JP2583072B2 - Metal detector - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial application field of the present invention> The present invention relates to a metal detector for detecting metal mixed in a test object, and more particularly, to a metal detector in a magnetic field between a transmission coil and a reception coil. A metal detection device that can prevent an error from occurring in the metal detection due to noise generated when the detection unit detects the metal mixed into the test object from a signal based on the induced voltage of the receiving coil. About the machine.

<Prior art> (FIGS. 4 to 6) A metal detector used for detecting minute metals mixed in a product (for example, ham, sausage, miso, etc.) generally has a detection principle shown in FIG. Depending on.

In FIG. 4, reference numeral 1 denotes a detecting unit, which is a transmitting coil P which is supplied with an alternating signal from an oscillator to generate an alternating magnetic field;
Opposite to the transmission coil P, two reception coils S ₁ , S ₂ are arranged such that the lines of magnetic force of the alternating magnetic field generated by the transmission coil P intersect and the induced voltages E ₁ , E ₂ generated by the alternating magnetic field become equal. It has. The test object W is transported at a predetermined speed between the transmitting coil P and the receiving coils S ₁ and S ₂ arranged in this way from one receiving coil S ₁ to the other receiving coil S ₂ at a predetermined speed. (Not shown). If a metal is mixed in the test object W, the metal causes a change in the line of magnetic force. That is, when the iron object to be inspected W is mixed, as shown in Figure 5, is deformed magnetic path by the presence of iron when the inspection object W is passing through the receiving coil S ₁ example receiving The lines of magnetic force intersecting the coil S ₁ increase and the induced voltage E ₁
Increases, becomes larger than the induced voltage E ₂ of the other receiving coil S _2. When a non-ferrous metal is mixed, an eddy current flows in the non-ferrous metal as shown in FIG. 6, an electromagnetic flux is consumed as energy of the eddy current, and magnetic lines of force intersecting one of the receiving coils S ₁ decrease. reduces the induced voltage E _1, it becomes smaller than the induced voltage E ₂ of the other receiving coil S _2. As described above, when metal is mixed in the inspection object W, a difference occurs between the induced voltages E ₁ and E ₂ of the first and second receiving coils S ₁ and S ₂ when passing through the detecting unit. The metal is detected by outputting the difference voltage as an imbalance signal.

<Problems to be Solved by the Present Invention> By the way, the signals of the induced voltages are generated until the induced voltages of the receiving coils S ₁ and S ₂ are sent to and processed by a circuit for detecting the presence or absence of a rear metal. , Noise may be added to the metal detection result.

As such noise, for example, noise generated when switching the detection head (coil) in a device that switches the number of turns of the transmission coil P and the reception coils S ₁ and S ₂ according to the material of the inspection object W, In the case of a device that changes the phase of the induced voltage of the receiving coils S ₁ and S ₂ by a tuning circuit according to the material of the device under test W and sends the output signal to a subsequent filter circuit, the capacitance of the capacitor of the tuning circuit There is noise generated when switching is performed by a manual switch or an automatic switch. (If this noise is sent to the subsequent filter circuit, it becomes a signal with a large signal width such that the object under test W contains metal when the transmission speed of the filter is low, resulting in an error. There is also noise that occurs when the motor speed is switched to switch the speed of the belt conveyor that transports the test object W according to the material of the test object W.

As described above, before the signal due to the induced voltage of the receiving coils S ₁ and S ₂ is transmitted to the detection circuit for the presence or absence of metal, various noises adversely affect the signal transmission speed, gain, pass band, and other device characteristics. May cause an error in the metal detection result.

The present invention has been made in order to solve the above-described problems, and when detecting conditions of metal detection are switched by a switching control circuit, a detection signal line is short-circuited to a reference level signal line to prevent the influence of noise, An object of the present invention is to provide a metal detector in which no error is detected in metal presence detection.

<Means for Solving the Problems> In order to solve the problems, the metal detector of the present invention detects a magnetic field change when the test object passes through a magnetic field, and responds to the magnetic field change. In a metal detector for detecting mixed metal of the inspection object based on a detection signal, a switching control circuit for changing a detection condition of the mixed metal, receiving a signal from the switching control circuit, and detecting mixed metal A short-circuit circuit for short-circuiting the detection signal line to a reference level signal line when the condition is changed.

<Operation> Due to such a configuration, in the metal detector of the present invention,
When the detection condition of the mixed metal is switched, noise generated in the detection signal line is removed by the short circuit, and a detection error due to the noise is prevented.

<Embodiment of the Present Invention> (FIGS. 1 to 3) Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

FIG. 1 is a block circuit diagram of the metal detector. In FIG. 1, reference numeral 11 denotes an oscillator that outputs an oscillation signal having a predetermined frequency. Reference numeral 12 denotes a transmitting coil P driven by the oscillation signal to generate an alternating magnetic field, and first and second transmitting coils P arranged oppositely to each other so that the lines of magnetic force of the alternating magnetic field intersect with each other so that each induced voltage becomes equal. This is a detection unit including reception coils S ₁ and S ₂ .
Reference numeral 13 denotes a tuning circuit for the receiving coils S ₁ and S ₂ .

The tuning circuit 13 is configured so that the phase of the induced voltage of the receiving coil can be changed by switching the capacitance of the capacitor with a changeover switch. The changeover switch is a manual switch (not shown) for coarse adjustment which is manually switched for each type of the inspection object W and a fine adjustment switch which is switched by a switching control signal from a control circuit (not shown). (Not shown). Therefore, in the drawing, a switching control circuit 22 outputs a switching signal corresponding to the output of the changeover switch to the tuning circuit 13.

Reference numeral 14 denotes a first-stage amplifier circuit that amplifies a difference voltage signal from the receiving coils S ₁ and S ₂ output from the tuning circuit 13. A circuit 15 adjusts the phase of the oscillation signal of the oscillator 11 by 90 degrees. Therefore, assuming that the oscillation signal of the oscillator 11 is a reference signal (0 degree), the output signal of the phase shift circuit 15 is a signal whose phase is shifted by 90 degrees.

The auto balance circuit 16 is a circuit that corrects a fluctuation due to a temperature change of the differential voltage between the receiving coils S ₁ and S ₂ . In this case, the difference voltage is normally 0 except when the test object W passes through the detection unit 12, but the difference voltage may not be 0 depending on the temperature, and therefore, the difference voltage is corrected. Therefore, the oscillation output (reference signal 0 degree) of the oscillator 11, the output of the first-stage amplifier circuit 14, and the signal shifted by 90 degrees from the phase shift circuit 15 are input to the auto balance circuit 16, respectively. The output signal of the auto balance circuit 16 is input to the differential amplifier circuit 17 together with the output signal of the first stage amplifier circuit 14. The differential amplifier circuit 17 differentially amplifies the input signal and supplies it to the synchronous detection circuits 18A and 18B.

Synchronous detection circuit 18A, filter / amplifier circuit 19A, rectifier circuit
20A is a circuit for detecting the presence or absence of an iron component in the test object W. In contrast, a synchronous detection circuit 18B,
The amplifying circuit 19B and the rectifying circuit 20B are circuits for detecting the presence or absence of a non-ferrous component in the test object W. In this case, the output of the phase shift circuit 15 (a signal obtained by shifting the phase of the reference signal by 90 degrees) is input to the synchronous detection circuit 18A.
A reference signal (0 degree) is input to B. The synchronous detection circuits 18A and 18B, the filter / amplifier circuits 19A and 19B, and the rectifier circuits 20A and 20B are substantially the same circuit. The filter / amplifier circuits 19A and 19B are provided with the short-circuit signal from the switching control circuit 22 when the capacitor of the tuning circuit 13 is switched, and clear the noise generated at that time.

Reference numeral 21 denotes an object detector that detects when the inspection object W passes through the detection unit 12 and outputs a detection signal. In this case, the test subject W is transferred from the receiver coil S ₁ side to the receiving coil S ₂ side by the belt conveyor (not shown). Then, the transfer speed is changed by automatically switching the motor speed of the belt conveyor according to the type of the inspection object W automatically read by, for example, a bar code reader.

FIG. 2 shows an example of the configuration of the filter / amplifier circuits 19A and 19B. As shown, the filter / amplifier circuits 19A and 19B comprise a series circuit of a low-pass filter 31, a high-pass filter 32, an amplifier circuit 33, a high-pass filter 34, and low-pass filters 35 and 36. . In this case, in this embodiment, a short-circuit signal is supplied to the low-pass filter 35 from the switching control circuit 22 in order to remove noise generated when the tuning circuit 13 switches the capacitor.

That is, FIG. 3 shows a specific circuit configuration of the low-pass filter 35, and as shown, an operational amplifier OP, a resistor R ₁ ,
R ₂ , R ₃ , R ₄ , capacitors C ₁ , C ₂ , and a field effect transistor FET. The field-effect transistor FET forms the short circuit of this embodiment. The short-circuit signal is applied to its gate terminal, and when noise occurs, the transistor FET conducts and the low-pass filter 35 is short-circuited. That is, noise is quickly removed.

<Operation of Embodiment> Next, the operation of the above embodiment will be described. When starting detection of metal or non-metal in the object W, the capacitance of the tuning circuit 13 is switched by operating a manual switch (not shown) for coarse adjustment according to the type of the object W. Keep it. Then sequentially one by one inspection object W, receives the detection unit 12 put on the belt conveyer from the receiver coil S ₁ side coil
To transfer to the S ₂ side. At this time, the object to be inspected W is the object detector 21.
Rectifier circuit for each detection signal output
The output of the iron component and the output of the non-ferrous component from 20A and 20B are taken into a control circuit (not shown), for example, when it is determined that there is an iron component, a notification to that effect is made.

That is, when the test object W passes, the phases of the induced voltages of the receiving coils S ₁ and S ₂ are roughly adjusted by the tuning circuit 13, and the output signal is then AC-amplified by the first-stage amplifier circuit 14,
Further, the signal is differentially amplified by the differential amplifier circuit 17. At this time, the differential amplifier circuit 17 adjusts the temperature-dependent change in the difference voltage between the induced voltages of the receiving coils S ₁ and S ₂ according to the output of the auto balance circuit 16. The output of the differential amplifier circuit 17 is input to synchronous detection circuits 18A and 18B.
B, a ferrous component signal output and a non-ferrous component signal output are obtained by each operation of the rectifier circuits 20A and 20b.

During the execution of the above-described operation, for example, the type of the inspection object W is changed, and the change is determined by, for example, a bar code reader, and a signal for switching the capacitance of the tuning circuit 13 is output from the switching control circuit 22. And At this time, when noise occurs due to the switching of the capacitance of the capacitor, the switching control circuit 22 outputs a short-circuit signal to the low-pass filter 35 in response to the occurrence of the noise, and the transistor FET shown in FIG.
Is made conductive. Therefore, the signal line of the low-pass filter 35 is short-circuited to the ground line (that is, the reference level signal line), whereby the generated noise is quickly eliminated and the noise does not become an error signal indicating the presence of an iron component. Prevention of occurrence is performed.

As described above, since the low-pass filter 35 is provided with the transistor FET that is turned on by the short-circuit signal as shown in FIG. 3, the signal line of the high-pass filter 35 is short-circuited when noise occurs, thereby preventing the occurrence of error due to noise. Things.

That is, since the circuit configuration of the conventional low-pass filter 35 does not include the transistor FET, the low-pass filter 35 cannot perform a short-circuit operation when noise occurs.

<Another embodiment of the present invention> In the above embodiment, an example is shown in which the filters in the filter / amplifier circuits 19A and 19B are short-circuited for noise generated when the capacitor of the tuning circuit 13 is switched. Noise generated when switching the motor of the belt conveyor or the like that transports the inspection object W, and the detection head (coil)
Noise generated at the time of switching may be prevented from occurring by short-circuiting the filter in the same manner.

In addition to the filter circuit, other circuits may be short-circuited to remove generated noise.

Further, apart from the method of short-circuiting a predetermined circuit when noise occurs, by switching the transmission characteristics of the signal, such as the transmission speed, the transmission band, and the gain, it is possible to prevent the occurrence of a detection error of the presence or absence of a foreign substance such as an iron component. You may.

<Effects of the present invention> As described above, the metal detector of the present invention detects a magnetic field change when an object to be inspected passes through a magnetic field, and performs an inspection based on a detection signal corresponding to the magnetic field change. In a metal detector for detecting a mixed metal in a body, a switching control circuit for changing a detecting condition of the mixed metal and a signal from the switching control circuit are received, and a detection signal line is switched when the detecting condition of the mixed metal is switched. Since a short circuit is provided for short-circuiting to the reference level signal line, noise generated in the detection signal line when the detection condition is switched by the switching control circuit is removed, and the occurrence of an error in metal presence detection is ensured. There is an advantage that can be prevented.

[Brief description of the drawings]

1 is a block circuit diagram of a metal detector according to one embodiment of the present invention, FIG. 2 is a specific circuit diagram of filter / amplifier circuits 19A and 19B, FIG. 3 is a specific circuit diagram of low-pass filter 35,
4 to 6 are diagrams showing the operation principle of the metal detector. 11 ... Oscillator, 12 ... Detector, 13 ... Tuning circuit, 15 ...
Phase shift circuit, 17 ... Differential amplifier circuit, 18A, 18B ... Synchronous detection circuit, 19A, 19B ... Filter / amplifier circuit, 20A, 20B ... Rectifier circuit, 21 ... Object detector, 22 ... Switch Control circuit, 31,3
5,36 low-pass filter, 32,34 high-pass filter, P transmission coil, S ₁ , S ₁ reception coil, W
Inspected body.

Claims (1)

(57) [Claims] 1. A metal detector for detecting a change in a magnetic field when an object to be inspected passes through a magnetic field and detecting a mixed metal of the object to be inspected based on a detection signal corresponding to the magnetic field change, A switching control circuit for changing a metal detection condition, and a short circuit that receives a signal from the switching control circuit and short-circuits the detection signal line to a reference level signal line when the mixed metal detection condition is switched. A metal detector.