JP2004125526A - High-precision detection method of magnetic substance - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To remove noise precisely by enhancing the S/N ratio by multiplying an electric signal outputted from a reading medium by a rectangular wave synchronized with the frequency of an excitation wave by paying attention to the fact that a peak output from a magnetic body appears synchronously with the frequency of an alternating-current excitation wave whereas disturbance noise has no periodicity. <P>SOLUTION: This high-precision detection method of the magnetic substance comprises a step for imparting a magnetic field changing periodically to a time in order to magnetize the magnetic substance, a step for detecting a change with time of magnetization or a magnetic flux density having the magnetization and the magnetic field as components, and outputting a peak waveform changing periodically to the time, a step for digitizing the peak waveform, and a step for extracting signal data digitized in accordance with a time frequency, and removing a noise output other than the peak waveform output. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for increasing the detection accuracy when detecting a magnetic substance.
[0002]
[Prior art]
For detecting magnetic ink formed by printing and magnetic recording material formed by a transfer method, a magnetic sensor having the same sensitivity as a compensation coil without providing a compensation coil has been proposed. (For example, see Patent Document 1).
Further, in order to determine the authenticity of the ferromagnetic material storing the fixed and semi-fixed information formed on the safety protection paper, a magnetic change detecting means for reading information stored in the ferromagnetic material as a magnetic change, and reading as a magnetic characteristic An apparatus is disclosed in which a magnetization characteristic detecting unit compares the data with data stored and determined in advance (for example, see Patent Document 2).
[0003]
Some cash vouchers, such as banknotes, gift certificates, and stock certificates, include a thin plastic film coated with a ferromagnetic material to prevent forgery. It is known that the application of heat or an external force such as cutting to a ferromagnetic material changes the magnetic permeability and the saturation magnetization.
For example, when heat is applied to a ferromagnetic material, the magnetic permeability of the applied portion changes to 1/10 to 1/100 before the application.
Further, even when the ferromagnetic material is cut or pressed, a change in magnetoresistance occurs, and the ferromagnetic material can be stored as fixed or semi-fixed information in a part of the ferromagnetic material.
Further, the ferromagnetic material can be made into an alloy by changing the ratio of the metal contained therein, and the magnetic characteristics are changed by changing the ratio of the added metal. Can be used.
[0004]
Further, the characteristic of the saturation characteristic element of the magnetic material can be detected and used as a forgery prevention means.
For example, the coercive force (Hc) inherent to the magnetic material and the magnetic flux density (Br) of the magnetic material, which varies depending on the prescription of the magnetic material, can be used as the characteristics of the saturation characteristic element. It can be used as a means of preventing forgery.
[0005]
On the other hand, it is possible to measure the magnetic properties of the magnetic material by the technique disclosed in Patent Document 1 described above, but it is possible to measure the deviation of the magnetic force lines due to the ferromagnetic material or magnetic metal existing near the magnetic sensor, No measures have been taken with respect to maintaining the balance between the voltages generated by the detection coil and the compensation coil.
Further, when the balance is lost, the voltage component of the excitation power supply becomes larger than the excitation voltage component of the magnetic substance to be measured, which causes a deterioration in the S / N ratio of the magnetic characteristic measurement.
For this reason, when the magnetic sensor is incorporated in a device having a transport system in which many metal parts are used, it is pointed out that the mounting position is restricted in order to avoid generation of noise.
[0006]
[Patent Document 1]
Japanese Patent Application Laid-Open No. 9-211095 [Patent Document 2]
JP-A-7-32778
[Problems to be solved by the invention]
In the present invention, the peak output from the magnetic material is synchronized with the period of the magnetic field applied to magnetize the magnetic material, while the reading accuracy is not improved due to the noise due to the magnetic metal near the read head. Although appearing, attention is paid to the fact that the disturbance noise has no periodicity, the A / D conversion of the electric signal output from the reading medium is performed, and then the data synchronized with the period of the magnetic field applied to the magnetic material is extracted. An object of the present invention is to increase the / N ratio and accurately remove noise.
[0008]
[Means for Solving the Problems]
In order to achieve the object of the present invention, the method according to the first aspect of the present invention provides a method for detecting a magnetic substance with high accuracy, wherein a step of applying a magnetic field that changes periodically with time to magnetize the magnetic substance. Detecting a temporal change in magnetic flux density having the magnetization or the magnetization and the magnetic field as components, outputting a peak waveform that periodically changes with time, and digitizing the peak waveform. , Extracting the digitized data in accordance with a time period and removing noise output other than the peak waveform output.
[0009]
According to a second aspect of the present invention, in the first aspect of the invention, the time period when extracting the digitized data is the same as the period of the magnetic field provided to magnetize the magnetic material. In addition, the time width is not less than the half width of the peak waveform.
[0010]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, a method for detecting a magnetic substance with high accuracy according to the present invention will be described with reference to the drawings.
FIG. 1 is a conceptual diagram for explaining a preferred embodiment of a method for detecting a magnetic substance with high accuracy according to the present invention, FIG. 2 is an explanatory diagram of a conventional general signal detection method, and FIG. 4 is an example of an equivalent circuit diagram of an AC excitation type magnetic sensor, FIG. 4 is a diagram for explaining characteristics of a magnetic material used as a detection target, and FIG. FIG. 6 is a view for explaining an example of a cash voucher having a magnetic material formed on its surface, FIG. 7 is a cross-sectional view of a safety wire cut into paper, and FIG. It is sectional drawing of the magnetic material transcribe | transferred to the surface.
[0011]
A preferred embodiment of the method for detecting a magnetic substance with high accuracy according to the present invention will be described with reference to FIGS.
When the magnetic sensor is brought close to a predetermined position of the magnetic material and an AC magnetic field is applied, a waveform as shown in the output waveform diagram A of FIG. 1 appears.
The magnetic sensor is composed of three coils as shown in FIG. 3, and first supplies an alternating current to the exciting coil 41 to apply the aforementioned alternating magnetic field H to the magnetic material.
The magnetic flux density (magnetic permeability) Be of the magnetic material changes in a non-linear manner with respect to the applied AC magnetic field, and these changes in the magnetic flux density are detected by the detection coil 42.
The voltage ve detected by the detection coil can be obtained by the following equation (1) from Faraday's induction law.
ve = −d / dt (N∫ (Be + Bo) Hdv) (1)
[0012]
On the other hand, the voltage vo from the compensation coil 43 is adjusted so as to satisfy the following equation (2).
vo = −d / dt (N · ∫BoHdv) (2)
Therefore, the change in the magnetic flux density of the magnetic material according to the alternating magnetic field is detected as shown in Expression (3) by calculating the difference between the voltages detected from the detection coil and the compensation coil using Expressions (1) and (2). can do.
ve−vo = −d / dt (N · ∫Be · Hdv) (3)
When the squareness ratio of the magnetic material is high, a sharp pulse appears in synchronization with the period of the exciting AC magnetic field.
[0013]
When the magnetic sensor detects an electric noise generated in the reading circuit or a magnetic noise of disturbance, it is necessary to distinguish the signal from the signal of the formula (3).
Since the voltage obtained by equation (3) is detected in synchronization with the cycle of the voltage applied to the exciting coil, the output data is extracted in accordance with this cycle to increase the S / N ratio and remove noise. be able to.
At this time, the time width of the data to be extracted needs to be larger than the half width of the signal output obtained by the equation (3).
[0014]
A detection signal shown in the output waveform diagram B of FIG. 1 is generated at a position where the waveform shown in the output waveform diagram A of FIG. 1 and the coercive force value (+ Hc, -Hc) of the magnetic material to be detected intersect.
In the detected signal, a plus waveform and a minus waveform appear every half cycle.
Next, as shown in the output waveform diagram C of FIG. 1, a negative signal wave is converted into a positive signal wave.
[0015]
As shown in the output waveform diagram C, noise a1 and noise c such as electric noise generated by the reading circuit and magnetic noise due to disturbance appear irregularly in the signal wave b. May be approximate.
Therefore, as shown in the output waveform diagram D, the position of the signal output to be detected is determined by the holding force of the magnetic material, and the period of the signal output is determined by the period of the AC magnetic field applied to the magnetic material. The signal extraction wave d is generated at an interval of 1/2 pt.
As shown in the output waveform diagram E, an output difference between the signal wave b and the noise wave is generated by the signal voltage obtained in the output waveform diagram C and the signal extraction wave, and the noise wave 2a and the noise wave c are removed.
The height and width of the signal extraction wave in the output waveform diagram D are set to values larger than the half width of the signal wave in the output waveform diagram C.
[0016]
FIG. 2 is an explanatory diagram of a conventional general method for signal detection.
In the output waveform diagram, a signal wave f detected at a constant cycle and noises e and g generated at an irregular cycle are mixed, and the output level of the noise e is substantially equal to the output level of the signal f. It is.
At this time, if the peak output value is sliced by vS, the noise e is also captured as a signal, and it can be seen that a correct determination cannot be made.
[0017]
The characteristics of the magnetic material used as a detection target will be described with reference to FIG.
FIG. 4 shows BH characteristics measured for a ferromagnetic material and a general magnetic material such as pure iron in an overlapping manner.
In FIG. 4, the saturation magnetic flux of the ferromagnetic material and that of pure iron happen to be displayed at the same level, but they need not be the same.
A magnetic material is produced by applying a magnetic paint on a plastic film, paper, or the like, or by forming by a method such as vapor deposition. Generally, magnets are used to orient the magnetic particles in a certain direction during the coating and forming.
The magnetic material produced in this manner is made into small pieces of a fixed size, and the coercive force and the residual magnetic flux are measured by a measuring machine for measuring the BH characteristics.
A small piece of a magnetic substance coated by dissolving powder of the above-mentioned general pure iron in a resin is inserted between the coils, and a direct current is applied to the coils.
When the voltage is increased from 0 along the central arrow and the voltage is increased along the horizontal axis, the magnetization curve of the dotted line reaches the saturation point Bm. When the voltage is reduced from the saturation point Bm, the magnetization curve crosses the vertical axis at Br2 when the voltage becomes zero. When a further negative voltage is applied, the line crosses the horizontal axis at a certain point, as indicated by a dotted line along the left arrow, and saturates negatively at a certain point.
When the voltage is increased to 0 from the point of negative saturation, the voltage intersects the vertical axis, and when the voltage is further increased, the horizontal axis intersects the horizontal axis at Hc2 along the arrow on the right. When the voltage is further increased, the saturation point Bm is reached. Such a curve is called a BH curve.
[0018]
Bm is called a saturation magnetic flux, and Br2 is called a residual magnetic flux. Hc2 is called a coercive force. Also, Br2 / Bm is called a squareness ratio. Performing the above-described orientation treatment improves the squareness ratio. When a material having a large squareness ratio is used, the squareness ratio shows a large value without performing orientation.
For example, when a ferromagnetic material is used, a straight line BH curve shown by a solid line in FIG. 4 is drawn. In the case of this magnetic material, Bm and Br1 are substantially the same, and thus the squareness ratio approaches 1.
The above-described coercive force value is a value determined by the magnetic material constituting the magnetic body. When a magnetic material having a special coercive force is used, it can be used as a unique information for preventing forgery or the like by registering it in a sensor.
The above-mentioned residual magnetic flux value is used as a forgery prevention material because its specificity is exhibited by the prescription of the magnetic substance and by the magnetic material itself.
At that time, it is necessary to accurately grasp the signal detected from the magnetic body, and it is an important step to completely remove noise received from the detection device or the detection environment.
[0019]
With reference to FIG. 5, the procedure of the method for detecting a magnetic substance with high accuracy according to the present invention will be described.
First, an AC magnetic field is applied to a magnetic body as a detection target to generate a signal wave (S41).
Next, a peak value synchronized with the period of the magnetic field is specified to generate an outgoing wave, and a signal extraction wave is generated (S42).
Further, the time width of the signal extraction waveform is set to be equal to or greater than the half width of the output signal (S43).
Next, a signal wave is extracted according to the cycle and the position learned in order 43 (S44).
Finally, output waves other than the signal are removed (S45).
[0020]
A cash voucher, which is an embodiment of the detection target, will be described with reference to FIG.
The magnetic body 1 or the magnetic body 2 is formed on a part of the base material 3. The magnetic material 1 is a safety filament (hereinafter, referred to as a thread), and the magnetic material 2 is a thin film (transfer foil) of a magnetic material that is thermally transferred.
The base material 3 is often paper.
The thread is slit into a thin film of polyester having a thickness of 3 to 15 [mu] m by thinly applying, depositing, or sputtering a ferromagnetic material. The slit threads are made as part of the paper during the papermaking stage.
The paper is cut into a predetermined size and printed.
As shown in FIG. 6, the thread 1 is formed in a finished state in parallel with the short side direction as a gift certificate.
This thread is usually only exposed on one side of the gift certificate. The thread embedded in the gift certificate is read by a dedicated reader with a magnetic sensor and checked for authenticity.
The ferromagnetic material may be crystalline or amorphous, and may be a magnetic material composed of one or a combination of two or more of Fe, Co, and Ni, or a magnetic material composed of one or two of them. As main components, boron (B), carbon (C), magnesium (Mg), aluminum (Al), silicon (Si), phosphorus (P), sulfur (S), titanium (Ti), and vanadium (V ), Chromium (Cr), manganese (Mn), copper (Cu), zinc (Zn), yttrium (Y), zirconium (Zr), niobium (Nb), moridieven (Mo), palladium (Pd), silver (Ag) ), Indium (In), tantalum (Ta), tungsten (W), iridium (Ir), platinum (Pt), gold (Au), and lead (Pb). Thing Constructed.
[0021]
Separately from the thread form, when ferromagnetic materials are transferred by heat to various types of paper such as gift certificates, car horse tickets, soccer tickets, other admission tickets, seat reservation tickets, instant lottery tickets, ski lift tickets, viewing tickets, etc. There is.
For example, a magnetic material is thermally transferred as a part of the design beside the display of a gift certificate, and the transferred magnetic material is read by a magnetic sensor to check whether a legitimate material is used.
When the authenticity of the medium is confirmed using the medium shown in FIG. 6, information is written and read with a magnetic head on a thread or a portion where a magnetic material is formed by a transfer foil, or the material used is By checking the characteristics (coercive force, residual magnetic flux, frequency, etc. of the material), it is checked whether the medium is a proper medium.
[0022]
FIG. 7 is a cross-sectional view of a thread that is made into paper.
The thread 1 has a soft magnetic material 11 applied to a surface of a plastic base material 12.
The soft magnetic material is an amorphous metal mainly composed of a ferromagnetic material such as iron, nickel, and cobalt, and is characterized by high magnetic permeability, small coercive force, control of magnetostriction in a wide range, It has excellent properties such as high resistance, small temperature change, low hysteresis loss and low eddy current loss.
In particular, since it has a small coercive force, it is also used as a soft magnetic material for various magnetic signal reading sensors.
Further, since the magnetic permeability and the squareness ratio are high, it is also used as a magnetic signal detector by the large Barkhausen effect.
[0023]
FIG. 8 is a cross-sectional view of the transfer foil transferred to the surface of the cash voucher.
The transfer foil 2 is transferred as a part of the design to the front or back side of the printed matter mainly based on the paper 3.
The area of the transferred magnetic material 21 is selected to an appropriate size according to the detection sensitivity of the magnetic head and the track width. It is transferred to the surface of the paper in a shape designed with a thermoplastic adhesive 22. The magnetic material may be the above-described ferromagnetic material or a magnetic paint obtained by solidifying iron oxide with a resin. The thickness of the magnetic material 21 is set to 0.05 to 3 μm so that transfer can be performed in a free shape by thermal transfer.
[0024]
【The invention's effect】
3. The method according to claim 1, further comprising: applying a magnetic field that changes periodically with time to magnetize the magnetic material; A step of detecting a change and outputting a peak waveform that periodically changes with time; a step of digitizing the peak waveform; extracting the digitized data in accordance with a time period and excluding the peak waveform output And the time period when extracting the digitized data is the same as the period of the magnetic field applied to magnetize the magnetic material, and the time width is half of the peak waveform. Since the peak output from the magnetic material is not less than the value width, the peak output from the magnetic material appears in synchronization with the period of the magnetic field applied to magnetize the magnetic material. Therefore, after the A / D conversion of the electric signal output from the reading medium, the S / N ratio is increased by extracting data synchronized with the period of the magnetic field applied to the magnetic body, and noise is accurately removed. And a magnetic substance can be detected with high accuracy.
[Brief description of the drawings]
FIG. 1 is a conceptual diagram for explaining a preferred embodiment of the present invention; FIG. 2 is an explanatory diagram of a conventional general method for signal detection; FIG. 3 is an equivalent circuit of an AC excitation type magnetic sensor; FIG. 4 is a diagram for explaining characteristics of a magnetic material used as a detection target. FIG. 5 is an explanatory diagram of a procedure of a method for detecting a magnetic substance with high accuracy according to the present invention. FIG. 7 is a cross-sectional view of a safety thread (thread) formed on paper. FIG. 8 is a cross-sectional view of a magnetic material transferred to the surface of the cash voucher. Explanation of code]
1 safety line (thread)
2 Transfer foil (of magnetic material) 3 Paper (gift certificate)
11, 21 Magnetic material 12 Plastic base 22 Adhesive 41 Excitation coil 42 Detection coil 43 Compensation coils a1, a2, c, e, g Noise waveform b, f Signal waveform d Signal extraction wave vS Slice lines Br1, Br2 Residual magnetic flux Bm Saturation Magnetic flux Hc1, Hc2 Coercive force value

Claims (2)

Applying a magnetic field that varies periodically with time to magnetize the magnetic material;
Detecting a temporal change in the magnetic flux density having the magnetization or the magnetization and the magnetic field as components, outputting a peak waveform that periodically changes with time, and digitizing the peak waveform;
Extracting the digitized signal data in accordance with a time period and removing noise outputs other than the peak waveform output;
A method for detecting a magnetic substance with high accuracy, comprising: The time period at which the digitized data is extracted is the same as the period of the magnetic field applied to magnetize the magnetic material, and the time width is equal to or greater than the half width of the peak waveform. Item 6. A method for detecting a magnetic substance with high accuracy according to Item 1.

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