JP4137836B2 - Apparatus and method for measuring weak magnetic field - Google Patents

Description

  The present invention relates to a measurement apparatus and a measurement method for detecting a weak magnetic field, magnetism, and magnetic field while moving a superconducting quantum interference device (SQUID) magnetic sensor itself.

  In recent years, the importance of non-destructive inspection technology has increased with the demand for improvements in reliability and accuracy in the manufacture and maintenance of structural materials such as aircraft, railway vehicles, ships, power generation equipment, bridges and high-rise buildings. . In particular, large-scale facilities that constitute social infrastructures such as the energy and transportation fields have recently become difficult to secure new locations and land, so efficient operation of existing facilities and life extension are required. The potential needs for non-destructive inspection of materials and structures are high, and recently, problems such as cracks in core bulkheads (shrouds) of nuclear power plants and cracks in railway vehicles have been highlighted.

  For this reason, in addition to conventional nondestructive inspection methods such as X-rays, ultrasonic waves, and eddy current flaws, development of highly accurate nondestructive inspection techniques is expected.

A superconducting quantum interference device is an ultra-sensitive magnetic sensor capable of measuring a magnetic field as small as one hundred million to one billionth of geomagnetism (several tens of microtesla) (see, for example, Non-Patent Document 1).
It has become clear at the laboratory level that SQUID can be used for non-contact inspection of deterioration of materials such as fine cracks and damage, as well as scratches inside structures, etc. Recently, the SQUID magnetic sensor has entered the field of non-destructive inspection. The expectation of the use of is increasing.

  In the field of biomagnetism measurement such as magnetoencephalograph and magnetocardiograph, clinical equipment using SQUID has been commercialized and used for epilepsy treatment and analysis of higher brain function, and examination of heart disease. There are also attempts to use SQUID in the field of authentication of banknotes and securities.

  On the other hand, the SQUID magnetic sensor is highly sensitive and detects a magnetic field much smaller than the environmental magnetic noise as described above, so special equipment such as a magnetic shield room has been required. Also, due to environmental magnetic noise and vibration problems, it has been impossible to perform continuous inspection while moving the SQUID magnetic sensor itself. For this reason, the SQUID magnetic sensor is fixed and the subject itself is moved, or although it is very expensive, the SQUID magnetic sensor is fixed and a large number of sensors of about 50 to 100 channels are arranged as in biomagnetic measurement. The method of making has been taken for many years.

  In the conventional inspection method for fixing a single SQUID magnetic sensor, only a small object such as a millimeter to a centimeter can be handled as an object, for example, a power plant, a bridge, a high-rise building, an aircraft, etc. Large-scale testing of subjects cannot be performed non-destructively. Moreover, the inspection apparatus is a desktop stationary type, and there is a problem that installation on an inspection object (work) is not free in various fields such as a power plant and a construction site.

  Based on the above, (1) There are no restrictions on the size and dimensions of the subject, (2) The installation is free in various fields, and (3) The SQUID magnetic sensor itself moves. Development of mechanisms and mobile devices is desired.

  The SQUID is a magnetic sensor that uses the characteristics of superconductivity, and is an ultra-sensitive magnetic sensor that can measure extremely weak magnetic fields, from 1/100 to 1/100 million of geomagnetism (several tens of micro Tesla).

As shown in FIG. 1, a magnetic sensor using SQUID is mainly composed of a SQUID and a detection coil for detecting a magnetic field. SQUID is based on the phenomenon of superconductivity, which is the phenomenon of quantization of magnetic flux in the superconducting ring, and the output voltage changes in units of quantum flux Φ ₀ (2.07 × 10 ⁻¹⁵ Wb) as the external magnetic flux changes. It is a circuit to do. The operation principle will be briefly described below (for example, see Non-Patent Document 2).

A structure in which two superconductors are sandwiched by a barrier (for example, a thin insulator, a semiconductor, or a confinement of several hundred nm or less to the superconductor) is called a Josephson junction. Although details are omitted, there is an effect that the phase difference of the tunnel current at both ends of the Josephson junction depends on the location in the barrier plane in the magnetic field, and SQUID is an element that skillfully uses the magnetic field effect of this Josephson junction. ing. Among various SQUIDs, DC-SQUID has the highest sensitivity and is mainly used at present, so DC-SQUID will be described as an example. When operated to connect the power to the Josephson junction, if the impedance is constant current operation due to low number, I called the constant current and the bias current I _B.

For simplicity, if the junction capacitance and self-inductance are ignored, the maximum bias current I _B ^max that can flow in the superconducting state and the total magnetic flux φ have the following relationship.
I _B ^max = 2I ₀ | cos (πΦ / Φ ₀ ) |
That is, the superconducting critical current is greatest when a magnetic flux that is an integral multiple of the flux quantum Φ ₀ is sensed, becomes zero when it is a half-integer multiple, and periodically varies with the external magnetic flux penetrating the ring. Further, when the bias current is fixed to a certain value, the DC voltage of the element also periodically changes with respect to the external magnetic flux, reflecting the change of the critical current. Actually, in order to obtain an element capable of converting magnetic flux / voltage, it is necessary to consider various conditions such as the size of the superconducting ring related to magnetic field sensitivity and the magnitude of bias current that does not cause hysteresis. The above parameters can be solved analytically by numerical calculations. As an example, if R = 1Ω, self-inductance L = 1 nH, and bias current I _B = 1 μA because of the shunt resistance (because the Josephson junction tunnel resistance is large), an element that can obtain a voltage of 2 μV per Φ ₀ can be designed. .

As described above, since SQUID has a magnetic flux-voltage conversion action, it can be used in principle as a highly sensitive magnetic sensor if amplified using an amplifier. However, because (1) the relationship between magnetic flux and voltage is non-linear, accurate conversion is cumbersome and cannot be used for general purposes. (2) At low frequencies, the effect of amplifier drift is unavoidable. (3) Period There are drawbacks such as slight deviations from large characteristics resulting in large gain fluctuations, etc., and in general they cannot be used as they are. Therefore, as shown schematically in FIG. 2, by combining negative feedback, linearity is obtained over a wide input range, and the magnetic flux quantum Φ ₀ is one period, which is about 1 / 100,000. Is devised so that the magnetic flux resolution can be obtained. In the circuit block diagram shown in FIG. 2, the synchronous detection circuit and the modulation circuit within the broken line frame are not necessarily required.

In FIG. 2, first consider the case where the feedback circuit of the element is open. SQUID is flushed with bias current, flowing a modulation current of several 100kHz to modulation coil (feedback coil), the amplitude of the modulated magnetic flux is about ± Φ _0/4 penetrating the element AC modulation magnetic flux [Phi _m time-varying Add as follows. Relationship between the magnetic flux φs by the waveform and the measurement field of the SQUID output voltage V by receiving modulated flux [Phi _m, depending on φs changes as in FIG. 3 (a) ~ (c) . In the case of φs = 2nΦ ₀ (see FIG. 3A), an alternating voltage consisting only of harmonics that are an even multiple of the modulation current is generated at both ends of the element. Next, in the case of φs ≠ 2nΦ ₀ (see FIGS. 3B and 3C), the voltage across the element includes the frequency of the measured magnetic field, and this voltage is detected synchronously by a lock-in amplifier or the like. When passed through the circuit, as shown in FIG. 3D, a voltage signal including only the frequency component of φs is obtained. Here, the modulation current is shown as a sine wave, but a method of modulating with a rectangular wave to increase the detection efficiency is also conceivable. Also, the frequency of the modulation current is not particularly limited in the range of several kHz to several GHz. In addition, a synchronous detection circuit and modulation are not necessarily required, and a driving method for applying negative feedback without using the synchronous detection circuit and modulation is also possible. Also in this case, as shown in FIG. 3D, a voltage signal including only the frequency component of φs is obtained at the output of the amplifier.

Next, by closing the feedback circuit of FIG. 2, by reversing the sign of the signal, applying a different feedback unsigned equal φs and size mended the feedback magnetic flux phi _f. As a result, .phi.s is the larger the phi _f from point B as plotted decreased as an example in FIG. 3 (d), φs conversely becomes the phi _f is increased always constant the sum of .phi.s and phi _f small , It works to keep it at one point on the magnetic flux-voltage characteristics. That is, the feedback acts and the SQUID operates so that the magnetic flux penetrating the SQUID is in a limited range very close to the zero point in FIG. Here, point B is the operating point, this negative feedback is called a flux-locked loop (FLL), and the state where SQUID is operating normally by feedback is called the locked state. Actually, as shown in FIG. 3D, the φ-V characteristic is infinite, and there are an infinite number of feedback operating points (stable points) such as A, B, C. Whether the peak or valley is the operating point depends on the direction of the coil and the phase angle of detection. In the present invention, as will be described later, this infinitely-shaped φ-V characteristic is used. Furthermore, by using the feedback of the FLL system, the non-linear φ-V characteristic has a linearity as shown in FIG. 4A and can be used as a magnetic sensor. Although the modulation type has been described here, the principle of linearization by feedback is the same in the non-modulation type.

Next, let us consider the operation of SQUID when magnetic noise with a high slew rate (product of signal amplitude and frequency) enters. When the magnetic noise enters in a state in which the lock is applied, feedback takes it causes a situation occurs for only different operating point integer multiple of phi ₀ at its front and rear. For example, if the input magnetic flux of the SQUID operating at the operating point B in FIG. 3D changes by Δφs = 1 / 2φ _{0 to} 3 / 2φ ₀ at a certain time, the SQUID output is as shown in FIG. The circuit tries to return to the operating point B so that a large output corresponding to Δφs is shown and the actual input magnetic flux becomes zero. However, in this situation, feedback is no longer applied to the direction away from the operating point B, and a stable state occurs at another operating point C. The linearized φ-V characteristic is shown in FIG. Thus, an unnecessary offset is added. Thus, when magnetic noise with a high slew rate is sensed, the continuity of measurement is lost before and after that, and even a state where no lock is applied occurs depending on the magnitude of the magnetic noise. As described above, the characteristic of SQUID is that it essentially does not work when noise exceeding a certain threshold value enters SQUID. The reason why the inspection that moved the SQUID magnetic sensor itself for many years was not related to a fatal problem that goes into the operation principle of SQUID is described below.

  FIG. 5 schematically shows the relationship between the environmental magnetic noise and the signal level detected by the SQUID magnetic sensor. As the SQUID magnetic sensor itself moves, the spatial (and temporal) magnetic component due to environmental magnetic noise simplified as a sine wave (low frequency) changes greatly. On the other hand, the weak magnetic signal emitted from the subject is a component having a higher frequency added to the low frequency component. If it cannot be separated from ambient magnetic noise, the target magnetic signal will be buried in the noise.

Qualitatively, it was found that the SQUID magnetic sensor is weak against magnetic noise, so we will consider a specific example of geomagnetic noise. Usually, in order to detect a 1 mm flaw, the detection coil connected to the SQUID must be 1 mm or less. If the detection coil is 1 mmφ, the change in the quantum magnetic flux Φ _{0 corresponds to} a change of about 1 × 10 ⁻⁸ T (10 nanotesla) from simple area conversion.

In order to simplify the effects of moving the SQUID magnetic sensor itself while receiving environmental magnetic noise, we will evaluate only the geomagnetism. When the coil of 1mmφ is to be 1mm movement, since geomagnetism level is about 0.5 × ^{10 -4} T, seen from a simple calculation of undergoing a change in external magnetic flux corresponding to approximately 100Φ _0. In this situation, the SQUID reference point changes 100 times at a stretch, and a dynamic range of 100 db is required. Therefore, the fixed operating point is lost (the lock is released), and the SQUID does not operate normally. Does not work. As shown in FIG. 4, at the first point, there is no offset as shown in FIG. 4 (a), at the second point, FIG. 4 (b), at the third point, FIG. 4 (c),. Fortunately, even if several consecutive measurements can be performed, the influence of the magnetic field due to geomagnetism is accumulated more and more, and the sensor is finally shaken out. Depending on the magnitude of the ambient magnetic noise, it is often even impossible to make a single measurement using a SQUID magnetic sensor.

  The above is underestimation considering only geomagnetism. Actually, when the SQUID magnetic sensor itself is moved, it is affected by environmental magnetic noise that is more severe than this condition. As described above, the influence of the environmental magnetic noise generated by the movement of the SQUID magnetic sensor itself is extremely large, and in order to measure a magnetic signal smaller than the environmental magnetic noise, measurement by moving the magnetic sensor itself cannot be performed.

  For this reason, as a prior art, a measurement method in which a SQUID magnetic sensor is fixed and a subject is moved is generally performed. This is because if the magnetic sensor is fixed, the influence of environmental magnetic noise such as geomagnetism that changes due to movement can be reduced. However, this method has a problem that installation in a field where the size and dimensions of the subject are restricted is restricted.

  In the case of biomagnetic measurement using SQUID, a case of myocardial infarction has been reported as a clinical application example of magnetocardiography (see, for example, Non-Patent Document 3). The measurement disclosed in this document is performed in a magnetic shield room. The xyz coordinate of the chest is determined using the bed horizontal surface on which the subject (patient) is placed as the xy plane and the two places on the body surface as indices. The inspection results obtained by simultaneous measurement with high time resolution by arranging 64 channels (8 vertical x 8 horizontal) SQUIDs at 2.5 cm intervals are disclosed. In this examination, both the subject (patient) and SQUID are stationary at the time of measurement, and the measurement is not performed by moving the SQUID magnetic sensor itself.

Further, a simple inspection device has been disclosed in which the SQUID magnetic sensor unit is covered with a magnetic shielding container so that a magnetically stable inspection result can be obtained (see, for example, Patent Document 1). In the apparatus of Patent Document 1, when a linear object passes under the SQUID detection portion placed inside the magnetic shielding container, a system configuration that captures a change in magnetic signal when there is a foreign object or a defect. It has become. For this reason, the subject is a fiber, a cable, or a wire, and in this case, the subject is also moved and the SQUID is fixed. This system can be applied to inspection of rolled steel as a large-sized one. However, it cannot be applied to large-scale structures such as power plants, bridges, high-rise buildings, and aircraft.
As described above, in any of the preceding cases, there is no disclosure of a technique for performing continuous measurement by moving the SQUID magnetic sensor itself.
JP-A-7-146277 Saburo Tanaka "Possibility of new applied measurement using high temperature superconducting quantum interference device (SQUID)" Applied Physics Editorial Committee, Applied Physics, 2003, Vol. 72, No. 8, p. 1039-1045 Tsunetaro Sakudo, “Solid Physics (Magnetic / Superconducting)”, Soka Hana, September 25, 1993, p. 136-138 Satsuki Yamada, Keiji Tsukada, Satoshi Yamaguchi, "Arrhythmia diagnosis by magnetocardiography" Medical School, Respiration and Circulation, December 2000, Vol. 48, No. 12, p. 1207

  In the present invention, the SQUID magnetic sensor itself is moved on the surface of the subject using a superconducting quantum interference device (SQUID), and the magnetism of the sample to be examined is continuously detected in a non-contact manner to obtain the magnetic distribution of the subject. An inspection apparatus that performs nondestructive inspection and an inspection method thereof are proposed.

  If the SQUID magnetic sensor moves the magnetic sensor itself in the environment magnetic noise such as geomagnetism, the dynamic that the sensor is shaken off by the environmental magnetic noise such as geomagnetism or the fixed lock of the SQUID magnetic sensor is released as described above. The range problem needs to be solved.

  In order to solve the above problems, the present invention provides a sensor having a periodic input / output characteristic with respect to a magnetic field or a magnetic flux and an output from the sensor, thereby locking the operating point to a single point. In conjunction with the movement / stop by the movement control unit, a sensor driving mechanism for detecting a weak magnetic field generated by, a movement mechanism for moving at least the sensor, a movement control unit for controlling movement / stop of the movement mechanism, The weak magnetic field measuring device includes a measurement operation control unit that controls switching between the locked state and the unlocked state of the sensor driving mechanism.

  The present invention also includes a detection coil and a SQUID element (superconducting quantum interference element) that outputs an electric signal in accordance with a magnetic flux based on a magnetic field detected by the detection coil, and the electric signal output from the SQUID element By feeding back to the SQUID element as magnetic flux, the SQUID magnetic sensor for measuring the weak magnetic field generated by the subject by locking the operating point to one point on the periodic voltage / flux characteristics of the SQUID element, and the SQUID magnetic sensor A movement mechanism for moving, a movement control unit for repeatedly moving / stopping the movement mechanism, and scanning the magnetic field measurement range of the subject with the SQUID magnetic sensor, and movement / stopping of the SQUID magnetic sensor by the movement control unit Linked to the measurement operation control unit that repeatedly operates the unlocked state and the locked state of the SQUID magnetic sensor, and the AC signal that applies an AC signal to the subject. And applying mechanism, and a weak magnetic field measuring device with a.

  In addition, the present invention provides a sensor having a periodic input / output characteristic with respect to a magnetic field or magnetic flux, and a weak magnetic field generated by a subject by locking an operating point to one point by feeding back an output from the sensor. A sensor driving mechanism for detecting, when scanning and measuring the object by the moving mechanism, the sensor unlocking step, the moving step of moving the measurement position in the unlocked state, and the target measurement position The weak magnetic field measurement method includes a movement stop step for stopping movement, a lock step for bringing the sensor into a locked state, and a data acquisition step for measuring a magnetic field in the locked state.

  Furthermore, the present invention includes a detection coil and a SQUID element (superconducting quantum interference element) that outputs an electric signal according to a magnetic flux based on a magnetic field detected by the detection coil, and the electric signal output from the SQUID element is Using the SQUID magnetic sensor that measures the weak magnetic field generated by the subject by locking the operating point to one point on the periodic voltage / magnetic flux characteristics of the SQUID element by feeding back to the SQUID element as magnetic flux, Scans the magnetic field measurement range on the subject by repeatedly moving / stopping in the moving process and moving stop process, and at that time, the SQUID magnetic sensor is unlocked by the SQUID unlocking process at the time of movement. The SQUID magnetic sensor was locked by the SQUID lock process when stopped, and the magnetic field was measured in the data acquisition process.

  In the present invention, a SQUID magnetic sensor or a sensor having a periodic input / output characteristic with respect to magnetic flux such as SQUID is used, and the sensor or the SQUID magnetic sensor is scanned on the subject by a moving mechanism. The movement process that moves the sensor and the movement stop process that stops are controlled by the movement control unit, and the SQUID lock process and SQUID lock release process are switched by the measurement operation control unit in synchronization with the movement / stop. Control the operating state of the sensor. It was devised that the sensor or SQUID magnetic sensor is temporarily put into a dormant mode by setting it to the unlocked state while moving, and the measurement is performed in a locked state in which the magnetic field can be measured only when stopped. In the data acquisition process, an AC signal application mechanism that applies an AC signal to a non-analyte gives an AC stimulus such as current, voltage, magnetism, light, and heat, and measures a magnetic change that occurs in response. It is also possible.

A SQUID magnetic sensor will be described as an example.
First, as mentioned above, when the SQUID magnetic sensor moves from one A point to the next A 'point, the influence of environmental magnetic noise is large even if it is about 1 mm. Therefore, during the continuous measurement in which the sensor itself runs, the SQUID magnetic sensor continuously repeats the SQUID lock process and the SQUID lock release process as shown in FIG.

The changes in the environmental magnetic noise is expressed as B ^EX _n, a weak signal emanating from the object as a B ^INT _n, move the SQUID magnetic sensor, the measured value B ^total according to the prior art,
In the first point, B ^total _n = B ^INT _n , in the second point, B ^total _{n + 1} = B ^EX _{n + 1} + B ^INT _{n + 1} , and in the third point, B ^total _{n + 2} = (B ^EX _{n + 1} + B ^EX _{n + 2} ) + B ^INT _{n + 2 In} the fourth point, B ^total _{n + 3} = (B ^EX _{n + 1} + B ^EX _{n + 2} + B ^EX _{n + 3} ) + B ^INT _{n + 3} ... And B ^EX _n >> B ^INT _n as described above. Even if measurement up to the second point is possible, an offset occurs as shown in FIG. 4, and the dynamic range is always exceeded at some points.

On the other hand, the present invention is characterized in that when the SQUID magnetic sensor moves, the SQUID magnetic sensor is unlocked and an instantaneous measurement pause state is created so as not to detect environmental magnetic noise such as geomagnetism. As in the prior art, moving the SQUID magnetic sensor while locked SQUID magnetic sensor, based even when moved slightly 1mm as described above as an example is changed by about 100Fai ₀ also geomagnetism. Therefore, as a conventional method, the SQUID magnetic sensor is fixed and the measurement is performed by moving the subject in order to make the spatial change of the environmental magnetic noise constant. In this method, nondestructive inspection of a large subject is performed. Is impossible.

  On the other hand, in the present invention, during the movement of the SQUID magnetic sensor, the SQUID magnetic sensor is temporarily put into the sleep mode by temporarily setting the SQUID magnetic sensor to the unlocked state by the SQUID unlocking process. Scan.

By newly adding a means for providing such a temporary sleep mode, it is possible to avoid detecting unnecessary environmental magnetic noise such as geomagnetism that is detected when the SQUID magnetic sensor moves (for example, about 100Φ _{0 at} 1 mm). Only the weak magnetism generated from the subject can be taken out, and continuous measurement can be performed by repeating the locked state and the unlocked state during the scanning of the SQ UID magnetic sensor. On the circuit, as shown in Fig. 3 (d), the SQUID's φ-V characteristic has an infinitely periodicity. By utilizing this periodicity, the magnetic noise is avoided by releasing the lock once. In the locked state, a new point of infinite φ-V characteristics can be fixed as an operating point, and the SQUID magnetic sensor can be operated without causing an unnecessary offset.

That is, when the SQUID magnetic sensor is moved with the change in the environmental magnetic noise represented as B ^EX _n and the weak signal emitted from the subject as B ^INT _n , the measured value B ^total according to the present invention is as follows: B ^total _n = B ^INT _n , B ^total _{n + 1} = B ^INT _{n + 1 at} the second point, B ^total _{n + 2} = B ^INT _{n + 2 at} the third point, B ^total _{n + 3} = B ^INT _{n + 3} · at the fourth point As shown in FIG. 4D, the offset can be eliminated and only a weak signal emitted from the subject that should be measured can be measured. In this way, the method of instantaneously releasing the lock and pausing the operation has the effect of canceling the influence of environmental magnetic noise, and moving the SQUID magnetic sensor itself may cause the sensor to shake out or the SQUID locked state to be released. It can solve the problem of dynamic range.

  The inspection speed requires about 30 to 40 minutes in the case of measuring 10,000 points with a 1 mm pitch in a 10 cm square in the prior art. On the other hand, in the present invention, although means for newly providing a temporary pause mode of the SQUID operation is added, ON / OFF of the lock state can be performed instantaneously (<10 milliseconds) without delay. By adopting the present invention, the inspection time is not prolonged. On the other hand, the weight and dimensions of the subject are limited in the prior art because the test is performed by placing the subject on the xyz stage, but the present invention is not limited in principle because the SQUID magnetic sensor itself scans. . In one example, since the maximum load of a commercially available xyz stage is 7 kg in weight, with a general metal (specific gravity about 5 g), a size of about 40 cm square (thickness 1 cm) is the maximum inspection range. Such restrictions can be eliminated.

  According to the present invention, a nondestructive inspection using a SQUID magnetic sensor can be continuously performed while a magnetic sensor itself is self-propelling a large-scale object such as a power plant, a bridge, a high-rise building, and the like. There are no restrictions on the size and dimensions of the sensor, and nondestructive inspection using SQUID magnetic sensors is possible in various fields.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
FIG. 6 is a block diagram showing a configuration of a weak magnetic field measurement apparatus according to the present invention.
The sensor driving mechanism 2 drives the sensor 1 having periodic input / output characteristics. The driving is performed so that the output from the sensor is fed back and the operating point is fixed to one point of the periodic input / output characteristics of the sensor. Thereby, a magnetic field or magnetism is detected as a feedback amount. The moving mechanism 3 mechanically moves the entire movable component including the sensor 1 having periodic input / output characteristics or the sensor 1 having periodic input / output characteristics, and moves the sensor 1 along the subject. To move. Conceivable components include the sensor 1, the sensor driving mechanism 2, the sensor 1, the sensor driving mechanism 2, the measurement operation control unit 4, and the like. The movement control unit 5 controls the movement and stop of the moving mechanism 3 and enables scanning of a predetermined examination area with respect to the subject while repeating the movement and stop. The movement control unit 5 and the measurement operation control unit 4 operate in conjunction with each other. When the movable component including the sensor or the movable component moves, the measurement operation control unit 4 puts the sensor into the unlocked state, puts the sensor into the sleep mode, does not fix the operating point, and cycles according to the input to the sensor at that time To the point on the typical input / output characteristics. When the sensor or a movable component including the sensor is stopped, the measurement operation control unit 4 sets the sensor in a locked state and fixes the operation point of the sensor to one point on the periodic input / output characteristics, thereby adjusting the input / output characteristics. Linearize to make it possible to measure magnetic field or magnetism.

  Control that matches the mechanical movement controlled by the movement control unit 5 and switching between the locked state and the unlocked state of the sensor by the measurement operation control unit 4 is performed based on FIG. First, in the unlocking process, the sensor is brought into the unlocked state, and the measurement operation is set to the pause mode. Next, in the sensor moving step, the movable component including the sensor or the sensor is moved to the first or next measurement point. When the next measurement point is reached, the sensor movement stop process stops at the target measurement point. Next, in the locking process, the sensor is brought into a locked state and a measurement operation is started. The measurement operation is performed in the data acquisition process. By continuously repeating a series of measurement procedures performed in this way, the entire inspection range is scanned and measured with a sensor or a movable component including the sensor. After the data acquisition process, if there is the next measurement point, return to the first unlocking process, put the sensor in the unlocked state, set the measurement operation to the sleep mode, and then move to the next measuring point in the sensor moving process. , Moving the sensor or a movable component including the sensor. Thereby, the inspection of the entire inspection range is completed.

FIG. 7 is a block diagram showing a configuration of an inspection apparatus using a SQUID magnetic sensor.
Control that matches the mechanical movement for moving the SQUID magnetic sensor (A-1) and the switching of the locked state and the unlocked state of the operation of the SQUID magnetic sensor (A-1) is performed based on FIG. The region to be inspected (for example, 1 meter square) and the inspection interval (for example, 1 mm pitch) are input to the SQUID movement control unit (B-2) that moves the SQUID magnetic sensor (A-1) in the movement operation setting step. If necessary, an AC signal is applied to the subject by an AC signal application mechanism (A-4) by a method such as current, voltage, magnetism, light, or heat. Taking the case of a one-channel SQUID magnetic sensor as an example, first, the first point of the start start point is measured in the data acquisition process. Next, the SQUID magnetic sensor (A-1) is unlocked once in the SQUID unlocking process to be in the unlocked state, the operation of the SQUID magnetic sensor (A-1) is temporarily set to the sleep mode, and the inspection interval in the moving process. Move the SQUID magnetic sensor (A-1) by the same amount, stop the SQUID magnetic sensor (A-1) in the movement stop process, move it to the second point, and move the SQUID magnetic sensor (A-1) in the SQUID lock process. ) Is returned to the locked state, and measurement is performed in the data acquisition process. By repeating a series of measurement procedures performed instantaneously in this way, the inspection is completed when the SQUID magnetic sensor (A-1) itself scans the entire inspection range. Since the SQUID magnetic sensor (A-1) stops at the time of measurement, its movement is digitally discontinuous, and spending 0.2 seconds (reference value) per point when calculated backward from the measurement time. Needless to say, the upper limit of the inspection range and the lower limit of the inspection pitch depend on the mechanical performance of the SQUID moving mechanism (B-1) and must be selected according to the inspection object.

  Electronic control system (including A: A-1, A-2, A-3, A-4) and SQUID magnetic sensor (A-1) or SQUID magnetic sensor (A-) 1) including a moving system that mechanically moves the entire movable component (including B: B-1 and B-2), and a measurement control unit (C) that controls these based on the measurement procedure of FIG. It is divided roughly into. The movable component may be any combination of the SQUID magnetic sensor (A-1), the SQUID operation control unit (A-2), the magnetic measurement unit, and the AC signal application mechanism (A-4). A-1 is a SQUID magnetic sensor, and the type is selected according to the application, such as a magnet meter type or a differential type. A-2 is a SQUID operation control unit that controls a locked state of the SQUID that temporarily stops or resumes the operation of the SQUID. A -3 is a magnetic measurement unit that electrically obtains magnetic strength data. A-4 is an AC signal applying mechanism for applying an AC signal to a subject by a method such as current, voltage, magnetism, light, and heat. On the other hand, the moving system B includes a moving mechanism (B-1) that moves the SQUID magnetic sensor itself and a movement control unit (B-2) that controls the measurement range, measurement interval, and movement / stop. C is a measurement control unit that controls the electronic control system A and the moving system B.

As an example, the following table shows the specifications of the SQUID moving mechanism (B-1).
For example, the dotted line in Fig. 7 is the part that is self-propelled by the SQUID moving mechanism (B-1), but in the case of an inspection device that can be installed freely in various fields, each component of the inspection device can be reduced by making the equipment compact. It is arbitrary to move in whole or in part. Of course, when the number of SQUID channels is increased, the inspection speed and accuracy can be improved compared to the case of one.
In addition, applying an AC signal with the AC signal application mechanism (A-4) and synchronously detecting the corresponding change in the magnetic signal with the magnetic measurement unit (A-3) can improve S / N and measure it. This is effective for increasing the accuracy of continuity of values.

In addition, even when performing an inspection in which the SQUID magnetic sensor (A-1) itself moves as in the present invention, the smaller the influence of environmental magnetic noise is, the better, the main body of the high-sensitivity magnetic sensor is installed inside the magnetic shield, An inspection device and a differential magnetic sensor provided with a separate detection coil are effective.

  FIG. 10 shows that a high-sensitivity magnetic sensor is a differential-type magnetometer type SQUID magnetic sensor, and the SQUID is locked and unlocked as shown in FIG. This is an example in which the magnetic distribution is mapped by inspecting the state continuously and repeatedly. The scanning range is 100 mm × 40 mm, and the arrows correspond to defective portions observed visually. A clear change appears in the signal of the SQUID magnetic sensor corresponding to the defective part. As described above, the present invention can solve the problem that the SQUID is unlocked and cannot be measured or the dynamic range is exceeded due to the influence of environmental magnetic noise such as geomagnetism caused by moving the SQUID magnetic sensor itself. In addition, in this embodiment, it is shown that, even in a magnetometer type SQUID magnetic sensor, which is usually considered to be weak against environmental magnetic noise, the present invention enables inspection by moving the SQUID magnetic sensor itself.

  Next, FIG. 11 shows a test result obtained by mapping a magnetic distribution by measuring a 62 mm × 2 mm range using a differential SQUID magnetic sensor in the same manner. Also in this case, the signal of the SQUID magnetic sensor clearly changed at the same position as the visually observed defective part, as in FIG. 10, and the defective part of the subject could be inspected.

  As shown in the above embodiment, when the SQUID magnetic sensor moves, the measurement mode of the SQUID magnetic sensor is measured based on the block diagram shown in FIG. 8 by means of repeatedly locking and unlocking the SQUID. This demonstrates that non-destructive inspection is possible by continuously scanning the SQUID magnetic sensor itself.

  In addition, these inspection results shown in this example were measured in the daytime without any special magnetic noise reduction measures such as magnetic shielding, and equipment and machines not related to this inspection were operating normally. Yes, environmental magnetic noise during inspection is not small.

  Moreover, although the inspection range of the centimeter level is shown in the present embodiment, if the moving mechanism is made large, it can cope with non-destructive inspection of a large-scale subject such as an aircraft, a power plant, a bridge, and a high-rise building. Furthermore, if a small inspection device built into a self-propelled robot such as wireless control is manufactured, it can be freely installed on the inspection object (work) in various fields such as a power plant and a construction site. Thus, in principle, the present invention can realize a SQUID magnetic sensor that can measure the magnetic distribution over a wide range in various fields without any restrictions on the size and dimensions of the subject.

It is a schematic diagram which shows the basic structure of a SQUID magnetic sensor. FIG. 4 is a circuit block diagram of a DC-SQUID with feedback using magnetic flux modulation. It is a figure which shows the relationship between the waveform of the output voltage V of SQUID, and the magnetic flux (phi) s by a measurement magnetic field, and is a figure explaining a magnetic flux fixed loop (FLL). It is a figure which shows the φ-V characteristic linearized by feedback. It is the figure which showed typically an example of the relationship of the spatial distribution of environmental magnetic noise and the weak magnetic signal emitted from the subject, and shows the state where the weak signal is added to the low frequency environmental magnetic noise It is. It is a block diagram which shows the structure of the measuring apparatus of the weak magnetic field which concerns on this invention. It is a block diagram which shows the structure of the inspection apparatus by the SQUID magnetic sensor which concerns on this invention. It is a flowchart which shows the measurement procedure of the test | inspection apparatus by the SQUID magnetic sensor which concerns on this invention. It is a flowchart which shows the measurement procedure by the measuring apparatus of the weak magnetic field which concerns on this invention. It is a figure which shows the Example which driven the SQUID head itself using the magnetometer type | mold SQUID, and test | inspected the defective location of the subject. (The arrow indicates the defective part.) It is a figure which shows the Example which driven the SQUID head itself using the differential type | mold SQUID, and test | inspected the defective location of the subject. (The arrow indicates the defective part.)

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Sensor which has periodic input-output characteristic 2 Sensor drive mechanism 3 Movement mechanism 4 Measurement operation control part 5 Movement control part A-1 SQUID magnetic sensor A-2 SQUID operation control part A-3 Magnetic measurement part A-4 AC signal Application mechanism B-1 SQUID movement mechanism B-2 SQUID movement control section C Measurement control section

Claims (4)

A SQUID element that outputs an electrical signal in accordance with a detection coil and a magnetic flux based on a magnetic field detected by the detection coil;
A sensor driving mechanism for detecting the magnetic field generated by the subject by locking the operating point of the SQUID element at one point by feeding back the output from the SQUID element OLE_LINK1;
A moving mechanism for moving at least the SQUID element ;
A movement control unit for controlling movement / stop of the moving mechanism;
In conjunction with movement / stop by the movement control unit, a measurement operation control unit that controls switching between the locked state and the unlocked state of the sensor drive mechanism;
An apparatus for measuring a weak magnetic field, comprising: A detection coil and a SQUID element that outputs an electrical signal in accordance with a magnetic flux based on the magnetic field detected by the detection coil;
By feeding back the electric signal output from the SQUID element as a magnetic flux to the SQUID element, the operating point is locked to one point on the periodic voltage / magnetic flux characteristics of the SQUID element, and the weak magnetic field generated by the subject is measured. A SQUID magnetic sensor;
A moving mechanism for moving the SQUID magnetic sensor;
A movement control unit configured to repeatedly move / stop the movement mechanism and scan the magnetic field measurement range of the subject with the SQUID magnetic sensor;
In conjunction with the movement / stop of the SQUID magnetic sensor by the movement control unit, a measurement operation control unit that repeatedly operates the unlocked state and the locked state of the SQUID magnetic sensor;
An AC signal applying mechanism for applying an AC signal to the subject;
An apparatus for measuring a weak magnetic field, comprising: A SQUID element that outputs an electrical signal in accordance with a detection coil and a magnetic flux based on a magnetic field detected by the detection coil;
Using a sensor driving mechanism that detects the magnetic field generated by the subject by locking the operating point of the SQUID element to one point by feeding back the output from the SQUID element ,
When moving at least the SQUID element on the subject by the moving mechanism, the SQUID element is unlocked,
When at least the SQUID element is stopped at a target measurement position on a subject by a moving mechanism, the SQUID element is locked, and data for measuring a magnetic field by the SQUID element is acquired in the locked state. A method for measuring weak magnetic fields. A detection coil and a SQUID element that outputs an electrical signal in accordance with a magnetic flux based on the magnetic field detected by the detection coil;
By feeding back the electric signal output from the SQUID element as a magnetic flux to the SQUID element, the operating point is locked to one point on the periodic voltage / magnetic flux characteristics of the SQUID element, and the weak magnetic field generated by the subject is measured. Using a SQUID magnetic sensor,
The SQUID magnetic sensor scans the magnetic field measurement range on the subject by repeatedly moving / stopping in the movement step and the movement stop step,
At that time, when moving, the SQUID magnetic sensor is unlocked by the SQUID lock releasing process, and the magnetic field measurement is paused. When stopped, the SQUID magnetic sensor is locked by the SQUID locking process, and the magnetic field is measured by the data acquisition process. A method for measuring a weak magnetic field characterized by