JP6913847B2 - Metal detector and diagnostic method - Google Patents

Description

The present invention relates to a metal detection device and a diagnostic method.

Patent Document 1 describes a metal detection device that detects a metallic foreign substance contained in an object to be inspected by utilizing the interaction between a magnetic field and a metal. This device includes a transmitting circuit that generates a magnetic field and a receiving circuit that detects changes in magnetic flux. This device diagnoses the foreign matter detection function using a test piece containing metal.

Japanese Unexamined Patent Publication No. 2015-11107

In the method of diagnosing the foreign matter detection function using a test piece, a metallic foreign matter is intentionally thrown into the inspection area of the metal detection device, and it is determined whether or not the device can detect the metal foreign matter. In order to prevent foreign matter from adhering to or remaining in the inspection area, it is preferable that the number of diagnoses using the test piece is as small as possible. Further, when diagnosing the foreign matter detection function using the test piece, a person who throws in and collects the test piece is required.

An object of the present invention is to provide a metal detection device and a diagnostic method capable of determining whether or not a transmission circuit and a reception circuit are functioning normally without using a test piece.

The metal detection device according to the present invention includes a transmission coil, and outputs a transmission circuit that generates an alternating magnetic field by the transmission coil in an inspection region through which an object to be inspected passes, and a detection signal corresponding to a change in magnetic flux in the inspection region. An object to be inspected that passes through the inspection area based on a difference signal that is a difference between a reception circuit having one reception coil and a second reception coil and a detection signal of the first reception coil and a detection signal of the second reception coil. Sensitivity to change the sensitivity of either the first receiving coil or the second receiving coil so that different detection values are output for the same magnetic flux change with the metal detection unit that detects contained metallic foreign matter. When the changing part and the object to be inspected do not pass through the inspection area, the sensitivity changing part changes the sensitivity of either the first receiving coil or the second receiving coil, and the transmitting coil changes the sensitivity of the inspection area. It is provided with a diagnostic unit that generates an alternating magnetic field inside and determines whether or not the transmitting circuit and the receiving circuit are functioning normally.

In this device, the diagnostic unit generates an alternating magnetic field in the inspection area by the transmission coil when the object to be inspected does not pass through the inspection area, and either the first receiving coil or the second receiving coil. The sensitivity of the receiving coil changes. Therefore, the first receiving coil and the second receiving coil output different detection signals even if the magnetic flux changes are the same. That is, if the transmission circuit and the reception circuit are normal, the difference signal changes (has a value of 0 or more). By verifying the change in the difference signal, this device can determine whether or not the transmitting circuit and the receiving circuit are functioning normally without using a test piece.

In one embodiment, the diagnostic unit determines that the transmission circuit and the reception circuit are normal when the difference signal does not change in response to a change in the sensitivity of either the first reception coil or the second reception coil. It may be determined that it is not functioning. With such a configuration, an abnormality can be detected without using a test piece.

In one embodiment, the sensitivity changing unit includes a third receiving coil connected in series to one of the first receiving coil and the second receiving coil, and a switch for switching the series connection of the third receiving coil. May be provided. With this configuration, the sensitivity of either the first receiving coil or the second receiving coil can be switched by a switch.

In one embodiment, the sensitivity changing unit may include a resistor connected in series to any one of the receiving coil of the first receiving coil and the second receiving coil, and a switch for switching the series connection of the resistor. With this configuration, the sensitivity of either the first receiving coil or the second receiving coil can be switched by a switch.

In one embodiment, the metal detection device further includes a detection unit that detects an alternating current flowing through the transmission coil, and the diagnostic unit has a normal transmission circuit or reception circuit based on the detection result by the detection unit and the difference signal. It may be determined whether or not it is functioning. If the alternating current is detected, it can be determined that the transmission circuit is normal, and if the alternating current is not detected, it can be determined that the transmission circuit is not normal. Further, when the transmission circuit is normal and the difference signal does not change according to the change in the sensitivity of the reception coil (when it remains approximately 0), it can be determined that the reception circuit is not normal. can. In this way, it is possible to distinguish which of the transmitting circuit and the receiving circuit is abnormal.

In one embodiment, the metal detection device includes a case body portion having a space inside the metal detection device through which the object to be inspected passes by free fall, and the inspection area may be set to the space. A device for inspecting an object by dropping it requires more personnel for diagnosis using a test piece than a device for inspecting an object by moving it horizontally using a belt conveyor or the like. .. In an apparatus for inspecting an object to be inspected by dropping it, it is possible to reduce the number of personnel required for diagnosis by determining whether or not the transmitting circuit and the receiving circuit are functioning normally without using a test piece.

The diagnostic method according to the present invention includes a transmitting circuit having a transmitting coil, a receiving circuit having a first receiving coil and a second receiving coil, and is based on a change in magnetic flux caused by a metal foreign substance contained in an object to be inspected passing through an inspection area. This is a diagnostic method for a metal detection device that detects metal foreign matter, and is a step of generating an alternating magnetic flux in the inspection area by the transmission coil when the object to be inspected does not pass through the inspection area. The step of changing the sensitivity of one of the first receiving coil and the second receiving coil by the sensitivity changing unit, and the detection signal of the first receiving coil and the detection signal of the second receiving coil. It includes a step of determining whether or not the transmitting circuit and the receiving circuit are functioning normally based on the difference signal which is the difference.

According to this diagnostic method, the same effect as that of the metal detection device described above is obtained.

According to the present invention, it is possible to determine whether or not the transmitting circuit and the receiving circuit are functioning normally without using a test piece.

It is a conceptual diagram of the metal detection apparatus which concerns on 1st Embodiment. It is a graph explaining an alternating current, a current of a receiving circuit, and a difference output. It is a graph explaining the current of the receiving circuit and the difference output when the sensitivity of one receiving coil is changed. It is a conceptual diagram of the metal detection apparatus which concerns on a modification. It is a flowchart explaining an example of the diagnostic process of a metal detection apparatus. It is a conceptual diagram of the metal detection apparatus which concerns on 2nd Embodiment.

Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the following description, the same or equivalent elements will be designated by the same reference numerals, and duplicate description will be omitted.

[First Embodiment]
[Device configuration]
FIG. 1 is a conceptual diagram of the metal detection device 1 according to the embodiment. The metal detection device 1 shown in FIG. 1 is a device that detects metal foreign substances contained in the object S to be inspected. The object S to be inspected is, for example, food. Metal detection is a method of detecting foreign matter contained in an object to be inspected by utilizing the interaction between a magnetic field and a metal.

The metal detection device 1 includes a case main body 10. The case main body 10 has a space inside the case body S through which the object S to be inspected passes by free fall. An inspection area R for metal detection is set inside the case body 10. The metal detection device 1 includes a transmission circuit 2, a reception circuit 3, and a control unit 4.

The transmission circuit 2 includes a transmission coil 20. The transmission coil 20 is arranged corresponding to the inspection region R, and converts an amount of electricity into a magnetic field. The transmission circuit 2 determines, for example, an alternating current of a set waveform to be input to the transmission coil 20 based on a transmission signal having a set frequency and a set amplitude transmitted from the control unit 4. The alternating current of the set waveform is a current modulated so as to have a predetermined standard waveform. In the alternating current of the set waveform, the amplitude and frequency (period) are fixed values. The transmission circuit 2 generates an alternating magnetic field in the inspection region R through which the object S to be inspected passes by supplying the alternating current of the set waveform to the transmitting coil 20.

The receiving circuit 3 includes a first receiving coil 30a and a second receiving coil 30b as the search coil 30. The first receiving coil 30a and the second receiving coil 30b are arranged so as to correspond to the inspection area R. As an example, the first receiving coil 30a and the second receiving coil 30b are arranged to face the transmitting coil 20. The inspection region R is interposed between the first receiving coil 30a and the second receiving coil 30b and the transmitting coil 20.

Each of the first receiving coil 30a and the second receiving coil 30b outputs a detection signal according to the change in magnetic flux in the inspection region R. The detection signal is an electrical signal and may be a current or a voltage. The first receiving coil 30a and the second receiving coil 30b are configured with the same number of turns and length, and are differentially connected. Therefore, the first receiving coil 30a and the second receiving coil 30b have opposite polarities.

When the metal foreign matter does not pass through the inspection region R, the output signals of the first receiving coil 30a and the second receiving coil 30b have the same amplitude, the same phase, and opposite polarities. Therefore, the output (difference signal) obtained from the two differentially connected receiving coils is substantially 0. On the other hand, when the metal foreign matter passes through the inspection region R, the magnetic flux changes according to the movement of the metal foreign matter. Therefore, the equilibrium state of the outputs of the first receiving coil 30a and the second receiving coil 30b is lost, and the outputs obtained from the two differentially connected receiving coils become outputs having values (difference signals). Metallic foreign matter can be detected by determining the presence or absence of the value of this difference signal.

The receiving circuit 3 includes a sensitivity changing unit 5 that changes the sensitivity of one of the receiving coils of the first receiving coil 30a and the second receiving coil 30b so that different detection values are output for the same magnetic flux change. .. The sensitivity changing unit 5 has a third receiving coil 30c, a first switch SW1 and a second switch SW2. The third receiving coil 30c is connected in series with, for example, the second receiving coil 30b. The first switch SW1 and the second switch SW2 switch the series connection between the second receiving coil 30b and the third receiving coil 30c.

When the first switch SW1 is ON and the second switch SW2 is OFF, the third receiving coil 30c is electrically disconnected from the second receiving coil 30b. Therefore, the first receiving coil 30a and the second receiving coil 30b output the same detected value for the same magnetic flux change. When the first switch SW1 is OFF and the second switch SW2 is ON, the third receiving coil 30c is connected in series with the second receiving coil 30b. Therefore, the number of turns of the second receiving coil 30b is increased, and the sensitivity is improved. Therefore, the first receiving coil 30a and the second receiving coil 30b output different detection values for the same magnetic flux change.

During normal operation other than diagnosis, the first switch SW1 is turned on and the second switch SW2 is turned off.

The control unit 4 is connected to the transmission circuit 2 and the reception circuit 3. The control unit 4 has an input / output interface I / O that inputs / outputs signals to and from the outside, a ROM (Read Only Memory) that stores programs and information for processing, and a RAM that temporarily stores data. It has a storage medium such as (RandomAccess Memory) and HDD (Hard Disk Drive), a CPU (Central Processing Unit), and a communication circuit. The control unit 4 stores the input data in the RAM based on the signal output by the CPU, loads the program stored in the ROM into the RAM, and executes the program loaded in the RAM, thereby performing a function described later. To realize.

The control unit 4 includes a metal detection unit 40, a diagnosis unit 41, a storage unit 42, and a display unit 43 as functional configurations.

The metal detection unit 40 generates a transmission signal having a set frequency and a set amplitude. The metal detection unit 40 outputs a transmission signal generated to the transmission circuit 2 via a communication circuit or the like. Then, the metal detection unit 40 acquires a difference signal from the reception circuit 3 via a communication circuit or the like. The metal detection unit 40 detects a metal foreign substance contained in the object S to be inspected passing through the inspection area R based on the difference signal. In this way, in the metal detection device 1, metal detection is performed by the transmission circuit 2, the reception circuit 3, and the metal detection unit 40 operating in cooperation with each other.

As described above, when the metallic foreign matter does not exist in the inspection area R, the difference signal becomes substantially 0. In this case, it is difficult to determine whether the difference signal is approximately 0 because no metal foreign matter is mixed in, or whether the difference signal is approximately 0 because at least one of the transmission circuit 2 and the reception circuit 3 is disconnected. Is. The diagnosis unit 41 has a function of isolating such a situation.

The diagnosis unit 41 diagnoses the metal detection ability of the metal detection device 1. As a more specific example, the diagnostic unit 41 determines whether or not the transmission circuit 2 and the reception circuit 3 are functioning normally. To function normally means to operate without any trouble for detecting metal. When functioning normally, the transmitting circuit 2 forms an alternating magnetic field, and the receiving circuit 3 detects according to the alternating magnetic field. The diagnosis unit 41 makes a diagnosis when the object S to be inspected does not pass through the inspection area R. The diagnosis unit 41 may start the diagnosis according to a predetermined event or time such as when the power of the metal detection device 1 is turned on or at a fixed time, or the diagnosis may be started manually.

At the time of diagnosis, the diagnosis unit 41 operates the sensitivity changing unit 5 to change the sensitivity of the second receiving coil 30b. As a result, the absolute value of the detected value of the first receiving coil 30a and the absolute value of the detected value of the second receiving coil 30b are different, and even when the metal foreign matter passes through the inspection region R, the differential connection is made. The output from the receiving coil is an output (difference signal) having a value.

The diagnostic unit 41 acquires a difference signal from the receiving circuit 3 via a communication circuit or the like. The diagnosis unit 41 determines whether or not the transmission circuit 2 and the reception circuit 3 are functioning normally based on the difference signal. The diagnostic unit 41 determines that the transmission circuit 2 and the reception circuit 3 are not functioning normally when the difference signal does not change according to the change in the set waveform of the alternating current supplied to the transmission coil 20.

As an example, the diagnosis unit 41 determines that the transmission circuit 2 and the reception circuit 3 are functioning normally when the amplitude of the difference signal equal to or larger than a predetermined value can be confirmed. For example, when the circuit is broken, the value of the difference signal becomes 0 even if the amplitude of the alternating current is changed. When the amplitude of the difference signal equal to or larger than a predetermined value cannot be confirmed, the diagnosis unit 41 determines that the transmission circuit 2 and the reception circuit 3 are not functioning normally.

The storage unit 42 stores the diagnosis result of the diagnosis unit 41 in a storage medium. The display unit 43 displays the diagnosis result of the diagnosis unit 41 on a display such as a display.

[Example of each signal at the time of metal detection]
Each signal at the time of metal detection is outlined. FIG. 2 is a graph for explaining an alternating current, a current of a receiving circuit, and a difference signal. FIG. 2A is an example of alternating current. FIG. 2B is an example of detection signals of the first receiving coil 30a and the second receiving coil 30b. FIG. 2C is an example of a difference signal.

FIG. 2A shows an alternating current having a predetermined period and a predetermined amplitude. By supplying such an alternating current to the transmitting coil, an alternating magnetic field is generated. FIG. 2B is an example of the detection signals of the first receiving coil 30a and the second receiving coil 30b. The first receiving coil 30a detects the waveform of the solid line, and the second receiving coil 30b detects the waveform of the broken line. The solid line waveform has the same absolute value and phase of the amplitude as the broken line waveform, and the positive and negative amplitudes are reversed. By adding the waveform of the solid line and the waveform of the broken line by the differential connection, the difference signal shown in FIG. 2C is obtained. When the metallic foreign matter does not pass through the inspection region R, the alternating magnetic field is not distorted, so that the difference signal becomes substantially 0. On the other hand, when the metallic foreign matter passes through the inspection region R, the alternating magnetic field is distorted, so that the difference signal has a value. When the metal detection unit 40 detects a difference signal equal to or higher than a predetermined threshold value, it determines that a metal foreign substance is mixed.

[Example of each signal at the time of diagnosis]
Each signal at the time of diagnosis is outlined. FIG. 3 is a graph for explaining an alternating current, a current of a receiving circuit, and a difference signal when the sensitivity of one receiving coil is changed. FIG. 3A is an example of alternating current. FIG. 3B is an example of the detection signals of the first receiving coil 30a and the second receiving coil 30b. FIG. 3C is an example of a difference signal.

(A) of FIG. 3 is the same as the alternating current of FIG. At the time of diagnosis, the diagnosis unit 41 changes the first switch SW1 from ON to OFF and the second switch SW2 from OFF to ON. As a result, the sensitivity of the second receiving coil 30b changes. FIG. 3B is an example of the detection signals of the first receiving coil 30a and the second receiving coil 30b. The first receiving coil 30a detects the waveform of the solid line, and the second receiving coil 30b detects the waveform of the broken line. Compared with FIG. 2B, the amplitude of the detection signal of the second receiving coil 30b is larger. Therefore, the difference signal shown in FIG. 3C is obtained. When the diagnosis unit 41 detects a difference signal of a predetermined threshold value A or more, it determines that the transmission circuit 2 and the reception circuit 3 are functioning normally. When the diagnosis unit 41 does not detect a difference signal equal to or higher than a predetermined threshold value A, the diagnosis unit 41 determines that the transmission circuit 2 and the reception circuit 3 are not functioning normally. When the maximum value of the difference signal is in the range of the predetermined threshold value A or more and the predetermined threshold value B or less, the diagnosis unit 41 may determine that the transmission circuit 2 and the reception circuit 3 are functioning normally.

[Modification example of sensitivity change part]
Next, a modified example of the sensitivity changing portion will be described. FIG. 4 is a conceptual diagram of the metal detection device according to the modified example. In the metal detection device 1A according to the modified example, as compared with the metal detection device 1 according to the first embodiment, the third receiving coil 30c of the sensitivity changing unit 5 is a resistor R1 (sensitivity changing unit 5A). , The first receiving coil 30a and the second receiving coil 30b are different in that they are connected to the ground, and the others are the same. In this case, since the resistance value of the second receiving coil 30b changes, the difference signal of the differential circuit is output from the differentially connected receiving coil even when the metal foreign matter passes through the inspection region R. Is an output (difference signal) with a value. Other functions are the same as those in the first embodiment.

[Diagnosis method]
Next, the diagnostic method of the metal detection device 1 will be described. FIG. 5 is a flowchart illustrating an example of a diagnostic process of the metal detection device. The flowchart shown in FIG. 5 may be started according to a pre-scheduled event or time such as when the power of the metal detection device 1 is turned on or at a fixed time, or may be started manually. The first switch SW1 is in the ON state, and the second switch SW2 is in the OFF state.

As shown in FIG. 5, the diagnosis unit 41 determines whether or not the inspected object S has passed through the inspection area R by the determination process (step S10). For example, the diagnostic unit 41 determines whether or not the object S to be inspected has passed through the inspection area R based on the detection result of a sensor (not shown) provided in the inspection area R. Alternatively, the diagnostic unit 41 may input the determination result of another component or the worker to determine whether or not the inspected object S has passed through the inspection area R.

When the object S to be inspected does not pass through the inspection area R (step S10: NO), the diagnostic unit 41 has a predetermined period used in metal detection as a magnetic field generation process (step S12: step to generate an alternating magnetic field). A transmission signal is generated and output to the transmission circuit 2 so that an alternating current having a predetermined amplitude is generated. As a result, an alternating magnetic field is generated in the inspection area R.

Subsequently, the diagnostic unit 41 changes the first switch SW1 from ON to OFF and the second switch SW2 from OFF to ON as a switch process (step S14: step of changing the sensitivity of the receiving coil).

Subsequently, as a diagnostic process (step S16: determination step), the diagnostic unit 41 determines whether or not the transmission circuit 2 and the reception circuit 3 are functioning normally based on the difference signal acquired from the reception circuit 3. do. When the amplitude of the difference signal equal to or larger than a predetermined value can be confirmed, the diagnosis unit 41 determines that the transmission circuit 2 and the reception circuit 3 are functioning normally. When the amplitude of the difference signal equal to or larger than a predetermined value cannot be confirmed, the diagnosis unit 41 determines that the transmission circuit 2 and the reception circuit 3 are not functioning normally. If necessary, the diagnosis unit 41 stores the determination result in the storage unit 42 and causes the display unit 43 to display the determination result.

When the object S to be inspected has passed through the inspection area R (step S10: YES) or the diagnostic process (step S16) is completed, the flowchart shown in FIG. 5 is terminated.

As described above, according to the metal detection device 1 and the diagnosis method according to the first embodiment, when the object S to be inspected does not pass through the inspection area R by the diagnosis unit 41, the transmission coil 20 enters the inspection area R. As the alternating magnetic field is generated, the sensitivity of the second receiving coil 30b changes. Therefore, the first receiving coil 30a and the second receiving coil 30b output different detection signals even if the magnetic flux changes are the same. That is, if the transmission circuit and the reception circuit are normal, the difference signal changes (has a value of 0 or more). By verifying the change in the difference signal, this device can determine whether or not the transmitting circuit and the receiving circuit are functioning normally without using a test piece.

Further, the metal detection device 1 can reduce the number of personnel required for diagnosis by determining whether or not the transmission circuit and the reception circuit are functioning normally without using a test piece.

[Second Embodiment]
Compared to the metal detection device 1 according to the first embodiment, the metal detection device 1B according to the second embodiment is equipped with a detector that detects an alternating current flowing through the transmission coil 20, and has a diagnostic unit. The difference is that it is determined whether or not the transmission circuit or the reception circuit is functioning normally by using the detection result of the detector, and the others are the same.

FIG. 6 is a conceptual diagram of the metal detection device according to the second embodiment. The metal detection device 1B has a detector 21 that detects an alternating current flowing through the transmission coil 20. The detector 21 may be an ammeter. The detector 21 outputs the detection result to the control unit 4.

The diagnosis unit 41A determines whether or not the transmission circuit or the reception circuit is functioning normally based on the detection result of the detector 21 and the difference signal. Similar to the first embodiment, the diagnosis unit 41A determines whether or not the transmission circuit 2 and the reception circuit 3 are functioning normally by using the difference signal. Here, when the difference signal is equal to or less than the threshold value, the diagnostic unit 41A determines whether or not the alternating current is detected by the detector 21.

When the difference signal is equal to or less than the threshold value and the alternating current is detected by the detector 21, the diagnostic unit 41A determines that the receiving circuit 3 is not functioning normally. When the difference signal is equal to or less than the threshold value and the alternating current is not detected by the detector 21, the diagnostic unit 41A determines that at least the transmission circuit 2 is not functioning normally. Other functions are the same as those in the first embodiment.

According to the metal detection device 1B according to the present embodiment, it is possible to distinguish which of the transmission circuit 2 and the reception circuit 3 is abnormal.

Although one embodiment of the present invention has been described above, the present invention is not limited to the above-described embodiment.

For example, an example of an apparatus for inspecting an object S to be inspected that naturally falls (a so-called circular inspection apparatus) has been shown, but the present invention is not limited to this. The metal detection device includes a conveyor that horizontally conveys the object S to be inspected and a case main body portion that passes the object S to be inspected conveyed by the conveyor, and an inspection area is set inside the case main body portion 10. You may.

Further, in the above embodiment, an example in which the third receiving coil 30c or the resistor R1 is connected in series to the second receiving coil 30b is shown, but the third receiving coil 30c or the resistor R1 is connected in series to the first receiving coil 30a. You may.

1 ... Metal detector, 2 ... Transmission circuit, 3 ... Reception circuit, 4 ... Control unit, 5, 5A ... Sensitivity change unit, 10 ... Case body, 20 ... Transmission coil, 21 ... Detector, 30 ... Search coil, 30a ... 1st receiving coil, 30b ... 2nd receiving coil, 40 ... metal detection unit, 41, 41A ... diagnostic unit.

Claims (7)

A transmission circuit that includes a transmission coil and generates an alternating magnetic field by the transmission coil in the inspection area through which the object to be inspected passes.
A receiving circuit including a first receiving coil and a second receiving coil that output a detection signal according to a change in magnetic flux in the inspection area, and a receiving circuit.
A metal detection unit that detects metal foreign matter contained in the object to be inspected passing through the inspection region based on a difference signal which is a difference between the detection signal of the first receiving coil and the detection signal of the second receiving coil. When,
A sensitivity changing unit that changes the sensitivity of either the first receiving coil or the second receiving coil so that different detection values are output for the same magnetic flux change.
When the object to be inspected does not pass through the inspection area, the sensitivity changing unit changes the sensitivity of either the first receiving coil or the second receiving coil, and the transmitting coil changes the sensitivity of the receiving coil. A diagnostic unit that generates an alternating magnetic field in the inspection area and determines whether or not the transmission circuit and the reception circuit are functioning normally.
A metal detector equipped with. When the difference signal does not change in response to a change in the sensitivity of either the first receiving coil or the second receiving coil, the diagnostic unit normally performs the transmitting circuit and the receiving circuit. The metal detection device according to claim 1, wherein it is determined that the device is not functioning. The sensitivity changing part
A third receiving coil connected in series to one of the first receiving coil and the second receiving coil, and the third receiving coil.
A switch for switching the series connection of the third receiving coil and
The metal detection device according to claim 1 or 2. The sensitivity changing part
A resistor connected in series to either the first receiving coil or the second receiving coil,
A switch that switches the series connection of the resistors,
The metal detection device according to claim 1 or 2. Further, a detection unit for detecting an alternating current flowing through the transmission coil is provided.
Any one of claims 1 to 4, wherein the diagnostic unit determines whether or not the transmission circuit or the reception circuit is functioning normally based on the detection result by the detection unit and the difference signal. The metal detector according to. The metal detection device according to any one of claims 1 to 5, further comprising a case main body having a space inside which the object to be inspected passes by free fall, and the inspection area is set in the space. The transmission circuit including the transmission coil and the reception circuit including the first reception coil and the second reception coil are provided, and the metal foreign matter is detected based on the magnetic flux change caused by the metal foreign matter contained in the object to be inspected passing through the inspection area. A diagnostic method for metal detectors
A step of generating an alternating magnetic field in the inspection area by the transmission coil when the object to be inspected does not pass through the inspection area.
A step of changing the sensitivity of one of the first receiving coil and the second receiving coil, and
A step of determining whether or not the transmission circuit and the reception circuit are functioning normally based on the difference signal which is the difference between the detection signal of the first reception coil and the detection signal of the second reception coil.
Diagnostic method with.

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