JP2005188947A - Magnetic detector - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To precisely detect a magnetic field of high frequency without being influenced by a low frequency field such as an environmental magnetic field even in a low magnetic shielding environment. <P>SOLUTION: This magnetic detector comprises a low-pass filter 1 extracting a predetermined low-pass component from output signal of an integrator 14 generating detection output voltage Vout according to an object of magnetic field detection. According to the output signal of the low-pass filter, a magnetic field corresponding to the extracted low-pass component is generated in a compensation coil 3 and applied to a SQUID 10. According to the output of the integrator, a magnetic field of the corresponding magnitude is generated by a modulation coil 16 and supplied to the SQUID 10. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a magnetic detection device using a magnetic sensor, and more particularly to a magnetic detection device using a superconducting quantum interference element (SQUID) as a magnetic sensor. More specifically, the present invention relates to a configuration for stably detecting a magnetic field to be measured without being affected by environmental magnetic noise such as an environmental magnetic field.

  SQUID (Superconducting Quantum Interference Device) is known as an element that utilizes the Josephson effect. The Josephson effect is a phenomenon in which when a thin insulating film is sandwiched between superconductors, current can flow without causing a voltage difference in the superconductor. When used as a magnetic sensor, the SQUID is generally roughly divided into a structure in which two Josephson junctions are connected in parallel and a structure in which one Josephson junction is used. A SQUID having two Josephson junctions is called a DC-SQUID, and a SQUID having one Josephson junction is called an RF-SQUID. Hereinafter, DC-SQUID will be described as an example of SQUID.

  FIG. 5 is a diagram schematically showing a configuration of a conventional magnetic detection device using a SQUID, which is disclosed, for example, in Patent Document 1 (Japanese Patent Laid-Open No. 2000-292511).

  In FIG. 5, the magnetic detection device includes a magnetic sensor (hereinafter simply referred to as SQUID) 10 composed of a SQUID, a bias current source 12 that supplies a bias current Ib having a constant magnitude to the SQUID 10, and a voltage across the SQUID 10. An amplifying circuit 13, an integrating circuit 14 for integrating the output signal of the amplifying circuit 13 to output a detected output voltage Vout, a feedback resistance element 15 for converting the detected output voltage of the integrating circuit 14 into a current, and the SQUID 10 And a feedback coil (modulation coil) 16 that generates a negative feedback magnetic field in accordance with a current supplied through the resistance element 15.

  The SQUID 10 has a ring shape and has two Josephson junctions 11a and 11b connected in parallel in the ring.

  The superconducting current flows due to the tunneling phenomenon of the Cooper pair that generates the superconducting state. When a current larger than the superconducting critical current value Ic of the SQUID 10 is supplied as the bias current Ib from the bias current source 12, a normal conducting current is generated in the SQUID 10, and a voltage Vs is generated across the SQUID 10. In this state, when a magnetic field φ is applied to the SQUID 10, a shielding current is generated in the SQUID 10 by the Meissner effect so as to prevent the magnetic field from entering the superconducting ring. Depending on the strength of the externally applied magnetic field φ, the phase difference Δφ of the Cooper pair in the Josephson junctions 11a and 11b changes. In this case, the shielding current Is is expressed by the following equation, where the superconducting critical current value is Ic.

Is = Ic · sinΔφ
In the superconductor, the magnetic field is quantized, and its minimum unit is called a quantized magnetic flux (flux quantum) φ0, and the phase difference Δφ is expressed by the following equation.

Δφ = 2 · n · π + 2 · π · (φ / φ0)
Therefore, the shielding current Is periodically changes in units of quantized magnetic flux φ0 (2 · 10 ^ (− 15) Wb: sign ^ indicates a power).

  That is, when the magnetic field φ is applied to the SQUID 10, the bias current value that can maintain the superconducting state is changed by the shielding current. At this time, if the bias current Ib is a constant value, the voltage Vs generated at both ends of the SQUID 10 also changes periodically with the quantized magnetic flux φ0 as a unit. FIG. 6 shows the relationship between the voltage Vs across the SQUID 10 and the applied magnetic field φ.

  As shown in FIG. 6, even if the magnetic field φ changes, if the voltage Vs changes periodically, the absolute amount of change in the magnetic field cannot be uniquely determined. Therefore, the operating point is set by adjusting the amount of current flowing through the modulation coil 16 using a bias circuit (not shown). A method of adjusting the bias current for setting the operating point is disclosed in the above-mentioned Patent Document 1.

  Further, in the voltage-magnetic field characteristic, since the voltage periodically changes, the linear region is narrow, and it is difficult to sufficiently secure the linearity of the characteristic. Therefore, the output signal of the amplifier circuit 13 is fed back to the modulation coil 16 via the integration circuit 14 and the feedback resistance element 15, and the modulation coil 16 generates a magnetic field that cancels the externally applied magnetic field φ. Method "(zero method)" is used to detect the magnetic field. A circuit composed of the amplifier circuit 13, the integrating circuit 14, the feedback resistor element 15, and the modulation coil 16 is called an FLL (flux (magnetic flux) / locked loop) circuit, constitutes a negative feedback circuit, and is externally applied in the SQUID 10. A compensation magnetic field is generated so as to cancel the magnetic field φ. That is, negative feedback is applied so that the sum of the feedback magnetic field and the signal magnetic field is constant, and accordingly, the operating point of the magnetic detection device is maintained constant.

  When the mutual inductance between the modulation coil 16 and the SQUID 10 is Mf, the voltage across the feedback resistance element 15 is represented by (Rf / Mf) φ, and therefore the output voltage Vout from the integration circuit 14 is applied from the outside. It is proportional to the magnetic field φ. By measuring this detection voltage Vout, the applied magnetic field φ can be detected.

During the measurement operation for detecting the magnetic field φ, the slope of the curve in the voltage-magnetic field characteristic, that is, the point where dV / dφ is the largest, for example, the point a shown in FIG. 6 is the operating point of the magnetic detection device (FLL circuit). Thus, the magnetic field can be detected by changing the detection output voltage Vout according to the applied magnetic field φ in the linear region. A magnetic field change smaller than the quantized magnetic flux φ0 can be detected. SQUID has a sensitivity of 10 ^ (-14) Tesla / √Hz when using a low-temperature superconductor and 10 ^ (-13) Tesla / √Hz or less when using a high-temperature superconductor, and is weak. It can measure magnetic fields and is used in various measuring devices and medical devices.
JP 2000-292511 A

  By using the integration circuit 14 shown in FIG. 5, a high frequency component (time differential component) of the output signal of the amplifier circuit 13 can be removed, and a stable detection output voltage Vout corresponding to the detection magnetic field can be generated. When this magnetic detection device operates in a completely magnetically shielded environment, as shown in FIG. 7, the detection output voltage is flat in the region from the direct current to the cutoff frequency fc determined by the amplifier circuit 13. Frequency characteristics can be obtained.

  Now, consider a region A in FIG. 7 as a frequency region of the magnetic field to be measured in a measurable frequency region, for example, in a range from DC to 600 kHz (or several MHz). For example, consider a case where measurement is performed using a magnetic field having a frequency of 100 kHz as a measurement target magnetic field in the measurement region A.

  When the magnetic shielding is insufficient, the environmental magnetic noise component in the frequency region indicated by the region B is superimposed on the signal magnetic field. Examples of such low-frequency environmental magnetic noise include an induced magnetic field of a commercial power line, geomagnetism, and 1 / f fluctuation.

  When the environmental magnetic noise in the low frequency region is superimposed on the signal magnetic field, it cannot be removed by the amplifier circuit 13 and the integrating circuit 14, and as a result, as shown by curve II in FIG. A noise component is superimposed on the output voltage Vout. Here, in FIG. 8, the horizontal axis represents time t, and the vertical axis represents the output voltage Vout.

  When the fluctuation of the environmental magnetic noise such as the environmental magnetic field is large, as shown by a point C in the curve II, the maximum value (power supply voltage) level of the output voltage of the amplifier circuit 13 which is a measurable voltage range is reached. The output voltage Vout corresponding to the signal magnetic field cannot be generated accurately. For example, in the case where the signal magnetic field is a characteristic curve as indicated by the curve I in FIG. There arises a problem that accurate measurement cannot be performed due to environmental magnetic noise.

  Further, when the slew rate of the SQUID 10 cannot follow the change in the environmental magnetic noise, the noise component due to the environmental magnetic noise is removed even if the FLL circuit including the integration circuit 14, the feedback resistance element 15, and the modulation coil 16 is used. The induced magnetic field of the modulation coil 16 is determined according to the voltage level corresponding to the slew rate of the SQUID 10. In this case, as indicated by points D and E in FIG. 8, the output voltage of the integration circuit 14 changes greatly (the operating point changes greatly), and accurate measurement cannot be performed. In particular, when the operating point changes greatly and the operating point moves to a region where the slope of the voltage-magnetic field characteristic curve is opposite, the feedback direction in the FLL circuit is reversed, and positive feedback is applied instead of negative feedback. A state occurs, and accurate magnetic flux measurement cannot be performed.

  In the above-mentioned Patent Document 1, a configuration is shown in which the bias current for the modulation coil is optimized to optimize the operating point of the magnetic detection device. However, environmental magnetic noise such as an environmental magnetic field in these low-frequency regions is present. No consideration is given to the effect.

  In order to remove external magnetic noise, a configuration called a gradiometer that collects a magnetic field with a pickup coil and transmits the magnetic flux to the SQUID with a magnetic flux transformer is also used. However, in this case as well, a similar problem arises because the pick-up coil transmits the environmental magnetic noise component in the low frequency region to the SQUID.

  In order to suppress the influence of such environmental magnetic noise, it is conceivable to complete the magnetic shielding. However, in this case, it is required to take measures such as shielding the entire measurement chamber with a material having high magnetic permeability such as permalloy, which increases costs and increases the scale of magnetic shielding. As a result, there is a problem that the use location of the magnetic detection device is limited.

  SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a magnetic detection device capable of stably detecting a signal magnetic field to be measured without being affected by a low frequency environmental magnetic noise component.

  A magnetic detection device according to the present invention includes a magnetic sensor that detects a magnetic field, and a detection output generation circuit that generates an electric signal corresponding to the detected magnetic field of the magnetic sensor in accordance with an output signal of the magnetic sensor and outputs the electric signal as a magnetic field detection signal. The low-pass filter that receives the magnetic field detection signal and passes the signal component of the low-frequency region of the frequency region of the detection output generation circuit, and the output signal of the low-pass filter according to the output signal of the low-pass filter A negative feedback circuit for generating a corresponding magnetic field and applying it to the magnetic sensor is provided.

  By extracting the low-frequency component from the detection output signal by the low-pass filter, the signal component corresponding to the environmental magnetic noise component can be extracted. By generating a magnetic field corresponding to the output signal of this low-pass filter and applying it to the magnetic sensor, the environmental magnetic noise component applied to the magnetic sensor can be canceled out, and the detection according to the magnetic field to be detected accurately. An output signal can be generated.

[Embodiment 1]
FIG. 1 schematically shows a structure of a magnetic detection device according to the first embodiment of the present invention. In addition to the configuration of the magnetic detection device shown in FIG. 5, the magnetic detection device shown in FIG. 1 further includes a low-pass filter (low-pass filter) 1 that receives the output signal of the integration circuit 14 and passes a predetermined low-frequency component. And a compensation resistive element 2 that converts the output signal of the low-pass filter 1 into a current, and a magnetic field that is magnetically coupled to the SQUID 10 and induces a magnetic field according to the current supplied through the compensation resistive element 2 and supplies the magnetic field generated in the SQUID 10 Compensating coil 3 is included. The other configuration of the magnetic detection device shown in FIG. 1 is the same as the configuration of the magnetic detection device shown in FIG. 5, and corresponding portions are denoted by the same reference numerals, and detailed description thereof is omitted.

  The low pass filter 1 passes a signal in a predetermined low frequency region including a frequency region of a low frequency noise component (hereinafter referred to as environmental magnetic noise) such as an environmental magnetic field.

  When the compensation coil 3 is not provided, the detection output voltage Vout from the integration circuit 14 includes a voltage component corresponding to a detection target magnetic field in a high frequency region and a voltage component corresponding to environmental magnetic noise. The low-pass filter 1 extracts a signal component in the frequency domain corresponding to this environmental magnetic noise domain (region B in FIG. 7), and supplies a current to the compensation coil 3 via the compensation resistor 2. Accordingly, a magnetic field that cancels environmental magnetic noise is generated from the compensation coil 3 and supplied to the SQUID 10. Thereby, the voltage Vs corresponding to the measurement target magnetic field in which the environmental magnetic noise component is suppressed is generated from the SQUID 10, and the detection output voltage Vout corresponding to the measurement target magnetic field can be generated from the integration circuit 14. . When the environmental magnetic noise is compensated by the compensation coil 3, the FLL circuit generates a magnetic field corresponding to the measurement target magnetic field in which the fluctuation component of the low-frequency magnetic field caused by the environmental magnetic noise is suppressed, and the SQUID 10 Apply.

  That is, the low-frequency component caused by the environmental magnetic noise which is a problem included in the detected output voltage Vout is extracted by the low-pass filter 1, and the magnetic field for environmental magnetic noise compensation is generated by the compensation coil 3. The fluctuation of the low frequency magnetic field caused by the magnetic noise can be suppressed, and the magnetic field to be measured can be accurately detected. In other words, it is possible to eliminate problems such as jumping of the operating point of the FLL circuit and saturation of the measurement voltage caused by environmental magnetic noise in a region of a frequency lower than the magnetic field to be measured. The magnetic field to be measured can be measured stably and accurately.

  As described above, according to the first embodiment of the present invention, the low frequency component corresponding to the environmental magnetic noise is extracted from the input / output signal by the low pass filter, and the compensation magnetic field corresponding to the extracted low frequency component is generated. Then, it is applied to the SQUID which is a magnetic sensor. Therefore, the influence of the fluctuation of the low frequency magnetic field caused by the environmental magnetic noise can be reliably suppressed, and the measurement target magnetic field in the high frequency region can be measured.

[Embodiment 2]
FIG. 2 schematically shows a configuration of a magnetic detection device according to the second embodiment of the present invention. In addition to the configuration of the magnetic detection device shown in FIG. 1, the magnetic detection device shown in FIG. 2 further includes an amplifier 4 that amplifies the output signal of the low-pass filter 1, and an attenuator (attenuator) that attenuates the output signal of the amplifier 4. An attenuator) 5 is provided. The output signal of the attenuator 5 is given to the compensating resistance element 2.

  The other configuration of the magnetic detection device shown in FIG. 2 is the same as the configuration of the magnetic detection device shown in FIG. 1, and corresponding portions are denoted by the same reference numerals, and detailed description thereof is omitted.

  The low-pass filter 1 extracts a signal component corresponding to environmental magnetic noise in a predetermined low-frequency region from the output voltage Vout of the integration circuit 14 as in the first embodiment. The amplifier 4 and the attenuator 5 adjust the output signal of the low-pass filter 1 to a predetermined magnitude, give it to the resistance element 2, generate a current, and give it to the compensation coil 3. By adjusting the amplification factor of the amplifier 4 and the attenuation factor of the attenuator 5 in accordance with the magnetic coupling coefficient (transconductance) between the compensation coil 3 and the SQUID 10, the low frequency magnetic field caused by the environmental magnetic noise is more accurately adjusted. Fluctuation can be suppressed. Thereby, the magnetic field to be measured in the high frequency region higher than the cutoff frequency of the low-pass filter 1 can be accurately detected.

  That is, when the low frequency component of the output signal voltage of the integration circuit 14 is extracted by the low pass filter 1, if the influence of the environmental magnetic noise remains largely, the feedback of the environmental magnetic noise is insufficient, and the amplifier 4 Increase the amplification factor. On the other hand, when the environmental magnetic noise is excessively compensated and the influence of the environmental magnetic noise occurs in the reverse direction and increases, the attenuation factor of the attenuator 5 is increased. By adjusting the amplification factor of the amplifier 4 and the attenuation factor of the attenuator 5 according to the operating characteristics of the amplifier circuit 13 and the integration circuit 14, the influence of environmental magnetic noise can be suppressed accurately.

  A configuration in which a phase adjustment circuit is arranged between the attenuator 5 and the compensation resistance element 2 so that the phase of the compensation magnetic field with respect to environmental magnetic noise is adjusted may be used.

  As described above, according to the second embodiment of the present invention, the amplitude of the signal component of the low-frequency magnetic field component extracted by the low-pass filter is further adjusted by the amplifier circuit and the attenuator. A compensation magnetic field according to the magnetic noise can be generated to suppress the environmental magnetic noise.

[Embodiment 3]
FIG. 3 schematically shows an overall configuration of the magnetic detection apparatus according to the third embodiment of the present invention. The magnetic detection device shown in FIG. 3 differs from the magnetic detection device shown in FIG. 1 in the following points. That is, the output current of the feedback resistance element 15 that receives the output voltage Vout of the integration circuit 14 and the output current of the compensation resistance element 2 that receives the output signal of the low-pass filter 1 are added and supplied to the modulation coil 16. In the magnetic detection device shown in FIG. 3, a dedicated compensation coil for compensating for environmental magnetic noise is not provided. The other configuration of the magnetic detection device shown in FIG. 3 is the same as the configuration of the magnetic detection device shown in FIG. 1, and corresponding portions are denoted by the same reference numerals, and detailed description thereof is omitted.

  In the magnetic detection device shown in FIG. 3, the current from the feedback resistance element 15 and the current from the compensation resistance element 2 are added and supplied to the modulation coil 16. Accordingly, a magnetic field corresponding to the measurement target magnetic field and a magnetic field corresponding to the low frequency magnetic field caused by the environmental magnetic noise are generated from the modulation coil 16 and supplied to the SQUID 10. Therefore, in the SQUID 10, even when the low-frequency magnetic field component is accurately suppressed and the slew rate of the SQUID 10 cannot follow the low-frequency magnetic field fluctuation, the output voltage Vs corresponds to the output voltage corresponding to the measurement target magnetic field. Thus, the detection output voltage Vout corresponding to the measurement target magnetic field in the high frequency region can be generated by the amplification circuit 13 and the integration circuit 14.

  In the case of the configuration of the magnetic detection device shown in FIG. 3, the modulation coil 16 is also used as a low-frequency magnetic field compensation coil, so that the number of circuit components can be reduced and the circuit scale can be reduced accordingly.

  In addition, it is not necessary to adjust the arrangement position for each of the compensation coil and the modulation coil in consideration of the magnetic coupling coefficient, and the magnetic field corresponding to the environmental magnetic field component is efficiently superimposed on the magnetic field corresponding to the measurement target magnetic field component. Accordingly, the low-frequency magnetic field noise component corresponding to the environmental magnetic noise can be suppressed.

[Embodiment 4]
FIG. 4 schematically shows an overall configuration of the magnetic detection apparatus according to the fourth embodiment of the present invention. The magnetic detection device shown in FIG. 4 differs in configuration from the magnetic detection device shown in FIG. 2 in the following points. That is, the output current of the compensation resistance element 2 that receives the output signal of the attenuator 5 is supplied to the modulation coil 16 together with the supply current of the feedback resistance element 15. In the configuration shown in FIG. 4, the dedicated environmental magnetic noise compensation coil (3) is not provided.

  The other configuration of the magnetic detection device shown in FIG. 4 is the same as the configuration of the magnetic detection device shown in FIG. 2, and corresponding portions are denoted by the same reference numerals, and detailed description thereof is omitted.

  The configuration of the magnetic detection device shown in FIG. 4 has the following effects in addition to the effects of the magnetic detection device shown in FIG. That is, the modulation coil 16 is also used as a low-frequency magnetic field component compensation coil including environmental magnetic noise, and accordingly, the number of circuits can be reduced and the device scale can be reduced. Moreover, a low frequency magnetic field component can be efficiently suppressed as in the configuration of the third embodiment shown in FIG.

  As described above, according to the fourth embodiment of the present invention, the amplifier and the attenuator for generating the compensation magnetic field corresponding to the low frequency magnetic field component are provided, and the modulation coil for detecting the measurement target magnetic field is further provided. Is also used as a low-frequency magnetic field compensation coil, and an unnecessary low-frequency magnetic field component can be accurately suppressed with a small number of circuits, and an output voltage corresponding to the measurement target magnetic field can be generated.

  The magnetic detection device according to the present invention can be used for a device that measures a magnetic field by SQUID, and can be applied to various uses such as biomagnetic field measurement and non-destructive inspection.

  In the above description, the SQUID is used as the magnetic sensor. As this SQUID, a high-temperature superconductor that becomes a superconductor state at a liquid nitrogen temperature may be used, or a low-temperature superconductor that becomes a superconductor state at a liquid helium temperature may be used.

  Further, as the SQUID, an RF-SQUID including one Josephson junction may be used.

  As a configuration of the magnetic detection device, a configuration may be used in which a measurement target magnetic field is detected using a pickup coil and the measurement target magnetic field is measured by magnetically coupling the pickup coil to a SQUID.

  The magnetic sensor may be an MI (magnetic impedance) sensor using an amorphous metal wire, a magnetic sensor using another semiconductor element, or a magnetic sensor using a magnetoresistive effect. .

It is a figure which shows roughly the whole structure of the magnetic detection apparatus according to Embodiment 1 of this invention. It is a figure which shows schematically the whole structure of the magnetic detection apparatus according to Embodiment 2 of this invention. It is a figure which shows roughly the whole structure of the magnetic detection apparatus according to Embodiment 3 of this invention. It is a figure which shows roughly the whole structure of the magnetic detection apparatus according to Embodiment 4 of this invention. It is a figure which shows schematically the whole structure of the conventional magnetic detection apparatus. It is a figure which shows the relationship between the output voltage of SQUID, and an applied magnetic field. It is a figure which shows the frequency characteristic of the conventional magnetic detection apparatus. It is a figure for demonstrating the problem of the conventional magnetic detection apparatus.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 Low pass filter, 2 Compensation resistance element, 3 Compensation coil, 4 Amplifier, 5 Attenuator, 10 SQUID, 12 Bias current source, 13 Amplification circuit, 14 Integration circuit, 15 Feedback resistance element

Claims (5)

A magnetic sensor for detecting a magnetic field,
A detection output generating circuit that generates an electric signal corresponding to the detected magnetic field of the magnetic sensor in accordance with the output signal of the magnetic sensor and outputs the electric signal as a magnetic field detection signal;
A low-pass filter that receives the magnetic field detection signal and passes a signal component in a low-frequency region of the frequency region of the detection output generation circuit; and an output of the low-pass filter according to the output signal of the low-pass filter A magnetic detection device comprising a low-frequency negative feedback circuit that generates a magnetic field corresponding to a signal and applies the magnetic field to the magnetic sensor.   The magnetic detection device according to claim 1, wherein the low-pass filter passes a signal in a frequency region lower than a frequency region of a magnetic field to be measured.   The magnetic detection apparatus according to claim 1, wherein the magnetic sensor includes a superconducting quantum interference element. The magnetic detection device further includes a feedback magnetic field generation circuit that generates a magnetic field corresponding to the magnetic field detection signal generated by the detection output generation circuit and applies the magnetic field to the magnetic sensor.
The detection output generation circuit includes:
An amplifier for receiving an output signal of the magnetic sensor;
An integration circuit that receives the output signal of the amplifier and generates the magnetic field detection signal;
The low-frequency negative feedback circuit is
The magnetic detection device according to claim 1, further comprising a compensation coil that generates a magnetic field corresponding to an output signal of the low-pass filter and applies the magnetic field to the magnetic sensor. The detection output generation circuit includes:
An amplifier for receiving an output signal of the magnetic sensor;
An integration circuit that receives the output signal of the amplifier and generates the magnetic field detection signal;
The low-frequency negative feedback circuit includes a feedback magnetic field generation coil that generates a magnetic field corresponding to a magnetic field detection signal generated by the integration circuit and an output signal of the low-pass filter and applies the magnetic field to the magnetic sensor. The magnetic detection device according to 1.

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