JPH09197058A - Metal detector - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a detector with no necessity of changing parts even with the change of conveyor speed by processing with digital filter the digital value converted from the signal after synchronous detection and making the filter characteristic of the digital filter freely variable. SOLUTION: Signals synchronously detected 7a and 7b are converted into digital values with A/D convertor circuits 11a and 11b. The converted digital values are stored in a memory means 101. To judge from the data converted to digital values after synchronous detection whether metal is mixed or not, it is necessary to extract only metal detection signals. Therefore, only frequency component about a few Hz to tens Hz which is the frequency component of metal detection signal is let pass in digital filter processing with an operation means 102. If the data of metal detection signals obtained due to the change of conveying velocity of a conveyor and the like is small, the coefficient of the digital filter can be made larger through a filter characteristics setting means 104 and a coefficient calculation means 105.

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 electromotive force induced by is input to the differential amplifier circuit 4, and a reception signal is obtained. 5 is a band pass filter having the same center frequency as the oscillation frequency, 6 is an amplifier circuit, 7a and 7b are synchronous detection circuits for performing synchronous detection by the first and second synchronization signals, 8a,
Reference numeral 8b is an analog filter having the same center frequency as the metal detection signal, 9a and 9b are voltage comparison circuits for determining the magnitude of the metal detection signal demodulated by the synchronous detection circuit, 10
Reference numerals a and 10b are first and second phase shift circuits that change the phases of the first and second synchronization signals supplied to the synchronous detection circuits 7a and 7b.

In the metal detecting apparatus having such a structure, the object to be inspected is passed between the oscillating coil 2 and the receiving coils 3a and 3b or in the oscillating coil and the receiving coil by using a conveying means such as a belt conveyor. Perform metal detection.
When metal is mixed in the device under test, the mixed metal changes the balance state of the magnetic field, and the magnitudes of the super powers induced in the two receiving coils become unequal. The difference between the electromotive forces of the two receiving coils is output.
This output signal is the metal detection signal modulated by the oscillation frequency, and the frequency is the same as the oscillation frequency. After passing through the band-pass filter 5 and the amplifier circuit 6, synchronous detection is performed by the synchronous detection circuits 7a and 7b in order to demodulate the metal detection signal. The frequency of the synchronizing signal used for synchronous detection is also the same as the oscillation frequency, and the phase of the synchronizing signal is adjusted by the first and second phase shift circuits 10a and 10b so as to be suitable for detecting magnetic and non-magnetic metals. It The signal after the synchronous detection passes through the analog filters 8a and 8b to become only the metal detection signal. The wave heights of the metal detection signals are compared by the voltage comparison circuits 9a and 9b. Is output to the outside. The analog filters 8a and 8b are circuits that pass only the demodulated metal detection signal of the output signals of the synchronous detection circuits 7a and 7b, and are designed so that the frequency of the metal detection signal falls within the pass frequency range. ing.

However, since the frequency of the metal detection signal demodulated by the synchronous detection circuits 7a and 7b changes depending on the conveying speed of the belt conveyor or the like which is the conveying means of the object to be inspected, the conveying speed of the belt conveyor or the like changes. In that case, there is a problem in that the frequency of the analog filter that allows the most signals to pass and the frequency of the metal detection signal are different, and the detection sensitivity fluctuates. Therefore, when it is desired to change the transport speed of the belt conveyor or the like, it is necessary to change the pass frequency range of the analog filters 8a and 8b. Analog filters 8a, 8b used in metal detectors so far
Is composed of circuit elements that include at least two of the three types of elements, capacitors, inductors, and resistors. To change the pass frequency range, these circuit elements must be replaced. I had no choice but to do the laborious work. It is an object of the present invention to provide a metal detection device that does not require parts to be changed 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 oscillating 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 a signal of an oscillating frequency only. Equipped with a band-pass filter that passes the signal and a synchronous detection circuit that demodulates the metal detection signal from the received signal, and when the object is passed, the presence or absence of metal contamination in the object is detected depending on whether the balance is lost or not. In the metal detection device of the type, an A / D conversion circuit that converts the signal after synchronous detection into a digital value, a storage unit that stores the digital value from the A / D conversion circuit, and a stored value A / D is provided with an arithmetic means for digital filter processing, a filter characteristic setting means for inputting a characteristic value of the filter, and a coefficient calculating means for obtaining a coefficient of the digital filter from the input characteristic value. To be able to extract the metal detection signal by which to digital values obtained by converting the signal after synchronous detection by the conversion circuit to a digital filtering,
Another object of the present invention is to provide a metal detection device characterized in that the filter characteristics of the digital filter can be freely changed.

[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, 7a, 7b, 10a and 10b are the same as those in FIG. 11a and 11b are A / D conversion circuits for converting the signal after synchronous detection into digital values, 101 is storage means for storing digital values, 102 is arithmetic means for digitally filtering the stored values, and 103 is a digital filter. Comparing means for judging the presence or absence of metal based on the calculation result, 104 is a filter characteristic setting means for inputting the characteristic value of the filter, 10
Reference numeral 5 is a coefficient calculating means for calculating the coefficient of the digital filter from the input characteristic value.

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 7a and 7b. 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 11a and 11b at a constant cycle and converted into a digital value. The converted digital value is sequentially stored in the storage unit 101.

The storage means 101 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, when the new data and _{x 1, x 2, x 3} ~x n sequentially, x _n-1 → x _n for each _{sampling, x n-2 → x n-} 1
The data is shifted like x ₂ → x ₃ , x ₁ → x ₂ , and new data is input to x ₁ .

As described in the prior art, the signal after the synchronous detection includes the metal detection signal, but it is very weak and includes an unnecessary signal. Therefore, the signal after the synchronous detection is detected. From the data x _{1 to} x _n converted into digital values, it is not possible to determine whether metal is mixed. In order to discriminate, it is necessary to extract only the metal detection signal from the obtained data x _{1 to} x _n , and the data x is calculated by the calculation means 102.
To ₁ ~x _n, can be achieved by applying a digital filtering process that passes only near several Hz~ number 10Hz is the frequency component of the metal detector signal.

[0010]

EXAMPLE A sampling frequency f _{S is set} to 20 Hz, and a lower limit frequency f _L and an upper limit frequency f _H of the filter are set to 2 respectively.
When the filter order is Hz, 6 Hz, and the filter order is 21, the digital filter process is represented by the following calculation 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]

When the frequency of the metal detection signal included in the signal after the synchronous detection is around 4 Hz, the data y ₁ after the digital filter obtained by performing the digital filter processing is the data representing only the metal detection signal. Becomes When the obtained metal detection signal data is small, the coefficients a _{1 to} a ₂₁ of the digital filter are doubled accordingly,
It should be 3 times larger. In this way, the data x _{1 to} x _n representing the signal after the synchronous detection is subjected to the digital filter process for each sampling, so that the same process as the analog filter can be performed, and the data y representing the metal detection signal is obtained. You can get ₁ . Then, the obtained metal detection signal data is input to the comparison means 103, and it is determined whether or not metal is mixed.

When the frequency of the metal detection signal is changed from 4 Hz to 6 Hz by changing the transport speed of the object to be inspected, the frequency of the metal detection signal is not included in the pass frequency range of the filter with the digital filter as it is. The metal detection sensitivity will decrease. Therefore, it is necessary to change the setting of the digital filter in accordance with the frequency of the metal detection signal changed to 6 Hz. In such a case, the filter characteristic setting means 104 sets the filter characteristic value such as the pass frequency range of the digital filter. The filter characteristic setting means 104 has a keyboard or a switch for inputting a numerical value in advance, and recognizes the input from the keyboard or the switch as the filter characteristic values such as the lower limit pass frequency f _L and the upper limit frequency f _H of the digital filter. . Then, the coefficient calculation unit 105 calculates the coefficient of the digital filter from the input filter characteristic value. For example, when it is desired to set the lower limit pass frequency f _L to 4 Hz and the upper limit frequency f _H to 8 Hz, the coefficient of the digital filter can be calculated by executing the following equation in the coefficient calculating unit 105. The calculated coefficients are shown in Table 2 (where the order k is 21
in the case of).

[0015]

[Equation 2]

[0016]

[Table 2]

The obtained coefficients a _{1 to} a ₂₁ are calculated by the calculating means 10
2 is stored in a rewritable memory inside. The coefficients a _{1 to} a ₂₁ stored in the memory are retained even when the power of the metal detection device is turned off, and are not changed until the filter characteristic value is input to the filter characteristic setting means again. Then, the arithmetic means 102 digitally filters the signals x _{1 to} x _n after the synchronous detection according to the arithmetic expression of Equation 1 using the stored coefficients a _{1 to} a ₂₁ . Since the digital filter with the obtained coefficient exhibits the characteristics of the band pass filter having the center frequency of 6 Hz as shown in FIG. 4, therefore, the same detection sensitivity as before the conveyance speed of the object to be inspected is obtained. become. Thus, the filter characteristic setting means 104 for inputting an arbitrary filter characteristic value and the coefficient calculating means 1 for calculating the coefficient of the digital filter from the input filter characteristic value.
Even if the conveyance speed of the object to be inspected is changed and the frequency of the metal detection signal is changed by a series of processes of the arithmetic means 102 which performs the arithmetic processing of the digital filter according to the coefficient calculated as 05, the user causes the filter characteristic setting means 104 accordingly. It is possible to change to the optimum detection sensitivity simply by inputting the filter characteristic value. A storage means 101 for storing the digital value from the A / D conversion circuit, a calculation means 102 for digitally filtering the stored value, a comparison means 103 for judging metal detection from the calculated value, Filter characteristic setting means 104 capable of inputting a characteristic value of the filter,
The coefficient calculating means 105 for obtaining the coefficient of the digital filter from the input characteristic value does not need to be configured by each individual circuit, and for example, all the circuits may be configured by one integrated processing device such as a microcomputer.

[0018]

As described in detail above, the present invention replaces the analog filter for extracting the metal detection signal from the signal after the synchronous detection with the digital filter processing performed by the arithmetic means, and the filter characteristic setting means. By calculating the coefficient of the digital filter by the coefficient calculating means based on the characteristic value input to, the characteristic of the digital filter is changed according to the change of the conveyance speed of the object to be inspected and the frequency of the metal detection signal. Therefore, it is not necessary to replace the circuit element like the conventional analog filter.

[Brief description of 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 supplementary explanatory view showing an embodiment of the present invention.

FIG. 4 is a supplementary explanatory view showing an embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 oscillating circuit 2 oscillating coil 3a, 3b receiving coil 4 differential amplifying circuit 5 band pass filter 6 amplifying circuit 7a, 7b synchronous detecting circuit 8a, 8b analog filter 9a, 9b voltage comparing circuit 10a, 10b first and second transfer Phase circuit 11a, 11b A / D conversion circuit 101 Storage means 102 Calculation means 103 Comparison means 104 Filter characteristic setting means 105 Coefficient calculation means

Claims (1)

[Claims] 1. An oscillating 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 only a signal of an oscillating frequency passes. It is equipped with a bandpass filter that allows it and a synchronous detection circuit that demodulates a metal detection signal from the received signal, and detects whether metal is mixed in the inspected object by whether or not the balance is disrupted when passing through the inspected object. In the metal detection device of the type, an A / D conversion circuit that converts the signal after synchronous detection into a digital value, a storage unit that stores the digital value from the A / D conversion circuit, and a stored value A / D is provided with an arithmetic means for digital filtering, a filter characteristic setting means for inputting a characteristic value of the filter, and a coefficient calculating means for obtaining a coefficient of the digital filter from the input characteristic value. The metal value is characterized by allowing the metal detection signal to be extracted by digitally filtering the digital value obtained by converting the signal after synchronous detection by the conversion circuit, and also by freely changing the filter characteristics of the digital filter. Detection device.