JP2990458B2 - Metal detector - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a metal detector for determining the presence or absence of a metal contained in a test object by detecting the influence of the test object on a magnetic field.

[0002]

2. Description of the Related Art In a metal detector of this kind, a current is applied to a coil or the like to generate a magnetic field, and a magnetic sensor such as a coil is disposed at a position receiving the magnetic flux of the magnetic field. A change in magnetic flux due to the body is detected by a signal from a magnetic sensor, and the presence or absence of metal in the test object is determined based on the detection signal.

FIG. 4 shows a configuration of a conventional metal detector for detecting metal by an alternating magnetic field.

A magnetic field generator 1 of the metal detector converts a reference pulse signal of a predetermined frequency output from a reference signal generator 2 into a sine wave by a filter 3, and amplifies the sine wave signal by a power amplifier 4. A predetermined exciting current is caused to flow through the transmission coil 5 to generate a magnetic field from the transmission coil 5.

[0005] The magnetic sensor of the metal detector is formed by a pair of receiving coils 6 and 7 arranged at positions receiving equal amounts of magnetic flux of a magnetic field generated from the transmitting coil 5.

[0006] Voltage signals of the same magnitude induced in the receiving coils 6 and 7 are input to a differential amplifier 8. Therefore, when the device under test W is at a position where it does not intersect the magnetic field, the output of the differential amplifier 8 becomes zero.

Here, when the test object W moves between the transmitting coil 5 and one of the receiving coils 6, the test object W and the metal contained in the test object W affect the magnetic field.
The magnetic flux crossing the two receiving coils 6, 7 becomes unbalanced.

As a result, a difference occurs between the voltages induced in the receiving coils 6 and 7, and the differential signal is output from the differential amplifier 8 to the detection circuit 9.

On the other hand, a phase shift reference signal in which the phase of the reference pulse signal is changed is input from the phase shift circuit 10 to the detection circuit 9.

The detection circuit 9 performs synchronous detection of the differential signal by the phase shift reference signal, and outputs the detection output signal to the determination means 12 via an LPF (low-pass filter) 11.

When the level of the input detection output signal exceeds a reference value, the judging means 12 outputs a judgment signal indicating that metal is mixed in the inspection object W. The level display 13 displays the output level of the LPF 11.

In the metal detector configured as described above, in order to detect metal contamination with high sensitivity, the object W
It is necessary to adjust the phase shift circuit 10 to a phase that has the least effect on its own magnetic field.

That is, the magnitude of the differential signal by the device under test itself changes according to the detection phase, as shown in FIG. 5, and the initial detection phase S for this device under test has a minimum level from the beginning. Hardly coincides with the phase P.

For this reason, in the conventional metal detector, the level display 11 is viewed each time the test object containing no mixed metal is passed through the magnetic field, and the phase shift circuit 10 is operated until the display level becomes the minimum value. The display level is varied by a predetermined phase step Δα, and when the display level is reduced, the phase is varied by a smaller phase step Δβ to drive the phase P to the minimum level.

[0015]

However, in the metal detector configured as described above, the initial phase S and the final phase P
If the distance between the target and the target is greatly different, the target must be passed through the magnetic field several tens of times to reach the final phase, and it takes a long time to start the actual inspection.

For this reason, it is conceivable to increase the phase step Δα, but when the phase step is increased,
In some cases, the number of times over the target final phase increases, and on the contrary, the number of times before reaching the final phase increases. An object of the present invention is to provide a metal detector that solves this problem.

[0017]

In order to solve the above-mentioned problems, a metal detector according to the present invention is provided with a magnetic field generating section for generating a magnetic field having the same frequency as a reference signal having a predetermined frequency, and receiving an equal amount of magnetic flux of the magnetic field. A pair of magnetic sensors disposed at a position, a differential output circuit that outputs a differential component of the pair of magnetic sensors, and a first phase shifter that shifts the reference signal in accordance with input phase data. A first phase shift circuit that outputs a phase shift reference signal, a second phase shift circuit that outputs a second phase shift reference signal that is 90 degrees different in phase from the first phase shift reference signal, A first detection circuit for synchronously detecting a differential signal from a differential output circuit using the first phase shift reference signal; and a second phase shift reference signal for detecting a differential signal from the differential output circuit. A second detection circuit for synchronous detection by the A peak value detecting means for detecting a peak value of said first output signal of the peak value and the second detection circuit of the output signal of the detection circuit when it passes through the Sakainai,
A tangent angle of a triangle having two peak values detected by the peak value detection means as orthogonal sides is calculated as a phase difference between the differential signal and the reference signal due to the passage of the test object that does not contain the mixed metal. Phase difference calculating means for outputting phase data corresponding to the phase difference to the first phase shift circuit;
After the phase data is input from the phase difference calculating means to the first phase shift circuit, the presence or absence of a mixed metal in the test object passing through the magnetic field is determined by the first detection circuit or the second detection circuit. Determining means for determining based on the output signal.

[0018]

In the metal detector constructed as described above, when the test object containing no mixed metal is passed through the magnetic field once, the differential signal is converted by the first detection circuit and the second detection circuit. The peaks of the respective detection output signals are detected at phases different from each other by 90 °, and the tangent angle of a triangle having the two peak values as orthogonal sides is the differential value due to the passage of the test object containing no mixed metal. The phase difference between the signal and the reference signal is calculated, and phase data corresponding to the difference is input to the first phase shift circuit. For this reason, when the next test object passes through the magnetic field, the detection output of the first detection circuit or the second detection circuit is such that the component due to the influence of the test object itself decreases to a level close to the minimum level. Then, based on the level of the detection output, the presence or absence of the mixed metal contained in the inspection object is determined.

The phase difference calculating means calculates a phase difference between the reference signal and the differential signal based on the detection output signal, and inputs phase data corresponding to the difference to the first phase shift circuit. For this reason,
Next, the output of the differential signal component due to the test object passing through the magnetic field decreases to a level close to the minimum level, and only the differential signal component due to the influence of the mixed metal included in the test object,
The detection is performed by the first detection circuit or the second detection circuit, and the presence or absence of mixed metal is determined based on the level of the detection output signal.

[0020]

An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram showing a configuration of a metal detector according to one embodiment of the present invention. A magnetic field generator 1 receives signals from a reference signal generator 2 similarly to the metal detector shown in FIG. Is converted into a sine wave by the filter 3, and the sine wave signal is amplified by the power amplifier 4 to generate a magnetic field from the transmission coil 5. Further, the induced voltages of the receiving coils 6 and 7 are differentially amplified by the differential amplifier 8.

On the other hand, the reference pulse signal from the reference signal generator 2 is input to the first phase shift circuit 20. The first phase shift circuit 20 outputs a first phase shift reference signal obtained by delaying the reference pulse signal by a phase angle corresponding to phase data Φ from a phase setting unit 30 described later.

The first phase shift reference signal is input to a second phase shift circuit 21. The second phase shift circuit 21
And outputs a second phase-shifted reference signal obtained by delaying the phase-shifted reference signal by 90 degrees.

The differential signal from the differential amplifier 8 is input to a first detection circuit 23 and a second detection circuit 26.

The first detection circuit 23 synchronously detects the differential signal with the first phase shift reference signal, and outputs the detection output signal L
The signal is output to the A / D converter 25 via the PF 24.

The second detection circuit 26 synchronously detects the differential signal by the second phase shift reference signal, and outputs the detection output signal to L.
The signal is output to the A / D converter 28 via the PF 27.

The outputs of the A / D converters 25 and 28 are input to the judgment means 29. Upon receiving a determination start signal from a mode switching unit 40 described later, the determination unit 29
Based on the output values of the A / D converters 25 and 28, the presence or absence of metal contained in the test object is determined.

The outputs of the A / D converters 25 and 28 are
It is input to the peak value detecting means 31 of the phase setting unit 30.

The peak value detecting means 31 detects and outputs the peak values A and B of the outputs of the A / D converters 25 and 28 each time the test object passes through the magnetic field.

The phase difference calculating means 32 calculates, as the initial phase data θ, the tangent angle of a triangle having the first input peak values A ₀ and B ₀ as orthogonal sides after receiving the reset signal from the mode switching means 40. And output.

After receiving the mode switching signal, the fine adjustment means 33 performs a predetermined phase step Δγ every time a second or subsequent peak value is input so that, for example, A of the peak value becomes minimum. The fine adjustment data K (= ± NΔγN is an integer) is variably output, and when the peak value becomes minimum, a setting completion signal is output.

The adder 34 outputs the sum of the initial phase data θ and the fine adjustment data K to the first phase shift circuit 20 as phase data Φ.

When the phase setting switch 41 is turned on, the mode switching means 40 outputs a reset signal to the phase difference calculation means 32 and the fine adjustment means 33,
The determination operation by the determination means 29 is stopped, and the fine adjustment means 3
When the setting completion signal is received from the control unit 3, the determination unit 29 outputs a determination start signal to the determination unit 29 to start the determination operation.

The setting completion notifier 43 converts the setting completion signal from the fine adjustment means 33 into a sound or light to notify that the setting has been completed.

Next, the operation of the metal detector will be described. When an object to be inspected whose optimum detection phase is not known is to be inspected, the phase setting switch 41 is turned on in advance to allow the object to be inspected that does not contain mixed metal to pass through the magnetic field.

At this time, the phase data Φ of the first phase shift circuit 20 is zero, and the reference pulse signal is directly input to the first detection circuit 23 as the first phase shift reference signal.
The second phase-shift reference signal obtained by shifting the phase of the second phase-shift reference signal by 90 degrees is input to the second detection circuit 24.

At this time, as shown in FIG. 2, the differential signal of the test object containing no mixed metal is represented by the phase θ ₁ on the xy coordinate of the reception phase based on the state where the test object is not present. And the coordinate XY based on the phase of the reference pulse signal has a predetermined phase difference with respect to the xy coordinate. The phase angle θ of the differential signal on the coordinates XY corresponds to the tangent angle between the peak value A ₀ of the output of the first detection circuit 23 and the peak value B ₀ of the output of the second detection circuit 24. ing.

By this one pass of the test object, an initial phase θ for making one of the peak values A zero is calculated by the phase difference calculating means 32. This initial phase θ is obtained by calculating the phase data Φ. Is output to the first phase shift circuit 20 (at this time, the fine adjustment data K is zero).

For this reason, the XY coordinates are rotated by the initial phase θ, and the X-axis is substantially coincident with the phase of the differential signal. It will be detected.

Accordingly, as shown in FIG. 3, the second detection phase changes by θ from the first detection phase, and the output of the first detection circuit 23 becomes almost the minimum level.

After the second passage of the test object, the fine adjustment means 33 converts the phase data Φ into a minute phase step Δγ.
And the phase that minimizes the influence of the test object itself is determined. Therefore, by passing the test object containing no mixed metal through the magnetic field several times, the setting completion signal from the fine adjustment means 33 is obtained. Is output.

When the setting completion signal is output from the fine adjustment means 33, the phase data Φ for the first phase shift circuit 20 is held, and the determination means 29 is in a state where it can determine the presence or absence of the mixed metal. The device 43 notifies the completion of the phase setting.

Thereafter, when the test object is sequentially passed through the magnetic field, the test object itself has the least influence, that is,
Metal detection is performed in a state where the detection sensitivity to the mixed metal is the highest.

In the metal detector of the above-described embodiment, the magnetic sensor is constituted by the pair of receiving coils 6 and 7 arranged so as to face the transmitting coil 5, but a semiconductor sensor such as a Hall element is used. The present invention can be similarly applied to a metal detector used as a magnetic sensor.

The present invention can be similarly applied to a metal detector in which a transmitting coil and a receiving coil are coaxially arranged, and a metal detector in which a receiving coil is arranged in the same direction inside a large-diameter transmitting coil. .

In the above-described embodiment, the phase is finely adjusted by the fine adjustment means after the first phase setting. The phase setting may be terminated only by the above operation.

[0047]

As described above, the metal detector according to the present invention outputs signals from the first detection circuit and the second detection circuit when the test object containing no mixed metal is passed through the magnetic field. The peak value of each of the detected signals is detected, and the tangent angle of a triangle having the two peak values as orthogonal sides is calculated as the phase difference between the differential signal and the reference signal due to the passage of the test object. Is input to the first phase shift circuit to set the detection phase for the next device under test so that the effect of the device under test itself is minimized.

For this reason, the detection phase can be set to a value that can detect the mixed metal with the highest detection sensitivity for the test object only by passing the test object through the magnetic field once or several times. Inspection operation can be started at the time.

[Brief description of the drawings]

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

FIG. 2 is a diagram for explaining a relationship between a detection output and a phase according to one embodiment.

FIG. 3 is a diagram for explaining the operation of one embodiment.

FIG. 4 is a block diagram showing a configuration of a conventional device.

FIG. 5 is a diagram for explaining the operation of the conventional device.

[Explanation of symbols]

 Reference Signs List 1 magnetic field generating unit 2 reference signal generator 5 transmitting coil 6, 7 receiving coil 8 differential amplifier 20 first phase shift circuit 21 second phase shift circuit 23 first detection circuit 25 A / D converter 26 second Detection circuit 28 A / D converter 29 determination means 30 phase setting unit 31 peak value detection means 32 phase difference calculation means 33 fine adjustment means 34 adder 40 mode switching means 43 setting completion alarm

Continuation of the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) G01V 3/10 G01V 3/08

Claims (1)

(57) [Claims] 1. A magnetic field having the same frequency as a reference signal having a predetermined frequency.
And a pair of magnets arranged at positions receiving equal amounts of magnetic flux of the magnetic field.
And a differential output circuit that outputs a differential component of the pair of magnetic sensors.
Road and the reference signalEnterPhase shift according to the phase data
A first phase shift circuit for outputting a first phase shift reference signal, and a first phase shift circuit having a phase difference of 90 degrees with respect to the first phase shift reference signal.
A second phase shift circuit for outputting a second phase shift reference signal, and a differential signal from the differential output circuit, the first phase shift circuit
A first detection circuit that performs synchronous detection based on a quasi-signal, and a differential signal from the differential output circuit,
A second detection circuit that performs synchronous detection based on the quasi-signal, and that an object to be inspected containing no mixed metal has passed through the magnetic field.
Output signal of the first detection circuitPeak value and the
And the peak for detecting the peak value of the output signal of the detection circuit 2
Value detection means; Two peaks detected by the peak value detecting means
The tangent angle of a triangle whose value is an orthogonal side is calculated by
No The differential signal due to the passage of the test objectIssueThe reference signal
Phase differenceAsCalculated, and the phase data corresponding to the phase difference is calculated.
Phase difference calculating means for outputting data to the first phase shift circuit; and a phase data from the phase difference calculating means to the first phase shift circuit.
Of the test object passing through the magnetic field after the
The first detection circuit or the second detection
Determining means for determining based on an output signal of the circuit.
Metal detector.