JP2001135973A - Magnetic shielding device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a magnetic shielding device which is equipped with a cylinder, whose ends are both open so as to accurately and quickly carry out an operation, capable of improving shielding inner residual magnetic field in uniformity, keeping it at a high shield ratio, and giving a place where a feeble magnetic field can be measured accurately. SOLUTION: If the axial length, diameter, wall thickness, and relative permeability of a cylinder of magnetic material whose both ends are open, are represented by L, D, t, and μ, and these are so set as to nearly satisfy the equation, L/D=1+log (μt/D)+C, where C is a constant is within the range of 0.5 to 1.2.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001] The present invention relates to a magnetic shield device, in particular, a rectangular magnetic thin strip having a rectangular hysteresis characteristic is fixed to a cylinder having both ends open, and a coil is wound therearound. The present invention relates to a magnetic shield device which creates a space which is not affected by an external magnetic field.

[Industrial applications]

[0002]

2. Description of the Related Art It is required in many applications to measure a minute change in a magnetic field without being affected by an external magnetic field. For example, in magnetic measurement, biomagnetic measurement, brain magnetic field measurement, precision physical measurement, etc.
In order to accurately evaluate the performance of a magnetometer, it is necessary to have a space that shields the earth's magnetic field and the environmental magnetic field and minimizes the noise magnetic field.

Conventionally, a magnetic shield device of a type in which a necessary space is completely covered with a magnetic material has been used exclusively. However, since the magnetic shield device completely covers the space, the workability is low, and an object inside the magnetically shielded space is not used. There is a problem that it is not possible to accurately grasp the situation such as the arrangement of the objects.

In order to solve such a problem, a magnetic shield device made of a cylindrical permalloy having both open ends and a magnetic shield device in which a rectangular magnetic ribbon is helically wound around the outer peripheral surface of the cylinder are proposed. Have been. In particular, a cylindrical magnetic shield device having both ends open is used for measuring a weak magnetic field because of its simple structure.

[0005]

In designing the magnetic shield device made of a cylindrical permalloy having both ends open, the ratio S (of the external magnetic field intensity H _O to the leakage magnetic field intensity H _I inside the magnetic shield device) is set as follows. = H _O / H _I ).
Attention has not been paid to the uniformity of the residual magnetic field in the shield. In such a magnetic shield device using a magnetic cylinder having an open both ends, not only the shield ratio but also the uniformity of the residual magnetic field in the shield is an important parameter.

For example, for measurement of an extremely weak magnetic field such as a brain magnetic field measurement, a first-order differential type pickup (called a gradiometer) having an effect of canceling out the influence of a uniform external magnetic field is widely used. Therefore, if there is a gradient in the residual magnetic field in the shield, there is a problem in that the gradient value is sensitive to the differential value and accurate measurement of the magnetic field cannot be performed. Under such circumstances, development of a magnetic shield device having a magnetic cylinder having both ends opened, having a high shield ratio, and having excellent uniformity of the residual magnetic field in the shield has been awaited.

[0007] It is therefore an object of the present invention to provide a cylinder with open ends at both ends so that the work can be carried out accurately and quickly, with a very high shield ratio and excellent uniformity of the residual magnetic field in the shield. The present invention intends to provide a magnetic shield device suitable for biomagnetism measurement, in which magnetic measurement can be performed with high accuracy and accurate measurement of a weak magnetic field is particularly required.

[0008]

According to a first aspect of the present invention, there is provided a magnetic shield device comprising a single cylindrical body made of a magnetic material and having both ends opened, wherein the axial length of the cylindrical body is L, and the diameter of the cylindrical body is L. D, when the thickness of the cylindrical body is t, the relative magnetic permeability of the magnetic material constituting the cylindrical body is μ, and C is a constant in the range of 0.5 to 1.2, the following expression L / D = 1 + log (Μt / D) + C 2.

In the magnetic shield device according to the first aspect of the present invention, the shield ratio is set by setting the relationship between the axial length L of the cylinder and the diameter D so as to substantially satisfy the above-mentioned equation. The uniformity of the residual magnetic field in the shield can be secured while maintaining the value at a high value. That is, the magnetic field that has entered the inside of the cylinder from the open end of the cylinder becomes dense on the axis of the cylinder and becomes sparser toward the cylinder wall. On the other hand, the magnetic field that penetrates the cylindrical wall and penetrates into the cylindrical wall becomes dense at the cylindrical wall, and becomes sparser toward the cylindrical axis.
Thus, the distribution of the magnetic field penetrating from the open end of the cylinder in the radial direction of the cylinder, and the magnetic field penetrating through the cylinder wall,
Similarly, the distribution viewed in the radial direction of the cylinder is opposite to each other, and the larger the axial length L, the larger the magnetic field that passes through the cylindrical wall and leaks into the cylinder.
By appropriately setting the ratio between the axial length L of the cylinder and the diameter D, the residual magnetic field in the shield in the radial direction of the cylinder can be made substantially uniform. The value of the above constant C is
A value in the range of 0.75 to 1.0 is particularly preferable because the uniformity of the residual magnetic field in the shield can be improved to a high degree while the shield ratio is maintained at a high value.

Further, the value of the constant C in the above equation is set to 0.
By limiting the ratio to the range of 5 to 1.2, the condition for obtaining the maximum shield ratio is L / D = 1 + log (μt
/ D), the shield ratio can be maintained at a high value. In the present invention, the value of the constant C is limited to the range of 0.5 to 1.2 as described above. If the value is smaller than 0.5, the shield ratio approaches the maximum value. Uniformity cannot be obtained. When the value of C is larger than 1.2, the shield ratio becomes excessively small and the uniformity of the residual magnetic field in the shield decreases.

Further, the magnetic shield device according to the second aspect of the present invention comprises a plurality of cylinders each made of a magnetic material, open at both ends, and arranged concentrically. The signs of the gradients of the magnetic field in the cylinder in the axial direction and / or in the radial direction are opposite to each other.
Into two groups, and the magnitude of the gradient of the synthetic magnetic field in the cylinder of the cylinder of one group is approximately equal to the magnitude of the gradient of the synthetic magnetic field in the cylinder of the other group of cylinders. It is characterized in that the ratio between the axial length L and the diameter is set.

In the magnetic shield device according to the second aspect of the present invention, the gradient of the magnetic field in the cylinder is one or more cylinders having a positive sign and one or more cylinders having a negative sign. In combination, the magnitude of the gradient of the combined magnetic field in the cylinder of the cylinder exhibiting the positive sign gradient and the magnitude of the gradient of the combined magnetic field in the cylinder of the cylinder exhibiting the negative sign are substantially equalized to cancel each other, and as a whole the cylinder The gradient of the internal magnetic field can be made almost zero. According to the second aspect of the present invention, the shield ratio for each cylinder is determined by the first aspect of the present invention using a single cylinder.
Although it is somewhat smaller than the magnetic shield device due to the characteristics of the above, the overall shield ratio when viewed as a whole multiple cylinders is proportional to the product of the shield ratio of each cylinder, so a sufficiently large shield ratio Can be obtained. Therefore, by appropriately combining a plurality of cylinders according to the respective applications, the gradient of the residual magnetic field in the shield in the radial direction, the gradient of the distribution in the axial direction or the radius can be maintained while maintaining a sufficiently large shield ratio. The gradient of the distribution in both the direction and the axial direction can be substantially zero.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a cylindrical body 1 constituting one embodiment of a cylindrical magnetic shield device with open both ends according to the first feature of the present invention. The cylindrical body 1 is made of a magnetic material such as permalloy. The axial length is L, the diameter is D, the thickness of the cylindrical body is t, and the relative permeability of the magnetic material forming the cylindrical body is μ. In addition, the axial direction of the cylindrical body 51 is x
, The radial direction is y, and the center axis of the cylindrical body 1 is the origin.

FIG. 2 qualitatively shows the principle of operation with respect to a magnetic field applied in the axial direction of the cylindrical body 1. The distribution of the magnetic field is shown by magnetic flux lines. The magnetic field has a property that parallel magnetic flux lines repel each other and try to increase the distance between them. Further, in a magnetic material having a high relative magnetic permeability μ, the repulsive force decreases in inverse proportion to the relative magnetic permeability, so that many magnetic flux lines can be accommodated. As shown in FIG. 2, the magnetic flux lines entering the cylindrical body from the opening of the cylindrical body 1 are sucked into the wall of the cylindrical body 1 made of a magnetic material. The magnetic flux lines near the center are redistributed. As a result, in the vicinity of the opening end, as shown in FIG. 2, the distribution decreases toward the back of the cylindrical body 1,
In the vicinity of the center of the cylindrical body, the interval between the magnetic flux lines is increased. An increase in the distance between the magnetic flux lines means a decrease in the magnetic flux density, which indicates the existence of a shielding effect.

On the other hand, outside the cylindrical body 1, the magnetic flux lines located near the outer wall of the cylindrical body are. It is pushed by a number of magnetic flux lines existing further outside and is sucked into the wall of the cylindrical body 1. When this is viewed from the opening toward the center along the outer wall of the cylindrical body 1, the magnetic flux lines are successively approached to the outer wall of the cylindrical body and are sucked. Such a relationship between the magnetic flux lines and the cylinder 1 is greatly affected by the axial length L of the cylinder. The magnetic flux lines are most concentrated near the center of the cylinder,
When the axial length L of the cylindrical body increases, the degree of concentration increases, so that the entire magnetic flux lines cannot be accommodated in the cylindrical body wall, and eventually leaks to the inside of the cylindrical body. From this, it is easily understood that there is an axial length L that maximizes the shield ratio, and A. Mager has already described the IEEE Tran.
s. Magn. Vol. 6, No. 1, 1970.

As a result of further careful consideration of the distribution of magnetic flux lines inside the cylindrical body 1 described above, by adjusting the axial length L with respect to the diameter D of the cylindrical body, the distribution of the magnetic flux as viewed in the radial direction of the cylindrical body is adjusted. It has been found that the distribution can be made uniform. That is, the magnetic field that has entered the inside of the cylindrical body from the open end of the cylindrical body 1 becomes densest on the axis of the cylindrical body, and becomes sparser toward the wall of the cylindrical body. On the other hand, the greater the axial length L of the magnetic flux lines sucked into the cylindrical body wall from the outside of the cylindrical body 1, the greater the percentage of leakage through the cylindrical body wall and leaking into the cylindrical body. That is, the magnetic flux line distribution inside the cylinder due to the leakage becomes dense on the inner wall side of the cylinder, and becomes sparser toward the central axis. Thus, the cylindrical body 1
The distribution of magnetic flux lines penetrating from the open end of the cylindrical body when viewed in the radial direction of the cylinder and the distribution of magnetic flux lines penetrating through the cylindrical wall and viewed in the radial direction of the cylindrical body are mutually contradictory, and The larger the axial length L, the larger the magnetic field that passes through the cylindrical body wall and leaks into the cylindrical body. Therefore, by appropriately setting the ratio between the axial length L of the cylindrical body and the diameter D, two fields can be obtained. The distribution of magnetic flux lines can be substantially canceled, and the residual magnetic field in the shield in the radial direction of the cylindrical body can be made substantially uniform.

FIG. 3 shows a value (μt / D) obtained by normalizing the product of the relative permeability μ of the magnetic material constituting the cylinder 1 and the wall thickness t of the cylinder with the diameter D of the cylinder. The numerical value of the shield ratio in the axial direction at the center position on the axis of the cylindrical body using the ratio (L / D) of the axial length L to the diameter (L / D) of the cylindrical body 1 as an independent variable. It is. The shield ratio initially increases with L / D, takes a peak value, and then gradually decreases. In the graph of FIG. 3, the dashed line A indicates the connection between the L / D values at which the shield ratio has a peak value when the parameter (μt / D) is changed. A broken line B indicates a position where the distribution of the residual magnetic field in the shield in the radial direction is most uniform when the parameter (μt / D) is changed. The radial distribution of the residual magnetic field in the shield is calculated by the finite element method at the center of the cylinder. As is clear from comparison between these curves A and B, the L / D value at the position where the distribution of the residual magnetic field in the shield is the most uniform is larger than the L / D value at the position where the shield ratio is the maximum. It's a bit big.

In FIG. 3, the value of the ratio between the axial length L and the diameter D of the cylindrical body 1 that maximizes the shield ratio in the axial direction is given by the following equation (1). L / D = 1 + log (μt / D) (1) Although the equation (1) is extremely simplified,
A. Mag in IEEE Trans. Magn. Vol. 6, No. 1, 1970
This is almost the same as that given by the complex approximate analysis formula shown by er. In the present invention, as shown in FIG. 3, the ratio of the axial length L to the diameter D that makes the shield ratio sufficiently high and makes the distribution of the residual magnetic field in the shield uniform is from 0.75 to 1.0. When a constant within the range was obtained, it was found that the following equation (2) was given. L / D = 1 + log (μt / D) + C (2) Here, the reason for setting the value of the constant C in the range of 0.5 to 1.2 is as described above, By setting the range, the reduction from the maximum shield ratio is small, and the uniformity of the radial distribution of the residual magnetic field in the shield can be secured. In particular, the value of the constant C is set to 0.7
The range of 5 to 1.0 is preferable because the uniformity of the residual magnetic field in the shield can be highly improved while maintaining the shield ratio at a high value.

FIG. 4 shows an embodiment of the magnetic shield device according to the second feature of the present invention. In this example, the first
And the second cylindrical bodies 11 and 12 are arranged concentrically. The axial lengths of these first and second cylindrical bodies 11 and 12 are L ₁ and L ₂ , the diameters are D ₁ and D ₂ ,
The thicknesses are t ₁ and t _2, and the relative magnetic permeability of the magnetic material forming the cylindrical body is μ ₁ and μ ₂ . However, in this example,
The thicknesses of the two cylinders 11 and 12 are made equal (t ₁
= T ₂ ) and the same magnetic material (μ ₁ = μ)
₂ ).

In a second aspect of the present invention, these
Remaining in the shield of the first and second cylindrical bodies 11 and 12
Configure the gradient of the magnetic field to have a different sign and
This realizes uniformity of the stay magnetic field. That is, inside
Axial length L of the first cylindrical body 11 ₁And the diameter D₁And the ratio (L
₁/ D₁) And the axial length L of the outer second cylindrical body 12₂When,
Diameter D₂And the ratio (L₂/ D₂) Is the graph shown in FIG.
From both sides of the curve B shown by the broken line
To ensure uniformity of the residual magnetic field in the shield
It is. For example, the ratio between the axial length and the diameter of the first cylindrical body 11
L₁/ D₁Is selected from the area on the right side of the curve B
Is the ratio L between the axial length and the diameter of the second cylindrical body 12₂/ D₂To
What is necessary is just to select from the area | region on the left side of the curve B.

FIGS. 5A and 5B show an outer second cylinder.
12 axis length L₂The axial length L of the inner cylindrical body 11 based on
₁Of the magnetic field distribution in the axial direction x when
And the gradient of the magnetic field distribution in the radial direction y.
It is. In these graphs, curve A represents the outer
The gradient of the magnetic field distribution in the second cylindrical body 12 is shown.
is there. The axial length L of the outer second cylindrical body 12₂Inside
Axial length L of cylindrical body 11 ₁Ratio (L₁/ L₂) To 0.
Inside the shield as it changes to 4, 0.5, 0.6
Although the gradient of the residual magnetic field distribution changes, the direction of the axis shown in FIG.
With respect to the gradient of the magnetic field distribution in the
The gradient of the magnetic field distribution of the first second cylindrical body 1 is positive.
The gradient of the magnetic field distribution of No. 2 is negative.

FIG. 5C shows the first and second cylindrical bodies 11 and
And 12 are arranged concentrically, the resultant magnetic field distribution
It shows the gradient in the axial direction, and the curve C₁And C₂
Is the axial length L of the second cylindrical body 12₂First cylindrical body for
11 axis length L₁The ratio L₁/ L ₂Are 0.5 and
And 0.6 are set respectively. This group
As is evident from the rough, the ratio L₁/ L₂Near 0.6
The gradient of the magnetic field distribution in the axial direction by setting
It can be zero.

FIG. 5D shows the first and second cylindrical bodies 11 and
And 12 are arranged concentrically, the resultant magnetic field distribution
It shows the radial gradient and shows the curve C₁And C₂
Is the axial length L of the second cylindrical body 12₂First cylindrical body for
11 axis length L₁The ratio L₁/ L ₂Are 0.5 and
And 0.6 are set respectively. This group
As is clear from the rough, the ratio L₁/ L₂With 0.5
Set to a value intermediate to 0.6, that is, a value near 0.5
To make the gradient of the magnetic field distribution in the radial direction almost zero.
can do.

As described above, according to the second feature of the present invention, the first and second cylindrical bodies 11 arranged concentrically are provided.
By appropriately setting the axial length ratio L ₁ / L ₂ of the _{first and second} embodiments, the axial gradient or the radial gradient or the axial and radial gradients of the residual magnetic field distribution in the shield can be reduced to almost zero. , The uniformity of the magnetic field remaining in the magnetic field can be realized. In this case, whether to realize axial uniformity, radial uniformity, or both axial and radial uniformity of the remanent magnetic field in the shield depends on each individual. It can be appropriately selected according to the application.

The present invention is not limited to the above-described embodiment, and many modifications and variations are possible. For example, in the above-described second embodiment, a double cylinder structure having two cylinders is used, but three or more cylinders may be arranged concentrically. In this case, which cylinder is to be the first group and which cylinder is to be the second group can be appropriately selected. Whatever combination is selected, the axial length of the cylindrical body is made longer from the inside to the outside. In any case, the uniformity of the magnetic field distribution in the radial direction and the axial direction is of course ensured in the region where all the cylinders overlap.

[0026]

As described above, according to the magnetic shield device of the first aspect of the present invention, the relationship between the axial length L of the cylindrical body and the diameter D is determined by setting the value of the constant C to 0.5 to 1 .2, especially 0, a value in the range of 75 to 1.0, the condition L / D = 1 + log (μt / D) +
By configuring so as to substantially satisfy C, uniformity of the residual magnetic field in the shield can be secured while maintaining the shield ratio at a high value.

Further, according to the magnetic shield device of the second aspect of the present invention, at least one cylindrical body having a gradient of the internal magnetic field having a positive sign and at least one cylindrical body having a negative sign are combined. By canceling the gradient of the synthetic magnetic field inside the cylinder having the sign gradient and the gradient of the synthetic magnetic field inside the cylinder having the negative sign, the gradient of the magnetic field inside the cylinder can be made almost zero as a whole, and the residual in the shield can be reduced. The uniformity of the magnetic field can be realized.

[Brief description of the drawings]

FIG. 1 is a diagram showing the configuration of a first embodiment of a magnetic shield device according to the present invention having a single cylindrical body.

FIG. 2 is a diagram showing distribution of magnetic flux lines inside and outside a cylindrical body.

FIG. 3 is a graph showing a change in a shield ratio in a radial direction with respect to a ratio between an axial length and a diameter of a cylindrical body.

FIG. 4 is a diagram showing the configuration of a second embodiment of the magnetic shield device of the present invention having two cylindrical bodies.

5A to 5D are graphs showing distributions of a residual magnetic field in a shield in a radial direction and an axial direction using a ratio of an axial length of two cylindrical bodies as a parameter.

[Explanation of symbols]

 1,11,12 Cylindrical body

Claims (4)

[Claims] 1. A 1 made of a magnetic material and having both ends opened.
With two cylindrical bodies, the axial length of this cylindrical body is L, the diameter is D,
When the thickness of the cylindrical body is t, the relative magnetic permeability of the magnetic material constituting the cylindrical body is μ, and C is a constant in the range of 0.5 to 1.2, the following expression is used: L / D = 1 + log (μt / D) + C. 2. The magnetic shield device according to claim 1, wherein the value of the constant C is set to a value within a range of 0.75 to 1.0. 3. A plurality of cylinders, each made of a magnetic material, open at both ends and arranged concentrically, said plurality of cylinders being arranged in a cylindrical magnetic field in the axial and / or radial direction of the cylinders. Are divided into two groups in which the signs of the gradients are opposite to each other, and the magnitude of the gradient of the combined magnetic field in the cylinder of the cylinder of one group and the magnitude of the gradient of the combined magnetic field in the cylinder of the other group are substantially the same. A magnetic shield device wherein the ratio of the axial length L and the diameter of each cylindrical body is set to be equal. 4. The magnetic shield device according to claim 3, wherein the first and second concentrically arranged cylindrical bodies are made of the same magnetic material and have the same thickness.