JP2019023582A - Magnetic sensor and magnetic measurement method - Google Patents

Abstract

To provide a magnetic sensor capable of high-sensitively detecting a signal magnetic field up to a low frequency area in an excitation magnetic field using a high temperature superconducting coil.SOLUTION: The magnetic sensor comprises: a coil L1 constituted of a high temperature superconducting conductor capturing a signal magnetic flux as the object to be measured; a coil L2 constituted of a high temperature superconducting conductor, which is connected to the coil L1 in series to detect the signal magnetic flux captured by the coil L1; a micro resistance Rc which is connected in series to the coil L1 and the coil L2 to form a closed circuit; a detection part 12 configured to measure the signal magnetic flux transferred from the coil L1 to the coil L2 after converting the same into a voltage signal; and modulation means for changing the inductance on the coil L2 in time.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an inductance modulation type magnetic sensor using a high-temperature superconducting coil.

  Magnetic sensors are applied in a wide range of fields such as medical / bio testing, nondestructive testing, material property analysis, resource exploration, and geophysics. In these applications, in order to enhance the inspection and analysis performance, it is necessary to measure a weak magnetic signal with high accuracy, and a high-sensitivity magnetic sensor for this purpose is required.

  As a general known magnetic sensor, there is an electromagnetic induction type magnetic sensor. FIG. 9 is a basic configuration diagram of a conventional electromagnetic induction type magnetic sensor. In FIG. 9A, assuming that the cross-sectional area of the coil is S, the number of turns is N, the magnetic field applied from the outside is B, and the magnetic flux linked to the coil is Φ (= B × N × S), the voltage across the coil V changed Φ over time

  Which is proportional to the frequency f. In other words, the relationship between the frequency f and the voltage V is a proportional relationship as shown in the graph of FIG. 9B, and in the low frequency region, the voltage becomes small and the sensitivity becomes low, so it cannot function as a sensor. There is a problem.

  Here, magnetic measurement methods for measuring a magnetic signal from an object are roughly classified into two cases. One is a case of measuring spontaneous magnetization spontaneously generated from the object. In this case, it is only necessary to measure a weak magnetic signal with high accuracy, and various sensors have been developed. The other is a case where an excitation magnetic field is applied to magnetize the object and the magnetization signal of the object is measured. In this case, it is necessary to measure a weak magnetic signal in a state where a large excitation magnetic field is applied, but there are very few magnetic sensors that satisfy this requirement.

  The reason is that the magnetic sensor has a normally operating magnetic field range (minimum value and maximum value), and the maximum value of the magnetic field that can be measured is smaller as the magnetic sensor is more sensitive. That is, when the magnetic sensor is directly exposed to a large excitation magnetic field, magnetic saturation occurs and normal operation of the magnetic sensor cannot be performed. For this reason, in magnetic measurement using an excitation magnetic field, it is an important issue how to remove the influence of the excitation magnetic field while maintaining high sensitivity of the sensor.

  One method for solving this problem is to separate the capture and detection of the signal magnetic field. In this method, a signal magnetic field is captured by a detection coil, and the signal magnetic field is transmitted to a magnetic sensor for measurement. In this method, since the magnetic sensor can be installed outside the excitation magnetic field, the adverse effect of the excitation magnetic field can be eliminated. A magnetic sensor using a superconducting coil and a superconducting quantum interference element (SQUID) operating at a liquid helium temperature is known as a technique for solving this problem and the problem of the above-described electromagnetic induction type magnetic sensor ( Non-patent documents 1 to 3).

FIG. 10 is a basic configuration diagram of a conventional SQUID. In FIG. 10A, a closed circuit of R _c = 0 is formed using a superconducting coil, and measurement is performed by reading current. Here, the current I flowing through the closed circuit is obtained when R _c = 0.

The magnetic flux Φ _{2 of the} detection coil is

  Is proportional to Φ and does not depend on the frequency f (see FIG. 10B). That is, by detecting the current I, measurement independent of the frequency f is possible.

  In addition, a device that transmits a magnetic signal by combining a high-temperature superconducting coil and a normal conductive coil and detects this magnetic signal with a high-temperature superconducting SQUID has been developed (Non-Patent Documents 4 to 10). Applications such as medical / bio-inspection, non-destructive inspection, and material property analysis using this device are being studied.

M. Mossle, WR. Myers, SK. Lee, N. Kelso, M. Hatridge, A. Pines, and J. Clarke, "SQUID-detected in vivo MRI at microtesla magnetic fields", IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, vol. 15, no. 2, pp. 757-760, 2005 A. H. Trabesinger, R. McDermott, SKLee, M. Mu1ck, J. Clarke, and A. Pines, "SQUID-Detected Liquid State NMR in Microtesla Fields", J. Phys. Chem. A 2004 108, 957-963, 2014 NL. Adolphi, KS. Butler, DM. Lovato, TE. Tessier, JE. Trujillo, HJ. Hathaway, DL. Fegan, TC. Monson, TE. Stevens, DL. Huber, J. Ramu, ML. Milne, SA. Altobelli, HC. Bryant, RS. Larson, and ER. Flynn, “Imaging of Her2-targeted magnetic nanoparticles for breast cancer detection: comparison of SQUID-detected magnetic relaxometry and MRI”, Contrast Media & Molecular Imaging vol. 7, 308- 319, 2012 A. Tsukamoto, T. Hato, S. Adachi, Y. Oshikubo, W. Cheng, K. Enpuku, K. Tsukada, and K. Tanabe, "Eddy Current Testing System Using HTS-SQUID With External Pickup Coil Made of HTS Wire ", IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, vol. 27, no. 4 1600505, 2017 MM. Saari, K. Sakai, T. Kiwa, A. Tsukamoto, S. Adachi, K. Tanabe, A. Kandori, and K. Tsukada, "Development of a Compact Moving-Sample Magnetometer Using High-Tc Superconducting Quantum Interference Device ", JAPANESE JOURNAL OF APPLIED PHYSICS, vol. 51, no. 4, 046601, 2012 DF. He, M. Tachiki and H. Itozaki, "Detecting the N-14 NQR signal using a high-Tc SQUID", IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, vol. 17, no. 2, pp. 843-845, 2007 H. Dong, Y. Zhang, HJ. Krause, X. Xie, AI Braginski and A. Offenhaeusser, "Suppression of ringing in the tuned input circuit of a SQUID detector used in low-field NMR measurements", Superconductor Science and Technology, Vol. 22, No. 12, 125022, 2009 T. Mizoguchi, A. Kandori, R. Kawabata, K. Ogata, T. Hato, A. Tsukamoto, S. Adachi, K. Tanabe, S. Tanaka K. Tsukada, and K. Enpuku, "Highly Sensitive Third-Harmonic Detection Method of Magnetic Nanoparticles Using an AC Susceptibility Measurement System for Liquid-Phase Assay ”, IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, vol. 26, no. 5, 1602004, 2016 HH. Chen, KW. Huang, HC. Yang, HE. Horng, SH. Liao, "Optimization of the detection coil of high-Tc superconducting quantum interference device-based nuclear magnetic resonance for discriminating a minimum amount of liver tumor of rats in microtesla fields ", JOURNAL OF APPLIED PHYSICS, vol. 114, no. 6, 064701, 2013 SY. Yang, JJ. Chieh, CC. Yang, SH. Liao, HH. Chen, HE. Horng, HC. Yang, CY. Hong, MJ. Chiu, TF. Chen, KW. Huang, and CC Wu, "Clinic Applications in Assaying Ultra-Low-Concentration Bio-Markers Using HTS SQUID-Based AC Magnetosusceptometer ", IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, vol. 23, no. 3, 1600604, 2013

  However, the SQUID operating at the liquid helium temperature requires liquid helium as a refrigerant, and has the problems that the price and maintenance cost of the apparatus are high and the handling is complicated.

  In addition, the method using the high-temperature superconducting coil has a problem that the signal transmission efficiency is extremely deteriorated at a low frequency of several tens of hertz or less, and a highly sensitive magnetic sensor for a low-frequency magnetic field cannot be realized.

  The present invention provides a magnetic sensor that can detect a signal magnetic field up to a low frequency region in an excitation magnetic field with high sensitivity using a high-temperature superconducting coil.

  A magnetic sensor according to the present invention is connected in series to a first coil made of a high-temperature superconductor that captures a signal magnetic flux to be measured, and detects the signal magnetic flux captured by the first coil. A second coil made of a high-temperature superconductor, a minute resistor that is connected in series to the first coil and the second coil to form a closed circuit, and a signal magnetic flux transmitted from the first coil to the second coil is converted into a voltage signal. Voltage measuring means for measuring, and modulation means for changing the inductance of the second coil over time.

  As described above, in the magnetic sensor according to the present invention, the first coil made of the high-temperature superconductor that captures the signal magnetic flux to be measured and the first coil are connected in series and captured by the first coil. Temporarily changing the inductance of the second coil in the second coil made of a high-temperature superconductor that detects the signal magnetic flux and the minute resistor that is connected in series to the first coil and the second coil to form a closed circuit. Thus, it becomes possible to measure a voltage that does not depend on the frequency, and the magnetic field can be detected with high sensitivity even at an extremely low frequency.

  In addition, by forming the first coil and the second coil with a high-temperature superconducting coil, it becomes possible to use liquid nitrogen as a refrigerant, reducing the price and maintenance cost of the apparatus, and simplifying the handling. There is an effect that can be.

  A magnetic sensor according to the present invention is connected in series to a first coil made of a high-temperature superconductor that captures a signal magnetic flux to be measured, and detects the signal magnetic flux captured by the first coil. A second coil made of a high-temperature superconductor; a third coil magnetically coupled to the second coil; a micro-resistor connected in series to the first coil and the second coil to form a closed circuit; Voltage measuring means for measuring the signal magnetic flux transmitted to the coil as a voltage signal of the third coil that is magnetically coupled to the second coil, and modulation means for temporally changing the mutual inductance between the second coil and the third coil; Is provided.

  As described above, in the magnetic sensor according to the present invention, the first coil made of the high-temperature superconductor that captures the signal magnetic flux to be measured and the first coil are connected in series and captured by the first coil. A second coil made of a high-temperature superconductor that detects the signal magnetic flux, and a first coil and a minute resistor connected in series to the second coil form a closed circuit, and a third coil that is magnetically coupled to the second coil is provided. By changing the mutual inductance between the second coil and the third coil over time, it becomes possible to measure a voltage that does not depend on the frequency, and a magnetic field is highly sensitive even at an extremely low frequency. There is an effect that it can be detected.

  In addition, by forming the first coil, the second coil, and the third coil with a high-temperature superconducting coil, it becomes possible to use liquid nitrogen as a refrigerant, reducing the price and maintenance cost of the apparatus, and handling it. There is an effect that it can be simplified.

  In the magnetic sensor according to the present invention, the minute resistance is made of a normal conductor connecting the first coil and the second coil, and the resistance value is adjusted according to the measurement frequency.

  As described above, in the magnetic sensor according to the present invention, the minute resistance is made of a normal conductor connecting the first coil and the second coil, and the resistance value is adjusted according to the measurement frequency. There is an effect that it becomes possible to measure at an appropriate frequency according to the object.

  In the magnetic sensor according to the present invention, the modulation means includes a magnetic body inserted into the second coil, and a current source that supplies current to the magnetic body.

  As described above, in the magnetic sensor according to the present invention, the modulation means includes the magnetic body inserted into the second coil and the current source that supplies current to the magnetic body, so that the current is controlled by the current source. Thus, there is an effect that the inductance of the second coil can be easily modulated.

  In the magnetic sensor according to the present invention, the modulation means includes a magnetic body inserted into the third coil, and a current source that supplies current to the magnetic body.

  As described above, in the magnetic sensor according to the present invention, the modulation means has the magnetic body inserted into the third coil and the current source that supplies current to the magnetic body, so that the current is controlled by the current source. Thus, there is an effect that the mutual inductance between the second coil and the third coil can be easily modulated.

It is a figure which shows the circuit structure of the magnetic sensor which concerns on 1st Embodiment. It is a figure which shows the relationship between a frequency in case there exists contact resistance, and an electric current. It is a figure which shows the circuit structure of the magnetic sensor which concerns on 2nd Embodiment. In an Example, it is a figure which shows the measurement result of the change of an inductance when a DC bias current is sent through a magnetic wire. In an Example, it is a figure which shows the measurement result of the frequency spectrum of a voltage when the frequency of a signal magnetic flux is 2 kHz. In an Example, it is a figure which shows the result of a frequency characteristic when the frequency of a signal magnetic flux is changed. In an Example, it is a figure which shows the result of having measured the magnetic field sensitivity. In an Example, it is a figure which shows the result of having measured the noise characteristic. It is a basic block diagram of the electromagnetic induction type magnetic sensor of a prior art. It is a basic composition figure of SQUID of a prior art.

  Embodiments of the present invention will be described below. Also, the same reference numerals are given to the same elements throughout the present embodiment.

(First embodiment of the present invention)
A magnetic sensor according to this embodiment will be described with reference to FIGS. 1 and 2. The magnetic sensor according to the present embodiment can measure a low-frequency signal magnetic field with high sensitivity in an excitation magnetic field using a high-temperature superconducting coil.

  FIG. 1 is a diagram illustrating a circuit configuration of a magnetic sensor according to the present embodiment. The magnetic sensor 1 includes a coil L1 formed of a high-temperature superconductor that captures an external signal magnetic field B as a signal magnetic flux Φ, and a magnetic flux that is connected in series to the coil L1 and is formed of the high-temperature superconductor and captured by the coil L1. And a resistor Rc connected in series to the coil L1 and the coil L2 to form a closed circuit.

The coil L2, to modulate the inductance L ₂ of the coil L2, the magnetic wire 10 in the coil (e.g., amorphous magnetic wires, etc.) is inserted, a current source 11 for energizing current to the magnetic wire 10 Is provided. The signal magnetic field captured by the coil L1 is transmitted to the coil L2, and is measured as a voltage signal by the detection unit 12.

Modulation of the inductance of the coil L2 is performed by passing a bias current I _B in the magnetic wire 10. When a current is passed through the magnetic wire 10, the magnetic wire 10 is magnetized by this current and its permeability changes. Inductance of the coil L2, in order to change by a change in magnetic permeability of the magnetic wire 10, thereby the inductance L ₂ of the coil L2 temporally changed by the bias current I _B.

Hereinafter, the operation principle of the magnetic sensor 1 according to the present embodiment will be described in detail. The inductance of the coil L1 in FIG. 1 _{L 1,} the inductance of the coil L2 and _{L 2.} The coils L1 and L2, since each consisting of high-temperature superconductors connection resistance R _c occurs always. The frequency f of the signal magnetic field to be detected is

In this case, since the influence of the connection resistance on the signal transmission can be ignored, here, R _c = 0 is described for the sake of simplicity.

Here, the relationship between the connection resistance _Rc and the frequency f will be described. In FIG. 10, a SQUID using a superconducting coil operating at a liquid helium temperature is described, but this can realize a superconducting loop of R _c = 0 by using liquid helium as a refrigerant. . That is, as described above, in FIG. 10, it is possible to read the current I without depending on the frequency, and it is possible to detect with high sensitivity even if f is a low frequency.

On the other hand, since the magnetic sensor according to the present embodiment uses a high-temperature superconducting coil, the connection resistance R _c > 0 occurs as described above. In FIG. 10, when R _c > 0, the current I uses ω = 2πf,

Thus, the relationship between the frequency f and | I | is a graph as shown in FIG. As can be seen from the graph of FIG. 2, when the frequency is less than fc, the current | I | fluctuates greatly, and the sensitivity becomes poor, so that the magnetic sensor cannot function. In addition, according to the following formula, the frequency fc increases as R _c increases. Therefore, in order to perform measurement at a low frequency, it is desirable to reduce R _c as much as possible.

Returning to Equation (1), as described above, here, a method in which R _c = 0 is set and the detection unit 12 measures the signal magnetic field by reading the voltage (V _s ) will be described. Considering the case where the signal magnetic flux Φ is linked to the coil L1, the current I flowing in the closed circuit in FIG.

Given in. When the magnetic flux is interlinked with the coil L2 and [Phi _2,

Given in. Here, if the inductance _{L 2} of the coil L2 is constant,

Since it is proportional to the frequency, it is difficult to measure with high sensitivity in the low frequency region. Therefore, the inductance _{L 2} of the coil L2

Let us consider a case where modulation is performed at time t so that However, ω _m is an angular frequency of modulation, and K represents a modulation degree. At this time, the voltage V _s induced between the terminals of the coil L2 shown in FIG. 1,

Given in. However, with respect to time change of the signal magnetic flux [Phi, modulation of the inductance L ₂ is made sufficiently fast. Therefore, using equation (3), the following equation is obtained when K <1.

As shown in the above equation, when L ₂ is modulated at the angular frequency ω _m and the signal magnetic flux Φ is a direct current, the angular frequency of the voltage V _s is ω _m . When the angular frequency of the signal magnetic flux Φ is ω _s , the angular frequency of the voltage V _s is ω _m ± ω _s , and the voltage V _s is expressed as an AM modulated wave of the signal magnetic flux Φ. That is, the voltage V _s is not proportional to Φ as shown in the first term of the equation (4), but is proportional to Φ. In this embodiment, the voltage value V _s proportional to the signal magnetic flux Φ is measured. It becomes possible to do.

Next, an inductance modulation method will be described. As a modulation method of the inductance of the coil L2, as shown in FIG. 1, the magnetic wire 10 is inserted into the coil L2, flows a bias current I _B in the magnetic wire 10. As described above, it is known that when a current flows through the magnetic wire 10, the magnetic wire 10 is magnetized by this current and its magnetic permeability changes (Reference 1: I. Sasada and S. Harada, “ Fundamental Mode Orthogonal Fluxgate Gradiometer ”, IEEE, TRANSACTIONS ON MAGNETICS, vol.50, No.11, 4007404, 2014, Reference 2: I. Sasada,“ Orthogonal fluxgate mechanism operated with dc biased excitation ”, JOURNAL OF APPLIED PHYSICS, vol .91, No. 10, pp. 7789-7791, 2002). Inductance of the coil L2, in order to vary the magnetic permeability of the magnetic wire 10, it is possible to change the inductance of the coil L2 by the bias current I _B.

Therefore, if the current I _B (t) that changes with time is supplied as the bias current, the inductance of the coil L2 can be temporally modulated as in the following equation.

Here, g is a function representing the dependence of the current I _B of the inductance.

Incidentally, as shown in FIG. 2, in the magnetic sensor according to the present embodiment, depending on the frequency range to be measured, the resistance R _c may be configured to allow adjustment. That is, in the connection resistance between the coil L1 and the coil L2, to adjust the resistance R _c by varying the contact area may be able to measure the frequency fc matching the frequency domain to be measured.

  In the present embodiment, in order to change the inductance of the coil L2 with time, a magnetic wire is inserted into the coil and a current is passed through the magnetic wire to modulate the inductance. The inductance of the coil L2 may be temporally changed by applying an external magnetic field for modulation to the magnetic wire without using a current source.

(Second embodiment of the present invention)
A magnetic sensor according to the present embodiment will be described with reference to FIG. The magnetic sensor according to the present embodiment is a modification of the magnetic sensor according to the first embodiment. In addition, in this embodiment, the description which overlaps with the said 1st Embodiment is abbreviate | omitted.

  FIG. 3 is a diagram illustrating a circuit configuration of the magnetic sensor according to the present embodiment. The magnetic sensor 1 differs from the case of FIG. 1 in the first embodiment in that the self-inductance of the coil L2 is temporally changed in the case of FIG. 1, whereas in the case of FIG. The coil L3 is magnetically coupled to L2, and the mutual inductance between the coil L2 and the coil L3 is changed with time. Moreover, in order to change the mutual inductance of the coil L2 and the coil L3 temporally, the magnetic wire 10 is inserted not in the coil L2 but in the coil L3. The coil L3 may be either a superconducting coil or a normal conducting coil.

In FIG. 3, when the magnetic flux interlinking with the coil L3 is Φ ₃ ,

  Given in. The mutual inductance M between the coil L2 and the coil L3 is

  Let us consider a case where modulation is performed at time t so that At this time, the voltage Vs induced between the terminals of the coil L3 is

  Given in. However, it is assumed that the modulation of M is sufficiently fast with respect to the time change of the signal magnetic flux Φ. Therefore,

It becomes. As in the case of the self-inductance modulation type in the first embodiment shown by the equation (5), the voltage Vs is expressed as an AM modulation wave of the signal magnetic flux Φ. That is, the voltage V _s is proportional to Φ, and the voltage value V _s proportional to the signal magnetic flux Φ can be measured also in this embodiment.

Note that, in the present embodiment as well, as in the case of the first embodiment, the resistance _Rc may be adjusted according to the frequency region to be measured. That is, in the connection resistance between the coil L1 and the coil L2, to adjust the resistance R _c by varying the contact area may be able to measure the frequency fc matching the frequency domain to be measured.

  Also in the present embodiment, in order to change the mutual inductance between the coil L2 and the coil L3 with time, a magnetic wire is inserted into the coil L3, and a current is passed through the magnetic wire, thereby reducing the inductance. For example, the mutual inductance between the coil L2 and the coil L3 may be temporally changed by applying an external magnetic field for modulation to the magnetic wire without using a current source. .

  The following experiment was conducted on the magnetic sensor according to the present invention.

(1) The modulation method of the inductance L ₂ of the coil L2 in the circuit of the inductance modulation Figure 1 of the coil L2, the following experiment was performed. The magnetic wire 10 in FIG. 1 by flowing a bias current _{I B,} changes the inductance _{L 2.} Figure 4 shows the results of measurement of change in inductance when a current of a DC bias current I _B in the magnetic wire 10.

As shown in FIG. 4, when the current I _B flowing through the magnetic wire 10 is I _B = 0, the inductance L ₂ of the coil L ₂ is maximized. This value is L _2,0 . Since the magnetic permeability of the magnetic wire 10 when an electric current is applied I _B is reduced, as shown in FIG. 4, the coil inductance is also reduced. The vertical axis of FIG. 4 indicates the inductance change rate ΔL / L _2,0 . In this experiment, it is understood that when a bias current of I _B = 50 mA is passed, the inductance changes by about 12%. . Note that the sign of the bias current changes symmetrically.

This result, as a method the application of the bias current I _B, 2 two methods can be considered. One is a case where an alternating current is superimposed on a direct current as I _B (t) = I _DC + I _AC sin (ω _m t). In this case, as can be seen from Figure 4, the inductance L ₂ of the coil L2 is changed as follows.

  Therefore, from the equation (5), the signal voltage is given by the following equation.

This is because when the angular frequency of the signal magnetic flux Φ is ω _s , the frequency of the voltage V _s is ω _m ± ω _s . That is, the voltage V _s is expressed as an AM modulated wave of the signal magnetic flux Φ.

On the other hand, when the bias current I _B is I _B (t) = I _AC sin (ω _m t), as can be seen from FIG. 4, the inductance L _{2 of the} coil L2 changes at 2ω _m . That is,

  In this case, the signal voltage is given by the following equation.

This is because when the angular frequency of the signal magnetic flux Φ is ω _s , the frequency of the voltage V _s is 2ω _m ± ω _s . That is, the voltage V _s is expressed as an AM modulated wave of the signal magnetic flux Φ.

(2) Operation experiment of magnetic sensor A magnetic sensor based on the circuit diagram of FIG. 1 was prepared. The coils L1 and L2 made of a high-temperature superconductor are made of a Bi-based high-temperature superconducting tape, and the coil parameters are as follows.
Wire: Sumitomo Electric Co., Ltd., Bi HTS tape, width 3 mm
Inner diameter: Di = 40 mm, outer diameter: Do = 60 mm
Number of windings: N = 27 (pancake coil)

The parameters of the magnetic wire are as follows.
Magnetic wire: Aichi Steel Co., Ltd., amorphous wire (diameter: 100 μm)
Wire resistance: 185 Ω wrapped around HTS coil for 10 turns

Bias current I _B of the amorphous wire is given in the following conditions. In I _B (t) = I _DC + I _AC sin (ω _m t), frequency f _m = 50 kHz, I _DC = −12 mA (direct current), and I _AC = 12 mA (alternating current amplitude). The signal magnetic flux Φ was applied to the coil L1 by a magnetic field application coil.

(2-1) Result of Signal Spectrum FIG. 5 shows the measurement result of the frequency spectrum of the voltage Vs when the frequency of the signal magnetic flux Φ is set to f _s = 2 kHz. As shown in FIG. 5, the signal voltage V _s has a component having a frequency of f _m ± f _s , and it has been clarified that this result agrees with the content of the equation (12). Note that the _{V s} is also present component of the frequency _{f m.} This component part of the magnetic flux generated by the modulation current I _B flowing through the amorphous wire is to interlinked with the coil L2.

(2-2) to low-frequency magnetic field results far above magnetic sensor of the frequency characteristics determine be measured, to measure the voltage V _s of the frequency changes of the signal magnetic flux [Phi. The frequency of the signal magnetic flux Φ was changed to f _s = 0.01, 0.1, 1, 10 Hz, the signal voltage V _s was input to the lock-in amplifier, and synchronous detection was performed at the frequency f _m = 50 kHz.

FIG. 6 is a diagram showing the results of the frequency characteristics. As shown in FIG. 6, a constant signal voltage (lock-in amplifier output V _out ) is obtained regardless of the signal frequency f _s . This indicates that the magnetic sensor according to the present invention can measure a low frequency magnetic field up to 0.01 Hz.

  Further, from the circuit diagram of FIG. 1, the frequency at which the influence of the connection resistance of the coils L1 and L2 can be ignored is

It becomes. and the influence of the resistance _{R c} to f = 0.01 Hz is negligible, considering that the value of the inductance _L 1 + _{L 2} is about 100 .mu.H, the value of _{R c} is _can be evaluated to be a _R c _<6μΩ.

(2-3) In order to examine the magnetic field sensitivity of the results magnetic sensor of the magnetic field sensitivity was measured voltage V _s of when changing the magnitude of the signal magnetic flux [Phi. The frequency of the signal magnetic flux Φ was fixed at f _s = 3 Hz, the amplitude of Φ was changed, the signal voltage V _s was input to the lock-in amplifier, and synchronous detection was performed at the frequency f _m = 50 kHz.

The results are shown in FIG. In FIG. 7, the horizontal axis is obtained by converting the input magnetic flux Φ into the magnetic flux density B by Φ = (π / 4) D _av ² NB using the average radius D _av = 50 mm of the coil L1 and the number of turns N = 27. Yes, the vertical axis represents the input voltage of the lock-in amplifier.

As shown in FIG. 7, a linear relationship is obtained in a wide range between the signal magnetic field B and the output voltage Vs. From this inclination, the sensitivity of the magnetic sensor is V _s /B=24.4 (V / T).

Next, the noise characteristics of the magnetic sensor were measured. With the 3 Hz signal magnetic flux applied, the output of the lock-in amplifier was input to the spectrum analyzer, and the noise spectrum was measured. The result is shown in FIG. In FIG. 8, the horizontal axis represents frequency and the vertical axis is converted to magnetic field noise S _B ^1/2 using the sensor voltage sensitivity given by V _s /B=24.4 (V / T).

As shown in FIG. 8, white noise occurred at f> 5 Hz. The value of this white noise was √S _B = 40 pT / √Hz. At f ≦ 5 Hz, the noise slightly increased as the frequency decreased to √S _B = 100 pT / √Hz (at 1 Hz). From the results of FIG. 6 and FIG. 8, the 3 Hz signal is clearly measured, and the magnetic field noise in the low frequency region is clearly reduced compared to the conventional electromagnetic induction type sensor, enabling high sensitivity detection. It was shown that

L1, L2, L3 Coil Rc Resistance 1 Magnetic sensor 10 Magnetic wire 11 Current source 12 Detector

Claims (7)

A first coil made of a high-temperature superconductor that captures a signal magnetic flux to be measured;
A second coil made of a high-temperature superconductor connected in series to the first coil and detecting the signal magnetic flux captured by the first coil;
A minute resistor that is connected in series to the first coil and the second coil to form a closed circuit;
Voltage measuring means for converting the signal magnetic flux transmitted from the first coil to the second coil into a voltage signal and measuring it;
A magnetic sensor comprising: modulation means for temporally changing the inductance of the second coil. A first coil made of a high-temperature superconductor that captures a signal magnetic flux to be measured;
A second coil made of a high-temperature superconductor connected in series to the first coil and detecting the signal magnetic flux captured by the first coil;
A third coil magnetically coupled to the second coil;
A minute resistor that is connected in series to the first coil and the second coil to form a closed circuit;
Voltage measuring means for measuring a signal magnetic flux transmitted from the first coil to the second coil as a voltage signal of a third coil that is magnetically coupled to the second coil;
A magnetic sensor comprising modulation means for temporally changing the mutual inductance between the second coil and the third coil. The magnetic sensor according to claim 1 or 2,
A magnetic sensor in which the minute resistance is made of a normal conductor connecting the first coil and the second coil, and the resistance value is adjusted according to the measurement frequency. The magnetic sensor according to claim 1,
A magnetic sensor in which the modulation means has a magnetic body inserted into the second coil and a current source for passing a current through the magnetic body. The magnetic sensor according to claim 2,
A magnetic sensor in which the modulation means includes a magnetic body inserted into the third coil and a current source that supplies current to the magnetic body.   A first coil made of a high-temperature superconductor that captures a signal magnetic flux to be measured, and a first coil made of a high-temperature superconductor that is connected in series to the first coil and detects the signal magnetic flux captured by the first coil. A signal transmitted from the first coil to the second coil by temporally changing the inductance of the second coil in a closed circuit formed by two coils and a minute resistor connected in series to the first coil and the second coil A magnetic measurement method comprising measuring magnetic flux by converting it into a voltage signal. A first coil made of a high-temperature superconductor that captures a signal magnetic flux to be measured, and a first coil made of a high-temperature superconductor that is connected in series to the first coil and detects the signal magnetic flux captured by the first coil. A closed circuit is formed by two coils and a minute resistor connected in series to the first coil and the second coil, and the mutual inductance between the second coil and the third coil magnetically coupled to the second coil is temporally determined. A magnetic measurement method, characterized in that a signal magnetic flux transmitted from a first coil to a second coil is measured as a voltage signal of a third coil that is magnetically coupled to the second coil.