metal detector

Abstract

To provide a metal detector that improves productivity and reduces waste. [Solution] The magnetic field output unit 3 generates an alternating magnetic field in the transport path of the product under inspection W. The magnetic field detection units 4 and 5 detect changes in the magnetic field caused by the product under inspection W passing through the alternating magnetic field and output detection signals DI and DQ. The control unit 6 detects the presence or absence of metallic foreign matter mixed in the product under inspection W based on the detection signals DI and DQ. The storage unit 7 sequentially stores phase information, amplitude information, or influence value information including Lissajous waveforms, which are obtained based on the detection signals DI and DQ when a good product (product under inspection W, which is detected as free of metallic foreign matter) passes through the alternating magnetic field. When the influence value information changes beyond a predetermined judgment criterion, the control unit 6 determines that the state of the product under inspection W has changed. When the notification unit 9 determines that the state of the product under inspection W has changed, it notifies the user of this fact.

Description

Technical Field

[0001] The present invention relates to a metal detector.

Background Art

[0002] In a production line such as for food, a metal detector is used to inspect on the line whether a test object contains metal. Further, the metal detector has an auto-setting function that uses a master work (a reference object for the test object) to determine internal parameters so that the magnetic field change due to the influence of the master work is minimized (Patent Document 1). Also, it is known to generate a reference reject waveform from a detection signal obtained using a master work (a reference object for the test object) and determine the presence or absence of a foreign object by setting a determination line (Patent Document 2). With such techniques, it has become possible to accurately detect the metal contained in the test object while easily performing the setting operation.

[0003] However, for example, if the state of the test object changes over time compared to when the internal parameters were determined, such as when a frozen product as the test object melts or the moisture content of the test object changes, the magnetic field change when a good product of the test object passes may vary. In this case, there was a possibility of false detection where even a test object that would otherwise not contain a metal foreign object would appear to have a large magnetic field change and be erroneously detected as having a metal foreign object.

[0004] This is not an obvious external change in appearance such that the change in the state of the test object can be visually recognized. Conventionally, when a metal detector makes a false detection, for example, determinations of having a metal foreign object occur frequently in a short time, and it is found retrospectively that the cause was a change in the state of the test object that occurred only after the confirmation that they were originally good products. Therefore, it takes time to find out that there has been a change in the state of the test object, and during that time, there is a risk that good products will be discarded, leading to problems such as a decrease in productivity and waste loss.

Prior Art Documents

Patent Documents

[0005] [Patent Document 1] Patent No. 4188283 [Patent Document 2] International Publication No. WO2015 / 49766 [Overview of the project] [Problems that the invention aims to solve]

[0006] This invention has been made in view of the circumstances described above, and its purpose is to provide a metal detector that improves productivity and reduces waste. [Means for solving the problem]

[0007] To achieve the aforementioned objectives, the metal detector according to the present invention is characterized by the following [1] to [4]. [1] A magnetic field output unit (3) generates an alternating magnetic field in the transport path of the product under inspection (W), A magnetic field detection unit (4, 5) detects changes in the magnetic field caused by the inspected item passing through the alternating magnetic field and outputs detection signals (DI, DQ), The system includes a metal determination unit (61) that determines the presence or absence of metal mixed in the inspected product based on the detection signal, In the metal detector (1), A storage unit (7) sequentially stores influence value information, including phase information and amplitude information, which is determined based on the detection signal when the good product under inspection, which has been determined by the metal determination unit to be free of metal contamination, passes through the alternating magnetic field, A state change determination unit (62) determines whether the state of the inspected product has changed based on whether the influence value information exceeds a predetermined determination criterion, The system further includes a notification unit (9) that notifies the user when it is determined that the state of the inspected item has changed, It must be a metal detector. [2] In the metal detector described in [1], The storage unit stores a predetermined number of types of influence value information, The state change determination unit determines that the state of the inspected product has changed when all of the multiple types of influence value information exceed the predetermined determination criteria. It must be a metal detector. [3] In the metal detector described in [1], The storage unit stores the Lissajous waveform, which includes the phase information and the amplitude information, as the influence value information. It must be a metal detector. [4] In the metal detector described in [3], The state change determination unit sets limit lines (L1 to L3) as predetermined determination criteria on the coordinates of the Lissajous waveform, and determines that the state of the inspected product has changed when the Lissajous waveform intersects with any of the limit lines. It must be a metal detector.

[0008] According to the configuration described in [1] above, changes in the condition of the inspected product can be grasped before a good product is mistakenly identified as a defective product, allowing for a review of the production line's operating conditions and the temperature control of the inspected product, or readjustment of the metal detector's settings. As a result, productivity can be improved and waste loss can be reduced. According to the configuration described in [2] above, even if one of the multiple influence value pieces changes without being caused by a change in the state of the product under inspection, the change in the state of the product under inspection will not be over-determined, and it will be possible to determine with greater accuracy that the state of the product under inspection has changed. According to the configuration described in [3] above, it is possible to determine with greater accuracy whether the state of the inspected product has changed based on phase information and amplitude information using the Lissajous waveform. According to the configuration described in [4] above, predetermined criteria for determining changes in the state of the inspected product can be easily set on the coordinate axes of the Lissajous waveform. [Effects of the Invention]

[0009] According to the metal detector of the present invention, it has the effect of improving productivity and reducing waste loss.

[0010] As described above, the present invention has been briefly described. Furthermore, the details of the present invention will be further clarified by reading through the embodiments for carrying out the invention described below (hereinafter referred to as "embodiments") with reference to the attached drawings.

Brief Explanation of Drawings

[0011] [Figure 1] FIG. 1 is a block diagram showing an embodiment of the metal detector of the present invention. [Figure 2] FIG. 2 is a graph showing the time-series transitions of the phase, amplitude of the received signal R, detection signals DI, DQ, and the resurge waveform shown in FIG. 1. [Figure 3] FIG. 3 is a flowchart showing the procedure of the state change detection process executed by the control unit constituting the metal detector shown in FIG. 1. [Figure 4] FIG. 4 is an explanatory diagram for explaining the state change determination process executed in S3 of FIG. 3. [Figure 5] FIG. 5 is an explanatory diagram for explaining the determination criteria of the state change determination process using the resurge waveform executed in S3 of FIG. 3. [Figure 6] FIG. 6 is a block diagram showing another embodiment of the detector of the present invention.

Embodiments for Carrying out the Invention

[0012] Specific embodiments of the present invention will be described below with reference to each drawing.

[0013] The metal detector 1 shown in FIG. 1 detects metal foreign substances mixed in the inspected product W. As shown in the figure, the metal detector 1 includes a signal generator 2, a magnetic field output unit 3, a magnetic field reception unit 4, a detection unit 5, a control unit 6, a storage unit 7, an operation unit 8, and a display unit 9.

[0014] The signal generator 2 outputs a signal of a predetermined frequency. The magnetic field output unit 3 receives the signal from the signal generator 2 and generates an alternating magnetic field of a predetermined frequency on the transport path through which the object under inspection W is transported. The magnetic field receiving unit 4 outputs a received signal R corresponding to the change in the magnetic field caused by an object passing through the alternating magnetic field. The magnetic field receiving unit 4 has two receiving coils (not shown) that receive the alternating magnetic field generated by the magnetic field output unit 3.

[0015] The two receiving coils are positioned to receive equal amounts of the alternating magnetic field and are aligned along the transport direction of the product W under inspection. Furthermore, the two receiving coils are differentially connected. Therefore, when there is no influence on the alternating magnetic field from the product W under inspection or metallic foreign matter, the amplitudes of the signals induced in the two receiving coils are equal and their phases are inverted, resulting in zero amplitude for the received signal R.

[0016] In this embodiment, the case in which two receiving coils are differentially connected is described, but the magnetic field receiving unit 4 may be configured to subtract the signals induced in the two receiving coils using an analog subtractor. Also, if the magnetic fields received by the two receiving coils are not equal in magnitude, the difference in the signals induced in the two receiving coils may be corrected using a variable resistor or amplifiers with different gains.

[0017] The control unit 6 is composed of a microcomputer including a CPU, RAM, and ROM. The CPU executes a control program stored in the ROM while exchanging data with the RAM. The control unit 6 has a functional block as a metal detection unit 61. When the item to be inspected W enters the transport path, it captures detection signals DI and DQ, and compares the captured signals with a preset determination value to determine, for example, whether or not a metallic foreign object is mixed in with the item to be inspected W. The control unit 6 also displays the determination result (for example, OK or NG) of whether or not a metallic foreign object is mixed in with the item to be inspected W, as determined by the metal detection unit 61, on the display unit 9.

[0018] The control unit 6 is composed of a microcomputer including a CPU, RAM, and ROM. The CPU executes the control program stored in the ROM while exchanging data with the RAM. The control unit 6 functions as a metal foreign object detection unit. When the item under inspection W enters the transport path, it captures detection signals DI and DQ, and compares the captured signals with a preset judgment value to determine whether or not a metal foreign object is present in the item under inspection W. If the control unit 6 determines that a metal foreign object is present in the item under inspection W, it displays this information on the display unit 9.

[0019] The control unit 6 is connected to the operation unit 8 and the display unit 9, and when a setting mode is specified by the operation unit 8, it performs setting processing for various parameters. When an inspection mode is specified by the operation unit 8, the control unit 6 causes the metal detection unit 61 to perform inspection processing for the presence of metal foreign matter in the inspected product W, and performs display processing to display the inspection result on the display unit 9.

[0020] The inspection parameters required for detecting the presence of metallic foreign matter include the length and transport speed of the item W under inspection, the frequency of the signal used to generate the alternating magnetic field, the detection phase of the reference signal (the amount of phase shift of the reference signal relative to the signal output from the signal generator 2), and a determination threshold for determining the presence or absence of metallic foreign matter.

[0021] The control unit 6 performs an automatic setting process to set the inspection parameters necessary for inspecting for the presence of metal foreign matter for each type of product W under inspection.

[0022] When the operation unit 8 instructs the execution of the auto-setting process, the control unit 6 switches the metal detector 1 from inspection mode to setting mode and starts the auto-setting process. Based on the phase and amplitude of the detection signals DI and DQ when a good product of the product to be inspected W passes through the alternating magnetic field, the control unit 6 determines the phase of the reference signal to be output to the orthogonal detection unit 51, determines a judgment threshold for determining the presence or absence of metallic foreign matter, stores it in the storage unit 7 in association with the type of product to be inspected W, and returns to inspection mode.

[0023] The memory unit 7 sequentially stores influence value information, including phase information and amplitude information of the received signal R, when a good product W, which is free of metallic foreign matter, passes through an alternating magnetic field. Influence value information is information that changes in response to changes in the state of the product W. In this embodiment, the influence value information sequentially stored in the memory unit 7 is phase information, amplitude information (see Figures 2(A) and (B)), or Lissajous waveform (see Figure 2(E)).

[0024] The above-mentioned phase and amplitude can be determined from the detection signals DI and DQ. Furthermore, the Lissajous waveform is obtained by plotting the detection signals DQ and DI (see Figures 2(C) and (D)) sampled while a good product is passing through an alternating magnetic field as data pairs on a graph with the detection signal DQ on the vertical axis and the detection signal DI on the horizontal axis, in chronological order. For this reason, the Lissajous waveform can also be said to contain information that includes both phase and amplitude.

[0025] As shown in Figure 2(E), the Lissajous waveform is, for example, roughly figure-eight shaped, but it can take on different shapes depending on the type of product W being inspected. Multiple Lissajous waveforms obtained each time a good product W passes through the alternating magnetic field are sequentially stored in the memory unit 7.

[0026] Furthermore, the control unit 6 has a functional block that functions as a state change determination unit 62, which executes a state change determination process to determine changes in the state of the inspected product W during the inspection process. This state change determination process will be described below with reference to the flowchart in Figure 3.

[0027] First, the control unit 6 samples the detection signals DI and DQ at predetermined intervals while a product W under inspection is passing through an alternating magnetic field and the received signal R is synchronously detected and output. The control unit 6 stores the sampled data in the storage unit 7 at least for the duration that the product W under inspection is passing through the alternating magnetic field (S1). Next, the control unit 6 reads the sampled data related to the product W under inspection from the storage unit 7 at the moment the product W passes through and performs signal processing. That is, it processes multiple detection signals DI and DQ to obtain the phase, amplitude, or Lissajous waveform of the received signal R and stores it in the storage unit 7 as detection data (S2).

[0028] In S2, the control unit 6 does not necessarily have to store the phase, amplitude, or Lissajous waveform of the received signal R when a defective product of the inspected product W, in which metal foreign matter has been detected, passes through as detection data in the storage unit 7. As a result, the phase, amplitude, or Lissajous waveform of the received signal R when a good product of the inspected product W passes through the alternating magnetic field is sequentially stored in the storage unit 7.

[0029] Next, the state change determination unit 62 of the control unit 6 reads the phase, amplitude, or Lissajous waveform of the received signal R from the storage unit 7 and determines whether there has been a change in the phase or amplitude immediately after the auto-setting process, based on whether it exceeds a predetermined determination criterion (S3). If it determines that the predetermined determination criterion has been exceeded (Y in S3), the control unit 6 determines that the state of the inspected product W has changed and notifies the user of this fact using the display means 91, which acts as the notification unit 9 (S4). The control unit 6 also uses the display means 91 to remind the user that the state of the inspected product W has been determined to have changed and that the auto-setting process should be redone (S5), before terminating the process. If it is not determined that the phase, amplitude, or Lissajous waveform exceeds the predetermined determination criterion (N in S3), the control unit 6 terminates the state change determination process of the state change determination unit 62 for the inspected product W. The term "exceeding the determination criterion" refers to a displacement from one side to the other, with the determination criterion as the boundary.

[0030] Next, we will explain S3 detection in detail.

[0031] First, let's explain the state change determination process based on phase and amplitude. As shown in Figure 4, the state change determination unit 62 of the control unit 6 determines that the state of the inspected product W has changed when the phase and amplitude of the received signal R, which changes over time, cross predetermined threshold values ​​Tp and Ta, respectively. The threshold values ​​Tp and Ta should be determined based on the phase and amplitude values ​​of the received signal R immediately after the auto-setting process.

[0032] For example, Figures 4(A) and (B) show the time-series changes in phase and amplitude as a certain type of inspected product W passes through the metal detector 1 sequentially. These graphs illustrate an example where the phase tends to gradually decrease while increasing and decreasing, and the amplitude tends to gradually increase while increasing and decreasing. The thresholds Tp and Ta, shown by dotted lines in each graph, are predetermined criteria for determining whether the state of the inspected product W has changed based on the time-series changes in phase and amplitude.

[0033] As shown in Figure 4(A), in the case of this type of product W under inspection, the lower limit value TpL as the threshold Tp is set to a value smaller than the phase immediately after the auto-setting process by the operation input of the operation unit 8. The state change determination unit 62 of the control unit 6 turns on the phase change flag when the phase of a product W under inspection falls below the lower limit value TpL, and turns off the phase change flag when the phase of another product W under inspection subsequently exceeds the lower limit value TpL.

[0034] Furthermore, as shown in Figure 4(B), in the case of this type of inspected product W, the upper limit value TaU as the threshold Ta is set to a value greater than the amplitude immediately after the auto-setting process by the operation input of the operation unit 8. The state change determination unit 62 of the control unit 6 turns on the amplitude change flag when the amplitude of one inspected product W exceeds the upper limit value TaU, and then turns off the amplitude change flag when the phase of another inspected product W falls below the upper limit value TaU. When both the phase change flag and the amplitude change flag are turned on, the state change determination unit 62 of the control unit 6 determines that the state of the inspected product W has changed.

[0035] Furthermore, the state change determination unit 62 of the control unit 6 may turn ON the phase change flag and the amplitude change flag, respectively, when the phase and amplitude continuously exceed or fall below the thresholds Tp and Ta for a certain period of time or longer. This suppresses the excessive detection of single, short-period fluctuations in phase and amplitude caused by allowable variations in individual inspected items W being sequentially transported, or disturbances in the transport posture, and enables accurate detection of changes in phase and amplitude.

[0036] Furthermore, the state change determination unit 62 of the control unit 6 may update the phase change flag and amplitude change flag from OFF to ON, or from ON to OFF, when the average (e.g., moving average or batch average) of a predetermined interval (e.g., predetermined time or predetermined number) in the time-series changes of phase and amplitude exceeds or falls below the thresholds Tp and Ta. In this case as well, it is possible to accurately detect changes in phase and amplitude as trends while suppressing the excessive detection of large, one-off, short-period fluctuations in phase and amplitude as changes.

[0037] Furthermore, an example of a process for determining the state change of the inspected product W based on upper and lower limits set using the phase and amplitude of the received signal R immediately after the auto-setting process will be explained.

[0038] The state change determination unit 62 of the control unit 6 determines that a state change exceeding the acceptable range has occurred in the inspected product W when the phase exceeds the upper phase limit (threshold TpU) or falls below the lower phase limit (threshold TpL). Furthermore, the state change determination unit 62 of the control unit 6 determines that a state change exceeding the acceptable range has occurred in the inspected product W when the amplitude exceeds the upper amplitude limit (threshold TaU) or falls below the lower amplitude limit (threshold TpL). This allows for the setting of predetermined criteria used in the state change determination process based on upper and lower limits derived from the phase and amplitude of the received signal R immediately after the auto-setting process using a good product W, even without knowing how the amplitude and phase tend to change in response to the state change of the inspected product W.

[0039] For example, when an automatic setting process is performed using a good sample of a certain type of product W under inspection, if the phase of the received signal R from this product W is determined to be θa, then the upper limit phase value TpU is set to θa + θd and the lower limit phase value TpL is set to θa - θd as predetermined criteria used in the state change determination process. This θd is often set in advance in the control unit 6 as an allowable value for phase variation, and multiple values ​​may be set to be selectable depending on the type of product W under inspection. Similarly, for amplitude, the upper limit amplitude value TaU and the lower limit amplitude value TaL can be set using an allowable value for amplitude variation.

[0040] Next, as another example, we will explain the process of determining a state change using a Lissajous waveform.

[0041] As shown in Figure 5(A), limit lines L(L1~L3) are set on the coordinates of the Lissajous waveform as predetermined judgment criteria. Limit lines L1~L3 are set so that they do not intersect with the Lissajous waveform immediately after the auto-setting process performed using a good product of the inspected product W. When a state change occurs, a change occurs in the Lissajous waveform, and as shown in Figures 5(B) and 5(C), the Lissajous waveform begins to intersect with the limit lines L1~L3. At this time, the state change detection unit 62 of the control unit 6 turns on the Lissajous waveform change flag and detects that a change has occurred.

[0042] Here, the number of limit lines L is not necessarily limited to the three L1 to L3 exemplified. Specifically, it is advisable to compare the Lissajous waveform of the inspected product W used in the auto-setting process with the Lissajous waveform of a sample of the inspected product W whose state has been intentionally changed to that of a defective product, and set the limit lines L in the region that occurs between the Lissajous waveform of the good product and the Lissajous waveform of the defective product. In this case, if multiple samples of inspected product W in different states, both good and defective, can be prepared, the Lissajous waveforms of the good and defective products can be overlaid to understand the variability, and the limit lines L can be set with greater accuracy.

[0043] In the embodiments described so far, the metal detector 1 was used to determine whether or not there were any metallic foreign objects mixed in the item W to be inspected. However, instead, it may be used to determine whether or not there are any necessary metallic objects in the item W to be inspected.

[0044] According to the embodiment described above, the state change determination unit 62 of the control unit 6 detects the change in the magnetic field caused by the passage of the inspected product W, and when the influence value information including the phase, amplitude, or Lissajous waveform obtained by detecting this change exceeds a predetermined determination criterion, it determines that the state of the inspected product W has changed and notifies the system accordingly. This makes it possible to grasp state changes of the inspected product W that are not visible to the naked eye. For example, before a good product W is mistakenly judged as containing a metal foreign object, the state management of the inspected product W on the production line can be reviewed, or the auto-setting process of the metal detector 1 can be reset. As a result, productivity of the inspected product W can be improved and waste loss can be reduced.

[0045] According to the embodiment described above, the state change determination unit 62 of the control unit 6 determines that the state of the inspected product W has changed when all of the predetermined multiple types of influence value information, including phase, amplitude, or Lissajous waveform, which are obtained by detecting changes in the magnetic field due to the passage of the inspected product W, exceed a predetermined determination criterion. As a result, if one of the influence value information (phase, amplitude, or Lissajous waveform) changes due to variations in the size of the inspected product W itself or variations in the transport conditions that are not caused by a change in state, and exceeds the predetermined determination criterion, the state change determination unit 62 does not determine that the state of the inspected product W has changed. Instead, it determines that the state of the inspected product W has changed when the other influence value information changes. Therefore, the state change of the inspected product W is not excessively detected, and the state change of the inspected product W can be detected with greater accuracy.

[0046] According to the embodiment described above, the state change determination unit 62 of the control unit 6 stores the Lissajous waveform as influence value information. This makes it possible to determine with greater accuracy whether the state of the inspected product W has changed based on the phase information and amplitude information.

[0047] According to the embodiment described above, the state change determination unit 62 of the control unit 6 sets limit lines L1 to L3 on the coordinates of the Lissajous waveform, and determines that the state of the inspected product W has changed when the Lissajous waveform intersects with the limit lines L1 to L3. This makes it easy to set predetermined determination criteria for determining the state change of the inspected product W on the coordinate axes of the Lissajous waveform.

[0048] Furthermore, the present invention is not limited to the embodiments described above, and can be modified, improved, etc., as appropriate. In addition, the material, shape, dimensions, number, placement, etc. of each component in the embodiments described above are arbitrary and not limited, as long as they can achieve the present invention.

[0049] In the embodiment described above, the phase, amplitude, and Lissajous waveform of the received signal R were used as influence value information, but this is not the only option. The phase and amplitude can be obtained from the detection signals DI and DQ, and the detection signals DI and DQ may also be used as influence value information. In other words, at least one of the phase, amplitude, Lissajous waveform, and detection signals DI and DQ of the received signal R can be used as influence value information that is substantially equivalent in that it includes phase information and amplitude information.

[0050] In the above-described embodiment, the state change determination unit 62 of the control unit 6 detected that the state of the product under inspection W had changed when the phase and amplitude of the received signal R changed beyond a predetermined determination criterion, but it is not limited to this. For example, depending on the characteristics of the production process of the product under inspection W, in order to maintain good metal detection sensitivity, the control unit 6 may track the phase of the reference signal output to the orthogonal detection unit 51 within a predetermined range based on the detection signals DI and DQ output by the detection unit 5. In such cases, the phase change flag is turned ON regardless of whether or not a change in the phase of the received signal R is detected by the threshold Tp, and in practice, when a change in amplitude is detected by the threshold Ta and the amplitude change flag is turned ON, it is determined that the state of the product under inspection W has changed.

[0051] In the embodiments described above, a metal detector 1 that generates an alternating magnetic field of a single frequency was described, but it is not limited to this. As shown in Figure 6, a metal detector 1B that generates alternating magnetic fields of multiple frequencies (two types, frequencies f1 and f2, in the example shown in Figure 6) may be used depending on the type of metallic foreign object (for example, whether it is iron or a non-ferrous metal).

[0052] The metal detector 1B comprises two signal generators 2,2 corresponding to the frequencies f1 and f2 of the alternating magnetic field, a magnetic field output unit 3 that generates alternating magnetic fields of both frequencies f1 and f2 on the transport path, a magnetic field receiving unit 4 that outputs received signals R1 and R2 of frequencies f1 and f2 as received signals R, and two detection units 5, 5 that receive the received signals R1 and R2, respectively. In this case, the state change detection unit 62 of the control unit 6 detects a state change in the inspected object W when it detects a state change based on either the phase, amplitude, or Lissajous waveform of the received signal R1 of frequency f1 or the phase, amplitude, or Lissajous waveform of the received signal R2 of frequency f2. [Explanation of Symbols]

[0053] 1. Metal detector 3. Magnetic field output section 4. Magnetic field receiving section 5. Detection Unit 6 Control Unit 7 Memory section 9. Notification Unit (Display means, notification signal output means) 61 Metal determination section 62 State Change Determination Unit R Received signal W Inspected item

Claims

1. A magnetic field output unit (3) generates an alternating magnetic field in the transport path of the product under inspection (W), A magnetic field detection unit (4, 5) detects changes in the magnetic field caused by the inspected item passing through the alternating magnetic field and outputs detection signals (DI, DQ), The system includes a metal determination unit (61) that determines the presence or absence of metal mixed in the inspected product based on the detection signal, In the metal detector (1), A storage unit (7) sequentially stores influence value information, including phase information and amplitude information, which is determined based on the detection signal when the good product under inspection, which has been determined by the metal determination unit to be free of metal contamination, passes through the alternating magnetic field, A state change determination unit (62) determines whether the state of the inspected product has changed based on whether the influence value information exceeds a predetermined determination criterion, The system further includes a notification unit (9) that notifies the user when it is determined that the state of the inspected item has changed, Metal detector.

2. In the metal detector according to claim 1, The storage unit stores a predetermined number of types of influence value information, The state change determination unit determines that the state of the inspected product has changed when all of the multiple types of influence value information exceed the predetermined determination criteria. Metal detector.

3. In the metal detector according to claim 1, The storage unit stores the Lissajous waveform, which includes the phase information and the amplitude information, as the influence value information. Metal detector.

4. In the metal detector according to claim 3, The state change determination unit sets limit lines (L1 to L3) as predetermined determination criteria on the coordinates of the Lissajous waveform, and determines that the state of the inspected product has changed when the Lissajous waveform intersects with any of the limit lines. Metal detector.

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