JP2001091662A - Metal detector - Google Patents

Abstract

PROBLEM TO BE SOLVED: To positively detect fine metal pieces. SOLUTION: In this metal detector, a transmission coil 6 and a reception coil 7 are arranged along a conveyance path of an inspection objective body 1, synchronous detections are performed on an output signal b of the reception coil by an excitation signal and a 90 deg. phase-shifted signal thereof, a Lissajou's waveform 15 is drawn on an orthogonal coordinate plane by each synchronous detection signal, and a metal piece included in the inspection objective body 1 is detected on the basis of this drawn Lissajou's wave form. It is provided with a coordinate segmenting means 35 segmenting the orthogonal coordinate plane into a plurality of zones, an area calculating means 28 calculating each area belonging to each zone of the Lissajou's waveform drawn on the orthogonal coordinate plane as each segment area, an area comparing means 32 comparing each segment area calculated by this area calculating means and each reference segment area in regard to a reference inspection objective body not including a metal piece, and a determining means 34 determining whether or not a metal piece is included in the inspection objective body on the basis of a comparison result of this area comparing means.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a metal detector for inspecting whether an object to be inspected such as a food conveyed on a conveyor path such as a belt conveyor contains a metal piece.

[0002]

2. Description of the Related Art In recent years, HACCP (Hazard Analysis Critical Control) has been used as a system for ensuring food safety.
Point Risk analysis critical control point) has been proposed. this
As part of HACCP, food is required to be checked for metal fragments.

[0003] Various methods have been proposed for inspecting whether or not a test piece such as food contains a metal piece. However, the method utilizes the fact that the magnetic field is disturbed by the metal entering the magnetic field. The metal detection technique described above is common.

[0004] A metal detector adopting this detection method is constructed, for example, as shown in FIG. A frame 3 is provided so as to surround a belt conveyor 2 as a transport path for transporting an inspection object 1 such as food in a certain direction. An operation panel 4 is provided on the front of the frame 3.

FIG. 6 is a block diagram showing a schematic configuration of a metal detector incorporated in the frame 3. As shown in FIG.

[0006] The exciting current a output from the AC power supply 5 is
Transmission coil 6 arranged in the conveying direction of belt conveyor 2
Is applied to Therefore, a constant magnetic field is formed in the transport path of the device under test 1 by the transmission coil 6. A receiving coil 7 is provided at a position opposite to the transmitting coil 6 so that a pair of coils 7a and 7b are differentially connected so that the winding directions are opposite to each other. An output signal b of the receiving coil 7 is taken out from a variable terminal of a variable resistor 8 connected between the output terminals of the receiving coil 7 and is output to a first synchronous detector 9.
a and the second synchronous detector 9b.

Each of the coils 7a and 7b of the receiving coil 7 detects a magnetic field generated in the transmitting coil 6 and generates an induced voltage. However, since the coils 7a and 7b have winding directions opposite to each other, The induced voltages cancel each other, and the signal level of the output signal b is 0. Specifically, the sliding position of the variable terminal of the variable resistor 8 is adjusted so that the signal level of the output signal b becomes zero.

Therefore, in a state where the test object 1 conveyed on the belt conveyor 2 does not include a metal piece, the output signal b of the receiving coil 7 is at the 0 level. However, when the test object 1 includes a metal piece, one of the coils 7
change in the induced voltage of a corresponding to the size of the metal piece + ΔE
Occurs. The induced voltage of the other coil 7b having a different winding direction also has a variation -ΔE corresponding to the size of the metal piece. As a result, the output signal b of the receiving coil 7 includes (+ 2Δ
The signal level of E) appears.

The exciting current a output from the AC power supply 5 is applied to the transmitting coil 6 and the first synchronous detector 9a
Is applied. Further, the excitation signal a is supplied to the 90 ° phase shift circuit 10.
Is shifted by 90 ° in the second direction as a new excitation signal a ₁ .
Is applied to the synchronous detector 9b.

The first synchronous detector 9a synchronously detects the output signal b of the receiving coil 7 with the excitation signal a. The output signal c1 of the _first synchronous detector 9a is input to the X-axis terminal of the determination device 13 having an oscilloscope after the noise component is removed by the BPF 11a and amplified by the amplifier 12a.

[0011] The second synchronous detector 9b is synchronously detected by the excitation signal a ₁ to the output signal b is phase-shifted by 90 ° in the receiving coil 7. The output signal c of the second synchronous detector 9a
₂ is a BPF 11b from which noise components are removed and an amplifier 12b
After that, the signal is input to the Y-axis terminal of the determination device 13 in which the above-described oscilloscope is incorporated.

The determination device 13 outputs an output signal c ₁ input from the X-axis terminal and an output signal c ₂ input from the Y-axis terminal.
And a point P at the tip of the combined vector is displayed on the XY orthogonal coordinate plane of the display screen 14.

Next, the first and second synchronous detectors 9a, 9
The reason for using b will be described. When the metal piece included in the test object 1 is a magnetic material such as iron, as described above, each coil 7
Since a change .DELTA.E of the induced voltage in the opposite directions is generated at a and 7b, a signal (+ 2.DELTA.E) appears in the output signal b.
In this case, the phase of the output signal b does not change with respect to the phase of the excitation signal a. Therefore, the output signal b may be synchronously detected with the excitation signal a.

On the other hand, when the metal piece included in the test object 1 is a non-magnetic material such as stainless steel or aluminum, an eddy current is generated in the non-magnetic material due to the presence of the magnetic field. The signal (−ΔE) appears in the output signal b of the receiving coil 7 due to the fact that the magnetic flux is consumed by the Joule heat of the eddy current. Further, in this case, due to the eddy current, the phase of the output signal b changes by 90 ° with respect to the phase of the excitation signal a. Therefore, it is sufficient synchronous detection at 90 ° phase-shifted excitation signal a ₁ to this output signal b of the original excitation signal a.

However, the material of the metal piece included in the test object 1 such as food does not completely separate into a magnetic material and a non-magnetic material, and has characteristics of both a magnetic material and a non-magnetic material. Therefore, when the test object 1 including the metal piece is passed through the installation position of the transmitting coil 6 and the receiving coil 7 on the belt conveyor 2, the output signal b of the receiving coil 7 changes its signal level and also changes its phase. I do. As a result, if the test object 1 contains a metal piece, the first and second synchronous detectors 9
The output signals c ₁ and c ₂ of a and 9b change together.

Further, when the inspected body 1 is a food, depending on the material (food material) of the food, the food often contains a metal component such as minute iron or the like. Further, in general, food is hygienically packaged and transported. Therefore, when a metal material such as aluminum vapor deposition is adopted as the packaging material, the influence is greatly affected.

In such a test object 1, even if no harmful metal piece to be removed is included, the output signal b of the receiving coil 7 includes the test object 1
A phase change occurs with respect to a signal level and an excitation signal corresponding to the magnetic characteristics and non-magnetic characteristics that the device itself has.

Therefore, in order to detect a minute metal piece included in the test object 1 with high accuracy, the signal level or phase change originally included in the output signal b is used to determine the signal level or the signal level caused by the harmful metal piece. It is necessary to distinguish and extract the phase change.

Next, a test object 1 which does not contain a metal piece but has a magnetic property or a non-magnetic property of the food itself including the package and serves as a normal reference is placed on a belt conveyor 2, and Display screen 1 of determination device 13 when test object 1 is moved between transmission coil 6 and reception coil 7
The locus of the point P at the tip of the composite vector displayed on the XY orthogonal coordinate plane 4 will be described with reference to FIGS. 7 (a) and 7 (b).

In the state where the test object 1 has not reached the coils 6 and 7 (state a), since the signal level of the output signal b is 0, no composite vector appears, and the point P is located on the XY orthogonal coordinates. It is located at the origin. When the test object 1 reaches the coils 6 and 7, the signal level of the output signal b of the transmission coil 7 increases, and the phase with respect to the excitation signal a also increases. As a result, the point P also moves away from the origin (b
Status).

As the device under test 1 moves to the center position between the coils 6 and 7, the signal level and phase of the output signal b increase. As a result, the point P also moves away from the coordinate origin (state c). When the device under test 1 reaches almost the center position of the coils 6 and 7, the signal level of the output signal b hardly changes, and only the phase greatly changes. Therefore, the point P rotates leftward on the XY orthogonal coordinate plane (state c).

Further, when the device under test 1 reaches the position of the coil 7b below the receiving coil 7, the signal level of the output signal b hardly changes, but only the phase further greatly changes. (D state). Further, when the test object 1 reaches the lower end position of the coil 7b below the receiving coil 7,
The signal level of the output signal b decreases and the phase also decreases (state e). Then, when the test object 1 deviates from the position of the coils 6 and 7, the signal level of the output signal b becomes 0, so that no combined vector appears and the point P returns to the origin of the XY orthogonal coordinates (f state).

As described above, when the test object 1 having the magnetic characteristics and the non-magnetic characteristics is caused to pass through the installation positions of the transmission coil 6 and the reception coil 7, the XY of the display screen 14 of the determination device 13 is displayed.
The Lissajous waveform 15 shown in FIG.
Is drawn.

The shape of the Lissajous waveform 15 is
It changes greatly depending on not only the neat elliptical shape shown in FIG. 7B but also the magnetic characteristics and non-magnetic characteristics of the device under test 1. Therefore, each test object 1 has an individual Lissajous waveform 15.

If the test piece 1 contains a metal piece, for example, the magnetic and nonmagnetic properties of the metal piece are smaller than the magnetic and nonmagnetic properties of the test piece 1 originally. Even if it is, as shown in FIG. 8B, it affects the Lissajous waveform 15 created by the magnetic characteristics and non-magnetic characteristics of the device under test 1, and a deformed portion 17 appears.

Therefore, in order to reliably detect the fine metal pieces contained in the test object 1, as shown in FIG. A tolerance frame 16 surrounding the Lissajous waveform 15 with a predetermined tolerance is set for the reference Lissajous waveform 15.

FIG. 8 shows a Lissajous waveform 15 obtained by measuring the device under test 1 in the normal measurement mode.
As shown in (b), a deformed portion 17 is generated and
Is determined, the judging device 13 judges that the measured object 1 contains a metal piece, and outputs a warning.

[0028]

However, FIG.
The metal detector configured as shown in FIG. 6 has the following problems to be solved.

That is, when a metal piece is included in the test object 1, the Lissajous waveform 15 created by the magnetic and non-magnetic characteristics of the test object 1 is affected, and the deformed portion 17 is formed.
Will appear. However, the deformed portion 17 caused by the metal piece does not always appear outside the reference Lissajous waveform 15 as shown in FIG. 8B, and as shown in FIG. In some cases, the Lissajous waveform 15 appears inside the Lissajous waveform 15, or the Lissajous waveform 15 itself greatly changes as shown in FIG. 9B.

In such a case, even if the test object 1 contains a metal piece, the Lissajous waveform 15 does not fall outside the allowable frame 16, so that it is erroneously determined that there is no metal piece. You.

The present invention has been made in view of such circumstances, and by comparing the area of a measured Lissajous waveform with that of a reference Lissajous waveform, for example, the magnetic properties and non-magnetic properties of the metal piece can be improved. Even if the magnetic characteristics are small compared to the magnetic and non-magnetic characteristics originally possessed by the test object, the presence of the metal piece can be reliably detected with a simple configuration, and the reliability can be greatly improved. It is an object to provide a metal detector.

[0032]

According to the present invention, a transmitting coil to which an AC excitation signal is applied and a receiving coil which is differentially connected are provided along a transport path of an object to be inspected. A synchronous detector synchronously detects the output signal of the receiving coil with an excitation signal,
The output signal of the receiving coil is synchronously detected by the second synchronous detector using a signal obtained by shifting the excitation signal by 90 °, and the output signal of the first synchronous detector and the output signal of the second synchronous detector are orthogonal coordinates. The present invention is applied to a metal detector that draws a Lissajous waveform on a surface and detects a metal piece included in the test object based on the drawn Lissajous waveform.

In order to solve the above-mentioned problems, in the metal detector according to the present invention, a coordinate plane dividing means for dividing a rectangular coordinate plane into a plurality of areas, and each of a Lissajous waveform drawn on the rectangular coordinate plane are provided. Area calculating means for calculating each area belonging to the region as each divided area, and comparing each of the divided areas calculated by the area calculating means with each of the reference divided areas in the reference test object not including the metal piece. An area comparing means is provided, and a judging means for judging whether or not the inspection object includes a metal piece based on a comparison result of the area comparing means.

In the metal detector configured as described above, the rectangular coordinate plane on which the Lissajous waveform is drawn is divided into a plurality of areas. First, an object to be inspected which does not include a metal piece is measured in advance, a Lissajous waveform is obtained, and each area belonging to each region of the reference Lissajous waveform is calculated as each section area, and each section of the reference is calculated. Store as area.

Then, an actual test object which may contain a metal piece is measured to obtain a Lissajous waveform, and each area belonging to each region of the Lissajous waveform is calculated as each section area. , And the calculated respective sectional areas are compared with the respective reference sectional areas stored and held. If each calculated section area is significantly different from the reference section area,
Assuming that a deformed portion caused by the metal piece exists in the Lissajous waveform, a metal piece detection warning is output.

[0036]

DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing a schematic configuration of a metal detector according to an embodiment of the present invention. The same parts as those of the conventional metal detector shown in FIG.
Detailed description of overlapping parts is omitted. The external view of this metal detector is the same as the external view shown in FIG.

In the metal detector of this embodiment, the operation mode is switched between a normal “measurement mode” and a “setting mode” for setting a reference Lissajous waveform by operating the operation panel 4 provided on the frame 3. Can be realized.

At a position near both ends of the transmitting coil 6 and the receiving coil 7 on the frame 3 in FIG. 5, a pair of position detectors 18a and 18b for detecting the test object 1 conveyed on the belt conveyor 2 are provided. ing. The position detector 18a on the entrance side includes the transmission coil 6 and the reception coil 7
The device under test detection signal e ₁ which indicates that close to the preparative and sends to the control unit 19 composed of a microcomputer. Also,
The position detector 18b on the exit side is configured such that the test object 1
That apart from the receiver coil 7 for the delivery to the control unit 19 to the device under test detection signal e ₂ consisting microcomputer indicating a.

As with the conventional metal detector shown in FIG.
The exciting current a output from the AC power supply 5 is applied to the transmission coil 6. An output signal b of the receiving coil 7 is output from a variable terminal of a variable resistor 8 connected between output terminals of the receiving coil 7 that is differentially connected so that the pair of coils 7a and 7b are wound in opposite directions. It is extracted and input to the first synchronous detector 9a and the second synchronous detector 9b.

The exciting current a output from the AC power supply 5 is applied to the transmitting coil 6 and the first synchronous detector 9a
Is applied. Further excitation current a output from the AC power source 5 is applied is phase-shifted by 90 ° with 90 ° phase shift circuit 10 as a new excitation signal a ₁ to the second synchronous detector 9b.

The first synchronous detector 9a synchronously detects the output signal b of the receiving coil 7 with the excitation signal a. The output signal c ₁ of the first synchronous detector 9 a is subjected to noise removal by the BPF 11 a and amplified by the amplifier 12 a.
At 0a, the data is converted into digital data and input to the control unit 19.

Further, the second synchronous detector 9b is synchronously detected by the excitation signal a ₁ to the output signal b is phase-shifted by 90 ° in the receiving coil 7. The output signal c of the second synchronous detector 9a
₂ is a BPF 11b from which noise components are removed and an amplifier 12b
, And is converted to digital data by the A / D converter 20 b and input to the control unit 19.

A control unit 19 comprising a microcomputer incorporated with an operation panel 4, a display unit 20, and a printer 21 exposed on the outer surface of the frame 3 shown in FIG.
It is configured as shown in FIG.

When the test object detection signal e ₁ is input from the position detector 18 a, the data fetch timing generation section 23 sends a data fetch start command to the data fetch section 24. Further, when the test object detection signal e ₂ is input from the position detector 18b, the data fetch timing generation unit 23 sends a data fetch end command to the data fetch unit 24.

The data fetching section 24 outputs each A during a period from when the data fetching start command is output from the data fetch timing generating section 23 to when the data fetching end command is output.
The digital output signals c ₁ and c _2, which have been A / D converted by the / D converters 20a and 20b, are fetched and sent to the Lissajous waveform generator 25.

The Lissajous waveform generator 25 performs vector synthesis of the input output signal c ₁ and output signal c _2, and determines a point P at the tip of the synthesized vector as shown in FIG.
Plot on the Y orthogonal coordinate plane. The period from when the data capture start command is output to when the data capture end command is output, that is, the test object 1 having the magnetic characteristics and the non-magnetic characteristics is moved to the installation positions of the transmission coil 6 and the reception coil 7. , A Lissajous waveform 15 shown in FIG. 3A is drawn on the XY orthogonal coordinate plane as described above.

When the operator has set the “setting mode” to the mode setting unit 22 from the operation panel 4, the Lissajous waveform 15 created by the Lissajous waveform creation unit 25 uses the metal piece. Are written to the reference Lissajous waveform memory 26 as the reference Lissajous waveform 15 of the DUT 1 that does not include The reference Lissajous waveform 15 written into the reference Lissajous waveform memory 26 is sent to the next waveform division unit 27.

The area memory 35 stores each area when the XY orthogonal coordinate plane is divided into a plurality of areas. In this embodiment, the XY orthogonal coordinate plane is divided into four regions from a first quadrant to a fourth quadrant having the X-axis and the Y-axis as boundaries, and each region from the first quadrant to the fourth quadrant is divided. Is stored.

The waveform dividing section 26 divides the inputted reference Lissajous waveform 15 into respective sections belonging to the respective areas stored in the area memory 35. In the embodiment, as shown in FIG. 3A, the Lissajous waveform 15 is divided into four sections 15a, 15b, 15c corresponding to the first quadrant (first area) to the fourth quadrant (fourth area). , 15d.

The section made of cotton section 28 calculates each section 15 a of the reference Lissajous waveform 15 created by the waveform dividing section 26,
Reference areas S ₁ , S ₂ , 15b, 15c, 15d,
S ₃ and S ₄ are calculated and written and stored in the reference section area memory 29.

Each of the reference areas S ₁ , S ₂ , and C ₁ corresponding to the reference Lissajous waveform 15 in the “setting mode” described above.
When the setting processing of the reference section area memory 29 in S ₃ and S ₄ is completed, the operator of the metal detector sets “measurement mode” in the mode setting unit 22 from the operation panel 4. .

In the "measurement mode" state,
The Lissajous waveform 15 created by the Lissajous waveform creation unit 25 is used as the Lissajous waveform 15 of the general test object 1 which may include a metal piece as shown in FIG. Written to The Lissajous waveform 15 of the measurement written in the measurement Lissajous waveform memory 30 is sent to the next waveform dividing unit 27.

The waveform dividing section 27 divides the input Lissajous waveform 15 of the measurement into respective sections belonging to the area memory 35. In the embodiment, as shown in FIG. 3B, the Lissajous waveform 15 of the measurement has four sections 15a, 15b, 15c, 1c corresponding to the first to fourth quadrants.
Divide into 5d.

The section made of cotton section 28 calculates each section 15a of the measured Lissajous waveform 15 created by the waveform dividing section 26,
Each area D ₁ , D ₂ , 15b, 15c, 15d measurement
D ₃ and D ₄ are calculated and written into the sectional area memory 31 once.

The comparing section 34 calculates the Lissajous waveform 15 of the measurement.
, The reference areas D ₁ , D ₂ , D ₃ , and D _{4 of the} sections 15a, 15b, 15c, and 15d, and the sections 15 of the reference Lissajous waveform 15 stored in the reference section area memory 29.
a, 15b, 15c, and 15d, reference areas S ₁ , S ₂ ,
S ₃ and S ₄ are compared and compared respectively.

The determination unit 43 determines whether any _{one of the} measurement areas D ₁ , D ₂ , D ₃ , and D ₄ is equal to the threshold memory 3.
3, each of the areas S ₁ , S ₂ , S
_3, if it is different from the S _4, it determines that the deformable portion 17 caused by the metal strip occurs in Lissajous waveform 15 as shown in FIG. 3 (b). When detecting the presence of the metal piece, the determination unit 43 displays a warning on the display unit 20 and sounds an alarm sound with a buzzer or the like. Further, if necessary, the detection record of the metal piece is printed out on a recording sheet by the printer 21.

In the metal detector configured as described above, when performing a metal detection operation for a new type of the test object 1, the operation mode is set to the “setting mode” and a metal piece is included. The metal detection is performed on the test object 1 of the reference that is not set. Then, the test object 1 of this standard
, A section 15a, 15b, 15c, 15d of the reference Lissajous waveform 15 is created.
The reference areas S ₁ , S ₂ , S ₃ , and S ₄ are written and stored in the reference section area memory 29.

Next, the operation mode is set to the "measurement mode", and the actual detection of the metal on the device under test 1 which may include a metal piece is performed. Then, the actual Lissajous waveform 15 of the measurement for the test object 1 is created, and the respective areas D ₁ , D ₂ , D ₃ , D ₄ of the measurements of the respective sections 15a, 15b, 15c, 15d are calculated, and the reference Each of the areas S ₁ , S ₂ , S ₃ , and S ₄ is individually compared. If at least one of the areas deviates by more than the threshold value stored in the threshold value memory 33, it is determined that “a piece of metal exists”. I have.

Therefore, as shown in FIGS. 3A and 3B, the deformed portion 17 of the Lissajous waveform 15 caused by the metal piece
Is formed into a shape that cuts inward, the deformed portion 17 is detected, and the “metal piece” can be reliably detected.

Further, as shown in FIGS. 4A and 4B,
FIG. 4A shows the reference Lissajous waveform 15 shown in FIG.
In the Lissajous waveform 15 of the measurement (b), even if two deformed portions 17 of unevenness are generated and the entire area of the Lissajous waveform 15 does not change, the respective areas D of the regions 15c and 15d do not change.
_{Since 3} and D ₄ have changed more than the threshold value with respect to the reference areas D ₃ and D ₄ , it is possible to reliably detect the “metal piece”.

As described above, the food including the packaging material itself has magnetic and non-magnetic characteristics, such as the food or the like in which the test object 1 to be subjected to metal detection is packaged. Can reliably detect a fine metal piece contained in the test object 1 with a high accuracy even if the value is not negligible compared to the magnetic and nonmagnetic properties of the harmful metal piece.

[0062]

As described above, in the metal detector according to the present invention, the presence or absence of a metal piece is determined by comparing the areas of the measured Lissajous waveform and the reference Lissajous waveform in each of the divided areas. Has been determined.

Therefore, even if the magnetic and non-magnetic characteristics of the metal piece are smaller than the magnetic and non-magnetic characteristics of the device to be inspected, the metal piece can be reliably formed with a simple configuration. The presence of the metal piece can be detected, and the reliability of the metal detector can be greatly improved.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a schematic configuration of a metal detector according to an embodiment of the present invention.

FIG. 2 is a block diagram showing a detailed configuration of a control unit of the metal detector according to the embodiment.

FIG. 3 is a view showing a Lissajous waveform measured by the metal detector according to the embodiment;

FIG. 4 is a diagram showing a Lissajous waveform measured by the metal detector according to the same embodiment.

FIG. 5 is an external view showing a general metal detector.

FIG. 6 is a block diagram showing a schematic configuration of a conventional metal detector.

FIG. 7 is a diagram showing an order in which a Lissajous waveform is created in a metal detector.

FIG. 8 is a diagram showing a Lissajous waveform measured by a conventional metal detector.

FIG. 9 is a diagram showing a Lissajous waveform measured by a conventional metal detector.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Test object 2 ... Belt conveyor 4 ... Operation panel 5 ... AC power supply 6 ... Transmission coil 7 ... Receiving coil 8 ... Variable resistance 9a ... 1st synchronous detector 9a ... 1st synchronous detector 10 ... 90 degree shift Phaser 11a, 11b BPF 12a, 12b Amplifier 15 Lissajous waveform 17 Deformation unit 18a, 18b Position detector 19 Control unit 20 Display unit 21 Printer 22 Mode setting unit 23 Data generation timing generation Unit 24 Data acquisition unit 25 Lissajous waveform creation unit 26 Reference Lissajous memory 27 Waveform division unit 28 Segmented area calculation unit 30 Measurement Lissajous memory 32 Ratio units 34 Determination unit

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Norihiko Nagaoka 5-10-27 Minamiazabu, Minato-ku, Tokyo Anritsu Corporation (72) Inventor Satoshi Mitani 5-10-27 Minamiazabu, Minato-ku, Tokyo Anritsu Co., Ltd. F-term (reference) 2G005 CA02

Claims (1)

[Claims] A transmission coil (5) to which an AC excitation signal is applied and a reception coil (7) that is differentially connected are provided along a transport path (2) of the device under test (1). A first synchronous detector (9a) synchronously detects the output signal of the receiving coil with the excitation signal, and a second synchronous detector (9b) shifts the output signal of the receiving coil by 90 ° with the excitation signal. Synchronous detection is performed with the synchronized signals, and a Lissajous waveform (15) is drawn on a rectangular coordinate plane using the output signal of the first synchronous detector and the output signal of the second synchronous detector. A metal detector for detecting a metal piece included in the object to be inspected based on a coordinate plane dividing means (35) for dividing the rectangular coordinate plane into a plurality of regions; and a Lissajous drawn on the rectangular coordinate plane. Each area belonging to each area of the waveform is defined as each section area An area calculating means (28) for calculating by calculation, an area comparing means (32) for comparing each of the divided areas calculated by the area calculating means with each of the reference divided areas in the reference test object not including the metal piece. A determining means (34) for determining whether or not the inspection object contains a metal piece based on a comparison result of the area comparing means.