JP2016217947A - Metal detection machine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a metal detection machine that can achieve improvement in detection sensitivity of metal in an inspected object and space saving.SOLUTION: A metal detection machine comprises: an excited magnetic coil 21 that generates a magnetic field annularly wound in an in-plane parallel with respect to a conveyance face conveying an inspected object in a prescribed conveyance direction; and a detection coil 22 that detects metal provided inside the excited magnetic coil 21. The detection coil 22 comprises: a rectangular flat magnetic core that is arranged so that a long-side direction runs along the conveyance direction, and has a plate thickness direction in a direction parallel with a face of the excited magnetic coil, and orthogonal to the conveyance direction; and at least a pair of coils that is wound around the magnetic core in a mutually opposite direction, and is connected in series.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a metal detector that detects the presence or absence of metal in an object to be inspected, such as food or clothing.

  Conventionally, a metal detector disclosed in Patent Document 1 is known as this type of metal detector. The conventional one described in Patent Document 1 is arranged at the front and rear positions with respect to the conveyance direction in the same plane as the transmission coil, the belt for conveying the inspection object, the transmission coil arranged in a plane parallel to the belt. And two receiving coils having the same shape.

  Specifically, the transmission coil has a rectangular shape, is connected to an oscillator, and generates a magnetic field. The two receiving coils have a rectangular shape and are arranged so as to bisect the area surrounded by the transmitting coils so as to detect the magnetic flux. The transmission coil and the two reception coils are disposed in the vicinity of the lower side of the belt in parallel with the transport surface. The two receiving coils are connected to a differential amplifier. The differential amplifier is adjusted so that the differential voltage between the two receiving coils is zero volts.

  With this configuration, when an object to be inspected containing metal appears in the vicinity of the receiving coil, a signal corresponding to the size and position of the metal is output from the differential amplifier. Can be detected.

JP 04-116493 A

  However, in the conventional apparatus, if the number of turns of the receiving coil is increased in order to further improve the metal detection sensitivity, the dimension in the direction perpendicular to the belt conveyance surface increases, and thus space saving is achieved. It became difficult to improve the situation.

  The present invention has been made in view of the circumstances as described above, and an object thereof is to provide a metal detector capable of improving the detection sensitivity of a metal in an object to be inspected and saving space.

  The metal detector according to claim 1 of the present invention is a metal detector for detecting the presence or absence of the metal (6) in the inspection object by passing the inspection object (5) through the magnetic field. A transport means (41) having a transport surface (41a) for transporting an inspection object in a predetermined transport direction, and an annular winding in one of a plane parallel to the transport surface and a plane perpendicular to the transport surface. An excitation coil (21) that generates a magnetic field, and a detection coil (22) that is provided inside the excitation coil and detects the metal, the detection coil being arranged such that the longitudinal direction is along the transport direction. A rectangular flat magnetic core (22a, 23a, 24a) having a plate thickness direction in a direction parallel to the surface of the exciting coil and perpendicular to the transport direction, and wound around the magnetic core in opposite directions. At least a pair connected in series It has a coil (22b and 22c, 62a and 62b, 63a and 63 b), a configuration with a.

  With this configuration, the metal detector according to claim 1 of the present invention is arranged so that the longitudinal direction thereof is along the transport direction, and the plate thickness direction is set in a direction parallel to the surface of the excitation coil and perpendicular to the transport direction. And a detection coil having a rectangular flat magnetic core and at least a pair of coils wound in opposite directions around the magnetic core and connected in series. The detection sensitivity can be further improved.

  Therefore, the metal detector according to claim 1 of the present invention does not need to increase the number of turns of the receiving coil in order to further improve the metal detection sensitivity, and is perpendicular to the transport surface of the transport means. Since the dimension in the direction does not increase, the metal detection sensitivity can be improved and the space can be saved.

  The metal detector according to claim 2 of the present invention has a configuration in which the coil intervals of the pair of coils (62a and 62b, 63a and 63b) are different from each other.

  With this configuration, the metal detector according to claim 2 of the present invention can have sensitivity in a wider sensitivity region by the detection coil having coils with different coil intervals.

  The metal detector according to claim 3 of the present invention has a configuration in which a plurality of the magnetic cores are arranged side by side in the plate thickness direction.

  With this configuration, the metal detector according to the third aspect of the present invention can increase the volume of the magnetic material, thereby further improving the metal detection sensitivity.

  The metal detector according to claim 4 of the present invention may have a configuration in which a plurality of the magnetic cores (23a, 24a) are arranged in the transport direction.

  The metal detector according to a fifth aspect of the present invention has a configuration in which a plurality of the detection coils are arranged in the plate thickness direction inside the excitation coil.

  With this configuration, the metal detector according to claim 5 of the present invention can perform inspection by arranging inspected objects in parallel on the transport surface of the transport means, so that metal can be detected efficiently. it can.

  The metal detector according to claim 6 of the present invention is connected to each of the pair of coils, and amplifying means (32a to 32d) for amplifying a voltage at both ends of the pair of coils, and the amplifying means amplifies. And a determination means (35a to 35d) for determining the presence or absence of metal in the inspection object based on the voltage.

  With this configuration, the metal detector according to claim 6 of the present invention can determine the presence or absence of metal in the inspection object for each detection coil.

  The present invention can provide a metal detector having the effect of improving the metal detection sensitivity and saving space.

It is a figure which shows typically the structure in 1st Embodiment of the metal detector which concerns on this invention. It is a figure which shows typically the metal detection processing apparatus in 1st Embodiment of the metal detector which concerns on this invention. It is a figure which shows typically the structure of the detection coil in 1st Embodiment of the metal detector which concerns on this invention. It is explanatory drawing of the principle of the metal detection in 1st Embodiment of the metal detector which concerns on this invention. It is a figure which shows an example of the arrangement | sequence of the magnetic core in 1st Embodiment of the metal detector which concerns on this invention. In 1st Embodiment of the metal detector which concerns on this invention, it is a figure which shows typically the metal detection processing apparatus in the 1st other aspect. In 1st Embodiment of the metal detector which concerns on this invention, it is a figure which shows typically the metal detection processing apparatus in the 2nd other aspect. In 1st Embodiment of the metal detector which concerns on this invention, it is a figure which shows typically the metal detection processing apparatus in the 3rd other aspect. It is a figure which shows typically the structure in 2nd Embodiment of the metal detector which concerns on this invention.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

(First embodiment)
First, the structure in 1st Embodiment of the metal detector which concerns on this invention is demonstrated using FIGS. 1-3.

  As shown in FIG. 1, the metal detector 1 in the present embodiment includes a metal detection processing device 10 that performs detection processing of metal that is a foreign object, and a conveyor 40 as a transport unit, The presence or absence of the metal 6 is detected.

  The metal detection processing apparatus 10 includes a metal detection unit 20 and a metal detection circuit 30. The conveyer 40 includes an endless annular conveyer belt 41 as conveying means for conveying the inspection object 5 in the illustrated conveying direction, and conveying rollers 42 and 43. The conveyor belt 41 has a transport surface 41a on which the inspection object 5 is placed and transported in the transport direction.

  As shown in FIGS. 1A and 1B, the metal detection unit 20 is disposed at a position near the conveyor belt 41 and substantially parallel to the transport surface 41 a. The metal detection unit 20 is connected to the metal detection circuit 30.

  As shown in FIG. 2, the metal detection circuit 30 includes an oscillator 31, a differential amplifier 32 (32a to 32d) as an amplifying unit, a detection unit 33 (33a to 33d), and an ADC (analog / digital converter) 34 (34a to 34a). 34d), a determination unit 35 (35a to 35d) as a determination unit, and a result display unit 36 are provided.

  The metal detection unit 20 includes an exciting coil 21 that generates a magnetic field, a direction that is substantially parallel to the surface of the exciting coil 21, and a direction that is substantially perpendicular to the conveying direction (sometimes referred to as “width direction of the conveying surface 41 a”). And detection coils 22 arranged in four rows. In the present embodiment, an example in which the detection coils 22 are arranged in four rows is given, but the present invention is not limited to this, and may be one row, for example. In the drawing, the width direction of the transport surface 41 a is simply indicated as “width direction”.

  The exciting coil 21 has a rectangular shape and is wound in a ring shape in a plane near the transport surface 41a (see FIG. 1) and substantially parallel to the transport surface 41a. The number of turns of the exciting coil 21 is, for example, about 1 to 5 turns. The exciting coil 21 is connected to an oscillator 31 and generates a magnetic field in a region through which the inspection object 5 passes. The shape of the exciting coil 21 is not limited to a rectangular shape, and may be a circular shape, an elliptical shape, or the like.

  As shown in FIG. 3, the detection coil 22 includes a plurality of elongated rectangular flat magnetic cores 22a and a pair of coils 22b and 22c wound around the magnetic core 22a.

  The magnetic core 22a is formed of a flat-shaped magnetic body, for example, an amorphous magnetic body, and is arranged so that the longitudinal direction is along the transport direction. The magnetic core 22a has a plate thickness direction in the width direction of the transport surface 41a, and a plurality of magnetic cores 22a are stacked in the plate thickness direction. The magnetic core 22a may have a single configuration.

  Each of the coils 22b and 22c is wound around the stacked magnetic cores 22a in opposite directions and connected in series, and has a configuration of a pair of anti-series connection coils. One end of the coil 22b is connected to one input terminal of a differential amplifier 32 (see FIG. 2). The other end of the coil 22b is connected to one end of the coil 22c. The other end of the coil 22c is connected to the ground.

  Returning to FIG. 2, the oscillator 31 generates an oscillation signal having a frequency of several kHz to 10 MHz, for example, and outputs the oscillation signal to the excitation coil 21 and the detection unit 33 of the metal detection unit 20.

  The differential amplifier 32 includes differential amplifiers 32a to 32d, each having two input terminals and one output terminal.

  In each of the differential amplifiers 32a to 32d, one input terminal is connected to one end of the coil 22b, and the other input terminal is connected to the other end of the coil 22c through the ground, and two coils 22b and 22c forming a pair are formed. The signal voltage difference from is taken and amplified.

  The detection unit 33 includes detection units 33a to 33d. The detectors 33a to 33d are connected to the oscillator 31 and the output terminals of the differential amplifiers 32a to 32d, respectively, and synchronously detect the output signals of the differential amplifiers 32a to 32d in synchronization with the output of the oscillator 31. It has become.

  The ADC 34 includes ADCs 34a to 34d. The ADCs 34a to 34d convert the output signals of the detectors 33a to 33d from analog signals to digital signals, respectively.

  The determination unit 35 includes determination units 35a to 35d. The determination units 35a to 35d are connected to the output terminals of the ADCs 34a to 34d, respectively, and based on these output signals, determine the presence or absence of the metal 6 in the inspection object 5 with reference to a predetermined determination threshold value. It is supposed to be.

  The result display unit 36 displays the determination result of the presence or absence of the metal 6 by the determination units 35a to 35d, for example, on a liquid crystal screen.

  Next, the principle of metal detection of the metal detection unit 20 will be described with reference to FIG. FIG. 4A is a plan view of the metal detection unit 20. FIG. 4B is a cross-sectional view in the transport direction in FIG. FIG. 4C is a cross-sectional view in the transport direction when the inspection object 5 includes the metal 6. For the sake of simplicity, it is assumed that there is one detection coil 22 having one magnetic core 22 a inside the excitation coil 21.

  As shown in FIGS. 4A and 4B, the longitudinal direction of the magnetic core 22a having the plate thickness direction in the width direction of the transport surface 41a is along the transport direction in the detection coil 22 inside the excitation coil 21. Has been placed. The coils 22b and 22c are wound in opposite directions and connected in series.

  That is, the direction of the magnetic field generated by the excitation coil 21 and the sensitivity axis of the detection coil 22 are orthogonal. The arrangement position of the detection coil 22 is adjusted so that no induced voltage is generated even when an alternating current is passed through the excitation coil 21. Normally, if the detection coil 22 is arranged at the center between the left coil 21a and the right coil 21b of the exciting coil 21, no induced voltage is generated. Here, that the induced voltage does not occur does not only mean that the induced voltage is zero volts, but means that the induced voltage is low enough to detect the metal 6.

  Further, in addition to the above arrangement, the metal detector 1 has a detection coil 22 connected in a differential configuration, so that high balance can be realized and highly sensitive detection is possible.

  Furthermore, since the metal detector 1 uses the thickness direction of the magnetic core 22a as the width direction of the transport surface 41a, the cross-sectional area through which the magnetic flux generated by the exciting coil 21 can pass is extremely narrow, and the magnetic core 22a is swirled. It is a structure that does not easily generate current. As a result, compared to the case where the plate thickness direction of the magnetic core 22a is orthogonal to the transport surface 41a, the metal detector 1 (1) makes it difficult for the power of the exciting coil 21 to be consumed by eddy currents, and thus achieves low power consumption. (2) Since eddy currents are less likely to occur in the magnetic core 22a, the output signals of the detection coils 22 can be easily balanced, and the output signals can be stabilized.

  Furthermore, since the metal detector 1 can increase the volume of the magnetic material by laminating the magnetic cores 22a in the plate thickness direction, the metal detection sensitivity can be further improved.

  The coil wire used between the coil 22b and the coil 22c or at the connection portion from the detection coil 22 to the outside is preferably a twisted pair wire in order to avoid the influence of the magnetic field.

  As shown in FIG. 4C, when the metal 6 passes through the upper part of the metal detection unit 20, a new magnetic field is generated in the metal 6 by the magnetic field generated by the excitation coil 21. In this case, the mechanism differs depending on whether the metal 6 is a conductor or a magnetic material. In the case of a magnetic material, the metal 6 is magnetized to generate a new magnetic field, and in the case of a conductor, an eddy current is generated in the metal 6 to generate a new magnetic field. When the metal 6 passes near the center of the coils 22b and 22c, in the state shown in FIG. 4C, the newly generated magnetic field is in the direction opposite to the conveying direction in the coil 22b and in the conveying direction in the coil 22c. Flowing into. As a result, an induced voltage generated by a new magnetic field is generated in the detection coil 22, and the induced voltage of the detection coil 22 is added by the differential amplifier 32 and synchronously detected by the detection unit 33, thereby causing a spike-like voltage signal ( (See FIG. 2).

  Next, a modification of the detection coil 22 will be described with reference to FIG. In the above description, the detection coil 22 has a configuration in which the coils 22b and 22c are wound around one magnetic core 22a. This invention is not limited to this, It can also be set as the structure which has arrange | positioned the several magnetic core side by side in the conveyance direction. Hereinafter, an example in which two magnetic cores are arranged in the transport direction will be described.

  FIG. 5A shows an example in which the detection coils 23 and 24 are arranged in a line substantially along the transport direction. The detection coils 23 and 24 have magnetic cores 23a and 24a, respectively. The magnetic cores 23a and 24a have coils 23b and 24b wound individually. The coils 23b and 24b are wound in opposite directions and connected in series. FIG. 5B shows a configuration in which the detection coils 23 and 24 are arranged substantially along the conveyance direction, and the detection coil 24 is arranged inclined at a predetermined angle with respect to the conveyance direction. FIG. 5C shows a configuration in which the detection coils 23 and 24 are both arranged in a line inclined at substantially the same angle with respect to the transport direction. FIG. 5D shows a configuration in which the detection coils 23 and 24 are arranged at different angles with respect to the transport direction.

  Next, operation | movement of the metal detector 1 in this embodiment is demonstrated using FIG.1 and FIG.2.

  The oscillator 31 generates an oscillation signal having a frequency of, for example, 100 kHz, and outputs the oscillation signal to the excitation coil 21 and the detection unit 33 of the metal detection unit 20. As a result, the exciting coil 21 generates a magnetic field in a region through which the inspection object 5 passes.

  When the inspection object 5 including the metal 6 passes near the upper part of the detection coil 22 in a state where the excitation coil 21 generates a magnetic field, a new magnetic field is generated by the metal 6. An induced voltage generated by the new magnetic field is generated in the detection coil 22.

  The differential amplifier 32 adds and amplifies each induced voltage generated by the two coils 22 b and 22 c forming a pair, and outputs the amplified voltage to the detector 33.

  The detector 33 outputs a spike-like voltage signal to the ADC 34 by synchronously detecting the signal amplified by the differential amplifier 32 based on the transmission signal from the oscillator 31.

  The ADC 34 converts the voltage signal from the detection unit 33 from an analog value to a digital value signal and outputs the signal to the determination unit 35.

  The determination unit 35 determines the presence / absence of the metal 6 in the inspection object 5 based on the output signal of the ADC 34 and a predetermined threshold for determination, and outputs data of the determination result to the result display unit 36.

  The result display unit 36 displays the determination result by the determination unit 35 on, for example, a liquid crystal screen. Here, the result display unit 36 has a configuration for displaying the determination result for each detection coil 22, and preferably displays the position where the metal 6 in the inspection object 5 is detected.

  As described above, the metal detector 1 according to the present embodiment is arranged such that the longitudinal direction thereof is along the transport direction, the plate thickness direction in a direction substantially parallel to the surface of the excitation coil 21 and a direction substantially orthogonal to the transport direction. And a pair of coils 22b and 22c that are wound around the magnetic core 22a in opposite directions and connected in series to each other, the detection coil 22 has a pair of coils 22b and 22c. The detection sensitivity of the metal 6 can be further improved based on the differential output voltage.

  That is, the metal detector 1 according to the present embodiment does not need to increase the number of turns of the receiving coil in order to further improve the detection sensitivity of the metal 6, and is perpendicular to the conveyance surface 41a of the conveyor belt 41. Directional dimensions do not increase.

  Therefore, the metal detector 1 in the present embodiment can improve the detection sensitivity of the metal 6 in the inspection object 5 and save space.

  Next, another aspect of the metal detector 1 in the present embodiment will be described with three examples. In addition, the same code | symbol is attached | subjected to the structure similar to the above-mentioned metal detector 1, and the description is abbreviate | omitted.

[First other aspect]
FIG. 6 shows a metal detection processing apparatus according to the first other aspect. The metal detection unit 20 of this metal detection processing apparatus includes one detection coil 25. In the detection coil 25, a plurality of cores 22a are stacked to the same extent as the length in the width direction of the transport surface 41a (see FIG. 1). Coils 22b and 22c are wound in opposite directions and connected in series to the laminated magnetic core 22a. One differential amplifier 32 is connected to the coils 22b and 22c. With this configuration, the metal detection processing device according to the first other aspect can be configured by one each from the differential amplifier 32 to the determination unit 35, so that the circuit configuration can be simplified.

[Second other embodiment]
FIG. 7 shows a metal detection processing apparatus according to a second other embodiment. This metal detection processing apparatus has two metal detection units 20 on the upper side and the lower side that face each other across the conveyor belt 41. The upper metal detection unit 20 is provided, for example, in the upper part of the tunnel part through which the inspection object 5 can pass. With this configuration, the metal detection processing apparatus according to the second other aspect can detect the metal 6 included in the inspection object 5 from both the upper and lower directions of the inspection object 5.

[Third Other Mode]
FIG. 8 shows a metal detection processing apparatus according to a third other aspect. This metal detection processing apparatus has two metal detection units 20 provided in the vicinity of the conveyor belt 41 and facing each other in a direction orthogonal to the transport direction. That is, the two metal detectors 20 are each arranged on a plane substantially perpendicular to the transport surface 41a. With this configuration, the metal detection processing device according to the third other aspect can detect the metal 6 included in the inspection object 5 from both directions in the width direction of the transport surface 41a perpendicular to the transport direction. In addition, the structure which arrange | positions the one metal detection part 20 on the plane substantially perpendicular | vertical to the conveyance surface 41a may be sufficient.

(Second Embodiment)
The structure of the metal detector in this embodiment is demonstrated using FIG.

  As shown in FIG. 9, the metal detector in the present embodiment is different in that a metal detection processing device 50 is provided instead of the metal detection processing device 10 (see FIG. 2) in the first embodiment. Therefore, the description of the same structure as the first embodiment is omitted.

  The metal detection processing device 50 includes a metal detection unit 60 and a metal detection circuit 70. The metal detection unit 60 includes an excitation coil 21 and a detection coil 61. The metal detection circuit 70 includes an oscillator 31, a differential amplifier 32 (32a to 32b) as an amplification unit, a detection unit 33 (33a to 33b), an ADC 34 (34a to 34b), and a determination unit 35 (35a to 35b) as a determination unit. ) And a result display unit 36.

  The detection coil 61 is arranged in a direction substantially coinciding with the transport direction. In the present embodiment, an example is described in which one detection coil 61 is arranged in a direction substantially orthogonal to the conveyance direction. However, the present invention is not limited to this, and a plurality of detection units may be arranged. .

  The detection coil 61 includes a core 22a laminated in the same manner as in the first other aspect of the first embodiment (see FIG. 6), two pairs of coils 62a and 62b wound around the laminated magnetic core 22a, a coil 63a and 63b.

  In the coils 62a and 62b, which are one pair of coils, one end of the coil 62a is connected to one input terminal of the differential amplifier 32b, the other end of the coil 62a is connected to one end of the coil 62b, and the other end of the coil 62b. Is connected to the other input terminal of the differential amplifier 32b through a ground.

  In the coils 63a and 63b, which are the other pair of coils, one end of the coil 63a is connected to one input terminal of the differential amplifier 32a, the other end of the coil 63a is connected to one end of the coil 63b, and the other end of the coil 63b. Is connected to the other input terminal of the differential amplifier 32a through a ground.

  In the present embodiment, the detection coil 61 has two pairs of coils 62a and 62b and coils 63a and 63b, the coil interval between one pair of coils 62a and 62b, and the coil between the other pair of coils 63a and 63b. The intervals are different from each other. That is, the coil interval between one pair of coils 62a and 62b is wider than the coil interval between the other pair of coils 63a and 63b.

  With this configuration, the metal detection processing device 50 according to the present embodiment has sensitivity in a wider sensitivity region by the two pairs of coils 62a and 62b and coils 63a and 63b having different coil intervals. Specifically, the metal detection processing device 50 has a metal detection sensitivity at a relatively distant position by one pair of coils 62a and 62b, and a metal detection sensitivity at a relatively close position by the other pair of coils 63a and 63b. Have

  Therefore, the metal detector in the present embodiment can improve the detection sensitivity of the metal 6 in the inspection object 5 and save space, and can reduce the cost. Has sensitivity in the area.

  In the second embodiment, two pairs of coils have been described as examples. However, the present invention is not limited to this, and a configuration in which three or more pairs of coils are provided in the magnetic core 22a is also possible. Good. The metal detector having this configuration has a sensitivity in a wider sensitivity region than that using two pairs of coils.

  As described above, the metal detector according to the present invention has the effect of improving the metal detection sensitivity and saving space, and detects the presence or absence of metal in the inspected object such as food or clothing. It is useful as a metal detector.

DESCRIPTION OF SYMBOLS 1 Metal detector 5 Inspected object 6 Metal 10, 50 Metal detection processing apparatus 20, 60 Metal detection part 21 Excitation coil 22, 23, 24, 25, 61 Detection coil 22a, 23a, 24a Magnetic core 22b, 22c, 23b, 24b, 62a, 62b, 63a, 63b Coil 30, 70 Metal detection circuit 31 Oscillator 32 (32a-32d) Differential amplifier (amplifying means)
35 (35a-35d) determination part (determination means)
41 Conveyor belt (conveying means)
41a Conveying surface

Claims (6)

A metal detector for passing the object to be inspected (5) through a magnetic field and detecting the presence or absence of the metal (6) in the object to be inspected,
A transport means (41) having a transport surface (41a) for transporting the inspection object in a predetermined transport direction;
An exciting coil (21) that is annularly wound in one of a plane parallel to the transport surface and a plane perpendicular to the transport surface to generate the magnetic field;
A detection coil (22) provided inside the excitation coil for detecting the metal;
With
The detection coil is
A rectangular flat plate-shaped magnetic core (22a, 23a, 24a) having a plate thickness direction in a direction parallel to the surface of the excitation coil and perpendicular to the transport direction, the longitudinal direction being arranged along the transport direction When,
At least a pair of coils (22b and 22c, 62a and 62b, 63a and 63b) wound in opposite directions around the magnetic core and connected in series;
A metal detector characterized by comprising:   2. The metal detector according to claim 1, wherein the pair of coils (62 a and 62 b, 63 a and 63 b) have different coil intervals.   The metal detector according to claim 1, wherein a plurality of the magnetic cores are arranged side by side in the plate thickness direction.   The metal detector according to any one of claims 1 to 3, wherein a plurality of the magnetic cores (23a, 24a) are arranged side by side in the transport direction.   5. The metal detector according to claim 1, wherein a plurality of the detection coils are arranged in the plate thickness direction inside the excitation coil. 6. Amplifying means (32a to 32d) connected to each of the pair of coils, respectively, for amplifying the voltage at both ends of the pair of coils;
Determination means (35a to 35d) for determining the presence or absence of metal in the inspection object based on the voltage amplified by the amplification means;
The metal detector according to any one of claims 1 to 5, further comprising:

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