JP2017062122A - Magnetic field detector - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a magnetic field detector capable of accurately detecting a magnetic field without using a magnetic shield.SOLUTION: A magnetic field detector comprises a pair of a measurement magnetic sensor 14a and a reference magnetic sensor 14b for detecting magnetism, and detects a magnetic field by differentially operating outputs of the pair of magnetic sensors 14a, 14b by use of an operation amplifier 44. The magnetic field detector further comprises: a feedback coil 20 capable of applying a common magnetic field Bf to the pair of magnetic sensors 14a, 14b; and a feedback circuit 60 for supplying an electric current to the feedback coil 20 based on an output Vout_b of the reference magnetic sensor 14b of the pair of magnetic sensors 14a, 14b. When the detector differentially operates the outputs of the pair of magnetic sensors 14a, 14b so as to detect the magnetic field ΔB of a measured object, the detector can detect the magnetic field at the pair of magnetic sensors 14a, 14b by considering the magnetic field Bf applied by the feedback coil 20.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a magnetic field detection device, and more particularly to a magnetic field detection device that can detect a minute magnetic field with high accuracy.

In recent years, there has been a demand for a magnetic field detection device capable of detecting a magnetic field with high accuracy, for example, high resolution, such as detection of a magnetic signal (magnetic field) generated by a living cell. In such a situation, for example, when an external magnetic field is applied to a SQUID (superconducting quantum interference device) or an amorphous wire (wire-shaped amorphous alloy) through which a high-frequency current is applied, the impedance changes (MI; Magneto-Impedance). ) Magnetic impedance sensors utilizing the effect have been proposed. For example, the magnetic sensor described in Patent Document 1 is an example of a magnetic impedance sensor. According to such a magnetic impedance sensor, a minute magnetic field change can be grasped as an impedance change of the amorphous wire, so that a magnetic field detection with high resolution can be performed.

  On the other hand, when measuring a measurement object that generates a minute magnetic field, it is necessary to reduce the influence of an external magnetic field such as geomagnetism in addition to using a sensor with high resolution as described above. As a method for reducing the influence of the external magnetic field, a method using a shield device has been conventionally used. This shield device aims to shield the external magnetic field as much as possible in the shield device and reduce its influence by surrounding the sensor with a shield made of a ferromagnetic material such as permalloy, for example. It is. In addition to the shield device, a so-called active shield device has been proposed which aims to further reduce the influence of the external magnetic field in the shield body by applying a magnetic field having an opposite phase to the external magnetic field in the vicinity of the shield device. . Patent Document 2 is an example of such an active shield device.

International Publication No. 2005/019851 JP 2002-094280 A

  By the way, in order to configure the above-described shield device, for example, it is necessary to provide a shield having a thickness capable of shielding magnetism, so that the entire device becomes large-scale or expensive. There was a problem.

  The present invention has been made in the background of the above circumstances, and the object of the present invention is to measure a magnetic field to enable so-called casual sensing, which can accurately measure a minute magnetic field by a simpler method. To provide an apparatus.

  In order to achieve this object, the invention according to claim 1 includes (a) a pair of magnetic sensors for detecting magnetism, and (b) detecting the magnetic field by differentially outputting the outputs of the pair of magnetic sensors. And (c) a feedback coil capable of applying a magnetic field common to the pair of magnetic sensors, and (d) based on an output signal of one of the pair of magnetic sensors. And a feedback circuit for supplying a current to the feedback coil.

  According to the first aspect of the present invention, a current is supplied to the feedback coil based on an output signal of one of the pair of magnetic sensors by the feedback circuit, and the pair of magnetic sensors is fed by the feedback coil. A common magnetic field is applied to the pair of magnetic sensors. Therefore, when the magnetic field is detected by differentially outputting the outputs of the pair of magnetic sensors, the magnetic field applied by the feedback coil is taken into account in the pair of magnetic sensors. Can be detected.

  Here, preferably, the feedback circuit determines the current so that an output of the one magnetic sensor becomes zero. In this way, when the outputs of the pair of magnetic sensors are operated, the output of the one magnetic sensor becomes 0, so that the influence of the external magnetic field can be reduced and the sensor is not magnetically saturated. Since the measurement can be performed in a range of a strong magnetic field strength, the resolution can be improved.

  Further preferably, a gradiometer is configured by using the pair of magnetic sensors. In this way, when the magnetic field generation source to be measured is close to one of the pair of magnetic sensors, the magnetic field gradient is measured based on the output of the pair of magnetic sensors, so that the geomagnetism, etc. The S / N ratio can be increased by removing the external magnetic field from far away (magnetic field generated by a source different from the measurement target, disturbance magnetic field).

Preferably, each of the pair of magnetic sensors is a magnetic impedance sensor. In this case, since the pair of sensors has a very high resolution, for example, 1 pT / Hz ^1/2 , the effect of making these differential is increased. Although a high-sensitivity magnetic sensor other than the magneto-impedance sensor can be configured as a gradiometer, for example, a magnetic sensor such as a fluxgate sensor having a magnetic field resolution of several pT level has a relatively large element size. Therefore, it is not suitable for so-called casual sensing with simple equipment.

  Preferably, the pair of magnetic sensors are arranged so that axes indicating their directivities are parallel to each other. In this way, when the external magnetic field has a uniform distribution, each of the pair of magnetic sensors penetrates substantially equally, so when the output of the one magnetic sensor is based on the external magnetic field, The influence of the external magnetic field can be reduced even in the output of the other magnetic sensor by the magnetic field applied by the feedback coil.

  Preferably, the pair of magnetic sensors are arranged so that axes indicating their directivities are on a common straight line. In this way, since the external magnetic field penetrates the pair of magnetic sensors in a so-called series, when the output of the one magnetic sensor is based on the external magnetic field, the other magnetic field is applied by the feedback coil. Even in the output of the magnetic sensor, the influence of the external magnetic field can be reduced.

  More preferably, each of the magneto-impedance sensors constituting the pair of magnetic sensors has an amorphous wire for detecting a magnetic field and a coil for detecting a change in magnetic flux generated by the amorphous wire. The wire is a member common to the pair of sensors. In this way, since the external magnetic field penetrates each of the pair of magnetic sensors so as to penetrate the amorphous wire, the external magnetic field is common to the pair of magnetic sensors, and the magnetic field applied by the feedback coil The effect becomes remarkable.

  Preferably, each of the magnetic impedance sensors constituting the pair of magnetic sensors includes an amorphous wire for detecting a magnetic field and a coil for detecting a magnetic flux change generated by the amorphous wire. Each of the pair of sensors is provided separately. In this way, the degree of freedom in design of the pair of magnetic sensors is improved while assuming that the pair of magnetic sensors is arranged so that an external magnetic field equally affects the pair of magnetic sensors. To do.

It is a figure explaining an example of composition of a sensor head of a magnetic field detection device of the present invention. In the magnetic field detection apparatus of a present Example, while inputting the electrical signal for driving a sensor head, it is a figure explaining the principal part of a structure and function of a circuit part for processing the output signal from a sensor head. It is a figure explaining the relationship between the repetition frequency of the pulse signal applied to an amorphous wire, and the sensitivity of a sensor. It is the figure which contrasted and showed the pulse signal applied to an amorphous wire, and the waveform of the induced voltage which generate | occur | produces in a detection coil. It is a figure explaining the principal part different from FIG. 2 among the structures of the circuit part in the magnetic field detection apparatus of a present Example. It is a block diagram explaining the function of a feedback circuit part among the circuit parts shown in FIG. It is a figure explaining the result of the experiment example of the magnetic field detection by the magnetic field detection apparatus of a present Example. It is a figure explaining the generation | occurrence | production cause of magnetic noise. It is a figure explaining the result of another experiment example 2 of the magnetic field detection by the magnetic field detection apparatus of this invention. It is a figure explaining the result of another experiment example 3 of the magnetic field detection by the magnetic field detection apparatus of this invention, Comprising: It is a figure which shows the detection result of the magnetic noise in a DC magnetic field. It is a figure explaining the result of another experiment example 4 of the magnetic field detection by the magnetic field detection apparatus of this invention, Comprising: It is a figure which shows the comparison with respect to the presence or absence of a magnetic shield. It is a figure explaining the structure of the sensor head in another Example of this invention. It is a figure explaining the structure of the sensor head in another Example of this invention.

  Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings.

  FIG. 1 is a diagram showing an example of the configuration of the magnetic detection device of the present invention. As shown in FIG. 1, the magnetic detection device 10 in this embodiment supplies a sensor head 12 for detecting a magnetic field, and an electric signal output from the sensor head 12 while supplying a current for driving the sensor head 12. And a circuit unit 30 for processing. Among these, the sensor head 12 includes a pair of magnetic sensors 14a and 14b (hereinafter referred to as “magnetic sensor 14” when they are not distinguished from each other). Among the magnetic sensors 14, the magnetic sensor 14 a is a measurement sensor 14 a for detecting a magnetic field generated by the measurement object 50 (see FIG. 5 and the like), and the other magnetic sensor 14 b is connected to the measurement object 50. The reference sensor 14b detects an external magnetic field that does not detect a magnetic field but detects a magnetic field generated by something other than the measurement object 50 such as geomagnetism. In this embodiment, the reference sensor 14b corresponds to one magnetic sensor of the present invention. Both magnetic sensors 14 are arranged so as to achieve such an object, that is, the reference magnetic sensor 14 b does not detect the magnetic field from the measurement object 50. The measurement sensor 14a and the reference sensor 14b are disposed so as to be separated by a predetermined distance such that the magnetic field gradient can be measured by the outputs of the both, specifically, for example, by 50 mm therebetween.

In the present embodiment, these magnetic sensors 14 are magnetic impedance sensors, and each of the amorphous wires 16 (referred to as “amorphous wires 16” when amorphous wires 16a and 16b described later are not distinguished) and the amorphous wires 16 are used. It includes a detection coil 18 that detects a change in magnetic flux (referred to as “detection coil 18” when detection coils 18a and 18b described later are not distinguished from each other). Each of the detection coils 18 is provided in the shape of a hollow coil so that the voltage at both ends of the coil can be detected by using an electric circuit described later. In this embodiment, one of the detection coils 18 is grounded. Has been. Specifically, the potential difference _{v out_b} across the detection coil 18b provided in the potential difference _{v out_a} and the reference sensor 14b at both ends of the detection coil 18a provided in the measurement sensor 14a is detectable. The detection coil 18a provided in the measurement sensor 14a and the detection coil 18b provided in the reference sensor 14b have the same shape, for example, a coil having a wire diameter of 30 μm, a winding number of 700, and a length of 10 mm.

  An amorphous wire 16 is passed through the hollow portion of the detection coil 18. In the present embodiment, as shown in FIG. 1, the amorphous wire 16 has a rod-like shape extending in the longitudinal direction, and the amorphous wires 16a and 16b respectively correspond to the hollow portion of the detection coil 18a and the hollow portion of the detection coil 18b. It is arranged to pass through. Further, the morphological wires 16a and 16b are electrically connected in series, and an exciting current is passed through them.

In this embodiment, the amorphous wire 16 is, for example, one having a length of 10 mm and a wire diameter of 30 μm. Electrodes and wirings are provided at both ends of the amorphous wire 16 so that a current i _in can be applied to the amorphous wire 16. In the example of FIG. 1, a current i _in from an oscillator 32 described later is applied to one end (right end) of the amorphous wire 16a, and one end (right end) of the amorphous wire 16b connected by a conducting wire from the other end (left end). To be applied. The other end of the amorphous wire 16b is grounded.

  Further, the sensor head 12 in this embodiment has a feedback coil 20. The feedback coil 20 is disposed so as to include both of the pair of magnetic sensors 14 therein. For example, when a pair of magnetic sensors are arranged on one axis as in this embodiment, the feedback coil 20 may be provided in a solenoid shape. Preferably, the pair of magnetic sensors 14 are arranged at both ends of the pair of magnetic sensors 14 so that the magnetic field generated by the feedback coils 20 can be detected equally in each of the pair of magnetic sensors 14. The magnetic field generated by the solenoid-like feedback coil 20 is disposed at a position that is a certain distance or more inward from the portion, in other words, at a position that is substantially parallel to the longitudinal direction of the feedback coil 20. Specifically, for example, a coil having a diameter of 25 mm, a length of 80 mm, and 800 turns is used. In the drawings, the size of each component is shown in a different size from the actual size in order to explain the structure of the apparatus.

FIG. 2 is a diagram for explaining the sensor head 12 and a part of the configuration of the circuit unit 30 that performs input / output with the sensor head 12 in the magnetic detection device 10 of the present embodiment shown in FIG. It is. The circuit unit 30 inputs the electric signal i _in for driving the sensor head 12 and processes the output signals V _{out_a} and V _{out_b} from the sensor head 12, and information on the magnetic field strength detected in the sensor head 12. Is calculated.

In the circuit unit 30 shown in FIG. 2, the oscillator 32 generates a pulse signal that is a source of current i _{in and the} like that is passed through the amorphous wire 16, that is, a rectangular wave. The rectangular wave is amplified by the amplifier 34 and applied to the amorphous wire 16. In the present embodiment, for example, amplification is performed so that the amplitude of the pulse signal is 2 to 3V. FIG. 3 is a diagram for explaining an example of the relationship between the repetition frequency of the pulse signal applied to the amorphous wire and the sensitivity of the sensor. A repetition frequency at which the sensitivity of the sensor shown in FIG. 3 is good is selected. Specifically, in the example of FIG. 3, since the sensitivity of the sensor is substantially constant when the repetition frequency is 10 kHz or more, the repetition frequency is set to 10 kHz. Further, the pulse width is a value obtained experimentally or by simulation in advance so that the magnetic impedance sensor has high sensitivity. Specifically, when the frequency at which the impedance change of the amorphous wire 16 is most remarkable is 10 MHz, the pulse width is 50 ns and the duty ratio is 0.0005.

  The sample hold circuits (analog switches) 36 and 40 are inputted with a potential difference between both ends of the detection coils 18a and 18b, that is, a voltage difference (electromotive force) between both ends. In the sample and hold circuits 36 and 40, the induced voltage generated in the coil by the rise (start of energization) of the pulse signal applied to the amorphous wire 16 peaks from the rise (time t1 in FIG. 4) (time t2 in FIG. 4). In the time range including), output is integrated. Specifically, for example, the time range is set to 10 ns to 50 ns. Therefore, the pulse signal output from the oscillator 32 is input to the sample and hold circuits 36 and 40, and the sample and hold circuits 36 and 40 operate with the rise of the pulse signal as a switch. The buffer amplifiers 38 and 42 send the outputs of the sample and hold circuits 36 and 40 to the differential amplifier 44, respectively.

  In the detection coil 18, as shown in FIG. 4, the waveform of the induced voltage generated in the detection coil 18 by the rise (start of energization) in the pulse signal applied to the amorphous wire 16 and the fall (energization) in the pulse signal. The fluctuation of the induced voltage generated in the detection coil 18 due to the interruption is continuously generated, that is, the waveform of the induced voltage generated in the detection coil 18 due to the rise (start of energization) in the pulse signal and the fall in the pulse signal. There is no time for the induced voltage to remain at 0, for example, between the waveform of the induced voltage generated in the detection coil 18 due to (energization interruption). The coil having a wire diameter of 60 μm, a winding number of 700, and a length of 10 mm exemplified as the shape of the detection coil 18 described above satisfies this condition in the present embodiment.

Returning to FIG. 2, the differential amplifier 44 amplifies the difference between the outputs of the sample hold circuits 36 and 40 output via the buffer amplifiers 38 and 42 and outputs the amplified difference as v _out . Specifically, the output of the sample hold circuit 36 via the buffer amplifier 38 associated with the detection coil 18a of the measurement sensor 14a and the sample hold circuit via the buffer amplifier 42 associated with the detection coil 18b of the reference sensor 14b. The difference from the output of 40 is calculated and output.

FIG. 5 is a diagram for explaining another main part of the configuration of the circuit unit 30 in the magnetic detection device 10 of the present embodiment. Specifically, FIG. 3 is a diagram illustrating a portion where further processing of the output v _out of the differential amplifier 44 of FIG. 2 is performed. That is, the circuit unit 30 includes the circuit shown in FIG. 2 and the circuit shown in FIG. Note that the differential amplifier 44 is depicted redundantly in both FIG. 2 and FIG.

Specifically, the output v _out of the differential amplifier 44 is blocked by the high-pass filter 46 from a frequency component lower than a predetermined frequency, for example, 0.3 Hz. The amplifier 48 performs predetermined amplification. Subsequently, the low-pass filter 50 blocks a frequency component higher than a predetermined frequency, for example, 30 Hz. Furthermore, after a specific frequency, for example, a frequency component of 60 Hz is cut off by the notch filter 52, predetermined amplification is performed by the amplifier 54, and another specific frequency, for example, a frequency component of 180 Hz is cut off by the notch filter 56. Output Eout (V) is output. By converting this output Eout (V) into magnetic field intensity by a conversion method obtained in advance, the magnetic field intensity generated by the measurement object can be obtained.

Returning to FIG. 2, the circuit unit 30 includes a feedback circuit 60. The feedback circuit 60, the output signal V _{out_b} sensor for reference 14b based on a signal V ₂ obtained by amplifying by the buffer amplifier 42, to generate a magnetic field by supplying a current to the above-described feedback coil 20.

Specifically, the feedback circuit 60 includes an amplifier 62 and a feedback resistor 64. The amplifier 62 supplies a voltage V ₂ ′ obtained by amplifying the output V2 of the buffer amplifier 42 with a predetermined amplification factor. The feedback resistor 64 is provided with a predetermined resistance value [a voltage V ₂ ′ that changes in accordance with the output signal V _{out —} b of the reference sensor 14 b and is a current proportional to the magnitude of the voltage V ₂ ′. Ω] resistance. That is, the feedback resistor 64 functions as a voltage-current converter that outputs a current proportional to the input voltage. With this configuration, the feedback circuit 60 can apply a current corresponding to the output signal V _{out —} b of the reference sensor 14 b to the feedback coil 20.

FIG. 6 is a block diagram for specifically explaining the operation of the magnetic field detection apparatus 10 of the present embodiment including the feedback circuit 60 in the present embodiment. As described above, in the feedback circuit 60, amplifier 62, sample-and-hold circuit 40 and based on the output signal V ₂ from the reference sensor 14b output through a buffer amplifier 42, the amplification factor A ₁ a predetermined Only the amplified voltage V ₂ ′ is output. That is,
V ₂ '= A ₁ * V ₂ (1)
It is. The amplification factor A1 may be determined as a ratio or a percentage (%), or may be determined as a gain in dB.

The resistance value of the feedback resistor 64 is set to Rf [Ω] in advance, and a feedback current If [A] corresponding to the voltage V ₂ ′ output from the amplifier 62 is generated. This feedback current If flows through the feedback coil 20 to generate a feedback magnetic field Bf. At this time, as shown in FIG. 6, the direction of the feedback magnetic field Bf is set to cancel the environmental magnetic field (external magnetic field). This example may be realized by the winding direction and the arrangement direction of the feedback coil 20, by reversing the sign of the value of the amplification factor A ₁ in the amplifier 62 is also realized by the direction of current in the opposite Good.

  In the embodiment, since the magnetic field generated from the measurement target is smaller than the external magnetic field, the magnetic field ΔB generated from the measurement target is detected only by the measurement sensor 14a, while the external magnetic field is external. The magnetic field Bn is detected by both the measurement sensor 14a and the reference sensor 14b. Therefore, the magnetic field detected by the measurement sensor 14a is the sum of the external magnetic field Bn, the magnetic field ΔB from the measurement object, and the feedback magnetic field Bf, and the direction of the feedback magnetic field Bf is opposite to the external magnetic field Bn as described above. Since it is generated, its size is Bn + ΔB−Bf.

It will be described below that the effect of the magnetic field detection device 10 of the present embodiment can be obtained by providing the feedback circuit 60 and the feedback coil 20 as shown in FIG. First, when the measurement sensor 14a, the sample hold circuit 36, and the buffer amplifier 38 are regarded as one body, and the sensitivity, that is, the voltage generated with respect to the detected magnetic field strength is S1 [V / T], the buffer amplifier 38 the output _{V 1} using the sensitivity _{S 1,}
V ₁ = S ₁ * (Bn + ΔB−Bf) (2)
It becomes.

On the other hand, since the magnetic field detected by the reference sensor 14b is the sum of the external magnetic field Bn and the feedback magnetic field Bf, its magnitude is Bn−Bf. If the sensitivity when the reference sensor 14b, the sample hold circuit 40, and the buffer amplifier 42 are regarded as one unit is S ₂ [V / T], the output V ₂ of the buffer amplifier 42 uses this sensitivity S ₂ . And
V ₂ = S ₂ * (Bn−Bf) (3)
It becomes.

Further, operation amplifier 44 amplifies the output V ₁ of the measurement sensor 14a side of the buffer amplifier 38, only the amplification factor A ₂ set in advance the difference between the output V ₂ of the reference sensor 14b side of the buffer amplifier 42 And output as Vout. That is,
Vout = A ₂ * (V ₁ −V ₂ ) (4)
It is.
Here, when the above equation (4) is rewritten using the above equations (2) and (3),
A ₂ * {S ₁ * (Bn + ΔB−Bf) −S ₂ * (Bn−Bf)} = Vout (5)
It becomes.

Further, as a characteristic of the feedback resistor 64 and the feedback coil 20, when the magnitude of the feedback magnetic field Bf is proportional to the voltage V ₂ ′ input to the feedback resistor 64, the coefficient is β.
Bf = β * V ₂ '(6)
It becomes. Here, β is, for example, β = μ ₀ * n / Rf
Where μ ₀ is the permeability [H / m] in vacuum, and n is the number of turns of the coil per unit length of the feedback coil 20. Rf [Ω] is the resistance value of the feedback resistor 64 as described above.

From the above equations (1) and (3), the output voltage V ₂ ′ of the buffer amplifier 42 on the reference sensor 14b side is
V ₂ ′ = A ₁ * S ₂ * (Bn−Bf) (7)
It is. From the above formulas (6) and (7), V ₂ ′ = A ₁ * S ₂ * (Bn−β * V ₂ ′) (8)
Become
V ₂ ′ = A ₁ * S ₂ * Bn / (1 + A ₁ * S ₂ * β) (9)
It becomes.

In addition, the equation (5) is modified,
A ₂ * {(S ₁ −S ₂ ) * (Bn−Bf) ++ ΔB * S ₁ } = Vout (10)
When applying formulas (6) and (9) to this,
A ₂ * [(S ₁ −S ₂ ) * Bn * {1−β * A ₁ * S ₂ / (1 + β * A ₁ * S ₂ )}] + ΔB * S ₁ * A ₂ = Vout (11 )
And when further deformed,
A ₂ * [Bn * {(S ₁ −S ₂ ) / (1 + β * A ₁ * S ₂ )}] + ΔB * S ₁ * A ₂ = Vout (12)
It becomes. Here, the first term on the left side of the equation (12) is an output voltage caused by an external magnetic field in the output voltage Vout of the differential amplifier 44, and the second term on the left side is an output voltage caused by a magnetic field generated from the measurement target. It is.

On the other hand, since the output voltage Vout of the operational amplifier 44 without the feedback circuit 60 corresponds to the case where β or A ₁ is 0 in the above equation (12), the output voltage Vout_n of the differential amplifier 44 in this case is
A ₂ * Bn * (S ₁ −S ₂ ) + ΔB * S ₁ * A ₂ = Vout_n (13)
It is expressed as As in the case of equation (12), the first term on the left side is the output voltage due to the external magnetic field in the output voltage Vout_n of the differential amplifier 44, and the second term on the left side is due to the magnetic field generated from the measurement target. Output voltage.

By the way, although the pair of magnetic sensors 14a and 14b in the magnetic field detection apparatus 10 of the present embodiment has the same configuration, it can be assumed that an error occurs in the output value due to manufacturing variations and the like. Such errors, for example, and thus appear as a difference in sensitivity S ₁ and S ₂ described above. If ideally no the variation, since the same sensitivity S ₂ of the reference magnetic sensor 14b and the sensitivity S ₁ of the measuring magnetic sensor 14a, the (12) or (13) the left-hand side in Formula 1 The term becomes 0, and the output voltages Vout and Vout_n of the differential amplifier 44 are only voltages caused by the magnetic field ΔB generated by the measurement target, and there is no need to provide the feedback circuit 60 or the feedback coil 20. However, in practice, the above-described variation occurs, and thus the output voltage Vout or Vout_n of the differential amplifier includes a voltage due to an external magnetic field. That is, the (12) or (13) the first term left side in the expression, it can be said that even in the voltage caused by the variation of the sensitivity S ₂ of the reference magnetic sensor 14b and the sensitivity S ₁ of the measuring magnetic sensor 14a.

Here, when calculating the ratio of the output voltage due to the external magnetic field in the case where there is the feedback circuit 60 and in the case where there is no feedback circuit 60,
(Output voltage due to an external magnetic field when the feedback circuit 60 is present) / (Output voltage due to an external magnetic field when the feedback circuit 60 is not present) = 1 / (1 + β * A ₁ * S ₂ ) (14 )
It becomes. Here, when (1 + β * A ₁ * S ₂ ) is sufficiently large, the value of the equation (14) becomes large. Therefore, the measurement object which is the second term on the left side in the equations (12) and (13) On the other hand, the voltage due to the magnetic field ΔB causing the change does not change, but the first term on the left side in the equation (13) becomes small, so the influence of the external magnetic field on the output voltage Vout of the differential amplifier 44 becomes small.

This, in other words, as to the variation of the sensitivity _{S 2} of the reference magnetic sensor 14b and the sensitivity _{S 1} of the measuring magnetic sensor 14a is present if is sufficiently large _{(1 + β * A 1 *} S 2) However, its influence on the output voltage Vout of the differential amplifier 44 is negligible. Therefore, when the resolution of the sensor 14 is very high, the magnetic field can be detected in detail up to the resolution level.

Preferably, the amplification factor _{A 1} of the aforementioned amplifier 62, as the magnetic field detected in the reference sensor 14b (Bn-Bf) is 0, the output V2 of the reference sensor 14b of side buffer amplifier 42 in other words, It is set to be zero. In this way, since the magnitude of the external magnetic field Bn is sufficiently larger than the magnetic field ΔB generated by the measurement object, it is realized that (1 + β * A ₁ * S ₂ ) in the above-described equation (14) is sufficiently large. Yes. Further, in the measurement sensor 14a, the external magnetic field Bn is canceled by the magnetic field Bf applied by the feedback coil 20, and the influence of the external magnetic field Bn is reduced, and the minute magnetic field ΔB generated mainly by the measurement object is detected. Therefore, for example, even when the sensor 14 has magnetic saturation, the magnetic field can be detected in a region where good output voltage characteristics can be obtained with respect to the externally applied magnetic field, and the detection accuracy is improved. .

[Experimental Example 1]
Next, an experimental example performed by the inventors using the magnetic field detection device 10 of this embodiment will be described. In this experiment, a solenoid-like feedback coil 20 is provided, and a measurement sensor 14 a and a reference sensor 14 b are arranged inside the feedback coil 20. The feedback coil has a length of 50 mm, a diameter of 25 mm, and 800 windings. Each of the measurement sensor 14a and the reference sensor 14b includes amorphous wires 16a and 16b having a diameter of 25 μm and a length of 10 mm and detection coils 18a and 18b. In addition, the measurement sensor 14a and the reference sensor 14b are arranged so that their longitudinal directions coincide with the longitudinal direction of the feedback coil 20, in other words, the direction of the magnetic field Bf generated by the feedback coil 20, the measurement sensor 14a, and the reference sensor 14b. The sensor 14b is arranged so that the direction (directivity) of the magnetic field detectable by the sensor 14b matches.

  FIG. 7 is a diagram showing the output of the magnetic field detection device (in other words, the operational amplifier 44) in the state where the external magnetic field Bn of 1 μT and 10 Hz is applied and the magnetic field ΔB from the measurement target is not provided in the configuration in this experimental example FIG. 7A shows the noise spectrum, and FIG. 7B shows the output voltage. 7A and 7B, a solid line represents a case where the feedback circuit 60 of this embodiment is operated, and a broken line represents a case where the feedback circuit 60 is not operated or not provided. As shown in FIG. 7A, the output is low when the feedback circuit 60 is operated, particularly at 10 Hz corresponding to an external magnetic field. Also, as shown in FIG. It can be seen that the output of 44 is also reduced. As can be seen from these experimental results, by using the magnetic field detection device 10 of the present embodiment having the feedback circuit 60 and the feedback coil 20, it is more effective than simply making the measurement sensor 14a and the reference sensor 14b differential. It can be seen that magnetic noise is reduced.

  The magnetic noise is caused by variations in sensitivity of the measurement sensor 14a and the reference sensor 14b as described above. FIG. 8 is a diagram in which changes in the output voltages of the measurement sensor 14a and the reference sensor 14b are recorded without operating the feedback circuit 60 in the configuration of the experimental example 1 described above. The sensor sensitivity on the specifications of the measurement sensor 14a and the reference sensor 14b was 0.951 μT / V. When a magnetic field of 1 μT and 10 Hz was applied as an external magnetic field, the output voltage was as shown in FIG. Specifically, since the amplitude (average value) of the output voltage of the measurement sensor 14a is 1.02 V, the measured magnetic field is 1.08 μT, while the amplitude (average value) of the output voltage of the reference sensor 14b. Value) was 0.900 V, and the measured magnetic field was 0.946 μT. This difference of 0.134 μT appears as noise in the output of the magnetic field detection apparatus 10.

[Experiment 2]
In this experimental example, a similar experiment was performed under the same conditions as in Experimental Example 1 except that the length of the solenoid-like feedback coil 20 was set to 80 mm. At this time, the measurement sensor 14a and the reference sensor 14b are arranged at the same interval in the longitudinal direction of the feedback coil 20 (near the center) as in the first experimental example. FIG. 9 shows the output of the magnetic field detection apparatus in the above-described configuration in this experimental example in a state where an external magnetic field Bn of 1 μT and 10 Hz is applied and the magnetic field ΔB from the measurement target is not provided as in the first embodiment. FIG. 9A shows the noise spectrum, FIG. 9B shows the output voltage, and corresponds to FIGS. 7A and 7B in Experimental Example 1 described above. . 9A and 9B, a solid line indicates a case where the feedback circuit 60 of this embodiment is operated, and a broken line indicates a case where the feedback circuit 60 is not operated or not provided. Similarly to Experimental Example 1, it can be seen that by operating the feedback circuit 60, noise is reduced more effectively than when the two measurement sensors 14a and the reference sensor 14b are simply differentiated. In addition, by comparing FIGS. 7 (a) and 7 (b) with FIGS. 9 (a) and 9 (b), a feedback having a sufficient length with respect to the lengths of the measurement sensor 14a and the reference sensor 14b. It can be seen that by providing the coil 20, noise can be more effectively reduced. This is considered because the feedback coil 20 can apply a uniform magnetic field Bf to each of the measurement sensor 14a and the reference sensor 14b.

[Experiment 3]
This Experimental Example 3 is an example in which a DC magnetic field is applied as the external magnetic field Bn instead of the AC magnetic field in the above Experimental Example 1 in the same configuration as in the above Experimental Example 1.

  FIG. 10 illustrates the output of the magnetic field detection device (in other words, the operational amplifier 44) in a state where a 10 μT DC magnetic field is applied as the external magnetic field Bn and no magnetic field ΔB is provided from the measurement target in the above-described configuration of this experimental example. FIG. 10A shows an output noise spectrum, and FIG. 10B shows an output obtained by making the measurement sensor 14a and the reference sensor 14b differential with respect to the magnetic field intensity of the DC magnetic field. The change of FIG. 1 etc. is shown. In FIG. 10A, the solid line represents the case where the feedback circuit 60 of this embodiment is operated, and the broken line represents the case where the feedback circuit 60 is not operated or not provided. As shown in FIG. 10, it can be seen that the noise is low when the feedback circuit 60 is operated as a whole, except in the vicinity of an extremely low frequency close to 1 Hz. As can be seen from these experimental results, even when the external magnetic field is a DC magnetic field including the geomagnetism, by using the magnetic field detection device 10 of the present embodiment having the feedback circuit 60 and the feedback coil 20, the measurement sensor 14a and It can be seen that magnetic noise is reduced more effectively than when the reference sensor 14b is differentiated. Further, as shown in FIG. 10B, the differential output Eout [V] of the measurement sensor 14a and the reference sensor 14b is almost equal to the strength [μT] of the magnetic field sensitivity in a DC magnetic field (uniform magnetic field). It can be seen that the relationship is linear. In other words, it is understood that accurate measurement can be performed based on the output voltage Eout of the sensor with respect to the change in the strength of the magnetic field.

[Experimental Example 4]
In Experimental Example 4, magnetic field detection is performed by using the magnetic field detection device 10 in the present embodiment in each of a magnetically shielded environment and a non-magnetically shielded environment. FIG. 11 is a diagram for explaining a magnetic field detection result using the magnetic field detection device 10 configured in the same manner as in Experimental Example 1 described above, and FIG. 11A shows an experimental result performed outside the magnetic shield device. FIG. 11B shows the results of experiments conducted in the magnetic shield device.

  In Experimental Example 4, a DC magnetic field of 2.4 to 3.0 μT is applied as the external magnetic field Bn outside the magnetic shield device. Further, the influence of the external magnetic field Bn is reduced inside the magnetic shield device. In addition, as a magnetic field corresponding to the magnetic field ΔB from the measurement object, a magnetic field having an amplitude of 200 pT was generated at 10 Hz by a one-turn (one turn) coil. The outputs of the magnetic field detection apparatus of this example at this time are shown in FIGS. 11 (a) and 11 (b), respectively.

  As can be seen from FIGS. 11A and 11B, both waveforms are waveforms corresponding to the magnetic field ΔB from the measurement target of 10 Hz. That is, according to the magnetic field detection apparatus 10 of the present embodiment, in the presence of a constant DC magnetic field, the magnetic field ΔB from the measurement target can be detected in the same manner as when the magnetic shield is provided without providing the magnetic shield. It is understood that is possible.

  According to the above-described embodiment, the magnetic field detection device 10 of the present embodiment includes a pair of magnetic sensors for measurement 14a and a reference magnetic sensor 14b for detecting magnetism, and the pair of magnetic sensors 14a by the operation amplifier 44. , 14b are differentially detected to detect a magnetic field, the feedback coil 20 capable of applying a magnetic field Bf common to the pair of magnetic sensors 14a, 14b, and the pair of magnetic sensors 14a, 14b. The feedback circuit 60 further supplies a current to the feedback coil 20 based on the output Vout_b of the reference magnetic sensor 14b or the output V2 of the buffer amplifier 42 that amplifies the output of the reference magnetic sensor 14b. In this way, when the magnetic field ΔB to be measured is detected by differentially outputting the outputs of the pair of magnetic sensors 14a and 14b, the magnetic field Bf applied by the feedback coil 20 is taken into consideration and the pair of magnetic sensors The sensors 14a and 14b can detect a magnetic field.

  Further, according to the above-described embodiment, the feedback circuit 60 determines the current to be supplied to the feedback coil 20 so that the output of the reference magnetic sensor 14b becomes zero, so that the outputs of the pair of magnetic sensors 14a and 14b are activated. In this case, since the output of the reference magnetic sensor 14b becomes 0, the influence of the external magnetic field Bn can be reduced, and the measurement can be performed in a magnetic field intensity range in which the sensors 14a and 14b are not magnetically saturated. Therefore, the resolution can be improved.

  Further, according to the above-described embodiment, since the gradiometer is configured using the pair of magnetic sensors 14a and 14b, the magnetic field for measurement in which the magnetic field generation source to be measured is one of the pair of magnetic sensors 14a and 14b. In the case of being close to the sensor 14a, the magnetic field gradient is measured based on the outputs of the pair of magnetic sensors 14a and 14b, thereby removing the influence of the external magnetic field Bn from a distant place such as geomagnetism, and the S / N ratio. Can be increased.

  In addition, according to the above-described embodiment, each of the pair of magnetic sensors 14a and 14b is a magnetic impedance sensor, and thus has a very high resolution such as a nano Tesla (nT) level. The effect is increased when the magnetic field Bf is applied by the feedback coil 20 by making the differential.

  Further, according to the above-described embodiment, the pair of magnetic sensors 14a and 14b are arranged so that the axes indicating the directivities thereof are on a common straight line, so that the external magnetic field Bn is the pair of magnetic sensors 14a and 14b. So that the output of the reference magnetic sensor 14b is based on the external magnetic field Bn, the output Vout_a of the measurement magnetic sensor 14a or the measurement magnetic sensor 14a is determined by the magnetic field Bf applied by the feedback coil 20. The influence of the external magnetic field Bn can also be reduced at the output V1 of the buffer amplifier 38 that amplifies the output of the current.

  Further, according to the above-described embodiment, each of the magnetic impedance sensors constituting the pair of magnetic sensors 14a and 14b includes the amorphous wires 16a and 16b for detecting the magnetic field, and the magnetic flux change generated by the amorphous wires 16a and 16b. The coils 18a and 18b for detection are provided, and the amorphous wires 16a and 16b are provided separately for the pair of sensors 14a and 14b, respectively, so that the external magnetic field Bn affects the pair of magnetic sensors 14a and 14b equally. The degree of freedom in designing the pair of magnetic sensors 14a and 14b is improved.

  Subsequently, another embodiment of the present invention will be described. In the following description, portions common to the embodiments are denoted by the same reference numerals and description thereof is omitted.

  FIG. 12 is a diagram for explaining an aspect of the sensor head 112 in another embodiment of the present invention, and corresponds to the sensor head 12 in the first embodiment. In the present embodiment, the measurement magnetic sensor 14a and the reference magnetic sensor 14b, which are a pair of magnetic sensors 14, are not arranged on the same axis, and both are arranged in series and on one axis. Different from Example 1.

  That is, the pair of magnetic sensors 14 are disposed at positions where their magnetically detectable directions (directivity) are parallel to each other. Preferably, as shown in FIG. 12, if it is arranged at a position that receives the external magnetic field Bn evenly, the position is not limited, and the degree of freedom in design is improved. Further, the shape of the feedback coil 20 is not limited as long as the magnetic field Bf applied by the feedback coil 20 equally applies a magnetic field to each of the pair of magnetic sensors 14. For example, when the feedback coil 20 is provided in a solenoid shape as in the first embodiment, the pair of magnetic sensors 14 are provided at equal positions from the center of a circle that is a cross section perpendicular to the longitudinal direction of the feedback coil 20. It only has to be done. In FIG. 12, the feedback coil 20 is omitted to clarify the configuration of the sensor head.

  Here, the exciting current energized to the amorphous wires 16a and 16b in each of the pair of magnetic sensors 14 may be energized by providing both the amorphous wires 16 in parallel as shown in FIG. Alternatively, although not shown, the two amorphous wires 16 may be energized by being electrically connected in series.

  According to the sensor head 112 of the above-described second embodiment, the pair of magnetic sensors 14 are arranged so that the axes indicating their directivities are parallel to each other, so that the external magnetic field Bu is positioned at the position of the pair of magnetic sensors 14. If the outputs Vout_b or V2 of the reference magnetic sensor 14b are based on the external magnetic field Bn, they are applied by the feedback coil 20. The influence of the external magnetic field Bn can be reduced also in the output Vout_a or V1 of the measurement magnetic sensor 14a by the magnetic field Bf.

  FIG. 13 is a diagram for explaining an aspect of the sensor head 212 according to still another embodiment of the present invention, and corresponds to the sensor head 12 according to the above-described first embodiment. In this embodiment, the measurement magnetic sensor 14a and the reference magnetic sensor 14b, which are a pair of magnetic sensors 14, are arranged on one axis, while a single amorphous wire 116 common to the pair of magnetic sensors is provided. This is different from the first embodiment described above.

  That is, in the sensor head 212 of the present embodiment, one amorphous wire 116 passes through the measurement coils 18a and 18b of the measurement magnetic sensor 14a and the reference magnetic sensor 14b, respectively. The high-frequency excitation current supplied to the amorphous wire is supplied from one end (left end in FIG. 13), and the other end is grounded. That is, unlike the first embodiment described above, the amorphous wire 116 is composed of one member, so that it is not necessary to electrically connect the two amorphous wires as in the first embodiment. In FIG. 13, the feedback coil 20 is omitted to clarify the configuration of the sensor head.

  In the sensor head 212 of this embodiment, each of the magnetic impedance sensors 14 constituting the pair of magnetic sensors 14a and 14b includes an amorphous wire 116 for detecting a magnetic field, and a coil 18 for detecting a magnetic flux change generated by the amorphous wire 116. The amorphous wire 116 is a member common to the pair of magnetic sensors 14a and 14b. In this way, since the external magnetic field penetrates each of the pair of magnetic sensors 14a and 14b while reducing leakage of magnetic flux, the external magnetic field Bn becomes common to the pair of magnetic sensors 14a and 14b, and the feedback coil 20 The effect of the magnetic field Bf applied by is significant.

  As mentioned above, although the Example of this invention was described in detail based on drawing, this invention is applied also in another aspect.

  For example, in the above-described embodiment, the magnetic impedance sensor is used as the measurement sensor 14a and the reference sensor 14b. However, the present invention is not limited to such a sensor. Also good. In particular, if the sensor is a highly accurate sensor such as a fluxgate sensor having a high resolution, the same effect as that of the above-described embodiment can be obtained.

  In the above-described embodiment, the feedback coil 20 is provided in a solenoid shape, but the present invention is not limited to this. The shape is not limited as long as the magnetic field Bf can be equally applied to the pair of magnetic sensors 14a and 14b.

  In addition, the source of the minute magnetic field that can be measured in the present embodiment is not limited and can be used for various purposes. That is, the application of the magnetic field detection device 10 of the present embodiment is not limited to the detection of the magnetic field generated from the living body.

  In the above-described embodiment, the reference sensor 14b corresponds to one magnetic sensor of the present invention. However, the present invention is not limited to this, and the measurement sensor 14a may correspond to one magnetic sensor. Good. In this case, the same result as in the above-described embodiment can be obtained by reversing the sign of the output of the operation amplifier 44.

  In addition, although not illustrated one by one, the present invention is implemented with various modifications within a range not departing from the gist thereof.

10: Magnetic field detector 14: Magnetic sensor 14a: The other magnetic sensor (magnetic sensor for measurement)
14b: One magnetic sensor (magnetic sensor for reference)
20: Feedback coil 60: Feedback circuit Bf: Magnetic field generated by the feedback coil ΔB: Magnetic field generated by the detection target

Claims (6)

Having a pair of magnetic sensors for detecting magnetism;
A magnetic field detection device that detects a magnetic field by differentiating outputs of the pair of magnetic sensors,
A feedback coil capable of applying a magnetic field common to the pair of magnetic sensors;
A feedback circuit for supplying a current to the feedback coil based on an output signal of one of the pair of magnetic sensors;
Magnetic field detection device characterized by the above. The feedback circuit determines the current so that an output of the one magnetic sensor becomes zero;
The magnetic field detection apparatus according to claim 1. The magnetic field detection apparatus according to claim 1, further comprising a graviometer using the pair of magnetic sensors. 4. The magnetic field detection device according to claim 1, wherein each of the pair of sensors is a magnetic impedance sensor. 5. The pair of sensors are arranged so that axes indicating their directivities are parallel,
The magnetic field detection apparatus according to any one of claims 1 to 4. The pair of sensors are arranged so that their directivity axes are on a common straight line,
The magnetic field detection apparatus according to any one of claims 1 to 4.

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