JPH0619467B2 - Metal detector - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Field of Use of the Present Invention> The present invention relates to a metal detection device for detecting whether or not a metal is contained in a test object packaged with a conductive metal foil. In particular, the present invention relates to a metal detection device that is not affected by low frequency noise.

<Prior Art> Conventionally, the magnetic field generated by the magnetic field generating means is passed through the object to be inspected, and the metal in the object to be inspected is unbalanced by the unbalanced outputs of the two magnetic field detecting means by the metal in the object to be inspected. Metal detection devices for detecting are well known. It is known that in this type of metal detection device, in the case of a non-ferrous metal, the detection sensitivity is good in an AC magnetic field in which the directions of magnetic force lines are alternately inverted, and the detection sensitivity is lowered in a low frequency magnetic field or a DC magnetic field. Has been.

For this reason, in order to detect a metal in an object to be inspected that is wrapped with a non-ferrous metal such as aluminum such as chocolate, if an alternating magnetic field in which the directions of magnetic lines of force are alternately reversed is used, it is The equilibrium output becomes large, and it becomes difficult to detect minute metals in the inspection object.

For this reason, conventionally, a metal detection device for detecting a metal contained in an object to be inspected packaged with a metal foil of a non-ferrous metal such as aluminum has a DC magnetic field that is least affected by the packaging metal. Since the detection sensitivity of iron contained in the object to be inspected is maximized, as shown in (a) of FIG. 2, a direct current magnetic field in which the directions of magnetic force lines are always the same is used, and the configuration as shown in FIG. Has become.

That is, the DC power source 1 is connected to the transmission coil 2 which is the magnetic field generating means, and the transmission coil has a DC line in which the direction of the magnetic field lines is always constant and the magnetic field strength is the same as shown in FIG. 2 (a). Generate a magnetic field.

The first and second receiving coils 3a and 3b are arranged to face the transmitting coil 2. The first and second receiving coils 3a and 3b are positioned so that the magnetic lines of force of the DC magnetic field generated by the transmitting coil 2 intersect in equal amounts.

The first and second receiving coils 3a and 3b are reversely connected to form a differential circuit that outputs unbalanced signals of both coils.

Since the DC magnetic field is used in this way, the DC amplifier 4 is used to amplify the unbalanced signal. The DC amplifier 4 is connected to a low-frequency band-pass filter 5 that passes a low-frequency unbalanced output signal that is determined by the speed at which the metal in the inspection object W passes, and the level of the unbalanced signal is used as a reference. A comparator 6 that outputs a metal detection signal in comparison with the reference voltage of the voltage generator 7 is connected.

Next, the principle of metal detection by the conventional metal detection device having such a configuration will be described.

The transmitter coil 2 generates a DC magnetic field as shown in FIG. The device under test W, which is wrapped with aluminum or other metal foil, extends between the transmission coil 2 and the first and second reception coils 3a and 3b in the direction from the first reception coil 3a to the second reception coil 3b. Then, it is conveyed by a conveying device (not shown) at a predetermined speed.

At this time, the transmitter coil 2 and the first and second receiver coils 3
When no metal is mixed in the object W to be inspected that passes between the first and second receiving coils 3a and 3b.
There is no change in the magnetic field lines intersecting with and no unbalanced signal is output, but when metal is mixed, the magnetic field lines change and an unbalanced signal is generated.

When the metal mixed in the object to be inspected is iron, the lines of magnetic force intersecting with the receiving coil increase and pass through the center of the first receiving coil 3a, as shown by the part (a) in FIG. The induced voltage E ₁ of the first receiving coil 3a becomes maximum when
As shown in the part (a) of FIG. 2C, when passing through the center of the second receiving coil 3b, the second receiving coil 3b
The induced voltage E _{2 of} is maximum.

Therefore, when iron is mixed, as shown in the part (a) of FIG. 2D, when passing through the center of the first receiving coil 3a, E ₁ −E ₂ = e, When passing through the center of the coil 3b, a low-frequency unbalanced signal, which is determined by the passing speed of the device under test, is E ₁ −E ₂ = −e is output to the DC amplifier 4.

This unbalanced signal is amplified by the DC amplifier 4, passed through the low-frequency bandpass filter 5, and then compared with the reference voltage supplied by the reference voltage generator 7. A metal detection signal is output from the comparator 6.

Even in an object to be inspected packaged with a non-ferrous metal such as aluminum foil, the metal in the object to be inspected can be detected by using a DC magnetic field whose magnetic lines always have the same direction as shown in (a) of FIG. It

<Problems to be Solved by the Present Invention> However, the conventional metal detecting device using the DC magnetic field having the configuration shown in FIG. 1 has the following problems.

That is, in this metal detection device, since the unbalanced signal is amplified by the DC amplifier using the DC magnetic field, the noise peculiar to the DC amplifier 4 contains many low frequency components which are the same as or lower than the unbalanced signal detected by the metal. . Therefore, the noise component from such a DC amplifier is mixed in the output signal of the bandpass filter 5, the S / N ratio is deteriorated, and the metal detection sensitivity is limited.

Also, when there is a motor, solenoid valve, radio wave transformer, etc. in the surroundings, or when other metal around them moves,
The noise components due to these are (b) and (c) in FIG.
The first and second receiving coils 3 as shown in the part B of FIG.
a, 3b, and therefore two receiving coils 3
As the unbalanced signal outputs of a and 3b, unbalanced noise as shown in the part (b) of FIG. 2 may be output. The frequency component of this unbalanced noise is also a low frequency that is similar to the unbalanced signal for metal detection shown in part (d) of FIG. Such a noise component is mixed in, the detection sensitivity is deteriorated, and this causes a malfunction.

As a method for solving these problems, conventionally, a method using a low-noise, low-drift DC amplifier or a band-pass filter having a narrow frequency band of 7 to 15 Hz (portion in FIG. 3a) as shown in FIG. Although a method using a filter has been used, a method using a DC amplifier with low noise and low drift has a drawback that it is very expensive, and a method using a bandpass filter has a drawback that a frequency component of a noise signal is metallic. Since it is similar to the detected signal (portion in FIG. 3b), it was difficult to completely remove noise even through the bandpass filter.

SUMMARY OF THE INVENTION It is an object of the present invention to solve the above-mentioned problems and to provide a metal detecting device which can detect a metal without being affected by noise as described above even when a DC magnetic field is used, and which is not expensive. I am trying.

<Means for Solving the Problems> In order to solve the problems, in the metal detection device of the present invention, a direct-current magnetic field is generated from the magnetic field generating means, and the direct-current magnetic field is received by the receiving coil to be inspected. In the metal detection device for detecting mixed metal, the magnetic field generating means is provided with an oscillator, the strength of the DC magnetic field is changed at a constant cycle by an output signal output from the oscillator, The receiving coil is arranged so as to receive the magnetic field generated by the magnetic field generating means equally, and is composed of first and second receiving coils that are differentially connected, and the unbalance detected by the first and second receiving coils. An AC amplifier that receives a signal and amplifies a signal having a predetermined frequency or higher, and an unbalanced signal amplified by the AC amplifier based on an output signal output from the oscillator, A synchronous detector that performs synchronous detection in accordance with a change in the DC magnetic field generated from the magnetic field generating means, from the output signal of the synchronous detector, a filter that passes only a low frequency band determined by the passing speed of the DUT, And a comparator that outputs a metal detection signal when the voltage level of the output signal of the filter is equal to or higher than the reference level.

<Operation> In the metal detection device of the present invention having such a configuration, the strength of the DC magnetic field generated from the magnetic field generation device changes in a constant cycle, so the unbalanced signal detected by the receiving coil changes in a constant cycle. The low frequency signal due to the influence of metal is superimposed on the high frequency signal.

This unbalanced signal is amplified by an AC amplifier and then synchronously detected in accordance with the change in the DC magnetic field. Then, only the output generated by the metal is taken out through the filter that passes only the low frequency band determined by the passing speed of the inspection object.
The voltage level of this output is compared with the reference voltage by the comparator, and if it is equal to or higher than the reference voltage, the metal detection signal is output.

On the other hand, when the output of the receiving coil fluctuates due to noise, not due to the metal in the object to be inspected, the unbalanced signal caused by noise is a high-frequency component due to the change in DC magnetic field because it is irrelevant to the change in the DC magnetic field. Since it does not include
No metal detection signal is output.

<Examples of the Present Invention> Examples of the present invention will be described below with reference to the drawings. In the embodiments described below, the same parts as those of the conventional device shown in FIG. 1 are designated by the same reference numerals, and the details thereof will be omitted.

FIG. 4 shows the structure of the metal detecting device according to the first embodiment of the present invention.

In the figure, a DC power supply 1, a transmitting coil 2 as magnetic field generating means, first and second receiving coils 3a and 3b, a comparator 6, and a reference voltage generator 7 are the same as those of the conventional device.
The second bandpass filter 5 is also the same as the bandpass filter 5 of the low frequency band determined by the passing speed of the device under test of the conventional apparatus shown in FIG.

The terminal of the transmission coil 2 is connected to the DC power supply 1 via a relay switch 8 driven by a rectangular wave oscillator 9. The output of the rectangular wave oscillator 9 is the synchronous detector 1
It is also used as a 0 sync signal. The oscillating frequency of the rectangular wave oscillator 9 is set to be much higher than the frequency determined by the speed at which the metal in the inspection object W passes.

The first and second receiving coils 3a and 3b are differentially connected,
An unbalanced signal output means for generating an unbalanced output of both coils is configured.

The AC amplifier 41 is a high frequency amplifier capable of amplifying a signal having a predetermined frequency or higher. The first bandpass filter 51 is a relatively high-frequency bandpass filter that passes a signal in the same frequency band as the oscillation frequency of the rectangular wave oscillator 9. The second bandpass filter 5 has a frequency of 7 to 15 Hz, which is determined by the speed at which the metal in the inspection object W passes, as in the case of FIG.
It is a low-frequency bandpass filter that passes low-frequency signals. The synchronous detector 10 is a detector that simultaneously detects the output signal of the rectangular wave oscillator 9 and detects the input signal of the synchronous detector 10 only while the output voltage of the oscillator 9 is equal to or higher than a certain voltage. Is.

Next, the operation of the metal detection device of the first embodiment will be described.

The inspection object W is passed at a predetermined speed between the transmission coil 2 and the first and second reception coils 3a and 3b.

From the transmission coil 2, by opening and closing the switch 8 by the rectangular wave oscillator 9 as described above, the direction of the magnetic force lines is always constant and the strength of the magnetic field is intermittent at a constant cycle, as shown in FIG. DC magnetic field is generated. The cycle of this change is set to a short cycle that is much higher than the frequency determined by the speed at which the metal in the inspection object W passes. Therefore, the first and second receiving coils 3a,
The output due to the induced voltage from 3b changes in this constant cycle.

Therefore, when the iron-based metal is mixed in the inspection object, the induced voltage E ₁ of the first reception coil 3a is positive when the metal in the inspection object W passes through the first reception coil 3a. Of the second receiving coil 3 in the direction of
induced voltage E ₂ of the second receiving coil 3b when passing through b
Changes in the negative direction. This change occurs only while the non-zero constant voltage is applied to the transmission coil 2, and therefore the induced voltage E of the first and second reception coils 3a and 3b is generated.
_As shown in (b) and (c) of FIG. 5, the signal waveforms of ₁ and E ₂ are high-frequency signals of the same frequency as the signal applied to the transmission coil 2, and the metal in the inspection object W is , The waveform is amplitude-modulated by a low-frequency signal induced when passing through the second receiving coils 3a and 3b.

As a result, the output of the unbalanced signal output means has a difference waveform between the waveforms shown in FIG. 5B and FIG. 5C, as shown in FIG. 5D. An unbalanced signal that becomes zero at a constant period is output.

This unbalanced output signal, after being amplified by the AC amplifier 41, passes through a relatively high-frequency first bandpass filter 51 capable of passing a signal in a frequency band substantially the same as the frequency of the rectangular wave oscillator 9, It becomes an amplified signal as shown in FIG.

The output signal of the first bandpass filter 51 is synchronously detected by the synchronous detector 10 which is synchronized with the output voltage of the rectangular wave oscillator 9 having the same cycle as the applied voltage of the transmission coil 2 shown in FIG. , And is converted into a signal as shown in FIG.

The synchronously detected signal is passed through a second band-pass filter 5 having a low frequency capable of passing only a low-frequency component of a frequency band determined by the passing speed of the object to be inspected. The component is removed, and as shown in FIG. 5 (g), it is converted into a low frequency signal due to the metal in the object to be inspected. When this signal level is higher than the reference voltage in the comparator 6, the metal detection signal is detected It is output from 6.

Next, the operation of the metal detecting device according to the first embodiment as described above when the low frequency noise as described above is mixed in the receiving coil will be described with reference to FIG.

As shown in FIG. 6 (h), low-frequency noise having substantially the same frequency and amplitude as the signal generated by the metal in the object W to be inspected is mixed into the first and second receiving coils 3a and 3b. The case will be described.

The voltage waveform of the first receiving coil 3a when noise is mixed is shown in FIG. 6 (b), and the voltage waveform of the second receiving coil 3b is shown in FIG. 6 (c). That is, (b) of FIG.
In (c), the portion a is the first and second receiving coils 3
When low-frequency noise as shown in (h) of FIG. 6 is mixed in both a and 3b, the part of B is the first receiving coil 3
The waveform when only low-frequency noise as shown in (h) of FIG. 6 is mixed in only a is shown.

As shown in (b) and (c) of FIG. 5, unlike the case where the low-frequency signal from the metal in the object W to be inspected is superimposed so as to amplitude-modulate the high-frequency signal, that is, the magnetic field is zero. In contrast to the fact that there is no change in the part of, when noise is directly mixed in the first and second receiving coils 3a and 3b in this way,
Since the output voltages of the second receiving coils 3a and 3b directly fluctuate in level due to noise, the first and second receiving coils 3
The voltage waveforms of a and 3b are, as shown in FIGS. 6 (b) and (c), the sum of the high frequency rectangular pulse signal shown in FIG. 6 (a) and the noise shown in FIG. 6 (h). It becomes a waveform.

Therefore, the output waveform of the unbalanced signal when such noise is mixed has a waveform as shown in FIG.

That is, when the same noise is mixed in the first and second receiving coils 3a and 3b in the same phase at the same time, as shown in (b) and (c) of FIG. As shown in the part (a) of d), the outputs of both coils cancel each other out to become a signal with neither periodic fluctuation due to interruption nor fluctuation due to noise.

As shown in (b) and (c) of FIG. 6, noise mixed in one of the first and second receiving coils 3a and 3b is the noise of (d) of FIG. As shown in the part (3), the periodic fluctuations due to the intermittent output of both coils cancel each other out, and only the low-frequency noise component is output as unbalanced noise.

However, the low-frequency noise signal in the part of (b) in FIG. 6D is generated by the AC amplifier 41, the first band-pass filter 51, and the synchronous detector 10 in the same frequency band as the oscillation frequency of the oscillator 9. To be removed.

Therefore, even when noise is mixed in both the first and second receiving coils 3a and 3b in the same phase, or when only one of the receiving coils is mixed, (e) in FIG.
As shown in (f) and (g), since no output due to noise occurs, the comparator 6 does not output an erroneous metal signal due to noise.

Since the noise component is removed by the synchronous detector 10 as described above, the first bandpass filter 51 may be omitted.

As described above, in this metal detection device, when the object W to be inspected containing a metal passes, the unbalanced signal generated due to this passage is generated by the transmission coil 2 as shown in (d) of FIG. The low frequency signal due to the passage of the metal is superimposed and output on the high frequency signal applied to the first and second signals.
When noise is directly mixed into the receiving coils 3a and 3b, a low frequency signal which is not superimposed on the high frequency signal is output as shown in the part (b) of FIG. 6 (d).

Therefore, by distinguishing the low-frequency signal and the simple low-frequency signal thus superimposed on the high-frequency signal by the first bandpass filter 51, the noise and the unbalanced signal due to the metal can be reliably identified. Be done. Therefore, a small amount of metal in an object wrapped with aluminum or other metal foil that can be detected only in a DC magnetic field can be detected without being affected by noise by changing this DC magnetic field at a constant cycle, The detection sensitivity and S / N ratio are also significantly improved.

FIG. 7 is a block diagram showing the circuit configuration of the second embodiment of the present invention.

In this embodiment, in order to periodically change the DC magnetic field of the transmission coil 2, the output signal of the oscillator 91 is amplified by the first amplifier 42 and then directly applied to the transmission coil 2. As the oscillator 91, various oscillators that output a waveform as shown in FIG. 8 are used. However, other than the oscillator shown in FIG. 8, any oscillator that outputs a waveform that periodically changes with the same polarity is also used. be able to. Note that, in FIG. 7, the output of the oscillator 91 is used as a synchronizing signal of the synchronous detector 10 as in the case of FIG. 4, but unlike the case of FIG. Since it has various waveforms as shown in FIG. 8, it is supplied to the synchronous detector 10 after passing through the waveform shaper 11.

The above embodiment is a case where the transmitting coil 2 and the first and second receiving coils 3a and 3b are used for transmitting and receiving the magnetic field, but the transmitting coil 2 or the first and second receiving coils 3a, Other circuit elements may be used instead of 3b. FIG. 9 is a block diagram showing a circuit configuration of a third embodiment of the present invention. Instead of the first and second receiving coils 3a and 3b in FIG. 4, first and second magnetoresistive elements or holes are provided. The elements 13a and 13b are used. The common terminals of the first and second magnetoresistive elements or Hall elements 13a and 13b are grounded via the DC power supply 14. In addition,
Each of these elements 13a and 13b may be composed of a plurality of elements.

FIG. 10 is a block diagram showing the circuit configuration of the fourth embodiment of the present invention, in which an electromagnet 15 is used instead of the transmission coil 2 in FIG. The electromagnet 15 is superposed and excited by the output signal of the oscillator 91 which is obtained by winding an exciting coil around the permanent magnet and amplified by the first amplifier 42.

Thus, in both the third and fourth embodiments, the first and second receiving coils 3a of the first and second embodiments of the present invention,
3b or transmitter coil 2 instead of first and second respectively
The structure is the same as that of the first and second embodiments except that the magnetoresistive elements or Hall elements 13a and 13b or the electromagnet 15 are used, and the same effects as those of the first and second embodiments are obtained.

<Effects of the Present Invention> In the metal detection device of the present invention, the magnetic field generation means includes an oscillator, and the strength of the DC magnetic field is changed at a constant cycle by the output signal output from the oscillator. The coil is arranged so as to receive the magnetic field generated by the magnetic field generating means equally, and is composed of first and second receiving coils that are differentially connected, and receives the unbalanced signal detected by the first and second receiving coils. Then, an unbalanced signal signal amplified by the alternating current amplifier based on an output signal output from the oscillator and an alternating current amplifier that amplifies a signal of a predetermined frequency or more, and a DC magnetic field generated from the magnetic field generating means. A synchronous detector that performs synchronous detection according to changes, a filter that passes only the low frequency band determined by the passing speed of the device under test from the output signal of the synchronous detector, and the output of the filter. With a comparator that outputs a metal detection signal when the voltage level of the force signal is equal to or higher than the reference level, the low-frequency external noise that appears in the unbalanced signal is not output after being cut by the AC amplification and the synchronous detector. Since the unbalanced signal and the noise at the time of metal detection are clearly distinguished, even noise that can be detected only by a DC magnetic field, such as metal in an inspected object packaged with a non-ferrous metal such as aluminum foil, is noisy. It is possible to detect with high accuracy without being affected.

Further, while performing metal detection using a DC magnetic field, an unbalanced signal can be amplified by an AC amplifier without using a DC amplifier as in the conventional case, so that it is not affected by noise peculiar to the DC amplifier. Moreover, since the AC amplifier is significantly cheaper than the AC amplifier, there is an advantage that the metal detection device as a whole can be provided at a low cost.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a conventional metal detector, and FIG.
FIG. 3 is a time chart showing the operation of the figure, FIG. 3 is a diagram showing frequency characteristics of the bandpass filter in FIG. 1, FIG. 4 is a circuit diagram showing a first embodiment of the present invention, FIG. 5 and FIG. Is the fourth
FIG. 5 is a time chart showing the operation of each part of the circuit shown in FIG. 5, where FIG. 5 shows the case of detecting metal and FIG. 6 shows the case of noise. FIG. 7 is a circuit diagram showing a second embodiment of the present invention, and FIG.
The figure is a waveform diagram showing the output signal of the oscillator of FIG. 9 and 10 show the third and fourth aspects of the present invention, respectively.
3 is a circuit diagram showing an embodiment of FIG. 1, 14 ... DC power source, 2 ... Transmitting coil, 3a ... First receiving coil, 3b ... Second receiving coil, 4 ... DC amplifier, 41 ... AC amplifier or second amplifier, 4
2 ... first amplifier, 5 ... bandpass filter or second bandpass filter, 51 ... first bandpass filter, 6 ... comparator,
7 ... Reference voltage generator, 8 ... Relay switch, 9 ...
... rectangular wave oscillator, 91 ... oscillator, 10 ... synchronous detector, 11 ... waveform shaper, 13a ... first magnetoresistive element or Hall element, 13b ... second magnetoresistive element or Hall element, 15 ... Electromagnet.

Claims (1)

[Claims] 1. A magnetic field generating means generates a direct current magnetic field in which the directions of magnetic force lines are always the same, and the direct current magnetic field is received by a magnetic field detector to detect metal mixed in an object to be inspected passing through the direct current magnetic field. In the metal detection device, the magnetic field generating means includes at least an oscillator and a transmission coil, based on an output signal output from the oscillator,
The strength of the DC magnetic field generated from the transmission coil is changed in a constant cycle, and the magnetic field detector is arranged so as to receive the magnetic field generated by the magnetic field generation means equally and is differentially connected. An AC amplifier for receiving an unbalanced signal detected by the first and second magnetic field detectors and amplifying a signal having a predetermined frequency or higher; and an output from the oscillator. A synchronous detector for synchronously detecting the unbalanced signal signal amplified by the AC amplifier according to the change of the DC magnetic field generated from the magnetic field generating means, and the output signal of the synchronous detector. From a filter that passes only a low frequency band determined by the passing speed of the object to be inspected, and a comparison that outputs a metal detection signal when the voltage level of the output signal of the filter is equal to or higher than a reference level. Metal detecting apparatus having and.