JPH05196711A - Magnetism measuring apparatus - Google Patents

Abstract

PURPOSE:To measure the absolute value of three-dimensional magnetic-field vector highly accurately by using the correcting coefficients, which are obtained by calibration with a reference-magnetism measuring apparatus beforehand, and operating the value with the specified expression. CONSTITUTION:On the three surfaces of a magnetism sensor 10, which are intersected at a right angle, three kinds of large, medium and small SQUIDs (superconductive quantum interference devices) are provided, respectively. The outputs of the SQUIDs are electrically coupled in a cascade pattern. A preamplifier 13 amplifies the outputs of the sensor 10. A sensor driver 14 converts the outputs into the voltage signals and sends the signals into an operator 12. The analog voltage signals from a magnetic-field detector 11 are converted into the digital data with an A/D converter 15. Components Hx, Hy and Hz of the geomagnetism in the axial directions of X, Y and Z are computed by an integrated cascade computing unit 16. The components are sent into a vector-synthesis computing unit 17. The absolute value H of the vector of the magnetic field is computed in this unit 17. At this time, correcting coefficients (a)-(f) are obtained beforehand by using a reference magnetism measuring apparatus and stored in memories 18, 19 and 20 of the operator 12. The correcting coefficients (a)-(f) are used, and the measured values Hx, Hy and Hz are substituted into the expression. Thus, the correct abolute value H of the magnetic-field vector is obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to SQUID (Supercon
The present invention relates to a magnetic measuring device (magnetometer) using a ducting Quantum Interference Device.

[0002]

2. Description of the Related Art SQUID utilizes the property that a magnetic flux enters and exits a superconducting ring having a Josephson junction in units of magnetic flux quantum, and has a characteristic that even a very small magnetic field can be detected. Have. Currently, the SQUIDs are rfSQUID and dcSQUI
There are two types, ID, but both types have the same principle of magnetic field detection, although there are differences in the excitation method.

The SQUID only detects the component of the magnetic field in the direction perpendicular to the ring surface, and cannot detect the magnitude of the magnetic field (absolute value of vector) which is a three-dimensional vector quantity. Therefore, when it is necessary to measure the absolute value of the magnetic field vector, three axes (X, Y, Z) orthogonal to each other are used.
SQUIDs for detecting the magnetic field component in the (axis) direction are provided respectively, and the absolute value of the magnetic field vector is calculated from the detection values Hx, Hy, Hz by each SQUID by the following equation. H = (Hx ² + Hy ² + Hz ² ) ^1/2 (1)

Next, in the magnetic measuring device using the SQUID, since the phenomenon that the magnetic flux goes in and out in the unit of magnetic flux quantum as described above, the magnetic field to be measured basically corresponds to the magnetic flux quantum. It is a deviation from a reference point that exists discretely (discretely) for each unit magnetic field. Therefore, a plurality of types of SQUID rings having different sizes are provided so that the unit magnetic field of the SQUID ring having the smallest area is larger than the absolute value of the magnetic field to be measured (for example, when measuring the earth magnetism, the unit magnetic field is The magnitude of the earth's magnetism should be about 50,000 γ or more.) First, the absolute magnitude of the magnetic field (not the relative value of deviation from the reference point, but the value measured from the state of zero magnetic field. , Not the absolute value of the magnetic field vector,
The magnitude of the component in the direction normal to the surface of the SQUID ring. ) With low sensitivity. Then, the measurement accuracy is increased by the SQUID ring having a large area in order, and finally the outputs of the plurality of SQUIDs are cascade-coupled (described later) to accurately (absolutely) measure the magnetic field. A method of measuring is considered (see Japanese Patent Laid-Open No. 3-33669).

[0005]

As described above, the SQUI
By cascading D, the magnitude of each axial component of the magnetic field can be measured with extremely high accuracy.
When the SQUIDs provided for the X, Y, and Z axes are not strictly perpendicular to each other, the accuracy of the magnitude of the absolute value H of the magnetic field vector obtained by the above equation (1) becomes questionable. In addition, due to factors such as differences in the accuracy of SQUIDs for each axis, or mutual interference between SQUIDs for each axis, even if careful measurement is performed using highly accurate SQUIDs, The accuracy of the absolute value of the magnetic field vector obtained by the calculation is low.

The present invention has been made to solve the above problems, and an object of the present invention is to provide a magnetic measuring device capable of measuring the magnitude of a three-dimensional magnetic field vector with high accuracy. To do.

[0007]

SUMMARY OF THE INVENTION The present invention, which has been made to solve the above-mentioned problems, has at least one SQUID arranged in each of three axial directions which are not parallel to each other, and each SQUID is arranged.
In the magnetic measuring device for detecting the axial direction components Hx, Hy, Hz of the magnetic field vector by D, the absolute values H of the magnetic field vector are corrected coefficients a, b, c, d previously obtained by calibration with the reference magnetic measuring device. , e, using f, H = (a · Hx 2 + b · Hy 2 + c · Hz 2 + d · Hx · Hy + e · Hy · Hz + f · Hz · Hx) 1/2 ... (2) an arithmetic unit for calculating the It is characterized by being provided.

[0008]

The magnetic field component measurement values Hx, Hy, Hz of the respective axes of the magnetic measuring device according to the present invention are substituted in the right side of the above equation (2), and the value H measured by the reference magnetic measuring device is put in the left side. Such calibration is performed at least 6 times by changing the attitude of the magnetic measuring apparatus of the present invention, and the six correction coefficients a, b,
c, d, e, f are calculated in advance. When actually measuring the absolute value (magnitude) of the magnetic field vector with the magnetic measurement device of the present invention, the arithmetic unit calculates the correction coefficients a, b, c, d, e, By using f, the magnetic field component measurement values Hx, Hy, and Hz of each axis are substituted into the above equation (2) to calculate a correct value in which the effects of various error factors are corrected.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An outline of a geomagnetism measuring device for aircraft mounting according to an embodiment of the present invention will be described with reference to FIG. The geomagnetism measuring device of this embodiment is composed of a magnetic sensor 10, a magnetic field detector 11, a calculator 12, a display device (not shown), and the like.

The magnetic sensor 10 is formed by forming SQUIDs on three surfaces of a cubic quartz block having a side length of 20 mm which are orthogonal to each other. SQU of these three sides
The IDs are in the X, Y, and Z directions of the magnetic field (geomagnetism) (however, the XYZ coordinate axes are fixed to the aircraft on which the geomagnetic measurement device is mounted and change with respect to the earth). It measures the components. In this embodiment, dcSQUID is used.

Before explaining the geomagnetism measuring apparatus of this embodiment, the principle of magnetic field measurement by SQUID will be explained. SQ
The UID measures the magnitude of the external magnetic field by utilizing the property that the magnetic flux generated by the external magnetic field can enter and leave the superconducting ring having the Josephson junction only in units of the magnetic flux quantum φ0. When the area of the SQUID ring is S and the magnitude of the external magnetic field (magnetic flux density) in the normal direction of the ring surface is B, the magnitude of the magnetic flux φ input to the ring is represented by φ = B · S. As described above, the magnetic flux φ input to the SQUID ring is discrete with the magnetic flux quantum φ0 (= 2.07 × 10 ^-15 Wb) as a unit, such as φ = φ0 × n (n = 0, ± 1, ± 2, ...). It can only change. Therefore, from B · S = φ0 × n B = (φ0 / S) · n, in the SQUID with the area S, the step of the external magnetic field corresponding to the step of change of the magnetic flux entering and leaving the ring (for each flux quantum φ0) is BUNIT = φ0 / S (this BUNIT is called a unit magnetic field).

When the external magnetic field given to the SQUID is gradually increased from 0, the magnetic flux is prevented from entering the ring.
A superconducting current Is flows through the SQUID ring, and this current Is increases as the external magnetic field increases. However, when this current Is exceeds the critical current value Ic limited by the Josephson junction, the magnetic flux is SQU by one magnetic flux quantum φ0.
Current Is flowing through the ring as it enters the ID ring
Is reduced. When the external magnetic field is further increased, the above process is repeated, and eventually the current flowing through the ring changes periodically with the magnetic flux quantum φ0 as a unit.

When a magnetic field is measured using a dcSQUID (having two Josephson junctions) as in this embodiment, a direct current (bias current) Ib slightly smaller than the critical current Ic is applied to the SQUID ring in advance. deep. The magnetic flux φ input to the ring is exactly an integral multiple of the magnetic flux quantum φ0 (φ =
.phi.0.n), the bias current Ib is the critical current Ic
Because of the following, the ring is in a superconducting state, and no voltage appears on both sides of the junction (solid line in FIG. 3). However, when the magnetic flux φ input to the ring is no longer an integral multiple of the magnetic flux quantum φ0, as described above, the current Is for canceling the input magnetic field exceeding the integral multiple of the magnetic flux quantum φ0 is generated, and one of the Josephson At the junction, (Ib + Is) exceeds the critical current Ic (the case where the input magnetic field is a half integer multiple of the magnetic flux quantum φ0 φ = φ0 · (n + 1/2) is shown by the dotted line in FIG. 3). Therefore, the ring becomes normally conductive, and the voltage V appears on both sides of the Josephson junction. After normal conduction, the voltage generated on both sides of the Josephson junction is a function of the magnitude of the input magnetic field φ that exceeds an integral multiple of the magnetic flux quantum φ0, as seen in the straight lines on both sides of the curve in Fig. 3. Becomes Therefore, the voltage (output voltage) on both sides of the Josephson junction is
As shown in FIG. 4, the input magnetic flux φ changes in a triangular wave shape with the magnetic flux quantum φ0 as a period, as shown in FIG.

Generally, in a magnetic measurement device using SQUID, an exciting coil called a modulation coil 23 is provided near the SQUID ring 22 as shown in FIG.
Magnetic fields having opposite directions and the same magnitude are generated so as to cancel the magnetic field input to the ring 22. That is, SQU
The current flowing through the modulation coil 23 is feedback-controlled so that the magnetic field input to the ID ring 22 is always zero.
The circuit that performs this control is FLL (Flux Locke = Flux Locke
d Loop) circuit, the operation of this circuit is shown in FIG. 2 and FIG.
Will be described. 2 shows in detail the part corresponding to one SQUID of the magnetic sensor 10 and the magnetic field detector 11 of FIG. 1, and FIG. 5 shows an enlarged part of FIG. ..

When an alternating current is passed from the af oscillator 27 constituting the FLL circuit to the modulation coil 23, small amplitude AC magnetic fluxes 30a to 30d (FIG. 5) are applied to the SQUID 22. SQUID2 for this AC magnetic flux 30a to 30d
The response (variation in output voltage V) of 2 is the magnitude of the external magnetic field input to the SQUID 22 (position on the characteristic curve 31).
Depends on That is, when the external magnetic field is on the characteristic curve 31 at a location where (dV / dφ) <0 (a location with a downward slope in FIG. 5), when the AC magnetic flux 30a is superimposed, the output voltage V becomes 32a. Changes in reverse phase. (D
When the AC magnetic flux 30b is superposed at a position where V / dφ)> 0 (a part having an upward slope), the output voltage V becomes 3
2c changes in the same phase. When they are superposed at the peaks above or below the characteristic curve 31, either one of them is folded like 32b or 32d. Therefore, by comparing the alternating current applied to the modulation coil 23 and the phase of the fluctuation of the output voltage of the SQUID, the magnitude of the magnetic field input to the SQUID 22 should be locked so that it is always at the peak or valley of the characteristic curve 31. You can

By detecting the magnitude of the current flowing through the SQUID 22 in the state of being locked at this peak or valley position, the magnitude of the external magnetic field with respect to the lock position (deviation from an integral multiple of the unit magnetic field) Can be measured.

In the present embodiment, three types of S of large, medium and small are respectively provided on the three surfaces of the cubic magnetic sensor 10 which are orthogonal to each other.
A QUID is provided. Among them, the large SQUID has the smallest ring area S and has a characteristic that the sensitivity is rough but the measurement range is wide (BUNIT = φ0 / S). The small SQUID has the largest area S of the ring, and has a characteristic that the measurement range is narrow but the sensitivity is high. Medium SQUIDs have properties in between. In the geomagnetism measuring apparatus of this embodiment, the outputs of the SQUIDs having these three different measurement ranges are electrically cascade-coupled (details will be described later), so that the magnitude of the absolute value of the geomagnetism can be set in a wide range. It is possible to measure with high sensitivity. In addition, the SQUI for each surface
The number of D may be determined according to the required measurement accuracy, 1 type, 2
It may be one kind or four or more kinds.

The magnetic field detector 11 comprises a preamplifier 13 and a sensor driver 14 provided for each SQUID (3 × 3 = 9 in this embodiment) and amplifies the signal from the sensor 10. As a result, it outputs a signal proportional to the input magnetic field and also serves to excite the modulation coil 23 of the magnetic sensor 10.

The signal processing in this embodiment will be described with reference to FIG. The preamplifier 13 is composed of an af amplifier 24, and uses an electronic element of low noise and high amplification degree to amplify a weak electric signal output from the SQUID 22. The sensor driver 14 includes a phase detector 25, a dc amplifier 26, an af oscillator 27, an output resistor 28, and a DC current source 21 with high stability. DC current source 21
Is for supplying the bias current Ib to the SQUID ring 22. The voltage appearing at the junction of the SQUID 22 is amplified by the af amplifier 24, and the phase detector 25
In, the phase of the AC magnetic flux (30a to 30d in FIG. 5) applied from the af oscillator 27 to the modulation coil 23 is compared. The dc amplifier 26 uses the SQUID
A feedback current is applied to the modulation coil 23 so that the operating point of is the peak point. Since this current value is a function of the deviation of the external magnetic field from the reference point, it is converted into a voltage signal by the resistor 28 and sent to the calculator 12.

The calculator 12 calculates the magnitude (absolute value) of the magnetic field vector from the signals of the three-axis orthogonal components (X, Y, Z) of the magnetic field vector. The arithmetic unit 12 is a microcomputer (for example, M which is a 32-bit processor).
C68030), the absolute value of the magnetic field vector is calculated by performing cascade coupling calculation and vector combination calculation on the signals from each SQUID. The analog voltage signal from the magnetic field detector 11 is first converted into digital data by the A / D converter 15 and sent to the cascade coupling calculation unit 16.

The cascade coupling calculation is a calculation in which the outputs of three kinds of SQUIDs of large, medium and small are combined in order in order to detect the components of each axis of the magnetic field in a wide range and with high sensitivity. The method will be described with reference to FIG. As mentioned above
The large SQUID has the smallest loop area S,
Therefore, it has the largest unit magnetic field BUNIT-L. In the magnetic measuring device of the present embodiment, since the geomagnetism is the measurement target, the magnitude of the unit magnetic field BUNIT-L of the large SQUID is the magnitude of the geomagnetism (about 50,000γ.1γ = 10 ^-5 G = 10 ^-9 T). It is set to be the above (for example, to be 70,000γ). Therefore, by measuring the amount of current flowing through the large SQUID, the absolute magnitude of the external magnetic field (ie,
(Size from zero) can be measured. But,
Since the large SQUID has low measurement sensitivity (about 70γ), a value different from the true value of the external magnetic field (geomagnetism) (first step in FIG. 6) may be detected (second step).

Although the detection accuracy of the large SQUID is not so high, the reference point of the signal from the medium SQUID can be determined. That is, the signal from the middle SQUID is B = BUNIT-M × n (BUNIT-M is the middle SQUID.
Unit magnetic field of D. n = 0, ± 1, ± 2, ...) Any of the signals (the accuracy 0.3γ) from the reference operating point (closest to the external magnetic field) is a signal (third stage in FIG. 6). From the detection result of the large SQUID, it is possible to determine to which value of n the reference point of the output of the medium SQUID corresponds. Therefore, by combining the outputs of the large SQUID and the medium SQUID, the absolute magnitude of the external magnetic field (magnitude from zero) can be measured with an accuracy of 0.3γ.

Similarly, a small SQ is output by outputting the medium SQUID.
UID (ring area S is maximum, therefore unit magnetic field BUNIT
It is possible to measure the magnitude of the external magnetic field (geomagnetism) with a sensitivity of 0.001γ by specifying the reference operating point (-S is the minimum) (4th step in Fig. 6) and combining it with the output of the small SQUID. (Bottom row of FIG. 6).

Returning to FIG. 1, the magnitude of the component in each axial direction of the external magnetic field calculated by the cascade coupling calculation unit 16 for each of the X, Y, and Z axes is sent to the vector synthesis calculation unit 17, where The absolute value of the external magnetic field (geomagnetism) vector is calculated. Here, ideally, the absolute value of the external magnetic field | H0 | simply ^{| H0 | = (Hx 2 +} Hy 2 + Hz 2) 1/2 ( where, Hx, Hy, Hz each axial direction of the external magnetic field Component) is calculated, but in reality,

(A) Three small SQUIDs on the X, Y and Z axes
(Determines final sensitivity as this is the most sensitive)
The sensitivity is not uniform (not the same). (B) The three axes of X, Y, and Z are not strictly perpendicular to each other (the SQUID arrangement planes of the cube of the magnetic sensor 10 are not completely perpendicular to each other). (C) There is mutual interference between SQUIDs on each axis. For this reason, correction is necessary when calculating | H0 | from the measured values Hx, Hy, and Hz of each axis.

[0026] including the correction term | H0 | of the calculation equation | H0 | = (a · Hx 2 + b · Hy 2 + c · Hz 2 + d · Hx · Hy + e · Hy · Hz + f · Hz · Hx) 1/2 … (3) Here, a to f are correction parameters, and a,
b and c are correction parameters mainly corresponding to the above cause (a), and d, e and f are correction parameters mainly corresponding to the above causes (b) and (c). These correction parameters a
˜f is calibrated using a standard magnetic measuring device. The method will be described below.

Now, the magnetic sensor 10 of the geomagnetism measuring device to be calibrated is directed in at least six different directions in the earth's magnetism ("6" corresponds to the number of unknown correction parameters a to f), 3-axis magnetic field component value H for each direction
Measure x, Hy and Hz. At the same time, the magnitude of geomagnetism | H0 | The parameters a to f are obtained by substituting the six sets of measurement data into both sides of the above equation (3) and solving the simultaneous equations of six. By storing these parameter values in the memories 18, 19, 30 of the arithmetic unit 12 of the geomagnetism measuring device, the correct absolute value of the geomagnetism can be measured only by the geomagnetic measuring device thereafter.

Next, a method of measuring the absolute value of the earth's magnetism by the reference magnetic measuring device will be described with reference to FIG. The standard magnetic measurement device is a highly accurate standard SQUID
And a cryogenic container for accommodating the same, and a highly accurate reversing mechanism. In order to measure the earth's magnetism with this reference magnetism measuring device, firstly, the reference SQUID (the normal line of the ring surface) is directed in the direction of the earth's magnetism, and then the reference SQUID is turned 180 degrees to be reversed and the position between both positions Difference in magnetic field magnitude 2H
Measure 0. By multiplying this value by 1/2, the absolute value H0 of the geomagnetism can be obtained. Here, since the measurement accuracy of the geomagnetism measuring device is 0.001γ, the reference magnetism measuring device must have at least a measurement accuracy higher than that. Assuming that the absolute value of the earth's magnetism is about 50,000γ, in order to secure the measurement accuracy of 0.001γ, the standard S
The direction of the QUID must be adjusted to the direction of the geomagnetism with an accuracy of 0.6 minutes in angle, and the accuracy of reversal is also the same.
Must be paid within 0.6 minutes. Therefore, the mechanical accuracy of the reversing mechanism of the reference magnetic measurement device (rotation axis and reference SQUID
Of the squareness with the ring surface and the rotation angle positioning accuracy)
Is less than 0.6 minutes.

First, in order to correctly align the reference SQUID with the direction of the geomagnetism, the reference magnetism measuring device uses a highly accurate reference SQUID for geomagnetism measurement (the normal direction of this SQUID ring surface is the Z axis). And two auxiliary SQUIDs are provided in the right-angled direction (X, Y axes). When the output of these auxiliary SQUIDs on the X and Y axes becomes 0, Z
The reference SQUID of the axis correctly points in the direction of the earth's magnetism. When the reference SQUID is inverted, the base of the reference SQUID is inverted over 180 degrees, and the output of the reference SQUID is maximized during that period. In addition, the reference magnetic measurement device is equipped with a device that counts the number of times the unit magnetic field of the reference SQUID is exceeded, which allows the high-accuracy reference SQUID measurement range to exceed ± 70,000γ, which exceeds the magnitude of the earth's magnetism. It is expanding.

Although the dcSQUID is used in the above embodiment, the present invention can be similarly applied to the magnetic measurement device using the rfSQUID. In this case, the magnetic field detector 11 needs to be changed, but the calculator 12 can be used as it is.

The use of the magnetic measuring device utilizing the present invention is not limited to the aircraft mounting, but it can be used for various moving bodies such as automobiles and ships. Further, it may be carried not only by being fixed to the moving body but also by being housed in an enclosure provided with an appropriate handle. Moreover, the measurement target is not limited to the geomagnetism, and the magnitude of the magnetic field due to any cause can be measured.

[0032]

In the magnetic measuring device of the present invention, calibration with a magnetic measuring device serving as a reference is performed in advance to correct error factors such as deviation of orthogonality among three axes, difference in accuracy, and mutual interference. You can always find an accurate value.

[Brief description of drawings]

FIG. 1 is a functional block diagram of a magnetic measurement apparatus that is an embodiment of the present invention.

FIG. 2 is a block diagram showing the configuration of one SQUID and a magnetic field detector corresponding thereto.

FIG. 3 is a graph showing a relationship between an input magnetic flux amount and an output voltage of SQUID.

FIG. 4 is a graph showing current-voltage characteristics of the SQUID ring when the input magnetic flux amount is an integral multiple of a magnetic flux quantum and a half integral multiple thereof.

FIG. 5 is a graph illustrating how the output voltage changes when the detected magnetic flux is superimposed on the input magnetic flux of the SQUID.

FIG. 6 is an explanatory diagram of a method of measuring the magnitude of an external magnetic field by cascade-connecting the outputs of three types of large, medium, and small SQUIDs.

FIG. 7 is an explanatory diagram of a geomagnetism measuring method using a reference magnetism measuring device.

[Explanation of symbols]

10 ... Magnetic sensor 11 ... Magnetic field detector 12 ... Arithmetic unit 13 ... Preamplifier 14 ... Sensor driver 15 ... A / D converter 16 ... Cascade coupling calculation unit 17 ... Vector composition calculation unit 18, 19, 20 ... Correction parameter memory

Claims (1)

[Claims] 1. A magnetic measuring device in which at least one SQUID is arranged in each of three axial directions that are not parallel to each other, and each axial direction component Hx, Hy, Hz of the magnetic field vector is detected by each SQUID. Using the correction coefficients a, b, c, d, e and f obtained by previously calibrating H with the reference magnetic measurement device, H = (a · Hx ² + b · Hy ² + c · Hz ² + d · Hx · Hy + e * Hy * Hz + f * Hz * Hx) ^{1/2 The} magnetic measuring device provided with the calculating device.