JP3658523B2 - Metal detector - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a metal detector for inspecting whether or not an object to be inspected, such as food, conveyed on a conveyance path such as a belt conveyor contains a metal piece.
[0002]
[Prior art]
In recent years, HACCP (Hazard Analysis Critical Control) has been proposed as a system for ensuring food safety. As part of this HACCP, it is proposed to inspect foods for metal fragments.
[0003]
Various methods have been proposed to inspect whether or not a metal piece is contained in an object to be inspected such as food, but a metal detection method using the fact that the magnetic field is disturbed by the metal entering the magnetic field. Is common.
[0004]
A metal detector employing this detection method is configured as shown in FIG. 7, for example. A frame 3 is installed so as to surround a belt conveyor 2 as a conveyance path for conveying an object to be inspected 1 such as food. An operation panel 4 is provided in front of the frame 3.
[0005]
FIG. 8 is a block diagram showing a schematic configuration of the metal detector incorporated in the frame 3.
[0006]
The excitation current a output from the AC power supply 5 is applied to the transmission coil 6 disposed in the conveying direction of the belt conveyor 2. Therefore, a constant magnetic field is formed in the conveyance path of the device under test 1 by the transmission coil 6. A receiving coil 7 that is differentially connected so that the winding directions of the pair of coils 7 a and 7 b are opposite to each other is disposed at a position opposite to the transmitting coil 6. The output signal b of the receiving coil 7 is taken out from the variable terminal of the variable resistor 8 connected between the output terminals of the receiving coil 7 and input to the first synchronous detector 9a and the second synchronous detector 9b. The
[0007]
The coils 7a and 7b of the receiving coil 7 generate an induced voltage by detecting the magnetic field generated in the transmitting coil 6. However, since the winding directions of the coils 7a and 7b are opposite to each other, the induced voltage is The signal level of the output signal b is 0 because they cancel each other. 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.
[0008]
Therefore, in a state where the object to be inspected 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 a metal piece is included in the device under test 1, a change (+ ΔE) corresponding to the size of the metal piece occurs in the induced voltage of one coil 7a. A change (-ΔE) corresponding to the size of the metal piece also occurs in the induced voltage of the other coil 7b having a different winding direction. As a result, a signal level of (+ 2ΔE) appears in the output signal b of the receiving coil 7.
[0009]
The excitation current a output from the AC power supply 5 is applied to the transmission coil 6 and to the first synchronous detector 9a. Further, the excitation signal a is phase-shifted by 90 ° by the 90 ° phase-shift circuit 10 and applied to the second synchronous detector 9b as a new excitation signal a ₁ .
[0010]
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 9a is subjected to noise component removal by the BPF 11a and amplified by the amplifier 12a, and then input to the determination circuit 13a as the detection signal d ₁ .
[0011]
The second synchronous detector 9b synchronously detects the output signal b of the receiving coil 7 with the excitation signal a ₁ whose phase is shifted by 90 °. After this output signal c ₂ of the second synchronous detector 9a is a noise component is amplified by the removed amplifier 12b in BPF11b, it is input to the determination circuit 13b as a detection signal d _2.
[0012]
When the signal level of the input detection signal d ₁ exceeds a predetermined threshold value (allowable limit), the determination circuit 13a determines that there is a metal piece and outputs a determination signal e ₁ . Further, when the signal level of the input detection signal d ₂ exceeds a predetermined threshold value (allowable limit), the determination circuit 13b determines that there is a metal piece and outputs a determination signal e ₂ .
[0013]
The OR circuit 14 outputs a logical sum of the input determination signal e ₁ and determination signal e ₂ as a determination result.
[0014]
Next, the reason why the first and second synchronous detectors 9a and 9b are used will be described. When the metal piece contained in the device under test 1 is a magnetic material such as iron, as described above, a change ΔE in the induced voltage is generated in the coils 7a and 7b in opposite directions, so that the output signal b has (+ 2ΔE). ) Appears. In this case, as shown in FIG. 9, the phase of the output signal b does not change with respect to the phase of the excitation signal a. Therefore, this output signal b may be detected synchronously with the excitation signal a.
[0015]
On the other hand, when the metal piece included in the device under test 1 is a nonmagnetic material such as stainless steel or aluminum, an eddy current is generated in the nonmagnetic material due to the presence of a magnetic field. Due to the fact that the magnetic flux is consumed by the eddy current Joule heat, a signal (−2ΔE) appears in the output signal b of the receiving coil 7. 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 as shown in FIG. Therefore, the output signal b may be synchronously detected with the excitation signal a ₁ that is 90 ° phase shifted from the original excitation signal a.
[0016]
Accordingly, by performing a logical OR operation on the determination signals e ₁ and e ₂ in the OR circuit 14, even if any metal piece of a magnetic material or a non-magnetic material is included in the device under test 1. These metal pieces can be reliably detected.
[0017]
[Problems to be solved by the invention]
However, the metal detectors shown in FIGS. 7 and 8 still have the following problems to be solved.
[0018]
That is, only the inspected object 1 such as food that does not include a metal piece has characteristics of both a magnetic body and a non-magnetic body, and these characteristics are randomly distributed in the ratio inspecting body 1. Accordingly, when the device under test 1 is passed through the installation position of the transmission coil 6 and the reception coil 7 by the belt conveyor 2, the output signals c ₁ and c ₂ of the first and second synchronous detectors 9a and 9b are sometimes changed. It changes every moment.
[0019]
Further, when an impact or the like is applied to the frame 3, the positional relationship between the coils 6, 7 a, and 7 b changes. Therefore, even if a metal piece is not included in the device under test 1, the first and second synchronizations are performed. Vibration noise appears in the output signals c ₁ and c ₂ of the detectors 9a and 9b.
[0020]
As described above, the output signals c ₁ and c ₂ of the first and second synchronous detectors 9a and 9b each have a magnetic property inherent to the device under test 1 other than a signal caused by a metal piece to be originally detected. A material signal due to characteristics and non-magnetic characteristics and a vibration signal due to vibration are included. Therefore, if the metal piece is weak, the signal level of the signal caused by the metal piece is small compared to the signal level of these material signals and vibration signals, so the presence of this fine metal piece cannot be detected reliably. The object to be inspected that does not include the metal piece is erroneously detected when the metal piece is present.
[0021]
The present invention has been made in view of such circumstances, and a signal waveform that appears in the output signal of the synchronous detector due to the material and vibration of the object to be inspected and a signal waveform that appears in the output signal due to the metal piece. By detecting this error, the magnetic properties and non-magnetic properties of the metal piece are small compared to the magnetic properties and non-magnetic properties that the test object originally has. Even so, an object of the present invention is to provide a metal detector that reliably detects a metal piece and does not erroneously detect it as a metal piece even if vibration noise is mixed.
[0022]
[Means for Solving the Problems]
In the present invention, a transmission coil and a reception coil to which an alternating excitation signal is applied are arranged along the conveyance path of the object to be inspected, and the output signal of the reception coil is synchronously detected by the excitation signal with a synchronous detector, The present invention is applied to a metal detector that detects a metal piece contained in an object to be inspected based on an output signal of a synchronous detector.
[0023]
And in order to eliminate the said subject, in the metal detector of this invention, a receiving coil is arranged so that winding direction may become a predetermined pattern along a conveyance path , and a metal piece may be connected. It comprises a plurality of forward coils (15a) having a winding direction in which a positive induced voltage is generated when passing and a plurality of reverse coils (15b) having a winding direction in which a negative induced voltage is generated.
Furthermore, the reference signal memory for storing the signal pattern of the reference signal output from the synchronous detector when the reference metal piece is transported to the transport path, and the signal pattern of the output signal of the synchronous detector and the reference signal memory are stored. A correlation calculation unit that obtains and outputs a correlation signal with the signal pattern of the reference signal , and a metal piece is included in the object to be inspected when the signal value of the correlation signal output from the correlation calculation unit exceeds a predetermined threshold value And a determination unit for determining that.
[0024]
In addition, the pattern in the winding direction along the conveyance path formed by the winding direction of the plurality of forward coils and the winding direction of the plurality of reverse coils has a maximum autocorrelation value and a second maximum value. A large difference is desirable. For example, a pseudo-random signal pattern is suitable.
[0025]
In the metal detector configured as described above, the principle that a small metal piece buried in a large object to be inspected can be detected, and the object to be inspected that does not contain a metal piece when vibration noise due to an impact occurs, includes the metal piece. Explain the principle of not detecting by mistake.
[0026]
First, before the operation of the metal detector, the output signal of the synchronous detector when the reference metal piece is caused to flow through the transport path is stored in the reference signal memory in advance.
[0027]
The output signal of the synchronous detector at this time is that the induced voltage ΔE is generated in the corresponding coil every time the reference metal piece constitutes the receiving coil—that is, (+) or ( The polarity of-) is the pattern in the winding direction of the receiving coil.
[0028]
Therefore, consider the case of an object to be inspected including a metal piece.
The output signal of the synchronous detector in this case is a superposition of the signal from the metal piece and the signal from the object to be inspected.
[0029]
The output signal of the synchronous detector by the metal piece has a waveform similar to the signal by the reference metal piece. Therefore, the output signal of the correlation calculation unit using the metal piece becomes large at the moment when the reference signal and the signal from the metal piece coincide with each other, and becomes 0 at other times. That is, the signal has a sharp peak.
[0030]
On the other hand, the object to be inspected has characteristics of both a magnetic body and a non-magnetic body, and these characteristics are randomly distributed over the entire object to be inspected. In addition, the object to be inspected is larger than the metal piece and is generally larger than each coil constituting the receiving coil. For the above two reasons, the output signal of the detector by the object to be inspected is not similar to the signal by the reference metal piece. Therefore, the output signal of the correlation calculation unit by the object to be inspected is smaller than the signal by the metal piece.
[0031]
Further, since the BPF is designed to pass only the frequency component of the correlation peak in the reference signal by the reference metal piece, only the correlation peak by the metal piece appears in the output of the BPF, and the correlation calculation unit output by the object to be inspected. The signal is greatly attenuated. Therefore, a minute metal piece buried in a large object to be inspected can be detected.
[0032]
Next, consider a case where an impact is applied to the frame when an object to be inspected that does not include a metal piece passes through the receiving coil.
When an impact is applied to the frame, the receiving coil and the transmitting coil vibrate, and vibration noise is generated at the output of the synchronous detector. Since the vibration noise and the reference signal from the reference metal piece have no correlation, no vibration noise is generated in the output of the correlation calculation unit. Therefore, even when an impact is applied to the frame when an object to be inspected that does not include a metal piece passes through the receiving coil, it is not erroneously detected that the object to be inspected includes a metal piece.
[0033]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, an embodiment of the present invention will be described 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. 8 are denoted by the same reference numerals, and detailed description of the overlapping parts is omitted. The external view of this metal detector is the same as the external view shown in FIG.
[0034]
In the metal detector of this embodiment, the receiving coil 15 disposed at a position opposite to the transmitting coil 6 is composed of N coils with different winding directions arranged along the transport direction of the device under test 1. ing. Specifically, as shown in FIG. 2, a plurality of forward coils 15a having a winding direction in which an induced voltage of (+ ΔE) is generated when the metal piece passes through the coil position, and the metal piece passes through the coil position. A plurality of reverse coils 15b having a winding direction in which an induced voltage of (−ΔE) is sometimes generated.
[0035]
For convenience, the signal value that generates an induced voltage of (+ ΔE) when the metal piece passes the coil position is defined as [1], and the induced voltage of (−ΔE) when the metal piece passes the coil position is defined as [1]. The resulting signal value is defined as [0].
[0036]
The forward coil 15a and the reverse coil 15a are arranged in a fixed pattern. The pattern in the winding direction is set substantially equal to the M-sequence pseudo-random signal pattern in which each signal value is represented by a binary value of [1] or [0]. The pattern in the winding direction is not limited to a pseudo random signal pattern, and may be a signal pattern having a good autocorrelation such as a gold code.
[0037]
In the vicinity of the transmission coil 6 and the reception coil 15 on the frame 3 in FIG. 7, a position detector 16 including an optical sensor that detects the inspection object 1 conveyed on the belt conveyor 2 is disposed. The position detector 16 sends a test object detection signal j indicating that the test object 1 has approached the transmission coil 6 and the reception coil 15 to the control unit 17 composed of a micro computer.
[0038]
As in the conventional metal detector shown in FIG. 8, the excitation current a output from the AC power supply 5 is applied to the transmission coil 6. The output signal b ₁ of the receiving coil 15 is taken out from the variable terminal of the variable resistor 8 connected between the output terminals of the receiving coil 15 and input to the first synchronous detector 9a and the second synchronous detector 9b. The
[0039]
The excitation current a output from the AC power supply 5 is applied to the transmission coil 6 and to the first synchronous detector 9a. Further, the excitation current a output from the AC power source 5 is phase-shifted by 90 ° by the 90 ° phase shift circuit 10 and applied to the second synchronous detector 9b as a new excitation signal a ₁ .
[0040]
The first synchronous detector 9a is synchronously detects the output signal b ₁ of the receiver coil 7 by the excitation signal a. The output signal c ₁ of the first synchronous detector 9a is amplified by the amplifier 12a, A / D converted by the A / D converter 20a, and then input to the first matched filter 18a as a correlation calculating unit. Is done.
[0041]
The first tap coefficient memory 19a as the reference signal memory includes a reference metal piece 29 made of, for example, a 0.5 mmφ iron ball in the inside shown in FIG. when obtained by conveying the rectangular parallelepiped plastic reference inspection object 1a formed in the belt conveyor 2, the first synchronous detector 9a is as Figure 3 the output signal c ₁ shown in (b) a first output from the The reference signal h ₁ is stored and held in advance. The first reference signal h ₁ stored in the first tap coefficient memory 19a is applied as a tap coefficient value to the first matched filter 18a.
[0042]
The first matched filter 18a calculates the correlation of every moment between the first reference signal h ₁ for the output signal c ₁ of the first synchronous detector 9a stored in the first tap coefficient memory 19a The correlation signal g ₁ shown in FIG. 5 is output.
[0043]
Accordingly, the signal waveform (signal pattern) of the output signal c ₁ output from the first synchronous detector 9a is the signal waveform (signal pattern) of the first reference signal h ₁ stored in the first tap coefficient memory 19a. when the case almost coincides, as shown by a characteristic in FIG. 5, a large peak value to the signal value of the correlation signal g ₁ in matched time (correlation value R) is generated.
[0044]
On the other hand, the signal waveform (signal pattern) of the first reference signal h ₁ stored in the first tap coefficient memory 19a is the signal waveform (signal pattern) of the output signal c ₁ output from the first synchronous detector 9a. 5, the signal value (correlation value R) of the correlation signal g ₁ is small and no large peak value is generated, as shown by the B characteristic in FIG.
[0045]
Specifically, unlike the inspected object 1 shown in FIG. 4, a metal piece is not included, but the metal component 30 included as the material of the inspected object 1 is randomly distributed over the entire conveyance direction of the inspected object 1. Distributed. Therefore, the waveform of the output signal c ₁ of the first synchronous detector 9a when the inspection object 1 is conveyed does not match the signal waveform (signal pattern) of the reference signal h ₁ described above.
[0046]
Further, even if vibration noise caused by vibration generated during conveyance of the inspection object 1 is mixed in the output signal c ₁ of the first synchronous detector 9a, the signal waveform of the vibration noise includes the above-described reference signal. h does not match the _first signal waveform (signal pattern).
[0047]
That is, a large peak value is generated in the signal value (correlation value R) of the correlation signal g ₁ only in the inspected object 1 including the metal piece.
[0048]
The correlation signal g ₁ output from the first matched filter 18 a is input to the control unit 17 after the noise component is removed by the BPF 11 a.
[0049]
The second synchronous detector 9b is synchronously detects the output signal b ₁ a 90 ° phase-shifted by the excitation signal a ₁ of the receiving coil 15. The output signal c2 of the _second synchronous detector 9a is amplified by the amplifier 12b, A / D converted by the A / D converter 20b, and then input to the second matched filter 18b as a correlation calculating unit. Is done.
[0050]
In the second tap coefficient memory 19b as a reference signal memory, when the reference object 1a shown in FIG. 3A is conveyed by the belt conveyor 2, it is output from the second synchronous detector 9b. The output signal c ₂ is stored and held in advance as the second reference signal h ₂ . Then, the second reference signal h ₂ stored in the second tap coefficient memory 19b is applied as a tap coefficient value to the second matched filter 18b.
[0051]
The second matched filter 18b calculates a correlation momentarily between the second reference signal h ₂ for the output signal c ₂ of the second synchronous detector 9b stored in the second tap coefficient memory 19b To output a correlation signal g ₂ .
[0052]
Also in the correlation signal g ₂ output from the second matched filter 18a, only the inspected object 1 including the metal piece is the same as the correlation signal g ₂ output from the first matched filter 18b described above. large peak value to the signal value of the correlation signal g ₂ (correlation value R) is generated.
[0053]
The correlation signal g ₂ output from the second matched filter 18 b is input to the control unit 17 after the noise component is removed by the BPF 11 b.
[0054]
A control unit 17 including a microcomputer including the operation panel 4, the display unit 21, and the printer 22 exposed on the outer surface of the frame 3 shown in FIG. 7 is configured as shown in FIG. 6, for example.
[0055]
When the inspected object detection signal j is input from the position detector 16, the data acquisition timing generation unit 23 sends a data acquisition start command to the data acquisition unit 24.
[0056]
When the data take-in start command is output from the data take-in timing generation unit 23, the data take-in unit 24 takes in the digital correlation signals g ₁ and g ₂ and sends them to the determination unit 26. As shown in FIG. 5, when the signal levels of the input correlation signals g ₁ and g ₂ exceed a threshold value (allowable limit) Rm predetermined in the threshold value memory 27, the determination unit 26 detects the metal Judge that there is a piece.
[0057]
When the determination unit 26 determines that there is a metal piece, the output unit 28 outputs a warning that there is a metal piece on the display unit 21 and generates a warning sound. Further, it is printed out to the printer 22.
[0058]
In the metal detector configured as described above, the reception coil 15 disposed opposite to the transmission coil 6 is not composed of only a pair of differentially connected coils 7a and 7b, but is wound. For example, a plurality of forward coils 15a and a plurality of reverse coils 15b having different winding directions are arranged so as to have a predetermined pattern such as a pseudo random signal pattern.
[0059]
As described above, the receiving coil 15 and the first and second matched filters 18a and 18b in which the winding directions of the plurality of coils 15a and 15b are arranged in a predetermined pattern, and the first and second matched filters 18a and 18b are used. Correlation signals g1 and g2 output as to whether or not the output signals c ₁ and c ₂ of the two synchronous detectors 9a and 9b contain signal waveforms (signal patterns) caused by the presence of harmful fine metal pieces. The signal value (correlation value R) is determined.
[0060]
Therefore, even if a minute metal piece is included in the inspected object 1 containing a metal component as a material or vibration noise is mixed, it is possible to reliably detect only the presence of the minute metal piece.
[0061]
【The invention's effect】
As described above, in the metal detector of the present invention, the signal waveform that appears in the output signal of the synchronous detector due to the material and vibration of the object to be inspected and the signal that appears in the output signal due to the harmful metal piece Differences from the waveform are detected using a correlation calculation method.
[0062]
Therefore, even if the magnetic properties and non-magnetic properties of the metal piece are smaller than the magnetic and non-magnetic properties of the object to be inspected, the presence of the metal piece is reliably detected. it can.
[0063]
Moreover, even if the vibration noise is larger than the magnetic characteristics and nonmagnetic characteristics of the object to be inspected, it is possible to suppress the occurrence of erroneous detection in the noninspected object that does not include a metal piece.
[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 diagram showing an arrangement of coils constituting a receiving coil of the metal detector. FIG. 4 is a diagram showing a reference object to be inspected and a reference signal waveform used in FIG. 4. FIG. 5 is a diagram showing an object to be inspected measured by the metal detector. FIG. FIG. 6 is a block diagram showing a detailed configuration of a control unit of the metal detector. FIG. 7 is an external view showing a general metal detector. FIG. 8 is a block diagram showing a schematic configuration of a conventional metal detector. FIG. 9 is a diagram showing the relationship between the phase of the excitation signal applied to the transmission coil and the phase of the output signal output from the reception coil.
DESCRIPTION OF SYMBOLS 1 ... Test object 1a ... Reference | standard test object 2 ... Belt conveyor 4 ... Operation panel 5 ... AC power supply 6 ... Transmission coil 8 ... Variable resistance 9a ... 1st synchronous detector 9a ... 1st synchronous detector 10 ... 90 ° Phase shifters 11a, 11b ... BPF
12a, 12b ... Amplifier 15 ... Receiving coil 15a ... Forward coil 15b ... Reverse coil 16 ... Position detector 17 ... Control unit 18a ... First matched filter 18a ... First matched filter 19a ... First tap coefficient memory 19b ... second tap coefficient memory 23 ... data take-in timing generator 24 ... data take-in unit 26 ... determination unit 28 ... output unit

Claims (2)

A transmission coil (6) to which an alternating excitation signal is applied and a reception coil (15) are arranged along the conveyance path (2) of the object to be inspected (1), and the synchronous detectors (9a, 9b). In the metal detector for synchronously detecting the output signal of the receiving coil with the excitation signal and detecting the metal piece included in the object to be inspected based on the output signal of the synchronous detector,
The receiving coils are connected in series and arranged in a winding pattern along the transport path so as to form a predetermined pattern , and the receiving coils have a winding direction in which a positive induced voltage is generated when the metal piece passes. Forward coil (15a) and a plurality of reverse coils (15b) having a winding direction in which a negative induced voltage is generated,
A reference signal memory (19a, 19b) for storing a signal pattern of a reference signal output from the synchronous detector when the reference metal piece (29) is transferred to the transfer path;
Correlation calculating unit for outputting the correlation signal of the signal pattern of the synchronous detector of the output signal of the signal pattern and the reference signal memory to the stored reference signal (18a, 18b),
A metal detector comprising: a determination unit (26) that determines that a metal piece is included in the object to be inspected when a signal value of a correlation signal output from the correlation calculation unit exceeds a predetermined threshold value. The pattern in the winding direction along the conveyance path formed by the winding direction of the plurality of forward coils and the winding direction of the plurality of reverse coils is a pseudo-random signal pattern. Item 1. A metal detector according to Item 1.

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