JP2003269905A - Electrostatic capacity type sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To simplify a control circuit required for collection of detection data in a capacitance type sensor unit. <P>SOLUTION: In the electrostatic capacity type sensor unit in the present invention, sensor electrodes arrayed two-dimensionally are coated with a dielectric protection film to constitute a sensor, the each sensor electrode is charged from a constant-voltage source by an input signal, and a capacitor of a time- voltage converting circuit is also charged at the same time. A potential data required as an output of the time-voltage converting circuit is provided only by imparting a detection starting signal, by controlling a charge time for the capacitor with a discharge time of the sensor. Design and production for the electrostatic capacity type sensor are facilitated thereby. Since data are held only by a pulse signal, the control is also simplified. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrostatic capacitance type sensor, and simplifies the circuit configuration of a control circuit necessary for collecting data to facilitate the design and control of the device. The sensor device of the present invention is suitable for detecting an object having fine irregularities such as a fingerprint.

[0002]

2. Description of the Related Art A conventional electrostatic capacitance type sensor, for example, a fingerprint sensor, detects electrostatic capacitance generated between a finger and an electrode by inputting power as shown in US Pat. No. 6,049,620. After charging the battery, the capacitance is calculated from the difference between the voltage and the voltage after discharging at a constant current for a predetermined time.The difference between the capacitances is used to derive the unevenness of the fingerprint and collect it as fingerprint data. is there.

That is, the conventional method is based on the following principle. First, the capacitance is calculated by C = q / (Vi-Vf) .... (However, q: discharged charge Vi: initial potential
Vf: potential after discharge) Next, the distance between the finger and the electrode is derived by C = εS / d ... And fingerprint data is taken. (However, C: capacitance ε: permittivity S: area of the sensor electrode d: distance between the finger and the sensor electrode) However, in this method, the charging current selector switch and the circuit that gives the control signal of the sample & hold circuit are Since they are required separately, the control signal generation circuit becomes complicated,
The charge / discharge circuit becomes complicated. Furthermore, if a shift occurs due to a signal delay among the plurality of control signal generating circuits that are routed around the electrodes of the fingerprint detection sensor, there is a risk that an accurate capacitance cannot be measured.

[0004]

SUMMARY OF THE INVENTION The present invention is intended to solve the above-mentioned drawbacks in the conventional capacitance type sensor device, and simplifies the configuration of the circuit for supplying the control signal to facilitate the design of the device. And the control is simple.

[0005]

A capacitance type sensor according to the present invention comprises a sensor electrode coated with a dielectric protective film, a constant voltage source for charging a capacitance generated between the sensor electrode and a detection object,
A first constant current source for discharging electric charges from the sensor electrode with a constant current; and a detection circuit having a detection means for detecting the start of constant current discharge of the charges charged in the sensor electrode and the fact that a predetermined voltage has been reached by the discharge. A time-voltage conversion circuit that charges the conversion capacitor from the second constant current source during the time from the start of discharge of the sensor electrode until the detection means reaches a predetermined voltage, and the charge of the conversion capacitor is discharged. It is characterized by having a discharge circuit. That is, in response to the input signal, the sensor electrode is charged by the constant voltage source so as to charge the electrostatic capacitance generated between the detection object and the sensor electrode, and discharged from the constant current source to a constant potential. At the same time, the capacitor of the time-voltage conversion circuit is charged from another constant current source. The charging time is controlled by the detection circuit so that necessary potential data can be obtained as the output of the time-voltage conversion circuit.

[0006]

BEST MODE FOR CARRYING OUT THE INVENTION As an embodiment of the present invention, in a capacitance type sensor, a constant voltage source is connected to a sensor electrode covered with a dielectric protective film through a switch and a non-inverting input terminal of a comparator. Connected, the inverting input terminal of the comparator is connected to the reference potential, the first constant current source is connected to the sensor electrode, and the comparator starts the constant current discharge of the charge charged in the sensor electrode. And a detection circuit for detecting that a predetermined voltage is reached by discharge, the output of the comparator is input to a logic circuit, and the output of the logic circuit is input to a switch circuit connected to a second constant current source, A time-voltage conversion circuit that charges the conversion capacitor by the second constant current source during the time from the start of discharge of the sensor electrode until the comparator reaches a predetermined voltage, and the charge of the conversion capacitor. Discharge circuit to discharge, Those having.

As another embodiment, in a capacitance type sensor, a constant voltage source is connected via a switch to a sensor electrode covered with a dielectric protective film and a non-inverting input terminal of a comparator. The inverting input terminal of the comparator is connected to the reference potential, the first constant current source is connected to the sensor electrode, and the constant current discharge start and discharge of the charge charged in the sensor electrode by the comparator Detection circuit that detects that a predetermined voltage has been reached, the output of the comparator is input to the output switch of the integration circuit, and the output terminal of the integration circuit that starts integration by the input signal is converted via the output switch. Connected to the capacitor for conversion, the time-voltage conversion circuit that charges the conversion capacitor with the output of the integration circuit during the charging time determined by the detection circuit, and the charge of the conversion capacitor. Discharge circuit for electric, and has a.

In still another embodiment, in a capacitance type sensor, a constant voltage source is connected via a switch to a sensor electrode covered with a dielectric protective film and an input terminal of a logic circuit, and a first constant current is applied. A source is connected to the sensor electrode,
A detection circuit for detecting that the sensor electrode has reached a predetermined voltage by comparing the discharge start of the voltage appearing at the sensor electrode by the logic circuit and the threshold voltage of the input of the logic circuit, and the output of the logic circuit is the second Time-voltage conversion circuit, which is input to a switch circuit connected to the constant current source and charges the conversion capacitor by the second constant current source during the charging time determined by the logic circuit, and the conversion circuit. And a discharge circuit for discharging the electric charge of the capacitor for use.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIGS. 1 to 5 using a fingerprint sensor as an example. The capacitance type sensor of the present invention constitutes a sensor array with a large number of two-dimensionally arranged sensor electrodes and a dielectric protective film covering them.
Each sensor electrode is arranged at an interval of 300 dpi to 500 dpi, which is narrower than the thickness of the line expressed by the unevenness of the fingerprint so that the image of the fingerprint can be expressed. The dielectric film covers and protects the sensor electrode and forms a capacitor between the finger and the sensor electrode.

This sensor array, as shown in FIG.
When the finger 1 is placed on the dielectric film 2, since the finger 1 is in the grounded state, a capacitance is generated between the finger 1 and the sensor electrode 3.
The capacitance is large in the convex portion of the fingerprint, that is, the portion where the distance between the finger 1 and the sensor electrode 3 is short, and is small in the concave portion of the fingerprint, that is, the portion where the distance between the finger and the sensor electrode is long.

FIG. 2 shows a unit circuit constituting a fingerprint sensor. As shown in FIG. 2, the unit circuit includes a detection circuit and a time-
It is composed of a voltage conversion circuit. The detection circuit K includes a constant voltage source 15 for charging with a constant voltage, a switch 6, and a sensor electrode 3 that generates an electrostatic capacitance between the finger 1 and the finger 1 and the sensor electrode 3.
The comparator 7 includes a comparator 7 which discharges the electrostatic capacitance generated by the constant current source 5 to detect that the voltage of the sensor electrode has reached a predetermined voltage.

The time-voltage conversion circuit TVC includes a conversion capacitor 13, a constant current source switch 12 for charging the capacitor 13 with a constant current source 11 for a charging time, and a constant current source switch 12 for a charging period. It consists of a logic circuit 9 (NAND circuit) that turns on. The discharging circuit H is for resetting the conversion capacitor 13. The discharge switch 4 discharges the electric charge of the conversion capacitor 13, and is therefore connected in parallel to the conversion capacitor 13. Reference numeral 14 is a switching signal source of the discharging circuit H.

With the finger placed on the dielectric film of the sensor array, the apparatus operates as follows to obtain a fingerprint image. The conversion capacitor 13 is grounded by the grounding switch 4 to discharge the residual charge. Also, the sensor electrode
3 turns on the charging switch 6 and charges the power supply voltage source 15. After charging, the charging switch 6 is turned off, the sensor electrode 3 is disconnected from the power supply voltage source 15, and the grounding switch 4 switches the conversion capacitor 13 from the grounded state to the open state. Since the sensor electrode 3 is discharged by the constant current source 5, the voltage gradually drops. Also, the logic circuit 9 and the constant current source switch 12
The conversion capacitor 13 is also charged by the constant current source 11 via the. When the discharged voltage of the sensor electrode 3 reaches the reference voltage of the standard power supply 8, the comparator 7, the logic circuit 9, and the constant current source switch 12 operate to end the charging of the conversion capacitor 13.

While the constant current source switch 12 is ON / OFF,
That is, the discharge time of the conversion capacitor 13 is proportional to the magnitude of the electrostatic capacitance according to the following formula. Capacitance is Q ₁ = C ₁ V ₁ (1) where Q ₁ : Discharged electric charge C ₁ : Capacitance of finger
V ₁ : The voltage difference reduced by discharge, expressed as The discharged charge is Q ₁ = I ₁ t ··· (2) where Q _{1 is the} discharged charge I _{1 is} the first constant current
t: From the discharge time (1) (2), I ₁ t = C ₁ V ₁ ... (3), and if discharged to a predetermined voltage with a constant current, the discharge time t is proportional to the charge Q ₁ . I know what to do. The charge charged in the time-voltage conversion circuit TVC is Q ₂ = I ₂ t = C ₂ V ₂ (4) However, Q ₂ : Charge charged by the conversion capacitor C ₂ :
Capacitance of conversion capacitor V ₂ : Voltage generated in conversion capacitor, therefore, from (3) (4) (5), V ₂ = (V ₁ I ₂
/ C ₂ I ₁ ) C ₁ (5) Since V _1, C _2, I _{1, and} I ₂ are constant, the voltage V across the conversion capacitor 13
₂ is a value proportional to the discharge time, and fingerprint data is generated based on this value. FIG. 3 shows a typical example showing the relationship between the capacitance generated between the finger 1 and the sensor electrode 3 and the discharge time.

Since the detection circuit K and the time-voltage conversion circuit TVC are prepared for the number of pixels, that is, the number of sensor electrodes, parallel processing is performed until the voltage corresponding to the discharge time is held in the conversion capacitor 13. The capacitance of all sensor electrodes can then be captured and retained. A fingerprint authentication system is constructed by using a sensor array in which a large number of the unit circuits described above are integrated and configured as shown in the block diagram of FIG. In the figure, 16 is a sensor, 17 is an address decoder, 18 is a selector, 19 is an A / D converter, 20 is an interface,
And 21 is a computer. The output voltage of the conversion capacitor 13 of the time-voltage conversion circuit TVC corresponding to the desired detection circuit K in the sensor array is selected via the address decoder 17 and the selector 18. Selected conversion capacitor 13
The output voltage of is input to the A / D converter 19. The data digitally converted by the A / D converter 19 is the interface
Delivered to computer 21 via 20. In this way, the computer 21 processes the data obtained from each sensor electrode and arranges it in two dimensions to complete the fingerprint image.

The operation of the unit circuit shown in FIG.
Is shown in the timing chart of the operation signal. In the figure,
Signal 22 is the waveform of input signal 10. The signal 23 is a voltage related to the electrostatic capacitance generated in the finger 1 and the sensor electrode 3. The signal 24 is an output signal of the logic circuit 9 and corresponds to the charging time.
The signal 25 is the voltage across the converting capacitor 13. The broken line shows a case where the electrostatic capacitance generated in the finger 1 and the sensor electrode 3 is large.

When the discharge is started at the rising edge of the input signal 22 and the potential 23 of the electrostatic capacitance between the finger and the sensor electrode is gradually reduced to a threshold value or less, the output signal 24 of the logic circuit becomes high level. . The period during which the output signal 24 is at a low level is the discharge time, and the conversion capacitor 13 is charged. The voltage 25 of the conversion capacitor 13 rises during the discharge period, and is held until the data is taken in after the discharge period ends. Therefore, the output voltage of the conversion capacitor 13 becomes data proportional to the discharge of the electrostatic capacitance generated between the finger and the sensor electrode as seen in FIG.

Although an example in which a logic circuit is used as the time-voltage conversion circuit TVC has been described above, another circuit capable of converting time into voltage may be used. FIG. 6 shows a circuit diagram of a time-voltage conversion circuit using an integrating circuit. in this case,
The input signal 10 is connected to the resistor 64 and the inverting input terminal of the amplifier 61 via the integration start switch 63, and the output of the amplifier 61 is connected to the switch 12 which receives the output from the comparator 7 of the detection circuit K. A capacitor 62 is connected between the inverting input terminal and the output terminal of the amplifier 61, and the non-inverting input terminal is grounded.
An integrating circuit composed of the resistor 64, the amplifier 61 and the capacitor 62 outputs a voltage proportional to the charging time. The symbols other than the changed portions are shown by the same symbols as in FIG.

Further, as shown in FIG. 7, a logic circuit 71 may be used as the detection means of the detection circuit K to replace the reference potential with the threshold voltage of the input of the NAND circuit. As a result, the number of transistors used can be reduced.

Although the fingerprint sensor device has been described above as an example, the fingerprint sensor device is useful not only for fingerprints but also for inspection of other electronic parts having minute irregularities. In this case, the object for detecting the unevenness has the property of a conductor that can have the same potential by applying a ground or a voltage. For example, it can also be used to detect scratches on a metal case.

[0021]

According to the present invention, the data can be stored in the time-voltage conversion circuit only by the detection start signal when the detection data is collected, so that the electrostatic capacitance type sensor having the simple control circuit can be constructed and designed. , Easy to manufacture. In addition, since the signal to the sensor electrode is only a pulse signal and data is retained, it is not necessary to consider the delay with other signals.
Easy to control. Since the reference voltage and the constant current source in the detection circuit are in a proportional relationship with the voltage of the capacitor that stores the voltage to be data, it is easy to control the range of the voltage generated by the change in the capacitance. And so on.

[Brief description of drawings]

FIG. 1 is an explanatory diagram of a state where a finger is applied to a sensor array.

FIG. 2 is a circuit diagram showing a unit circuit of the capacitance type sensor according to the present invention.

3 is a timing chart showing voltage changes during fingerprint detection operation of the circuit of FIG.

FIG. 4 is a graph showing the relationship between the capacitance between the finger and the sensor electrode and the discharge time.

FIG. 5 is a block diagram of a fingerprint recognition system using the sensor device of the present invention.

FIG. 6 is a sensor unit circuit diagram of another embodiment of the present invention.

FIG. 7 is a sensor unit circuit diagram of yet another embodiment of the present invention.

[Explanation of symbols]

K detection circuit TVC time-voltage conversion circuit H discharge circuit One finger 2 Dielectric protective film 3 sensor electrodes 4 Discharge switch 5 First constant current source 6 Sensor electrode selection switch 7 comparator 8 Reference voltage 9 logic circuits 10 Input signal 11 Second constant current source 12 Switch for constant current source 13 Conversion capacitor 14 Discharge signal source 15 constant voltage source

   ─────────────────────────────────────────────────── ─── Continued front page    F term (reference) 2F063 AA43 BA29 BA30 BB02 BD05                       BD11 DA02 DA05 DD07 HA04                       KA02 LA08 LA09 LA10 NA06                 4C038 FF01 FF05 FG00                 5B047 AA25 BA02 BB10 BC01 CA04

Claims (4)

[Claims] 1. In a capacitance type sensor, a sensor electrode covered with a dielectric protective film, a constant voltage source for charging a capacitance generated between the sensor electrode and a detection object, and a constant charge from the sensor electrode. A first constant current source for discharging with a current, a detection circuit having a detection means for detecting the start of constant current discharge of the charges charged in the sensor electrode and the fact that a predetermined voltage has been reached by the discharge, and the discharge of the sensor electrode A time-voltage conversion circuit for charging the conversion capacitor from the second constant current source during the time from the start until the detection means reaches a predetermined voltage, and a discharge circuit for discharging the charge of the conversion capacitor,
An electrostatic capacitance type sensor having: 2. In a capacitance type sensor, a constant voltage source is connected via a switch to a sensor electrode covered with a dielectric protective film and a non-inverting input terminal of a comparator, the inverting input terminal of the comparator being The first constant current source is connected to the reference potential, and the first constant current source is connected to the sensor electrode, and the comparator detects the start of constant current discharge of the charge charged in the sensor electrode and the fact that a predetermined voltage has been reached by the discharge. The output of the comparator is input to a switch connected to the second constant current source, and the comparator outputs a predetermined value from the start of discharge of the sensor electrode. A time-voltage conversion circuit for charging the conversion capacitor by the second constant current source during the time until the voltage is reached, and a discharge circuit for discharging the electric charge of the conversion capacitor. Capacitive sensor 3. In a capacitance type sensor, a constant voltage source is connected via a switch to a sensor electrode covered with a dielectric protective film and a non-inverting input terminal of a comparator, the inverting input terminal of the comparator being The first constant current source is connected to the reference potential, the first constant current source is connected to the sensor electrode, and the predetermined voltage is reached by the start of constant current discharge of the electric charge charged in the sensor electrode by the comparator and the discharge. The detection circuit for detecting the output of the comparator, the output of the comparator is input to the output switch of the integration circuit, the output terminal of the integration circuit that starts integration by the input signal is connected to the conversion capacitor via the output switch. A time-voltage conversion circuit that charges the conversion capacitor with the output of the integration circuit during the charging time determined by the detection circuit, and a discharge circuit that discharges the electric charge of the conversion capacitor. Capacitive sensor device comprising and. 4. In a capacitance type sensor, a constant voltage source is connected via a switch to a sensor electrode covered with a dielectric protective film and an input terminal of a logic circuit, and a first constant current source is the sensor electrode. And a detection circuit for detecting that the sensor electrode has reached a predetermined voltage by comparing the discharge start of the voltage appearing at the sensor electrode by the logic circuit and the threshold voltage of the input of the logic circuit, and the logic circuit. The output of the circuit is input to the switch circuit connected to the second constant current source, and the conversion capacitor is charged by the second constant current source during the charging time determined by the logic circuit. circuit,
And a discharge circuit that discharges the electric charge of the conversion capacitor.