JPH03125987A - Metal detector - Google Patents

Abstract

PURPOSE:To achieve a higher foreign metal detection sensitivity by removing a detec tion signal or the like of an object to be inspected itself with a digital filter having a passage band of a detection signal determined by an optimum characteristic judging means and a filter constant altering means. CONSTITUTION:An optimum filter characteristic judging means 12 comprises an equip ment characteristic judging means to determine a filter characteristic from a detection head structure or the like, a product characteristic judging means to determine a filter characteristic by a test operation of frequency characteristics of three signals - a detection signal of a product alone, a detection signal of a foreign matter alone and a detection signal of a product containing a foreign metal and a vibration charac teristic judging means to obtain a frequency characteristic of a detection signal attributed to a floor vibration or the like. A filter constant altering means 13 is made up of an increment key, an input key control circuit and a lower limit frequency and an upper limit frequency of a pass bands of digital filters 10a and 10b is graduated by 0.1Hz with the increment key to be moved to a direction of an increase in frequency while it is graduated by 0.1Hz conversely with a decrement key and moved in a direc tion of a decrease therein to determine. A means 13 is used when a filter characteristic determined by the means 12 is changed by a human judgment.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application 1] The present invention relates to a metal detector for detecting whether metal is mixed in an object to be inspected that is being conveyed by a conveyor or the like.

[Prior Art] First, an overview of a conventionally used metal detector will be explained with reference to FIG. In this figure, 1 is an oscillator, 2 is a transmitting coil connected to the oscillator 1, and 3 is a transmitting coil connected to the oscillator 1.
3a and 3b are receiving coils arranged opposite to the transmitting coil 2, and the receiving coils 3a and 3b are placed in the alternating magnetic field of the transmitting coil 2, and are arranged so that their magnetic pole lines are equally interlinked. has been done. 4a and 4b are the receiving coils 3
Volumes for adjusting the phase and amplitude of induced voltages e+ and ex of a and ab are shown, and by adjusting these volumes 4a and 4b, they are normally set so that -6□=0. 5 is an amplifier that amplifies the differential induced voltage 6°62;
a and 8b are synchronous detectors that detect ferrous and nonferrous metals, respectively; 7a;

7b is an analog filter of about 2 to 20Hz, 8a, 8
b is a discrimination circuit. Note that 9a and 9b are the first synchronous signals that are supplied to the synchronous detectors 6a and 6b. Second
shows a phase shifter. Further, 6r@=g is a synchronous detection signal.

In a metal detector having such a configuration, an object to be inspected W passes between the transmitting coil 2 and receiving coils 3a and 3b, and when metal is mixed in the object to be inspected, the type of metal (ferrous or non-ferrous) is detected. ) generates a detection signal in the discrimination circuits 8a and 8b.

To explain this point using the vector diagrams in FIGS. 11(a) and 11(b), normally, the induced voltage "er" in the receiving coils 3a, 3b,
"er is set so that er-ez=o on the input side of the amplifier 5, but when the inspected object W containing iron passes from the direction of the arrow, first, as shown in FIG. 11(a), As shown, the induced voltage 6. of the receiving coil 3a increases to 1, and then the induced voltage 62 of the receiving coil 3b increases to δ゛2.

Therefore, the differential induced voltage of er e*=eot is input to the synchronous detector 6a, and detected by only the synchronous detection signal of the same phase supplied to the synchronous detector 6a.

On the other hand, when the inspected object W containing non-ferrous metal (stainless steel, aluminum, etc.) passes through, an eddy current flows in the non-ferrous metal under the influence of the alternating current magnetic field of the oscillator 1. Then, due to the influence of this eddy current, the induced voltage e+ in the receiving coils 3a and 3b increases.
The phase of Hez will change.

That is, as shown in FIG. 11(b), the receiving coil 3
When the phase of the induced voltage a changes to a'', a differential induced voltage e''*-e z = 6°1 is generated at a point shifted in phase by approximately 90'', as shown.

So, look at this differential induced voltage. Non-ferrous metals can be detected by phase detection in the synchronous detector 6b, which is supplied with the synchronous detection signal shown by t, which is approximately in phase with t.

[Problems to be Solved by the Invention] As described above, conventional metal detectors perform synchronous detection with phases that enable highly sensitive detection of both iron (magnetic material) and non-ferrous metals. However, if the product to be inspected W contains moisture and salt, such as ham, the product itself is conductive, so a detection signal similar to that of the nonferrous metal to be detected may be generated. In addition, if the product is seaweed or sugar, the product itself contains iron, so a detection signal similar to that of iron (lifl type) is generated, but as shown in the example shown in Figure 9, this The detection signal generated by the electromagnetic characteristics of the product itself is in a lower frequency range than the detection signal of metal alone, and products containing foreign metals have a frequency range intermediate between the two.

Furthermore, the detection signal characteristics of the product itself are determined by the shape of the product, the conveyor speed, the transmitter coil 2 and the receiver coils 3a and 3b.
It varies depending on the structure of the detection head, including the For this reason, conventionally, in order to determine the band frequencies of the analog filters 7a and 7b shown in FIG. 10, it was necessary to repeat trial and error by replacing parts of the filter circuit and measuring the effects. In addition, floor vibrations at the location where the metal detector is installed and mechanical surfaces on the metal detector may affect the detection signal, but when determining the passband of the filter to eliminate this influence, the above-mentioned It had the disadvantage of requiring similar trial and error.

In addition, in order to avoid the above trial and error, conventionally the passband was set to a large value of 2 to 20 Hz as shown in Figure 9, but the detection signal due to factors other than foreign metal becomes relatively large, Metal detection ability was often sacrificed.

This invention was made with the aim of alleviating the above problems, and it was discovered that there is a difference in frequency characteristics between the detection signal generated by the product itself or the detection signal caused by vibration, and the detection signal of foreign metal. Based on this knowledge, we have made it possible to increase the signal-to-noise ratio of the foreign metal detection signal, and
The present invention provides a metal detector that can be easily and optimally reset whenever the object to be inspected changes.

[Means for Solving the Problems] A metal detector according to the present invention is provided with a digital filter, an optimum filter characteristic determining means, and a filter constant changing means in order to easily detect foreign metals with high sensitivity. It is.

[Effect]

In this invention, the detection signal obtained by the detection head is processed by a digital filter having a passband of the detection signal determined by the optimum filter characteristic determining means and the filter constant changing means. Since invalid signals such as detection signals generated by the target surface can be removed, the detection sensitivity of foreign metals can be easily and effectively improved.

〔Example〕

FIG. 1 shows an embodiment of the present invention, in which 10a, 1
0b is a digital filter which is the main part of this invention, l
1a and 11b are A/D converters, 12 is optimum filter characteristic determining means, and 13 is filter constant changing means. The rest is the same as the conventional example shown in FIG.

FIG. 2 shows the digital filters 10a and 10b of FIG.
This is a configuration example showing details of low-pass filters 101 and 10.
2. It is composed of four stages of digital filters including bypass filters 103 and 104, each of which has the configuration shown in FIG. Further, LPF indicates a low-pass filter, and HPF indicates a bypass filter.

In FIG. 3, 111 and 112 are adders, 113 to 1
17 is a multiplier, 118 and 119 are delay devices, and the value A of each multiplier 113-117. .

By changing A, , A, , Bl, and B*, the cutoff frequency, LPF or HPF, gain, etc. can be changed.

FIG. 6 shows a digital filter 10a used in this invention.
10b is a diagram illustrating an example of a bandwidth of 10b.

In this example, digital filters 10a and 10b have a sampling frequency of 200Hz and a passband of 5 to 8Hz.
This is the case.

In FIG. 6, the horizontal axis represents the frequency, and the vertical axis represents the output of the detection signal. The detection signal of the object W to be inspected itself, shown by the solid curved line in this figure, belongs to a lower frequency band than the detection signal from the object W to be inspected containing foreign metal, shown by the dotted curve 2. The curves I and II change depending on the type and shape of the object W to be inspected, and are not fixed at all. That is, the passbands of the digital filters 10a and 10b must be reset for each object W to be inspected, that is, each product.

In this case, the digital filter 10a shown in FIG.
Values A0 to A of each multiplier 113 to 117 configuring 10b
, ,B, ,By adjusting ,B,, the frequency setting can be easily changed.

Specifically, the frequency of the digital filters 10a and 10b is changed to pass the object W to be inspected, and the magnitude of the influence of the object W to be inspected and the influence of the object W contaminated with metal are compared. The frequency may be adjusted to the frequency at which the difference in the detection signal between the case of only W and the case of the object to be inspected W containing metal is the largest.

As shown in FIG. 4, the optimum filter characteristic determining means 12 includes an apparatus characteristic determining means 121 that determines filter characteristics from the detection head structure and conveyor conveyance speed, and a device characteristic determining means 121 that determines the filter characteristics from the detection head structure and conveyor conveyance speed, and a device characteristic determining means 121 that determines the filter characteristics based on the detection head structure and conveyor conveyance speed, and the apparatus characteristic determining means 121 that determines the filter characteristics from the detection head structure and conveyor conveyance speed, and the device characteristic determining means 121 that determines the filter characteristics from the detection head structure and conveyor conveyance speed, and the device characteristic determining means 121 that determines the filter characteristics from the detection head structure and conveyor conveyance speed, and the device characteristic determination means 121 which determines the filter characteristics from the detection head structure and conveyor conveyance speed, and the device characteristic determination means 121 which determines the filter characteristics from the detection head structure and the conveyor conveyance speed. Product characteristic determining means 1 for determining filter characteristics by performing test operation to obtain the frequency characteristics of three types of signals of detection signals of products containing the product.
22 and 1. It consists of a vibration characteristic determining means 123 that obtains and determines the frequency characteristics of the detection signal due to floor vibration and the influence of the mechanical surface on the metal detector by measurement.

Further, as shown in FIG. 5, the filter constant changing means 13 is composed of two keys, an up key 131 and a down key 132, and an input key control circuit 133, and is configured to adjust the lower limit frequency of the pass band of the digital filters 10a and 10b. The up key 131 moves the frequency in increments of O, lHz in the direction of increase, and the down key 132 conversely moves the frequency up to O, lHz.
The frequency is determined by moving the frequency in a decreasing direction in 1Hz steps. This filter constant changing means 13 is used when changing the filter characteristics determined by the optimum filter characteristic determining means 12 based on human judgment, but even when the optimum filter characteristic determining means 12 is not present, changing the filter passband is key. Needless to say, it has the advantage that it can be easily changed only by operation, and therefore has sufficient effects.

Next, the digital filter 10a is used when there is a change in the transport speed when transporting the inspected object W.

The frequency setting method of 10b will be explained.

FIG. 7 shows an example of how the frequency of the foreign metal detection signal changes when the conveyance speed changes. That is, the induced voltage e H obtained in the receiving coils 3a and 3b depending on the magnitude of the conveyance speed
, e*, the frequency characteristics of the digital filters 10a and iob change accordingly, for example, 2.
Change the settings within the range of ~20Hz to maintain the highest detection sensitivity. Even when the conveyance speed (speed of the belt conveyor) is changed in this way, according to the present invention, it is possible to immediately adapt to the change.

FIG. 8 is an explanatory diagram of a method for preventing errors caused by floor vibration depending on the setting location. In other words, in the past, analog filters were used as described above, so the frequency was fixed and had to be wideband, so the induced voltage due to floor vibration would be as shown in curves III- and III-b in Figure 8. Once this occurs, it is inevitable that the detection sensitivity will decrease. Therefore, conventional methods have been to measure the frequency characteristics of vibrations using another device, and then change the analog filter, or try using several types of printed circuit boards with different analog filters, and then select the best analog filter. etc.

However, according to the present invention, the frequencies of the digital filters 10a and 10b are set so that they fall between the curves m, .
For example, by simply setting the frequency within the range of 2 to 20 Hz, such a decrease in sensitivity due to floor vibration can be prevented.

In addition, if the above figures 6, 7, and 8 overlap, basically, give priority to figure 4, and then solve the problems in figures 7 and 8. , digital filter 10a.

10b is set as described above.

〔Effect of the invention〕

As described in detail above, the present invention uses a digital filter having a detection signal pass band determined from the signal obtained by the receiving coil by the optimum filter characteristic determining means and the filter constant changing means to generate the detected signal of the object to be inspected itself. Also, by removing detection signals generated by floor vibrations and mechanical surfaces, there is an advantage that the detection sensitivity of foreign metals can be easily and effectively improved.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing one embodiment of the present invention, FIGS. 2(a) and (b) are block diagrams showing the detailed configuration of the digital filter in FIG. 1, and FIG. 3 is a block diagram showing the configuration of the digital filter. FIG. 4 is a diagram showing the configuration of each individual filter.
FIG. 1 is a block diagram showing details of the configuration of the optimum filter characteristic determining means, FIG. 5 is a block diagram showing details of the configuration of the filter constant changing means, and FIG. 6 is an example of frequency setting of the digital filter used in the present invention. Figure 7 is a diagram showing an example of frequency setting when changing the conveyance speed.
Figure 8 is a diagram showing an example of frequency setting when there is floor vibration, Figure 9 is a diagram showing the relationship between the frequency of a conventional detection signal and detection sensitivity, and Figure 10 is a diagram showing the configuration of a conventional metal detector. The block diagrams shown in FIGS. 11(a) and 11(b) are diagrams for explaining the detection operation in FIG. 10. In the figure, 1 is an oscillator, 2 is a transmitting coil, and 3a. 3b is a receiving coil, 4a, 4b are volumes, 5 is an amplifier, 6a, 6b are synchronous detectors, 7a, 7b are analog filters, 8a, 8b are discrimination circuits, 9a, 9b are first... The second phase shifter, 10a, 10b is a digital filter, l
1a and 11b are A/D converters, 12 is optimum filter characteristic determining means, and 13 is filter constant changing means. Fig. 1 Fig. Fig. 3 Fig. Fig. Fig. 111m soft chart - *i Ei Fig. 8 Fig. Z number per country (Hz) Fig. 1° Fig. 1 Fig. (a) (b)

Claims (1)

[Claims] The object to be inspected is passed between a transmitting coil that generates an alternating magnetic field and a receiving coil that receives this alternating magnetic field, and a discrimination circuit discriminates the magnitude of the detection signal obtained from the receiving coil to detect foreign metal. What is claimed is: 1. A metal detector comprising a digital filter, an optimum filter characteristic determining means, and a filter constant changing means, the metal detector being characterized in that it passes only a foreign metal detection signal obtained by the receiving coil.