of the eddy current shield, so that the internal magnetic field gradient due to the active shield can be reduced.
[0050]
FIG. 5 shows an example of an electrode part, 38 is a low conductivity connection part, 39 is a resistance group for setting a distributed current, and 40 is a high conductivity connection electrode for connecting them.
By increasing the current flowing from the resistance at the end portion toward the intermediate portion among the resistors constituting the resistance group 39, the current value at the intermediate portion of the coil 33 can be suppressed and a uniform magnetic field distribution can be obtained. The resistance value of the resistors constituting the resistor group 39 may be fixed or variable. Further, if the resistance value and the number distribution of the coil 33 itself are created, the same effect can be obtained.
[0051]
In this way, the current distribution is connected via the low-conductivity electrode, so that a uniform magnetic field distribution can be obtained with a simple configuration, unlike a finite-length solenoid.
[0052]
FIG. 6 shows a further example of the electrode section, 38 is a low conductivity connection section, 39 is a resistance group for setting a distributed current, and 41 is a driver connected to each resistance of the resistance group 39 for driving. It is. By providing the driver 41, it is possible to independently set the current injected into the coil 33 via each resistance of the resistor group 39, and the distributed current can be freely controlled, and the magnetic field distribution inside the coil is uniform. It is possible to control not only the setting but also the magnetic field gradient created by the high permeability layer (inner wall 12, outer wall 13, see FIG. 2).
[0053]
In this way, the current distribution is connected via the low-conductivity electrode, so that a uniform magnetic field distribution can be obtained with a simple configuration, unlike a finite-length solenoid.
[0054]
FIG. 7 shows a configuration in which an induction coil 42 is provided at the upper end of the magnetic shield room 11 to detect a magnetic field flowing into the magnetic shield wall, and a canceling magnetic field generating current is supplied to the distributed current coil via a feedback circuit. is there.
[0055]
The configuration includes an induction coil 42 that is wound around the outer periphery of the upper end portion of the magnetic shield room 11, a sheet-like coil 33 that is arranged so as to cover the periphery of the outer wall of the magnetic shield room 11, and an induction coil 42. The control unit 51 amplifies the detected signal and feeds it back to the coil 42.
[0056]
The control device 51 includes an amplifier 52 that amplifies a weak signal detected by the induction coil 42, an integrator 54 that integrates the output signal of the amplified induction coil 42, an amplifier 55, and an adder / subtractor 53. The feedback circuit, a regulator 56 for adjusting the balance of current supplied to the coil 33, and coil drivers 57a and 57b for supplying current to the coil 33, so that the output signal of the induction coil 42 is substantially minimized. The current supplied to the coil 33 is subjected to negative feedback control.
[0057]
In this induction coil 42, when current is supplied from the outside to the coil 33 covering the outer periphery of the magnetic shield room 11, magnetic noise detected by the induction coil 42 in the same direction as the magnetic flux detected by the induction coil 42 is generated. Generate a magnetic field to cancel.
[0058]
As described above, by detecting the current to be generated by the induction coil 42, feedback can be performed with high linearity from a low frequency to a high frequency in the same manner as a magnetic field entering the conductive layer.
[0059]
Next, the shield state of the entrance / exit part 15 of the magnetic shield room 11 will be described.
FIG. 8 shows a state of connection of the entrance / exit portion 15 of the magnetic shield room 11. The fixed electrodes 58 a and 58 b provided on both side ends of the entrance / exit portion 15, and the coil 33 a are attached to the opening / closing movable portion of the entrance / exit portion 15. It consists of electrodes 58c and 58d provided on both ends of the floor. The electrode 58a and the electrode 58c provided at the entrance / exit part 15 are connected by a conductive cable 59a, and the electrode 58b and the electrode 58d of the entrance / exit part 15 are electrically connected by a conductive cable 59b.
[0060]
In this way, the coil 11 wound around the magnetic shield room 11 is once connected at the entrance / exit part 15 to the electrodes 58a and 58c, 58b and 58d with the conductive cables 59a and 59b. By connecting in this way, the spatial shape of the current distribution can be maintained. The vertical connection can be made in the same way.
[0061]
Next, a modified example of the sheet-like coil will be described.
As shown in FIG. 9, the sheet-like coil 33B has a structure in which a slit 60 is provided in a sheet-like conductor 59, and has an anisotropic conductivity as a whole. The coil is configured as a shield coil. The same can be done with an anisotropic conductive film having properties similar to those described above.
[0062]
Next, a magnetic shield device that can compensate not only the zeroth-order component of the spatial magnetic field but also the first-order gradient will be described.
[0063]
FIG. 10 schematically shows a configuration in which intrusion magnetic fields are inductively detected at both ends of the magnetic shield room, and currents are injected into the coil array independently of the zero-order component and the first-order gradient component. The configuration is that detection coils 64a and 64b having opposite polarities and a detection coil 64a as an actuating coil are provided in series on the wall of the magnetic shield room in a position spaced apart in the axial direction, and the outputs of the detection coils 64a and 64b are output. Based on the difference, a control device 71 is provided which corrects the magnetic field gradient in the axial direction of the detection coils 64a and 64b in the magnetic shield room 11 and controls the current flowing through the cancel coils (Helmholtz coils) 63a and 63b. Yes.
[0064]
The control device 71 is connected to both ends of the detection coil 64b, amplifies the output of the amplifier 72, an integrator 74 that integrates the output of the amplified detection coil 64b, and the balance of current supplied to the Helmholtz coils 63a and 63b. A regulator 76 for adjusting the current, coil drivers 78 and 79 for supplying current to the Helmholtz coils 63a and 63b, an amplifier 73 connected to both ends of the detection coils 64a and 64b connected in series to amplify the output, and an amplifier 73, an integrator 75 for integrating the output of 73, regulators 76a, 76b for adjusting the balance of current supplied to the Helmholtz coils, a coil driver 79 for supplying current to the Helmholtz coils 63a, 63b, and outputs of the regulators 76a, 76b. Addition / subtraction to input / output to the coils constituting the Helmholtz coils 63a, 63b Vessel 77a, is made from the 77b.
[0065]
Here, when the magnetic flux passing through the wall has a gradient in the environmental magnetic field (magnetic field noise), there is a difference between the incident magnetic flux Φc and the outgoing magnetic flux Φc ′ to the wall w1.
The detection coil 64 a as a differential coil detects a magnetic flux reflecting this difference, and the detection coil 64 b detects a component proportional to the zero-order component B in the magnetic shield room 11.
An output obtained by connecting the detection coils 64a and 64b in series provides a first-order differential component in the shield wall surface, which is proportional to the difference ΔB in the density of the leakage magnetic flux in the magnetic shield room 11.
[0066]
Then, the magnetic field gradient in the axial direction of the detection coils 64a and 64b can be corrected by adding and subtracting the outputs of the adjusters 76a and 76b by the adders / subtractors 77a and 77b and feeding back to the Helmholtz coils 63a and 63b.
[0067]
As described above, since the coil current can flow spatially independently, it is possible to positively generate a gradient magnetic field by detecting the magnetic field gradient and cancel the proximity magnetic field.
Although only primary gradient detection has been dealt with, it goes without saying that secondary and higher components can be canceled by controlling the current distribution.
[0068]
Next, a modification in which circular coils are provided on the upper and lower surfaces of the magnetic shield room will be described with reference to FIG.
[0069]
The circular coils 33C and 33D may be provided as spiral planar coils or concentric coils, or square coils having different side lengths may be arranged concentrically. Although not shown in the figure, each coil is distributed so that the outside is large and the inside is small in the same direction as the current wound around the body, or the current density is distributed so that the same effect is produced. You may give it. The induction coil may be wound around the entire magnetic shield room.
In addition to the induction coil, the sensor may be provided with a fluxgate magnetometer on a panel surface on a plane constituting the coil. An internal SQUID magnetometer may be used. Furthermore, the coil current may be controlled by an open loop without using feedback.
[0070]
As described above, the eddy current shield constitutes a current distribution in the entire conductive layer of the magnetic shield room 11 so that the internal magnetic field gradient can be eliminated. Therefore, since the current distribution closer to the eddy current of the conductive layer can be simulated, the internal magnetic field gradient can be further reduced.
[0071]
Next, a further modification of the coil in which the magnetic shield room is a spherical magnetic shield room will be described with reference to FIG.
[0072]
In the coil configuration in the spherical magnetic shield room 11A, currents are distributed so as to repel each other, thereby generating an eddy current distribution, which has a high density at the upper and lower ends and a smaller value in the circumferential direction. The same applies to the box-shaped magnetic shield room described above. By generating such coil arrangement and current distribution, high magnetic shielding performance can be obtained even at low frequencies.
In the cylindrical shield, a circular coil may be provided at the opening end with the configuration shown in FIG.
[0073]
Next, the structure of the conductive layer constituting the magnetic shield room will be described for the various coil shapes described above. The structure of the conductive layer is assumed to be bulky and thick, but if this is made extremely thin, another merit is generated.
Although not shown, when combined with a mesh or a thin conductive sheet, an eddy current effect is generated at a high frequency, so that the phase around the eddy current can be prevented from affecting the control system at a low frequency range. In addition, the magnetic shield can be realized at low cost and at low cost.
[0074]
【The invention's effect】
As described above, in the magnetic shield device according to the present invention, a one-turn coil assembly is wound around the entire magnetic shield room or previously incorporated into a panel to form a finite-length solenoid coil, which is the same as or similar to an eddy current shield. By artificially generating a simulated current distribution and generating a uniform magnetic field, the passive eddy current shield reduces the eddy current for generating a counter magnetic field as the frequency decreases. Since a magnetic field is measured and a current can be actively passed through each coil, there is an effect that even a thin conductive layer can be covered from a low range to a wide range.
[0075]
Further, unlike the magnetic field generated by the Helmholtz coil, the current distribution is close to that of the eddy current shield, so that the internal magnetic field gradient caused by the active shield can be reduced.
[0076]
Furthermore, the current distribution has an effect that a uniform magnetic field distribution can be obtained with a simple configuration, unlike a finite length solenoid, by connecting via a low conductivity electrode.
[0077]
In addition, by detecting the current to be generated by the induction coil, it is possible to feed back with high linearity from low frequency to high frequency as well as the magnetic field entering the conductive layer, while the coil current can flow spatially independently. Therefore, it is possible to positively generate a gradient magnetic field by detecting the magnetic field gradient and to cancel the near magnetic field.
[Brief description of the drawings]
FIG. 1 is an explanatory view showing a solenoid-like coil employed in a magnetic shield device according to the present invention.
2 is a cross-sectional view of the solenoid-like coil in FIG. 1. FIG.
FIG. 3 is an explanatory diagram in which the coil is formed in a mesh shape.
FIG. 4 is an explanatory view showing a connection example of end portions of coils that have made the same round.
FIG. 5 is an explanatory view showing an example of the electrode part.
FIG. 6 is an explanatory view showing an example of the electrode part.
FIG. 7 is a schematic configuration diagram for flowing a current for generating a canceling magnetic field in a magnetic shield room using the same induction coil.
FIG. 8 is an explanatory view showing a connection state of the entrance / exit part.
FIG. 9 is an explanatory view in which a coil is formed by providing a slit in the sheet-like conductor.
FIG. 10 is an explanatory view schematically showing a configuration in which an intruding magnetic field is detected and canceled at both ends of the magnetic shield room.
FIG. 11 is an explanatory diagram showing a configuration in which circular coils are provided on the upper and lower surfaces.
FIG. 12 is an explanatory diagram assuming a case of the same spherical shield room.
FIG. 13 is an explanatory view showing an appearance of a magnetic shield room.
14 is a cross-sectional view of the shield room in FIG.
FIG. 15 is an explanatory view showing a state of electromagnetic shielding in the box.
FIG. 16 is a graph showing an example of magnetic field noise inside and outside a magnetic shield room.
FIG. 17 is an explanatory diagram showing a state of a magnetic field in a magnetic shield room in the prior art.
FIG. 18 is an explanatory diagram showing a state of a magnetic field in a magnetic shield room in the prior art.
FIG. 19 is an explanatory diagram showing a state of a magnetic field in a magnetic shield room in the prior art.
FIG. 20 is an explanatory diagram showing a state of a magnetic field in a magnetic shield room in the prior art.
[Explanation of symbols]
11 Magnetic shield room
12 inner wall
13 Exterior wall
14 High conductivity layer
15 gateway
33 coils
33A coil
33B coil
34 First electrode
35 Second electrode
36 First leader line
37 Second leader line
38 Low conductivity connection
39 Resistance group
40 connection electrode
41 drivers
42 induction coil
51 Control device
52 Amplifier
53 Adder / Subtractor
54 Integrator
55 Amplifier
56 Adjuster
57a Coil driver
57b Coil driver
58a electrode
58b electrode
58c electrode
58d electrode
59a Conductive cable
59b Conductive cable
60 Conductor
60a slit
63a Helmholtz coil
63b Helmholtz coil
64a detection coil
64b detection coil
71 Controller
72 Amplifier
73 Amplifier
74 Integrator
75 integrator
76a regulator
76b regulator
77a Adder / Subtractor
77b Adder / Subtractor
78 Coil driver
79 Coil driver

Claims (12)

  A high-permeability represented by permalloy, which is formed in a sealed room and includes at least one high-conductivity layer formed of a high-conductivity member containing at least aluminum or copper, and does not include the high-conductivity member. Magnetic shield room comprising a magnetic shield room having a wall made of a high permeability layer formed of a magnetic permeability member, and a magnetic shield means for shielding by an eddy current shielding effect due to conductivity of the high permeability layer of the wall. A shield device, wherein the magnetic shield means includes a coil that is arranged on the wall and that passes a current that generates a magnetic field distribution similar to a repulsive magnetic field due to an eddy current distribution formed by the high conductivity layer forming the wall A magnetic shield device comprising:   The magnetic shield device according to claim 1, wherein the coils are arranged around the wall by gathering one-turn coils into a solenoid shape and supplying a non-uniform current to each coil of the one-turn coils. Magnetic shield device characterized in that an electrode for connecting is connected.   3. The magnetic shield device according to claim 2, wherein the electrodes are connected so that a current value at a solenoid end of a coil obtained by collecting the one-turn coils into a solenoid shape is larger than a current distribution of the solenoid body. A magnetic shield device.   3. The magnetic shield device according to claim 2, wherein the one-turn coil is concentrically arranged on a surface covering the magnetic field generating surface, separately from the coil arranged in the form of a solenoid by collecting the one-turn coil around the wall. A magnetic shield device characterized in that a distributed current similar to an eddy current flows through the coil.   2. The magnetic shield device according to claim 1, wherein the magnetic shield means includes means for measuring a magnetic field at one end of the wall with an induction coil, and negatively feeding back the measured signal to control a current flowing through the coil. A magnetic shield device characterized by that.   3. The magnetic shield device according to claim 2, wherein an input magnetic field at each end face of the wall is measured by an induction coil, and the measured signal is negatively fed back to flow through each solenoid-like coil composed of the one-turn coil assembly. A magnetic shield device, wherein a magnetic field gradient is corrected by providing a component corresponding to the magnetic field intensity of each end face by controlling a distributed current.   2. The magnetic shield device according to claim 1, wherein a magnetometer is installed outside the magnetic shield room, a magnetic field is detected by the installed magnetometer, and open loop compensation is performed.   2. The magnetic shield device according to claim 1, wherein a SQUID magnetometer is installed inside the magnetic shield room, and a feedback system is configured so that closed loop compensation is performed by the installed SQUID magnetometer.   3. The magnetic shield device according to claim 2, wherein the coil formed by gathering the one-turn coils into a solenoid is a sheet coil made of a thin wire made of vertical and horizontal mesh-like conductive materials. .   3. The magnetic shield device according to claim 2, wherein said one-turn coil is assembled into a solenoid-like coil, and a slit is formed in the conductive film so that a current distribution is formed in one direction. apparatus.   2. The magnetic shield device according to claim 1, wherein a conductive connection is made at an entrance / exit part provided in the magnetic shield room.   2. The magnetic shield device according to claim 1, wherein the high-conductivity layer is thinned to such an extent that an eddy current can be generated, or an eddy current shield layer made of a cloth-like mesh having conductivity in a direction perpendicular to each other; A magnetic shield device in which a switching point between an active shield and a passive shield is provided on the high frequency side by combining coils.

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