MAGNETIC FIELD CALIBRATION DEVICE AND METHOD OF CALIBRATING MAGNETISM MEASUREMENT DEVICE USING THE SAME

Abstract

A magnetic field calibration device is used to calibrate a magnetism measurement device having a plurality of magnetic sensors and includes a first holder having a first holding surface, a second holder having a second holding surface having a fixed relative positional relation with the first holding surface, and magnetism generating parts fixed to the first holding surface and the second holding surface. Thus, calibration can be completed with a single operation by assigning the first and second holding surfaces of the magnetic field calibration device respectively to the first and second measurement surfaces of the magnetism measurement device. In addition, since the relative positional relation between the first and second holding surfaces is fixed, measurement results obtained from the individual measurement surfaces match each other.

Claims

1. A magnetic field calibration device used to calibrate a magnetism measurement device having a plurality of magnetic sensors, the magnetic field calibration device comprising: a first holder having a first holding surface; a second holder having a second holding surface having a fixed relative positional relation with the first holding surface; at least one first magnetism generating part fixed to the first holding surface; and at least one second magnetism generating part fixed to the second holding surface.

2. The magnetic field calibration device as claimed in claim 1, wherein each of the first and second magnetism generating parts includes a first coil wound in a first axis direction, a second coil wound in a second axis direction perpendicular to the first axis direction, and a third coil wound in a third axis direction perpendicular to both the first axis direction and second axis direction.

3. The magnetic field calibration device as claimed in claim 1, wherein a plurality of the first magnetism generating parts are fixed to the first holding surface of the first holder, and a plurality of the second magnetism generating parts are fixed to the second holding surface of the second holder.

4. The magnetic field calibration device as claimed in claim 3, wherein the plurality of first magnetism generating parts are arranged in an array, and wherein the plurality of second magnetism generating parts and some of the plurality of magnetism generating parts are arranged in an array.

5. The magnetic field calibration device as claimed in claim 1, wherein the first and second holding surfaces are perpendicular to each other.

6. A calibration method of a magnetism measurement device having first and second measurement surfaces each provided with a plurality of magnetic sensors, the calibration method comprising: generating magnetic fields from the first and second magnetism generating parts in a state where the magnetic field calibration device as claimed in claim 1 is fixed to the magnetism measurement device such that the first holding surface of the first holder faces the first measurement surface and that the second holding surface of the second holder faces the second measurement surface to calibrate the plurality of magnetic sensors.

7. The magnetic field calibration device as claimed in claim 2, wherein a plurality of the first magnetism generating parts are fixed to the first holding surface of the first holder, and a plurality of the second magnetism generating parts are fixed to the second holding surface of the second holder.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0018] FIG. 1 is a schematic perspective view illustrating the outer appearance of a magnetic field calibration device 1 according to a preferred embodiment of the present invention.

[0019] FIG. 2 is an xy plan view of the magnetic field calibration device 1 as viewed in the z-direction.

[0020] FIG. 3 is a yz plan view of the magnetic field calibration device 1 as viewed in the x-direction.

[0021] FIG. 4 is a schematic perspective view for explaining the structure of the magnetism generating part M.

[0022] FIG. 5 is a schematic perspective view illustrating the outer appearance of a magnetism measurement device 2 which is subjected to calibration by the magnetic field calibration device 1.

[0023] FIG. 6 is a schematic perspective view illustrating a state where the magnetic field calibration device 1 is used to calibrate the magnetism measurement device 2.

[0024] FIG. 7 is a schematic diagram illustrating a first modification.

[0025] FIG. 8 is a schematic diagram illustrating a second modification.

[0026] FIG. 9 is a schematic diagram illustrating a third modification.

[0027] FIG. 10 is a schematic diagram illustrating a fourth modification.

[0028] FIG. 11 is a schematic diagram illustrating a fifth modification.

[0029] FIG. 12 is a schematic diagram illustrating a sixth modification.

[0030] FIG. 13 is a schematic diagram illustrating a seventh modification.

MODE FOR CARRYING OUT THE INVENTION

[0031] Preferred embodiments of the present invention will be explained below in detail with reference to the accompanying drawings.

[0032] FIG. 1 is a schematic perspective view illustrating the outer appearance of a magnetic field calibration device 1 according to a preferred embodiment of the present invention. FIG. 2 is an xy plan view of the magnetic field calibration device 1 as viewed in the z-direction, and FIG. 3 is a yz plan view of the magnetic field calibration device 1 as viewed in the x-direction.

[0033] The magnetic field calibration device 1 according to the present embodiment is used in calibration for a magnetism measurement device and has, as illustrated in FIGS. 1 to 3, a first holder 10, a second holder 20, and a plurality of magnetism generating parts M fixed to a first holding surface S1 of the first holder 10 and a second holding surface S2 of the second holder 20. The first and second holders 10 and 20 each have a plate-like shape made of a nonmagnetic material such as acrylic and are fixed to each other. The first holding surface S1 of the first holder 10 constitutes the xy plane, and the second holding surface S2 of the second holder 20 constitutes the yz plane.

[0034] The first holding surface S1 of the first holder 10 has nine magnetism generating parts M.sub.111, M.sub.121, M.sub.131, M.sub.211, M.sub.221, M.sub.231, M.sub.311, M.sub.321, and M.sub.331 fixed thereto. The second holding surface S2 of the second holder 20 has six magnetism generating parts M.sub.312, M.sub.322, M.sub.332, M.sub.313, M.sub.323, and M.sub.333 fixed thereto. The subscript xyz added to each magnetism generating part M indicates x-, y-, and z-coordinate positions. In the present specification, when there is no particular need to make a distinction between the plurality of magnetism generating parts M, they are collectively referred to as “magnetic generating part M” or “magnetic generating parts M”.

[0035] As illustrated in FIG. 2, the nine magnetism generating parts M.sub.111, M.sub.121, M.sub.131, M.sub.211, M.sub.221, M.sub.231, M.sub.311, M.sub.321, and M.sub.331 fixed to the first holding surface S1 constitute a first group G1 arranged in an array on the xy plane. The nine magnetism generating parts M constituting the first group G1 are the same in z-coordinate and different at least in one of x- and y-coordinates from one another. As illustrated in FIG. 3, the three magnetism generating parts M.sub.311, M.sub.321, and M.sub.331 fixed to the first holding surface S1 and six magnetism generating parts M.sub.312, M.sub.322, M.sub.332, M.sub.313, M.sub.323, and M.sub.333 fixed to the second holding surface S2 constitute a second group G2. The nine magnetism generating parts M constituting the second group G2 are the same in x-coordinate and different at least in one of y- and z-coordinates from one another. Thus, the three magnetism generating parts M.sub.311, M.sub.321, and M.sub.331 fixed to the first holding surface S1 belong to both the first and second groups G1 and G2.

[0036] FIG. 4 is a schematic perspective view for explaining the structure of the magnetism generating part M.

[0037] As illustrated in FIG. 4, the magnetism generating part M has a spherical bobbin 30, first, second, and third coils Cx, Cy, and Cz wound around the bobbin 30, and eight leg parts 31 to 38 connected to the bobbin 30. The first coil Cx is wound with the coil axis thereof directed along the x-axis direction, the second coil Cy is wound with the coil axis thereof directed along the y-axis direction, and the third coil Cz is wound with the coil axis thereof directed along the z-axis direction. The first, second, and third coils Cx, Cy, and Cz are independent of one another and can generate desired magnetic fields in the three-axis directions when applied with a current. Some of the leg parts 31 to 38 have screw holes 39 for fixing the magnetism generating part M to the first holder 10 or second holder 20.

[0038] FIG. 5 is a schematic perspective view illustrating the outer appearance of a magnetism measurement device 2 which is subjected to calibration by the magnetic field calibration device 1.

[0039] The magnetism measurement device 2 illustrated in FIG. 5 has a first measurement surface 41 constituting the xy plane and a second measurement surface 42 constituting the yz plane. On each of the first and second measurement surfaces 41 and 42, sensor heads of a plurality of magnetic sensors 50 are arranged in an array. Upon actual use of the magnetism measurement device 2, an object to be measured, e.g., a subject is placed in an area facing the first and second measurement surfaces 41 and 42, and a magnetic field is measured using the plurality of magnetic sensors 50 arranged in an array on the first measurement surface 41 and the plurality of magnetic sensors 50 arranged in an array on the second measurement surface 42. The magnetic sensor 50 has one or two or more magneto-sensitive elements which are MR sensors, for example. As the MR sensor, for example, a GMR sensor, a TMR sensor, and an AMR sensor may be used. Further, a fluxgate sensor and a magneto-impedance sensor may be used as an alternative to the MR sensor.

[0040] On the first measurement surface 41 of the magnetism measurement device 2, the sensor heads of the magnetic sensors 50 are arranged in an array in the xy-direction; on the second measurement surface 42 of the magnetism measurement device 2, the sensor heads of the magnetic sensors 50 are arranged in an array in the yz-direction. The use of the thus configured two measurement surfaces 41 and 42 allows a weak magnetic field generated from the object to be measured.

[0041] Although the position, inclination, and sensitivity of each magnetic sensor 50 are known to some degree at the manufacturing stage, the actual position, inclination, and sensitivity may include variations. Thus, when the magnetic field generated from the object to be measured is very weak, the variations result in large measurement error. Therefore, upon actual use of the magnetism measurement device 2, a calibration operation needs to be performed in advance. The magnetic field calibration device 1 according to the present embodiment is used for such a purpose.

[0042] FIG. 6 is a schematic perspective view illustrating a state where the magnetic field calibration device 1 is used to calibrate the magnetism measurement device 2.

[0043] As illustrated in FIG. 6, when calibration is performed, the magnetic field calibration device 1 is fixed to the magnetism measurement device 2 such that the first holding surface S1 of the first holder 10 faces the first measurement surface 41 of the magnetism measurement device 2 and that the second holding surface S2 of the second holder 20 faces the second measurement surface 42 of the magnetism measurement device 2. The magnetic field calibration device 1 need not strictly be positioned with respect to the magnetism measurement device 2 and may be fixed to any position as long as the relative positional relation between the magnetic field calibration device 1 and the magnetism measurement device 2 does not change during calibration.

[0044] Thus, magnetic fields generated from the nine magnetism generating parts M.sub.111, M.sub.121, M.sub.131, M.sub.211, M.sub.221, M.sub.231, M.sub.311, M.sub.321, and M.sub.331 constituting the first group G1 are mainly detected by the plurality of magnetic sensors 50 arranged on the first measurement surface 41, and magnetic fields generated from the nine magnetism generating parts M.sub.311, M.sub.321, M.sub.331, M.sub.312, M.sub.322, M.sub.332, M.sub.313, M.sub.323, and M.sub.333 constituting the second group G2 are mainly detected by the plurality of magnetic sensors 50 arranged on the second measurement surface 42.

[0045] In calibration, current is made to flow individually in the coils Cx, Cy, and Cz included in each magnetism generating part M, magnetic fields generated at this time are measured by the magnetic sensors 50, whereby magnetic field data is acquired. The above operation is performed for each of the coils Cx, Cy, and Cz included in each magnetism generating part M, and the least square method is used to solve an inverse problem based on the acquired magnetic field data, whereby calibration for each magnetic sensor 50 is achieved.

[0046] Specifically, assuming that the output of the magnetic sensor 50 is V.sub.meas and that a magnetic field generated at the position of the magnetic sensor by each coil is B.sub.meas,

[0047] B.sub.meas=g.Math.V.sub.meas is satisfied, where g is the sensitivity of the magnetic sensor 50. The position, inclination, and sensitivity of each magnetic sensor 50 is known to some degree, so that an estimated magnetic field B.sub.cal that will be given from each coil to each magnetic sensor 50 can be calculated by circle integration based on the position (x, y, z) from each coil, inclination (θ, ϕ), and sensitivity (G). The inclination θ indicates an angle in the z-direction about the x-axis, and the inclination ϕ indicates an angle in the y-direction about the x-axis. Then, by performing calculation using the least square method so as to minimize the value E in the following expressions (1), the actual position (x, y, z) and inclination (θ, ϕ) of the magnetic sensor 50 can be calculated.

[00001]$\begin{array}{cc}\left[\mathrm{Numeral}1\right]& \\ E=1-\frac{{\left(B_{\mathrm{cal}}.Math.V_{\mathrm{meas}}\right)}^2}{{.Math.B_{\mathrm{cal}}.Math.}^2{.Math.V_{\mathrm{meas}}.Math.}^2}& \left(1\right)\end{array}$

[0048] Further, by solving the following expression (2), the actual sensitivity g of the magnetic sensor 50 can be calculated.

[00002]$\begin{array}{cc}\left[\mathrm{Numeral}2\right]& \\ g=\frac{{.Math.B_{\mathrm{cal}}.Math.}^2}{B_{\mathrm{cal}}.Math.V_{\mathrm{meas}}}& \left(2\right)\end{array}$

[0049] After the position, inclination, and sensitivity of each magnetic sensor 50 are thus acquired, the actual measurement values are corrected based on the obtained values, whereby accurate magnetism measurement can be performed.

[0050] As described above, the magnetic field calibration device 1 according to the present embodiment has a structure in which the first holder 10 having the first holding surface S1 constituting the xy plane and the second holder 20 having the second holding surface S2 constituting the yz plane are fixed to each other, and the first and second holding surfaces S1 and S2 each have the plurality of magnetism generating parts M, thereby allowing a calibration operation to be performed at a time for the magnetism measurement device 2 having the first measurement surface 41 constituting the xy plane and the second measurement surface 42 constituting the yz plane. In addition, the first and second holders 10 and 20 are fixed to each other, so that the measurement result obtained by the first measurement surface 41 and that obtained by the second measurement surface 42 match each other, that is, no error occurs between them.

[0051] Further, in the magnetic field calibration device 1 according to the present embodiment, the three magnetism generating parts M.sub.311, M.sub.321, and M.sub.331 fixed to the first holding surface S1 belong to both the first and second groups G1 and G2, allowing the number of components to be reduced.

[0052] At least six coils are required in order to determine the six parameters of the position (x, y, z), inclination (θ, ϕ), and sensitivity (G). Thus, as long as the magnetism generating part M has the three coils Cx, Cy, and Cz, and magnetic fields generated from the coils can be detected by each magnetic sensor 50, it is possible to determine the above six parameters by providing at least one magnetism generating part M on each of the first and second holding surfaces S1 and S2.

[0053] Although the first and second holders 10 and 20 are directly fixed to each other in the above embodiment, they may be indirectly fixed to each other through a connecting member 60 as in a first modification illustrated in FIG. 7.

[0054] Further, although the first holding surface S1 of the first holder 10 and the second holding surface S2 of the second holder 20 are perpendicular to each other in the above embodiment, they need not be perpendicular as in a second embodiment illustrated in FIG. 8. Although the angle formed by the first holding surface S1 and second holding surface S2 is obtuse in the example of FIG. 8, it may be acute. Further, the first and second holding surfaces S1 and S2 may be parallel to each other as in a third modification illustrated in FIG. 9. In the example of FIG. 9, both the first and second holding surfaces S1 and S2 constitute the yz plane.

[0055] Further, although both the first and second holding surfaces S1 and S2 are formed into a flat surface in the above embodiment, one or both of the first and second holding surfaces S1 and S2 may be formed into a curved surface as in a fourth modification illustrated in FIG. 10. In the example of FIG. 10, the first holding surface S1 is formed into a curved surface, and the second holding surface S2 is formed into a flat surface. Further, as in a fifth modification illustrated in FIG. 11, the first holder 10 may have both the first and second holding surfaces S1 and S2 which are formed in a continuous manner.

[0056] Further, although the magnetism generating parts M are provided on the first and second holding surfaces S1 and S2 in the above embodiment, while the magnetic generating parts M may be provided on three surfaces as in a sixth modification illustrated in FIG. 12. In the example of FIG. 12, the first holding surface S1 constitutes the xy plane, and the second and third holding surfaces S2 and S3 constitute the yz plane. Further, as in a seventh modification illustrated in FIG. 13, the first holding surface S1, the second holding surface S2, and the third holding surface S3 may constitute the xy plane, yz plane, and xz plane, respectively. Further, although not illustrated, the magnetic field calibration device may have four or more surfaces to each of which the magnetism generating parts M are fixed.

[0057] It is apparent that the present invention is not limited to the above embodiments, but may be modified and changed without departing from the scope and spirit of the invention.

REFERENCE SIGNS LIST

[0058] 1 magnetic field calibration device [0059] 2 magnetism measurement device [0060] 10 first holder [0061] 30 second holder [0062] 31-38 leg part [0063] 39 screw hole [0064] 41, 42 measurement surface [0065] 50 magnetic sensor [0066] 60 connecting member [0067] Cx, Cy, Cz coil [0068] G1 first group [0069] G2 second group [0070] M magnetism generating part [0071] S1 first holding surface [0072] S2 second holding surface [0073] S3 third holding surface