JPS6168582A - Metal detector - Google Patents

Abstract

PURPOSE:To enable the detection to meet the requirement of the highest detection sensitivity according to the quality of an object to be inspected, by switching an alternating magnetic field over to a DC magnetic field varying periodically in the intensity of the magnetic field to measure an induced voltage by metal in the object being inspected passing through the magnetic field. CONSTITUTION:When an aluminum foil packed product is run to a line, selector switches 4 and 10 are turned to the position (a) and a transmission coil 5 is driven with a square wave of an oscillator 1 overlapping the voltage of an DC power source 3 to generate a DC magnetic field. For other products, the selector switches 4 and 10 are turned to the position (b) and the coil 5 is driven with a square wave signal alternating from positive to negative from the oscillator 1 to generate an AC magnetic field. When a product is mixed with metal, an output signal changes as an object to be inspected pass through receiving coils 6a and 6b and a differential circuit outputs an unbalance signal. This signal is synchronously detected with a detector 11 by output signals of shifters 9a and 9b via an amplifier 7 and a band filter 8 and compared with a reference voltage by a comparator 13 via a low-pass filter 12 and when it exceeds the reference voltage, a metal detection signal is outputted.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a metal detection signal that detects metal in a test object by passing the test object through a magnetic field and based on changes in the magnetic field caused by the metal in the test object. .

Metal detection using this type of metal detection signal is performed according to the principle shown in FIG. That is, two receiving coils S+ and 82 are arranged opposite to the transmitting coil P, which is supplied with current and generates a magnetic field, so that the lines of magnetic force are equally spaced, and the object W to be inspected is passed through this magnetic field. If no metal is mixed in the test object W, the test object W has no effect on the magnetic field lines, but if metal is mixed, the magnetic field lines change due to the metal, and the two receiving coils S+ , 82 are no longer of equal magnitude. For this reason, the presence of metal causes an induced voltage E+ in both receiving coils S+ and S2.
, E2.

However, with this metal detection signal, the metal detection sensitivity is
It varies significantly depending on the type of magnetic field generated by the transmitting coil, and when using an alternating magnetic field in which the directions of magnetic lines of force alternate in opposite directions, the higher the frequency, the higher the detection sensitivity for nonferrous metals, but if the object to be inspected is conductive. be affected by it.

For this reason, even if an alternating magnetic field is used to detect metal mixed in aluminum-packaged products, it will react strongly to aluminum foil, reducing the ability to detect metal in the product or making it impossible to detect it. Become. On the other hand, in the case of detecting iron, the detection frequency is approximately constant regardless of the frequency of the magnetic W. For this reason, conventionally, metal detection signals that generate a DC magnetic field in which the direction of the magnetic field lines does not change are used to detect metal in products wrapped in aluminum foil, and an alternating magnetic field is used to detect metal in other products. If you want to use different metal detection signals to run aluminum packaged products and non-aluminum packaged products on the same line, install two metal detection signals, one for aluminum foil packaged products and one for other products, and switch between them. I was using it.

This requires a large amount of installation space, requires the installation cost of two units, requires more maintenance, and has the disadvantage of doubling the probability of failure.

It is an object of the present invention to correct the above-mentioned drawbacks and provide a metal detection signal that can be used to detect metals in both products packaged with aluminum foil and other products.

Hereinafter, one embodiment of the present invention will be described based on the drawings.

FIG. 2 is a block diagram showing one embodiment of the present invention.

In the figure, 1 is an oscillator that outputs a rectangular wave signal, 2 is an amplifier, 3 is a DC power supply that is superimposed on the rectangular wave signal from the oscillator 1 via a changeover switch 4, and 5 is an oscillator that is driven by the output signal of the amplifier 2. This is a transmission that generates a magnetic field. Reference numerals 6a and 5h designate first and second receiving coils that are arranged so that the lines of magnetic force of the magnetic field generated by the transmitting coil 5 intersect with each other, and are reversely connected to form a differential circuit.

7 is an amplifier for amplifying the output signal of the differential circuit consisting of receiving coils 6a and 6b, and 8 is a bandpass filter for passing the signal at the oscillation frequency of the oscillator 1.

9a and 19b are phase shifters that change the phase of the output j signal from the oscillator 1, 10 is a changeover switch, and 11 is an output voltage of the phase shifter 9a or 9b in synchronization with the output signal of the phase shifter 9a or 9b. This is a detector that detects the input signal from the bandpass filter 8 only during a period when the voltage is above a certain certain level.

The reason why the phase of the output signal of the oscillator 1 is shifted by the phase shifters 9a and 9b is that the phase shift of the output signal n on the receiving coil side differs depending on the metal to be detected, so the signal is shifted in accordance with this shift. In order to detect metals most accurately through synchronous detection, the phase shifter 9a is set at a phase shift angle φ1 suitable for detecting iron in aluminum foil packaging products, and the phase shifter 9b is set at a phase shift angle φ1 suitable for detecting iron in aluminum foil packaging products. A phase shift angle φ2 suitable for detection is set, and the changeover switches 4 and 10 are switched in conjunction.

12 is a low-pass filter that passes a low-frequency signal determined by the speed at which the inspected object W passes; 13 is a metal detection device that compares the level of the output signal of the low-pass filter 12 with a reference voltage and detects metal when the level of the output signal exceeds the reference voltage. It is a comparative heat output signal.

Next, the operation of the above embodiment will be explained.

When sending products other than aluminum foil packaging products to the line,
Connect the changeover switches 4 and 10 to the b side. When the changeover switch is set to the b side, the DC voltage is not superimposed, so the third
As shown in (A) of the figure, the transmitting coil 5 is driven by a rectangular wave signal that alternates between positive and negative, and generates an alternating magnetic field in which the directions of the lines of magnetic force are alternately opposite.

The object W to be inspected is passed between the transmitting coil 5 and the first and second receiving coils 6a, 6b at a predetermined speed.

When the object W to be inspected contains metal, the first receiving coil 6
The output voltage of a changes as shown in Fig. 3 (b), and the output voltage of the test object W
When the signal passes through the second receiving coil 61), the output voltage of the second receiving coil 6b changes as shown in FIG. 3(C).

Therefore, the output waveform of the differential circuit that outputs the unbalanced signals of the receiving coils 5a and 6b is as shown in FIG. 3(d).

This unbalanced output signal is amplified by an amplifier 7 and then passed through a relatively high-frequency bandpass filter 8 that allows a signal having the same frequency as that of the oscillator 1 to pass through.
The signal will be as shown in Figure (E). The output signal of the bandpass filter 8 is synchronously detected by a detector 11 synchronized with a signal obtained by shifting the output signal of the oscillator 1 with the same period as the voltage applied to the transmitting coil 5 by a phase shift angle φ2 by a phase shifter 9b. It is converted into a signal as shown in the figure. The synchronously detected signal is passed through a low-pass filter 12 that allows only low-frequency components to pass, thereby removing high-frequency components and creating the signal shown in FIG. 3 (G). The level of the value is compared by a comparator 13, and a metal detection signal is output when it is equal to or higher than a reference voltage.

When the aluminum foil packaged product is to be sent to the line, the changeover switches 4 and 10 are switched to the a side. When the selector switch 4 is set to the a side, the DC voltage from the DC power source 3 is superimposed on the rectangular wave from the oscillator 1, and the transmitting coil 5 is driven by a signal as shown in FIG. A pulsating DC magnetic field is generated.

If metal is mixed into the product wrapped in aluminum foil,
When the inspected object W passes through the first receiving coil 6a, the fourth
The output voltage of the second receiving coil 6b changes as shown in (b) in the figure, and the output voltage of the second receiving coil 6b changes as shown in FIG.
It changes as shown in c).

Therefore, the differential circuit outputs an unbalanced output signal shown in (d) of FIG.
The signal shown in (e) in FIG. 3 is obtained, and is synchronously detected by the detector 11 using the output signal of the phase shifter 9a, resulting in the signal shown in (e) in FIG. The signal shown in (g) of FIG. 4 is obtained through the above steps, and the level of the peak value of this signal is compared in the comparator 13, and if the level is higher than the reference voltage, a metal detection signal is output.

Note that the unbalanced output signal from the low-pass filter 12 is a low-frequency signal determined by the moving speed of the object W to be inspected.
Due to the presence of electromagnetic waves, solenoid valves, transformers, etc., noise of the same amplitude and containing frequency components of the same order as this low frequency may be mixed into the receiving coils 5a and 3h (output of 3h and output 1). For example, in part A in FIGS. 5(b) and 5(c), if low frequency noise as shown in FIG. 5(f)-8 is mixed in both the first and second receiving coils 6a and 6b, Part B is a waveform when low frequency noise as shown in B in FIG. 5 is mixed only in the first receiving coil 6a. Regardless of whether the selector switch 4 is connected to the a side or the b side,
As shown in FIGS. 4(b) and 4(c), unlike the case where the low frequency signal due to the metal in the object W to be inspected is superimposed so as to modulate the amplitude of the high frequency signal, the first and second If noise directly mixes into the first and second receiving coils 6a, 61), regardless of the voltage of the transmitting coil 5.

Since the voltage at 6b fluctuates due to noise, the voltage waveforms at the first and second receiving coils (3a and 5b are shown in Figure 5 (b).
, (c), the waveform is the sum of the high frequency rectangular pulse signal shown in FIG. 5(a) and the noise shown in FIG. 5(f). When such noise is mixed, the output waveform of the differential circuit becomes a waveform as shown in FIG. 5 (2). That is,
If the same noise mixes in the first and second receiving coils 6a, (3b) in the same phase as shown in part A of Fig. 5(d), Noise is never output as shown in the section.

As shown in part 8 of Fig. 5 (b) and (c), the first,
The noise mixed into one of the second receiving coils 6a16b is output as unbalanced noise as shown at part 8 in FIG. 5(d). However, this kind of noise
As shown in part 8 of the figure (E), 50KH2 to 500
Since the signal is removed by a relatively high frequency bandpass filter 8 that passes only signals having a frequency equal to or close to the oscillation frequency of the oscillator 1 of about KH7, noise with a lower frequency is not output.

FIG. 6 is a block diagram showing another embodiment of the transmitting coil 5.
The switch 15 switches the switches 4 and 10 to the a side and the b side using the output signal of the switching signal generator 14, which repeats turning on and A several dozen times while the inspected object W passes through the receiver coil 6a16b. Operate.

As shown in FIG. 7 (B), while the changeover switch 4.10 is switched to the b side by the changeover device 15, the alternating signal shown in FIG. 7 (B) is transmitted. coil 5
While the transmitting coil 5 is being switched to the a side, a pulsating DC signal is applied to the transmitting coil 5 as shown in FIG. 7(a).

The actual detection signal when the inspected object W passes through the magnetic field is obtained by switching the changeover switch 4.10 in the directions a and b by operating the switch 15, as shown in (b) of FIG. , the output of the detector 11 has the waveform shown in FIG. 8(a). 8th
In the figure (a), the waveform Δ shows the iron component detection signal while the changeover switch 4.10 is switched in the direction a, and the waveform B shows the detection signal of the iron component while the changeover switch 4.10 is switched in the direction b. Indicates the detection signal of non-ferrous components during the change.

This example is used for products other than aluminum foil packaging, and iron detection uses a DC magnetic field, so even if the product is conductive, it will not be affected by it, and non-ferrous metals. The detection used a high-frequency alternating magnetic field with high sensitivity.
It can be operated independently under the conditions with the highest detection sensitivity for ferrous and non-ferrous metals.

Figure 9 is a graph showing the frequency of the magnetic field used and the sensitivity index.In the case of the conventional alternating magnetic field, both ferrous ('Fe) and non-ferrous metals (SUS) have a balanced sensitivity to the output signal from the product itself. Since the commonly obtained frequency of fl was used, the sensitivity index was B'; however, in this example, for nonferrous metals, [2 was used, and the sensitivity index larger than B' was (q, and iron was B'. A sensitivity index of A, which is almost the same as -, is obtained. Conventional metal detectors for aluminum foil-wrapped products are affected by electronic circuits and ambient noise, so in reality they have a sensitivity of A', which is smaller than 8. I couldn't get 1.

In addition, after detecting that the object to be inspected approaches the metal detector, and before it passes the metal detector, the selector switch 4.10 is automatically switched and both measurements are taken to determine the presence or absence of metal. It can also be detected. This means that the detection sensitivity is 1q higher than when using the conventional DC l1iis19 with constant strength.

In the above embodiment, the signal applied to the transmitting coil 5 is a rectangular wave, but other signals such as a sine wave may also be used. Furthermore, the low-pass filter 12 is also a bandpass filter that passes a frequency component determined by the speed at which the object W passes.

In this way, with the metal detection signal of the above embodiment, by changing the changeover switch, it is possible to detect aluminum foil packaged products and other products with one device, and it is also possible to prevent false detection due to other noises. In addition, by switching a plurality of phase shifters, the optimum phase can be selected for the metal to be detected and the material of the object W to be inspected, and detection can be performed with high sensitivity.

As explained above, according to the present invention, one device can detect metals for products packaged with aluminum foil and other products, so the installation space can be halved, and costs and maintenance can also be halved. can do. Furthermore, by switching between an alternating magnetic field and a direct current magnetic field while a single product is passing through, it is also possible to detect different types of metals, such as nonferrous metals and iron, in a single product under conditions that provide the highest detection sensitivity.

[Brief explanation of the drawing]

Fig. 1 is a diagram that does not show the detection principle of metal detection signals, Fig. 2 is a block diagram showing an embodiment of the present invention, Fig. 3 is a time chart when an alternating magnetic field is used, and Fig. 4 is a direct current magnetic field. Fig. 5 is a time chart when noise is mixed in. Fig. 6 is a block diagram showing an embodiment of the present invention. Fig. 7.8 shows switching between a DC magnetic field and an alternating magnetic field. The time chart in FIG. 9 is a graph showing the frequency of the magnetic field and the sensitivity index. 1...Oscillator, 2...Amplifier, 3...
...DC power supply, 4... Selector switch, 5.
...Transmission coil, 6a16b...Reception coil, 7...Amplifier, 8...Band filter, 9a, 9b...Phase shifter, 10...
・Selector switch, 11...Detector, 12...
...Low pass filter, 13...Comparator, 14.
...Switching signal generator, 15...Switching device. Patent Applicant Anritsu Electric Co., Ltd. Representative Patent Attorney Makoto Hayakawa No. 31 (G) No. 48 (D) (G)

Claims (1)

[Claims] a magnetic field generating means; a magnetic field detecting means located within the magnetic field generated by the magnetic field generating means; and a magnetic field detecting means located within the magnetic field generated by the magnetic field generating means; In a metal detection device comprising: a detection means for outputting a detection signal; the magnetic field generation means;
A metal detection device characterized by being able to switch between an alternating magnetic field and a direct current magnetic field in which the strength of the magnetic field changes periodically.