metal detector

Abstract

This metal detector can detect minute metallic foreign objects as small as tens of microns, which was not possible with conventional metal detectors. [Solution] The metal detector 10 of the present invention is equipped with a transport unit 11 for transporting an item to be inspected, and three or more magnetic detection elements 12 for detecting magnetic field fluctuations originating from metal foreign matter mixed in the item to be inspected are arranged in a continuous line along the transport direction of the item to be inspected. The device is characterized by having a Fourier transform unit 17 that performs a Fourier transform on detection signals of two or more periods obtained from three or more of the aforementioned magnetic detection elements 12.

Description

Technical Field

[0001] The present invention relates to a metal detector for detecting metallic foreign matter such as iron mixed in articles such as food, medicines, and chemical products.

Background Art

[0002] A metal detector is a device that is installed on production lines for food, medicines, chemical products, and the like to detect metallic foreign matter such as iron that has accidentally been mixed into articles. Among conventional metal detectors, the method that captures fluctuations in a static magnetic field is called a needle detector or iron piece detector. As disclosed in Patent Documents 1 and 2, a permanent magnet and a pair of detection coils are arranged so as to sandwich an inspection space through which the inspection article passes.

[0003] Also, the method that captures fluctuations in an alternating magnetic field is similarly called a metal detector. As disclosed in Patent Document 3, a receiving coil for the alternating magnetic field, a generating coil, and another receiving coil are arranged coaxially in this order with respect to the inspection space through which the inspection article passes, and changes in a signal induced by the pair of receiving coils are captured. In addition, the method using a magnetic sensor is called a magnetizing metal detector. As disclosed in Patent Document 4, metallic foreign matter is magnetized by a magnetizing device, and fluctuations in a magnetic field caused by magnetic lines of force emitted from the metallic foreign matter are captured by a magnetic sensor.

[0004] All of these Patent Documents 1 to 4 capture fluctuations in a magnetic field by using a single magnetic detection element such as a magnetic sensor or an induction coil. Alternatively, two magnetic detection elements are arranged along the passing direction of the inspection article to calculate a difference between signals obtained from the pair of magnetic detection elements, thereby canceling out persistent noise coming from the surrounding environment. Furthermore, by utilizing the fact that the target signal obtained from the passage of metallic foreign matter has a difference in signal output time between the pair of magnetic detection elements, the persistent noise is differentiated from the target signal to detect the metallic foreign matter. The target signal in the above prior art does not have multi-cycle periodicity. Also, means for suppressing persistent noise has been limited to signal processing in the real time domain, such as low-pass filters and band-pass filters. Furthermore, in today's market, there is a growing demand for high-precision inspection of metal foreign objects in materials, intermediate processed products, and final products that make up lithium-ion batteries. However, conventional metal detectors have difficulty detecting metal foreign objects as small as tens of microns. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2008-232745 [Patent Document 2] Patent No. 7537669 [Patent Document 3] Patent No. 5576226 [Patent Document 4] Patent No. 4395559 [Overview of the project] [Problems that the invention aims to solve]

[0006] The problem that this invention aims to solve is, in view of the problems of the prior art described above, to provide a metal detector that can detect minute metallic foreign objects of several tens of microns in size, which could not be achieved with conventional metal detectors. [Means for solving the problem]

[0007] As a first means for solving the above problems, the present invention provides a conveying unit for conveying an inspection item, and arranges three or more magnetic detection elements in a continuous line along the conveying direction of the inspection item to detect magnetism originating from metallic foreign matter mixed in the inspection item. The objective is to provide a metal detector that includes a Fourier transform unit that performs a Fourier transform on detection signals of two or more periods obtained from three or more of the aforementioned magnetic detection elements. According to the first method described above, highly sensitive foreign object detection can be achieved by performing a Fourier transform on a detection signal of two or more periods (multiple periods) obtained by arranging three or more magnetic detection elements (multiple arrangement).

[0008] As a second means for solving the above problems, the present invention provides a metal detector characterized in that, in the first means, it is equipped with a storage unit that stores repeating time waveforms of two or more periods obtained from three or more magnetic detection elements. According to the second method described above, highly sensitive foreign object detection can be achieved by storing a detection signal of two or more periods (multiple periods) obtained by arranging three or more magnetic detection elements (multiple-unit arrangement) and performing a Fourier transform. The time waveform stored in the storage unit is stored as a long continuous waveform even if it is intermittent in time.

[0009] As a third means to solve the above problems, the present invention provides a metal detector characterized in that, in the second means, it includes a determination unit that limits the frequency within a certain bandwidth from the sample frequency determined by the movement speed of the inspection item and the distance between the magnetic detection elements, and determines whether the respective thresholds are exceeded by utilizing the calculation results for amplitude and phase, thereby determining whether the metallic foreign object has been detected. According to the third method described above, by performing a Fourier transform on a detection signal of two or more periods (multiple periods) obtained by arranging three or more magnetic detection elements (multiple arrangement), the signal strength due to noise at each frequency caused by ubiquitous noise is stored, and by evaluating the increase in signal strength at each frequency obtained when a metallic foreign object passes through, the presence or absence of a metallic foreign object can be determined with high sensitivity.

[0010] As a fourth means for solving the above problems, the present invention provides a metal detector characterized in that, in any one of the first to third means, the magnetic detection element is an annular detection coil, and three or more of them are arranged alternately with hollow cylindrical permanent magnets, and the hollow portions of the detection coil and the permanent magnets are used as a transport path for the inspected article. According to the fourth method described above, by using the hollow portion of the permanent magnet and detection coil as an inspection space, it is possible to strengthen the inspection magnetic field, or in other words, improve the detection sensitivity by using a strong magnetic field.

[0011] As a fifth means to solve the above problems, the present invention provides a metal detector characterized in that, in the fourth means, the permanent magnets are arranged such that when three or more permanent magnets are lined up along the transport direction of the inspection item, the like poles of adjacent permanent magnets face each other. According to the fifth method described above, the magnetic field lines generated by each permanent magnet are guided into the inspection space and simultaneously focused to strengthen the inspection magnetic field lines, thereby increasing the induced electromotive force of the detection coil and achieving high sensitivity. Furthermore, by placing the detection coil in the air gap between three or more permanent magnets arranged in a continuous configuration (multi-unit arrangement), the reversal of the magnetic field inside the detection coil obtained as a metallic foreign object passes through can be efficiently detected, and a large induced electromotive force can be obtained, thereby achieving high sensitivity.

[0012] As a sixth means to solve the above problems, the present invention provides a metal detector characterized in that, in the fifth means, the detection coils and permanent magnets arranged in a row of three or more are arranged such that the detection coils are positioned alongside the permanent magnets along the inner circumference or outer circumference of the permanent magnets, or along the transport direction of the inspected article. According to the sixth method described above, by positioning the detection coil in close proximity to the space where the inspection magnetic flux is concentrated, and efficiently capturing magnetic field fluctuations in the strong magnetic field region generated by the permanent magnet at close range, highly sensitive detection of metallic foreign objects can be achieved.

[0013] As a seventh means to solve the above problems, the present invention provides a metal detector characterized in that, in the sixth means, the detection coils have the same polarity, the spacing between the detection coils and the permanent magnets is set so that the response waveform of any one of the detection coils and the response waveform of a detection coil adjacent to any one of them are added in phase, and the transport unit moves the inspection item at a specified speed. According to the seventh method described above, even if the object being inspected rotates while moving, the risk of missed inspections is reduced by averaging through measurement with multiple coils. Each time the object approaches the magnet, magnetization and demagnetization are repeated, and the change in magnetic flux linkage during the magnetization and demagnetization process is measured, resulting in a large signal amplitude. Furthermore, because magnetization and demagnetization are repeated, even if the object being inspected rotates while moving, the easy and difficult magnetization axes change, thus reducing the risk of missed inspections. In addition, because magnetization and demagnetization are repeated, it is equivalent to applying an alternating magnetic field, making detection by eddy currents possible even for conductive nonmagnetic materials.

[0014] As an eighth means to solve the above problems, the present invention provides a metal detector characterized in that, in the sixth means, the detection coils are arranged in an even number with alternating opposite polarities, the spacing between the detection coils and the permanent magnets is set such that the response waveform of any one of the detection coils and the response waveform of a detection coil adjacent to any one of them are added in phase and environmental noise is canceled out with opposite polarity, and the transport unit moves the inspection item at a specified speed. According to the eighth method described above, even if the object being inspected rotates while moving, the risk of missed inspections is reduced by averaging through measurement with multiple coils. Each time the object approaches the magnet, magnetization and demagnetization are repeated, and the change in magnetic flux linkage during the magnetization and demagnetization process is measured, resulting in a large signal amplitude. Furthermore, because magnetization and demagnetization are repeated, even if the object being inspected rotates while moving, the easy and difficult magnetization axes change, thus reducing the risk of missed inspections. In addition, because magnetization and demagnetization are repeated, it is equivalent to applying an alternating magnetic field, making detection by eddy currents possible even for conductive nonmagnetic materials. [Effects of the Invention]

[0015] According to the present invention, for example, it is possible to achieve highly sensitive inspection performance for minute metallic foreign objects of several tens of microns in size, which is required for inspection performance of lithium-ion battery materials and intermediate processed products. [Brief explanation of the drawing]

[0016] [Figure 1] FIG. 1 is an explanatory diagram of Example 1 of the metal detector according to the present invention, wherein (1) is a view taken along line A-A of (2), and (2) is a side view. [Figure 2] FIG. 2 shows a waveform obtained by measuring the electromotive force waveform of one detection coil among multiple detection coils arranged in series. [Figure 3] FIG. 3 shows the electromotive force waveform of the adjacent detection coil. [Figure 4] FIG. 4 is a connection diagram in which winding ends (B terminals) of eight detection coils are connected to an operational amplifier. [Figure 5] FIG. 5 is an explanatory diagram showing the mutual addition effect between waveforms from each detection coil. [Figure 6] FIG. 6 is a connection diagram for differential processing between adjacent detection coils. [Figure 7] FIG. 7 is a connection diagram that achieves a cancellation effect on inherent noise and simultaneously reduces the number of operational amplifiers used. [Figure 8] FIG. 8 is a connection diagram for performing differential processing with a detection coil 30 that is one or more positions ahead. [Figure 9] FIG. 9 is a connection diagram in which several detection coils are grouped, and differential processing is performed between the groups. [Figure 10] FIG. 10 is an explanatory diagram of an output waveform obtained by dividing 28 detection coils into groups of 14 coils each and connecting the groups to operational amplifiers. [Figure 11] FIG. 11 is an explanatory diagram of a frequency spectrum in a section where a target signal exists, obtained by performing Fourier transform on the waveform of FIG. 10. [Figure 12] FIG. 12 is an explanatory diagram of a frequency spectrum in a section where only inherent noise exists, obtained by performing Fourier transform on the waveform of FIG. 10. [Figure 13] FIG. 13 is an explanatory diagram of an inherent noise signal of one detection coil when differential arithmetic processing is performed between adjacent detection coils. [Figure 14] FIG. 14 is an explanatory diagram of an inherent noise signal of the other detection coil when differential arithmetic processing is performed between adjacent detection coils. [Figure 15] FIG. 15 is an explanatory diagram obtained by performing differential arithmetic processing on FIG. 13 and FIG. 14. [Figure 16]This diagram illustrates differential calculation processing between distant detection coils, showing the signal of the ambient noise in a detection coil 200 mm away, assuming one of the detection coils is as shown in Figure 13. [Figure 17] Figure 16 is an explanatory diagram of the differential calculation results. [Figure 18] This diagram illustrates the optimization of the spacing between pairs of permanent magnets and detection coils so that the signals obtained between adjacent detection coils are in opposite phase. (1) is a view along arrow AA in (2), and (2) is a side view. [Figure 19] This diagram illustrates how a detection coil is wound around the inside of a hollow, donut-shaped permanent magnet, bringing the detection coil 30 close to the space through which the inspected item passes. (1) is a view along arrow AA in (2), and (2) is a side view. [Figure 20] This diagram illustrates how a detection coil is wound around the outside of a hollow permanent magnet, and how the hollow permanent magnet 20 is positioned in close proximity to the space through which a metallic foreign object passes. (1) is a view along arrow AA in (2), and (2) is a side view. [Figure 21] This diagram illustrates the relationship between metal detection waveforms obtained from adjacent detection coils, which are shifted by half a wave and have opposite polarity, and ultimately form in-phase waveforms. [Figure 22] The waveform obtained when the distance between adjacent detection coils is increased by half a wave of the detection waveform, and the waveform, which has been shifted by one wavelength and has opposite polarity, is inverted and input to the operational amplifier, is ultimately shown in the diagram illustrating the in-phase relationship. [Figure 23] This is an explanatory diagram showing a conventional coaxial coil arrangement. [Figure 24] This diagram illustrates how to obtain a waveform variation equivalent to two cycles as a differential output from four receiving coils. [Figure 25] This is an explanatory diagram of a method that uses magnetic sensors to detect fluctuations in the magnetic field. [Modes for carrying out the invention]

[0017] Embodiments of the metal detector of the present invention will be described in detail below with reference to the drawings. The metallic foreign matter contained in the inspected article of the present invention is a metal with magnetic properties, such as iron and nickel. In the case of the alternating magnetic field method, in addition to magnetic metals, non-ferrous metals such as aluminum and copper, which do not have magnetic properties, are also targeted. The metal detector 10 of the present invention includes a transport unit 11 for transporting an item to be inspected, and has three or more magnetic detection elements 12 arranged in a continuous line along the transport direction of the item to be inspected for detecting magnetic field fluctuations originating from metallic foreign matter mixed in the item to be inspected, and a Fourier transform unit 17 that performs a Fourier transform on detection signals of two or more periods obtained from the three or more magnetic detection elements 12. The magnetic detection element 12 of the present invention has three configurations: one that detects fluctuations in a static magnetic field, one that detects fluctuations in an alternating magnetic field, and one that detects fluctuations in a magnetic field using a magnetic sensor.

[0018] (Example 1: Static magnetic field) Example 1 describes the configuration of an iron piece detector that captures fluctuations in a static magnetic field, which is a type of metal detector. Figure 1 is an explanatory diagram of Example 1 of the metal detector of the present invention, where (1) is a view along arrow AA in (2), and (2) is a side view. In the case of a static magnetic field, the magnetic detection element 12 is configured by stacking hollow permanent magnets 20 in directions that repel each other, and winding a detection coil 30 in the air gap between the magnets, as described below. In the metal detector 10 of Example 1, hollow permanent magnets 20 are stacked in directions that repel each other, and the arrangement of the magnetic poles is a repeating configuration of NS (air gap) SN (air gap) NS (air gap) SN (air gap)... (6 units in Figure 1). Detection coils 30 are wound around the air gaps between the stacked hollow permanent magnets 20, which are arranged in directions that repel each other (with like poles facing each other) (5 coils in Figure 1). An iron ball is passed through the hollow portions of the permanent magnets 20 and detection coils 30, and the electromotive force waveform of one of the detection coils 30 is measured and shown in Figure 2. In Figure 2, the vertical axis is voltage (V) and the horizontal axis is elapsed time (seconds) (the same applies below). The waveform of the electromotive force waveform of the adjacent detection coil 30 is shown in Figure 3. When comparing the waveforms in Figure 2 and Figure 3, they are in phase, and the same relationship was observed for all the detection coils 30 arranged in the multi-unit configuration.

[0019] Based on this waveform relationship, when the winding ends (B ends) of the eight detection coils 30 are connected to the operational amplifier (amplifier) ​​14 as shown in Figure 4, the mutual addition effect of the waveforms from each detection coil 30 shown in Figure 5 is obtained, and a waveform with periodicity is obtained at the same time. By performing a Fourier transform on the time domain interval in the Fourier transform unit 17, the presence or absence of the target signal in the frequency domain can be evaluated, making it possible to check for the presence or absence of the target signal more efficiently than conventional time domain signal evaluation. In this case, when inspecting an item for the presence of metallic foreign matter, if it is transported by a conveyor, the inspection can be performed at a constant speed. Therefore, once the transport speed is determined in relation to the width of the permanent magnet, the coil, and the mounting pitch of the coil, it is relatively easy to extract the target signal generated by the passage of metallic foreign matter by signal processing with a general bandpass filter. However, when inspected items are passed along inclined plates or pipes for transport, inspection in a transport path arranged within the hollow structure of the present invention involves transport by liquid, air pressure, dropping on an inclined surface, and vertical free fall, so the transport speed of the metallic foreign object will not be perfectly constant. In this case, when extracting the target signal generated by the passage of the metallic foreign object into the time domain using a bandpass filter composed of an electronic circuit, it is conceivable that the target signal may fall outside the passband, so stable inspection performance cannot be expected.

[0020] Therefore, by evaluating the multi-period target signal generated from a combination of a multi-periodic permanent magnet 20 and a detection coil 30 as in the present invention using a Fourier transform in the frequency domain, and comparing the increase in spectral components at specific frequencies due to the target signal with the discrete spectral spectra of the ambient noise in the steady state, the periodicity of the target signal can be maximized by making the length of the coefficient sequence in the Fourier transform close to the duration of the periodic signal from the multi-coil system. Furthermore, even if the period of the target signal changes due to a change in transport speed, performing the evaluation using a Fourier transform reduces the impact on inspection performance compared to a fixed-frequency evaluation method such as a bandpass filter that extracts only fixed frequencies.

[0021] In Example 1, the periodicity of the target signal is ensured by arranging three or more detection coils 30 along the transport direction of the inspected item (multi-coil configuration). However, when increasing the number of detection coils 30 connected to the operational amplifier 14 in Figure 4 during the multi-coil configuration, the ambient noise from the surrounding environment also increases proportionally. As shown in Figure 5 above, when comparing the waveform measured with 16 detection coils connected to the operational amplifier 14 in Figure 4 with the ambient noise amplitude in the 0.0 to 1.0 second and 1.6 to 2.4 second intervals, which are periods when there is no target signal (detection signal for metal foreign objects), the amplitude of ambient noise measured with only one detection coil in Figure 2 is larger.

[0022] Since ambient noise originates outside the permanent magnet 20 and from a distance compared to the transport path (transport area) formed in the hollow portion of the permanent magnet 20 through which the metallic foreign object is transported, a common method is to cancel out the ambient noise by differential processing between adjacent detection coils 30, as shown in Figure 6. By performing differential processing, the ambient noise between the detection coils 30 becomes in phase and is canceled out. However, since there is a time difference in the signal obtained when the metallic foreign object passes, it is not canceled out like ambient noise, and the effect of residual target signal is often used to address this. In the case of the multi-unit permanent magnet 20 and detection coil 30 structure according to the present invention, there are cases where the signals between adjacent detection coils 30 become in phase. In this case, if differential processing is performed between adjacent detection coils 30, there will be a time difference in the signal when the metallic foreign object passes, and complete cancellation will not be achieved. However, the signals between the differential coils become in phase, and attenuation occurs in the signal resulting from the differential processing. In this case, by using the method in Figure 4 and configuring the wiring of the detection coil 30 as shown in Figure 7, it is possible to obtain the same cancellation effect against ambient noise as in Figure 6, and at the same time reduce the number of operational amplifiers 14 used. However, as in the case of Figure 6, the signals between the differential coils become in phase, and attenuation occurs in the signal resulting from the differential processing.

[0023] Next, we will describe the input connection method for each detection coil 30 to the operational amplifier 14 in the more useful configurations shown in Figures 4 and 7. As mentioned above, in the structure shown in Figure 1 of Example 1, differential processing between adjacent detection coils 30 results in the target signal being attenuated due to differential processing because the target signal obtained from the detection coils 30 has a common-phase component. To improve this, differential processing is performed with one or more detection coils 30 further away, as shown in Figure 8, or several detection coils 30 are grouped together, forming groups of the same number, and differential processing is performed between these groups, as shown in Figure 9. This configuration cancels out ambient noise and enables the acquisition of the target signal using Fourier transform by obtaining a signal (with periodicity) when a metallic foreign object passes through. Figure 9 shows an example where 16 detection coils 30 are divided into groups of 8 and connected to the operational amplifier 14. This is a wiring diagram where coils 5-12 are connected to the B terminal and coils 1-4 and 13-16 are connected to the A terminal to ensure balance against ambient noise and achieve the maximum cancellation effect against ambient noise. The operational amplifier 14 has an A / D conversion unit 15, a memory unit 16, a Fourier transform unit 17, and a determination unit 18 in its downstream stages. The A / D conversion unit 15 performs the process of converting an analog signal into a digital signal. The memory unit 16 is a memory that stores repeating time waveforms of two or more periods obtained from three or more magnetic detection elements 12. The Fourier transform unit 17 performs a Fourier transform on the detection signals of two or more periods obtained from three or more magnetic detection elements 12. The determination unit 18 distinguishes between the inspection item and noise based on the frequency determined by the coil period and the passage speed of the inspection item, and determines that a metal foreign object is present when the signal intensity at each frequency of the frequency spectrum obtained by the Fourier transform exceeds a threshold (for example, the measurement value of a good product that does not contain a metal foreign object). The determination unit 18 can also determine that a metal foreign object has been detected by limiting the frequency within a certain bandwidth from the sample frequency determined by the movement speed of the inspection item and the spacing between the magnetic detection elements 12, and using the calculation results regarding amplitude and phase to determine whether the respective thresholds are exceeded.

[0024] Figure 10 shows the output waveforms when the 28 detection coils 30 are divided into groups of 14 and connected to the operational amplifier 14. Figures 11 and 12 illustrate the performance improvement achieved by applying a Fourier transform to a multi-period waveform, which is the most distinctive feature of this invention. A Fourier transform is applied to the waveform in Figure 10, and the frequency spectra of the section containing only ambient noise and the section containing the target signal are compared. In Figures 11 and 12, the vertical axis represents signal intensity, and the horizontal axis represents frequency (Hz). In Figure 10, the detection margin (signal / noise ratio) of the target signal in the time domain was 248 / 35 = 7.1 times, given that the noise was 35 mVp-p relative to the target signal of 248 mVp-p. A Fourier transform was then performed on the same signal waveform in Figure 10 to examine the detection margin of the target signal in the frequency domain. In this case, as shown in Figure 11, when a Fourier transform was performed on a 0.5-second interval starting from 1.15 seconds into the waveform of Figure 10, where the target signal obtained by the passage of the metallic foreign object continues, the relative signal intensity at 14 Hz, which was the peak of the resulting spectrum, was 22.4.

[0025] To compare this and confirm the degree of improvement in inspection performance, Figure 12 shows the frequency spectrum obtained by performing a Fourier transform on a 0.5-second interval starting from 0.20 seconds in the waveform of Figure 10, in the case where only noise due to ambient noise is present. By focusing on the spectral components due to the ubiquitous noise at 14 Hz, where the target signal was present in Figure 11, a comparison was made. The relative signal intensity of the noise shown in Figure 12 was 0.12, resulting in a detection margin of 22.4 / 0.12 = 187 times. Therefore, compared to the conventional time-domain inspection performance, a performance improvement of 187 / 7.1 = 26 times was achieved.

[0026] The 14Hz spectral peak shown in Figure 11 changes according to the speed at which the metallic foreign object passes through the multi-coil detection system. However, for each of the discretely existing Fourier transform evaluation results, the signal intensity due to ambient noise at each frequency, as shown in Figure 12, is stored. By evaluating the increase in signal intensity at each frequency obtained when the metallic foreign object shown in Figure 11 passes through, the determination unit 18 can determine the presence or absence of the metallic foreign object with high sensitivity. Furthermore, because this method evaluates the increase or decrease in signal strength at each frequency even when the speed at which a metallic foreign object passes through multiple detection coils changes, it offers excellent responsiveness to speed fluctuations and simultaneously enables the realization of a highly sensitive metal detector.

[0027] According to the invention of this embodiment 1, since it is an electromagnetic induction type, it can handle high sensitivity at high speeds. Conventional iron piece detectors use an iron core or similar in the detection coil to increase the amount of magnetic flux passing through it. However, this also attracts ambient noise from the surrounding environment. As a result, the detection coil has poor resistance to environmental noise. However, according to Example 1, the strong magnetic field inside the hollow shape of a permanent magnet is used as the inspection space (conveyor path), and by stacking the permanent magnets in a repulsive direction, the inspection magnetic flux can be further guided into the inspection space and focused. Furthermore, by arranging the detection coil attached to the permanent magnet, which is the magnetic force source, the inspection magnetic flux passing through the detection coil is enhanced, eliminating the need for an iron core. Therefore, the detection coil can be formed as an air-core coil, achieving high resistance to environmental noise.

[0028] Furthermore, since the relative permeability of a saturated permanent magnet is close to 1, and there is no termination point for magnetic flux induction from the surrounding magnetic field, a uniform magnetic field environment can be formed from the surrounding magnetic field for all of the multiple detection coils, and improved environmental noise immunity can be achieved through differential calculation between the detection coils. Because the structure lacks the magnetic core material and magnetic termination points, it becomes easy to add noise reference coils and provide a means to actively cancel out environmental noise. Furthermore, by connecting the detection coils 30 in parallel as shown in Figure 4, the effect of reducing the overall inductance through the parallel connection of inductors is obtained, making it possible to handle even when the induced signal is at a high frequency. Therefore, high sensitivity can be maintained even when metal foreign objects are moving at high speed. In addition to the circular detection coil attached to the circular permanent magnet shown in Figure 1, the detection coil of Example 1 may also be in the form of an elliptical or rectangular (polygonal) permanent magnet with an elliptical or rectangular (polygonal) detection coil attached.

[0029] (Example 2: Equally spaced static magnetic fields) Example 2 describes an iron piece detector that uses a method of detecting fluctuations in a static magnetic field at equal intervals. In Example 1, with the coil connection shown in Figure 9, the signal from a single detection coil shown in Figure 2 can be obtained as an in-phase signal between adjacent detection coils. Then, with the coil connection shown in Figure 9, the signals from each detection coil are added together to obtain a large detection signal as shown in Figure 10. By obtaining a multi-period signal suitable for Fourier transform, the signal is strengthened and the signal-to-noise ratio is improved. However, in the case of Example 1, it was necessary to add the signals between adjacent coils in order to obtain a multi-period detection signal, and differential signal processing to suppress ambient noise from the surrounding environment had to be performed between the coil that obtained the multi-period signal and the coil that received the signal. In the configuration of the 16-coil detection coil shown in Figure 9 for Embodiment 1, differential signal processing is performed by adding the A end (start of winding) of coils 1 to 4, the B end (end of winding) of coils 5 to 11, and the A end (start of winding) of coils 12 to 16. In this case, since the detection coils that perform differential signal processing are far apart, the effect of suppressing ambient noise by differential calculation is limited.

[0030] Here, we illustrate the difference between the results of differential calculation between adjacent detection coils and the results of differential calculation between distant detection coils. Figure 13 shows the ambient noise signal of one detection coil undergoing differential calculation between adjacent detection coils. Figure 14 shows the ambient noise signal of the other adjacent coil. Since Figures 13 and 14 have almost the same waveform, performing differential calculation on them results in Figure 15, demonstrating that the ambient noise suppression effect of differential calculation is sufficiently obtained. In contrast, when differential calculation processing is performed between distant detection coils as shown in Figure 9 according to Example 1, if one detection coil is as shown in Figure 13, the ambient noise signal at the detection coil 200 mm away becomes as shown in Figure 16. In this case, the differential calculation result shows that the ambient noise suppression effect is limited, as shown in Figure 17.

[0031] While Example 1 focused on improving the signal-to-noise ratio by emphasizing the target signal, Example 2 aimed to improve the signal-to-noise ratio by suppressing noise. Assuming differential signal processing between adjacent detection coils, two methods were constructed to realize Example 2, which is a means of obtaining a multi-period signal suitable for Fourier transform. Method 1 optimizes the spacing between pairs of permanent magnets 20 and detection coils 30, as shown in Figure 18, so that the signals obtained between adjacent detection coils 30 are in opposite phase, while also acquiring signal periodicity.

[0032] Method 2, as shown in Figure 19, involves winding a detection coil 30 inside the inner diameter of a hollow, donut-shaped or frame-shaped permanent magnet 20, bringing the detection coil 30 close to the space through which the inspected object passes. This configuration efficiently captures fluctuations in the magnetic field generated when a metallic foreign object passes, improving detection performance, while simultaneously optimizing the stacking distance of the permanent magnet 20 and detection coil 30 pairs to generate a periodic signal. In addition, as shown in Figure 20, by reversing the inner and outer relationship between the permanent magnet 20 and the detection coil 30, winding the detection coil 30 outside the outer diameter of the hollow permanent magnet 20, and positioning the hollow permanent magnet 20 close to the space through which the metallic foreign object passes, detection performance can be improved by applying a stronger inspection magnetic field to the metallic foreign object. According to this embodiment 2, the arrangement of the permanent magnet 20 and the detection coil 30 is optimized so that the signals obtained from adjacent detection coils 30 are in opposite phase. By performing differential calculations on the signals between adjacent detection coils 30 and canceling out ambient noise from the surrounding environment, noise components can be suppressed, and the signal-to-noise ratio can be improved. In addition, as in embodiment 1, an A / D conversion unit, a storage unit, a Fourier transform unit, and a determination unit are installed after the operational amplifier that connects the detection coils 30.

[0033] (Example 3: A method that combines the signal summing effect achieved by having adjacent coils in phase as in Example 1 with the improved disturbance immunity achieved by differential processing by alternately inverting the connection of adjacent coils as in Example 2.) Example 3 is a means of simultaneously obtaining the signal addition effect due to the detection signals between adjacent detection coils being in phase as in Example 1, and the cancellation effect of ambient noise coming from the surrounding environment due to the inverted wiring of adjacent detection coils as in Example 2. In Example 1, the magnets were connected with alternating polarity, and all coils were connected with the same polarity. In this case, the detection waveform when a metallic foreign object passed through two adjacent detection coils is shown in Figure 21. In this case, by adjusting the distance between adjacent coils to secure a distance equivalent to half a period t of the detection waveform, the waveforms obtained from adjacent detection coils become in phase. As explained in Example 1, the connection shown in Figure 4 has the advantage of obtaining a signal waveform summation effect and a larger amplitude. However, because the coils are in phase, there was no effect of suppressing conventional noise. In Example 2, we focused on the effect of suppressing ambient noise and suppressed it by performing the differential connection shown in Figures 6 to 8. However, in this case, the detection signal obtained when a metallic foreign object passed through was also out of phase between the two adjacent coils, so the signal waveform summing effect like in Example 1 could not be obtained.

[0034] In contrast, in Example 3, the spacing between the coils in Figures 18 to 20 is increased to 2t, which corresponds to half a period t of the detection waveform in Figure 22. When a metallic foreign object passes, the detection waveform obtained from the adjacent detection coil is in opposite phase, and the connection to the operational amplifier is differentially connected as shown in Figure 6 or Figure 7 to perform operational amplification in phase, making it possible to obtain the same signal waveform summation effect as in Example 1. In this case, the adjacent detection coils are in a differential operation relationship, which allows for a cancellation effect against ambient noise coming from the surrounding environment. Ultimately, a summation effect is obtained for the detection waveform when a metallic foreign object passes, and a cancellation effect due to differential operation is obtained against ambient noise simultaneously. However, since the peak points of the detection waveforms obtained from adjacent coils are separated by one period, the final summation effect is inferior to that of Example 1. Furthermore, following the operational amplifier to which the detection coil 30 is connected, an A / D conversion unit, a storage unit, a Fourier transform unit, and a determination unit are installed, similar to those in Embodiment 1.

[0035] To elaborate on the optimization of the spacing between adjacent detection coils based on the actual waveform at this time, the actual waveform exhibits the characteristics shown in Figure 2. Unlike the sinc function, which has a wide central portion, and unlike the cosine function, which exhibits damped oscillations with a constant period, the waveform has a wider period at the base than at the center. Therefore, when superimposing the waves, 2t is not strictly optimal, and there is a slight deviation. In practice, it is possible to achieve the maximum effect by ensuring a slightly longer spacing between detection coils than 2t, so that the larger parts of the waves overlap. When optimizing the above for Example 1, the principal maximum value (the peak of maximum amplitude in the center) of the nth stage detection coil and the first secondary minimum value (the next peak of opposite polarity) of the (n±1) stage detection coil should be made to coincide. Since the polarity of the magnet is reversed, the polarity of the first secondary minimum will actually be the same as the principal maximum value, and the optimization will be completed. Similarly, when performing optimization for Example 3, the principal maximum value (the peak of maximum amplitude in the center) of the nth stage detection coil and the second secondary minimum value (the next-nth peak of the same polarity) of the (n±1) stage detection coil are made to coincide. The polarity of the magnet is reversed, the polarity of the second secondary minimum is the same, and the coil has opposite polarity, so it becomes the same polarity as the principal maximum value, and the optimization is completed.

[0036] (Example 4: Alternating magnetic field) Example 4 describes a method for detecting fluctuations in an alternating magnetic field among metal detectors. In the case of the metal detector 10A that utilizes an alternating magnetic field, the coaxial coil arrangement suitable for using the present invention will be described. The magnetic detection element 12 consists of a receiving coil and a transmitting coil. Figure 23 shows a conventional coaxial coil arrangement. With this configuration, the change in the induction signal due to the passage of a metallic foreign object can be obtained as a waveform fluctuation of one period as a differential output of two receiving coils. In contrast, the coil arrangement according to the present invention is as shown in Figure 24, and the change in the induced signal due to the passage of metallic foreign matter can be obtained as a waveform fluctuation for two periods as a differential output of four receiving coils 40. In addition, as in Embodiment 1, an A / D conversion unit, a storage unit, a Fourier transform unit, and a determination unit are installed after the differential amplifier. By obtaining a multi-period signal and performing a Fourier transform, sensitivity performance can be improved. In the case of Figure 24, an improvement in detection performance can be obtained by applying a Fourier transform to a multi-period signal compared to Figure 23.

[0037] (Example 5: Magnetic Sensor) Example 5 describes a method of detecting fluctuations in the magnetic field using a magnetic sensor 50 in the metal detector 10B. The magnetic detection element 12 consists of a magnetic sensor 50 and a magnetizing device. As shown in Figure 25, when the magnetic sensor 50 is mounted upright relative to the transport surface of the inspected item, in order to obtain the multi-period signal of the present invention, the magnetic sensor 50 is mounted while reversing its magnetic poles in the order of positive, then negative, then positive, along the transport direction of the inspected item. In addition, an A / D conversion unit, a storage unit, a Fourier transform unit, and a determination unit are installed after the operational amplifier, as in Embodiment 1. This means that for every doubling of the number of magnetic sensors 50, an improvement in detection performance due to the Fourier transform for multi-period signals can be obtained.

[0038] According to this invention, it is possible to achieve highly sensitive inspection performance for minute metallic foreign objects. Preferred embodiments of the present invention have been described above. However, the present invention is not limited in any way to the above embodiments, and various modifications are possible without departing from the spirit of the invention. Furthermore, the present invention is not limited to the combinations shown in the embodiments, but can be implemented using various combinations. [Explanation of symbols]

[0039] 10, 10A, 10B Metal Detectors 11 Conveying section 12 Magnetic detection element 14 Operational Amplifier 15 A / D conversion section 16 Memory section 17. Fourier Transform Section 18 Judgment section 20 permanent magnets 30 detection coils 40 Receiving coil 50 Magnetic Sensors

Claims

1. The system includes a transport unit for transporting the inspected items, and three or more magnetic detection elements are arranged in a continuous line along the transport direction of the inspected items to detect magnetic field fluctuations originating from metallic foreign matter mixed in the inspected items. A metal detector characterized by comprising a Fourier transform unit that performs a Fourier transform on detection signals of two or more periods obtained from three or more of the aforementioned magnetic detection elements.

2. A metal detector according to claim 1, A metal detector characterized by comprising a storage unit that stores repeating time waveforms of two or more periods obtained from three or more of the magnetic detection elements.

3. A metal detector according to claim 2, A metal detector characterized by comprising a determination unit that determines whether a metallic foreign object has been detected by limiting the frequency within a certain bandwidth from the sample frequency determined by the movement speed of the inspected item and the spacing of the magnetic detection elements, and using the calculation results for amplitude and for phase to determine whether the respective thresholds are exceeded.

4. A metal detector according to any one of claims 1 to 3, The metal detector is characterized in that the magnetic detection element is an annular detection coil, and three or more of these are arranged alternately with hollow cylindrical permanent magnets, and the hollow portions of the detection coils and permanent magnets serve as a transport path for the inspected item.

5. A metal detector according to claim 4, The metal detector is characterized in that, when three or more permanent magnets are lined up along the transport direction of the inspected item, the like poles of adjacent permanent magnets face each other.

6. A metal detector according to claim 5, A metal detector characterized in that three or more detection coils and permanent magnets are arranged in a row, with the detection coils positioned alongside the permanent magnets along the inner circumference, outer circumference, or transport direction of the inspected item.

7. A metal detector according to claim 6, The metal detector is characterized in that the detection coils have the same polarity, the spacing between the detection coils and the permanent magnets is set so that the response waveform of any one of the detection coils and the response waveform of a detection coil adjacent to any one of the detection coils are added in phase, and the transport unit moves the object to be inspected at a specified speed.

8. A metal detector according to claim 6, The metal detector is characterized in that the detection coils are arranged in an even number of alternating opposite polarities, the spacing between the detection coils and the permanent magnets is set such that the response waveform of any one of the detection coils and the response waveform of a detection coil adjacent to any one of them are added in phase and environmental noise is canceled out in opposite polarities, and the transport unit moves the object to be inspected at a specified speed.

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