Method and unit for sensing physical quantity using capacitive sensor

• US 20050016273A1
• Filed: 07/22/2004
• Published: 01/27/2005
• Est. Priority Date: 07/25/2003
• Status: Active Grant

• Current Assignee: DENSO Corporation
• Original Assignee: DENSO Corporation
• Inventors: Murata, Minoru, Kawai, Takaaki
• Granted Patent: US 7,168,320 B2
• Time in Patent Office: Days
• US Class Current: 73/514.320
• CPC Class Codes: G01P 15/125 by capacitive pick-up
• CPC Class Codes: G01P 15/131 with electrostatic counterbalancing means

Abstract

A physical quantity (e.g., acceleration) sensing unit is provided in this unit, a capacitive sensor has first and second fixed electrodes and a physical-quantity-sensitive movable electrode disposed between the first and second fixed electrodes. An adjusting circuit first adjusts a first bias voltage applied between the first fixed electrode and the movable electrode and a second voltage applied between the movable electrode and the second fixed electrode so that the movable electrode is brought into contact with either the first or second fixed electrode. The adjusting circuit then adjusts the first and second bias voltages to return the movable electrode to its original position. A detecting circuit detects an output on a capacitance relationship among the first and second fixed electrodes and the movable electrode. The output is subjected to determination of whether or not the output is out of order, when tested.

Claims

1. A unit for sensing physical quantity comprising:

a capacitive sensor equipped with a first fixed electrode and a second electrode disposed face to face and a movable electrode disposed between the first and second fixed electrodes to be movable in response to the physical quantity and to be face to face between the first and second fixed electrodes;

first adjusting means for adjusting a first bias voltage applied between the first fixed electrode and the movable electrode and a second voltage applied to between the movable electrode and the second fixed electrode so that the movable electrode is brought into contact with either of the first and second fixed electrodes;

second adjusting means for adjusting the first and second bias voltages to return the movable electrode to an original position thereof, the first adjustment preceding the second adjustment; and

means for detecting an output signal corresponding to a relationship of capacitance among the first and second fixed electrodes and the movable electrode, the output signal being subjected to determination of whether or not the output signal is out of order when a test mode of the unit is instructed.

2. A unit for sensing physical quantity comprising:

a capacitive sensor equipped with a first fixed electrode and a second electrode disposed face to face and a movable electrode disposed between the first and second fixed electrodes to be movable in response to the physical quantity and to be face to face between the first and second fixed electrodes;

an adjusting circuit adjusting, in a first step, a first bias voltage applied between the first fixed electrode and the movable electrode and a second voltage applied between the movable electrode and the second fixed electrode so that the movable electrode is brought into contact with either of the first and second fixed electrodes, and adjusting, in a second step, the first and second bias voltages to return the movable electrode to an original position thereof, the first step preceding the second step; and

a detecting circuit detecting an output signal corresponding to a relationship of capacitance among the first and second fixed electrodes and the movable electrode, the output signal being subjected to determination of whether or not the output signal is out of order when a test mode of the unit is instructed.

3. The unit according to claim 2, wherein, in the first step, the adjusting applies a potential of higher than Vcc between the first fixed electrode and the movable electrode so that the first bias voltage becomes zero and applies a potential of zero to the second fixed electrode so that the second bias voltage becomes a voltage of Vcc, the Vcc being a power supply voltage to be applied to the unit.

4. The unit according to claim 3, wherein the potential of higher than the voltage of Vcc is a potential predetermined to ensure that the movable electrode is brought into contact with the first fixed electrode.

5. The unit according to claim 3, wherein the adjusting circuit is provided with a first circuit applying between the first and second fixed electrodes a voltage signal whose frequency is higher than a resonance frequency of the movable electrode and whose polarities are inverted at intervals between zero and Vcc and a second circuit applying to the movable electrode a voltage of Vcc/2, the Vcc being a power supply voltage to be applied to the unit in the second step.

6. The unit according to claim 2, wherein, in the first step, the adjusting circuit applies a potential of higher than Vcc between the second fixed electrode and the movable electrode so that the second bias voltage becomes zero and applies a potential of zero to the first fixed electrode so that the first bias voltage becomes a voltage of Vcc, the Vcc being a power supply voltage to be applied to the unit.

7. The unit according to claim 6, wherein the potential of higher than Vcc is a potential predetermined to ensure that the movable electrode is brought into contact with the second fixed electrode.

8. The unit according to claim 2, further comprising a determination circuit performing the determination.

9. The unit according to claim 8, wherein the determination circuit is configured to examine the output signal for being fixedly biased toward a level higher or lower than a normal level detected when the movable electrode is at an intermediate position between the first and second fixed electrodes.

10. The unit according to claim 2, further comprising a measuring circuit operative when the unit is used for measuring the physical quantity, wherein

the measuring circuit is provided with a first circuit applying between the first and second fixed electrodes a voltage signal whose frequency is higher than a resonance frequency of the movable electrode and whose polarities are inverted at intervals between zero and Vcc and a second circuit applying to the movable electrode a voltage of Vcc/2, the Vcc being a power supply voltage to be applied to the unit.

11. The unit according to claim 2, wherein the physical quantity is acceleration so that the physical quantity sensing unit is dedicated to sensing the acceleration.

12. An on-vehicle airbag system comprising:

an airbag unit mounted on a vehicle and formed to have an airbag;

a physical quantity sensing unit sensing a physical quantity as a signal indicative of the sensed physical quantity; and

a controller controlling a development of the airbag in response to the signal outputted by the sensing unit,

wherein the physical quantity sensing unit comprises;

a capacitive sensor equipped with a first fixed electrode and a second fixed electrode disposed face to face and a movable electrode disposed between the first and second fixed electrodes to be movable in response to the physical quantity and to be face to face between the first and second fixed electrodes;

an adjusting circuit adjusting, in a first step, a first bias voltage applied between the first fixed electrode and the movable electrode and a second voltage applied between the movable electrode and the second fixed electrode so that the movable electrode is brought into contact with either of the first and second fixed electrodes, and adjusting, in a second step, the first and second bias voltages to return the movable electrode to an original position thereof, the first step preceding the second step;

a detecting circuit detecting the signal corresponding to a relationship of capacitance among the first and second fixed electrodes and the movable electrode; and

a determining unit determining whether or not the output signal detected by the detecting circuit on completion of the second adjustment is out of order.

13. A method of sensing physical quantity using a capacitive sensor equipped with a first fixed electrode and a second fixed electrode disposed face to face and a movable electrode disposed between the first and second fixed electrodes to be movable in response to the physical quantity and to be face to face between the first and second fixed electrodes, comprising;

first adjusting a first bias voltage applied between the first fixed electrode and the movable electrode and a second voltage applied between the movable electrode and the second fixed electrode so that the movable electrode is brought into contact with either of the first and second fixed electrodes;

second adjusting the first and second bias voltages to return the movable electrode to an original position thereof, the first adjustment preceding the second adjustment; and

detecting an output signal corresponding to a relationship of capacitance among the first and second fixed electrodes and the movable electrode, the output signal being subjected to determination of whether or not the output signal is out of order when a test mode is instructed.

14. The method according to claim 13, wherein the first adjusting step applies a potential of higher than Vcc between the first fixed electrode and the movable electrode so that the first bias voltage becomes zero and to apply a potential of zero to the second fixed electrode so that the second bias voltage becomes a voltage of Vcc, the Vcc being a power supply voltage.

15. The method according to claim 14, wherein the potential of higher than the voltage of Vcc is a potential predetermined to ensure that the movable electrode is brought into contact with the first fixed electrode.

16. The method according to claim 14, wherein the second adjusting step applies between the first and second fixed electrodes a voltage signal whose frequency is higher than a resonance frequency of the movable electrode and whose polarities are inverted at intervals between zero and Vcc and then applies to the movable electrode a voltage of Vcc/2, the Vcc being a power supply voltage to be applied to the unit.

17. The method according to claim 13, wherein the first adjusting step applies a potential of higher than Vcc between the second fixed electrode and the movable electrode so that the second bias voltage becomes zero and to apply a potential of zero to the first fixed electrode so that the first bias voltage becomes a voltage of Vcc, the Vcc being a power supply voltage.

18. The method according to claim 17, wherein the potential of higher than Vcc is a potential predetermined to ensure that the movable electrode is brought into contact with the second fixed electrode.

19. The method according to claim 13, further comprising a step of performing the determination.

20. The method according to claim 19, wherein in the determination, it is determined that the output signal for being fixedly biased toward a level higher or lower than a normal level detected when the movable electrode is at an intermediate position between the first and second fixed electrodes.