JP5085123B2 - Magnetic detection method for magnetic metal pieces - Google Patents

Description

  The present invention relates to a method for magnetically inspecting foods, and is useful, for example, for detecting magnetic metal pieces mixed in packed juice.

In conveyor lines for the production of packed foods and beverages such as packed fruit juice, the parts of the manufacturing equipment, such as bearings, bolts, nuts and screws, or mechanical parts, after filling the pack and before the pack is closed There is a possibility that foreign matter such as debris may enter.
Conventionally, eddy current type metal detectors are used to detect defective products that have entered the foreign matter.
In this system, a transmission coil and a pair of reception coils are disposed around the transfer path, and the pair of reception coils are differentially connected. Therefore, for a normal product running without intrusion of metal foreign objects, the outputs of both receiving coils are the same and the differential output is zero. On the other hand, when a defective product that has entered a metal foreign object is traveling, the penetrating magnetic flux changes with the displacement of the metal foreign object due to the eddy current flowing in the metal foreign object, and as a result, the differential output of the pair of receiving coils Change. It is detected from the change in the differential output that a defective product passes, and the defective product is excluded from the conveyor line.
However, in the use of this eddy current metal detection method, when the pack is a metal composite type, for example, an aluminum vapor-deposited paper container or an aluminum foil laminated paper container, aluminum is conductive and eddy current flows, Since the penetrating magnetic flux is also changed by the aluminum foil, it is difficult to detect a metallic foreign object with high accuracy.

  The metal foreign matter that enters the container during the manufacturing process is a bearing, bolt, nut, screw, etc., which is a part of the manufacturing apparatus, and is iron or ferrite-based stainless steel. Magnetized due to distortion. Even austenitic stainless parts with weak magnetism are magnetized to some extent due to plastic deformation.

Therefore, it is conceivable to detect the metal foreign matter in the container in the product as a magnetized material and detect it magnetically.
Conventionally, in the production line, it has been proposed to magnetize a metal foreign matter that has entered a product by a magnetic means, and to detect the magnetized foreign matter by a magneto-impedance effect sensor and eliminate it from the production line (Patent Document 1).
According to this detection method, since the container is non-magnetic, there is no response of the magneto-impedance effect sensor to the container, and there is no problem even if the container is conductive such as an aluminum composite structure.

The magneto-impedance effect element uses an amorphous alloy wire with zero magnetostriction or negative magnetostriction, which has an outer shell portion in which magnetic domains whose spontaneous magnetization directions are opposite to each other in the circumferential direction of the wire are separated by domain walls. Has been.
The inductance voltage component in the output voltage between both ends of the wire generated when a high frequency current is applied to the zero magnetostrictive or negative magnetostrictive amorphous magnetic wire is easily increased in the circumferential direction by the circumferential magnetic flux generated in the cross section of the wire. It occurs due to the magnetized outer shell being magnetized in the circumferential direction. Accordingly, the circumferential magnetic permeability μθ depends on the circumferential magnetization of the outer shell.
Therefore, when an external magnetic field is applied to the energized amorphous wire, the magnetic flux acting on the outer shell portion having the easily magnetizable property in the circumferential direction is obtained by synthesizing the circumferential magnetic flux and the external magnetic flux by the energization. Is deviated from the circumferential direction and magnetization in the circumferential direction is less likely to occur, the circumferential permeability μθ is changed, and the inductance voltage is changed.

Further, when the frequency of the energization current is in the order of MHz, a high-frequency skin effect appears greatly, and the skin depth δ = (2ρ / wμθ) ^1/2 (μθ is the circumferential permeability, as described above, and ρ is (The electrical resistivity, w indicates the angular frequency, respectively) varies with μθ, and this μθ varies with the external magnetic field as described above, so that the resistance voltage component in the output voltage across the wire also varies with the external magnetic field. Become.

  Therefore, an external magnetic field detection method using the magneto-impedance effect element (for example, see Patent Document 2) and an external magnetic field detection method using the magnetic inductance effect (for example, see Patent Document 3) have been proposed.

  In the above, the positive and negative of the external magnetic field causes the circumferential shift φ of the magnetic field to be positive or negative, but the reduction factor cos (± φ) of the circumferential magnetic field does not change, so the degree of decrease in μθ is positive or negative in the direction of the external magnetic field. Does not change. Therefore, the external magnetic field-output characteristics are substantially symmetrical with respect to the y axis as shown in FIG. 4A described later when the magnetic field is taken on the x axis and the output is taken on the y axis. In addition, as shown in FIG.

The magnetic field detection circuit using this magneto-impedance effect element basically has (1) a high-frequency current source 2 ′ for applying a high-frequency excitation current to the magneto-impedance effect element 1 ′, as shown in FIG. (2) a magneto-impedance effect element 1 ′, (3) a detection circuit 3 ′ that demodulates a modulated wave obtained by modulating the high-frequency excitation current (carrier wave) with an external magnetic field Hex applied to the magneto-impedance effect element, and (4) demodulation. It comprises an amplifier 4 'for amplifying a wave and (5) a detection output terminal 5'.
In FIG. 5B, (a) shows a detected magnetic field (external magnetic field) Hex applied to the magneto-impedance effect element, (b) shows a high-frequency excitation current wave (carrier wave) Ic passed through the magneto-impedance effect element, and (c). Represents a modulated wave as an output of the magneto-impedance effect element, and (d) represents a demodulated wave.

The relationship between the amplitude H _{ex of the} detected magnetic field and the amplitude of the output E _out can be represented as shown in FIG.
Therefore, in the circuit of FIG. 5-1, a negative feedback coil indicated by 6 'is subjected to negative feedback to linearize the characteristics as shown in FIG.
Furthermore, as shown in FIG. 4C, the characteristic shown in FIG. 4B is moved in the direction of the arrow by the bias magnetic field Hb so that the polarity can be discriminated.

  6A and 6B show a magneto-impedance effect element with a coil in the magneto-impedance effect sensor, and in the case shown in FIG. 6A, a negative feedback coil 6 ′ and a bias are included in the magneto-impedance effect element. In the case shown in FIG. 6B, the magnetic impedance effect element 1 ′ is disposed on one surface of the substrate 100 ′, and the iron core 103 ′ is disposed on the other surface 100 ′ of the substrate. Is arranged so as to form a magnetic loop circuit with the magnetic impedance effect 1 ′, and a negative feedback coil 6 ′ and a bias magnetic field coil 7 ′ are wound around the iron core 103 ′ (Patent Document 4). .

JP 2006-98117 A JP 7-181239 A JP-A-6-283344 JP 2002-289940 A

In the magneto-impedance effect element with a coil shown in FIG. 6 (a), even if a magnetic flux enters from the middle of the element in the axial direction of the element as indicated by h ', an eddy current is generated in the coil and the magnetic flux cancels out. Therefore, the magnetic flux mainly passes from both ends of the magneto-impedance effect element in the element axial direction.
On the other hand, in the magneto-impedance effect element with a coil shown in FIG. 6B, the magnetic flux passes not only at both ends of the element but also from the intermediate surface of the element in the axial direction of the element.
Therefore, the magneto-impedance effect element with a coil shown in FIG. 6B has a larger amount of magnetic flux to be detected and has better detection sensitivity than the magneto-impedance effect element with a coil shown in FIG. . Further, the area that can be magnetically detected by one magneto-impedance effect element is vast.

By the way, the length of the magneto-impedance effect element is restricted in production. For example, in order to connect both ends of a magneto-impedance effect element to an electrode, a welding method is used in which a welding electrode is energized by contacting a + electrode on one end of the magneto-impedance effect element and a-electrode on the other end. The length of the element is limited. Usually 6 mm or less.
Therefore, even in the magneto-impedance effect element with a coil shown in FIG. 6B, there is a limitation in improving the detection sensitivity and expanding the detection area.

  An object of the present invention is to enable high-sensitivity detection of a magnetic metal piece mixed in a packed food or beverage by a magneto-impedance effect sensor.

The magnetic metal piece magnetic detection method according to claim 1 is a method of detecting a magnetic metal piece mixed in the inspection object by moving the magneto-impedance effect sensor relative to the inspection object, An iron core is provided on the magneto-impedance effect element so as to form a magnetic loop circuit, and two iron-impedance magneto-impedance effect elements each having a negative feedback coil and a bias magnetic field coil wound around the iron core are provided. And a sensor head in which the mutual spacing in the magnetosensitive axis direction of the magneto-impedance effect element is shorter than the length of the magneto-impedance effect element, and the detected magnetic field applied to each magneto-impedance effect element is Using a magneto-impedance effect sensor that detects each detected circuit and differentially amplifies and detects the detected output, the relative movement direction Against, wherein the directing direction of the magneto-impedance effect element of the sensor head in the perpendicular direction.
The magnetic metal piece magnetic detecting method according to claim 2 is the magnetic metal piece magnetic detecting method according to claim 1, wherein the magnetic metal piece is applied to a packaged food or beverage as an object to be inspected by applying a magnetic field. It is characterized by being magnetized.
A magnetic metal piece magnetic detection method according to claim 3 is the magnetic metal piece magnetic detection method according to any one of claims 1 and 2, wherein a magnetic impedance effect sensor head is formed on a surface to be magnetically inspected. The surface on which the magneto-impedance effect element is attached faces each other.
According to a fourth aspect of the present invention, there is provided a magnetic metal strip magnetic detection method in which a negative feedback coil and a bias magnetic field coil of a magnetic impedance effect element with a series of coils are connected in series, and the magnetoimpedance effect elements 1a and 1b face each other. Of the ends a and b, the end a of the magneto-impedance effect element 1a is grounded, connected to the differential amplifier side of the element 1a with respect to the end a of the magneto-impedance effect element 1a, and the end b of the magneto-impedance effect element 1b is A differential amplifier is connected to the alligator, and the other end of the element 1b with respect to the end b of the magneto-impedance effect element 1b is grounded .

A negative feedback is applied to the magneto-impedance effect element and a bias magnetic field is applied. However, since the negative feedback coil and the bias magnetic field coil are not directly wound around the magneto-impedance effect element, the element is arranged in the axial direction of the magneto-impedance effect element. The magnetic flux of the detected magnetic field passes not only from both ends but also from the intermediate surface.
Even if the length of the magneto-impedance effect element itself is constrained, increasing the number of columns of the magneto-impedance effect element can substantially increase the total length of the magneto-impedance effect element and increase the amount of magnetic flux pick-up of the detected magnetic field. The detection sensitivity can be increased.
In addition, since the interval in the magnetosensitive axis direction between the tandem elements is sufficiently narrowed, the detection region can be well suppressed and enlarged. Therefore, the magnetic metal piece mixed in the packed food or beverage can be reliably detected.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1-1 shows an example of a circuit diagram of a magneto-impedance effect sensor used in the present invention.
1-1, 1 is a magnetoimpedance effect element, 6 is a negative feedback coil, 7 is a bias magnetic field coil, 1a and 1b are coiled magnetoimpedance effect elements, and A is two coiled magnetoimpedance effect elements 1a, 1 shows a magneto-impedance effect sensor head in which 1b is connected in series in series.
1-2 (a) is a side view showing an example of the coiled magneto-impedance effect element, FIG. 1-2 (b) is a bottom view, and FIG. 1-2 (c) is FIG. 1-2. It is a ha sectional drawing in (b).
In FIG. 1-2, 100 is a board | substrate chip and can use a ceramic board, for example. Reference numeral 101 denotes an electrode provided on one side of the substrate piece, and includes a magneto-impedance effect element connecting projection 102. This electrode can be provided by printing and baking a conductive paste, for example, a silver paste. Reference numeral 1 denotes a magneto-impedance effect element connected between the protrusions 102 and 102 of the electrodes 101 and 101 by welding, and an amorphous wire, amorphous ribbon, sputtered film, or the like having zero or negative magnetostriction can be used. 103 is a C-type iron core made of iron, ferrite, etc., 6x is a negative feedback coil wound around the C-type iron core, and 7 is a bias magnetic field coil. The magneto-impedance effect element 1 and the C-type iron core 103 The both ends of the C-type iron core 103 are fixed to the other surface of the substrate piece 100 with an adhesive or the like so as to constitute a loop magnetic circuit. The iron core material may be a magnetic material having a small residual magnetic flux density. Examples thereof include permalloy, ferrite, iron, amorphous magnetic alloy, magnetic powder mixed plastic, and the like.

The magneto-impedance effect element 1 includes a zero magnetostrictive or negative magnetostrictive amorphous alloy wire having an outer shell portion in which magnetic domains whose spontaneous magnetization directions are opposite to each other in the circumferential direction of the wire are alternately separated by a domain wall. used. The inductance voltage component in the output voltage between both ends of the wire generated when a high-frequency excitation current is passed through an amorphous magnetic wire having zero magnetostriction or negative magnetostriction is obtained by the circumferential magnetic flux generated in the cross section of the wire. This occurs due to the magnetization of the easily magnetizable outer shell in the circumferential direction. Accordingly, the circumferential magnetic permeability μθ depends on the circumferential magnetization of the outer shell. Thus, when a signal magnetic field is applied in the axial direction of the amorphous wire being energized, the outer shell portion having the easily magnetizable property in the circumferential direction is obtained by synthesizing the circumferential magnetic flux and the signal magnetic field magnetic flux by the energization. The direction of the magnetic flux acting on the magnetic field is deviated from the circumferential direction, and magnetization in the circumferential direction is less likely to occur, the circumferential permeability μθ is changed, and the inductance voltage is changed. This fluctuation phenomenon is called a magnetic inductance effect, which can be said to be a phenomenon in which the high-frequency excitation current (carrier wave) is modulated by a detected magnetic field (signal wave). Further, when the frequency of the energizing current is in the order of MHz, a high-frequency skin effect appears greatly, and the skin depth δ = (2ρ / wμθ) ^1/2 (μθ is the circumferential permeability, as described above, and ρ is the electrical conductivity. (Resistivity, w indicates angular frequency) varies with μθ, and μθ varies with the signal magnetic field as described above, so the resistance voltage component in the output voltage across the wire also varies with the signal magnetic field. . This fluctuation phenomenon is called a magneto-impedance effect, which can be said to be a phenomenon in which the high-frequency excitation current (carrier wave) is modulated by a signal magnetic field (signal wave).

  In the magneto-impedance effect sensor head, the magneto-impedance effect elements are arranged in series, and the mutual interval in the magnetosensitive axis direction of the magneto-impedance effect element is shorter than the length of the magneto-impedance effect element. It is preferable to set it to 1/4 or less of the length of the effect element. For example, the interval is set to 1 mm for a length of 5 mm of the magneto-impedance effect element.

  1-1, 2 is a high-frequency current source circuit for applying a high-frequency excitation current to the magneto-impedance effect element, and 3 is a detected magnetic field H (signal wave) acting in the axial direction of the magneto-impedance effect element. A detector circuit for demodulating the modulated wave modulated by (carrier wave), 4 an amplifier circuit for amplifying the demodulated wave, 5 a detection output terminal, 60 a negative feedback circuit, and 70 a bias magnetic field + Vcc power source.

1-3 (a) is a front view of the magneto-impedance effect sensor having the circuit of FIG. 1-1, and (b) in FIG. 1-3 is a side view of the same.
In FIG. 1-3, B is a main body board on which the high-frequency current source circuit 2, the detection circuit 3, the amplification circuit 4, the detection output terminal 5, the negative feedback circuit 60, the bias magnetic field + Vcc power source and the like are mounted. .
A is a sensor head, and the magnetic impedance effect elements 1a and 1b with coils are placed on the head substrate C so that the magneto-impedance effect element 1 is exposed. The distance between the effect elements 1 and 1 is set to ¼ or less of the length of the magneto-impedance effect element 1, and the magnetic force of the sensor head with respect to the relative movement of the sensor along the electromagnetic detection surface is mounted. It is connected to the sensor body B by a conductor joint d,... In such a direction that the impedance effect can be brought close to the electromagnetic detection surface and relatively scanned.

2A is a side view showing a method for inspecting food by the method according to the present invention using the magneto-impedance effect sensor described with reference to FIGS. 1-1 to 1-3, and FIG. 2B is a plan view. P,... Represents packed fruit juice, c represents a conveyor, magnetic metal pieces mixed in the pack p are magnetized by the magnetizing device m upstream of the conveyor, and the magnetic impedance of the sensor head A connected in series The packs p,... Are transferred by the conveyor c so that the effect elements 1, 1 are oriented in a direction perpendicular to the transfer direction x and the magneto-impedance effect elements 1, 1 of the sensor head A are brought close to the bottom surface of the pack p. Then, the axial component of the magnetic field generated from the mixed magnetic metal piece with respect to the magneto-impedance effect elements 1 and 1 is picked up to detect the mixing of the magnetic metal piece. In this case, as described above, the amount of magnetic flux picked up by the tandem magneto-impedance effect element can be increased, and the distance between the magneto-impedance effect element of the sensor head and the magnetic metal piece settled on the bottom surface of the pack is sufficiently short. Since a strong detected magnetic field can be applied, highly sensitive detection is possible. The magneto-impedance effect sensor can be moved without moving the pack.
If the magnetic metal piece is a ferromagnetic material, for example, ferritic stainless steel, it is usually magnetized considerably due to processing strain or strain in use, and thus magnetization may be omitted. Magnetization is indispensable when the magnetism is weak, for example, austenitic stainless steel.

  In the above embodiment, the number of columns of the magneto-impedance effect elements with coils is two, but it can be three or more.

In the above embodiment, the magneto-impedance effect elements with coils are connected in series, and the output of the series-connection elements is detected by the detection circuit. As shown in FIG. The modulated waves at the output ends of the elements 1a and 1b are detected by the detection circuits 3a and 3b, the detected outputs are differentially amplified by the differential amplifier circuit 40, and the differential amplified output is provided as a magneto-impedance effect element with each coil. The negative feedback coils 6 and 6 of 1a and 1b can be negatively fed back.
In this case, noise such as geomagnetism can be removed because the geomagnetism acting on both magnetoimpedance effect elements and the noise magnetic field emitted from the surrounding iron-based structure are equivalent.

In order to verify the differential detection method described with reference to FIG. 3-1, an amorphous wire of composition Co _70.5 B ₁₅ Si ₁₀ Fe _4.5 having a length of 5 mm and an outer diameter of 30 μmφ is applied to each magneto-impedance effect element of the head. Using a sensor head having an inter-amorphous wire spacing of 3 mm and a 1 mmφ iron ball magnetized with a magnet having a surface magnetic flux density of 4000 G, (a) (plan view) and (b) in FIG. When the distances y and x shown in [Side view] are moved with y = 3 mm and x = 3 mm, the detection output is as shown in FIG. 3-3 (a), and the movement is performed with y = 5 mm and x = 1 mm. As a result, the detection output was as shown in (b) of FIG. 3-3, and it was confirmed that the detection could be performed with sufficiently high sensitivity.

In the above embodiment, the magneto-impedance effect elements with coils are arranged in a straight line so that the mutual interval in the magneto-sensitive effect direction of the magneto-impedance effect elements is smaller than the length of the magneto-impedance effect element. It is also possible to dispose the magnetic impedance effect elements in a column by shifting them in directions other than the magnetosensitive axis direction while keeping the mutual distance in the axial direction smaller than the length of the magnetoimpedance effect element.
In the present invention, two magneto-impedance effect sensor heads in which magneto-impedance effect elements with coils arranged in series are connected in series are used, the modulation wave at the output end of each sensor head is detected by each detection circuit, and these detection outputs are output. Differential amplification is performed by a differential amplifier circuit, and the differential amplification output is negatively fed back to the negative feedback coil of the magneto-impedance effect element with each coil, and both sensor heads are oriented in the same direction and connected to the main body by a conductor joint. You can also.
In this case, it is possible to equalize the geomagnetism acting on both sensor heads and the noise magnetic field generated from the surrounding iron-based structure, so that noise such as geomagnetism can be removed.

  As the magneto-impedance effect element, an amorphous ribbon, an amorphous sputtered film, or the like can be used in addition to zero magnetostrictive or negative magnetostrictive amorphous wires.

The magneto-impedance effect element 1 is an alloy of transition metal and non-metal having a non-metal composition of 10 to 30 atomic%, particularly an alloy of transition metal and non-metal having a non-metal content of 10 to 30 atomic%. The transition metal is Fe and Co and the nonmetal is B and Si, or the transition metal is Fe and the nonmetal is B and Si. For example, the composition Co _70.5 B ₁₅ Si ₁₀ Fe _4.5 , length 2000 μm to 6000 μm, outer diameter 30 μm to 50 μmφ can be used.

For the high-frequency excitation current, a normal high frequency such as a continuous sine wave, a pulse wave, or a triangular wave can be used. As the high-frequency excitation current source, for example, a Hartley oscillation circuit, a Colpitts oscillation circuit, a collector tuned oscillation circuit, a base tuned oscillation circuit In addition to the normal oscillation circuit, a square wave generator that integrates the square wave output of the crystal oscillator through a DC component cut-off capacitor with an integration circuit and amplifies the triangular wave of this integration output with an amplification circuit, and uses a CMOS-IC as the oscillation unit Can be used.

As the detection circuit, for example, a configuration in which a modulated wave is half-wave rectified by an operational amplifier circuit and the half-wave rectified wave is processed by a parallel RC circuit or an RC low-pass filter to obtain an envelope output of the half-wave rectified wave, the modulated wave The half-wave rectified wave is processed by a diode, and the half-wave rectified wave is processed by a parallel RC circuit or an RC low-pass filter to obtain an envelope output of the half-wave rectified wave.
Further, it is possible to use tuning detection in which a signal wave is sampled by multiplying the modulated wave by a square wave having a frequency fs tuned to the modulated wave (frequency fs).
In the above embodiment, the signal magnetic field (signal wave) is extracted by demodulating the modulated wave. However, the present invention is not limited to this, and the high-frequency excitation current modulated by the signal magnetic field (signal wave) acting on the magneto-impedance effect element. As long as the signal magnetic field can be detected from the wave (carrier wave), an appropriate detection means can be used.

It is a circuit diagram which shows an example of the magneto-impedance effect sensor used in this invention. It is drawing which shows an example of the magneto-impedance effect sensor with a coil in the magneto-impedance effect sensor of FIGS. 1-1. It is drawing which shows the external appearance of the magneto-impedance effect sensor of FIGS. 1-1. 1 is a drawing showing a food magnetic inspection method according to the present invention. It is a circuit diagram which shows an example different from the above of the magneto-impedance effect sensor used in this invention. It is drawing which shows the verification point of another Example of this invention using the sensor of FIGS. It is drawing which shows the verification result of another Example same as the above. It is drawing which shows the output characteristic of a magneto-impedance effect element. 1 is a diagram illustrating a conventional magneto-impedance effect sensor. It is drawing which shows the input / output waveform in various places in the conventional magnetic impedance effect sensor. 6 is a diagram illustrating a magneto-impedance effect element with a coil in a conventional magneto-impedance effect sensor.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Magnetoimpedance effect element 1a Magnetoimpedance effect element 1b with a coil Magnetoimpedance effect element 2 with a coil Excitation current source 3 Detection circuit 3a Detection circuit 3b Detection circuit 4 Amplifier 40 Differential amplifier 5 Detection output terminal 6 Negative feedback magnetic field coil 7 Bias Magnetic field coil A Sensor head B Body substrate C Head substrate d Conductor joint

Claims (4)

A method of detecting a magnetic metal piece mixed in a test object by moving a magneto-impedance effect sensor relative to the test object, and forming a magnetic loop circuit with an iron core in the magneto-impedance effect element Two magnetic impedance effect elements with a coil in which a negative feedback coil and a bias magnetic field coil are wound around the iron core, the magnetic impedance effect elements are arranged in tandem, and the magnetic impedance effect elements The sensor head is arranged with the mutual interval shorter than the length of the magneto-impedance effect element, the detected magnetic field applied to each magneto-impedance effect element is detected by each detection circuit, and the detection output is differentially amplified. The magnetic impedance effect of the sensor head with respect to the relative movement direction Magnetic detection method of the magnetic metal pieces, characterized in that directing the direction of the child in the perpendicular direction. 2. A magnetic metal piece magnetic detection method according to claim 1, wherein the magnetic metal piece is magnetized by applying a magnetic field to a packed food or beverage to be inspected . 3. The magnetism of a magnetic metal piece according to claim 1, wherein a surface on which a magneto-impedance effect element is attached in the magneto-impedance effect sensor head is opposed to a detection surface to be magnetically inspected. Detection method. The opposite ends of the negative feedback coils of the magnetic impedance effect elements with the coils in the column are connected to each other, and the opposite ends of the bias magnetic field coils of the magnetoimpedance effect elements with the same coil are connected to each other. The end a of the magneto-impedance effect element 1a out of the ends a and b facing each other is grounded and connected to the differential amplifier side of the element 1a with respect to the end a of the magneto-impedance effect element 1a. 2. The magnetic metal piece according to claim 1, wherein the end b is connected to a differential amplifier, and the other end of the element 1b with respect to the end b of the magneto-impedance effect element 1b is grounded. Detection method.

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