JPS6341502B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a metal detection device for detecting whether metal is mixed in an object to be inspected (particularly food) being conveyed by a conveyor or the like.

First, an overview of a conventionally used metal detection device will be explained with reference to FIG.

In this figure, 1 is an oscillator, 2 is a transmitting coil connected to the oscillator 1, 3a and 3b are receiving coils arranged opposite to this transmitting coil 2, and these receiving coils 3a and 3b are It is placed in the alternating magnetic field of the coil 2 and arranged so that the lines of magnetic force intersect equally.

4a and 4b indicate adjustment volumes for the phase and amplitude of the induced voltages e〓 ₁ and e〓 ₂ of the receiving coils 3a and 3b, and by adjusting the adjustment volumes 4a and 4b, the differential voltage of the receiving coils 3a and 3b is The induced voltage is e〓 ₁ −
It is set so that e〓 ₂ =0. 5 is an amplifier that amplifies the differential induced voltage e〓 ₁ −e〓 ₂ ; 6a and 6b are synchronous detectors that detect ferrous and nonferrous metals, respectively; 7a,
7b is a low-pass filter, and 8a and 8b are discrimination circuits. Note that 9a and 9b are the synchronous detectors 6
Fig. 6 shows first and second phase shifters that form synchronization signals supplied to terminals a and 6b.

The metal detection device having such a configuration has an object W to be inspected between the transmitting coil 2 and the receiving coils 3a and 3b.
When the inspected object W is contaminated with metal, a detection signal is generated in the discrimination circuits 8a and 8b depending on the type of metal (ferrous or non-ferrous).

To explain this point using the vector diagrams in Fig. 2 a and b, normally, the induced voltages e〓 ₁ ,
e〓 ₂ is on the input side of the amplifier 5 and is set so that e〓 ₁ −e〓 ₂ = 0, but when the object W containing iron passes from the direction of the arrow, first the As shown in a, the induced voltage e〓 ₁ of the receiving coil 3a increases to e〓' ₁ , and then the induced voltage e〓 ₂ of the receiving coil 3b increases. Therefore, the differential induced voltage of e〓′ ₁ −e〓 ₂ =e〓 _Df is input to the synchronous detector 6a, and this synchronous detector 6a
It is detected by the in-phase synchronous detection signal e〓 _F supplied to the.

On the other hand, when the inspected object W containing non-ferrous metals (stainless steel, aluminum, etc.) passes through, eddy currents flow in the non-ferrous metals under the influence of the alternating current magnetic field of the oscillator 1. Then, due to the influence of this eddy current, the phases of the induced voltages e〓 ₁ , e〓 ₂ of the receiving coils 3a, 3b change.

That is, when the phase of the induced voltage _e〓1 of the receiving coil 3a changes to e〓''1 as shown in Fig. 2b, the differential induced voltage _e〓''1 - _e〓2 = _{e〓DS becomes} Almost like I did
Occurs at a point with a 90° phase shift. Therefore, non-ferrous metals can be detected by performing phase detection in the synchronous detector 6b, which is supplied with a signal for synchronous detection indicated by _e〓S having approximately the same phase as this differential induced voltage e〓 _DS . (When the induced voltage e〓 ₂ changes to e〓″ ₂ , it can be detected for the same reason.) The metal detection operation of the inspected object W has been briefly explained above, but next, the frequency supplied from the oscillator 1 and metal detection sensitivity.

FIG. 3 shows the sensitivity index for ferrous (Fe), non-ferrous (stainless steel (SUS)), and a product (sausage) when the frequency of the oscillator 1 is changed. From this figure, iron (Fe) shows a constant sensitivity index even when the frequency (horizontal axis) changes, and the detection sensitivity does not change, but non-ferrous (SUS) and products have eddy current loss that increases to the square of the frequency. Because of the proportionality, as the frequency increases, the sensitivity index becomes higher than that of iron, and in the end, in order to detect iron (Fe), it is better to use a low frequency signal ( _l ) that does not generate eddy currents in the product. I know it's good.

On the other hand, when comparing products with non-ferrous metals (SUS), as shown in Figure 4, the sensitivity index changes depending on the phase of the detection signal, with SUS reaching a maximum around 0° and lowest around 90°. However, in the case of products, they are non-ferrous (SUS) due to their physical properties as shown by the broken line.
The lowest point exists in a phase different from .

It can also be seen that the phase difference between the non-ferrous metal (SUS) and the point at which the product has the lowest sensitivity becomes larger as the frequency _h becomes higher than that of the lower frequency _l .

In other words, the sensitivity difference between nonferrous (SUS) and the product is
At low frequency _l , if the phase difference of the synchronous detection signal is chosen to be around 80°, the magnitude will be between a and b, but at high frequency _h , the sensitivity difference will be between a' and b' (the phase is approximately 70°). It can be seen that the higher the frequency, the more advantageous it is when distinguishing between products and non-ferrous metals.

According to the present invention, the metal detection device is operated in a time-sharing manner so that the sensitivity index for both ferrous and non-ferrous metals becomes high based on the data analysis results as described above.

Hereinafter, one embodiment of the metal detection device of the present invention will be explained with reference to the block circuit diagram of FIG.

In this circuit, 10 is an oscillator, 11 is 1/
12 is a first frequency divider of n, and 12 is a second frequency divider of 1/m. The first frequency divider 11 is the frequency of the oscillator 10
The second frequency divider 12 forms a frequency _l lower than _h , and the second frequency divider 12 is connected to a time division switching switch 2, as will be described later.
0 to 22 drive pulses (timing pulses) are formed.

Reference numerals 13 and 14 indicate the above-mentioned transmitting coil and receiving coil, and 15 is an amplifier that outputs the differential induced voltage of the receiving coil 14.

16 is a synchronous detector, 17a and 17b are band pass filters, and 18a and 18b are discrimination circuits.

The synchronous detector 16 includes two phase shifters 19.
Two signals for synchronous detection are supplied from a and 19b.

Next, the operation of the above embodiment will be explained based on the waveform diagram of FIG.

The signal waveform B of the oscillator 10 is set to a frequency _h at which the difference in sensitivity index between non-ferrous metal (SUS) and the product becomes large, as explained using the data in FIG. This frequency _h
is stepped down by the first frequency divider 11, and is divided to a frequency _l at which the difference in sensitivity index between ferrous and non-ferrous (products) explained using the data in FIG. 3 becomes large.
Figure 6A shows the frequency _l when the division ratio is set to 1/2.
The waveform is shown.

These two signals of frequencies _h and _l are supplied to the transmitting coil 13 via the switch 20 which is switched by the timing pulse H formed by the second frequency divider 12, so that the supplied waveform C _{The h} and _l frequency components are output alternately at the period _T0 of the timing pulse H.

When the object W to be inspected does not exist between the transmitting coil 13 and the receiving coil 14, the differential output of the induced voltage of the receiving coil 14 is adjusted to be zero, so there is no effect on the synchronous detector 16. No detection signal is generated.

Now, when the inspected object W containing iron passes in this state as shown by the arrow, the induced voltage in the receiving coil 14
Since the balance between e〓 ₁ and e〓 ₂ , especially the balance of the amplitudes, is lost, a difference signal as shown in waveform C' in FIG. 6 is output.

As shown in the waveform D in FIG. 6, the synchronous detector 16 receives synchronous detection signals e〓 _F and e〓 _S with periods corresponding to the first frequency _l and the second frequency _h , respectively, through the phase shifter 18a. , 18b to the timing pulse H
Since the difference signal C' is supplied from the switch ₂₁ which _switches at
The second synchronous detection signal e〓 _S and the second synchronous detection signal e〓 _F are used for synchronous detection.

Then, the synchronously detected waveform E is further filtered through the bandpass filter 17a and the switch 22.
Since the input waveforms are separated and inputted to 17b, the input waveforms become detected signals shown in F and G, respectively.

Since the phase of the first synchronous detection signal e〓 _S is delayed by approximately 70° with respect to the signal of frequency _h , the average value of the detection signal F is approximately zero, and the non-ferrous discrimination circuit 18
a does not work. However, the second synchronous detection signal e〓 _F
The average value of the detected signal G detected by is
Since it becomes E _f , the discrimination circuit 18b operates and outputs a signal indicating that iron is mixed.

The operation is almost the same when non-ferrous metal (SUS) is mixed in the object W to be inspected, but in this case, as explained in the data diagram of Fig. 3, the eddy _current is Small effect, frequency
It is considered that _{the l} component is canceled within the receiving coil 14 and is hardly output.

Therefore, in FIG. 6, a differential output is generated in the induced voltage of the receiving coil 14 due to the eddy current flowing in the non-ferrous metal (SUS) and the product only during the period T _h , but this differential output is caused by phase fluctuation. Therefore, as shown in the data diagram of Figure 4, the first synchronous detection signal e _s is set at a phase (for example, 70°) that has the lowest sensitivity index for the product. Therefore, the detection sensitivity corresponds to point b' in FIG. 4, and non-ferrous metals (SUS) can be detected as seen in the waveform due to phase fluctuation shown by the dotted line in FIG. 6F.

Furthermore, even if non-ferrous metal is not mixed in the object to be inspected W, if the object to be inspected is conductive, a detection signal is output due to eddy current, but in the case of the present invention, the first The phase of the synchronous detection signal e〓 _s is set to the lowest sensitivity index for the product, and
Since the frequency is set to be high so as to increase the sensitivity difference between non-ferrous metals and the product, the detection signal becomes extremely small on the object W to be inspected, and it is possible to determine with high accuracy whether or not non-ferrous metals are mixed in the object W to be inspected. be able to.

FIG. 7 shows another embodiment of the present invention in which a changeover switch is also provided in the transmitter/receiver coil for time-division operation.

That is, two transmitting coils 13a and 13b are formed, and switches 23 and 24 are used to use only the transmitting coil 13a when the frequency is _h , and to connect the transmitting coils 13a and 13b in series when the frequency is _l . At the same time, the switch 25 controls the resonance capacitors C ₁ and C ₂ for the frequencies _h and _l using a timing pulse H.

The receiving coils 14a and 14b are further divided into two parts, 14a ₁ , 14a ₂ , and 14b ₁ , 14b ₂ , which are controlled by switches 26 and 27 so that they are connected in series when the frequency is _l .

In this way, the impedance matching of the transmitting and receiving coils can be controlled to be optimal for both the two frequencies _h and _l , so that the detection sensitivity of the metal detection device can be further improved.

Note that since the phase at which the lowest sensitivity is obtained differs depending on the type of product, a signal different from the frequency _h may be used in that case.

As explained above, the metal detection device of the present invention drives the transmitting coil with signals of at least two frequencies _h and _l , and detects iron at the lower frequency _l that does not affect the product. , To detect non-ferrous metals, detect at a high frequency _h that has a high sensitivity index and allows good phase discrimination between the product and non-ferrous metal, and selects the phase that has the lowest sensitivity index for the product as a synchronous detection signal. Since it is added, iron,
Nonferrous metals can also be detected with high sensitivity.

Moreover, since the two frequency signals are supplied in a time-division manner and the detection is performed in a time-division manner, most parts of the metal detection device can be configured in the same way as before. It is something that you have.

[Brief explanation of the drawing]

Figure 1 is a block diagram of a conventional metal detection device, Figures 2 a and b are vector diagrams for detecting ferrous and non-ferrous metals, and Figures 3 and 4 are for ferrous and non-ferrous metals (SUS).
5 is a block diagram of a metal detection device showing an embodiment of the present invention, and FIG. 6 is a diagram of the main waveforms of FIG. 5. , FIG. 7 is a switching diagram of transmitting and receiving coils showing another embodiment of the present invention. In the figure, 10 is an oscillator, 11 is a first frequency divider, 1
2 is a second frequency divider, 13 is a transmitting coil, 14 is a receiving coil, 15 is an amplifier, 16 is a synchronous detector, 2
0 to 27 indicate switches operated by time-division timing pulses.

Claims (1)

[Claims] 1 A transmitting coil to which signals of a first frequency ( _l ) and a second frequency ( _h ) higher than the first frequency are alternately supplied at a predetermined timing, and a signal that intersects with the magnetic field of the transmitting coil. , two receiving coils placed in positions where the induced voltage changes in response to the passing object to be inspected and the metal contained in the object to be inspected, and the difference signal between the signals induced in these two receiving coils. an amplifier that outputs a signal, a synchronous detector that detects the output of the amplifier, and a switch that switches the output of the synchronous detector in synchronization with the timing and supplies each to two discrimination circuits; corresponds to the first frequency ( _l ) and has a phase such that the detection signal for _magnetic metal is almost the highest; A metal detection device characterized in that a second synchronous detection signal having a phase such that a detection signal for a body is the lowest is supplied in synchronization with the timing.