JPH03279888A - Metal detector - Google Patents

Abstract

PURPOSE:To prevent misdetection due to vibration by providing a frequency distribution characteristic waveform detector, a memory stored with characteristic waveforms, a comparator, a vibration discriminating circuit, and a signal short-circuit circuit which removes a signal of vibration. CONSTITUTION:The outputs of analog filters 7a and 7b are converted by an A/D converter 10 into digital signals, whose frequency distribution waveforms are detected by a frequency distribution characteristic waveform detector 11 and stored. Then the output of the detector 11 is compares with frequency distribution characteristic waveforms of articles and metal stored previously in the memory 12. In this case, when a detection signal corresponds to an article or metal, the output of the comparator 13 is almost ceased. Further, when the detection signal is generated owing to vibration, the difference between the output of the detector 11 and the output of the memory 12 is calculated and a peak remains on the high-frequency side. A vibration discriminating circuit/4, therefore, decides that the detection signal is generated because of the vibration and performs control so that the output is sent to none of the filters and signal short-circuit circuits 15a and 15b. Thus, even if the detection signal is generated owing to the vibration, it is ignored and misdetection is prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a metal detector that detects the presence or absence of foreign metal in an object to be inspected.

[Prior Art 1] First, an outline 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 3a
, 3b are receiving coils arranged opposite to this transmitting coil 2;
They are placed in an alternating magnetic field and arranged so that the lines of magnetic force are equally interlinked. 4a and 4b are the receiving coils 3a
, 3b induced voltage 61. 2 shows a volume for adjusting the phase and amplitude of the signal 2, and is normally set so that e+ez=0 by adjusting the volumes 4a and 4b. 5 is the differential induced voltage.

6a and 6b are synchronous detectors that detect ferrous and non-ferrous 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.

e　F　l　e　g is a synchronous detection signal.

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

To explain this point using the vector diagrams in FIGS. 4(a) and 4(b), normally, the induced voltage e++eg of the receiving coils 3a and 3b
is set so that 61e m = O on the input side of the amplifier 5, but when the object W to be inspected containing iron passes from the direction of the arrow.

First, as shown in FIG. 4(a), the induced voltage in the receiving coil 3a increases to m', and then the induced voltage in the receiving coil 3b increases to a'.

Therefore, a differential induced voltage of Mi1 - Mito = Miot is input to the synchronous detector 6a, and detected by the same phase synchronous detection signal 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 magnetic field of the oscillator 1. Then, due to the influence of this eddy current, the induced voltage e+ in the receiving coils 3a, 3b,
The phase of ex will change.

That is, as shown in FIG. 4(b), the receiving coil 3a
When the phase of the induced voltage a″t e2″ebs changes to , the differential induced voltage a″t e2″ebs becomes approximately 90° as shown in the figure.
``The differential induced voltage is generated at a point where the phase is shifted.Therefore, this differential induced voltage is almost in phase with the non-ferrous metal. can be detected.

[Problems to be Solved by the Invention] In the conventional metal detector described above, when vibrations such as floor vibrations caused by the belt for conveying the object W to be inspected are applied to the detection part, a detection signal is sent to the receiving coils 3a and 3b. Occurred,
This caused a problem of false detection.

FIG. 2 shows a detection signal generated due to vibration and a detection signal generated due to an article (non-inspected object W) or metal. Curve I shows the outline of the detection signal of articles and metals, and as mentioned above, it is possible to distinguish between articles and metals.Curves 2 and 2 indicate detection signals caused by impact. As will be seen, the frequency distribution of the detection signal is different between an impact and an article/metal. curve H
In case of shock, the filter makes the song easier!
It can be distinguished from the detection signal of articles/metals in I.
In the case of the impact shown by curve (2), even if it can be removed to some extent, the low frequency component passes through, resulting in a problem that the S/N ratio of the detection signal of the article/metal deteriorates. In addition, curve I
The frequency of the peak value of is approximately 8 to 10 Hz, and the highest frequency of curve (2) is approximately 50 Hz.

This invention was made to solve the above problems, and an object thereof is to provide a metal detector that does not malfunction due to vibration.

[Means to solve problems]

The metal detector according to the present invention includes a frequency distribution characteristic waveform detector of a detection signal of an object to be inspected, a memory storing in advance a frequency distribution characteristic waveform of a detection signal of the object to be inspected, and an output of the frequency distribution characteristic waveform detector. and the output of the memory, a vibration discrimination circuit that determines whether the detected signal is due to vibration from the frequency component of the output of the comparator, and a signal short circuit that removes the vibration signal. It has been established.

[Effect]

In this invention, the vibration of the object to be inspected is detected from the frequency distribution characteristics of the detection signal.

[Example 1 FIG. 1 is a block diagram showing one embodiment of the present invention.

In this figure, symbols 1 to 7a, 7b and 9a, 9b are the third
It is the same as the figure, 1o is an A/D converter, 11 is a frequency distribution characteristic waveform detector, and fast Fourier transform (FFT)
Processing is applied to the digitized signal, and the peak value for each frequency is digitally detected and stored. The frequency distribution characteristic waveform detector 1 12 digitally detects the frequency distribution characteristic waveform of the object W (article/metal) to be inspected in advance.
This is a memory that stores data in the same way as 1. 13 is a comparator which determines the difference between the output of the frequency distribution characteristic waveform detector 11 and the output of the memory 12, that is, the difference between peak values at the same frequency. Reference numeral 14 denotes a vibration discrimination circuit, which judges whether or not the detection signal is caused by vibration from the frequency of the peak value of the output of the comparator 13, and stops the output of the filter and signal shorting circuits 15a and 15b if it is judged to be vibration. let

Next, the operation of the embodiment shown in FIG. 1 will be explained. The outputs of the analog filters 7a and 7b are exchanged into digital signals by the A/D converter 10, and are converted into digital signals by the frequency distribution characteristic waveform detector 1.
1, the frequency distribution characteristic waveform is detected and stored. Next, the output of the frequency distribution characteristic waveform detector 11 is compared with the frequency distribution characteristic waveform of the article or metal stored in the memory 12 in advance. In this case, if the detection signal is caused by an article or metal, the output of the comparator 13 will almost disappear, but if it is caused by vibration, the curve
Alternatively, since the difference between ■ and curve ■ is calculated, a peak remains on the high frequency side of both curves ■ and ■. Therefore, the vibration discrimination circuit 14 determines that the detection signal is vibration, and controls the filter and signal shorting circuits 15a and 15b so that they do not output. In this way, even if a detection signal due to vibrations occurs, it is ignored, thereby preventing false detections.

[Effect of the Invention 1] As explained in detail above, the present invention includes a frequency distribution characteristic waveform detector of a detection signal of an object to be inspected, a memory that stores in advance a frequency distribution characteristic waveform of a detection signal of an object to be inspected, a comparator that takes the difference between the output of the frequency distribution characteristic waveform detector and the output of the memory; a vibration discrimination circuit that determines whether the detected signal is due to vibration from the frequency component of the output of the comparator; Since a signal short circuit is provided to remove the signal, erroneous detection due to vibration can be reliably prevented and metal detection can be performed with high precision, which is an excellent effect.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention, FIG. 2 is a waveform diagram for explaining the operation of the embodiment of FIG. 1, and FIG. 3 is a block diagram showing an example of a conventional metal detector. Figure 4 (a
) and (b) are block diagrams for explaining the detection operation in FIG. 3. In the figure, 1 is an oscillator, 2 is a transmitting coil, and 3a. 3b is a receiving coil, 4a and 4b are volumes, 5 is an amplifier, 6a and 6b are synchronous detectors, and 7a. 7b is an analog filter, 9a and 9b are first . A second phase shifter, 10 is an A/D converter, 11 is a frequency distribution characteristic waveform detector, 12 is a memory, 13 is a comparator, 14 is a vibration discrimination circuit, 15a and 15b are a filter and a signal short circuit. . Figure 2 (a) 2 ei'

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. In the metal detector, a frequency distribution characteristic waveform detector of the detection signal of the object to be inspected, a memory storing in advance a frequency distribution characteristic waveform of the detection signal of the object to be inspected, and an output of the frequency distribution characteristic waveform detector and the A comparator that takes the difference from the output of the memory, a vibration discrimination circuit that determines whether the detected signal is due to vibration from the frequency component of the output of the comparator, and a signal short circuit that removes the vibration signal are provided. A metal detector characterized by: