JPH09203782A - Metal detecting device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable filter characteristics to be changed automatically by providing A/D converter circuits for converting signals after synchronous detection into digital values, a storage means for storing the digital values, a computing means for subjecting the stored values to digital filter processing, etc. SOLUTION: Signals after synchronous detection are sampled in a certain period and converted into digital values by A/D converter circuits 11a, 11b. A storage means 101 has an area to store (n) data in such a way that data originated by sampling are stored from new ones. Even if the speed at which a subject for inspection is conveyed is varied, the sensitivity of metal detection can be held constant by varying the characteristic of a digital filter automatically through a series of processes by a rotary encoder 21 and a frequency calculation means 104, for calculating the frequencies of metal detection signals, a coefficient calculation means 105 for calculating the coefficient of the digital filter from the calculated filter characteristic value, and a computing means 102 for performing computations about the digital filter in accordance with the calculated coefficient.

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. Reference numeral 20 is a kind of belt conveyor, which is a means for conveying the object to be inspected.

In the metal detecting device having such a structure, a conveyor means such as a belt conveyor 20 is used to pass the object to be inspected between the oscillation coil 2 and the receiving coils 3a and 3b or between the oscillation coil and the receiving coil. , Metal detection. When a metal is mixed in the inspected body, the mixed metal changes the balance state of the magnetic field and the electromotive forces 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 the analog filters are arranged so that the frequency of the metal detection signal falls within the pass frequency range. The circuit elements such as capacitors, inductors, and resistors that form 8a and 8b are determined.

However, the frequency of the metal detection signal demodulated by the synchronous detection circuits 7a and 7b depends on the carrier speed of the belt conveyor 20 or the like, which is a carrier of the object to be inspected, and the oscillation.
It is determined by the dimensions such as the interval and shape of the receiving coil.
Therefore, when changing the transport speed of the object to be inspected, the frequency of the metal detection signal changes and the detection sensitivity decreases,
At the same time, it is necessary to change the pass frequency range of the analog filter. As a method therefor, the following procedure has been conventionally performed.

For example, first, a conveyor tachometer of a metal detecting device is measured using a portable tachometer or the like, and the frequency of the metal detecting signal is derived from the size of the oscillation, the interval between the receiving coils, the shape and the like. Alternatively, the frequency of the metal detection signal is directly obtained by a frequency analyzer such as a spectrum analyzer. And
An analog filter having that frequency as the pass frequency range was designed, the circuit constants constituting the analog filter were determined, and the circuit elements were replaced. If there is no special device such as a portable tachometer for measuring the transport speed or a frequency analyzer, check the detection sensitivity while changing the circuit element of the analog filter little by little to make the detection sensitivity the highest. The circuit constant was decided. In any case, there was no choice but to prepare special equipment, perform circuit constant calculations, switch elements, and perform other extremely time-consuming work.

The present invention solves this problem and automatically changes the filter characteristics without changing the circuit element of the filter even when the frequency of the metal detection signal changes due to the change of the transport speed of the object to be inspected. It is an object of the present invention to provide a metal detection device that keeps the metal detection sensitivity constant.

[0007]

[Means for solving the problem]

(1) In order to solve the above problems, in the metal detection device of the present invention, an A / D conversion circuit that converts the signal after synchronous detection into a digital value, and a storage unit that stores the digital value,
Calculating means for digitally filtering the stored value,
A speed detector for detecting the speed at which the object to be inspected is conveyed between the oscillation coil and the reception coil or in the oscillation coil and the reception coil, and the frequency of the metal detection signal is determined by the speed signal from the speed detector. It is provided with a frequency calculating means for obtaining and a coefficient calculating means for obtaining a coefficient of the digital filter having the frequency obtained by the frequency calculating means within the pass frequency range.

(2) A / D conversion circuit for converting the signal after synchronous detection into a digital value, storage means for storing the digital value, arithmetic means for digitally filtering the stored value, and At least two object detectors, which are arranged at intervals, between the oscillation coil and the receiving coil on which the object to be inspected is conveyed, or on the conveying path in the oscillation coil and the receiving coil, and from the object detector. The transport speed is calculated by each passage detection signal of the inspected object during transport of
It is provided with a frequency calculation means for obtaining the frequency of the metal detection signal from the transport speed and a coefficient calculation means for obtaining the coefficient of the digital filter having the frequency obtained by the frequency calculation means within the pass frequency range.

In this way, the pass frequency range of the filter for extracting the metal detection signal is changed to an optimum value while detecting the transport speed of the object to be inspected, so that the transport speed changes and the frequency of the metal detection signal changes. Even if it changes, the detection sensitivity can be automatically kept constant.

[0010]

1 is a block diagram showing an embodiment of the present invention, in which reference numerals 1, 2, 3a, 3b, 4, 5, 6, 7a,
7b, 10a, 10b and 20 are the same as those in FIG.
Reference numeral 21 is a rotary encoder that detects the transport speed from the driving portion of the belt conveyor 20, 11a and 11b are A / D conversion circuits that convert the signal after synchronous detection into a digital value, 1
Reference numeral 01 is a storage means for storing a digital value, 102 is a calculation means for digitally filtering the stored value, 103 is a comparison means for judging the presence or absence of metal based on the calculation result of the digital filter, and 104 is a speed signal from the rotary encoder. Frequency calculation means for further obtaining the frequency of the metal detection signal, and 105 is coefficient calculation means for obtaining the coefficient of the digital filter having the frequency obtained by the frequency calculation means within the pass frequency range.

When the object to be inspected 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, x ₁ from the new data in order, x _2, x ₃ When _{~x n, 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 and its problems,
The signal after synchronous detection includes the metal detection signal, but it is also very weak and includes an unnecessary signal. Therefore, data x _{1 to} x obtained by converting the signal after synchronous detection into digital values
_It is not yet possible to judge from _n whether metal is mixed. Therefore, in order to extract only the metal detection signal from the obtained data x ₁ ~x _n, the calculation means 102, the data x ₁ ~x
_{For n} , several Hz which is the frequency component of the metal detector signal
Digital filter processing is performed to pass only around several tens Hz.

FIG. 3 is a block diagram showing another embodiment of the present invention, wherein reference numerals 1, 2, 3a, 3b, 4, 5, 6, 6 and 7 are used.
a, 7b, 10a, 10b, 20, 101, 102, 1
Reference numerals 03 and 105 are the same as those in FIG. 1, and a description thereof will be omitted. Reference numerals 22a and 22b denote two photoelectric switches as an example of an object detector disposed on the belt conveyor 20, and 104 is a conveyance speed according to each passage detection signal of the object to be inspected during conveyance from the photoelectric switch. Is a frequency calculating means for calculating the frequency of the metal detection signal from the transport speed.

The photoelectric switches 22a and 22b are arranged at intervals between the oscillating coil and the receiving coil or on the carrier path between the oscillating coil and the receiving coil, and the object to be inspected is the above-mentioned. Belt conveyor 20
Passage is detected when it is transported over. The photoelectric switches 22a and 22b are used for the belt conveyor 20.
A proximity switch or the like may be used as long as it can detect the passage of the inspection object conveyed above.

In the frequency calculating means, the photoelectric switch 22a, 22b is connected to the photoelectric switch 22a, 22b.
22b is input, a conveyance speed of the belt conveyor 20 is calculated from the time difference between the passage detection signals, and the conveyance speed, the oscillation, the interval between the receiving coils,
The frequency of the metal detection signal is obtained based on the shape and the like. The coefficient of the digital filter is obtained by the coefficient calculating means 105 according to the frequency of the metal detection signal, and the digital filter processing is performed by the calculating means 102 using the coefficient to extract the metal detection signal. Detailed operations of the coefficient calculation means 105 and the calculation means 102 are the same as those in the above-mentioned embodiment.

In this way, the photoelectric switches 22a, 22b,
The frequency calculating means 104, the coefficient calculating means 105, and the calculating means 102 detect the conveying speed of the object to be inspected, and automatically and optimally change the passing frequency range of the digital filter to convey the belt conveyor 20. The metal detection sensitivity is kept constant according to the fluctuation of speed.

[0018]

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.

[0019]

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

[0020]

[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.

The conveyance speed at which the object to be inspected is conveyed by the belt conveyor 20 is always detected and output as a speed signal by the rotary encoder 21, and the frequency calculation means 104 is used.
Is input to The frequency calculating means calculates the frequency of the metal detection signal based on the input speed signal, the oscillation, the interval between the receiving coils, the shape, etc., and sends the value to the coefficient calculating means. Then, the coefficient calculating means calculates the coefficient of the digital filter having the frequency of the metal detection signal in the pass frequency range, and sends the coefficient to the calculating means.

For example, the conveyance speed of the belt conveyor 20 is changed and the frequency of the metal detection signal is changed from 4 Hz to 6 Hz.
In the case of the change to, since 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 is lowered. The rotary encoder 21 and the frequency calculation means 104 detect that the frequency of the metal detection signal has changed from 4 Hz to 6 Hz,
The frequency is sent to the coefficient calculation means 105. The coefficient calculating means 105 has a lower limit frequency f _L and an upper limit frequency f _H of 4 Hz and 8 _H , respectively, so as to pass a metal detection signal of 6 Hz.
The coefficient of the digital filter to be z is calculated by executing the following formula. The obtained coefficients are shown in Table 2 (when the order k is 21).

[0024]

[Equation 2]

[0025]

[Table 2]

The calculated 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 unless the rotary encoder 21 and the frequency calculation means 104 detect that the frequency of the metal detection signal has changed. Not changed 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 . The digital filter with the obtained coefficient has a center frequency f _{0 of 6} Hz as shown in FIG.
It can be seen that the characteristics of the bandpass filter are shown. Therefore, it is possible to obtain the same detection sensitivity as that before the conveyance speed of the inspection object is changed.

As described above, the rotary encoder 21 and the frequency calculating means 10 for calculating the frequency of the metal detection signal.
4, the coefficient calculation unit 105 that calculates the coefficient of the digital filter from the calculated filter characteristic value, and the calculation unit 102 that calculates the digital filter according to the calculated coefficient, the transport speed of the inspection object changes. In this case, the characteristics of the digital filter can be changed automatically and the metal detection sensitivity can be kept constant.

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, and a comparison means for judging metal detection from the calculated value. 103, frequency calculation means 104 for obtaining the frequency of the metal detection signal from the speed signal from the rotary encoder, and coefficient calculation means 105 for obtaining the coefficient of the digital filter having the frequency of the metal detection signal as the pass frequency range are respectively independent circuits. It is not necessary to configure all the circuits, but all the circuits may be configured by one integrated processing device such as a microcomputer.

[0029]

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 also provides the speed detector, Alternatively, the object detector detects the conveying speed of the object to be inspected, and the pass frequency range of the digital filter can be changed based on this speed. Can be kept constant.

[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 configuration diagram showing another embodiment of the present invention.

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

FIG. 5 is a supplementary explanatory diagram 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 20 Belt conveyor 21 Rotary encoder 22a, 22b Photoelectric switch 101 Storage means 102 Calculation means 103 Comparison means 104 Frequency calculation means 105 Coefficient calculation means

Claims (2)

[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 for converting the signal after synchronous detection into a digital value, a storage means for storing the digital value, and an operation means for digitally filtering the stored value. A speed detector for detecting a speed at which the object to be inspected is conveyed between the oscillation coil and the reception coil or in the oscillation coil and the reception coil, and a speed signal from the speed detector. A frequency calculating means for calculating a frequency of the metal detection signal by,
A coefficient calculating means for calculating a coefficient of a digital filter having a frequency obtained by the frequency calculating means within a pass frequency range, and a digital value for a digital value obtained by converting the signal after the synchronous detection by the A / D conversion circuit. A metal detection device, wherein a metal detection signal can be extracted by performing a filtering process, and a pass frequency range of a digital filter can be changed according to a transportation speed of the object to be inspected. 2. 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 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 for converting the signal after synchronous detection into a digital value, a storage means for storing the digital value, and an operation means for digitally filtering the stored value. , At least two objects which are arranged at intervals between the oscillation coil and the receiving coil on which the object to be inspected is transported, or along a transportation path in the oscillation coil and the receiving coil. A detector and a frequency calculating means for calculating a carrying speed from each passing detection signal of the inspected object being carried from the object detector, and a frequency of the metal detection signal from the carrying speed, and the frequency calculating means. And a coefficient calculating unit for calculating a coefficient of a digital filter that sets the obtained frequency within the pass frequency range, and performs digital filter processing on the digital value obtained by converting the signal after the synchronous detection by the A / D conversion circuit. A metal detection device, characterized in that a metal detection signal can be extracted, and a pass frequency range of a digital filter can be changed according to a transportation speed of the object to be inspected.