JP3662841B2 - Metal detector - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a metal detector for inspecting whether or not a metal piece is contained in an object to be inspected such as food, medicine, clothing, etc. conveyed on a conveyance path such as a belt conveyor.
[0002]
[Prior art]
In recent years, HACCP (Hazard Analysis Critical Control Point) has been proposed as a system for ensuring food safety. As part of this HACCP, it is mandatory to inspect foods for metal fragments.
[0003]
Various methods have been proposed to inspect whether or not a metal piece is contained in an object to be inspected such as food, but metal detection utilizing the fact that the magnetic field is disturbed by the metal entering the magnetic field. The method is common.
[0004]
A metal detector employing this detection method is configured as shown in FIG. 8, for example. A frame 3 is installed so as to surround a belt conveyor 2 as a conveyance path for conveying an object to be inspected 1 such as food in a certain direction. An operation panel 4 is provided in front of the frame 3.
[0005]
FIG. 9 is a block diagram showing a schematic configuration of the metal detector incorporated in the frame 3.
The excitation current a output from the AC source 5 is applied to the transmission coil 6 disposed in the conveying direction of the belt conveyor 2. Therefore, a constant magnetic field is formed in the conveyance path of the device under test 1 by the transmission coil 6. A receiving coil 7 that is differentially connected so that the winding directions of the pair of coils 7 a and 7 b are opposite to each other is disposed at a position opposite to the transmitting coil 6. The output signal b of the receiving coil 7 is taken out from the variable terminal of the variable resistor 8 connected between the output terminals of the receiving coil 7 and input to the first synchronous detector 9a and the second synchronous detector 9b. The
[0006]
The coils 7a and 7b of the receiving coil 7 generate an induced voltage by detecting the magnetic field generated in the transmitting coil 6. However, since the winding directions of the coils 7a and 7b are opposite to each other, the induced voltage is The signal level of the output signal b is 0 because they cancel each other. Specifically, the sliding position of the variable terminal of the variable resistor 8 is adjusted so that the signal level of the output signal b becomes zero.
[0007]
Therefore, in a state where the object to be inspected 1 conveyed on the belt conveyor 2 does not include a metal piece, the output signal b of the receiving coil 7 is at the 0 level. However, when a metal piece is included in the device under test 1, a change + ΔE corresponding to the size of the metal piece is generated in the induced voltage of one coil 7a. A change −ΔE corresponding to the size of the metal piece also occurs in the induced voltage of the other coil 7b having a different winding direction. As a result, a signal level of (+ 2ΔE) appears in the output signal b of the receiving coil 7.
[0008]
The excitation current a output from the AC source 5 is applied to the transmission coil 6 and to the first synchronous detector 9a. Further, the excitation signal a is phase-shifted by 90 ° by the 90 ° phase-shift circuit 10 to generate a new excitation signal a.₁Applied to the second synchronous detector 9b.
[0009]
The first synchronous detector 9a synchronously detects the output signal b of the receiving coil 7 with the excitation signal a. The output signal c of the first synchronous detector 9a₁ After the noise component is removed by the BPF 11a and amplified by the amplifier 12a, it is input to the X-axis terminal of the determination device 13 incorporating the oscilloscope.
[0010]
The second synchronous detector 9b is an excitation signal a obtained by shifting the output signal b of the receiving coil 7 by 90 °.₁Synchronous detection with. The output signal c of the second synchronous detector 9a₂  The noise component is removed by the BPF 11b and amplified by the amplifier 12b, and then input to the Y-axis terminal of the determination device 13 in which the oscilloscope is incorporated.
[0011]
The determination device 13 outputs an output signal c input from the X-axis terminal.₁And an output signal c input from the Y-axis terminal₂And the point P at the tip of the combined vector is displayed on the XY orthogonal coordinate plane of the display screen 14.
[0012]
Next, the reason why the first and second synchronous detectors 9a and 9b are used will be described. When the metal piece contained in the device under test 1 is a magnetic material such as iron, as described above, a change ΔE in the induced voltage is generated in the coils 7a and 7b in opposite directions, so that the output signal b has (+ 2ΔE). ) Appears. In this case, the phase of the output signal b does not change with respect to the phase of the excitation signal a. Therefore, this output signal b may be detected synchronously with the excitation signal a.
[0013]
On the other hand, when the metal piece contained in the device under test 1 is a nonmagnetic material such as stainless steel or aluminum, an eddy current is generated in the nonmagnetic material due to the presence of a magnetic field. Due to the fact that the magnetic flux is consumed by the Joule heat of the eddy current, a signal (−ΔE) appears in the output signal b of the receiving coil 7. Furthermore, in this case, due to the eddy current, the phase of the output signal b changes by 90 ° with respect to the phase of the excitation signal a. Therefore, the excitation signal a obtained by shifting the output signal b by 90 ° with respect to the original excitation signal a.₁And synchronous detection.
[0014]
However, the material of the metal piece contained in the inspected object 1 such as food is not completely separated into a magnetic body and a non-magnetic body, and has characteristics of both a magnetic body and a non-magnetic body. Therefore, when the device under test 1 including a metal piece is passed through the installation positions of the transmission coil 6 and the reception coil 7 by the belt conveyor 2, the output signal b of the reception coil 7 changes in signal level and phase. To do. As a result, when a metal piece is included in the device under test 1, the output signals c of the first and second synchronous detectors 9a and 9b are output.₁, C₂Both change.
[0015]
Further, when the object to be inspected 1 is a food, depending on the material (foodstuff) of the food, there are many cases where the food originally contains a metal component such as fine iron. Furthermore, generally, food is packaged and transported hygienically. Therefore, when a metal material such as aluminum vapor deposition is used for the packaging material, this influence is greatly affected.
[0016]
In such an inspected object 1, even if the harmful metal piece to be removed is not included, the output signal b of the receiving coil 7 has magnetic characteristics inherent to the inspected object 1 itself. And a signal level corresponding to the nonmagnetic characteristic and a phase change with respect to the excitation signal occur.
[0017]
Therefore, in order to detect a minute metal piece included in the inspected object 1 with high accuracy, a signal level or phase change caused by a harmful metal piece is detected from a signal level or phase change originally included in the output signal b. It is necessary to distinguish and extract.
[0018]
Next, although the metal piece is not included, the object to be inspected 1 serving as a normal reference having the magnetic characteristics and non-magnetic characteristics of the food itself including the packaging is placed on the belt conveyor 2, and this object to be inspected. The locus of the point P at the tip of the combined vector displayed on the XY orthogonal coordinate plane of the display screen 14 of the determination device 13 when the body 1 is moved between the transmitting coil 6 and the receiving coil 7 is shown in FIG. ) (B).
[0019]
As shown in FIG. 10 (a), the relative position of the device under test 1 with respect to the coils 6 and 7 is (a) state, (b) state, (c) state, (d) state, (e) state. , (F) As the state changes, the signal level of the output signal b of the receiving coil 7 changes greatly, and the phase with respect to the excitation signal a also changes greatly. As shown in FIG. The composite vector tip P of b changes to (a) state, (b) state, (c) state, (d) state, (e) state, and (f) state, and becomes a Lissajous waveform 15.
[0020]
As described above, when the device under test 1 having magnetic characteristics and non-magnetic characteristics is passed through the installation positions of the transmission coil 6 and the reception coil 7, on the XY orthogonal coordinate plane of the display screen 14 of the determination device 13, FIG. A Lissajous waveform 15 shown in b) is drawn.
[0021]
The shape of the Lissajous waveform 15 varies greatly depending on the magnetic characteristics and nonmagnetic characteristics of the device under test 1. Therefore, each inspected object 1 has an individual Lissajous waveform 15.
[0022]
When the object to be inspected 1 includes a metal piece, even if the metal piece has a magnetic characteristic and a nonmagnetic characteristic that are smaller than the magnetic characteristic and the nonmagnetic characteristic originally possessed by the object to be inspected 1. Even so, as shown in FIG. 11B, the Lissajous waveform 15 created by the magnetic characteristics and non-magnetic characteristics of the device under test 1 is affected, and the deformed portion 16 appears.
[0023]
Therefore, the Lissajous waveform 15 of the inspected object 1 serving as a reference that does not include a metal piece as a foreign object is measured in advance, and the deformed portion 16 is converted into the measured Lissajous waveform 15 in the actual measurement of the inspected object 1. Is present, it is determined that the inspected object 1 includes a metal piece.
[0024]
[Problems to be solved by the invention]
However, the metal detector configured as shown in FIGS. 8 and 9 still has the following problems to be solved.
[0025]
That is, the metal detector is often incorporated in an inspection line in a series of production, inspection, packaging, and shipping lines in a food factory. Therefore, a large number of devices for carrying out the manufacturing, packaging, and shipping processes are installed around the metal detector. As a matter of course, motors are incorporated in these devices, and many relays and switches for controlling the operation of the devices are incorporated in these devices.
[0026]
These motors, relays, and switches radiate electromagnetic noise to the surrounding space during operation. It is very difficult to completely enclose this electromagnetic noise in the device.
[0027]
As described above, in the metal detector, the transmission coil 6 generates a magnetic field, and the reception coil 7 detects a change in the magnetic field. And in order to improve the detection sensitivity of the metal piece contained in the to-be-inspected object 1, it is necessary to detect a fine magnetic field change with the receiving coil 7 mentioned above.
[0028]
However, as described above, the noise detector around this metal detector is filled with noise electromagnetic waves radiated from each device. Therefore, if the detection sensitivity is set high for the purpose of improving the detection sensitivity of the metal piece, this noise is detected. There is a concern that a magnetic field change caused by electromagnetic waves may be erroneously determined that a metal piece is present on the inspection object 1. Therefore, there is a certain limitation on the detection accuracy of the metal detector.
[0029]
The present invention has been made in view of such circumstances. By using the characteristics of frequency spreading and frequency despreading for a signal, the signal received by the receiving coil is mixed between the transmitting coil and the receiving coil. An object of the present invention is to provide a metal detector that can greatly suppress the generated noise, greatly improve the S / N ratio of the signal received by the receiving coil, and thus improve the detection accuracy of the metal piece to the object to be inspected. To do.
[0030]
[Means for Solving the Problems]
According to the present invention, a transmission coil to which an alternating excitation signal is applied and a differentially connected reception coil are disposed along the conveyance path of the inspection object, and the inspection object is applied to the inspection object based on the output signal of the reception coil. The present invention is applied to a metal detector that detects a metal piece contained therein.
[0031]
  And in order to eliminate the above-mentioned subject, in the metal detector of the present invention,A random number generator that generates a random number at a fixed period and outputs a random number signal whose voltage value changes in accordance with the value of the random number, and changes according to each voltage value of the random number signal output from the random number generator A voltage-controlled oscillator that outputs an excitation signal having a frequency and applies it to the transmission coil;thisVoltage controlled oscillatorThe 90 ° phase shifter that shifts the phase of the excitation signal frequency-spread by 90 ° and the output signal of the receiving coil,Voltage controlled oscillatorThe first synchronous detector that performs synchronous detection and frequency despreading with an excitation signal that has been phase-spread by the 90 ° phase shifter and an excitation signal that is 90 ° phase shifted by the 90 ° phase shifter. A second synchronous detector for performing synchronous detection and frequency despreading, and a frequency component other than the frequency component of the excitation signal included in each output signal after frequency despreading by the first and second synchronous detectors. A pair of bandpass filters to be removed.
  Further, in the metal detector of another invention, a ramp waveform generating circuit that outputs a ramp waveform signal whose voltage value changes in a sawtooth shape at a constant cycle, and each voltage value of the ramp waveform signal output from the ramp waveform generating circuit A voltage-controlled oscillator that outputs an excitation signal having a frequency that changes in response to the voltage to be applied to the transmission coil, and a 90 ° phase shifter that shifts the phase of the excitation signal that is frequency-spread by the voltage-controlled oscillator by 90 ° The first synchronous detector for synchronously detecting and despreading the frequency of the output signal of the receiving coil with the excitation signal frequency-spread by the voltage controlled oscillator, and the phase of the output signal of the receiving coil by the 90 ° phase shifter Is a 90 ° phase-shifted excitation signal that is included in the second synchronous detector that performs synchronous detection and frequency despreading, and each output signal after frequency despreading by the first and second synchronous detectors Excitation signal And a pair of band-pass filter for removing the frequency components other than frequency components, respectively.
[0032]
First, the principle of operation that can suppress noise input between frequency spreading and frequency despreading by frequency spreading the signal, then frequency despreading the same signal, and passing through a bandpass filter is shown in FIG. I will explain.
[0033]
The frequency f output from the signal source 20_SThe signal 21 having the frequency is the frequency spreading unit 22 and the frequency is f._STo [(f_S-Δf) to (f_S+ Δf)]. Therefore, the average level of the signal also decreases. Noise 24 is superimposed on the frequency spread signal 23 output from the frequency spreader 22. The signal on which the noise 24 is superimposed is frequency despread by the next frequency despreading unit 25.
[0034]
When the component of the signal 23 that has been frequency spread first is frequency despread, the original frequency f_SHowever, the component of the noise 24 is inversely frequency-spread and has a low signal level [(f_S-Δf) to (f_S+ Δf)] of the wideband signal 24a.
[0035]
The frequency band of the output signal 26 of the frequency despreading unit 25 is converted to the original frequency f by a BPF (band pass filter) 27._SBy limiting to the vicinity, the S / N ratio of the signal 28 output from the BPF 27 can be greatly improved.
[0036]
This operating principle is applied to the metal detector of the present application.
The excitation signal applied to the transmission coil is frequency spread. And since the excitation signal of this frequency spread state is applied to the transmission coil, the magnetic field generated by the transmission coil is also frequency spread. A noise magnetic field is superimposed on the frequency-spread magnetic field from the outside.
[0037]
A current corresponding to the magnetic field on which the noise magnetic field is superimposed and frequency-spread flows through the receiving coil. Therefore, the output signal of the receiving coil includes a signal component from the transmission coil that has been subjected to frequency spreading and a noise component that has not been subjected to frequency spreading.
[0038]
Therefore, if the output signal of the receiving coil is subjected to frequency despreading, the signal component becomes a signal having a high signal level in the original narrow band, but the noise component is conversely frequency spread and has a low signal level. It becomes a broadband signal.
[0039]
Thus, if only a narrow band high signal level is extracted using a narrow band pass filter, the S / N ratio of the output signal of the receiving coil is greatly improved.
[0041]
According to still another aspect of the present invention, in the metal detector of the above-described invention, a tuning relaxation circuit for relaxing tuning characteristics is provided for the transmission coil and the reception coil.
[0042]
If the tuning relaxation circuit is provided in this way to relax the tuning characteristics, for example, even if the temperature of the object to be inspected changes, the phase characteristic change in the object to be inspected becomes gradual. Since the Lissajous waveform does not change greatly, the detection sensitivity does not change greatly.
[0043]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
(First embodiment)
FIG. 1 is a block diagram showing a schematic configuration of a metal detector according to the first embodiment of the present invention. The same parts as those of the conventional metal detector shown in FIG. 9 are denoted by the same reference numerals, and detailed description of the overlapping parts is omitted. The external view of this metal detector is the same as the external view shown in FIG.
[0044]
In the metal detector of the first embodiment, a signal synthesizer 30 is interposed between the AC source 5 and the transmission coil 6. A tuning capacitor 45 is connected in parallel to the transmission coil 6. Further, a tuning capacitor 44 is connected in parallel to the receiving coil 7.
[0045]
A signal synthesizer 32 is interposed between the variable resistor 8 of the receiving coil 7 and the pair of synchronous detectors 9a and 9b. A PN (pseudo noise) signal generation circuit 31 is connected to each signal synthesizer 30 and 32.
[0046]
The PN signal generation circuit 31 includes N registers connected in series and at least one EXOR (exclusive OR) gate, and (2^NA PN (pseudo noise) signal d having a bit period of 1) is generated and applied to each signal synthesizer 30 and 32.
[0047]
The signal synthesizer 30 outputs the frequency f output from the AC source 5._SAnd the PN signal d inputted from the PN signal generator 31 are combined. Thus, the frequency f_SThe excitation signal “a” having the above is frequency-spread by being synthesized with the PN signal “d”. An excitation signal a that is frequency-spread by the PN signal d is a new excitation signal a.₁Applied to the transmission coil 6.
Therefore, the signal synthesizer 30 and the PN signal generator 31 constitute frequency spreading means.
[0048]
The output signal b of the receiving coil 7 is output from the variable terminal of the variable resistor 8 connected between the output terminals of the receiving coil 7 which is differentially connected so that the winding directions of the pair of coils 7a and 7b are opposite to each other. The signal is taken out and input to the signal synthesizer 32.
[0049]
The signal synthesizer 32 synthesizes the frequency-spread output signal b output from the variable terminal of the variable resistor 8 of the receiving coil 7 and the PN signal d input from the PN signal generator 31. Thus, the output signal b that has already been frequency-spread with the PN signal d is synthesized with the same PN signal d, so that the original frequency f_SFrequency despread. The output signal b that has been frequency despread with the PN signal d is the new output signal b₁Applied to the first and second synchronous detection circuits 9a and 9b.
[0050]
The exciting current a output from the AC source 5 is applied to the signal synthesizer 30 and also to the first synchronous detector 9a. Further, the excitation current a output from the AC source 5 is phase-shifted by 90 ° by the 90 ° phase-shift circuit 10 to generate a new excitation signal a.₂Applied to the second synchronous detector 9b.
[0051]
The first synchronous detector 9a outputs the output signal b of the signal synthesizer 32.₁Is synchronously detected by the excitation signal a. The output signal c of the first synchronous detector 9a₁ After the noise component is removed by the BPF 11a and amplified by the amplifier 12a, it is converted to digital data by the A / D converter 34a and input to the control unit 35 comprising, for example, a microcomputer.
[0052]
The second synchronous detector 9b outputs the output signal b of the signal synthesizer 32.₁The excitation signal a₂Synchronous detection with. The output signal c of the second synchronous detector 9a₂  The sound component is removed by the BPF 11b, amplified by the amplifier 12b, converted into digital data by the A / D converter 34b, and input to the control unit 35 described above.
[0053]
As described above, the signal synthesizer 32 and the PN signal generator 31 constitute frequency despreading means for the signal component included in the output signal b of the receiving coil 7.
[0054]
The passing frequency of each BPF 11a, 11b is the frequency f of the excitation signal a._SIs set to Therefore, these 11a and 11b are the output signal b after the frequency is despread.₁The noise frequency component other than the frequency component of the excitation signal a contained in is removed, and a new output signal c₁, C₂To the amplifiers 12a and 12b.
[0055]
A control unit 35 composed of a microcomputer incorporates an operation panel 4 exposed on the outer surface of the frame 3 shown in FIG. The control unit 35 outputs the output signal c input from the A / D converter 34a.₁Is the X axis, and the output signal c input from the A / D converter 34b₂The Lissajous waveform 15 shown in FIG. Then, it is checked whether or not the deformed portion 16 exists in the created Lissajous waveform 15, and when the deformed portion 16 exceeding the predetermined allowable limit is detected, a metal detection warning is displayed on the display unit 26, and the printer Printed out at 37.
[0056]
In the metal detector according to the first embodiment configured as described above, the excitation signal a applied to the transmission coil 6 as described with reference to FIG.₁The signal synthesizer 30 performs frequency spreading with the PN signal d. Therefore, the magnetic field generated by the transmission coil 6 is also spread in frequency. A noise magnetic field is superimposed on the frequency-spread magnetic field from the outside. A current corresponding to the magnetic field on which the noise magnetic field is superimposed and the frequency is spread flows through the receiving coil 7. Therefore, the output signal b output from the variable terminal of the variable resistor 8 of the reception coil 7 includes a signal component from the transmission coil 6 subjected to frequency spreading and a noise component not subjected to frequency spreading.
[0057]
Therefore, if the signal synthesizer 32 again synthesizes the output signal b of this receiving coil with the PN signal d, the signal component is despread and becomes a signal having a high signal level in the original narrow band. On the contrary, the signal is frequency-spread to be a wideband signal having a low signal level.
[0058]
Therefore, the pass frequency is the frequency f of the excitation signal a._SIf only the narrow band high signal level is extracted using the narrow band BPFs 11a and 11b set to, most of the broadband noise components are removed, and the output signal b input to the amplifiers 12a and 12b.₂The S / N ratio is greatly improved.
[0059]
Therefore, the detection accuracy of the metal piece contained in the inspected object 1 of this metal detector can be greatly improved.
[0060]
Further, in the metal detector of the first embodiment, the PN signal generation circuit 31 is used as the frequency spreading means and the frequency despreading means. Since the PN signal generation circuit 31 can change the pattern of the PN signal generated simply, a plurality of metal detectors having different PN signal patterns can be arranged adjacent to each other. Therefore, the inspection processing efficiency of the product inspection line in the food manufacturing factory can be improved.
[0061]
(Second Embodiment)
FIG. 3 is a block diagram showing a schematic configuration of a metal detector according to the second embodiment of the present invention. The same parts as those of the metal detector of the first embodiment shown in FIG. 1 are denoted by the same reference numerals, and detailed description of the overlapping parts is omitted. The external view of this metal detector is the same as the external view shown in FIG.
[0062]
In the metal detector of the second embodiment, the excitation signal a frequency-spread by the signal synthesizer 30 using the PN signal d.₁Is applied to the transmission coil 6 and input to the first synchronous detector 38a. Further, this frequency spread excitation signal a₁Is shifted by 90 ° by the 90 ° phase shifter 10 and a new excitation signal a₂Is input to the second synchronous detector 38b.
[0063]
Since the first and second synchronous detectors 38a and 38b are formed of a kind of signal synthesizer, each of the output signals b output from the variable terminal of the variable resistor 8 of the receiving coil 7 and frequency-spread. Excitation signal a₁, A₂Are combined to detect the output signal b synchronously and despread the frequency. Therefore, in the first and second synchronous detectors 38a and 38b, the noise component caused by the noise magnetic field included in the output signal b of the receiving coil 7 is frequency-spread to obtain a wideband signal having a low signal level. Become.
[0064]
The output signal c output from the first and second synchronous detectors 38a and 38b.₁, C₂In the next BPF 39a, 39b, noise components having a wide frequency band other than the frequency component of the excitation signal a are removed. Each output signal c from which noise components have been removed by each BPF 39a, 39b₁, C₂Are amplified by the amplifiers 12a and 12b, converted into digital data by the A / D converters 34a and 34b, and then input to the control unit 35 comprising a microcomputer.
[0065]
Also in the metal detector of the second embodiment configured as described above, the noise component due to the noise magnetic field included in the output signal b of the receiving coil 7 is greatly suppressed, so the first embodiment shown in FIG. The same effects as the metal detector of the form are exhibited.
[0066]
Furthermore, in the metal detector according to the second embodiment, since the inverse frequency conversion is performed by the synchronous detector, the configuration can be simplified.
[0067]
(Third embodiment)
FIG. 4 is a block diagram showing an essential part of a metal detector according to the third embodiment of the present invention. The same parts as those of the metal detector of the second embodiment shown in FIG. 3 are denoted by the same reference numerals, and detailed description of the overlapping parts is omitted.
[0068]
  Metal detector of this third embodimentIn the AC sourceA random number generator 40 is employed, and a VCO (voltage controlled oscillator) 41 is employed as a frequency spreading means for spreading the frequency of the excitation signal applied to the transmission coil 6.
[0069]
  The random number generator 40 generates random numbers from 1 to N, for example, at a constant cycle. Specifically, the voltage value changes corresponding to the random number value.Random number signalSend to the next VCO 41. The VCO 41 has a frequency [f that changes corresponding to each voltage value of the input random number signal.₁, F₂... f_NExcitation signal a having₁Is output. Excitation signal a output from the VCO 41₁Is applied to the transmission coil 6.
[0070]
Therefore, the excitation signal a applied to the transmission coil 6₁Becomes a frequency-spread signal. Frequency-spread excitation signal a output from the VCO 41₁Is applied to the transmission coil 6 and input to the first synchronous detection circuit 38a. Further, this frequency spread excitation signal a₁Is shifted by 90 ° by the 90 ° phase shifter 10 and a new excitation signal a₂Is input to the second synchronous detector 38b.
[0071]
Also in the metal detector of the third embodiment configured as described above, an excitation signal a applied to the transmission coil 6.₁Since the output signal b of the receiving coil 7 is frequency despread by the respective synchronous detectors 38a and 38b, the noise component due to the noise magnetic field contained in the output signal b of the receiving coil 7 is greatly suppressed. Therefore, the same effects as the metal detector of the second embodiment shown in FIG.
[0072]
(Fourth embodiment)
FIG. 5 is a block diagram showing an essential part of a metal detector according to the third embodiment of the present invention. The same parts as those of the metal detector of the second embodiment shown in FIG. 3 are denoted by the same reference numerals, and detailed description of the overlapping parts is omitted.
[0073]
  In the metal detector of the fourth embodimentIn the exchange sourceA ramp waveform generation circuit 42 is employed, and a VCO (voltage controlled oscillator) 41 is employed as a frequency spreading means for spreading the frequency of the excitation signal applied to the transmission coil 6.
[0074]
  As shown in the figure, the ramp waveform generation circuit 42 has a constant period T₁The voltage value changes in a sawtooth shapeRamp waveform signal eOutput. The ramp waveform signal e output from the ramp waveform generation circuit 42 is input to the next VCO 41. The VCO 41 has an excitation signal a whose frequency changes in a sawtooth shape corresponding to each voltage value of a random number signal whose input voltage value changes in a sawtooth shape.₁Is output. Excitation signal a output from the VCO 41₁Is applied to the transmission coil 6.
[0075]
Therefore, the excitation signal a applied to the transmission coil 6₁Since the signal becomes a frequency-spread signal, it is possible to achieve substantially the same operation and effect as the metal detector of the third embodiment shown in FIG.
[0076]
(Fifth embodiment)
FIG. 6 is a block diagram showing a schematic configuration of a metal detector according to the fifth embodiment of the present invention. The same parts as those of the metal detector of the first embodiment shown in FIG. 1 are denoted by the same reference numerals, and detailed description of the overlapping parts is omitted. The external view of this metal detector is the same as the external view shown in FIG.
[0077]
In the metal detector of the fifth embodiment, a stagger tuning circuit 53 composed of a parallel circuit of a capacitor and a coil is provided adjacent to the transmission coil 6. This stagger tuning circuit 53 has a function of relaxing the tuning characteristics of the transmission coil 6.
[0078]
Further, grounding resistors 51 and 52 for relaxing the tuning characteristics of the receiving coil 7 are interposed between both ends of the receiving coil 7 and the ground. Accordingly, the stagger tuning circuit 53 and the ground resistors 51 and 52 constitute a tuning relaxation circuit that relaxes the tuning characteristics of the transmission coil 6 and the reception coil 7. Other configurations are almost the same as those of the metal detector of the first embodiment shown in FIG.
[0079]
Next, the reason why the tuning characteristic in the synthesized tuning circuit including the coils 6 and 7 is relaxed using this tuning relaxation circuit will be described with reference to FIG.
FIG. 7A is a frequency characteristic diagram of the output signal b in the tuning circuit in which the transmission coil 6 and the reception coil 7 are synthesized in a state where the tuning relaxation circuit is not incorporated. As shown, the amplitude characteristic is the resonance frequency f of the tuning circuit.₀A sharp peak waveform is shown in the vicinity. On the other hand, the phase characteristic is the resonance frequency f of the tuning circuit.₀The phase suddenly reverses 180 °. Thus, the resonance frequency f₀Since an output signal b having a large signal level can be obtained by using a tuning circuit having a sharp peak waveform and a sharp phase inversion characteristic in the vicinity, the S / N ratio of the output signal b can be improved, and the metal piece Detection accuracy can be improved.
[0080]
However, the resonant frequency f of this tuning circuit₀Changes slightly as the ambient temperature R changes. Resonant frequency f of tuning circuit₀Changes, the frequency position where the phase of the output signal b changes suddenly changes. As a result, as shown in FIG. 7C, the inclination angle of the Lissajous waveform 15 calculated by the control unit 35 with respect to the X axis and the Y axis greatly changes. As a result, it is not possible to accurately detect a delicate deformed portion caused by the metal piece included in the device under test 1.
[0081]
Therefore, in the metal detector of the fifth embodiment, the amplitude of the output signal b in the tuning circuit is provided as shown in FIG. 7B by providing the transmission coil 6 and the reception coil 7 with the above-described tuning relaxation circuit. The characteristic is the resonance frequency f of the tuning circuit.₀Although the signal level including is low, it shows a wide and gentle waveform. On the other hand, the phase characteristic is the resonance frequency f of the tuning circuit.₀The phase is gradually reversed in the vicinity.
[0082]
Thus, the resonance frequency f₀In the case where the phase is gradually reversed in the vicinity, even if the ambient temperature R changes, the resonance frequency f₀Even if has changed, the inclination angle of the Lissajous waveform 15 calculated by the control unit 35 shown in FIG. 7C with respect to the X axis and the Y axis does not change greatly. As a result, it is possible to accurately detect a delicate deformed portion caused by the metal piece included in the device under test 1.
[0083]
Although the signal level at the time of tuning of the output signal b decreases, the S / N ratio of the output signal b becomes sufficiently high through the frequency spreading and frequency despreading steps described above, so that the metal detector as a whole The S / N ratio does not decrease. Therefore, the detection precision of the metal piece contained in the to-be-inspected object 1 as the whole metal detector can be improved significantly.
[0084]
【The invention's effect】
As described above, in the metal detector of the present invention, the excitation signal applied to the transmission coil is frequency-spread and the output signal of the reception coil is frequency-spread.
[0085]
Therefore, the noise mixed between the transmission coil and the reception coil can be greatly suppressed from the signal received by the reception coil, and the S / N ratio of the signal received by the reception coil can be greatly improved. The detection accuracy of the metal piece with respect to the object to be inspected can be greatly improved.
[Brief description of the drawings]
FIG. 1 is a block diagram showing a schematic configuration of a metal detector according to a first embodiment of the present invention.
FIG. 2 is a diagram for explaining the operation principle of the present invention.
FIG. 3 is a block diagram showing a schematic configuration of a metal detector according to a second embodiment of the present invention.
FIG. 4 is a block diagram showing a schematic configuration of a metal detector according to a third embodiment of the present invention.
FIG. 5 is a block diagram showing a schematic configuration of a metal detector according to a fourth embodiment of the present invention.
FIG. 6 is a block diagram showing a schematic configuration of a metal detector according to a fifth embodiment of the present invention.
FIG. 7 is a view for explaining the operation principle of the metal detector according to the fifth embodiment;
FIG. 8 is an external view showing a general metal detector.
FIG. 9 is a block diagram showing a schematic configuration of a conventional metal detector.
FIG. 10 is a diagram showing the order in which Lissajous waveforms are created in a metal detector
FIG. 11 is a diagram showing a Lissajous waveform measured by a conventional metal detector.
[Explanation of symbols]
1 ... Inspection object
2 ... Belt conveyor
4. Operation panel
5 ... AC source
6 ... Transmitting coil
7 ... Receiving coil
8 ... Variable resistance
9a, 38a ... first synchronous detector
9a, 38b ... 1st synchronous detector
10 ... 90 ° phase shifter
11a, 11b, 39a, 39b ... BPF
12a, 12b ... amplifier
15 ... Lissajous waveform
16. Deformation part
30, 32 ... Signal synthesizer
31 ... PN signal generation circuit
33 ... BPF
34a, 34b ... A / D converter
35. Control unit
40 ... Random number generator
41 ... VCO
42. Ramp signal generation circuit
51, 52 ... Grounding resistance
53 ... Staga tuning circuit

Claims (3)

A transmission coil (6) to which an alternating excitation signal is applied and a differentially connected reception coil (7) are disposed along the conveyance path of the object to be inspected (1), and an output signal of the reception coil is provided. In a metal detector for detecting a metal piece contained in the object to be inspected based on:
A random number generator (40) for generating a random number at a constant period and outputting a random number signal whose voltage value changes in accordance with the value of the random number;
A voltage controlled oscillator (41) for outputting an excitation signal (a ₁ ) having a frequency that changes in accordance with each voltage value of the random number signal output from the random number generator and applying the excitation signal (a ₁ ) to the transmission coil (6);
A 90 ° phase shifter (10) for shifting the phase of the excitation signal (a ₁ ) frequency-spread by the voltage controlled oscillator by 90 °;
A first synchronous detector (38a) for performing synchronous detection and frequency despreading on the output signal (b) of the receiving coil with an excitation signal (a ₁ ) frequency-spread by the voltage controlled oscillator ;
A second synchronous detector (38b) that performs synchronous detection and frequency despreading on the output signal (b) of the receiving coil with the excitation signal (a ₂ ) phase-shifted by 90 ° by the 90 ° phase shifter. )When,
A pair of band-pass filters that respectively remove frequency components other than the frequency components of the excitation signal included in each output signal (c ₁ , c ₂ ) after frequency despreading by the first and second synchronous detectors (39a, 39b). A metal detector. A transmission coil (6) to which an alternating excitation signal is applied and a differentially connected reception coil (7) are disposed along the conveyance path of the object to be inspected (1), and an output signal of the reception coil is provided. In a metal detector for detecting a metal piece contained in the object to be inspected based on:
A ramp waveform generating circuit (42) for outputting a ramp waveform signal whose voltage value changes in a sawtooth shape at a constant cycle;
A voltage controlled oscillator (41) that outputs an excitation signal (a ₁ ) having a frequency that changes in accordance with each voltage value of the ramp waveform signal output from the ramp waveform generation circuit and applies the excitation signal (a ₁ ) to the transmission coil (6). When,
A 90 ° phase shifter (10) for shifting the phase of the excitation signal (a ₁ ) frequency-spread by the voltage controlled oscillator by 90 °;
A first synchronous detector (38a) for performing synchronous detection and frequency despreading on the output signal (b) of the receiving coil with an excitation signal (a ₁ ) frequency-spread by the voltage controlled oscillator ;
A second synchronous detector (38b) that performs synchronous detection and frequency despreading on the output signal (b) of the receiving coil with the excitation signal (a ₂ ) phase-shifted by 90 ° by the 90 ° phase shifter. )When,
A pair of band-pass filters that respectively remove frequency components other than the frequency components of the excitation signal included in each output signal (c ₁ , c ₂ ) after frequency despreading by the first and second synchronous detectors (39a, 39b). A metal detector. 3. The metal according to claim 1, wherein a tuning relaxation circuit (51, 52, 53) for relaxing tuning characteristics is provided for the transmission coil (6) and the reception coil (7). Detector.

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