WO2015060344A1

WO2015060344A1 - Magnetic field gradient sensor

Info

Publication number: WO2015060344A1
Application number: PCT/JP2014/078090
Authority: WO
Inventors: 笹田　一郎; 翔夢原田
Original assignee: 国立大学法人九州大学
Priority date: 2013-10-22
Filing date: 2014-10-22
Publication date: 2015-04-30
Also published as: JP6472140B2; JPWO2015060344A1

Abstract

A magnetic field gradient sensor (4) is provided with a first sensor head (1) and second sensor head (2) that each have a magnetic core (110, 120) to which excitation AC current and excitation DC current are applied and a detection coil (11, 12) wrapped around the magnetic core and that each output a detection voltage corresponding to a magnetic field in the directions of extension of the sensor head and a sensor circuit (3) that has, applied thereto, the combined voltage of the detection voltage output by the first sensor head and the detection voltage output by the second sensor head and outputs a magnetic field gradient detection signal corresponding to the combined voltage. The first sensor head and second sensor head are disposed so as to be separated and have directions of extension that are coaxial or parallel.

Description

Gradient magnetic field sensor

The present invention relates to a gradient magnetic field sensor.
This application claims priority on the basis of Japanese Patent Application No. 2013-2119016 for which it applied to Japan on October 22, 2013, and uses the content here.

Cardiac magnetic field measurement is an effective means for early detection of heart diseases and the like. Currently, a measurement system using SQUID (Superconducting Quantum Interference Device) is used, but this is not only expensive liquid helium for cooling and large dewar for cooling, but also a shield room This is necessary and requires a lot of introduction and operation costs. This hinders the spread of the magnetocardiogram measurement technique.

On the other hand, research on cardiomagnetic field measurement using various sensors instead of SQUID has been conducted (see Non-Patent Documents 1 to 3). In particular, an electrocardiogram measurement using a fundamental wave type orthogonal fluxgate sensor that does not require any cooling or heating has been attempted (see Non-Patent Document 4). For example, a fundamental wave type orthogonal fluxgate sensor energizes a magnetic wire by applying an AC excitation current (excitation AC current) with a DC current (excitation DC current) larger than the amplitude value as a bias. Thereby, not only Barkhausen noise is greatly reduced but also high sensitivity can be achieved (see Non-Patent Document 5), and a resolution of 1.8 pT / √Hz is obtained at 1 Hz (see Non-Patent Document 6). ).

In addition, various technologies related to the fluxgate sensor have been proposed (see Non-Patent Document 7 to Non-Patent Document 12).

However, in order to fully utilize this sensor, it is necessary to sufficiently shield external magnetic noise. For this reason, it is desired to reconfigure the fluxgate with the configuration of a gradiometer, which is also used in SQUID magnetocardiographs. There has already been a report (see Non-Patent Documents 7 and 8) on a gradiometer that obtains a differential output using a fluxgate sensor, and there is an example applied to pulmonary magnetogram measurement (see Non-Patent Document 9). However, what is reported here is that the output from the two sensors is differentiated by the circuit after synchronous detection, so the resolution cannot be gained, or the single core parallel fluxgate sensor is used as a gradiometer. A slight shift of the coil deteriorates the effect of reducing the uniform magnetic field and is difficult to adjust.

In addition, if a superconducting coil cooled to 4.2 Kelvin (= −269 ° C.) with liquid helium is used, a current proportional to the difference in magnetic field can be generated, and a differential pickup coil using this can be obtained. As is known, the accuracy of the difference is limited by the matching of the shapes of the two circular coils, and H is reduced to about 1/50 to 1/100. Also, the cost for cooling is extremely high.

The present invention provides a gradient magnetic field sensor capable of measuring a weak magnetic field easily and with high sensitivity.

According to the first aspect of the present invention, the gradient magnetic field sensor has a first magnetic core and a first detection coil wound around the first magnetic core, and the first magnetic core is energized for excitation. A first sensor head to which a current and an exciting direct current are applied and which outputs a detection voltage corresponding to a magnetic field in the extending direction; a second magnetic core; and a second detection coil wound around the second magnetic core; A second sensor head that applies an excitation AC current and an excitation DC current to the second magnetic core and outputs a detection voltage corresponding to a magnetic field in the extending direction; and the first sensor head outputs A sensor circuit that receives a composite voltage of a detection voltage and a detection voltage output from the second sensor head and outputs a gradient magnetic field detection signal corresponding to the composite voltage; and the first sensor head and the sensor circuit The second sensor heads are not separated from each other. Et al., With each of the extending direction are arranged to have the same axis or parallel, the detected voltage to be output to the same direction of the magnetic field are connected to cancel each other.

According to the second aspect of the present invention, each of the first detection coil and the second detection coil is connected at one end, and the other end of the first detection coil is connected to the ground. The other end of the detection coil is connected to the sensor circuit.

According to the third aspect of the present invention, the gradient magnetic field sensor is an excitation that can independently change the exciting direct current applied to at least one of the first magnetic core and the second magnetic core. An adjustment unit is further provided.

According to a fourth aspect of the present invention, the sensor circuit is a negative feedback circuit having a feedback resistor, and a displacement of a voltage generated between the feedback resistors in accordance with the combined voltage is detected by the gradient magnetic field detection signal. Output as.

The above-mentioned gradient magnetic field sensor can measure a weak magnetic field easily and with high sensitivity.

It is a figure which shows the function structure of the gradient magnetic field sensor which concerns on 1st Embodiment. It is a figure explaining the adjustment method of the gradient magnetic field sensor which concerns on 1st Embodiment. It is the 1st figure explaining the composition of the sensor head concerning a 1st embodiment. It is a 2nd figure explaining the structure of the sensor head which concerns on 1st Embodiment. It is a 3rd figure explaining the structure of the sensor head which concerns on 1st Embodiment. It is a 1st figure explaining the evaluation method of the gradient magnetic field sensor which concerns on 1st Embodiment. It is a 2nd figure explaining the evaluation method of the gradient magnetic field sensor which concerns on 1st Embodiment. It is a 1st figure explaining the evaluation result of the gradient magnetic field sensor which concerns on 1st Embodiment. It is the 2nd figure explaining the evaluation result of the gradient magnetic field sensor concerning a 1st embodiment. It is a 3rd figure explaining the evaluation result of the gradient magnetic field sensor which concerns on 1st Embodiment. It is a figure explaining the composition of the sensor head concerning a 2nd embodiment. It is a 1st figure explaining the evaluation result of the gradient magnetic field sensor which concerns on 2nd Embodiment. It is a 2nd figure explaining the evaluation result of the gradient magnetic field sensor which concerns on 2nd Embodiment. It is a 3rd figure explaining the evaluation result of the gradient magnetic field sensor which concerns on 2nd Embodiment. It is a figure which shows the function structure of the gradient magnetic field sensor which concerns on contrast. It is a figure explaining the adjustment method of the gradient magnetic field sensor which concerns on the modification of each embodiment.

<First Embodiment>
The gradient magnetic field sensor according to the first embodiment will be described below with reference to the drawings.

FIG. 1 is a diagram illustrating a functional configuration of the gradient magnetic field sensor according to the first embodiment.
As shown in FIG. 1, the gradient magnetic field sensor 4 (gradiometer) according to the first embodiment includes two sensor heads constituting a so-called fundamental mode orthogonal fluxgate sensor head (FM-OFG). 1 and 2 and a fluxgate sensor circuit 3 (sensor circuit).

The sensor head 1 (first sensor head) includes a magnetic core 110 (first magnetic core) and a detection coil 11 (first detection coil). The sensor head 2 (second sensor head) includes a magnetic core 120 (second magnetic core) and a detection coil 12 (second detection coil).
The magnetic core 110 and the magnetic core 120 are configured by, for example, a Co-based amorphous wire formed in a U-shape (or hairpin shape). In another embodiment, the material used for the magnetic core 110 and the magnetic core 120 (magnetic wire) is not limited to this as long as the material has high conductivity and appropriate soft magnetism.
The detection coil 11 is a coil wound around its extending direction (Z axis) so as to wrap around the magnetic core 110. Similarly, the detection coil 12 is a coil wound around the extending direction so as to surround the magnetic core 120.
For example, the detection coil 11 and the detection coil 12 have 1000 turns.

In the present embodiment, the sensor head 1 and the sensor head 2 are arranged so that the extending directions of the magnetic cores (magnetic cores 110 and 120) are on the same axis (Z-axis) line. At this time, the sensor head 1 and the sensor head 2 are spaced apart by a separation distance l (see FIG. 1). The sensor head disposed on the −Z direction side (the lower side of the drawing) is referred to as sensor head 1, and the sensor head disposed on the + Z direction side (the upper side of the drawing) is referred to as sensor head 2.

As shown in FIG. 1, the magnetic core 110 and the magnetic core 120 are connected in series with an AC power supply VEX and a DC power supply E having a value larger than its amplitude. When the AC power source VEX and the DC power source E apply a predetermined AC voltage and DC voltage to the magnetic core 110 and the magnetic core 120 and energize them, the

sensor heads

1 and 2 are excited. As a result, the sensor heads 1 and 2 can output a detection voltage corresponding to the magnetic field along each extending direction, so-called orthogonal fluxgate sensor (fundamental-mode orthogonal-fluxgate (MF-OFG)). ). Thereby, Barkhausen noise can be reduced and the sensitivity of the sensor can be increased.

Moreover, as shown in FIG. 1, each end of the detection coil 11 and the detection coil 12 is connected by electric wiring so as to be connected in series. The other end side of the detection coil 12 is connected to the fluxgate sensor circuit 3, and the other end side of the detection coil 11 is connected to the ground. Further, the detection coil 11 and the detection coil 12 are such that induced voltages (detection voltages V ₁ and V ₂ ) generated with respect to the magnetic field in the same direction cancel each other (so that the polarities of the detection coils 11 and 12 are opposite to each other). Connected. Thereby, for a magnetic field in the same direction, detection output from each of the sensor heads 1 and _{2 as} a combined voltage (sensor output) of the detection voltage V ₁ of the sensor head ₁ and the detection voltage V ₂ of the sensor head 2. A voltage difference (V ₂ −V ₁ ) appears.
In this way, uniform magnetic noise that reaches from a distance is picked up similarly by both the sensor heads 1 and 2 and does not appear in the sensor output. However, since a local magnetic field such as a cardiac magnetic field is picked up only by one sensor head (for example, sensor head 2), it is observed as a sensor output. As a result, uniform magnetic noise can be removed and a signal can be detected, and the anti-noise performance can be improved.

The fluxgate sensor circuit 3 includes a synchronous detection circuit 30 (PSD: Phase Sensitive Detector), a smoothing circuit 31 (smoothing filter), an error amplifier 32 (Error Amplifier), and a low-pass filter 33, and is a negative feedback circuit (non-patented). Reference 10).
Sensor output _V 2 -V ₁ from the

sensor head

1 and 2, capacitor C, the synchronous detection circuit 30: through (PSD Phase Sensitive Detector) and a smoothing circuit 31 (smoothing filter), corresponding to the sensor output _V 2 -V ₁ It becomes a constant voltage and is sent to an error amplifier 32 (Error Amplifier). Thereafter, the feedback current _if flows through the detection coils 11 and 12 through the feedback resistor _Rf so that the input to the error amplifier 32 (sensor output V ₂ −V ₁ ) becomes zero. The voltage displacement (gradient magnetic field detection signal Vo) generated between the feedback resistors _{Rf at} this time corresponds to the sensor output V ₂ −V ₁ . Note that the gradient magnetic field detection signal Vo is output via the low-pass filter 33 in order to reduce the switching ripple generated in the synchronous detection circuit 30.
A negative feedback current i _f, to generate a magnetic field n · i _f in the direction of strengthening the input field H of the magnetic core 110 in the sensor head 1, in the sensor head 2 input field H + field in a direction to cancel the lG n · i _f Is generated.

Here, it is assumed that the magnetic field gradient G is detected in the environment of the uniform magnetic field H when the above-described negative feedback configuration fluxgate sensor circuit 3 is used. Then, the magnitude ΔH1 of the magnetic field detected by the sensor head 1 and the magnitude of the magnetic field ΔH2 detected by the sensor head 2 are expressed as follows. Here, in each of the following equations, “l” is the separation distance 1 between the two

sensor heads

1 and 2, “n” is the winding density n of the detection coils 11 and 12, and “ _if ” is the fluxgate sensor. The feedback current _if from the circuit 3 to the sensor heads 1 and 2 is shown.

Assuming that the detection sensitivities of the sensor head 1 and the sensor head 2 are equal by a predetermined coefficient K, the detection voltage V1 of the sensor head ₁ and the detection voltage V2 of the sensor head 2 are the following from the equations (1) and (2) It is expressed by a formula.

From the expressions (3) and (4), the sensor outputs V ₂ -V ₁ of the detection coils 11 and 12 applied to the fluxgate sensor circuit 3 are as follows.

At this point, it can be seen that the uniform magnetic field H has been removed. In the flux gate sensor circuit 3 having the negative feedback configuration, the negative feedback current _if flows so that the sensor output V ₂ -V ₁ is zero. Thereby, the negative feedback current is derived as follows.

In this case, the gradient magnetic field detection signal Vo, which is the final output of the gradient magnetic field sensor 4, so given from the voltage R _f · i _f according to the feedback resistor R _f, the gradient magnetic field detection signal Vo, the distance l and a magnetic field gradient G Although it is proportional to (1G), it can be seen that it is not affected by the uniform magnetic field H. Thus, it can be confirmed from the formula that the gradient magnetic field G can be detected by canceling the uniform magnetic field H by using the two

sensor heads

1 and 2 as a gradiometer. Since the external magnetic noise can be regarded as a uniform magnetic field H, and the local magnetic field such as a cardiac magnetic field is equivalent to the magnetic field gradient G, signal detection that eliminates the influence of the external magnetic noise by using the gradient magnetic field sensor 4 according to this embodiment. Is possible.

FIG. 2 is a diagram illustrating a method for adjusting the gradient magnetic field sensor according to the first embodiment.
Next, a method for adjusting the sensitivity of the gradient magnetic field sensor 4 according to the first embodiment will be described with reference to FIG.
According to the gradient magnetic field sensor 4 according to the first embodiment, the expression (5) indicates that the influence of the uniform magnetic field H can be eliminated. However, the expression (5) is satisfied because the sensor head 1 and the sensor This is a case where the sensitivity (coefficient K) is completely equal in the head 2. If the sensitivities of the sensor heads 1 and 2 are different, the uniform magnetic field H is not completely canceled and affects the gradient magnetic field detection signal Vo. The actual sensitivities of the sensor heads 1 and 2 slightly differ between the sensor heads 1 and 2 due to individual differences that occur at the time of manufacture even if the excitation conditions of the

magnetic cores

110 and 120 are the same.

In the present embodiment, a method of adjusting the excitation current to the

magnetic cores

110 and 120 is effective. Since the gradient magnetic field sensor 4 according to the present embodiment forms a fundamental wave type orthogonal flux gate, the

magnetic cores

110 and 120 are biased with an AC voltage by the AC power supply VEX and a DC voltage by the DC power supply E, The

heads

1 and 2 are excited. In the fundamental wave type orthogonal flux gate, when the alternating current for excitation (current flowing through application of the alternating voltage of the AC power supply VEX) is constant, the sensitivity (coefficient K) of the sensor heads 1 and 2 is the direct current for excitation ( It is known to have a monotonically decreasing relationship with respect to a current that flows when a DC voltage is applied from the DC power source E).
Therefore, in the gradient magnetic field sensor 4 according to the present embodiment, the AC power supply VEX and the DC power supply E shown in FIG. 1 are actually configured as shown in FIG.

According to the circuit configuration shown in FIG. 2, the AC voltage from the AC power supply VEX is applied to each of the

magnetic cores

110 and 120 connected in series through the resistor R1. As a result, a common alternating current Iac (excitation alternating current) based on the resistors R2 and R3 flows through the

magnetic cores

110 and 120. A DC voltage from the DC power source E is also applied to each of the

magnetic cores

110 and 120 connected in series, whereby a common DC current Idc1 (excitation for excitation) is applied to the

magnetic cores

110 and 120 based on the resistors R2 and R3. DC current) flows.
Further, the DC voltage output from the DC power source E ′ (actually, a battery or the like) is applied only to the magnetic core 120 through the variable resistor Rv. As a result, a direct current Idc2 (exciting direct current) based on the variable resistance Rv further flows in the magnetic core 110.
As described above, the DC power supply E ′ and the variable resistor Rv function as an excitation adjusting unit that can change the excitation DC current applied to the magnetic core 110 of the sensor head 1 independently of the sensor head 2. However, the circuit configuration shown in FIG. 2 is an example of a unit that can independently change the exciting direct current applied to the magnetic core 110 (120) of the sensor head 1 (sensor head 2). It is not limited to the circuit configuration shown. Other circuit configuration examples will be described later (see FIG. 15).

The operator of the gradient magnetic field sensor 4 can independently adjust the excitation of the sensor head 2 by setting the resistance value of the variable resistor Rv as desired. That is, when the sensitivity (coefficient K) of the sensor head 2 is larger than the sensitivity of the sensor head 1, the sensitivity of the sensor heads 1 and 2 is increased by increasing the DC current Idc2 (decreasing the resistance value of the variable resistor Rv). Can be the same. On the contrary, when the sensitivity (coefficient K) of the sensor head 1 is larger than the sensitivity of the sensor head 2, the direct current Idc2 is decreased (the resistance value of the variable resistor Rv is increased), thereby Sensitivity can be the same.

3, 4, and 5 are a first diagram, a second diagram, and a third diagram illustrating the configuration of the sensor head according to the first embodiment, respectively.
FIG. 3 is a photograph showing a configuration example of the sensor head 1. As shown in FIG. 3, the length of the detection coils 11 and 12 in the extending direction (sensor head length L) is, for example, 30 mm. Further, the extension diameter of the detection coils 11 and 12 is, for example, 3 mm.
FIG. 4 shows an arrangement example of the two

sensor heads

1 and 2. In the present embodiment, as shown in FIG. 4, the sensor heads 1 and 2 are arranged on the coaxial line (on the Z-axis, see FIG. 1) with a separation distance l. Specifically, the separation distance l is a distance between the center positions of the sensor heads 1 and 2 and is, for example, 50 mm.
FIG. 5 shows an example of a holder for two

sensor heads

1 and 2. As shown in FIG. 5, two

sensor heads

1 and 2 are stored in a plastic cover so that the two

sensor heads

1 and 2 can be accurately arranged on the same axis (see FIG. 4). In this embodiment, the sensor head 1 is stored in the protrusion arranged on the left side of FIG. 5, and the sensor head 2 is stored in the plastic cover body on the right side of FIG.

The AC power supply VEX is adjusted so that, for example, an AC current having an effective value of 12 mA (excitation AC current) flows at 100 kHz. The direct current power source E is adjusted so that a direct current of about 40 mA flows.

6A and 6B are a first diagram and a second diagram, respectively, for explaining the evaluation method of the gradient magnetic field sensor according to the first embodiment.
The evaluation of the gradient magnetic field sensor 4 uses a Helmholtz coil 5 (see Non-Patent Document 11) in which three sets of circular coils are combined. The Helmholtz coil 5 can generate a uniform magnetic field when the exciting current ie flows in the same direction between the left and right coils (FIG. 6A). In this evaluation example, for example, a uniform magnetic field of 1.0 μT can be generated with an excitation current ie of 3.5 mA.
On the other hand, when the current is passed in the opposite direction, the magnetic field gradient with the central magnetic field between the Helmholtz coils 5 set to 0 can be generated (FIG. 6B). A magnetic field gradient of 2.25 μT / m can be generated with an excitation current ie of 3.5 mA.
In the evaluation, in order to remove the environmental noise component of the uniform magnetic field, the Helmholtz coil 5 is evaluated by flowing an alternating current of 5 Hz, for example. Further, in order to avoid the influence of geomagnetism, the sensor heads 1 and 2 are arranged so that the extending direction (Z axis, see FIG. 1) faces east and west.
Actually, a gradient magnetic field is generated using a Helmholtz coil, and the sensitivity of the gradient magnetic field sensor 4 to the gradient magnetic field is measured. As a result, 5.98 mV / (μT / m) can be obtained at a separation distance of 1 = 5 cm.

FIG. 7 is a first diagram illustrating an evaluation result of the gradient magnetic field sensor according to the first embodiment.
Specifically, in the graph shown in FIG. 7, the horizontal axis indicates the direct current Idc2 that flows only to the sensor head 2, and the vertical axis indicates the suppression ratio (Suppression ratio) corresponding to the direct current Idc2. Yes. Here, the “suppression ratio” refers to a case where the gradient magnetic field sensor 4 (gradiometer) according to the present embodiment is observed with respect to a uniform magnetic field of 1 μT, and a case where the sensor head 2 is removed and observed as a magnetometer. The gradient magnetic field detection signal Vo is a ratio.

The measurement result shown in FIG. 7 shows that each of the three sensor heads (sensor heads Head1, Head2, Head3) prototyped as the sensor head 2 used for the gradient magnetic field sensor 4 has a direct current to the sensor head 2 by the variable resistor R2. It is shown how the response of the sensor heads 1 and 2 to the uniform magnetic field changes as a result of adjusting Idc2 (see FIG. 2).
In the measurement shown in FIG. 7, only the output intensity (amplitude) of the gradient magnetic field detection signal Vo is acquired by FFT (Fast Fourier Transform) processing at 5 Hz. As shown in FIG. 7, all of the prototyped sensor heads Head1 to Head3 have V-shaped characteristics, and the DC current Idc2 is adjusted to a value corresponding to each of the sensor heads Head1 to Head3 within an adjustment range within 2 mA. By adjusting, the suppression ratio can be minimized.

FIG. 8 is a second diagram for explaining the evaluation result of the gradient magnetic field sensor according to the first embodiment.
Specifically, FIG. 8 shows the waveform of the gradient magnetic field detection signal Vo when the sensor head Head1 shown in FIG. 7 is used. As shown in FIG. 7, the optimum condition of the direct current Idc2 in the sensor head Head1 is Idc2 = 0.28 mA. In this case, FIG. 8 shows that the amplitude of the gradient magnetic field detection signal Vo is minimized. It can be seen that the output signal Vo amplitude at Idc2 = 0.28 mA can reduce the output intensity of the gradient magnetic field detection signal Vo based on the uniform magnetic field to 1/42 as compared with the case of Idc2 = 0 mA (no adjustment).

FIG. 9 is a third diagram illustrating the evaluation result of the gradient magnetic field sensor according to the first embodiment.
The table shown in FIG. 9 shows the reciprocal of the suppression ratio (1 / Supp.ratio) when the three sensor heads Head1, Head2, and Head3 are not adjusted (Idc2 = 0), and the sensor heads Head1 to Head3. And the reciprocal of the suppression ratio when the optimal condition is set.
According to FIG. 9, it can be seen that a more harmonious set requires less DC current Idc2. Even if the DC current Idc2 is not adjusted, the suppression ratio is 1/60 to 1/150.

<Second Embodiment>
Next, the case of the gradient magnetic field sensor 4 according to the second embodiment will be described.
FIG. 10 is a diagram illustrating the configuration of the sensor head according to the second embodiment.
In the gradient magnetic field sensor 4 according to the second embodiment, as shown in FIG. 10, the sensor heads 1 and 2 are arranged with a separation distance l (50 mm) so that their extending directions are parallel to each other. Yes.

11, FIG. 12, and FIG. 13 are a first diagram, a second diagram, and a third diagram, respectively, for explaining the evaluation results of the gradient magnetic field sensor according to the second embodiment.
In the graph shown in FIG. 11, the horizontal axis indicates the DC current Idc2 that flows only through the sensor head 2 as in FIG. 7, and the vertical axis indicates the suppression ratio (Suppression ratio) corresponding to the change in the DC current Idc2. Show.

Although the characteristics shown in FIG. 11 are slightly different from those in FIG. 7, even when the extending directions of the sensor heads 1 and 2 are arranged in parallel (second embodiment), they are arranged on the same axis. It can be seen that a high suppression ratio can be obtained in the same adjustment range as in the case (first embodiment).
Further, when the dependency between the uniform magnetic field and the direct current Idc2 is investigated, it has been found that the dependency is not high. For example, when the uniform magnetic field is in the range of 1 μT to 5 μT, the direct current Idc2 for obtaining a high suppression ratio remains only 5%.

FIG. 12 shows the reciprocal of the suppression ratio (1 / Supp.ratio) when the three sensor heads Head1, Head2, and Head3 are not adjusted (Idc2 = 0) and the optimum conditions for each of the sensor heads Head1 to Head3. And the reciprocal of the suppression ratio when set to.
As can be seen from FIG. 12, the gradient magnetic field sensor 4 according to the second embodiment can also obtain a suppression ratio in the same range as in the first embodiment. This means that the gradient magnetic field sensor 4 according to the present embodiment is highly resistant to magnetic noise coming from a remote place. In particular, in the case of this embodiment, a small magnetic source such as a magnetic piece can be sensed by reducing the separation distance l to 1 cm or less. However, in this case, the sensor heads 1 and 2 need to be arranged in a range of 1 cm to 5 mm or less with respect to the magnetic piece.

FIG. 13 shows the result of measuring the noise spectral density of the three prototype sensor heads Head1 to Head3 in the gradient magnetic field sensor 4 according to the second embodiment (separation distance l = 5 cm).

As described above, according to the gradient magnetic field sensor 4 according to the first embodiment and the second embodiment described above, the influence of the uniform magnetic field can be greatly reduced. For example, by appropriately adjusting the exciting direct current (DC current Idc2) for one sensor head, a suppression ratio of 1/40000 can be obtained at the maximum. Such adjustment is extremely simple because it is only necessary to change the exciting DC current (specifically, the variable resistor Rv (see FIG. 2)) while monitoring the sensor output (gradient magnetic field detection signal Vo). Can be done.
As described above, according to the gradient magnetic field sensor 4 according to the above-described embodiment, a weak magnetic field can be measured easily and with high sensitivity.

FIG. 14 is a diagram illustrating a functional configuration of the gradient magnetic field sensor according to the proportionality.
As an example of a conventional gradient magnetic field detection method, as in the gradient magnetic field sensor 9 shown in FIG. 14, two independent sensor heads 1 and 2 are arranged at a separation distance l, and the gradient magnetic field is determined from the difference between the detected voltages. Find by subtraction through working amplifier.
In the case of this method, it is necessary to prepare the fluxgate sensor circuit 3 for each of the sensor heads 1 and 2, so that not only the circuit scale is doubled but also the sensitivity of the sensor heads 1 and 2 is not matched. Not only the gradient magnetic field but also a part of the magnetic field at that point is detected. If the signal magnetic field is weak, the influence of the noise magnetic field becomes large and measurement becomes impossible. Further, if H >> l · G (H: uniform magnetic field, G: magnetic field gradient in the direction of the separation distance l), the sensor heads 1 and 2 are sensitive so that the output is not saturated with respect to the large uniform magnetic field H. Must be set small. As a result, l · G cannot be measured with high sensitivity.
The gradient magnetic field sensor 4 according to each of the above-described embodiments solves the above problem by canceling the uniform magnetic field H at the previous stage of the fluxgate sensor circuit 3, that is, at the stage of the sensor heads 1 and 2. Since the gradient magnetic field sensor 4 has only one fluxgate sensor circuit 3 as shown in FIG. 1, the circuit scale can be reduced.

<Modification of the first and second embodiments>
FIG. 15 is a diagram illustrating a method for adjusting a gradient magnetic field sensor according to a modification of each embodiment.
In FIG. 2, the gradient magnetic field sensor 4 according to the first and second embodiments includes an excitation adjustment unit that can independently change the excitation DC current applied to at least one of the magnetic core 110 and the magnetic core 120. It has been described that the DC power supply E ′ and the variable resistor Rv) are included.
On the other hand, as a modification of the first and second embodiments, for example, an excitation adjustment unit may be realized with a circuit configuration as shown in FIG.

Specifically, as shown in FIG. 15, the magnetic core 110 includes an alternating current Iac2 (exciting alternating current) based on the alternating current power supply VEX, a direct current Idc2 (exciting direct current) based on the direct current power supply E1, and Flows. The AC current Iac2 flows from the AC power supply VEX to the magnetic core 110 through the buffer amplifier Buf, the variable resistor Rv1, and the capacitor. On the other hand, the direct current Idc2 flows from the direct current power source E1 to the magnetic core 110 via the variable resistor Rv2.
Thereby, the operator of the gradient magnetic field sensor 4 can independently change the alternating current Iac2 based on the resistance value of the variable resistor Rv1, and can independently change the direct current Idc2 based on the resistance value of the variable resistor Rv2. Can be changed.

In addition, an AC current Iac1 (excitation AC current) based on the AC power supply VEX common to the magnetic core 110 and a DC current Idc1 (excitation DC current) based on the DC power supply E2 flow through the magnetic core 110. The AC current Iac1 flows from the AC power supply VEX to the magnetic core 120 through the buffer amplifier Buf, the variable resistor Rv3, and the capacitor. On the other hand, the direct current Idc1 flows from the direct current power source E2 to the magnetic core 120 via the variable resistor Rv4.
Thereby, the operator of the gradient magnetic field sensor 4 can independently change the AC current Iac1 based on the resistance value of the variable resistor Rv3, and can independently change the DC current Idc1 based on the resistance value of the variable resistor Rv4. Can be changed.

According to the gradient magnetic field sensor 4 according to this modification, all of the AC current Iac2, the DC current Idc2, and the AC current Iac1 and DC current Idc1 flowing through the sensor head 1 are independently adjusted. be able to. Thus, by generalizing the adjustment of the excitation AC current and the excitation DC current, based on various combinations of the excitation AC current and the excitation DC current (that is, the resistance values of the variable resistors Rv1 to Rv4). The suppression ratio can be further reduced.

Although several embodiments of the present invention have been described above, these embodiments are presented as examples and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the scope of the invention. These embodiments and modifications thereof are included in the invention described in the claims and equivalents thereof, as long as they are included in the scope and gist of the invention.

1 Sensor head (first sensor head)
2 Sensor head (second sensor head)
11 Detection coil (first detection coil)
12 Detection coil (second detection coil)
110 Magnetic core (first magnetic core)
120 Magnetic core (second magnetic core)
3 Fluxgate sensor circuit (sensor circuit)
30 Synchronous detection circuit 31 Smoothing circuit 32 Error amplifier 33 Low-pass filter 4 Gradient magnetic field sensor 5 Helmholtz coil 9 Gradient magnetic field sensor Rv, Rv1, Rv2, Rv3, Rv4 Variable resistance (excitation adjustment unit)

Claims

A first magnetic core and a first detection coil wound around the first magnetic core, wherein an exciting alternating current and an exciting direct current are applied to the first magnetic core, and an extension direction magnetic field is applied to the first magnetic core; A first sensor head that outputs a corresponding detection voltage;
A second magnetic core and a second detection coil wound around the second magnetic core, wherein an excitation AC current and an excitation DC current are applied to the second magnetic core, and an extension magnetic field is applied to the second magnetic core. A second sensor head for outputting a corresponding detection voltage;
A sensor circuit that receives a combined voltage of a detection voltage output from the first sensor head and a detection voltage output from the second sensor head and outputs a gradient magnetic field detection signal corresponding to the combined voltage;
With
The first sensor head and the second sensor head are
Gradient magnetic field sensors are arranged so that their extending directions are on the same axis or parallel to each other while being spaced apart from each other, and are connected so that the detection voltages output to the magnetic field in the same direction cancel each other. .
One end of each of the first detection coil and the second detection coil is connected, the other end of the first detection coil is connected to the ground, and the other end of the second detection coil is connected to the sensor circuit. The gradient magnetic field sensor according to claim 1.
The gradient magnetic field sensor according to claim 1, further comprising an excitation adjustment unit that can independently change the exciting DC current applied to at least one of the first magnetic core and the second magnetic core. .
The sensor circuit is
4. The negative feedback circuit having a feedback resistor, wherein a displacement of a voltage generated between the feedback resistors in accordance with the combined voltage is output as the gradient magnetic field detection signal. 5. Gradient magnetic field sensor.