JP7537669B2 - Metal detector - Google Patents

Description

Article 30, paragraph 2 of the Patent Act applies. Sales date: March 3, 2020. Sales location: Sanko Electronics Laboratory Co., Ltd., Shibata Building 2F, 2-6-4 Kanda, Chiyoda-ku, Tokyo.

The present invention relates to a metal detector that detects metallic foreign objects such as needles that are mixed into clothing or other items to be inspected.

There are metal detectors that use a method in which metallic foreign objects such as needles that have become mixed in with clothing or other items to be inspected are magnetized using a permanent magnet or the like, or that detect a change in magnetic flux density that occurs when a metallic foreign object passes through a magnetic field generated by a permanent magnet or the like.
FIG. 7 is an explanatory diagram 1 of a conventional metal detector. In the metal detector 1A, a permanent magnet 2 and a detection member 5 having a coil 4 wound around an iron core 3 are arranged to face each other with the transport path of the inspection object sandwiched between them. A plurality of detection members 5 are arranged side by side on the same plane (for example, as disclosed in FIG. 1 of Patent Document 1, FIG. 7 of Patent Document 2, and FIG. 13 of Patent Document 3). In the metal detector 1A configured in this way, the inspection object is moved between the detection member 5 and the permanent magnet 2. If a metal foreign object is mixed in the inspection object, a magnetic field fluctuation occurs when the metal foreign object passes through the magnetic field formed by the magnetic field lines generated between the N pole and S pole of the permanent magnet 2, causing a fluctuation in the magnetic field input to the iron core 3 of the detection member 5, and an induced electromotive force due to the magnetic field fluctuation is generated in the coil 4, which can be extracted as a detection signal for the metal foreign object. At this time, the difference between the coils 4 is taken to improve resistance to noise due to disturbances.

However, when the number of turns per unit length is increased to improve the detection performance of the coil 4, the outer diameter may become larger than the thickness of the coil 4. Coils arranged on the same plane (with a larger outer diameter due to an increased number of turns to improve performance) will have a clear difference in the distance from each coil 4 to the noise source depending on the installation position of the detector and the position of the noise source. In this case, since the amount of change in magnetic flux density that causes noise increases or decreases depending on the distance, differences will occur in the noise received by each coil 4, especially when the noise source is on the same plane, and even after the difference is taken, noise remains, leading to a decrease in the S/N ratio (signal-to-noise ratio).

Figure 8 is an explanatory diagram 2 of a conventional metal detector. In metal detector 1B, detection units 6A and 6B, each of which has a coil 4 wound around an iron core 3 protruding from a permanent magnet 2, are arranged above and below the path of the object being inspected (for example, as disclosed in FIG. 6 of Patent Document 2 and FIG. 2 of Patent Document 3). In metal detector 1B of this configuration, detection units 6A and 6B face each other across the path along which the object being inspected moves, and each iron core 3 is arranged concentrically so that the magnetic flux passing through detection units 6A and 6B is linear, providing uniform detection capability and preventing differences in detection sensitivity.

However, it is easily affected by factors such as the direction of the magnetic flux of the magnetized portion (permanent magnet), and the combination of the directions of the waveform when a metal foreign object is detected, the noise waveform generated by disturbance, and the noise waveform generated by vibration will not all be in the same direction or all in opposite directions in the upper coil (detection unit 6A) and the lower coil (detection unit 6B) on either side of the passage. Therefore, there is a problem that when a difference is taken between the upper coil and the lower coil, one of them will be weaker.

Japanese Patent Application Publication No. 8-291420 JP 10-121370 A JP 2006-113043 A

The problem that the present invention aims to solve is to provide a metal detector that has an improved S/N ratio against noise caused by external disturbances, in consideration of the problems of the conventional technology described above.

As a first means for solving the above problems, the present invention provides a magnet that magnetizes metallic foreign matter contained in an object to be inspected or generates a magnetic field in a space through which the object to be inspected passes;
A metal detector including a detection unit having a coil that faces the magnet across a hollow space through which the inspection object moves and generates an induced electromotive force due to a change in magnetic flux density caused by the metallic foreign object,
The object of the present invention is to provide a metal detector characterized in that the detection unit has multiple coils stacked one on top of the other in a direction perpendicular to the direction of movement of the object to be inspected, and the coils obtain a detection signal that cancels out external noise .
According to the first means, the S/N ratio for noise due to disturbances can be improved.
In addition, the system is more resistant to external disturbances, allowing the machine to be installed in a variety of locations where it previously could not be installed due to the effects of noise.
Furthermore, by reducing the possibility of malfunctions due to disturbances, inspection accuracy is improved and inspections can be performed more efficiently.
Since they are on the same axis, there is no phase difference, and noise signals such as vibrations can be cancelled out efficiently. In addition, errors such as time lags in the detection signals and noise signals are less likely to occur.
The effect of local magnetic fields generated by metal foreign objects passing near the coils causes large differences in signals due to magnetic changes that occur at the same timing due to the phase difference of the coils in the staggered arrangement, which reduces the cancellation of detection signals.
In a uniform magnetic field caused by a disturbance input from a distance, the effect of the phase difference of the coils due to the staggered arrangement is small, making it possible to cancel out noise effectively, and as a result, an improvement in the S/N ratio can be expected.

As a second means for solving the above problem, the present invention provides a metal detector as recited in the first means, characterized in that the detection unit has multiple coils stacked on top of each other and arranged on the same axis.
According to the second aspect, since the two sensors have the same axis, there is no phase difference, and noise signals such as vibrations can be cancelled efficiently. Also, errors such as time differences between the detection signals and noise signals are less likely to occur.

As a third means for solving the above problem, the present invention provides a metal detector as in the first means, characterized in that the detection unit is arranged in a staggered manner with the axes of the multiple stacked coils eccentrically arranged.
According to the third aspect, the influence of a local magnetic field generated by a metallic foreign object passing near the coil produces a large difference in signals due to magnetic changes that occur at the same timing due to the phase difference of the coils in the staggered arrangement, which reduces the cancellation of the detection signals.
In a uniform magnetic field caused by a disturbance input from a distance, the effect of the phase difference of the coils due to the staggered arrangement is small, making it possible to cancel out noise effectively, and as a result, an improvement in the S/N ratio can be expected.

As a fourth means for solving the above problem, the present invention provides a metal detector characterized in that, in any one of the first to third means, the detection unit has an iron core disposed within the coil.
According to the fourth aspect, the magnetic flux density passing through the coil is increased compared to a coil in which the center of the bobbin is hollow, making it possible to detect metallic foreign matter that is located farther away or is smaller.

As a fifth means for solving the above problem, the present invention provides a metal detector which, in any one of the first to fourth means, is characterized in that the detection unit has the coil wound around a bobbin.
According to the fifth aspect, by winding a coil around a bobbin having a shape suitable for winding the coil and placing the iron core at the center of the bobbin, it is possible to make the coil shape and characteristics uniform compared to a configuration in which the coil is wound directly around the iron core, and also to simplify the manufacturing process and reduce manufacturing costs.

As a sixth means for solving the above problem, the present invention provides a metal detector as described in any one of claims 2 to 4 and 5 when quoting claim 2, characterized in that the detection unit has multiple coils stacked on the same axis and wound around the same bobbin.
According to the sixth means described above, when vibrations occur, the multiple stacked coils (upper and lower) vibrate in the same manner, and the generated vibration noise also has a similar waveform, making it possible to cancel it out, which is expected to improve the S/N ratio.
Furthermore, compared to a configuration in which multiple coils are stacked, the number of bobbins can be halved, making it possible to reduce manufacturing costs.

As a seventh means for solving the above problem, the present invention provides a metal detector according to any one of the first to sixth means, characterized in that the coils of the detection unit have the same shape.
According to the seventh aspect, it is possible to reduce the unevenness in performance between a plurality of coils, thereby reducing the variation in detection signals, and also to reduce the management costs of coil manufacturing.

As an eighth means for solving the above problem, the present invention provides a metal detector according to any one of the first to seventh means, characterized in that the detection unit is connected to a signal detection unit which obtains a detection signal by taking the difference or sum between multiple stacked coils, or which obtains a detection signal by taking the difference or sum of signals from the coils in a circuit.
According to the eighth means described above, a detection signal is obtained by taking the difference or sum between multiple overlapping (upper and lower) coils, or by taking the difference or sum of signals from multiple overlapping (upper and lower) coils in a circuit to obtain a detection signal, thereby making it possible to cancel out external noise and to expect an improvement in the S/N ratio.

According to the present invention, the S/N ratio for noise due to disturbances can be improved.
In addition, the system is more resistant to external disturbances, making it possible to install the machine in various locations where it was previously not possible to do so due to the effects of noise.
Furthermore, by reducing the possibility of malfunctions due to disturbances, inspection accuracy is improved and inspections can be performed more efficiently.

1 is a schematic diagram of a configuration of a metal detector according to the present invention. FIG. 1 is an explanatory diagram of a metal detector in which upper and lower layers of coils are arranged in a staggered pattern with their axes offset. FIG. 4 is an explanatory diagram of a signal detection unit. An explanatory diagram of the arrangement of the upper and lower two-layer coils and the noise source. 5A and 5B are diagrams illustrating coil waveforms when disturbance and foreign object are detected. 11A and 11B are diagrams illustrating waveform samples when disturbance and foreign object are detected. FIG. 1 is an explanatory diagram of a conventional metal detector. FIG. 2 is an explanatory diagram 2 of a conventional metal detector.

The following describes in detail an embodiment of the metal detector of the present invention with reference to the drawings.

[Metal detector 10]
FIG. 1 is a schematic diagram of the configuration of the metal detector of the present invention. The direction perpendicular to the paper surface is the direction of movement of the object to be inspected, and the up-down direction of the paper surface is the direction perpendicular to the movement direction. As shown in the figure, the metal detector 10 of the present invention includes a magnet 20 that magnetizes a metallic foreign matter contained in the object to be inspected or generates a magnetic field in the space through which the object to be inspected passes, and a detection unit 30 that faces the magnet 20 across a hollow portion 12 through which the object to be inspected moves and has a coil 32 that generates an induced electromotive force due to a change in magnetic flux density caused by the metallic foreign matter. In the detection unit 30, the coils 32 are arranged in a multiple-layered manner in a direction perpendicular to the direction of movement of the object to be inspected. More specifically, the hollow portion 12 through which the object to be inspected moves on a belt conveyor is arranged in the center of the casing, and two layers of coils 32, upper and lower, are arranged in the upper head 14 above the hollow portion 12, and the magnet 20 is arranged in the lower head 16 below the hollow portion 12.

(Coil 32)
A plurality of coils 32 are attached to the upper head 14. The coils 32 are wound around bobbins and formed into a cylindrical shape.
The coil 32 is formed with an outer diameter larger than the thickness (outer diameter >> thickness) in order to increase the number of turns per unit length, giving it an optimal shape when obtaining the difference between the top and bottom.
An iron core 34 is attached to the center of the bobbin. This increases the magnetic flux density passing through the coil 32 compared to a coil with a hollow center of the bobbin, making it possible to detect metallic foreign objects that are located further away or that are smaller.
By winding the coil 32 around a bobbin having a shape suitable for winding the coil 32 and arranging the iron core 34 at the center of the bobbin in this way, it is possible to make the shape and characteristics of the coil 32 uniform compared to a configuration in which the coil is wound directly around the iron core. In addition, it is possible to simplify the manufacturing process and reduce manufacturing costs.

The coil 32 shown in Fig. 1 has a two-layer structure consisting of an upper layer and a lower layer in a direction perpendicular to the direction of movement of the object to be inspected (the vertical direction of the detector), and three coils are arranged side by side along the surface of the object to be inspected, for a total of six coils. The upper and lower two layers of coils 32 are arranged concentrically with the axis of the iron core 34, in other words, the coils 32 are arranged on the same axis. This configuration makes it difficult for errors such as time differences to occur in the detection signal or noise signal of the coils 32, and reduces them.
In addition, the cores 34 of the two upper and lower layers of coils 32 may be eccentric (shifted) and arranged in a staggered pattern. Fig. 2 is an explanatory diagram of a metal detector in which the coils are staggered and eccentric. With this configuration, the influence of a local magnetic field generated by a metal foreign object passing near the coils 32 will cause a large difference in signals due to magnetic changes that occur at the same timing due to the phase difference of the coils 32 caused by the staggered arrangement. This reduces the cancellation of detection signals.
In a uniform magnetic field caused by a disturbance input from a distance, the effect of the phase difference of the coils 32 due to the staggered arrangement is small, making it possible to cancel out noise effectively, and as a result, an improvement in the S/N ratio can be expected.

When the axes of the iron cores 34 of the upper and lower layers of coils 32 are arranged concentrically (the coils 32 are coaxial), they can be wound around the same bobbin.
This allows the multiple stacked coils (upper and lower) to vibrate in the same way when vibration occurs, and the generated vibration noise also has a similar waveform, making it possible to cancel it out, which is expected to improve the S/N ratio. Also, compared to a configuration with multiple stacked coils, the number of bobbins can be halved, which reduces manufacturing costs.
In addition, the shape of the multiple overlapping coils 32 may be the same. This can reduce the unevenness in performance between the multiple coils. This can reduce the variation in detection signals. In addition, the management cost of coil manufacturing can be reduced.

(Magnet 20)
A magnet 20 is attached to the lower head 16. The magnet 20 is a plate-shaped permanent magnet that is attached across the width of the transport surface of the inspection object (a direction perpendicular to the direction of movement of the inspection object).
The magnet 20 of this embodiment can take either of the following forms: a configuration for magnetizing metallic foreign bodies contained in the test object, or for generating a magnetic field in the space through which the test object passes, or a configuration for magnetizing metallic foreign bodies contained in the test object and for generating a magnetic field in the space through which the test object passes.

(Signal detection unit 40)
The detection unit 30 is connected to a signal detection unit 40 which obtains a detection signal by taking the difference or sum between a plurality of overlapping coils, or obtains a detection signal by taking the difference or sum of signals from the coils on a circuit. Fig. 3 is an explanatory diagram of the signal detection unit.
The signal detection unit 40 shown in FIG. 3(1) has a configuration in which the difference or sum between a plurality of overlapping coils is detected by coil wiring.
When the upper and lower coils 32A, 32B in the detection unit 30 are the same coils connected in the opposite directions, the difference between the coils is taken (cancelled out), and the signal detection unit 40 obtains a detection signal via a filter or the like.
When the upper and lower coils 32A, 32B in the detection unit 30 are reversely wound and connected in the forward direction, the coils are summed (cancelled out) and the signal detection unit 40 obtains a detection signal via a filter or the like.
The signal detection unit 40 shown in FIG. 3(2) shows a configuration in which a detection signal is obtained by taking the difference or sum of signals from a plurality of coils stacked on a circuit.
The upper and lower coils 32A, 32B in the detection unit 30 are the same coils and output signals to the signal detection unit 40, which takes the difference (cancels each other) on the circuit to obtain a detection signal.
The upper and lower coils 32A, 32B in the detection section 30 are reversely wound coils that output signals to a signal detection section 40, which then sums them up (cancels each other) on the circuit to obtain a detection signal.
With such a configuration, disturbance noise can be cancelled, and an improvement in the S/N ratio can be expected.
In the metal detector 10 configured as above, the inspection object 11 moves by the belt conveyor within the hollow portion 12. If a metallic foreign object is present in the inspection object, a detection signal is generated.

[Action]
4 is an explanatory diagram of the arrangement of the upper and lower two-layer coils and the source of disturbance. As shown in the figure, the distance L3 between the upper layer coil 32A and the lower layer coil 32B is set to be sufficiently short compared with the distances L1, L2 from the assumed source of disturbance to the coils (L1 or L2 >> L3). Also, the distance L4 between the object 11 and the coil 32 is set to be sufficiently short compared with the distances L1, L2 from the coil 32 to the source of disturbance (L1 or L2 >> L4), so that the object 11 moves close to the coil 32.

Fig. 5 is an explanatory diagram of the coil waveform when a disturbance and a foreign object are detected. Fig. 6 is an explanatory diagram of waveform samples, (1) when a metallic foreign object is detected, and (2) when a disturbance noise occurs.
When the inspection object 11 moving through the hollow portion 12 of the metal detector 10 contains a metallic foreign object, the proximity of the metallic foreign object has a large effect. In other words, the induced electromotive force generated in the lower layer coil 32B, which is close to the metallic foreign object, is large, and the induced electromotive force generated in the upper layer coil 32A, which is located farther away than the lower layer coil 32B, is smaller, resulting in a difference. For this reason, when the difference between the upper and lower two-layer coils 32 is taken, a large detection waveform remains and can be observed (see Figures 5 and 6 (1)).
When a disturbance occurs, the difference in distance L3 between the upper and lower two-layer coils 32A and B can be ignored compared to the distances L1 and L2 between the coil 32 and the source of disturbance 18, so the induced electromotive forces generated in the upper and lower two-layer coils 32A and B are approximately equal. Therefore, by taking the difference in output from the upper and lower two-layer coils 32A and B, noises caused by disturbances can be cancelled out, reducing their impact (see Figures 5 and 6 (2)). Regarding noises generated when the metal detector 10 body vibrates, the upper and lower two-layer coils 32A and B vibrate in the same way, so the generated noises can also be cancelled out.

In the metal detector of the present invention, when a metallic foreign object moves within the magnetic field generated by the magnet, the magnetic flux density in the iron core of the coil changes, which generates an induced electromotive force in the coil, which can be observed as a detection waveform.
In addition, when electromagnetic waves that cause disturbances occur, they affect the coils, generating induced electromotive forces. This becomes noise, leading to a decrease in the S/N ratio. To address this issue, multiple coils are stacked in a direction perpendicular to the direction of movement of the test object, and the difference between the coils in the upper and lower layers is taken, which cancels out and reduces the noise.
Although the preferred embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments and various modifications can be made without departing from the spirit and scope of the present invention.
Furthermore, the present invention is not limited to the combinations shown in the embodiments, but can be implemented in various combinations.

Reference Signs List 1A, 1B Metal detector 2 Permanent magnet 3 Iron core 4 Coil 5 Detecting member 6A, 6B Detecting section 10 Metal detector 11 Inspection object 12 Hollow section 14 Upper head 16 Lower head 18 Source of disturbance 20 Magnet 30 Detecting section 32 Coil 32A Upper coil 32B Lower coil 34 Iron core 40 Signal detecting section

Claims (8)

a magnet that magnetizes metallic foreign particles contained in an object to be inspected or generates a magnetic field in a space through which the object to be inspected passes;
A metal detector including a detection unit having a coil that faces the magnet across a hollow space through which the inspection object moves and generates an induced electromotive force due to a change in magnetic flux density caused by the metallic foreign object,
The detection unit is a metal detector characterized in that multiple coils are stacked one on top of the other in a direction perpendicular to the direction of movement of the object to be inspected, and a detection signal is obtained by canceling out external noise using the coils . 2. The metal detector according to claim 1,
The metal detector is characterized in that the detection unit has multiple coils stacked on top of each other along the same axis. 2. The metal detector according to claim 1,
The metal detector is characterized in that the detection unit has a staggered arrangement of multiple stacked coils with their axes offset from one another. 4. A metal detector according to claim 1,
A metal detector characterized in that the detection unit has an iron core disposed within the coil. 5. A metal detector according to claim 1,
A metal detector characterized in that the detection unit has the coil wound around a bobbin. A metal detector according to any one of claims 2 to 4 and 5 when claim 2 is recited ,
A metal detector characterized in that the detection unit has multiple coils stacked on the same axis and wound around the same bobbin. 7. A metal detector according to claim 1,
A metal detector characterized in that the coils of the detection unit have the same shape. 8. A metal detector according to claim 1,
The metal detector is characterized in that the detection unit is connected to a signal detection unit that obtains a detection signal by taking the difference or sum between multiple stacked coils, or that obtains a detection signal by taking the difference or sum of signals from the coils in a circuit.

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