KR100293899B1 - Method for sensing fingerprint by redistribution of electric charge - Google Patents

Abstract

PURPOSE: A fingerprint sensing method is provided to make an initial voltage of a fingerprint not have an influence on sensing the fingerprint, to output grey level value according to a fingerprint valley or a fingerprint ridge, and to detect electric characteristics of the fingerprint by scanning the fingerprint so that it can get detailed fingerprint distribution and reduce the number of sensing electrodes. CONSTITUTION: The method comprises steps of charging two or more capacitors to a certain voltage, distributing different charges to the capacitors according to the capacitance of the contacted capacitor when a finger is contacted, and measuring the amount of a voltage variation caused by the charge redistributed to the capacitors. When the capacitors are charged to a certain voltage, they are independent so that they are respectively charged to predetermined voltages. In the step of measuring the amount of the voltage variation, the independently charged capacitors are connected, and charges are differently redistributed according to the capacitance of the contacted capacitor. Two outputs at spatially adjacent points are compared and the difference between the two outputs is output in order to sense data on a grey level between a fingerprint ridge and a fingerprint valley as well as on the ridge and the valley.

Description

Fingerprint Detection Method by Charge Redistribution

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fingerprint detection method in a fingerprint recognition system, and more particularly, to a method for detecting a fingerprint without being affected by the initial voltage of a finger to be contacted by using a charge redistribution method.

The fingerprint recognition system is a system that recognizes a person by collecting and storing a fingerprint by using a property that fingerprints are different for each person, and taking an input fingerprint and comparing it with previously stored data. Such a fingerprint recognition system is mainly used for a security system for identifying a transaction party in electronic commerce, controlling access to a specific area, or controlling access to specific data. Unlike signatures, keys, or cards, the fingerprint recognition system is not duplicated or counterfeit, nor is it a way to enter a password, so there is no risk of losing or stealing keys or cards, and mistaken for mistaken passwords. There is no advantage that can be conveniently used compared to the conventional safety system.

The core of such a fingerprint recognition system is to accurately capture and store fingerprint data that can indicate the characteristics of an input fingerprint, or to identify the difference by comparing the collected fingerprint data with previously stored fingerprint data. The characteristics of the fingerprint grasped in a commonly used fingerprint recognition system are the shape of the valleys and the floor of the fingerprint, and the pattern extracted from the shape of the valleys and the floor is recognized by the image processing method.

The characteristic of the fingerprint is the distribution of the valley of the fingerprint and the floor of the fingerprint formed on the fingerprint. As a method of measuring and distributing the distribution of the valleys and the floor of the fingerprint, a commonly used method is to use the difference in the capacitance of the capacitor formed by contact of the fingerprint with the sensing electrode. That is, since the distance difference in the capacitor formed by the contact of the fingerprint is different depending on whether the portion where the fingerprint is in contact with the valley position of the fingerprint or the floor position of the fingerprint is different, the capacitance is used accordingly.

In general, the capacitance of a capacitor can be known by applying a voltage to the capacitor and measuring the charging current of the capacitor. Therefore, in the conventional method described above, the capacitance of the capacitor measured by the voltage of the finger is affected when the finger contacting the sensing electrode is charged in advance. In fact, since the finger is often charged due to static electricity or the like due to synthetic fibers, there is a concern that the conventional fingerprint sensing device malfunctions.

In addition, in the conventional fingerprint recognition method, in identifying the characteristics of the fingerprint, only data relating to whether a specific portion is a bone portion or a floor portion of the fingerprint is output. The fingerprint is a continuous connection from the bone site to the ridge site and a particular bone site of the fingerprint has a height distribution. According to the conventional method described above, the output according to the height distribution of the connection site or the valley itself, that is, the gray level cannot be detected.

1 is a view for explaining a sensing unit of a general fingerprint sensing device.

As shown in FIG. 1, a sensing unit of a general fingerprint sensing device includes a driving unit 10 for applying a voltage or a current to a sensing unit in order to detect electrical characteristics of a valley and a floor of a fingerprint. Due to the electrical output signal detected by the sensing electrode, it is separated enough to distinguish the valley and the floor of the fingerprint, and the sensing is large enough to grasp the fingerprint characteristics of the finger as the distribution of electrical output of the entire sensing electrode. And a detector 30 configured to detect the electrical output signal of the fingerprint output from the sensing electrodes of the detector.

In the sensing unit of the fingerprint sensing device as described above, the interval between the sensing electrodes constituting the sensing unit should be close enough to distinguish the valley and the floor of the fingerprint, and the fingerprint characteristics of the finger as the distribution of the electrical output of the entire sensing electrode. The sensing unit needs a sufficient number of sensing electrodes to be sensed.

The sensing electrode generally includes a transistor, and the sensing unit composed of the sensing electrodes is manufactured as an integrated circuit. In the case of an integrated circuit, the size of the sensing unit is directly related to the manufacturing cost and power consumption. The larger the size of the sensing unit is, the more expensive it is to manufacture into an integrated circuit and the greater the power consumption during circuit operation.

In the conventional general sensing unit, since the characteristic of the fingerprint is detected as a distribution of the electrical outputs of the entire sensing electrodes constituting the sensing unit, a sufficient number of sensing electrodes is required as described above, and thus, there is a limit in reducing the size of the sensing unit. There is also a limit to reducing the manufacturing cost.

The present invention is to solve the problems of the prior art as described above, it is an object of the present invention to provide a fingerprint detection method that is not affected by the initial voltage of the finger to be contacted using the charge redistribution method.

Still another object of the present invention is to provide a fingerprint sensing method for giving a gray level output value according to a valley part or a floor part of a fingerprint in order to grasp the distribution of the fingerprint in more detail.

Another object of the present invention is to solve the problem of the general detection unit as described above, the number of detection electrodes constituting the detection unit significantly reduced the fingerprint of the finger to be scanned by the detection unit of the fingerprint Another object of the present invention is to provide a fingerprint detection method capable of detecting electrical characteristics.

Another object of the present invention is to have a sensing unit with a reduced number of sensing electrodes can be made in a smaller size, and also can scan the fingerprint of the finger fingerprint sensing that can implement a portable fingerprint sensing device utilizing these features To provide a way.

1 is a view for explaining a detection unit of a general fingerprint detection device,

2 is a circuit diagram illustrating a charge redistribution method used in a fingerprint sensing method according to the present invention;

3 is an operating state diagram of the circuit diagram shown in FIG. 1;

4 is a view for explaining grasping a gray level in the fingerprint detection method according to the present invention;

FIG. 5 is an example of implementing a circuit for outputting a comparison value to grasp the gray level shown in FIG. 3.

6 is a view showing an example of a sensing unit implemented by a fingerprint sensing method according to the present invention;

7 is a state diagram used in the sensing unit shown in FIG.

Explanation of symbols on the main parts of the drawings

10: drive unit 20: detection unit

21: detection electrode 30: detection unit

40: fingerprint detection device

In order to achieve the object as described above, the fingerprint detection method according to the present invention comprises the steps of charging two or more set capacitors to a predetermined voltage in advance; After the charging step of the set capacitor is finished, the electrical charge is distributed differently according to the capacitance of the contact capacitor caused by the contact of the fingerprint of the finger when electrically connecting them, and measures the amount of voltage change due to the charge redistributed to the set capacitor Characterized in that it comprises a step.

In the fingerprint sensing method according to the present invention, the set capacitors are the first capacitor and the second capacitor which are independent of each other when charged to a predetermined voltage, and the step of charging the set capacitor is a first capacitor and a second capacitor It is characterized in that the step of charging to each of the predetermined voltage.

In the fingerprint sensing method according to the present invention, the step of measuring the amount of voltage change due to the redistribution of charge in the set capacitor independently connects the first capacitor and the second capacitor connected to a constant voltage, and the The charge is distributed differently according to the capacitance of the contact capacitor, and the step of measuring the amount of voltage change due to the redistributed charge in the first capacitor and the second capacitor.

The fingerprint detection method according to the present invention includes comparing two spatially adjacent output values in order to give not only information about the valley part and the floor part of the fingerprint, but also information on the gray level between the bone part and the floor part. And outputting a comparison value of which one of the two output values is larger when the difference between two spatially adjacent output values is smaller than a constant value or the difference between the two output values is larger than the constant value. do.

The fingerprint detection method according to the present invention is characterized in that it further comprises the step of reconstructing the gray level by the comparison value output in the step of outputting the comparison value.

In the fingerprint sensing method according to the present invention described above, connecting the set capacitors or keeping them independent of each other is implemented by a switching action by a transistor circuit.

The fingerprint detection method according to the present invention comprises the steps of scanning the fingerprint of the finger by the sensing unit including a sensing electrode consisting of the set capacitors; And obtaining an output signal of the entire fingerprint from the output signal obtained while the fingerprint of the finger is scanned by the sensing unit in the step.

In the fingerprint sensing method according to the present invention, the step of scanning the fingerprint of the finger by the sensing unit is achieved by rubbing the fingerprint of the finger with respect to the sensing electrode in the state in which the sensing unit is fixed, or to the fingerprint of the finger It is characterized in that the detection is achieved in the movement.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings.

2 is a circuit diagram for explaining a charge redistribution method used in the fingerprint sensing method according to the present invention.

A circuit capable of implementing a fingerprint detection method by a charge redistribution method according to an exemplary embodiment of the present invention includes a sensing electrode 11 to which a fingerprint of a finger contacts, as shown in FIG. The capacitor C ₁ and the second capacitor C ₂ are provided, and both capacitors are set regardless of whether the fingerprint of the finger contacts the sensing electrode, and thus are referred to herein as 'setting capacitors'. Both set capacitors are connected by a third switch SW ₃ as shown, and both set capacitors are connected to respective voltage sources through the switch, and the first capacitor C _{1 is} connected to the first switch SW _1. ) Is connected to the first voltage source V ₁ , and the second capacitor C ₂ is connected to the second voltage source V ₂ through the second switch SW ₂ .

When the fingerprint of the finger is in contact with the sensor unit 11, a capacitor is formed between the finger and the sensing electrode 11, which is referred to herein as a 'contact capacitor'. The capacitance of the contact capacitor varies according to the fingerprint portion of the finger in contact with the sensing electrode 11. In addition, the finger in contact with the sensing electrode 11 may be already charged by any path.

Figure 2b is equivalent to, Figure 2a to that the capacitance of the touch capacitor thus generating As an initial voltage of a print of a finger of the finger as V ₃ of the fingers in contact with the sensing electrodes 11 of Figure 2a in contact C ₃ It is a circuit diagram.

The circuit shown in FIG. 1 for implementing a fingerprint sensing method by charge redistribution according to the present invention has a two-stage operating state. The first state is for applying a voltage to the set capacitors by a voltage source, and the second state is for measuring the output value according to the capacitance C ₃ of the contact capacitor by redistributing the charge after disconnecting the voltage source. .

3 is an operating state diagram of the circuit diagram shown in FIG.

3A corresponds to a first state for applying a voltage to the set capacitors by a voltage source, and FIG. 3B corresponds to a second state for measuring an output value by redistributing charge after disconnecting the voltage source.

As shown in FIG. 3, the first state is a short state in which the first switch SW ₁ and the second switch SW ₂ are connected, and the third switch SW ₃ is open. On the contrary, the second state is a state in which the first switch SW ₁ and the second switch SW ₂ are open, and a state in which the third switch SW ₃ is connected.

Here, the first switch SW ₁ , the second switch SW ₂ , and the third switch SW ₃ may be implemented by a switching action by a transistor circuit.

The charge redistribution used in the present invention will now be described with reference to the circuit diagrams of the first state (FIG. 3A) and the second state (FIG. 3B) shown in FIG.

First, the charge amount Q ₁ charged in the first capacitor C ₁ in the first state is C ₁ V ₁ , and the charge amount Q ₂ charged in the second capacitor C ₂ is C ₂ V ₂ , The amount of charge Q ₃ charged in the third capacitor is C ₃ (V ₁ -V ₃ ). Therefore, the total charge amount Q _total charged in the first state may be expressed by Equation 1 below.

Q _total = Q ₁ + Q ₂ + Q ₃ = C ₁ V ₁ + C ₂ V ₂ + C ₃ (V ₁ -V ₃ )

When the output voltage is V _O in the second state, the charge amount Q ₁ ′ charged in the first capacitor C ₁ is C ₁ V _O and the charge amount Q ₂ ′ charged in the second capacitor C ₂ . ) Is C ₂ V ₀ , and the amount of charge Q ₃ ′ charged in the third capacitor C ₃ is C ₃ (V _O -V ₃ ). Accordingly, the total charge amount Q _total 'in the second state may be expressed as in Equation 2 below.

Q _total ′ = Q ₁ ′ + Q ₂ ′ + Q ₃ ′ = C ₁ V _O + C ₂ V _O + C ₃ (V ₀ -V ₃ )

Since the total charge amount Q _total of the first state and the total charge amount Q _total ′ of the second state are the same by the charge conservation law, the following equation (3) is established.

When the equation of Equation 3 is solved for V _O , it can be expressed as Equation 4 below.

Since the output voltage in the first state is V ₂ , as shown in FIG. 3A, the difference between the output voltage V _O of the second state and the output voltage V ₂ of the first state, which can be expressed by Equation 4 above. When ΔV, it can be expressed as Equation 5 below.

It can be seen that ΔV shown in Equation 5 is a value that is independent of the voltage V _{3 of the} finger contacting the sensing electrode and depends on the capacitance C ₃ of the capacitor formed by contact of the finger with the sensing electrode. . Since the capacitance C ₃ of the capacitor formed by the contact of the finger with the sensing electrode is different depending on whether the fingerprint of the finger in contact with the sensing electrode is a valley or a floor, C ₁ , C ₂ , V ₁ , V ₂ is a given value, so measuring the ΔV can detect the fingerprint of the finger.

In the present invention, instead of the conventional method of outputting one of two different output values depending on whether the portion of the finger that is in contact with the sensing electrode is the bone portion or the floor portion of the fingerprint, the bone portion and the floor portion to grasp the fingerprint in more detail. Allows detection of gray levels between sites.

4 is a view for explaining a gray level output in the fingerprint detection method according to the present invention.

As shown in FIG. 4A, the fingerprint has a gray level between the bone and the ridges because the bone and the ridges are connected in a natural curve. In the present invention, the gray level is extracted in order to identify the fingerprint in detail.

In order to extract gray levels, the present invention compares two spatially adjacent output values, and when the difference between the two spatially adjacent output values is smaller than a constant value, or when the difference between the two output values is greater than a constant value, which of the two output values is greater than the constant value. Output a comparison of whether one is greater.

For example, the measurement is started from P ₁ point if compared with the value (V _O ¹⁾ with a reference value and measurement value in the P ^₂ (V _{O 2)} a measured value of P ^₁ (V _{O 1).} Measured value of P ^₂ (V _{O 2)} is greater than the reference value preset as compared to measured values of P ^₁ (V _{O 1),} and outputs "+1" as a comparison value. The measured value of P ₃ (V _O ³ ) is compared with the measured value of P ₂ (V _O ² ), and the measured value of P ₃ (V _O ³ ) is a preset reference value compared to the measured value of P ₂ (V _O ² ) If greater than, it also prints '+1'. To compare the measured value of P ^₄ (V _{O 4)} is the measured value of P ^₃ (V _{O 3),} and outputs "0" as the comparison value, so the range is a predetermined difference value. Measured value of P ^₅ (V _{O 5)} is less than the reference value compared with the measured value of P ^₄ (V _{O 4)} as compared to a predetermined measured value of P ^₄ (V _{O 4)} as the comparison value to "-1" Output '-1' is output as a comparison value with respect to P ₆ and P ₇ in the same manner.

FIG. 4B is a gray level reconstruction using the measured values of R1 as reference values using the comparison values of '+1', '0', and '-1' as described above. Referring to the reconstructed gray level shown in Fig. 4b, the method according to the present invention detects not only the fingerprint of the finger touching the sensitive part but the bone or the floor, but also the gray level leading from the bone to the floor. I can see that I can.

FIG. 5 is an example of implementing a circuit for outputting a comparison value to grasp the gray level shown in FIG. 4.

If, if you want to print the comparison value for V _O ^n, the first comparator 41 and second comparator 42 to input the measured value of V _O ^n, respectively, and the other input terminal of the first comparator 41, A value obtained by adding a predetermined value α to a measurement value of the previous measurement value V _O ^n-1 , which is adjacent to V _O ⁿ , is input, and another input terminal of the second comparator 42 has V _O ^n-1. Subtract the preset value α to the measured value of. The first comparator 41 is less than V _O ⁿ a V _O ^n-1 (1 + α) is greater than the output to 'High' level, and V _O ⁿ a V _O ^n-1 (1 + α) 'Low 'Print the level. The second comparator 42 is V _O ⁿ a ^{_{V O n-1 (1-}} α) is greater than the outputs a 'High' level, V _O ⁿ is smaller than ^{_{V O n-1 (1-}} α) 'Low 'Print the level.

The first comparator 41 when the output value of the output value of the O _1, a second comparator 42 as O _2, the comparison value with the O ₁ and O ₂ can be obtained with the following Table 1.

Comparator output, O ₁ , O ₂ V _O ⁿ , V _O ^n-1 (1 + α), V _O ^n-1 (1-α) Comparison O ₁ = High, O ₂ = Low V _O ⁿ > V _O ^n-1 (1 + α) +1 O ₁ = Low, O ₂ = High V _O ^n-1 (1-α) <V _O ⁿ <V _O ^n-1 (1 + α) 0 O ₁ = Low, O ₂ = Low V _O ⁿ <V _O ^n-1 (1-α) -One

Since the circuit outputs '+1', '0' and '-1' as comparison values, the circuit may represent a comparison value with two bits. The gray level can be reconstructed with the comparison value represented by two bits.

In the present invention, in order to reduce the size of the sensing unit of the fingerprint sensing device, instead of the conventional manner in which the entire fingerprint is simultaneously contacted by the sensing unit and the electrical characteristics of the entire fingerprint of the finger are output therefrom, A part is brought into contact, but a fingerprint is scanned by the sensing unit, thereby introducing a method of obtaining data of the entire fingerprint.

Therefore, in the method according to the present invention, since the sensing unit does not need to contact the entire fingerprint of the finger at the same time, the data of the entire fingerprint is obtained by scanning the fingerprint, so that the size of the sensing unit can be reduced. Reducing the size of the sensing unit means that the number of sensing electrodes constituting the sensing unit can be reduced, thereby reducing the cost of manufacturing the sensing unit.

Obtaining electrical characteristic data of the entire fingerprint by scanning the fingerprint with a sensing unit having a size smaller than the total size of the fingerprint to be detected is a simple signal processing that collects and collects data of each sensing electrode of the sensing unit obtained during scanning. It can be implemented by.

Scanning of the fingerprint by the sensor is accomplished in two ways.

The first method is to rub the fingerprint of the finger with respect to the sensing unit while the sensing unit is fixed. In this method, a fingertip is placed on the sensing part having a size smaller than the size of the fingerprint to be detected, and the finger is rubbed until the electrical characteristics of the entire fingerprint appear. The finger moves relative to the sensing unit, but as a result, the entire fingerprint of the finger is scanned by the sensing unit.

The second method is to move the sensing unit with respect to the fingerprint of the finger. In particular, this method makes the fingerprint sensing device including the sensing unit easy to carry, so that a person who wants to detect a fingerprint can pick up the fingerprint sensing device and examine the fingerprint of the person who is detected. In this case, too, the sensing unit having a size smaller than the fingerprint is placed on the tip of the finger until the characteristic of the fingerprint is detected.

6 is a diagram illustrating an example of a sensing unit implemented by a fingerprint sensing method according to the present invention, and FIG. 7 is a state diagram of use of the sensing unit illustrated in FIG. 6.

The sensing unit 20 of the fingerprint sensing device 40 shown in FIG. 6 takes the above-described second method of moving the sensing unit 20 with respect to a fingerprint of a finger. The size of the sensing unit 20 is smaller than that of the finger fingerprint, and as shown in FIG. 7, the fingerprint is scanned until the electrical characteristics of the fingerprint are determined by moving the sensing unit 20 with respect to the fingerprint of the finger. do.

According to the fingerprint detection method according to the present invention, as shown in Equation 5 by the redistribution of the charge, it is possible to obtain a measurement value irrespective of the initial voltage of the contacted finger, and compare the measured values spatially adjacent to each other. As a result, the gray level between the bone portion and the ridge portion of the fingerprint can be detected.

Claims (8)

In the fingerprint detection method, Charging two or more set capacitors to a predetermined voltage in advance; After the charging step of the set capacitor is finished, the electrical charge is distributed differently according to the capacitance of the contact capacitor caused by the contact of the fingerprint of the finger when electrically connecting them, and measures the amount of voltage change due to the charge redistributed to the set capacitor Fingerprint detection method comprising the step of. The method of claim 1, The set capacitors are first and second capacitors independent of each other when charged to a predetermined voltage, and the step of charging the set capacitor is a step of charging the first capacitor and the second capacitor to respective predetermined voltages, respectively. Fingerprint detection method characterized in that. The method of claim 2, The step of measuring the voltage change due to the redistribution of the charge in the set capacitor is independently connected to the first capacitor and the second capacitor charged to a constant voltage, the charge is distributed differently according to the capacitance of the contact capacitor And measuring an amount of change in voltage due to charge redistributed to the first capacitor and the second capacitor. The method of claim 1, The fingerprint detection method includes comparing two spatially adjacent output values and giving spatial information as a result of the comparison, in order to give not only information about the valley part and the floor part of the fingerprint but also information on the gray level between the bone part and the floor part. And outputting a comparison value of which one of the two output values is larger when the difference between two adjacent output values is smaller than a constant value or the difference between the two output values is larger than the constant value. . The method of claim 4, wherein The fingerprint sensing method further comprises the step of reconstructing the gray level by the comparison value output in the step of outputting the comparison value. The fingerprint sensing method according to any one of claims 1 to 5, wherein the setting capacitors are connected to each other or are independent of each other. The method of claim 1, wherein the fingerprint sensing method comprises: scanning a fingerprint of a finger by a sensing unit including a sensing electrode formed of the set capacitors; And obtaining the output signal of the entire fingerprint from the output signal obtained while the fingerprint of the finger is scanned by the sensing unit in the step. The method of claim 7, wherein The step of scanning the fingerprint of the finger by the sensing electrode is achieved by rubbing the fingerprint of the finger with respect to the sensing unit while the sensing unit is fixed, or the movement of the sensing unit with respect to the fingerprint of the finger Fingerprint detection method.

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