JP2008134236A - Magnetic detector - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a small magnetic detector capable of detecting a variable magnetic field with high sensitivity without influence from the earth's magnetism. <P>SOLUTION: A magnetic detector includes a magnetic impedance element which applies a pulse current or a high-frequency current to an amorphous wire and generates, in a detector coil wound around the amorphous wire, a voltage corresponding to the strength of a peripheral magnetic field. The number of turns of the detector coil falls within a range of 10 to 500. The ratio of the diameter of the detector coil to the diameter of the amorphous wire falls within a range of 1.05 to 10. The ratio of the length of the coil to the diameter of the detector coil falls within a range of 10 to 200. Further, in the magnetic detector, the magnetic impedance element is connected to an input terminal of a differential amplifier via a high-frequency filter. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to high sensitivity of a magnetic detector and a technology for detecting foreign matter such as a minute iron piece.

  In a product inspection process such as a food factory or a clothing factory, it is necessary to inspect minute iron foreign matters mixed in the product. As a means for inspecting foreign matter, when a small amount of magnetism of a minute foreign matter in a product is transported by a conveyor, it is possible to detect that a weak magnetic field fluctuation occurs in the vicinity by a magnetic detection device using a magnetic sensor. Generally known.

However, since the magnetic field generated by the minute object such as the iron piece is local unlike the geomagnetism, a sensor having a large volume such as a fluxgate magnetic sensor has a low spatial resolution and is difficult to detect. On the other hand, an MI magnetic sensor composed of a magnetic impedance sensor element (hereinafter referred to as an MI element) is very small and has high spatial resolution and is suitable for detecting a minute object.
Here, the MI element outputs a voltage corresponding to a surrounding magnetic field to a detection coil wound around the amorphous wire when a pulse current or a high-frequency current is applied to the amorphous wire as a magnetic sensitive body.

  On the other hand, for example, in order to find a foreign matter having a size of around 0.2 mm, the magnetic detection device eliminates the influence of geomagnetism from a predetermined distance and further corresponds to about one thousandth of the geomagnetism. It is necessary to detect magnetic fluctuations of 100 μG (micro Gauss) or less.

As for the MI magnetic sensor as the magnetic detection device, a technique for reducing the thickness and increasing the sensitivity is disclosed, but as shown in FIG. 8, the output voltage per external magnetic field ± 2.5 G (Gauss) is at most ± 0. Since it is about 6 V, the sensitivity is about 0.24 V / G (Patent Document 1).
However, since the output voltage corresponding to the magnetic field of the order of 100 μG due to the 0.2 mm foreign matter is as small as several tens of μV, in reality, the MI element or an electronic circuit that drives the MI element is used. There is a possibility that practical detection cannot be performed because it is hidden by the same level of voltage noise. FIG. 9 shows a signal recorded after the MI element is housed in a magnetic shield case made of Permalloy capable of realizing a zero magnetic field environment and the output voltage of the MI element is amplified 100 times. Random noise of about 3 mVpp is recorded. Has been observed. This 3 mVpp voltage is equivalent to 120 μG in terms of magnetic field, and is equal to or higher than the magnetic field strength of the foreign matter having a size of 0.2 mm. Therefore, the signal corresponding to the foreign substance and the noise cannot be distinguished with sufficient accuracy. For this reason, it is necessary to increase the sensitivity so that the detection voltage of the MI element is 5 times or more larger than this noise, or to increase the accuracy so that the noise is 1/5 or less.

As shown in FIG. 10, the coil pulse voltage per applied magnetic field ± 80 A / m is about 0.35 V, and the sensitivity per Gauss is 0.35 V / G (Patent Document 2). Although this is about 1.5 times higher than the above-mentioned sensitivity of 0.24 V / G, it does not reach the above-mentioned sensitivity of 5 times, so that further enhancement of the sensitivity of the MI element is necessary.
Furthermore, in order to detect a weak fluctuating magnetic field, it is necessary to increase the amplification degree of the amplifier 6 in the electronic circuit shown in FIG. 11 (Patent Document 2). However, when the amplification factor of the amplifier is increased, the surrounding geomagnetic DC magnetic field component superimposed on the variable magnetic field component is also amplified. Therefore, when the variable magnetic field component is smaller than the geomagnetic component, the geomagnetic component reaches the saturation voltage of the amplifier before the variable magnetic field component is amplified to a sufficient magnitude, and the operating point becomes unstable and accurate. There is a problem that the fluctuation magnetic field component cannot be detected stably.
Therefore, it is necessary to accurately and stably detect only the variable magnetic field component.

  In addition, a technique for selectively detecting a magnetic signal component in a target frequency region by negative feedback with high sensitivity is disclosed (Patent Document 3). However, since a negative feedback coil and a negative feedback circuit are required, the circuit scale becomes large. Therefore, even if the size is reduced by the MI element, the entire magnetic detector is increased in size and cost, and a negative feedback current flows. As a result, the power consumption of the power source increases.

International Publication No. WO2003 / 071299 JP 2000-258517 A JP 2003-255029 A

  The present invention has been made in view of the above-described conventional problems, and an object thereof is to provide a magnetic detector that can detect a varying magnetic field with a small size, high sensitivity, and high accuracy.

The magnetic detection device of the present invention is a magneto-impedance element that applies a pulse current or a high-frequency current to an amorphous wire and generates a voltage corresponding to a magnetic field strength around the detection coil wound around the amorphous wire,
The number of turns of the detection coil is 10 to 500, the ratio of the diameter of the detection coil to the diameter of the amorphous wire is 1.05 to 10, and the ratio of the length of the detection coil to the diameter of the detection coil is 10 A magnetic detector comprising magnetic impedance elements of 200 to 200. (Claim 1)

  By setting the number of turns of the detection coil wound around the amorphous wire to 10 or more, the output of the MI element increases corresponding to the number of turns of the detection coil, so that a sensitivity capable of detecting a predetermined magnetic field fluctuation component can be obtained. On the other hand, since the inductance of the detection coil increases in proportion to the square of the number of turns, the generation of noise due to the formation of a resonance circuit with the stray capacitance of the circuit around the detection coil due to excessive inductance is avoided. The upper limit of the number of turns is 500.

This is because, when the ratio of the diameter of the detection coil to the diameter of the amorphous wire is set to 1.05 or more, it becomes possible to manufacture the MI element.
The following description will be made with reference to FIG. 1 showing a cross section of the MI element of the present invention.
In the MI element, a detection coil C having a diameter S and a length L is formed around an amorphous wire W having a diameter R via an insulator N having a predetermined thickness. The reason why the detection coil C generates a voltage based on the magnetic impedance effect is that the magnetic flux Fe generated by the amorphous wire W is linked to the detection coil C. However, if the diameter of the detection coil C is large, part of the magnetic flux Fc generated by the amorphous wire W passes through the inside of the detection coil C, so that effective linkage does not occur, and sensitivity is lowered. Therefore, in the present invention, the insulator N is made small, the diameter S of the detection coil is made small, and the detection coil is linked with more magnetic flux to achieve high sensitivity.
Therefore, the lower limit of the ratio of the diameter R of the amorphous wire to the diameter of the detection coil is set to 1.05 due to the manufacturing limit of the insulator N having a predetermined thickness.

  On the other hand, by setting the ratio of the diameter of the detection coil to the diameter of the amorphous wire to 10 or less, since the diameter of the detection coil does not become excessively large, a decrease in sensitivity of the MI element can be avoided in a practical range.

As described above, the sensitivity decreases as the ratio of the diameter S of the detection coil C to the diameter R of the amorphous wire W increases. However, by setting the ratio of the length of the coil to the diameter of the detection coil to 10 to 200, the detection coil The ratio between the diameter S of the coil and the length L of the coil is set to a predetermined value, and when the coil diameter S increases, the coil length L is increased to increase the number of turns, thereby increasing the sensitivity as an MI element. It can be kept almost constant and high sensitivity can be maintained.
Thus, the magnetic detector can be reduced in size and sensitivity.

  In addition, the magnetic detector is preferably a magnetic detector that applies a pulse current from a capacitor to the amorphous wire via an electronic switch.

  In this case, the capacitor that accumulates electric charges can flow an electric current instantaneously by being electrically connected to the amorphous wire using an electronic switch.

The MI element has a characteristic that the greater the change in the current flowing through the amorphous wire, that is, the greater the rise and fall speed, the greater the output, thereby increasing the sensitivity for magnetic field detection.
Thus, the sensitivity of the magnetic detector is increased.

  Preferably, the magnetic detector is a magnetic detector in which a voltage generated by a detection coil of the magneto-impedance element is accumulated in a hold capacitor via an electronic switch, and the hold capacitor and the amplifier are connected by a high frequency filter. .

  In this case, only the AC component, which is a variable magnetic field component of the signal voltage corresponding to the strength of the surrounding magnetic field accumulated by the hold capacitor, is transmitted to the input terminal of the amplifier by the high frequency filter. As a result, only the target magnetic field fluctuation signal to be detected can be amplified and output. On the other hand, DC signal components such as geomagnetism are blocked by a high-frequency filter, so even if the amplification level of the amplifier is sufficiently high, voltage saturation does not occur, and amplification can be performed accurately and with high stability. Can be detected.

  And since no negative feedback circuit is required, the scale of the electronic circuit is not increased, so that the magnetic detector can be realized in a small size and at a low cost, and further, no negative feedback current flows, so that the power consumption of the power supply can be reduced. Can be lowered.

  Preferably, the amplifier is a magnetic detector having a differential amplifier.

  In this case, the common-mode noise other than the voltage of the hold capacitor corresponding to the input detected magnetic field signal is canceled by the common-mode component suppression effect of the differential amplifier. In other words, since the pulse that drives the amorphous wire contains a high-frequency component, the component may be input to the amplifier as in-phase noise by electromagnetic induction or electrostatic induction, but the amplifier should be a differential amplifier. Therefore, a high amplification degree can be obtained stably, and a highly sensitive magnetic detector can be realized.

Moreover, it is preferable that it is a magnetic detector which connects each detection coil of the said two or more said magnetic impedance elements in series (Claim 5).
By connecting the detection coils in series, the voltages corresponding to the peripheral magnetic fields generated in the detection coils are added to each other, so that a highly sensitive magnetic detector can be realized.

Moreover, it is preferable that it is a magnetic detector which connects each detection coil of two or more said magnetic impedance elements in parallel (Claim 6).
By connecting the detection coils in parallel, the impedance of the detection coil as a signal source can be reduced, and the effect of reducing the random noise generated by the magnetic impedance sensor can be obtained, so a highly accurate magnetic detector can be realized. it can.

  Since the magnetic detector of the present invention can amplify only the variable magnetic field component with high accuracy without being affected by the DC magnetic field component such as geomagnetism, it detects the weak variable magnetic field component with high sensitivity and high accuracy. be able to. Moreover, since the scale of the electronic circuit can be reduced, it is possible to realize small size, low cost, and low power consumption.

    Hereinafter, embodiments of the present invention will be described with reference to FIGS.

FIG. 2 is a circuit diagram of the magnetic detector of the present invention. The MI element 1 that converts a detected magnetic field into a voltage includes an amorphous wire 11, a detection coil 12 and a current limiting resistor 13 that are wound around the amorphous wire 11. A driving current is applied to the amorphous wire 11 by the output pulse P1 of the driving circuit 2. Here, the amorphous wire 11 is made of a CoFeSiB alloy and has a length of 4 mm and a diameter of 20 μm. The detection coil 12 is formed by forming a coil having an average diameter (equivalent circle diameter) of about 100 μm and 93 turns and a length of about 4 mm around the amorphous wire 11 via an insulator. Thereby, a small MI element can be realized.
The detection coil 12 is connected to the electronic switch S1. The electronic switch S1 is controlled to be “open” or “closed” by the output pulse P2 of the drive circuit 2.

The drive circuit 2 includes an oscillator composed of inverters ICQ1 and Q2, a resistor R1, a capacitor C1, and inverters ICQ3, Q4, Q5, a resistor R3, a capacitor C3, a resistor Rp having one end connected to the power supply Vdd, a capacitor Cp, and an electronic switch S2. A pulse generator for outputting the pulse P1 to the amorphous wire 11, an inverter ICQ6, a resistor R4, and a capacitor C2, and a pulse generator for outputting the pulse P2.
Here, both pulses P1 and P2 are repeatedly output at a frequency of 1 MHz, but P1 and P2 are synchronized with each other with a predetermined time difference.
In addition, since the electric charge of the capacitor Cp is discharged to the amorphous wire 11 by the electronic switch S2, the pulse P1 energized to the amorphous wire 11 can be set to a fast rising current. This increases the sensitivity of the magnetic detector.

  The electronic switch S1 is always in the “open” state, but when it is “closed” for a predetermined time by the pulse P2, the magnetic field signal voltage detected by the detection coil 12 is accumulated in the hold capacitor Ch. As described above, the voltage accumulated in the hold capacitor Ch corresponds to the strength of the peripheral magnetic field detected by the MI element 1.

The hold capacitor Ch is connected to the input side of the high-frequency filter 3 composed of a capacitor C31 that determines a cutoff frequency and a resistor R31. This cut-off frequency is determined by 1 / (2π × R31 × C31).
Here, the cut-off frequency is determined from the lowest frequency of the signal component included in the magnetic fluctuation signal generated when the foreign matter is conveyed.

  Further, since the output side of the high frequency filter 3 is connected to the input terminal of the differential amplifier 4 composed of an instrumentation amplifier circuit, only the variable magnetic field component of the magnetic field signal voltage of the hold capacitor Ch is input to the amplifier 4 and the geomagnetism. Since the DC magnetic field component is excluded, it is not input to the amplifier 4. At this time, one terminal of the resistor R31 of the high frequency filter 3 is connected to a connection point between the resistors R61 and R62 constituting the bias voltage circuit 6. This bias voltage circuit 6 is realized by dividing the power supply voltage Vdd by resistors R61 and R62 so that a predetermined bias voltage required by the amplifier 4 can be supplied. As described above, only the magnetic field fluctuation signal can be amplified accurately and with high stability.

  The fluctuating magnetic field component input to the amplifier 4 is amplified with a predetermined amplification degree, and unnecessary high frequency noise is removed by the low frequency filter circuit 5 before being output from the output terminal 51.

  From the above description, the sensitivity of the MI element of this embodiment is 2 V / G, and a sensitivity 5 to 8 times higher than that of the conventional MI element can be obtained.

FIG. 3 is a measurement example using the magnetic detector of the present invention. This is a signal waveform obtained when a steel ball having a diameter of 0.2 mm passes 10 mm away from the magnetic detector. As a result of observation through a 100-fold amplifier, the maximum value is about 20 mV in 0.1 second. The fluctuating magnetic field signal can be obtained. In addition, random noise other than the magnetic field signal is reduced to 2 mV or less, and the magnetic field signal can be detected with high accuracy without being hidden by the noise.
This shows that high sensitivity and high accuracy can be achieved.

  FIG. 4 shows an example of measuring noise in a zero magnetic field environment of the magnetic detector of the present invention. The amplitude of the random noise is about 15 μG, which is greatly reduced compared to the random noise of 120 μG (3 mV) shown in FIG. 9, and it can be seen that high accuracy has been achieved.

  FIG. 5 shows an embodiment of the MI element 1a configured by using two MI elements as the MI element 1 and connecting the respective detection coils in series in order to realize a further highly sensitive magnetic detector. That is, the detection coil 12a wound around one amorphous wire 11a and the detection coil 12b wound around the other amorphous wire 11b are connected in series with each other.

As a result, it was possible to obtain a sensitivity approximately twice that of the above-described embodiment using one MI element shown in FIG.
Note that higher sensitivity can be realized as the number of MI elements is not limited to two.
In FIG. 5, the two amorphous wires 11a and 11b are connected in series to each other and connected to the current limiting resistor 13a. However, the amorphous wires 11a and 11b may be connected in parallel to each other.

  FIG. 6 shows an embodiment of an MI element 1c configured by using two MI elements as the MI element 1 and connecting the respective detection coils in parallel in order to realize a further highly accurate magnetic detector. That is, the detection coil 12c wound around one amorphous wire 11c and the detection coil 12d wound around the other amorphous wire 11d are connected in parallel to each other.

As a result, by reducing the impedance of the detection coil as a signal source to ½, the random noise from the MI element 1c can be reduced to approximately √ (½) = 0.7 times.
That is, the amplitude of the random noise in the zero magnetic field environment of the magnetic detector according to the present embodiment shown in FIG. You can see that As a result, it can be seen that higher accuracy has been achieved.
It should be noted that the number of MI elements is not limited to two, and higher accuracy can be realized as the number is larger.

  In FIG. 6, the two amorphous wires 11c and 11d are connected in parallel to each other and connected to the current limiting resistor 13c. However, the amorphous wires 11c and 11d may be connected in series to each other.

  Since the magnetic detector of the present invention is small and highly sensitive, it can be used as a general high-sensitivity magnetic sensor and further as a sensor for a magnetic gate for security to find possession of a gun / sword.

It is sectional drawing of MI element. It is a circuit diagram of the Example of this invention. It is a detection signal waveform of a steel ball having a size of 0.2 mm according to an embodiment of the present invention. It is the example which measured the noise in the magnetic field in the zero magnetic field environment of the magnetic detector by the Example of this invention. It is a block diagram of the circuit of MI element comprised by connecting the detection coil in series by the Example of this invention. It is a block diagram of the circuit of MI element comprised by connecting the detection coil in parallel by the Example of this invention. It is the example which measured the noise in the zero magnetic field environment of the magnetic detector by the Example of this invention. It is a diagram which shows the relationship between the external magnetic field and output voltage in the sensor of 1st Example shown by patent document 1, and the conventional bobbin type sensor. Random noise measured using an MI element. FIG. 6 is a characteristic diagram of a coil pulse voltage versus a magnetic field of the first example shown in Patent Document 2. 1 is a configuration diagram of a magneto-impedance effect micromagnetic (MI) sensor circuit showing a first embodiment shown in Patent Document 2. FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 MI element 2 Drive circuit 3 High frequency filter 4 Differential amplifier 5 Low frequency filter circuit 6 Bias voltage circuit 11 Amorphous wire 12 Detection coil
S1 electronic switch
S2 electronic switch
Ch hold capacitor
Cp capacitor

Claims (6)

A magneto-impedance element that applies a pulse current to an amorphous wire and generates a voltage corresponding to a surrounding magnetic field strength in a detection coil wound around the amorphous wire,
The number of turns of the detection coil is 10 to 500, the ratio of the diameter of the detection coil to the diameter of the amorphous wire is 1.05 to 10, and the ratio of the length of the detection coil to the diameter of the detection coil is 10 A magnetic detector comprising a magneto-impedance element, wherein the magnetic detector is 200 to 200.   The magnetic detector according to claim 1, wherein a pulse current is applied to the amorphous wire from a capacitor via an electronic switch.   3. The magnetic detector according to claim 1, wherein the voltage generated by the detection coil of the magneto-impedance element is accumulated in a hold capacitor via an electronic switch, and the hold capacitor and the amplifier are connected by a high frequency filter. A magnetic detector.   4. The magnetic detector according to claim 1, wherein the amplifier comprises a differential amplifier.   5. The magnetic detector according to claim 1, wherein two or more detection coils of the magnetic impedance element are connected in series.   5. The magnetic detector according to claim 1, wherein two or more detection coils of the magnetic impedance element are connected in parallel.