JPH0792273A - Metal detector - Google Patents

Abstract

PURPOSE:To provide a metal detector which is precisely controlled to quickly minimize the effects of an object on a magnetic field. CONSTITUTION:The metal detector comprises an oscillation coil 1 which generates an alternating magnetic field, a plurality of receiving coils 3a, 3b which detect a change in the magnetic flux of the magnetic field, an amplifier circuit 5 which amplifies a difference voltage induced in the receiving coils 3a, 3b, a first synchronous detection circuit 11a which synchronously detects the output signal of the amplifier circuit 5, and a first phase-shifting circuit 15a which changes the phase of a synchronizing signal supplied to the synchronous detection circuit 11a. In the metal detection apparatus, a second synchronous detection circuit 11b, a second phase-shifting circuit 15b and a computer 16 which controls the first and second phase-shifting circuits 15a, 15b so as to eliminate the influence of an object to be inspected on the magnetic field due to the magnitude of output signals of the first and second synchronous detection circuits 11a, 11b at a time when the object, to be inspected, to which no metal has been added, is passed are installed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is an accurate and short time test in which the influence of a magnetic field on an object to be inspected is minimized when inspecting the object to be inspected for metal contamination. The present invention relates to a metal detection device that can be set.

[0002]

2. Description of the Related Art First, an outline of a conventional metal detecting device will be described with reference to FIG. In this figure, 1 is an oscillation circuit and 2 is an oscillation coil connected to the oscillation circuit. The two receiving coils 3a, 3b for receiving changes in the magnetic field are placed in the alternating magnetic field of the oscillating coil 2,
It is arranged so that the magnetic fluxes are equally linked. The differential voltage of the electromotive force induced in the receiving coils 3a and 3b becomes 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 that amplifies the received signal, 6 is a synchronous detection circuit that performs synchronous detection with the synchronous signal, and 7 is a voltage comparison circuit that determines the magnitude of the signal demodulated by the synchronous detection circuit. Reference numeral 10 is a phase shift circuit that changes the phase of the synchronization signal supplied to the synchronous detection circuit 6, and is a phase shift circuit whose amount of phase shift can be adjusted by means such as manual operation.

In the metal detecting device having such a structure, when the metal is passed between the oscillation coil 2 and the receiving coils 3a, 3b, the two receiving coils 3a, 3 are made of metal.
Since the number of magnetic fluxes linked to b is unbalanced, the received signal is output. The output reception signal includes a metal detection signal modulated by the oscillation frequency (hereinafter referred to as a modulation signal). However, since the modulation signal is weak, the amplifier circuit 5
Amplify by. In order to extract the metal detection signal from the modulation signal, the coherent detection circuit 6 performs coherent detection at the same frequency as the oscillation frequency to demodulate the metal detection signal. The demodulated signal becomes only the metal detection signal by passing through the analog filter 7, and after being rectified by the rectifying circuit 8,
The voltage comparison circuit 9 outputs the metal detection result.

Next, a case will be described in which an object to be inspected that affects a magnetic field, such as a substance containing salt or a substance wrapped in a metal film, is passed through a metal detector. In the case of such an inspected object, unlike the inspected object such as a dried product, the inspected object affects the magnetic field, so that the inspected object reacts in the same manner as metal, and normal metal detection cannot be performed. Will end up. Therefore, in order to eliminate the influence of the object to be inspected and detect the metal mixed in the object to be inspected, the following operation is required before the inspection. The operation will be described below.

In the signal vector diagram with the phase of the synchronizing signal as the X axis (see FIG. 6), when the synchronous detection is performed by the modulated signal by the device under test, the magnitude of the output signal from the rectifier circuit is Absolute value of cosine of phase difference θ (| cos
It is known to be proportional to θ |) (see FIG. 7). Therefore, if the synchronous detection is performed with the synchronization signal having a phase difference of 90 ° with respect to the modulation signal of the inspection object, the influence of the inspection object can be minimized. When inspecting an object to be inspected that influences the magnetic field in this way, it is necessary to adjust the phase of the synchronizing signal to minimize the effect of the object to be inspected on the magnetic field and inspect the mixed metal.

Therefore, in the conventional metal detection apparatus, the magnitude of the output from the object to be inspected is checked by looking at the indicator attached to the voltage comparison circuit 9 every time the object to be inspected is passed, and the phase shift circuit 10 is checked. Was adjusted to find the phase that minimizes the magnitude of the output from the device under test. Next, the method will be described in detail. In the initial state, the phase to be adjusted cannot be estimated, so as shown in FIG.
Pass the object under inspection in the state adjusted to the second time) and check the magnitude of the output signal of the rectifier circuit at that time with the display. Next, the magnitude of the output when the phase is changed at a certain phase interval (second time in the figure) is confirmed, and the phase is changed in the direction in which the magnitude of the output decreases. When the output is near the phase where the output becomes the minimum, the phase interval is narrowed, and the output near the phase is confirmed, and the phase where the influence of the object to be inspected is minimized is found. Moreover, since the phase in which the influence of the inspection object can be erased differs depending on the inspection object, it is necessary to adjust the phase as described above each time the inspection object changes, and it takes considerable labor and time to perform the inspection. Was needed.

[0007]

SUMMARY OF THE INVENTION It is an object of the present invention to solve the above-mentioned problems and provide a metal detection device which can be accurately set in a short time to a phase that minimizes the influence of the object to be inspected. Has an aim.

[0008]

In order to solve the above-mentioned problems, in a metal detecting device of the present invention, an oscillating coil for generating an alternating magnetic field, a plurality of receiving coils for detecting a change in magnetic flux of the magnetic field, and the receiving coil. An amplifier circuit that amplifies the differential voltage induced in the circuit, a first synchronous detection circuit that synchronously detects the output signal of the amplifier circuit and outputs a metal detection signal, and a phase of the synchronous signal that is supplied to the synchronous detection circuit. In a metal detection device including a first phase shift circuit that changes, a second synchronous detection circuit that synchronously detects an output signal of the amplification circuit and outputs a metal detection signal, and the metal detection device is supplied to the synchronous detection circuit. A second phase shift circuit for changing the phase of the synchronizing signal; and a second phase shift circuit for the first and second synchronous detecting circuits when a DUT not mixed with metal is passed between the oscillation coil and the receiving coil. Depending on the magnitude of the output signal, A computer for controlling the first and second phase shift circuits so that the phase difference between the modulated signal and the synchronization signal is calculated, and the influence on the magnetic field of the device under test itself can be eliminated from the calculated phase value. It is characterized by being provided.

[0009]

[Operation] The metal detector configured in this way can be used by passing an inspection object that does not contain metal once.
A computer calculates the phase difference between the modulated signal and the simultaneous signal by the device under test. Then, the phase shift amount of the phase circuit is adjusted so that the phase shift difference between the modulation signal and the synchronization signal becomes 90 degrees by the computer. In this way, the influence of the inspection object can be minimized immediately,
It is possible to shorten the adjustment time until the inspection of the inspection object is actually started.

[0010]

1 is a block diagram showing an embodiment of the present invention, in which reference numerals 1, 2, 3a, 3b, 4 and 5 indicate the same constituent elements as in FIG. Reference numerals 11a and 11b are synchronous detection circuits for performing synchronous detection with the first and second synchronization signals, 12a and 12b are analog filters, 13a and 13b are rectifier circuits, and 14a and 14
b is an A / D conversion circuit, and 15a and 15b are first and second phase shift circuits for changing the phases of the first and second synchronization signals supplied to the synchronous detection circuits 11a and 11b, which are set by a computer. The phases of the first and second synchronization signals are changed according to the above. Reference numeral 16 is a computer that inputs the results measured by the two A / D conversion circuits, performs various calculations, and controls the phase shift amounts of the first and second phase shift circuits. Here, the path from the synchronous detection circuit 11a, passing through the analog filter 12a, to the rectifier circuit 13a is the first path, and the synchronous detection circuit 11b.
From the rectifier circuit 13 through the analog filter 12b.
The route up to b is called the second route.

The phase adjustment for eliminating the influence of the magnetic field of the device under test is roughly classified into a first stage and a second stage.

First, in the first phase adjustment, the second phase shift
The phase shift amount of the circuit 15b is set to a certain phase shift amount φ.
The phase shift amount φ is an arbitrary value from 0 to 360 degrees. First phase shift
The phase shift amount of the path 15a is 0 degree. Inspected object is a metal detector
When passed through, it will interlink with the two receiving coils 3a, 3b
The number of magnetic fluxes to be generated becomes unbalanced, and the received signal is output.
The received signal contains the modulated signal generated by the device under test.
The modulated signal is unique to the device under test with respect to the synchronization signal.
Has a phase difference. Synchronous detection with the first and second synchronization signals
The detected signal of the DUT that has been waved and demodulated is an analog signal.
The rectifier circuits 13a, 13 through the filters 12a, 12b.
Enter b. Output signals from the rectifier circuits 13a and 13b
The size of the signal is measured by the A / D conversion circuits 14a and 14b.
Is determined. Here, what is called the magnitude is the output signal
It is the peak value or the integrated value of the output waveform. With the first sync signal
The second synchronization signal has a degree of φ by the second phase shift circuit 15b.
Have a phase difference. As shown in Fig. 2, the phase shift of the first synchronization signal is on the X axis.
In the signal vector diagram to be taken, the modulation signal by the inspected object
And the phase difference between the first sync signal and θ₁Then, as mentioned above
The magnitude of the output signal depends on the phase difference θ between the modulation signal and the synchronization signal.
Since it is proportional to the absolute value of cosine | cos θ |
The magnitude of the output signal obtained is | cos θ₁Proportional to
The magnitude of the output signal obtained by the second path is | cos (φ−
θ₁) | = | Sin (90−φ + θ₁) |
Therefore, the magnitude of the output signal obtained on each path
From the relationship, the position of the modulation signal and the first synchronization signal depending on the device under test
Phase difference θ ₁Can be obtained by the following equation.

[0013]

[Equation 1]

The above equation is effective when the phase shift range of the second phase shift circuit 15b is within 90 degrees. If the phase shift amount of the second phase shift circuit can be set to 90 degrees, the following equation is obtained. Can be simplified.

[0015]

[Equation 2]

The above process is performed by the A / D conversion circuits 14a, 1
It is performed by computing the digital value sent from 4b by a computer.

In order to eliminate the influence of the object to be inspected, the phase difference between the modulation signal and the synchronizing signal may be set to 90 degrees, so that the computer uses θ ₁ − with respect to the current phase value of the first phase shift circuit 15a. By shifting the phase to 90 degrees or θ ₁ +90 degrees, the phase difference between the modulation signal and the synchronization signal becomes 90 degrees, and the influence of the inspection object is eliminated. Up to this point, the phase adjustment of the first stage is performed.

By such a method, it is possible to substantially match the phase with the influence of the object to be inspected. However, the measurement error in the A / D conversion circuit, the first and second
Since the phase does not always match perfectly with such a phase due to a gain error in the path or the like, the second-stage phase adjustment is performed when the phase is set with higher accuracy. The method of phase adjustment in the second stage will be described below.

In the first phase adjustment, the second phase shift circuit 1
Although the phase shift amount of 5b was φ (degrees), in the second stage, the phase shift amount is changed so that the phase shift amount becomes several degrees or less. In this state, the object to be inspected is passed through the metal detector again, and the magnitude of the final output output from the first path and the second path is A
Measurement is performed by the / D conversion circuit, and the magnitudes of the two output signals are compared by a computer. By comparing the magnitudes of the outputs of the first path and the second path, it is determined to which side the phase where the actual influence of the object to be inspected is minimized is shifted. For example, if the output on the first path side is large, it is determined that the phase is shifted to the second synchronization signal side, and if the output on the second path side is large, it is determined that the phase is shifted to the first synchronization signal side. Then, the phase is changed in the determined direction. Such phase adjustment is repeated several times, and when the difference in output magnitude between the first path and the second path falls within a certain range, the second phase adjustment is completed. The second-stage phase adjustment is similar to the conventional method, but the output is determined by one path in the related art, but by changing the phase shift amount of the second phase shift circuit 15b, The outputs at the two phase values can be compared at the same time.

In this way, following the first phase adjustment,
By performing the phase adjustment in the second stage, it is possible to accurately eliminate the influence of the inspection object in a short time.

As an embodiment for solving the problem, the above-mentioned phase adjustment can be similarly performed when the first phase shift circuit and the second phase shift circuit are arranged in parallel as shown in FIG.

Further, the phase adjustment of the second stage is performed by the ambient temperature,
This is also effective when the phase of the modulation signal of the object to be inspected changes for a long time due to the temperature of the object to be inspected and the setting of the object to be inspected changes.
The output magnitudes of the path and the second path are averaged N times, the average values are compared, and the phase is adjusted in the same manner as in the second step, so that the phase of the object to be inspected due to temperature changes automatically. The influence of the object to be inspected can be eliminated.

[0023]

As described above, according to the present invention, the influence of the object to be inspected on the metal detection signal can be accurately minimized, and further, the adjustment for the fluctuation of the temperature during operation can be automated. It is also possible to provide a highly useful metal detection device.

[Brief description of drawings]

FIG. 1 is a configuration diagram of a metal detection device according to the present invention.

FIG. 2 is an explanatory diagram of phase adjustment of the apparatus of FIG.

FIG. 3 is a flowchart showing an algorithm for phase tracking in the apparatus of FIG.

FIG. 4 is a diagram showing a modification of FIG.

FIG. 5 is a configuration diagram of a conventional example.

FIG. 6 is a diagram showing a phase of a modulation signal.

FIG. 7 is a diagram showing a relationship between a phase difference between an output signal and a modulation / synchronization signal.

FIG. 8 is a diagram illustrating a conventional phase adjustment method.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 oscillating circuit 2 oscillating coil 3a, 3b receiving coil 4 variable resistance for differential balance adjustment 5 differential amplifying circuit 6, 11a, 11b synchronous detection circuit 9 voltage comparing circuit 10 phase circuit 12a, 12b analog filter 13a, 13b rectifying circuit 14a, 14b A / D conversion circuit 15a, 15b Phase circuit 16 Computer

Claims (1)

[Claims] 1. An oscillating coil for generating an alternating magnetic field, a plurality of receiving coils for detecting a change in magnetic flux of the magnetic field, an amplifier circuit for amplifying a differential voltage induced in the receiving coils, and an output signal of the amplifier circuit. A metal detection device comprising a first synchronous detection circuit for synchronously detecting and outputting a metal detection signal, and a first phase shift circuit for changing the phase of a synchronization signal supplied to the synchronous detection circuit. A second synchronous detection circuit for synchronously detecting the output signal of the amplifier circuit and outputting a metal detection signal;
A second phase shift circuit for changing the phase of the synchronization signal supplied to the synchronous detection circuit, and the first phase shift circuit when a device under test containing no metal is passed between the oscillation coil and the reception coil. Also, the phase difference between the modulation signal by the device under test and the synchronization signal is calculated from the magnitude of the output signal of the second synchronous detection circuit, and the influence on the magnetic field of the device under test itself can be eliminated from the calculated phase value. And a computer for controlling the first and second phase shift circuits.