JP2003294406A - Capacitive sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To simplify a control circuit required for collecting detection data in a capacitive sensor. <P>SOLUTION: The capacitive sensor is constituted by covering sensor electrodes arranged two-dimensionally with a dielectric protective film to form a sensor and charging a sensor electrode from a constant voltage source on the basis of a start signal, and a condenser is also charged at the same time. The charge or discharge time of the condenser is controlled on the basis of the charge or discharge time of the sensor electrode, to obtain the data of charge necessary for the condenser by applying a detection start signal to the sensor. The control circuit is simple and the design and production of the capacitive sensor becomes easy. Control is also simple because the holding of data is performed only by a pulse signal. <P>COPYRIGHT: (C)2004,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 has a sensor electrode coated with a dielectric protective film and a fixed capacitance value that changes an output voltage in proportion to a change in charge. A capacitor, a switch circuit that inputs a start signal that switches the voltage applied to the electrostatic capacitance generated between the sensor electrode and the detected object, a switch circuit that inputs a start signal that sets the initial voltage to the capacitor, and the sensor electrode on the reference side. It has a current mirror circuit with a capacitor connected to the output side, and detects the voltage generated by the charge of the capacitor that is proportional to the charge that changes when the voltage applied to the electrostatic capacitance generated between the sensor electrode and the detected object changes It is characterized by that.

[0006]

BEST MODE FOR CARRYING OUT THE INVENTION As one embodiment of the present invention, in a capacitance type sensor of the present invention, a sensor electrode covered with a dielectric protective film is connected to a reference side transistor of a current mirror circuit, The output side transistor is connected to a capacitor, and the sensor electrode and the capacitor respectively input a start signal for switching the voltage applied to the electrostatic capacitance generated between the sensor electrode and the detected object,
It is characterized in that it detects a voltage generated by an electric charge accumulated in the capacitor, which is grounded through a switch circuit for inputting a start signal for setting an initial voltage to the capacitor and is in a proportional relationship with an electric charge charged in a sensor electrode. .

That is, in response to the start signal, the constant voltage source charges the sensor electrode to a constant potential so as to charge the electrostatic capacitance generated between the detected object and the sensor electrode.
At the same time, the constant voltage source charges the capacitor connected to the output side of the current mirror circuit. The charging time of the capacitor is controlled by the charge of the sensor electrode so that the voltage data required as the output can be obtained.

As another embodiment of the present invention, in the capacitance type sensor of the present invention, the sensor electrode covered with the dielectric protective film is connected to the reference side transistor of the current mirror circuit, and the output of the current mirror circuit is output. The side transistor is connected to the capacitor, the sensor electrode and the capacitor are connected to a constant voltage through the switches for inputting the start signal to each, and are reduced by the charge discharged from the capacitor that is proportional to the charge discharged to the sensor electrode. It is characterized by detecting a voltage.

[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. In the capacitance type sensor of the present invention, as shown in FIG. 1, a sensor array is formed by a large number of two-dimensionally arranged sensor electrodes 3 and a dielectric protective film 2 covering the sensor electrodes 3. Each sensor electrode 3 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 is
The sensor electrode is covered and protected and a capacitor is formed between the finger and the sensor electrode.

In this sensor array, each sensor electrode has a substantially uniform electrostatic capacitance. As shown in FIG. 1, when the finger 1 is placed on the dielectric film 2, since the finger 1 is in the grounded state,
An electrostatic 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 which constitutes one unit of a fingerprint sensor circuit, and is an example in which the circuit is constituted by MOS transistors. As shown in FIG. 2, this unit circuit is composed of a sensor electrode 3 and a capacitor that generate an electrostatic capacitance with the finger 1.
7, a current mirror circuit 5 for charging the sensor electrode 3 and the capacitor 7 from a low voltage source, and a discharge switch 4 for resetting the sensor electrode 3 and the capacitor 7. The reference side of the current mirror circuit is connected to the sensor electrode 3, and the output side is connected to the capacitor 7. One of the discharging switches is connected to the sensor electrode and the other is connected to the capacitor 7 in parallel, and both switches are connected to the charging start signal source 6.

With the finger placed on the dielectric film of the sensor array, the fingerprint image can be obtained by performing the following procedure. The grounding switch 4 connected to the sensor electrode 3 and the capacitor 7 is grounded by turning on the start signal source 6 to discharge the residual charge. Finger by this
The voltage at the capacitor end composed of 1 and the sensor electrode 3 is 0 [V]
Reset to. After discharging, the grounding switch 4 is turned off to switch the sensor electrode 3 and the capacitor 7 from the grounded state to the open state. Since the residual charge is discharged and the gate voltage of the transistor of the current mirror circuit 5 is low, the current mirror circuit 5 operates and current is supplied to the capacitor composed of the finger 1 and the sensor electrode 3 and the capacitor 7, and both capacitors Voltage rises.

For this charging, the terminal voltage of the capacitor composed of the finger 1 and the sensor electrode 3 changes from the power supply voltage to the current mirror circuit.
The process is performed until a predetermined voltage lower by the threshold voltage of the transistor forming 5 is reached. When this predetermined voltage is reached, the gate voltage of the transistor of the current mirror circuit 5 becomes high, so the operation of the current mirror circuit 5 stops. Since the supply of electric charge to the capacitor 7 is also stopped by stopping the current mirror circuit 5, the voltage generated in the capacitor 7 is held until the ground switch is switched to the ground state.
The electric charge that is charged until the supply of the electric charge to the capacitor 7 is stopped is an amount of electric charge that is proportional to the electric charge that is charged in the capacitor that is configured by the finger 1 and the sensor electrode 3. Capacitor 7
Since the charge charged in the current mirror circuit 5 is determined by the current ratio of the current mirror circuit 5, it is possible to change it to an arbitrary charge by changing the gate size of the transistor forming the current mirror circuit 5.

The charge charged in the capacitor composed of the finger 1 and the sensor electrode 3 is Qf = Cf · V (1) where Cf: finger 1-electrostatic capacity of the sensor electrode V: predetermined voltage It can be expressed as (constant), and an amount of charge proportional to this is charged in the capacitor 7, so the amount of charge is A Qf = Cp Vp (2) where Cp is the static of the capacitor 7. Capacitance (constant) Vp:
It can be expressed as a proportional constant determined by the voltage of the capacitor 7, A: current ratio of the current mirror circuit. (1),
From equation (2), Cf ・ V = (Cp / A) ・ Vp ・ ・ ・ (3), and rewriting the equation for the output voltage, Vp = ((V ・ A) / Cp) ・ Cf ・ ・ ・ ( 4) Vp = K · Cf (5) where K: proportional constant. Therefore, the output voltage Vp is found to be a value proportional to the electrostatic capacitance Cf that changes depending on the distance between the fingerprint and the sensor electrode 3, and fingerprint data can be generated from this data.

FIG. 3 shows a typical example in which the horizontal axis represents the electrostatic capacity of the capacitor composed of the finger 1 and the sensor electrode 3, and the vertical axis represents the output voltage corresponding to the electrostatic capacity. From the figure, it can be seen that the voltage generated across the capacitor 7 when the supply of electric charge is stopped shows a value proportional to the electrostatic capacitance of the capacitor composed of the finger 1 and the sensor electrode 3.

Regarding the unit circuit of the device of the present invention described above,
A timing chart of operation signals is shown in FIG. A signal 14 is a waveform generated from the signal source 6 in FIG. 2 and is a signal for switching ON / OFF of the discharge switch 4. Signal 15 is the voltage of the capacitor composed of finger 1 and sensor electrode 3. The signal 16 is the voltage of the capacitor 7, and the solid line and the broken line show the cases where the capacitance of the capacitor composed of the finger 1 and the sensor electrode 3 is large and small, respectively.

Since the discharge switch is ON while the start signal 14 is at the high level, both the capacitor composed of the finger 1 and the sensor electrode 3 and the capacitor 7 are grounded, and the waveforms 15 and 16 are It shows 0 [V]. Start signal is Low
When switched to the level, the discharge switch turns off and the capacitor starts charging. Capacitor voltage is a waveform
The voltage rises as indicated by 15 and 16, and this charging is performed until the voltage of the waveform 15 reaches the above-mentioned predetermined voltage. The capacitor 7 is also charged for the same period as this, and shows a different voltage depending on the capacitance of the capacitor formed by the finger 1 and the sensor electrode 3 as shown by the waveform 17. The voltage 16 of the capacitor 7 is held until the start signal 14 becomes High again and is discharged.

Since this unit circuit is prepared for the number of pixels, that is, the number of sensor electrodes, the voltage corresponding to each electrostatic capacity of the capacitor constituted by the finger 1 and the sensor electrode 3 is the capacitor.
It is possible to process in parallel until it is held at 7, and it is possible to collectively capture and hold the electrostatic capacitances of all sensor electrodes. A fingerprint authentication system can be 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, 6 is a signal source, 8 is a sensor array, 9 is an address decoder, 10 is a selector, 11 is an A / D converter, 12 is an interface, and 13 is a computer.

The output voltage of the capacitor 7 of the desired element from the sensor array 8 is applied to the address decoder 9 and the selector 10.
To choose via. The output voltage of the selected capacitor 7 is input to the A / D converter 11. The data digitally converted by the A / D converter 11 is delivered to the computer 13 via the interface 12. The computer 13 processes the data obtained from each sensor electrode element and arranges it in two dimensions to complete the fingerprint image.

FIG. 6 shows another embodiment of the present invention. The present invention is based on the same principle as that of the above embodiment, a circuit for detecting the charge of the capacitor connected to the output side of the current mirror when the discharge of the capacitor composed of the finger 1 and the sensor electrode 3 is completed. It is also possible to use. An example of this discharge type circuit is shown in FIG. The reference side of the current mirror circuit is connected to the sensor electrode 3, and the output side is connected to the capacitor 20. One of the charging switches 18 is connected in series to the sensor electrode, and the other is connected in series to the capacitor 7, the gates of both switches are connected to the discharge start signal source 17, and the constant voltage source is connected to the source. The operation of the circuit will be described below.

By turning on the switch 18 connected to the sensor electrode 3 and the capacitor 20, the power supply voltage is charged. When the switch 18 is turned off after charging, the current mirror circuit 19 operates because the gate voltage of the NMOS transistor of the current mirror circuit 19 is high, and discharge starts from the capacitor composed of the finger 1 and the sensor electrode 3 and the capacitor 20. To do. With this discharge, finger 1 and sensor electrode 3
And the voltage of the capacitor 20 and the capacitor 20 are reduced.

This discharge is performed until the voltage at the capacitor end formed by the finger 1 and the sensor electrode 3 becomes higher than the ground level by the threshold voltage of the transistor forming the current mirror circuit 19. When this discharge is completed,
Since the gate voltage of the NMOS transistor of the current mirror circuit 19 is low, the current mirror circuit 19
Will stop. By stopping the current mirror circuit 19,
Discharge from the capacitor 20 is also stopped. At this time, the amount of electric charge discharged from the capacitor 20 is proportional to the amount of electric charge discharged from the electrostatic capacity of the capacitor formed by the finger 1 and the sensor electrode 3.

The amount of electric charge discharged is the current mirror circuit 1
Since it is determined by the current ratio of the current mirror circuit 9, it is possible to obtain an arbitrary amount of charge by changing the gate size of the transistor forming the current mirror circuit 19. The voltage generated in the capacitor 20 when the discharge of the electric charge is completed shows a value proportional to the electrostatic capacity of the capacitor composed of the finger 1 and the sensor electrode 3. When this is read and processed, a fingerprint image is obtained.

Although the fingerprint sensor device has been described above as an example, the invention is not limited to fingerprints and is also useful for inspection of other electronic components 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 grounding or applying a voltage. For example, it can also be used to detect scratches on a metal case.

[0025]

According to the present invention, the capacitance type sensor can be constructed with a simple circuit, and the data can be accumulated only by the detection start signal when the detection data is collected. Manufacturing is easy. Further, since the signal to the sensor electrode is a pulse signal only and the data is held, it is not necessary to consider the delay with other signals, and the control is simple. Since the capacitance of the sensor electrode is proportional to the voltage of the capacitor that stores the voltage that becomes the data,
It is easy to control the range of voltage generated by the change in capacitance. Furthermore, since the sensor electrode is connected to the power supply voltage via the diode-connected current mirror, the time required for charging can be charged in a short time of several ns, and thus can be detected in a short time. 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 a capacitance type sensor according to one embodiment of the present invention.

FIG. 3 is a diagram showing a relationship between an electrostatic capacitance between a finger and a sensor electrode and an output voltage.

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

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

FIG. 6 is a circuit diagram showing a unit circuit of a capacitance type sensor according to another embodiment of the present invention.

[Explanation of symbols]

One finger 2 Dielectric protective film 3 sensor electrodes 4 Discharge switch 5 Current mirror circuit 6 Charge start signal source 7 capacitors 17 Discharge start signal source 18 Charging switch 19 Current mirror circuit 20 capacitors

Continued front page    F term (reference) 2F063 AA43 BA29 DA02 DA05 DD07                       HA04 LA30                 4C038 FF01 FF05 FG00                 5B047 AA25 AB02 BB10 BC01 CA04                       CB17

Claims (3)

[Claims] 1. In a capacitance type sensor, a sensor electrode covered with a dielectric protective film, a capacitor having a fixed capacitance value that changes an output voltage in proportion to a change in charge, the sensor electrode and detection. A switch circuit that inputs a start signal that switches the voltage applied to the electrostatic capacitance generated between objects, a switch circuit that inputs a start signal that sets the initial voltage to the capacitor, and the sensor electrode on the reference side to the output side. Detecting a voltage generated by the electric charge of the capacitor, which has a current mirror circuit connected to the capacitor and is proportional to the electric charge that changes when the voltage applied to the electrostatic capacitance generated between the sensor electrode and the detection object changes Capacitive sensor. 2. In a capacitance type sensor, a sensor electrode covered with a dielectric protective film is connected to a reference side transistor of a current mirror circuit, an output side transistor of the current mirror circuit is connected to a capacitor, and the sensor electrode is connected. And the capacitor are grounded via a switch for inputting a start signal to each of them, and a voltage generated by the charge accumulated in the capacitor proportional to the charge charged in the sensor electrode is detected. Capacitive sensor. 3. In a capacitance type sensor, a sensor electrode covered with a dielectric protective film is connected to a reference side transistor of a current mirror circuit, an output side transistor of the current mirror circuit is connected to a capacitor, and the sensor electrode is connected. And the capacitor are connected to a constant voltage via a switch for inputting a start signal to each of them, and detect a voltage reduced by the charge discharged from the capacitor in proportion to the charge discharged to the sensor electrode. Capacitive type sensor.