CAPACITANCE DETECTING APPARATUS

• US 20080252303A1
• Filed: 09/07/2007
• Published: 10/16/2008
• Est. Priority Date: 09/08/2006
• Status: Active Grant

• Current Assignee: Aisin Seiki Co., Ltd. (Aisin Corp.)
• Original Assignee: Aisin Seiki Co., Ltd. (Aisin Corp.)
• Inventors: Kato, Manabu, Fujioka, Eiji
• Granted Patent: US 7,768,272 B2
• Time in Patent Office: Days
• US Class Current: 324/661
• CPC Class Codes: H03K 17/955 using a capacitive detector
• CPC Class Codes: H03K 2017/9602 characterised by the type or shape of the sensing electrodes
• CPC Class Codes: H03K 2217/960725 Charge-transfer

Abstract

A capacitance detecting apparatus includes: a first on/off switch; a first reference capacitor; a second on/off switch; a third on/off switch; a first sensor electrode; a fourth on/off switch; a second reference capacitance; a fifth on/off switch; a sixth on/off switch; a second sensor electrode; a comparator; switch controlling means for alternately repeating a second switch operation and a third switch operation following a first switch operation; counting means for counting a number of times for repeating the second switch operation; and judging means for judging changes in capacitances related to the first and second sensor electrodes, based upon the number of times for repeating the second switch operation counted by the counting means before a level of voltage at the first input terminal and a level of voltage at the second input terminal are reversed.

Claims

1. A capacitance detecting apparatus comprising:

a first on/off switch connected to a first power-supply voltage at one end and to a first electrical node at the other end;

a first reference capacitor connected to the first electrical node at one end and to a third power-supply voltage;

a second on/off switch connected to the first electrical node at one end and to a second electrical node at the other end;

a third on/off switch connected to the second electrical node at one end and to a second power-supply voltage at the other end;

a first sensor electrode connected to the second electrical node and facing a measured object, a first capacitance being formed between the first sensor electrode and the measured object in response to a distance between the first sensor electrode and the measured object;

a fourth on/off switch connected to the second power-supply voltage at one end and to a third electrical node at the other end;

a second reference capacitance connected to the third electrical node at one end and to a fourth power-supply voltage at the other end;

a fifth on/off switch connected to the third electrical node at one end and to a fourth electrical node at the other end;

a sixth on/off switch connected to the fourth electrical node at one end and to the first power-supply voltage at the other end;

a second sensor electrode connected to the fourth electrical node and facing the measured object, a second capacitance being formed between the second sensor electrode and the measured object, the second sensor electrode being arranged adjacent to the first sensor electrode so as to define a fixed capacitance between the second sensor electrode and the first sensor electrode;

a comparator having a first input terminal connected to the first electrical node and a second input terminal connected to the third electrical node;

switch controlling means for alternately repeating a second switch operation, by which each of the second and fifth on/off switches is changed to a closed state and returned to an open state, and a third switch operation, by which each of the third and sixth on/off switches is changed to a closed state and returned to an open state, following a first switch operation, by which each of the first and fourth on/off switches is changed to a closed state and returned to an open state;

counting means for counting a number of times for repeating the second switch operation; and

judging means for judging changes in the first capacitance defined between the first sensor electrode and the measured object and changes in the second capacitance defined between the second sensor electrode and the measured object based upon the number of times for repeating the second switch operation counted by the counting means before a level of voltage at the first input terminal and a level of voltage at the second input terminal of the comparator are reversed.

2. A capacitance detecting apparatus comprising:

a first variable capacitor having a first sensor electrode at one end, the first sensor electrode being adapted to face a measured object and storing an electric charge at an amount corresponding to an electric potential of the first sensor electrode, the first capacitor being configured to output a first capacitance which varies in response to a distance between the first sensor electrode and the measured object;

a second variable capacitor having a second sensor electrode at one end, the second sensor electrode being adapted to face the measured object and storing an electric charge at an amount corresponding to an electric potential of the second sensor electrode, the second capacitor being configured to output a second capacitance which varies in response to a distance between the second sensor electrode and the measured object;

a first reference capacitor configured to be connected to the first variable capacitor; and

a second reference capacitor configured to be connected to the second variable capacitor;

wherein the electric charge stored at the first sensor electrode is discharged and stored at the first reference capacitor, the electric charge stored at the second sensor electrode is discharged and stored at the second reference capacitor, so that the first capacitance of the first variable capacitor and the second capacitance of the second variable capacitor are detected based upon a difference between an electric potential at one end of the first reference capacitor and an electric potential at one end of the second reference capacitor, and

wherein the electric potential of the first sensor electrode and the electric potential of the second sensor electrode change at an identical increment/decrement and in opposite directions.

3. A capacitance detecting apparatus according to claim 2, wherein an area of the first sensor electrode is equal to an area of the second sensor electrode.

4. A capacitance detecting apparatus according to claim 3, wherein the first sensor electrode is structured with an arbitrary quantity of electric conductor, the second sensor electrode is structured with an arbitrary quantity of electric conductor, and the center of mass of the first sensor electrode matches the center of mass of the second sensor electrode.

5. A capacitance detecting apparatus according to claim 4, wherein the first sensor electrode is symmetrical relative to at least two symmetry planes crossing the matched centers of mass, and the second sensor electrode is symmetrical relative to the at least two symmetry planes crossing the matched centers of mass.

6. A capacitance detecting apparatus comprising:

a capacitor having a sensor electrode at one end, the sensor electrode being adapted to face a measured object, the capacitor being configured to output capacitance in response to a distance between the sensor electrode and the measured object;

a reference capacitor connected to a first power-supply voltage at one electrode and a first electrical node at the other electrode;

a first on/off switch connected to the first power-supply voltage at one end and to the first electrical node at the other end;

a lowpass filter connected to the sensor electrode at an input terminal and to a second electrical node at an output terminal;

a second on/off switch connected to the first electrical node at one end and to the second electrical node at the other end;

a third on/off switch connected to the second electrical node at one end and to the a second power-supply voltage lower than the first power-supply voltage at the other end;

a first rectifying element connected between the second electrical node and the first power-supply voltage and adapted to be electrically conducted when an electric potential of the second electrical node is higher than the first power-supply voltage;

a second rectifying element connected between the second electrical node and the second power-supply voltage and adapted to be electrically conducted when the electric potential of the second electrical node is lower than the second power-supply voltage;

a comparator connected to the first electrical node at a first input terminal and to a reference power-supply voltage at a second input terminal; and

a controller controlling the first, second and third on/off switches, implementing a process for alternately repeating a second switch operation, by which the second on/off switch is changed to a closed state and returned to an open state, and a third switch operation, by which the third switch is changed to a closed state and returned to an open state, following a first switch operation, by which the first switch is changed to a closed state and returned to an open state, and implementing a process for counting a number of times for repeating the second switch operation and for judging changes in the capacitance at the capacitor based upon the number of times for repeating the second switch operation before a level of voltage at the first input terminal and a level of voltage at the second input terminal are reversed.

7. A capacitance detecting apparatus according to claim 6, wherein a cutoff frequency fc1 of the lowpass filter is set to satisfy 1/T2<

fc1<

1/(2×

T1) when T1 depicts a period of time defined by a time, where the third on/off switch is switched to an open state, and a time, where the second on/off switch is switched to a closed state, and T2 depicts a period of time for the second on/off switch to return to an open state from the closed state.

8. A capacitance detecting apparatus according to claim 6, further comprising:

a high-pass filter connected to the sensor electrode at one end and passing a high-frequency component on the sensor electrode; and

a buffer connected to the second electrical node at an input terminal and to the other end of the high-pass filter at an output terminal.