JP2009210399A - Eddy current sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an inexpensive eddy current sensor of good reproducibility using a signal processing circuit flat in frequency characteristics, small in size and simple in structure. <P>SOLUTION: The eddy current sensor includes an AC magnetic field generating coil for generating an eddy current in a detection material being a conductor by the application of an AC magnetic field and a magnetic sensor, which has sensitivity in a definite direction, for detecting the magnetic field by the eddy current. The AC magnetic field generating coil and the magnetic sensor are allowed to approach each other or arranged in a concentric circular state so that the magnetic field passes vertically with respect to the magnetism sensing direction of the magnetic sensor to detect the eddy current in the detection material passing the vicinity of the magnetic sensor. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an eddy current sensor used in an apparatus that needs to select the type of coins according to shape, material, and the like.

  In general, a method of measuring the magnetic characteristics of a coin to be conveyed with a coil is used to determine the type and authenticity of the coin. As shown in FIG. 5, an alternating current is passed through the coil, and the generated alternating magnetic field 31 is applied to the coin 13. Since the eddy current 32 generated when the magnetic field 31 passes through the coin 13 generates a magnetic field 33 due to the eddy current and the magnetic field in the coil changes, the impedance of the magnetic field generating coil or the separately mounted detection coil It is known that a change is detected as a voltage change at both ends of the coil, and the material, thickness, diameter, and the like of the coin are estimated from this output to determine the type and authenticity of the coin.

For example, the coin discriminating apparatus described in Patent Document 1 is used in public telephones, vending machines, etc., and the thickness or material of a coin is determined by a transmission / reception coil arranged in a coin trajectory to determine the authenticity and type of the coin. The present invention relates to a coin discriminating device for discriminating, and an eddy current is generated in a coin moving by an alternating magnetic field generated from a transmitting coil, and an output waveform of an induced voltage change generated in a receiving coil by a magnetic field generated by the eddy current is obtained Based on the output waveform, the authenticity and type of the coin are discriminated.
Japanese Patent Laid-Open No. 05-217050

また、巻数ｎ、断面積Ｓのコイルの断面に対して磁力線が垂直になるように設置したとき、コイルの両端に発生する電圧ｅは以下の式で表される。

よって、磁場Ｂに置かれたコイルの端子間に生じる電圧の最大値Ｖ０は、前記の数式（２）にｃｏｓ（ωｔ）＝１を代入することにより、次のように表すことができる。

Conventionally, as in Japanese Patent Application Laid-Open No. 2003-228620, many sensors determine the type of a coin based on data on the material, thickness, and diameter of the coin obtained from the detection output of an eddy current sensor using a coil.
Here, the magnetic field B changing at the frequency f is expressed by the following equation.

Further, when the magnetic field lines are installed so as to be perpendicular to the cross section of the coil having the winding number n and the cross-sectional area S, the voltage e generated at both ends of the coil is expressed by the following expression.

Therefore, the maximum value V0 of the voltage generated between the terminals of the coil placed in the magnetic field B can be expressed as follows by substituting cos (ωt) = 1 into the above equation (2).

  From Equation (3), the output of the coil used for eddy current detection is directly proportional to the product of the number of turns n, the cross-sectional area S, and the frequency f. In particular, the number of turns n and the cross-sectional area S are increased in the low-frequency measurement. Because it is necessary to apply the optimum magnetic field to the material and outer dimensions of the object to be detected, it is difficult to measure various frequencies with the same coil, and sufficient sensitivity cannot be obtained. was there.

  Further, an AC magnetic field is always applied to the detection coil, and there is a problem that a signal processing circuit becomes complicated in order to detect a weak change due to the presence or absence of a detection object. Furthermore, depending on the configuration of the coil, the output greatly depends on the interval between the coil and the coin, and thus there is a problem that it is difficult to determine the type and authenticity.

  The present invention has been made in view of the above problems, and an object of the present invention is to provide an eddy current sensor that has a flat frequency characteristic, is small and has a simple structure, and is inexpensive and has good reproducibility. Is.

  The invention according to claim 1 is an AC magnetic field generating coil for generating an eddy current in a detection object which is a conductor by applying an AC magnetic field, and a magnetic sensor having sensitivity in a certain direction for detecting the magnetic field due to the eddy current; In the vicinity of the magnetic sensor, the AC magnetic field generating coil and the magnetic sensor are arranged close to each other or concentrically so that the magnetic field passes perpendicularly to the magnetic sensing direction of the magnetic sensor. The eddy current sensor is characterized in that eddy current generated in a detection object passing through the slab is detected.

  According to a second aspect of the present invention, there is provided an AC magnetic field generating coil for generating an eddy current in a detection object as a conductor by applying an AC magnetic field, a magnetic sensor having sensitivity in a certain direction for detecting the magnetic field due to the eddy current, The AC magnetic field generating coil and the magnetic sensor are arranged so that a magnetic field passes in the same direction as the magnetic sensing direction of the magnetic sensor, and a magnetic flux line between the AC magnetic field generating coil and the magnetic sensor is arranged. An eddy current sensor is characterized in that an eddy current generated in a detection object that passes across is detected.

  According to a third aspect of the present invention, in addition to the first or second aspect, the applied magnetic field for generating the eddy current is a square wave or a pulse wave.

  According to a fourth aspect of the present invention, in addition to the first to third aspects, the magnetic sensor uses an anisotropic magnetoresistive element and applies a bias magnetic field by a permanent magnet, a coil, or the like. The eddy current sensor according to claim 1.

  According to the first aspect of the present invention, the AC magnetic field generating coil and the magnetic sensor are disposed close to each other or concentrically so that the magnetic field passes perpendicularly to the magnetic sensing direction of the magnetic sensor. Since the eddy current generated in the detected object passing in the vicinity of the magnetic sensor is detected, almost no signal is generated from the magnetic sensor even if the AC magnetic field generating coil is operating in the absence of the detected object. (There is no change), and the eddy current generated when the detection object passes changes the output of the magnetic sensor for the first time. Therefore, the signal processing circuit has a small and simple structure and is inexpensive and highly reproducible. A sensor can be realized.

  According to a second aspect of the present invention, the AC magnetic field generating coil and the magnetic sensor are arranged so that the magnetic field passes in the same direction as the magnetic sensing direction of the magnetic sensor, and the AC magnetic field generating coil and the magnetic sensor are arranged. The eddy current generated in the detected object passing across the magnetic flux line between the two is detected, so the magnetic field from the AC magnetic field generating coil is consumed by the detected object and the remaining magnetic field is detected by the magnetic sensor. Therefore, no matter what part between the AC magnetic field generating coil and the magnetic sensor passes through the detected object, the output of the magnetic sensor is not affected, and highly accurate measurement can be realized with a simple structure.

  According to the invention described in claim 3, since the applied magnetic field for generating the eddy current is a square wave or a pulse wave, the fundamental wave and the harmonics constituting the square wave are used to change the output of the magnetic sensor of the detected object. By setting the frequency and odd-order harmonics so that consumption due to eddy currents at the fundamental wave due to the skin effect is kept low, and consumption due to eddy currents due to the skin effect due to differences in the detected objects can be sufficiently identified, Stable detection is possible with only one measurement without depending on the output fluctuation due to the temperature characteristic or the like depending on the ratio of the detection signal.

  According to the invention of claim 4, the magnetic sensor uses an anisotropic magnetoresistive element and is configured by applying a bias magnetic field by a permanent magnet, a coil, etc., so that the sensor using a general coil is used. There is no frequency dependence of the sensor sensitivity that can be seen, and it is possible to realize high sensitivity and stable characteristics at low cost by using only a simple signal processing circuit to set the frequency according to various detection objects from good frequency characteristics.

  An eddy current sensor according to the present invention includes an AC magnetic field generating coil that generates an eddy current in an object to be detected by application of an AC magnetic field, a magnetic sensor having sensitivity in a certain direction for detecting the magnetic field due to the eddy current, and The AC magnetic field generating coil and the magnetic sensor are arranged so that a magnetic field passes in the same direction as the magnetic sensing direction of the magnetic sensor, and a magnetic flux line between the AC magnetic field generating coil and the magnetic sensor is arranged. An eddy current generated in a detection object that passes across is detected, the applied magnetic field that generates the eddy current is a square wave or a pulse wave, and the magnetic sensor is anisotropic. It is characterized by using a magnetoresistive element and applying a bias magnetic field by a permanent magnet, a coil or the like.

  FIG. 1A is a top view showing the configuration of the first embodiment of the eddy current sensor 10 of the present invention, and FIG. 1B is a schematic diagram showing the positional relationship when viewed from the A direction. It is. In FIG. 1, 11 is a magnetic sensor comprising an anisotropic magnetoresistive element biased with a weak magnetic field by a permanent magnet, 12 is a coil for generating an alternating magnetic field, and 13 is a detection object such as a coin. It is.

  FIG. 2 shows an example of the configuration of the magnetic sensor 11 of the present invention. FIG. 2A shows a basic configuration of the magnetic sensor 11 using an anisotropic magnetoresistive element, and four magnetoresistive elements 18, 19, 20 formed on a substrate 15 as an attachment surface. , 21 and a bias magnet 16 having a magnetic pole face 25 (for example, N pole) facing each other so as to be substantially parallel to the mounting surface on the substrate 15. Of the four magnetoresistive elements on the substrate 15, 18 and 21 are provided on the center line 23 passing through the center point 24 substantially at the center of the substrate 15 in the drawing and at symmetrical positions with respect to the center point 24. These magnetoresistive elements 18 and 21 are both extended in an angle direction of 45 ° with respect to the center line 23. The remaining 19 and 20 of the four magnetoresistive elements are provided on the center line 22 that passes through the center point 24 and is perpendicular to the center line 23 and is symmetrical to the center point 24. These magnetoresistive elements 19 and 20 are provided so as to extend in an angle direction of 45 ° with respect to the center line 22. Although the shape of the bias magnet 16 in FIG. 2A is square, the shape is not limited to this as long as the magnetic pole surface 25 faces the substrate 15. For example, the bias magnet 16 having a circular shape is used. Also good. As shown in FIG. 2D, magnetic vectors are generated radially from the magnetic center 26 from the magnetic pole face 25 of the bias magnet 16.

In the substrate 15 on which the four magnetoresistive elements 18 to 21 are arranged in this manner, the center point 24 and the magnetic center 26 on the magnetic pole surface 25 of the bias magnet 16 overlap as shown in FIG. Adjust the positional relationship. In this state, as shown in FIG. 2 (c), the magnetic sensor 11 is configured by being bonded to both surfaces of the lead frame 17 and being held by the mold package 14.
Further, as shown in FIG. 2B, the magnetoresistive elements 18 and 21 and 19 and 20 facing the center point 24 constitute half bridges, respectively, and outputs from these midpoints are OutA and OutB, respectively. Take out as. In this embodiment, the substrate 15 and the bias magnet 16 are held by the mold package 14, but the holding method is not limited as long as the positional relationship between the center point 24 on the substrate 15 and the bias magnet 16 is correct. .

  The eddy current sensor 10 in this embodiment discriminates the detected object 13 based on the difference output between OutA and OutB, which are the two outputs of the magnetic sensor 11, and the waveform of the difference output is shown in FIG. As shown.

When the detected object 13 passes through the vicinity of the magnetic sensor 11, a vertical magnetic field is applied from the AC magnetic field generating coil 12, and an eddy current is generated in the detected object 13. Here, since the surface on which the anisotropic magnetoresistive element of the magnetic sensor 11 is formed is installed perpendicular to the direction of the applied magnetic field from the alternating magnetic field generating coil 12, the applied magnetic field from the alternating magnetic field generating coil 12 is used. And the magnetic sensing direction of the anisotropic magnetoresistive element are orthogonal to each other, and the magnetic sensor 11 detects the magnetic field in the surface direction due to the eddy current and outputs an output signal as shown in FIG. Will increase.
Further, when the detected object 13 approaches the magnetic sensor 11 and approaches the position where the magnetic sensing center of the magnetic sensor 11 and the detected object 13 overlap, a relationship in which eddy currents are generated almost radially from the center of the AC magnetic field generating coil 12 is generated. In addition, since the horizontal magnetic field is reduced to a state in which only the vertical magnetic field is present, the output signal of the magnetic sensor 11 decreases as shown in FIG.
Thereafter, as the detected object 13 gradually moves away from the magnetic sensing center of the magnetic sensor 11, as shown in FIG. 1C, the horizontal magnetic field increases again and the output signal also increases. The output signal decreases as the distance increases.

  In the eddy current sensor 10 according to the present embodiment, the surface of the magnetic sensor 11 on which the anisotropic magnetoresistive element is formed and the direction of the applied magnetic field from the AC magnetic field generating coil 12 are perpendicular to each other. Then, since the magnetic flux lines passing through the magnetic sensor 11 are only in the vertical direction, the signal from the magnetic sensor is very weak. Therefore, it may be determined by the increase or decrease of the signal when passing through the detected object 13, and a signal processing circuit can be easily created.

  In FIG. 1, the AC magnetic field generating coil 12 is used. However, in order to make the applied magnetic field in the vertical direction a parallel magnetic field, two AC magnetic field generating coils 12 are used to form a Helmholtz coil. May be used in pairs.

FIG. 3A is a top view illustrating the configuration of the eddy current sensor 10 according to the second embodiment of the present invention, and FIG. 3B is a schematic diagram illustrating the positional relationship when viewed from the B direction. It is.
In FIG. 3, 11 is a magnetic sensor comprising an anisotropic magnetoresistive element biased with a weak magnetic field by a permanent magnet, 12 is a coil for generating an alternating magnetic field, and 13 is a detection object such as a coin. It is. As in the first embodiment, the magnetic sensor 11 uses an anisotropic magnetoresistive element (18 to 21) shown in FIG.
The second embodiment is characterized in that the detected object 13 passes between the AC magnetic field generating coil 12 and the magnetic sensor 11.

  When the detected object 13 passes between the magnetic sensor 11 and the AC magnetic field generating coil 12, the magnetic field from the AC magnetic field generating coil 12 is shielded by the detected object 13, and an eddy current is generated. In the magnetic field from the AC magnetic field generating coil 12 that reaches the magnetic sensor 11, a deficiency of the magnetic force converted into the eddy current is generated.

The AC magnetic field generating coil 12 generates a magnetic field corresponding to the current waveform to be applied. Here, assuming that the current flowing through the AC magnetic field generating coil 12 is a square wave, the frequency component y (t) of the generated magnetic field is expressed by the following formula (4).

この数式（５）からも分かるように、表皮深さｄと電気抵抗ρ（導電率σ）との関係は、一方が判明すれば他方が判明する関係にあることから、これを検出物１３の判定に役立てることが可能となる。 Further, when a square wave having a frequency component as described above is input, the skin depth d of the eddy current generated is expressed by the following formula (5). At this time, the skin depth with respect to the thickness of the detected object 13 is expressed. The frequency is selected so that the depth d is sufficiently deep.

As can be seen from this equation (5), the relationship between the skin depth d and the electrical resistance ρ (conductivity σ) is such that if one is found, the other is found. It can be used for judgment.

  Next, a signal processing circuit from the magnetic sensor 11 in the second embodiment will be described with reference to FIG. In the block diagram shown in FIG. 4A, the output signal from the magnetic sensor 11 is input to the amplifier 27 and simply AC amplified. Here, when two signals OutA and OutB are taken out as output signals of the magnetic sensor 11, AC amplification is performed for each of them. One of the outputs from the amplifier 27 is passed through a high pass filter (HPF) 28, and then the maximum value of the signal is detected by the peak detection circuit 29a. The peak detection circuit 29b is applied to the other output of the amplifier 27 as it is. The comparison circuit 30 divides the output of the peak detection circuit 29b by the output of the peak detection circuit 29a, thereby obtaining an output signal representing the ratio of fluctuation due to the detected object 13.

The signal from the magnetic sensor 11 is simply AC amplified by the amplifier 27, then the maximum value is measured by the peak detector 29b, and the signal is passed through a simple high-pass filter 28 that can separate the harmonic components from the fundamental wave. The detector 29a is applied to measure the maximum value of the harmonic component.
At the maximum value of the basic waveform, loss due to eddy current at the time of passage is negligible, so there is almost no change in the maximum value, but eddy current loss due to the skin effect is greatly generated for harmonic components. That is, since the loss of eddy current due to the skin effect varies depending on the material, thickness, and the like of the detected object 13, the detected object 13 can be determined based on this.

In the comparison circuit 30, by dividing the output of the peak detector 29b of the fundamental wave by the output of the peak detector 29a, the relationship of the fluctuation ratio of the eddy current due to the detected object is obtained. Since the output of the comparison circuit 30 is free from fluctuation factors due to temperature or the like, the material can be distinguished well by the difference in the electrical resistance of the detected object 13.
Since the magnetic sensor 11 uses an anisotropic magnetoresistive element, the frequency characteristic is flat from direct current to several MHz or more, and can be easily changed according to the thickness and electric resistance of the detected object 13.

  In the second embodiment, as shown in FIG. 3A, the magnetic sensor 11 and the AC magnetic field generating coil 12 are installed facing each other, and the detection object 13 passes between them. Example 2 is not limited to this. That is, the detected object 13 is configured to pass between the magnetic sensor 11 and the AC magnetic field generating coil 12, and the magnetic sensor 11 can detect a missing magnetic field due to the eddy current flowing through the detected object 13. Any positional relationship may be used. Thereby, since the freedom degree of positioning of an eddy current sensor becomes high when comprising a detection apparatus, versatility increases.

In the above-described embodiment, the magnetic sensor 11 uses an anisotropic magnetoresistive element. However, the magnetic sensor 11 is not necessarily limited to this. For example, a Hall element can be used manually. However, in consideration of frequency characteristics, it is preferable to use an anisotropic magnetoresistive element.
Moreover, in the said Example, although the magnetic sensor 11 was comprised using four anisotropic magnetoresistive elements, it is not limited to this, At least 1 or more anisotropic magnetoresistive element is included. If there is, it is configurable.

(A) is the top view showing the structure of Example 1 of the eddy current sensor 10 of this invention, (b) is the schematic diagram showing the positional relationship when it sees from A direction, (c ) Is an example of an output waveform of the magnetic sensor 11. It is explanatory drawing showing the basic composition of the magnetic sensor 11 using an anisotropic magnetoresistive element. (A) is the top view showing the structure of Example 2 of the eddy current sensor 10 of this invention, (b) is the schematic diagram showing the positional relationship when it sees from a B direction. (A) is a block diagram showing an example of a processing circuit for an output signal of the magnetic sensor 11, and (b) is an example of an output waveform of the eddy current sensor 10. FIG. 6 is a schematic diagram illustrating how eddy currents are generated in a detection object 13.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... Eddy current sensor, 11 ... Magnetic sensor, 12 ... Coil for alternating current magnetic field generation, 13 ... Detected object, 14 ... Mold package, 15 ... Substrate, 16 ... Bias magnet, 17 ... Lead frame, 18-21 ... Anisotropy Magnetoresistive element, 22 ... center line, 23 ... center line, 24 ... center point, 25 ... magnetic pole surface, 26 ... magnetic center, 27 ... amplifier, 28 ... high-pass filter, 29a, 29b ... peak detection circuit, 30 ... comparison Circuit, 31 ... alternating magnetic field, 32 ... eddy current, 33 ... magnetic field by eddy current.

Claims (4)

  An AC magnetic field generating coil that generates an eddy current in a detection object that is a conductor by applying an AC magnetic field, and a magnetic sensor that has sensitivity in a certain direction for detecting the magnetic field due to the eddy current. The AC magnetic field generating coil and the magnetic sensor are placed close to each other or concentrically arranged so that a magnetic field passes perpendicularly to the magnetic direction, and vortices generated in an object passing near the magnetic sensor. An eddy current sensor characterized by detecting an electric current.   An AC magnetic field generating coil that generates an eddy current in a detection object that is a conductor by applying an AC magnetic field, and a magnetic sensor that has sensitivity in a certain direction for detecting the magnetic field due to the eddy current. The AC magnetic field generating coil and the magnetic sensor are arranged so that the magnetic field passes in the same direction as the magnetic direction, and the detected object passes across the magnetic flux line between the AC magnetic field generating coil and the magnetic sensor. An eddy current sensor characterized by detecting an eddy current generated.   3. The eddy current sensor according to claim 1, wherein the applied magnetic field for generating the eddy current is a square wave or a pulse wave.   4. The eddy current sensor according to claim 1, wherein the magnetic sensor uses an anisotropic magnetoresistive element and is applied with a bias magnetic field by a permanent magnet, a coil or the like.

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