JPH09178865A - Metal detecting device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To dispense with the change of a part even when a conveyor speed is changed by A/D converting the signal after synchronous detection, and performing a digital filtering to extract a metal detection signal. SOLUTION: The magnetic fluxes interlinked with receiving coils 3a, 3b are unbalanced by the pass of a body to be inspected, whereby a received signal is outputted. This signal includes a modulated signal generated in the body to be inspected, and it is synchronously detected by synchronous detecting circuits 8a, 8b. The signals after synchronous detection include the metal detecting signal demodulated by synchronous detection. The signals after synchronous detection are A/D converted by A/D converting circuits 12a, 12b, and successively stored in a memory means 101. An arithmetic means 102 performs a digital filtering for passing only about the frequency component of the metal detection signal to these data to extract only the metal detection signal, and a comparing means 103 judges the detection of a metal from the result.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a metal detecting device.

[0002]

2. Description of the Related Art First, an outline of a conventional metal detecting device will be described with reference to FIG.
In FIG. 2, 1 is an oscillating circuit, 2 is an oscillating coil connected to the oscillating circuit, 3a and 3b are two receiving coils for receiving a change in magnetic field, and receiving coils 3a and 3b are alternating magnetic fields of the oscillating coil 2. It is placed inside, and the magnetic fluxes are arranged so as to link equally. This receiving coil 3a, 3b
The differential voltage of the electromotive force induced in is the received signal input to the signal processing circuit. Reference numeral 4 denotes a variable resistor for adjusting the differential balance, which is adjusted so that the amplitude of the reception voltage is minimized. Reference numeral 5 is an amplifier circuit, 6 is a bandpass filter having the same center frequency as the oscillation frequency, 7 is an amplifier circuit, 8a and 8b are synchronous detection circuits that perform synchronous detection by the first and second synchronization signals, and 9a and 9b are metal. Analog filters having the same center frequency as the detection signal, 10a and 10b are voltage comparison circuits for judging the magnitude of the metal detection signal demodulated by the synchronous detection circuit, and 11a and 11b are the synchronous detection circuits 8a and 8b.
To change the phase of the first and second synchronization signals supplied to
It is a second phase shift circuit.

In the metal detecting device having such a structure, after the object to be inspected is passed between the oscillation coil 2 and the receiving coils 3a and 3b or between the oscillation coil and the receiving coil,
When metal is mixed in the device under test, an unbalanced state occurs in the received signal due to the mixed metal. The signal generated by this unbalanced state is the metal detection signal modulated by the oscillation frequency, and the frequency is the same as the oscillation frequency. After passing through the amplifier circuit 5, the bandpass filter 6, and the amplifier circuit 7, the synchronous detection circuits 8a and 8b perform synchronous detection in order to demodulate the metal detection signal. The frequency of the synchronization signal used for synchronous detection is also the same as the oscillation frequency, and the phase of the synchronization signal is the first,
The second phase shift circuits 11a and 11b are adjusted to be suitable for detecting magnetic and nonmagnetic metals. The signal after the synchronous detection passes through the analog filters 9a and 9b to become only the metal detection signal, and the voltage comparison circuits 10a and 10b.
To output the metal detection result.

Therefore, in the conventional metal detection device shown in FIG. 2, an analog filter is required to extract the metal detection signal from the signal after the synchronous detection. However, since the components that make up the analog filter are determined by the frequency of the metal detection signal, if the frequency of the metal detection signal changes due to changes in the conveyor speed, the parts of the analog filter must be changed each time. It was It is an object of the present invention to provide a metal detection device that does not need to change parts even if the conveyor speed is changed.

[0005]

Means for Solving the Problems As a result of intensive studies for achieving the above-mentioned object, the inventors of the present invention converted a signal after synchronous detection into a digital signal by an A / D conversion circuit, and converted the digital value into a digital signal. The present invention has been completed based on the finding that a metal detection device that does not need to use an analog filter and therefore does not need to change parts even when the speed of a belt conveyor is changed can be obtained by performing a filtering process. Only That is, the gist of the present invention is to provide an oscillation coil that generates an alternating magnetic field, one or more sets of receiving coils that detect changes in the magnetic flux, an amplifier circuit that amplifies an unbalanced signal from the receiving coil, and an oscillation frequency signal. In a metal detection device of a type that includes a synchronous detection circuit for demodulation and detects the presence or absence of metal contamination of the inspected object depending on whether the balance is disrupted when passing through the inspected object, the signal after the synchronous detection An A / D conversion circuit for converting the digital value into a digital value, a storage means for storing the digital value from the A / D conversion circuit, and a calculation means for digitally filtering the stored value,
It is an object of the present invention to provide a metal detection device characterized in that a metal detection signal can be extracted by digitally filtering a digital value obtained by converting a signal after synchronous detection by an A / D conversion circuit.

[0006]

FIG. 1 is a block diagram showing an embodiment of the present invention, in which reference numerals 1, 2, 3a, 3b, 4,5 and 5.
6, 7, 8a, 8b, 11a and 11b are the same as those in FIG. Reference numerals 12a and 12b denote A / D conversion circuits for converting the signal after synchronous detection into digital values, 101 storage means for storing digital values, 102 arithmetic means for performing digital filter arithmetic on the stored digital values, 103 Is a comparing means for judging the presence or absence of metal from the calculation result of the digital filter.

When the inspection object is passed through the metal detector, 2
The number of magnetic fluxes interlinking the two receiving coils 3a and 3b is in an unbalanced state, and a received signal is output. The received signal includes a modulation signal generated by the device under test, and the modulation signal is synchronously detected by the synchronous detection circuits 8a and 8b. The signal after the synchronous detection contains the metal detection signal demodulated by the synchronous detection. The signal after the synchronous detection is sampled by the A / D conversion circuits 12a and 12b at a constant cycle and converted into a digital value. The converted digital values are sequentially stored in the internal storage means 101.

The storage means has an area for storing n pieces of data, and the data generated by sampling are stored in order from the newest one. Then, each time new data is generated, the data is shifted to the old one. For example, x ₁ from the new data in order, when the x _2, x ₃ ~x _n,
X _n-1 → x _n , x _n-2 → x _{n-1 for} each sampling
The data is shifted as x ₂ → x ₃ , x ₁ → x ₂ ,
New data is input to x ₁ .

As described in the prior art and its problems,
The signal after the synchronous detection includes the metal detection signal, but it is also very weak and includes an unnecessary signal. Therefore, the data after the synchronous detection is converted into a digital value (x ₁ to
It cannot be judged yet from x _n ) whether the metal is mixed. Therefore, since only the metal detection signal is extracted from the obtained data (x _{1 to} x _n ), the arithmetic means 102 calculates the frequency component (several to several tens Hz of the metal detection signal) of the data (x _{1 to} x _n ). ) Apply digital filtering that passes only the vicinity. The storage means 101 for storing the digital value from the A / D conversion circuit, the calculation means 102 for digitally filtering the stored value, and the comparison means 103 for judging the metal detection from the value after the calculation, It is not necessary to configure each circuit individually. For example, all the circuits may be configured by one integrated processing device such as a microcomputer.

[0010]

EXAMPLE When the sampling frequency f _S is 20 Hz, the cut-off frequencies f _L and f _H are 2 Hz and 6 Hz, respectively, and the filter order is 21, the digital filter processing is expressed by the following formula.

[0011]

[Equation 1] For example, when a _{1 to} a ₂₁ are set to the following numerical values, the digital filter has the filter characteristic shown in FIG.

[0012]

[Table 1]

By performing the digital filter processing, the obtained data y ₁ after the digital filter becomes the data representing only the metal detection signal. When the data of the obtained metal detection signal is small, the coefficients a _{1 to} a ₂₁ of the digital filter may be increased by 2 times, 3 times ... The data y ₁ of the obtained metal detection signal is input to the comparison means 103, and the presence or absence of metal is determined by comparing it with the threshold value of metal mixing. A / D conversion circuits 12a, 1
When a new sampling data is generated by 2b, the previous data x ₁ ~x ₂₀ is shifted in x ₂ ~x _21, it is stored in the new data x _1. Similarly, the digital filtering process is performed on the data of x ₁ to x ₂₁ to generate new metal detection signal data y ₁ . In this way, every time new data x ₁ is generated by sampling, the metal detection signal y ₁ is generated, and the comparing means 103 compares the magnitude of y ₁ to continuously determine the presence or absence of metal contamination. As described above, in the prior art method, the metal detection signal is obtained by obtaining the metal detection signal that is continuous in time by the analog filter, but in the embodiment, the metal detection signal that is discrete for each sampling time is used. It can detect metals in exactly the same way, except that it changes to detect. Thus, data (x ₁
.About.x _n ) is subjected to a digital filter process for each sampling, the same process as the analog filter can be performed, and the data y ₁ representing the metal detection signal can be obtained. Then, the obtained metal detection signal data y ₁ is input to the comparison means 103, and it is determined whether or not metal is mixed.

[0014]

As described in detail above, according to the present invention, the analog filter for extracting the metal detection signal from the signal after the synchronous detection can be replaced with the digital filter processing, and the conveyor speed can be improved. Even if the frequency component of the metal detection signal changes, there is an excellent effect that it can be dealt with only by changing the arithmetic expression of the digital filter.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing an embodiment of the present invention.

FIG. 2 is a configuration diagram showing an example of a conventional metal detection device.

FIG. 3 is a diagram showing frequency characteristics of a filter according to an embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 oscillation circuit 2 oscillation coils 3a, 3b reception coil 4 variable resistance for differential balance adjustment 5 amplification circuit 6 band pass filter 7 amplification circuits 8a, 8b synchronous detection circuits 9a, 9b analog filters 10a, 10b voltage comparison circuits 11a, 11b First and second phase shift circuits 12a, 12b A / D conversion circuit 101 storage means 102 calculation means 103 comparison means

Claims (1)

[Claims] 1. An oscillating coil for generating an alternating magnetic field, one or more sets of receiving coils for detecting changes in the magnetic flux, an amplifier circuit for amplifying an unbalanced signal from the receiving coil, and a signal of oscillating frequency only. A band-pass filter and a synchronous detection circuit that demodulates a metal detection signal from a received signal are provided, and depending on whether or not the balance is broken when the object to be inspected is passed,
In a metal detection device of a type that detects the presence or absence of metal contamination of the object to be inspected, an A / D conversion circuit that converts a signal after synchronous detection into a digital value and a digital value from the A / D conversion circuit are stored. A metal detection signal by digitally filtering a digital value obtained by converting the signal after synchronous detection by the A / D conversion circuit A metal detection device characterized by being able to extract.