JP3645093B2 - Magneto-impedance effect eddy current sensor - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an eddy current sensor, and more particularly to a pulse excitation type magnetic impedance effect eddy current sensor.
[0002]
[Prior art]
The conventional eddy current sensor mainly uses an air core coil to reduce the size of the head, and an alternating current is applied to the head to reduce the change in the magnetic flux of the air core coil due to the eddy current demagnetizing field from the metal surface. It is used for detection of proximity of metal, detection of electromagnetic characteristics and defects of metal surface, and the like.
[0003]
In this case, a high-permeability magnetic body can be installed as a magnetic core in the coil to increase sensitivity, but it is necessary to secure the length of the magnetic body in order to reduce the demagnetizing field in the coil axis direction inside the magnetic body. When there is a spatial restriction such as when a sensor is installed inside a thin metal tube, it is difficult to reduce the size and thickness of the head, and an air-core coil head is mainly used.
[0004]
However, the demand for advanced non-destructive inspection in recent years has become strict, and in order to detect fine scratches and impurities on the metal surface, the head size can be reduced to 1 mm or less, or the non-destructive inspection of a shallow metal surface. For example, there is an increasing number of cases where the conventional coil method cannot solve the problem, for example, a high-frequency technique that makes the skin effect remarkable is required.
[0005]
[Problems to be solved by the invention]
As described above, the conventional eddy current sensor generally employs a method in which the air-core coil is AC-excited and a decrease in the coil inductance is detected by the demagnetizing field due to the eddy current on the surface of the target metal. However, with this method,
(1) A coil diameter that is at least twice the distance from the metal surface to the coil end face (lift-off) is required. For example, a 2 mm lift-off requires a coil of 4 mm or more, and it is difficult to reduce the size of the head. Also, pinpoint detection on the metal surface is difficult.
[0006]
(2) In order to increase the lift-off, it is necessary to generate a coil magnetic field far from the end face of the coil and to increase the demagnetizing field from the metal surface, and thus the axial length of the coil is increased. This makes it difficult to reduce the size of the head.
(3) In order to increase the detection sensitivity and the signal-to-noise ratio, it is necessary to increase the number of turns of the coil, and it is difficult to reduce the size of the head.
[0007]
(4) When detecting fine cracks on a shallow surface of a sample having a smooth surface such as a uranium pellet tube for a nuclear reactor, it is necessary to increase the frequency in order to reduce the depth of the skin of the metal (for example, 20 MHz) However, because of the displacement current due to the stray capacitance of the coil, the detection circuit (lead wire) tends to become unstable, and detection accuracy cannot be obtained.
[0008]
However, these problems are still unresolved.
As described above, the conventional eddy current sensor mainly uses an air-core coil, and an alternating current is supplied to the coil, which is detected as a decrease in magnetic flux change in the air-core coil generated by the eddy current demagnetizing field from the metal surface. It is used for detection and detection of electromagnetic characteristics and defects of metal surfaces.
[0009]
In order to further increase the detection sensitivity, a high permeability magnetic material can be used for the coil core, but it is necessary to ensure the length of the magnetic material in order to reduce the demagnetizing field in the coil axis direction inside the magnetic material. Therefore, since it is difficult to reduce the size and thickness of the head, a head using an air-core coil is generally used.
Recently, high-frequency technology that makes the skin effect conspicuous for non-destructive inspection of shallow metal surfaces is required. However, conventional coil methods are difficult to solve the problem of stray capacitance and it is difficult to improve detection accuracy. There is something.
[0010]
Therefore, the present invention improves the above-described conventional coil-type eddy current sensor head and uses the magneto-impedance effect of the magnetic wire previously found by the inventor of the present invention, thereby significantly reducing the size of the head and reducing the metal shallow surface. We have proposed the detection of the electromagnetic characteristics in the pinpoint region of this and the stable and fast response eddy current sensor by exciting the head with a pulse current. This belongs to all technical fields where eddy current sensors are used industrially, and can be remarkably advanced.
[0011]
That is, an object of the present invention is to provide a magneto-impedance effect type eddy current sensor that eliminates the above-mentioned problems and can achieve downsizing of the head of the eddy current sensor, ensuring lift-off, and high frequency.
[0012]
[Means for Solving the Problems]
In order to achieve the above object, the present invention provides
[1] A magneto-impedance effect type eddy current sensor, a pulse voltage generation circuit for applying a sharp pulse current for generating a magnetic impedance effect due to a skin effect to a magnetic thin wire, and a length direction of the magnetic thin wire according to the pulse current Means for inducing a pulsed magnetic field, and a circuit for obtaining an output voltage by converting and amplifying the induced pulse voltage across the magnetic wire reduced by a demagnetizing field caused by an eddy current induced on the surface of a nearby metal by the pulsed magnetic field into a DC voltage It is made to comprise.
[0013]
[2] In the magneto-impedance effect type eddy current sensor according to [1], an amorphous magnetic wire is used as the magnetic wire.
[3] In the magneto-impedance effect eddy current sensor according to [1], the pulse voltage generation circuit includes a multivibrator circuit using a C-MOS inverter, a differentiation circuit, and a pulse waveform shaping amplification C-MOS inverter. A circuit is used.
[0014]
[4] In the magneto-impedance effect eddy current sensor described in [1] above, a coated conductor coil is installed around the magnetic wire as means for generating a pulse magnetic field in the length direction of the magnetic wire. is there.
[5] In the magneto-impedance effect eddy current sensor according to [1], a circuit comprising a Schottky barrier diode and a peak hold circuit is used as a circuit for converting the pulse voltage induced across the magnetic wire into a DC voltage. It is a thing.
[0015]
[6] In the magneto-impedance effect type eddy current sensor according to [1] above, a pair of the magnetic wires are used, the induced pulse voltage of each magnetic wire is converted into a DC voltage, and the difference between the voltages is output. It is what I did.
It should be noted that a high-frequency current that generates a skin effect may be used instead of the sharp pulse current of [1] from the viewpoint of the magnetic impedance (MI) effect.
[0016]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail.
FIG. 1 is a block diagram of a magneto-impedance effect type eddy current sensor according to the present invention.
In this figure, reference numeral 1 denotes a C-MOS IC (74AC04), which includes two inverters 2 and 3 (Q ₁ and Q ₂ ), a resistor 4 (R ₁ : 20 kΩ), and a capacitor 5 (C ₁ : 100 pF). It has a multivibrator and has other inverters 6, 7, 8, and 9 that shape and amplify the waveform.
[0017]
Further, 10 is a resistance (R ₂ : 1Ω), 11 is a zero magnetostrictive amorphous wire, 12 is a Schottky barrier diode (SBD), 13 is a capacitor (C ₂ : 1000 pF), 14 is a resistance (R ₃ : 520 kΩ), 15 Is a resistor (R ₅ : 10 kΩ), 16 is a resistor (R ₇ : 100 kΩ), 17 is a variable resistor (VR ₁ : 3 kΩ), 18 is a resistor (R ₄ : 10 kΩ), and 19 is a first differential amplifier (OP ₁ : LF356), 20 is a resistor (R ₆ : 100 kΩ), 21 is a resistor (R ₈ : 10 kΩ), 22 is a resistor (R ₁₀ : 10 kΩ), 23 is a second differential amplifier (OP ₂ : LF356), Reference numeral 24 denotes a resistor (R ₉ : 1 MΩ).
[0018]
The basic structure of the eddy current sensor circuit of the present invention shown in FIG. 1 is that a multiwave vibrator of two inverters 2 and 3 in the C-MOS IC 1 oscillates a square wave , and the pulse current flowing through the power line is long. A zero-magnetostrictive amorphous wire 11 having a thickness of 1 mm and a diameter of 30 μm is energized, and a pulse magnetic flux linked to a 30-turn coil installed on the zero-magnetostrictive amorphous wire 11 is applied to a target metal (not shown), and the eddy current reaction on the metal surface is counteracted. The voltage across the amorphous wire changed by the magnetic field is detected by the Schottky barrier diode 12 (SBD) and converted to a DC voltage by the peak hold circuit.
[0019]
DC voltage above, first, is applied to the second differential amplifier 19 and 23, when the head is sufficiently distant from the target, the sensor output voltage E _out is set to zero.
Instead of the pulse voltage across the head, the voltage across the coil can be used as the detection voltage.
The rise time of the pulse current of the head energization is 2 to 3 ns, which is equivalent to the energization of an alternating current of 100 to 200 MHz that is most sensitive to the generation of the magnetic impedance (MI) effect due to the skin effect of the amorphous wire.
[0020]
FIG. 2 is a magneto-impedance effect type eddy current sensor circuit diagram (No. 1) showing the first embodiment of the present invention. FIG. 2 is developed to differentiate the square wave voltage of the multivibrator with a differentiating circuit. After the amplification and shaping at, the zero magnetostrictive amorphous wire 38 is pulsed.
FIG. 3 is a diagram showing the experimental results of the magneto-impedance effect type eddy current sensor (No. 1), and FIG. 4 is a diagram showing the voltage across the amorphous wire and the voltage across the coil of the eddy current sensor.
[0021]
In FIG. 2, 31 and 32 are inverters, 33 is a resistor (51 kΩ), 34 is a capacitor (100 pF), and these constitute a multivibrator. 35 is a capacitor (100 pF), 36 is a resistor (200Ω), 37 is an inverter, 38 is a sensor head (zero magnetostrictive amorphous wire), 39 is a Schottky barrier diode (SBD), 40 is a capacitor (1000 pF), 41 is a resistor ( 51 kΩ), 42 is a variable resistor (VR ₁ : 200Ω), 43 is a differential amplifier (OP: AD524 100 times), 44 is an aluminum plate (60 × 100 × 1 mm).
[0022]
In this embodiment, as shown in FIG. 2, a sensor head 38 in which a 30 μm diameter amorphous wire with 1 mm between electrodes formed by soldering both ends is installed on a bobbin with an inner diameter of 1 mm wound with a 30-turn coil, It was made to approach at a distance x in parallel to a wide aluminum plate 44 having a thickness of 1 mm.
FIG. 3 shows the relationship between the sensor output voltage _Vout and the distance x in such a case.
[0023]
In FIG. 3, a curve a is a case where the angle θ _c formed by the amorphous wire and the central axis of the coil is 10 °, a curve b is 5 °, a curve c is 0 °, and a curve d is a case without a coil.
As shown in FIG. 3, V _{out is} about 4 mm. Changes are appearing. The noise voltage is 20 mV, and the distance detection resolution is about 15 μm.
[0024]
When the angle θ _c formed between the amorphous wire and the coil axis is 0 °, 5 °, and 10 °, the change width of V _out with respect to the distance x is the maximum when the angle θ _c is 10 ° (curve a). . This is because the flux change with respect to the energizing current of the amorphous wire is in the wire circumferential direction, so that the flux linkage change does not appear unless the coil lead wire and the flux change direction have an angle.
[0025]
When the coil is not applied (curve d), the change width of _Vout is as small as 1/3 to 1/4 of that when the coil is applied, but the change is a size that can be measured stably. This is presumably because twist stress is applied to the amorphous wire during electrode formation, a pulse magnetic field is generated in the wire length due to slight spiral magnetization, and a slight magnetic flux enters the metal plate.
[0026]
Although both ends of the coil were short-circuited, the characteristics were almost the same as in the case of opening. This is because the wire pulse current waveform is steep and the excitation frequency corresponds to a high frequency of 100 to 200 MHz. Therefore, the impedance of the stray capacitance in the coil is low with respect to the high frequency of the flux linkage change of the coil, and the LC in the coil is low. This is thought to be due to the formation of a closed circuit.
The voltage across the amorphous wire of the eddy current sensor is as shown in FIG. 4A (upper stage), and the voltage across the coil is as shown in FIG. 4B (lower stage). In FIG. 4, one scale is 10 ns.
[0027]
FIG. 5 is a magneto-impedance effect type eddy current sensor circuit diagram (part 2) showing the first embodiment of the present invention, and FIG. 6 is a diagram (part 2) showing an experimental result by the magneto-impedance effect type eddy current sensor. LC vibration waveform due to coil inductance and stray capacitance is observed.
In FIG. 5, 51 and 52 are inverters, 53 is a resistor (51 kΩ), 54 is a capacitor (100 pF), and these constitute a multivibrator. 55 is a capacitor (100 pF), 56 is a resistor (200Ω), 57 is an inverter, 58 is a sensor head (zero magnetostrictive amorphous wire), 59 is a Schottky barrier diode (SBD), 60 is a capacitor (1000 pF), and 61 is a resistor ( 51 kΩ), 62 is a variable resistor (VR ₁ : 1 kΩ), 63 is a resistor (760 Ω), and 64 is a differential amplifier (OP: AD524 100 times).
[0028]
FIG. 6 is a graph showing V _out -x characteristics when the coil of the zero magnetostrictive amorphous wire 58 is opened (curve a) and when a wire current is passed through the coil in series (curve b).
In the latter, the change ends at 0.5 mm, and the change width is about 1/4 of the former. This is because the former coil is almost free LC vibration as a secondary coil of a weakly coupled transformer, and is susceptible to the reaction of the eddy current demagnetizing field of the metal plate. In the latter, the coil current is the same as the wire current. This is thought to be due to being fixed.
[0029]
Next, a second embodiment of the present invention will be described.
FIG. 7 is a magneto-impedance effect type eddy current sensor circuit diagram showing a second embodiment of the present invention. A twin head which is a differential type by combining two single head type sensor circuits shown in FIG. 1 symmetrically. It is a shape eddy current sensor circuit diagram.
In FIG. 7, two inverters 71 and 72 (Q ₁ , Q ₂ ), a resistor 73 (R ₁ : 5.1 kΩ), a capacitor 74 (C ₁ : 100 pF) constitute a multivibrator, and a capacitor 75 (C ₂ : 100 pF), a resistor 76 (R ₂ : 200Ω), and other inverters 77 and 78 (Q ₃ and Q ₄ ) that shape and amplify the waveform. 79 is a variable resistance (VR ₁ : 200Ω), 80 is a twin head, 81 is a first coil thereof, 82 is a second coil thereof, 83 and 84 are Schottky barrier diodes (SBD), and 85 and 86 are Capacitors (C ₃ , C ₄ : 1000 pF), 87 and 88 are resistors (R ₃ and R ₄ : 510 kΩ), and 89 is a differential amplifier (OP: AD524 1000 times).
[0030]
The eddy current sensor according to the second embodiment has the following advantages.
When the twin head 80 is set equidistant from the surface of the target metal (not shown), the difference in electromagnetic characteristics between the two minute regions on the metal surface is detected without depending on the distance from the metal surface. Suitable for non-destructive detection of scratches and inclusions.
Further, since the impedance of the head also changes due to a disturbance magnetic field such as geomagnetism other than the eddy current demagnetizing field on the metal surface, the influence of the disturbance magnetic field can be offset by this twin head.
[0031]
In the sensor circuits of the first embodiment (FIG. 1) and the second embodiment (FIG. 7), only the power consumption due to a pulse current with a width of several ns is obtained. If the amplifier is also composed of a C-MOS amplifier, the power consumption of the sensor Is 10 mW or less, and can be configured as a small and light sensor with a small power consumption and easy to carry.
FIG. 8 is a waveform diagram of detection of a 100 μm diameter pinhole on the surface of an aluminum plate by a twin head type eddy current sensor according to a second embodiment of the present invention. FIG. ) Is for lift off 0.5 mm.
[0032]
Here, two amorphous wires having a diameter of 30 μm and a length of 1 mm were set in parallel at an interval of 2 mm, and parallel to the surface of the target aluminum plate with a lift-off of 1 mm. It can be seen that the eddy current near the pinhole is disturbed by the pinhole, causing a difference in the magnitude of the demagnetizing field applied to the two amorphous wires, and the pulse voltage is detected when the head is swept on the aluminum plate.
[0033]
As described above, in the present invention, problems that cannot be solved by the conventional sensor technology such as miniaturization of the head of the eddy current sensor, securing of lift-off, and high frequency are solved by the magneto-impedance effect (MI effect) type of zero magnetostrictive amorphous wire. It could be solved by the head and pulse electronic circuit technology.
Note that the present invention is not limited to the above-described embodiment, and based on the spirit of the present invention, such as a method of using a magnetic impedance (MI) effect by exciting a lead wire without applying a coil to an amorphous wire. Various modifications are possible and are not excluded from the scope of the invention.
[0034]
【The invention's effect】
As described above in detail, according to the present invention, it is possible to reduce the size of the head of the eddy current sensor, ensure the lift-off, and increase the frequency.
[Brief description of the drawings]
FIG. 1 is a configuration diagram of a magneto-impedance effect type eddy current sensor according to the present invention.
FIG. 2 is a circuit diagram (part 1) of a magneto-impedance effect type eddy current sensor showing a first embodiment of the present invention.
FIG. 3 is a diagram (part 1) showing a result of an experiment by a magneto-impedance effect type eddy current sensor according to the first embodiment of the present invention.
FIG. 4 is a diagram showing the voltage across the amorphous wire and the voltage across the coil of the magneto-impedance effect eddy current sensor according to the first embodiment of the present invention.
FIG. 5 is a magneto-impedance effect eddy current sensor circuit diagram (No. 2) showing the first embodiment of the present invention;
FIG. 6 is a diagram (part 2) illustrating an experimental result of the magneto-impedance effect type eddy current sensor according to the first embodiment of the present invention.
FIG. 7 is a magneto-impedance effect eddy current sensor circuit diagram showing a second embodiment of the present invention.
FIG. 8 is a detection waveform diagram of a 100 μm diameter pinhole on the surface of an aluminum plate by a twin head type eddy current sensor showing a second embodiment of the present invention.
[Explanation of symbols]
1 C-MOS IC (74AC04)
2, 3, 6, 7, 8, 9, 31, 32, 37, 51, 52, 57, 71, 72, 77, 78 Inverters 4, 10, 14, 15, 16, 18, 20, 21, 22, 24, 33, 36, 41, 53, 56, 61, 63, 73, 76, 87, 88 Resistor 5, 13, 34, 35, 40, 54, 55, 60, 74, 75, 85, 86 Capacitor 11 Zero Magnetostrictive amorphous wire (magnetic wire)
12, 39, 59, 83, 84 Schottky barrier diode (SBD)
17, 42, 62, 79 Variable resistance (VR ₁ )
19 First differential amplifier 23 Second differential amplifier 38, 58 Sensor head (zero magnetostrictive amorphous wire)
43, 64, 89 Differential amplifier 44 Aluminum plate 80 Twin head 81 Twin head first coil 82 Twin head second coil

Claims (6)

A magneto-impedance effect eddy current sensor,
(A) a pulse voltage generation circuit that applies a sharp pulse current that generates a magneto-impedance effect due to the skin effect on the magnetic wire;
(B) means for inducing a pulse magnetic field in the length direction of the magnetic wire according to the pulse current;
(C) a circuit for converting an induced pulse voltage across the magnetic wire, which is reduced by a demagnetizing field caused by an eddy current induced on the surface of a nearby metal by the pulse magnetic field, into a direct current voltage and amplifying it to obtain an output voltage. A magneto-impedance effect type eddy current sensor. 2. The magneto-impedance effect eddy current sensor according to claim 1, wherein an amorphous magnetic wire is used as the magnetic wire. 2. The magneto-impedance effect eddy current sensor according to claim 1, wherein the pulse voltage generation circuit uses an oscillation circuit comprising a multivibrator circuit using a C-MOS inverter, a differentiating circuit, and a pulse waveform shaping amplification C-MOS inverter. Magnetic impedance effect type eddy current sensor. 2. The magneto-impedance effect type eddy current sensor according to claim 1, wherein a coated conductor coil is installed around the magnetic wire as means for generating a pulse magnetic field in the length direction of the magnetic wire. Eddy current sensor. 2. The magneto-impedance effect eddy current sensor according to claim 1, wherein a circuit comprising a Schottky barrier diode and a peak hold circuit is used as a circuit for converting the pulse voltage across the magnetic wire into a DC voltage. Impedance effect eddy current sensor. 2. The magneto-impedance effect type eddy current sensor according to claim 1, wherein a pair of the magnetic thin wires are used, and an induced pulse voltage of each magnetic thin wire is converted into a DC voltage, and then a difference between the voltages is used as an output. Magnetic impedance effect type eddy current sensor.

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