JPH05134053A - Metal detecting apparatus - Google Patents

Abstract

PURPOSE:To cope with the frequency change even if the exciting frequency of an oscillating coil is changed into the frequency suitable for material under inspection without changing the elements of circuits constituting a signal processing system. CONSTITUTION:An oscillating coil L1 is excited with an oscillator, and an alternating field is generated. The difference frequency, which is obtained with two receiving coils L2 and L3 that are interlinked with the magnetic flux of the magnetic field, is detected with a synchronizing signal, whose phase is different. A piece of metal passing between the oscillating coil and the receiving coils is detected. The oscillating frequency of the difference signal is converted into the other constant frequency, which is different from that of the difference signal with frequency converting means 20 and 21.

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,
In particular, the present invention relates to a device that processes a signal from a detection unit including an oscillation coil and two receiving coils that are differentially connected to detect a metal mixed in an object to be inspected.

[0002]

2. Description of the Related Art Conventionally, FIG.
There is one shown in the configuration block diagram of. In the figure, the detection unit 1 includes an oscillating coil L ₁ for generating an alternating magnetic field,
The object to be inspected, which is made up of two receiving coils L ₂ and L ₃ arranged so as to interlink the magnetic flux of the magnetic field, is the oscillation coil L ₁ and the receiving coils L ₂ and L 3. Pass between ₃ and the direction of arrow A.

The oscillation coil L ₁ is, for example, 100 kHz.
It is excited by the z oscillation circuit 2 to generate a high frequency magnetic flux. The receiving coils L ₂ and L ₃ are arranged so that a part of this magnetic flux is linked in an equal amount. Then, the one end opposite polarities of the reception coil L _2, L _3, i.e. the difference is dynamically connected in series, and the other end is connected to both ends of the variable resistor 3 to each other, each receiving coil L _2, L ₃ is balanced and a differential voltage signal is obtained from the output terminal 4.

Next, as a signal processing system, 5 is an amplifier circuit in the first stage, 6 is a bandpass filter having the same center frequency as the output frequency of the oscillation circuit 2, 7 is an amplifier circuit in the second stage,
Reference numeral 9 is a synchronous detection circuit for demodulating the magnetic and non-magnetic metal detection signals, 10 and 11 are analog filters having the same center frequency as the metal detection signal, and 12 and 13 are voltage comparison circuits for magnetic and non-magnetic metal, respectively. is there.

Reference numerals 14 and 15 denote the synchronous detection circuit 8 and
First and second phase shift circuits for changing the phase of the sync signal supplied to the sync signal 9 and the sync signals having phases suitable for detecting magnetic and nonmagnetic metals by the first and second phase shift circuits 14 and 15. To form. Reference numeral 16 denotes a synchronizing signal generating circuit, which has the same frequency as the output frequency of the oscillating circuit 2 and is synchronized.

In the metal detecting device having such a structure, the object to be inspected is passed between the oscillation coil L ₁ and the receiving coils L ₂ and L ₃ in the direction of arrow A to detect the metal.
When metal is mixed in the inspection object, the received signal includes a metal detection signal for modulating the frequency for exciting the oscillation coil L ₁ . Since the signal also includes many unnecessary signals having different frequencies, these unnecessary signals are removed by the band-pass filter 6 that passes only signals near the excitation frequency.

In order to extract the metal detection signal from this signal, the metal detection signal is demodulated in the coherent detection circuits 8 and 9 by the coherent detection with the sync signals having the same frequency as the excitation frequency but different phases. To do. Then, only the metal detection signal is passed by the analog filters 10 and 11, and the detection result of the magnetic or non-magnetic metal is output by the voltage comparison circuits 12 and 13.

[0008]

However, when inspecting the object to be inspected for the presence of metal, it is necessary to perform the excitation at a frequency suitable for the object to be inspected. Therefore, when the excitation frequency is changed, in the conventional metal detection device, it is necessary to change the circuit constant that determines the center frequency of the bandpass filter 6 and the frequency of the synchronization signal for synchronous detection. In addition, the phase shift circuits 14 and 1 that form the synchronization signal
When 5 is composed of resistors, capacitors, coils, etc.,
Since the phase shift angle changes depending on the frequency, there is a problem in that the circuit constants forming the phase shift circuit must also be changed.

As described above, when changing the exciting frequency of the metal detecting device, the elements of each circuit constituting the signal processing system are changed each time. Therefore, it takes time to replace and adjust the elements, and it has been necessary to prepare elements of many kinds of values.

The present invention has been made in view of the above points,
The purpose is to solve the above-mentioned problems, and even when changing the excitation frequency suitable for the object to be inspected, it is possible to respond immediately without changing the elements of each circuit constituting the signal processing system such as a bandpass filter. It is to provide a detection device.

[0011]

The structure of the present invention for achieving the above-mentioned object is to excite an oscillating coil by an oscillator to generate an alternating magnetic field, and to generate an alternating magnetic field from two receiving coils interlinking with the magnetic flux of the magnetic field. The obtained difference signal is detected by a synchronization signal having a different phase from each other, from each output signal, in the device for detecting the metal passing between the oscillation coil and the receiving coil, the oscillation frequency of the difference signal, It is characterized in that it is converted into another fixed frequency different from the above frequency by the frequency conversion means.

[0012]

Since the present invention is constructed as described above, by performing frequency conversion of the frequency of the received signal output from the receiving coil, it becomes possible to perform signal processing at a constant frequency of the conventional device, and a bandpass filter. It is not necessary to replace the elements of each circuit that constitutes the equal signal processing system.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT A preferred embodiment of the present invention will be illustratively described in detail below with reference to the drawings. FIG. 1 is a block diagram showing the configuration of an embodiment of the metal detector of the present invention. The same parts as those in FIG. In FIG. 1, 20 is a frequency conversion signal generating circuit, and 21 is a frequency conversion circuit for converting a received signal into another frequency.

When the object to be inspected passes between the oscillating coil L ₁ and the receiving coils L ₂ and L ₃ in the direction of arrow A, the receiving signals from the receiving coils L ₂ and L ₃ are the metal detection signals at the excitation frequency. The carrier signal having the same frequency as that of (output frequency of the oscillation circuit 2) is output in the form of being modulated. Here, f _d is the frequency of the metal detection signal, and f _g is the excitation frequency for the carrier signal of the metal detection device. First, the received signal is amplified by the amplifier circuit 5 in the first stage.

The frequency conversion circuit 21 converts this signal into
A signal with a frequency of f _t + f _g or f _t −f _g from the frequency conversion signal generation circuit 20 is multiplied and converted to another frequency f _t . The frequency-converted received signal is in a form in which the carrier-frequency signal f _t after frequency conversion is modulated by the metal detection signal. Here, the frequency conversion circuit 21 is a circuit that mixes two signals, and includes, for example, a signal mixer circuit, a multiplication circuit, a balanced modulation circuit, a voltage control amplification circuit, and the like.

Now, when the excitation frequency f _g is changed to the frequency f _g1 , the frequency of the carrier signal component of the received signal is also changed to f _g1 , but the frequency conversion circuit 21 causes f _t + f _g1 or f _t.
If multiplied by -f _g1 , the frequency of the carrier signal component of the converted received signal becomes f _t , and even if the excitation frequency is changed,
After the frequency conversion, the frequency f _t of the carrier signal component can be kept constant.

The bandpass filter 6 has an effect of passing only a signal of a necessary frequency and removing a signal of an unnecessary frequency, and it is necessary to match the center frequency of the bandpass filter 6 with the frequency of the received signal. However, in the present embodiment, since the received signal is converted to the constant frequency f _t , the band filter 6 can fix the center frequency to the converted frequency f _t .

After passing through the bandpass filter 6, it is amplified by the second-stage amplifier circuit 7. Next, the amplified signal is detected by the synchronous detection circuits 8 and 9 to demodulate the metal detection signal. The synchronous signal for synchronous detection must have the same frequency as the carrier signal component of the detected signal, but the received signal is frequency-converted by the frequency conversion circuit 21, so that the frequency f _t after frequency conversion is synchronized. The signal can be synchronously detected, and the frequency of the synchronous detection can be the excitation frequency f.
_No need to change for _g1 .

If necessary, part of the output from the amplifier circuit 7 may be added to the synchronizing signal generating circuit 16 for synchronization. Further, since the frequency of the sync signal does not change, it is not necessary to change the circuit elements of the phase shift circuits 14 and 15 that change the phase of the sync signal.

As with the conventional metal detecting device, only the metal detection signal of the synchronously detected signal is passed by the analog filters 10 and 11, and the voltage comparison circuits 12 and 13 output the detection result of magnetic or non-magnetic metal. To do.

As described above, the metal detecting apparatus of this embodiment has a frequency conversion circuit for converting a received signal into another frequency, a bandpass filter having the converted frequency as a center frequency, and a metal detection signal from the frequency-converted received signal. The metal detection device is equipped with a synchronous detection circuit that detects the converted frequency in order to demodulate, and the elements of each circuit constituting the signal processing system are replaced in response to changes in the excitation frequency of the metal detection device. It is possible to respond immediately without doing such things.

Note that the technique of the present invention is not limited to the technique in the above-described embodiment, and may be implemented by means of another aspect having the same function, and the technique of the present invention can be variously modified within the scope of the above configuration. Can be changed or added.

[0023]

As is apparent from the above description, according to the metal detecting device of the present invention, the frequency of the received signal output from the receiving coil of the detecting portion is changed by the frequency converting means to another fixed frequency. Since the signal is converted into a frequency, even when the excitation frequency is changed to a value suitable for the object to be inspected in the detection section, it is possible to immediately respond without changing the elements of each circuit constituting the signal processing system that processes the received signal. There is an effect.

[Brief description of drawings]

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

FIG. 2 is a configuration block diagram of a conventional metal detection device.

[Explanation of symbols]

L ₁ oscillator coil L ₂ , L ₃ receiver coil 20 frequency conversion signal generation circuit 21 frequency conversion circuit

Claims (1)

[Claims] 1. An oscillator is used to excite an oscillating coil to generate an alternating magnetic field, and a differential signal obtained from two receiving coils interlinking with a magnetic flux of the magnetic field is detected by synchronizing signals having mutually different phases. In the device for detecting the metal passing between the oscillation coil and the receiving coil from the output signal of 1., the oscillation frequency of the difference signal is converted into another constant frequency different from the frequency by the frequency conversion means. Metal detection device characterized by.