JP2000166900A - Fingerprint sensor and method of generating fingerprint data thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To eliminate setting or adjustment from the outside. SOLUTION: When a finger touches at a fingerprint sensor, a main electrode A110 and a sub electrode B120 of a capacitance detecting means 100 detects the capacitance. The capacitance detected by the sub electrode B120 in a predetermined range is average, and a reference voltage is generated by a signal generating means 200. Also, the capacitance detected by the main electrode 110A is converted into voltage by the signal-generating means 200. A binary- coding means 300 subjects the main voltage value to binary coding, based on whether or not a main voltage is higher than a reference voltage with reference to the reference voltage generated by the signal generating means 200. This processing is executed for all the main electrodes arranged in the fingerprint sensor for generating fingerprint data.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a fingerprint sensor,
In particular, the present invention relates to a fingerprint sensor for detecting a fingerprint by using a capacitance difference caused by unevenness of a fingerprint, and a fingerprint data generating method thereof.

[0002]

2. Description of the Related Art Conventionally, a fingerprint authentication system for identifying an individual using a fingerprint has been used in a situation where high security is required, such as at the time of access control or access to a highly confidential system.

[0003] In the future, in addition to such business applications, portable telephones, PDAs (personal digital assistants: Persona)
It is anticipated that the use of such a fingerprint authentication system will increase as personal use such as l Digital Assistant) and personal computer login management. For a fingerprint authentication system used for personal use, a small-sized and low-cost fingerprint input device is essential.

As a fingerprint input device, a CCD (Char) is used.
Although the majority of the optical fingerprint sensors use a Ge Coupled Device, a fingerprint sensor using a capacitance that is advantageous in terms of miniaturization and cost reduction is expected to be the mainstream in the future.

A structure of a fingerprint sensor in which electrodes are formed on a silicon substrate and detection is performed by using a capacitance difference due to unevenness of a fingerprint is disclosed in, for example, Japanese Patent Application Laid-Open No. Hei 8-305832.

Normally, a change in the capacitance due to the unevenness of the fingerprint detected by the sensor unit is converted into an analog voltage, amplified and corrected by an amplifier circuit or the like, and then converted into an analog-digital (hereinafter, A / D). Conversion) and binarization (0 or 1) is performed. The main binarization processing method will be described.

As a first binarization processing method, there is a method in which a constant reference voltage set from the outside is used as a threshold value and 0 or 1 is converted with reference to this threshold voltage. Also, as a second binarization process, the fingerprint sensor is divided into certain areas,
There is a method in which the average value of each output voltage is calculated externally, optimized for each area and used as a reference value, and conversion of 0 or 1 is performed.

[0008]

However, in such a conventional fingerprint sensor utilizing the capacitance difference due to the unevenness of the fingerprint, there is a problem that it is not easy to binarize the output of the sensor.

In the first binarization process described above, a fixed threshold value is used. However, there are individual differences in fingerprints, and variations in output voltage cannot be avoided. Can not. Therefore, if you set a fixed threshold,
The accuracy of the fingerprint sensor deteriorates.

In the above-described second binarization processing,
In addition to the binarization system, software processing such as external calculation of the average value of each output voltage is required separately, so that the processing becomes complicated and it is difficult to realize low cost.

SUMMARY OF THE INVENTION The present invention has been made in view of such a point, and an object of the present invention is to provide a fingerprint sensor that does not require external setting or adjustment and a method of generating the fingerprint data.

[0012]

According to the present invention, in order to solve the above-mentioned problems, in a fingerprint sensor for detecting a fingerprint using a capacitance difference due to unevenness of the fingerprint, the fingerprint sensor is arranged on a plane where the fingerprint contacts. A capacitance detecting means composed of a main electrode and a sub-electrode arranged in the vicinity of the main electrode, and generating a reference voltage by averaging the capacitance detected by the sub-electrode in a predetermined range; Signal generating means for generating a main voltage from the capacitance detected by the main electrode; and binarizing the main voltage value by comparing the reference voltage with the main voltage.
And a fingerprint sensor, comprising:

In the fingerprint sensor having such a configuration, when a finger touches the fingerprint sensor, the main electrode and the sub-electrode serving as the capacitance detecting means detect the capacitance. The capacitance detected by the sub-electrode within a predetermined range is averaged by the signal generation means,
A reference voltage is generated. Further, the capacitance detected by the main electrode is also converted into a voltage by the signal generating means. 2
The binarizing unit binarizes the main voltage value based on whether the main voltage is higher than the reference voltage with reference to the reference voltage generated by the signal generating unit. The above process is performed on all the main electrodes arranged in the fingerprint sensor to detect a fingerprint.

In a fingerprint data generating method for a fingerprint sensor for detecting a fingerprint by utilizing a capacitance difference caused by unevenness of the fingerprint, the fingerprint sensor includes a main electrode and a vicinity of the main electrode on a plane where the fingerprint contacts. Are arranged with sub-electrodes,
The main electrode and the sub-electrode detect a capacitance due to contact with a fingerprint, generate a reference voltage by averaging the capacitance detected by the sub-electrode in a predetermined range, and detect the capacitance by the main electrode. Generating a main voltage from the measured capacitance, comparing the main voltage with the reference voltage, and binarizing the main voltage value based on whether the main voltage is higher than the reference voltage. And a fingerprint data generation method is provided.

[0015] The fingerprint data generating method of such a procedure is as follows.
The main electrode and the sub-electrode arranged in the fingerprint sensor detect the capacitance generated between the finger due to the contact of the fingerprint.
By averaging the capacitances detected by the predetermined range of the sub-electrodes, a reference voltage is generated. The main voltage generated from the capacitance detected by the main electrode is compared with a reference voltage to binarize the main voltage value. The above processing is performed for all the main electrodes arranged in the fingerprint sensor, thereby generating fingerprint data.

[0016]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a principle diagram of a fingerprint sensor according to an embodiment of the present invention. The fingerprint sensor according to the present invention includes: a capacitance detection unit 100 having a main electrode and a sub-electrode arranged on a plane contacting a fingerprint;
A signal generating means 200 for generating a main voltage and a reference voltage from the output of the circuit, and a binarizing means 30 for binarizing the main voltage value
0.

In the capacitance detecting means 100, a main electrode A and a sub-electrode B are arranged on a plane in contact with a fingerprint, and in the immediate vicinity of each main electrode A. The main electrode A and the sub electrode B are M
Although they are arranged in rows and N columns, here, for simplicity, the main electrode A110 and the sub-electrode B120 will be described one by one. The area of the sub-electrode B120 is 1 / D of that of the main electrode A110.
Is the size of The main electrode A110 is connected to the switch SWA, and the sub-electrode B is connected to the signal generating means 200 via the switch SWB. The signal generation means 200 includes an amplifier AMP2
10. + of the amplifier AMP210 via the switch SW2
Constant power supplies Vpr and Vc connected to the input terminal, switch S
Amplifier AMP21 via W3a and switch SW3b
It comprises capacitors C1 and C2 connected to the-input terminal of 0 and the output terminal A0 of the amplifier AMP210.
The binarizing means 300 is connected via switches SW4 to the two-system inputs of the sample and hold circuits SH1 and SH2 which hold the value of the amplifier output A0. The outputs SH1 and SH2 of SH1 and SH2 are connected to the comparator COM310. C3 and Cref of SH1 and SH2 are set to the same capacitance value. The comparator COM310 has the input S
Hi1 and SHi2 are compared. For example, when the potential of the input SHi1 is higher than the potential of the input SHi2, H (1) is output, and when it is lower, L (0) is output.

The operation of the fingerprint sensor having such a configuration and a method of generating fingerprint data will be described. While the finger is in contact with the electrode surface, SW2 of the signal generation means 200 is switched to the Vpr side, SW3a is switched to the A0 side, and SW3b is switched to the C2 side. At this time, when the SWB is turned on, a charge Qb is charged between the electrode B and the finger and between the electrode B and the SUB electrode. Next, SWB is turned off, SW2 is switched to Vc,
SW3a is turned off and SW3b is switched to C2. When SWB is turned on again, Qb moves and C2 is also charged. At this time, the voltage change of the output A0 of the amplifier AMP210 is set to V2. Next, SW4 is switched to the Cref side, and V2 is sampled and held at SH2. Subsequently, SWB is turned off, SW2 is switched to Vpr side, SW3a is switched to A0 side, and SW3b is switched to C1 side. At this time, when the SWA is turned on, the electrode A and the finger and the electrode A and the SU
Charge Qa is charged between the electrode and the B electrode. Next, SW
A is turned off, SW2 is switched to Vc, SW3a is turned off, and SW3b is switched to C1. When SWA is turned on again, Qa moves and C1 is also charged. At this time, the voltage change of the output A0 of the amplifier AMP210 is set to V1. Next, SW4 is switched to C3 side, and V1 is set to SH1.
Sample hold. Here, the fingerprint data can be binarized by comparing the voltage V2 with the comparator COM310 and outputting the result as 1/0. M
Main electrode Am-n and sub-electrode Bm-n arranged in row N columns
, The capacitances of all the main electrodes are binarized by sequentially switching the SWAm-n and the SWBm-n provided in.

In the above description, SWA, SWB, SW2,
By switching SW3 and SW4, the output signal and the reference voltage of the sub-electrode B are applied to the main electrode A by one amplifier AMP.
Although the processing is performed at 210 and held by the sample and hold circuit, a signal processing circuit may be provided for each of the main electrode A and the sub-electrode B.

Next, the configuration of the entire fingerprint sensor according to an embodiment of the present invention will be described. FIG. 2 is an overall configuration diagram of a fingerprint sensor according to an embodiment of the present invention. FIG.
The same parts as those described above are denoted by the same reference numerals, and description thereof will be omitted.

In the fingerprint sensor according to the present invention, the main electrodes Am-n (m = 1, 2,..., M,
n = 1, 2,..., N; M = the number of rows of electrodes, N = the number of columns of electrodes) are regularly arranged, and the sub-electrodes Bm-n are arranged immediately near each main electrode. One row of electrodes A and B arranged in a vertical or horizontal direction are connected to a common line L400.
Switches SWAm-n and SWBm provided respectively
-N. The switch SWAm-n provided on the main electrode A is provided for each electrode row, that is, SWA1.
-N, SWA2-n,..., SWAm-n can be independently switched. Further, the SWBm-n provided on the sub-electrode B is switched on a column basis or simultaneously on the whole. In the example of FIG. 1, the SWA1-1 and SWA1-2 or the SWA2-1 and SWA2-2 are provided with a timing control circuit (Timing Control Block).
(Column)) 510. SWB1-1 and SWB2-1 or SWB1-1
B1-2 and SWB2-2 are connected to a timing control circuit (Ti
Ming Control Block (Raw))
520 are switched simultaneously. Wiring L400
Is connected to the-input terminal of the amplifier AMP210.
Further, the minus input terminal of the amplifier AMP210 is connected to the capacitors of the capacitors C1 and C2 and the amplifier AMP2 via the switch SW3.
10 output A0. Also, the amplifier AMP210
Is connected to a constant power supply Vpr or Vc having a different voltage value via SW2. At this time, C1, C2
Sets the capacity so as to satisfy the following equation (1).

[0022]

(M / D) * C1 <C2 <((2Cp + Cs) / (Cp + Cs)) * (M / D) * C1 (1) Cs: Static between the convex portion of the fingerprint and the main electrode A Capacitance Cp: Parasitic capacitance per Cs The output A0 of the amplifier AMP210 is connected to two-system inputs of the sample and hold circuits SH1 and SH2 via SW4. SH1 and SHi2 which are the outputs of SH1 and SH2
Is connected to the comparator COM310. At this time,
C3 and Cref of SH1 and SH2 are set to the same capacitance value. The output of the comparator COM310 is, for example, the input S
When the potential of Hi1 is higher than the potential of input Shi2, H (1) is output when it is higher, and L (0) is output when it is lower.

The operation of the fingerprint sensor having such a configuration and a method of generating fingerprint data will be described. FIG. 3 is a timing chart of the fingerprint sensor according to one embodiment of the present invention. The operation of the fingerprint sensor according to the present invention includes a process [1] of generating a reference voltage from the sub-electrode Bm-n,
processing [2] for generating a main voltage serving as fingerprint data from mn.

The process [1] of generating a reference voltage from the sub-electrode Bm-n will be described. With the finger in contact with the electrode surface, switch SW2 to the Vpr side and switch SW3a to A0
To short. At this time, SW3b is connected to C2. At this time, when a predetermined range of SWBm-n is turned on, electric charges are charged between the sub-electrode B and the finger and between the electrode B and the SUB electrode. At this time, the charge amount Qb charged per column (M) is expressed by the following equation (2).

[0025]

Qb = (M * (Cp / D) + (Cs / D) * k + (Cs / D / x) * (Mk)) * Vpr (2) k: Sub-electrode B, Number of contacts with the convex portion of the fingerprint in M rows x: ratio of the distance between the concave portion and the electrode to the distance between the convex portion and the electrode of the fingerprint (x> 1) Next, SWBm-n is turned off. Then, switch SW2 to the Vc side. Thereafter, SW3a is turned off and SWBm-n is turned on again. At this time, since the amplifier AMP210 performs feedback with the voltage Vc, the charge Qb charged earlier
Moves and C2 is also charged. At this time, if the voltage of the output A0 of the amplifier AMP210 changes by V2, Qb is expressed by the following equation (3).

[0026]

Qb = (M * (Cp / D) + (Cs / D) * k + (Cs / D / x) * (M−k)) * Vc + C2 * V2 (3) Equation (2) From equation (3) and equation (3), V2 is represented by the following equation (4).

[0027]

V2 = (1 / C2) * (Vpr-Vc) / D * (M * Cp + Cs * k + (Cs / x) * (Mk)) (4) Here, SW4 is connected to the Cref side. Then, the voltage V2 is sampled and held by the sample and hold circuit SH2. After holding, SW4 is turned off.

Next, a process [2] of generating a main voltage serving as fingerprint data from the main electrode Am-n will be described. SW
Bm-n is turned off, SW2 is turned to the Vpr side, SW3 is turned to A0
And short. At this time, SW3b is short-circuited to C1 side. When only one of the M SWA control lines is turned on, charges are charged between the main electrode A and the finger and between the main electrode A and the SUB electrode. The charge amount Qa charged at this time is expressed by the following equation (5).

[0029]

Qa = (Cp + Cs) * Vpr (5) Next, SWAm-n is turned off, and SW2 is switched to Vc. Thereafter, SW3a is turned off and SWAm-n is turned on again. At this time, since the amplifier AMP210 performs feedback with the voltage Vc, the charge Qa
Moves, and C1 is also charged. At this time, if the voltage of the output A0 of the amplifier AMP210 changes by V1, Qa is expressed by the following equation (6).

[0030]

Qa = (Cp + Cs) * Vc * C1 * V1 (6) From equations (5) and (6), V1 is expressed by the following equation (7).

[0031]

V1 = (1 / C1) * (Vpr-Vc) * (Cp + Cs) (7) Here, assuming that k = M / 2 in the equation (4),

[0032]

C2 needs to be determined so that V2Ｃ (V1 + V1 / x) / 2 = V1 * (1 + 1 / x) / 2 (8) (4), (7)
From equation (8) and 1 <x <∞, it can be derived that C2 takes the value of equation (1).

Next, by switching SW4 to the C3 side, the voltage V1 can be sampled and held at SH1. Here, the voltage V1 and the reference voltage V
2 is compared by the comparator COM310, and is binarized by outputting 1/0. Hereinafter, the capacitances of all the main electrodes A can be binarized by repeatedly performing the processes [1] and [2] while sequentially switching the SWAm-n in the row direction.

In the above description, when the reference voltage is generated from the sub-electrode Bm-n, the sub-electrode Bm-n of the entire column is generated.
Although the reference voltage is generated by averaging the detected capacitances, it is also possible to generate the reference voltage by averaging the capacitances detected by all the sub-electrodes Bm-n.

It is also possible to divide a column into several parts and to generate a reference voltage by averaging the capacitances detected by the sub-electrodes Bm-n in each area. In this case, the capacity of C2 can be reduced.

The capacitance detected by the sub-electrodes Bm-n arranged in an area of a plurality of columns and a plurality of rows obtained by dividing a plane on which the sub-electrodes Bm-n are arranged into a predetermined area is averaged to obtain a reference voltage. Can also be generated. In this case, C2
Can be reduced, and an average value can be obtained in two dimensions.

Further, in the above description, the process [1] for generating the reference voltage and the process [2] for generating the main voltage are described.
Is repeated, the procedure may be such that a reference voltage is generated in the process [1], and the process [2] is repeated using the reference voltage. In this case, it is possible to reduce the time for obtaining fingerprint data.

[0038]

As described above, according to the present invention, the capacitance detection means detects the capacitance generated when the main electrode and the sub-electrode come in contact with the fingerprint. The capacitances detected by the sub-electrodes in the predetermined range are averaged by the signal generation means, and a reference voltage is generated. The binarizing means binarizes the main voltage value based on whether the main voltage detected by the main electrode is higher than the reference voltage. This process is performed for all the main electrodes arranged in the fingerprint sensor, and a fingerprint is detected. As described above, since the reference voltage is generated inside the fingerprint sensor at the time of fingerprint detection, it is possible to cope with the variation of the output voltage due to individual differences, and it is not necessary to change the setting from the outside and the cost is changed. Further, since external processing is not required, on-chip processing from analog output to binarization can be realized. Further, since the threshold value for binarization is automatically optimized as a reference voltage, a high-performance fingerprint sensor can be provided.

Further, according to the fingerprint data generating method of the present invention,
The main electrode and the sub-electrode arranged in the fingerprint sensor detect the capacitance generated between the finger and the finger due to the contact with the fingerprint. A reference voltage is generated from the capacitance detected by the sub-electrode, and the reference voltage is compared with a main voltage generated from the capacitance detected by the main electrode to binarize the main voltage value. This process is performed on all the main electrodes arranged in the fingerprint sensor to generate fingerprint data. in this way,
Since a reference voltage is generated inside the fingerprint sensor at the time of fingerprint detection, it is possible to cope with variations in output voltage due to individual differences. Further, since the threshold value for binarization is automatically optimized, fingerprint data with high accuracy can be obtained.

[Brief description of the drawings]

FIG. 1 is a principle diagram of a fingerprint sensor according to an embodiment of the present invention.

FIG. 2 is an overall configuration diagram of a fingerprint sensor according to an embodiment of the present invention.

FIG. 3 is a timing chart of the fingerprint sensor according to the embodiment of the present invention.

[Explanation of symbols]

100: capacitance detecting means, 110: main electrode A, 1
20 ... sub-electrode B, 200 ... signal generation means, 210 ...
· Amplifier AMP, 300 ··· Binarization means, 310 ··· Comparator COM, 400 ··· Wiring L, 510 ··· Timing control circuit (Timing Control Bl)
ock (Column)), 520... Timing control circuit (Timing Control Block)
(Raw))

Continued on the front page F term (reference) 4C038 FF01 FG00 5B043 AA09 BA02 DA07 EA04 5B047 AA25 BA02 BB10 DB06

Claims (9)

[Claims] 1. A fingerprint sensor for detecting a fingerprint using a capacitance difference caused by unevenness of a fingerprint, comprising: a main electrode disposed on a plane in contact with the fingerprint; and a sub-electrode disposed near the main electrode. And a capacitance detecting unit configured to average a capacitance detected by the sub-electrode in a predetermined range to generate a reference voltage and generate a main voltage from the capacitance detected by the main electrode. A fingerprint sensor comprising: signal generation means; and binarization means for comparing the reference voltage and the main voltage to binarize the main voltage value. 2. The method according to claim 1, wherein the signal generation unit averages a capacitance detected by the sub-electrodes arranged in a predetermined column or row among the sub-electrodes arranged on a plane contacting the fingerprint, and outputs a reference signal. 2. The fingerprint sensor according to claim 1, wherein the fingerprint sensor is means for generating a voltage. 3. The signal generation means divides a plane contacting the fingerprint into a predetermined area, and generates a reference voltage by averaging the capacitance detected by the sub-electrodes arranged in the area. 2. The fingerprint sensor according to claim 1, wherein the fingerprint sensor is a means. 4. The signal generating means according to claim 1, wherein the signal generating means generates a reference voltage by averaging the detected capacitances of all the sub-electrodes arranged on a plane contacting the fingerprint. Item 3. The fingerprint sensor according to Item 1. 5. The fingerprint sensor according to claim 1, wherein said binarizing means is a comparator circuit which outputs whether said main voltage is higher than said reference voltage with said reference voltage as a reference value. . 6. A fingerprint data generating method for a fingerprint sensor for detecting a fingerprint using a capacitance difference due to unevenness of a fingerprint, wherein the fingerprint sensor has a main electrode and a vicinity of the main electrode on a plane where the fingerprint contacts. The main electrode and the sub-electrode detect the capacitance due to the contact with the fingerprint, and average the capacitances detected by the sub-electrode in a predetermined range. Generating a voltage; generating a main voltage from a capacitance detected by the main electrode; comparing the reference voltage with the main voltage; determining whether the main voltage is greater than the reference voltage by 2; A fingerprint data generating method, wherein the step of converting into a value is performed for all main electrodes. 7. The step of generating the reference voltage includes dividing a plane contacting the fingerprint into a predetermined region, averaging capacitances detected by the sub-electrodes disposed in the region, and averaging a reference voltage. 7. The fingerprint data generation method according to claim 6, wherein the fingerprint data is generated. 8. The step of generating a reference voltage is a step of generating a reference voltage by averaging detected capacitances of all the sub-electrodes arranged on a plane contacting the fingerprint. The fingerprint data generation method according to claim 6, wherein 9. The procedure for generating the reference voltage is a procedure performed only once when a finger touches the fingerprint sensor, and a procedure for binarizing the main voltage value performed on all the main electrodes. 7. The fingerprint data generating method according to claim 6, wherein a reference voltage generated when the finger is in contact is used.