JPH0854475A - Metal detector - Google Patents

Abstract

PURPOSE:To immediately deal with change of frequency without altering element of an automatic balance regulator even when altering the frequency to an oscillating frequency suitable for an element to be detected by providing a frequency converting means. CONSTITUTION:The signal of a frequency generated by a frequency conversion signal generator 12 is multiplied by reception signal and a synchronizing signal by frequency converters 13a, 13b, and the reception signal and the synchronizing signal are frequency-converted into a signal of a certain frequency (ft). The frequency-converted reception signal is passed through a differential amplifier 5, a band bass filter 6, and an amplifier 7, synchronously detected by synchronous detectors 8a, 8b by using the frequency-converted synchronizing signal, and a metal detection signal is demodulated. Even if the oscillation frequency is altered to vary the element to be detected, the frequency at the rear stages of the converters 13a, 13b is held constant at ft, and hence necessary signal for an automatic balance regulator 100 becomes a predetermined frequency, and the circuit constant of the circuit 10 may not be altered, or regulated.

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 that includes 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, reference numeral 1 is an oscillating circuit, 2 is an oscillating coil connected to the oscillating circuit, 3a and 3b are receiving coils arranged opposite to or coaxial with the oscillating coil, and receiving coils 3a and 3b.
Is placed in the alternating magnetic field of the oscillating coil 2 and the two receiving coils are geometrically adjusted with respect to the oscillating coil so that their magnetic fluxes are equally linked. Further, the differential voltage obtained by adjusting the differential balance of the electromotive force induced in the two receiving coils by the variable resistor 4 becomes the received signal input to the signal processing circuit. 5 is a differential or addition amplifier circuit, 6 is a bandpass filter having the same center frequency as the oscillation frequency, 7 is an amplifier circuit, 100
Is an automatic balance adjustment circuit that generates a cancellation signal of the same phase or an opposite phase with the same amplitude as the signal that is steadily occurring in the received signal, and 8a and 8b are formed by the first and second phase shift circuits, A synchronous detection circuit for performing synchronous detection with the first and second synchronous signals, 9a and 9b are analog filters having the same center frequency as the metal detection signal, and 10a and 10b are magnitudes of the metal detection signal demodulated by the synchronous detection circuit. A voltage comparison circuit for determining whether or not the synchronous detection circuit 8 is provided.
They are first and second phase shift circuits that change the phases of the first and second synchronization signals supplied to a and 8b.

In the metal detecting device having such a structure, when the object to be inspected is passed between the oscillation coil 2 and the receiving coils 3a and 3b or between the oscillation coil and the receiving coil,
When metal is mixed in the device under test, an unbalanced state occurs in the received signal due to the mixed metal. The signal generated by this unbalanced state is the metal detection signal modulated by the oscillation frequency, and the frequency is the same as the oscillation frequency. In order to demodulate the metal detection signal from the received signal, synchronous detection is performed by the synchronous detection circuits 8a and 8b after passing through the differential amplifier circuit 5, the bandpass filter 6 and the amplifier circuit 7. Here, the frequency of the synchronization signal is also the same as the oscillation frequency, and the phase of the synchronization signal is adjusted by the first and second phase shift circuits 11a and 11b so as to be suitable for detecting magnetic and nonmagnetic metals. The signal after the synchronous detection passes through the analog filters 9a and 9b to become only the metal detection signal, and the voltage comparison circuit 10a and
The detection result of metal is output by 10b.

Normally, the received signal has a substantially zero amplitude (balanced state) due to the geometrical arrangement adjustment of the receiving coil in the magnetic field generated by the oscillation coil and the adjustment of the variable resistor for balance adjustment. The metal is detected by the unbalanced state caused by the metal. However, in the long term, the equilibrium state is disrupted due to changes in the constants of the receiving coil and variable resistance due to mechanical distortion and temperature changes of the oscillation coil and receiving coil, and a signal with a steady amplitude in the received signal Signal) will occur. When the steady signal becomes large, the noise of the detection signal output from the analog filter becomes large, so that the detection sensitivity decreases, and in the worst case, the amplitude of the signal is changed by the power supply voltage by the amplifier circuits 5 and 7. It may be saturated and normal signal processing may not be performed and metal detection may become impossible. The automatic balance adjustment circuit 100 is a circuit that corrects a steady signal generated due to the above reason to a balanced state, and outputs a cancel signal (OUT) having the same amplitude or the same phase as the steady signal from the signals IN1, IN2, and IN3. By generating the difference or sum of the cancel signal and the received signal by the differential or addition amplification circuit 5, the steady signal can be erased at the output of the differential or addition amplification circuit 5.

Here, as an example of the automatic balance adjustment circuit 100, two types of circuits are shown in FIGS. 3 and 4, and the internal circuit configuration and detailed operation will be described.

First, the operation of the automatic balance adjustment circuit of FIG. 3 will be described. Reference numeral 102 is a phase shift circuit having a 90 degree phase shift amount, 103a and 103b are synchronous detection circuits for detecting amplitude components in the 0 degree and 90 degree directions, respectively, and 104a and 104.
b is an integrating circuit having a time constant of several seconds to several tens of seconds, 105a, 1
Reference numeral 05b is a voltage control amplifier circuit capable of controlling the amplification degree by voltage, 106 is an addition amplifier circuit for adding two signals, 10
Reference numeral 7 is a band pass filter having the same center frequency as the oscillation frequency. In this automatic balance adjustment circuit, the signal of IN3 is synchronously detected by the synchronous detection circuits 103a and 103b to extract the amplitude components in the 0 ° and 90 ° directions of the stationary signal generated in the received signal, and the integration circuit 104a, 104b
Is used to extract only the long-term amplitude component due to temperature or the like.
These amplitude components are supplied to the voltage control amplifier circuits 105a and 105a.
0b reference signal (IN1) from the oscillator circuit 1 and 9
The 90-degree reference signal, which has been phase-shifted by 90 degrees by the 0-degree phase shift circuit 102, is multiplied by each, and the two output signals are added by the addition amplification circuit 106. Finally, the band-pass filter 107 is added.
To restore a signal (cancellation signal: OUT) that is constantly present in the received signal. By inputting the cancel signal to the differential amplifier circuit, the steady signal in the received signal is deleted from the received signal and the signal after differential amplification is brought into a balanced state. In the operation described above, since the negative feedback is provided in the path of the differential amplifier circuit 5 → the automatic balance adjustment circuit 100 → the differential amplifier circuit 5 in FIG. 1, the adjustment is in the convergence direction, and the balanced state of the received signal is lost. However, it is automatically adjusted to the equilibrium state. Since the balance adjustment only follows long-term changes in amplitude, changes in the amplitude and phase of the received signal due to the metal are not absorbed, and metal detection can be performed by the signal processing circuit in the subsequent stage.

Next, the operation of the automatic balance adjustment circuit shown in FIG. 4 will be described. 108 is a phase detection circuit that detects the phase difference between the received signal and the reference signal, 109 is an integration circuit that integrates the output from the phase detection circuit, and 110 is a voltage control phase shift that changes the amount of phase shift according to the output result from the integration circuit. Circuit, 11
Reference numeral 1 is a synchronous detection circuit that detects the amplitude of a received signal, 112 is an integration circuit that integrates the output from the synchronous detection circuit, 113 is a voltage control amplifier circuit that changes the signal amplitude according to the output result of the integration circuit, and 114 is an oscillation It is a bandpass filter having the same center frequency as the frequency.

The phase detection circuit is a circuit that outputs a voltage corresponding to the phase difference between the two signals, and detects the phase difference between the steady signal (IN2) and the reference signal (IN1) generated in the received signal by the phase detection circuit 108. To detect. The integrator circuit 109 does not follow the received signal due to the passage of metal, but only follows a long-term phase change due to temperature change or the like, and has a time constant of several tens of seconds or more. Then, by inputting the output voltage of the integrating circuit 109 to the voltage control phase shift circuit 110, the phase of the reference signal after the phase is made to follow and match the received signal for a long term. Next, the received signal (IN3) after the differential amplification is synchronously detected by the reference signal (IN1) after the phase shift adjustment, and the amplitude component of the steady signal is extracted. Among the extracted amplitude components, the integration circuit 112 is passed so as to follow a long-term amplitude change due to temperature or the like, and the output voltage thereof is multiplied by the reference signal after the phase shift adjustment by the voltage control amplifier circuit 113 to perform the phase shift adjustment. Adjust the amplitude of the subsequent reference signal. Finally, the signal that is constantly present in the received signal is restored by passing it through the bandpass filter 114. By inputting the restored cancel signal (OUT) to the differential amplifier circuit and applying negative feedback, the received signal can be converged to a balanced state.

As described above, although the internal circuit configurations of the automatic balance adjustment circuits of FIGS. 3 and 4 are different, cancel signals (for canceling the steady signal from the signals of IN1, IN2, IN3) ( OUT)), and the automatic balance adjustment circuit in FIG. 1 can automatically perform the balance adjustment regardless of which of FIGS. 3 and 4 is used.

[0010]

In the metal detecting device having such a structure, the metal is generally detected by changing the oscillation frequency according to the object to be inspected. This is because there is an oscillating frequency suitable for detection depending on the object to be inspected.For example, in the case of an object to be inspected that has salt or the packaging paper is conductive, the eddy current generated in the object to be inspected may not detect the mixed metal inside. Since it becomes difficult, the influence of the object to be inspected is reduced by lowering the oscillation frequency to detect the metal. On the contrary, when the inspection object itself such as a dried product is not affected by the magnetic field at all, a high oscillation frequency is used to increase the detection sensitivity by increasing the eddy current generated in the mixed metal.

As described above, when a plurality of objects to be inspected are inspected by one metal detector, it is necessary to change the oscillation frequency according to the object to be inspected. In the conventional metal detector, the received signals are balanced. In the automatic balance adjustment circuit that adjusts the state, the part where the circuit constant is determined by the oscillation frequency (for example, in FIG.
In the automatic balance adjustment circuit, the phase shift circuit 102 and the band pass filter 107, and in the automatic balance adjustment circuit in FIG. 3, the voltage control phase shift circuit 110 and the band pass filter 114 determine constants such as resistors, capacitors and inductors depending on the oscillation frequency. It Therefore, if the inspection object to be inspected by the metal detection device changes, in order to change the oscillation frequency suitable for the inspection object, the components of the automatic balance adjustment circuit must be changed or adjusted according to the oscillation frequency each time. did not become.

That is, when changing the oscillation frequency in the conventional metal detector shown in FIG. 1, the circuit constants constituting the automatic balance adjustment circuit must be changed to values determined by the oscillation frequency, and It was replaced and corresponded. Therefore, it takes time to replace the element, and many kinds of values have to be prepared.

The present invention has been made in view of the above points, and an object thereof is to solve the above-mentioned problems and to change the elements constituting the automatic balance adjustment circuit even when the oscillation frequency is changed so as to be suitable for the object to be inspected. It is to provide a metal detection device that can respond immediately without doing so.

[0014]

In order to solve the above-mentioned problems, in the metal detector of the present invention, an automatic balance adjustment circuit for eliminating the difference signal obtained from two receiving coils and the steady signal generated from the difference signal. By adding or subtracting the output of, the difference signal and the oscillation frequency of the synchronization signal obtained from the oscillation circuit for the oscillation coil is converted into another constant frequency by the frequency conversion means and input to the automatic balance adjustment circuit,
The feature is that even when the oscillation frequency for the oscillation coil is changed, it is possible to respond immediately without changing the elements constituting the automatic balance adjustment circuit.

[0015]

Since the present invention is configured as described above, even if the oscillation frequency is changed, the frequency of the automatic balance adjustment circuit at the stage subsequent to the frequency conversion circuit may be constant only by changing the frequency conversion signal generating circuit. It will be.

[0016]

1 is a block diagram showing an embodiment of the present invention, in which reference numerals 1, 2, 3a, 3b, 4, 5, 7, 8a and 8 are shown.
b, 9a, 9b, 10a, 10b, 11a and 11b are the same as those in FIG. 6 is a band pass filter having the same center frequency as the frequency ft after frequency conversion, 12 is a frequency conversion signal generation circuit, 13a is a frequency conversion circuit for converting the received signal into a certain frequency, and 13b is a synchronization signal. It is a frequency conversion circuit that converts to a constant frequency. Here, the frequency conversion circuit is a circuit that mixes two signals, and includes, for example, a signal mixer circuit, a multiplication circuit, a balanced modulation circuit, and a voltage control amplifier circuit.

The operation of the metal detecting device according to the embodiment will be described below. Here, fg is the oscillation frequency, and ft is the frequency after frequency conversion. In this metal detection device, ft + f generated by the frequency conversion signal generation circuit by the frequency conversion circuits 13a and 13b is applied to the reception signal and the synchronization signal.
Multiply the signal of the frequency of g or ft-fg, and the received signal and the synchronizing signal have a certain constant frequency (ft)
The frequency is converted to the signal. The frequency-converted received signal passes through the amplifier circuit 5, the bandpass filter 6 having the same center frequency as ft, and the amplifier circuit 7, and is synchronously detected by the synchronous detection circuits 8a and 8b using the frequency-converted synchronous signal. The metal detection signal is demodulated. The signal after the synchronous detection passes through the analog filters 9a and 9b to become only the metal detection signal, and the voltage comparison circuits 10a and 10b output the metal detection result.

Now, it is assumed that the received signal is out of balance due to external influences such as temperature, temperature, and stress change. The stationary signal in the received signal generated by the unbalanced state has the same frequency as the oscillation frequency fg, and this stationary signal is frequency-converted by the frequency conversion circuit 13a to be converted into a stationary signal of a certain constant frequency ft. The automatic balance adjustment circuit 100 is arranged in the latter stage of the frequency conversion circuit,
By generating a cancellation signal (OUT) having the same amplitude or the same phase as the stationary signal frequency-converted from the signals IN1, IN2, and IN3 and having the same phase and an opposite phase, and inputting the cancellation signal to the differential amplifier circuit 5, The amplitude of the signal after dynamic amplification is automatically corrected to zero. Automatic balance adjustment circuit 100
Has the internal circuit configuration shown in FIG. 3 or FIG. 4, and the automatic balance adjustment circuit can automatically perform the balance adjustment in the same manner.

Even if the oscillation frequency is changed from fg to fg1 in order to change the object to be inspected of the metal detection device, the frequency of the frequency conversion signal is changed to ft + fg1 or ft-fg1 so that the frequency in the subsequent stage of the frequency conversion circuit is changed. Since it is kept constant at ft, the signal (I
N1, IN2, IN3) have a constant frequency, and it is not necessary to change or adjust the circuit constants constituting the automatic balance adjustment circuit. That is, even if the oscillation frequency is changed because of the frequency conversion circuit, the frequency of the automatic balance adjustment circuit at the stage subsequent to the frequency conversion circuit may be constant only by changing the frequency conversion signal generation circuit.

[0020]

As described in detail above, according to the present invention, in a metal detection device provided with an automatic balance adjustment circuit for eliminating a stationary signal, a reception signal from a reception coil and an oscillation for the oscillation coil are provided. Since the oscillating frequency of the synchronizing signal obtained from the circuit is converted to another constant frequency different from the frequency by the frequency converting means, even when changing to the oscillating frequency suitable for the object to be inspected in the detecting section, the automatic balance adjusting circuit There is an excellent effect that it can be dealt with immediately without changing the element.

[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 an example of an automatic balance adjustment circuit.

FIG. 4 is a configuration diagram showing another example of the automatic balance adjustment circuit.

[Explanation of symbols]

 1 Oscillation circuit 2 Oscillation coil 3a, 3b Reception coil 4 Variable resistance for differential balance adjustment 5 Differential or summing amplification circuit 6 Band pass filter 7 Amplification circuit 8a, 8b Synchronous detection circuit 9a, 9b Analog filter 10a, 10b Voltage comparison Circuits 11a, 11b First and second phase shift circuits 12 Frequency conversion signal generation circuits 13a, 13b Frequency conversion circuits

Claims (1)

[Claims] 1. An automatic oscillator for erasing a difference signal obtained from two receiving coils interlinking with a magnetic flux of the magnetic field by exciting an oscillating coil by an oscillator to generate an alternating magnetic field, and a steady signal generated from the difference signal. In the device for detecting the metal passing between the oscillation coil and the receiving coil from each output signal after adding or subtracting the output of the balance adjustment circuit and detecting with the synchronization signals having mutually different phases, the difference A metal detection device, characterized in that the oscillation frequency of a synchronizing signal obtained from an oscillation circuit for a signal and an oscillation coil is converted into another constant frequency by a frequency conversion means and is input to an automatic balance adjustment circuit.