Piezoresistive force rebalance accelerometer

• US 5,596,144 A
• Filed: 08/04/1995
• Issued: 01/21/1997
• Est. Priority Date: 08/04/1995
• Status: Expired due to Fees

• Current Assignee: Delco Electronics Corporation (Motors Liquidation Co. GUC Trust)
• Original Assignee: Delco Electronics Corporation (Motors Liquidation Co. GUC Trust)
• Inventors: Swanson, Gregory D.
• Primary Examiner(s): NOT, DEFINED
• Assistant Examiner(s): NOT, DEFINED
• Application Number:
  US08/511,215
• Time in Patent Office: 536 Days
• Field of Search: 73/514.17, 73/514.18, 73/514.32, 73/514.33, 73/514.34, 73/862.61, 73/862.627, 338/2, 338/5
• US Class Current: 73/514.18
• CPC Class Codes: G01P 15/123 by piezo-resistive elements, e.g. semiconductor strain gauges
• CPC Class Codes: G01P 15/131 with electrostatic counterbalancing means
• CPC Class Codes: G01P 2015/0828 the mass being of the paddle type being suspended at one of its longitudinal ends
• CPC Class Codes: G01P 2015/0882 for providing damping of vibrations

Abstract

A motion sensor is provided for sensing motion or acceleration of a body, such as an accelerometer for use in an on-board automotive safety control system or navigational system. The motion sensor is a piezoresistive motion sensor that operates in a closed loop force rebalance mode. As such, the motion sensor may be considered a hybrid of piezoresistive sensors and capacitive force rebalance sensors. The sensor achieves this novel combination through a mechanically-biased proof mass that enables-the use of a single electrode for maintaining the proof mass in a null position during the operation of the sensor.

Claims

1. A piezoresistive force rebalance motion sensor comprising:

a cantilevered proof mass having a first electrode;

a second electrode disposed adjacent the proof mass so as to form a gap between the second electrode and the first electrode, the second electrode being adapted to generate an electrostatic force that electrostatically attracts the proof mass in a direction toward the second electrode in an energized state of the second electrode;

means associated with the proof mass for deflecting the proof mass in a direction away from the second electrode such that the proof mass is deflected away from the second electrode in absence of an electrostatic force on the proof mass;

means for sensing strain induced by a deflection of the proof mass relative to the second electrode and for producing an output signal therefrom; and

means for controlling the electrostatic force generated by the second electrode based on the output signal of the strain sensing means, the controlling means serving to return the proof mass to a null position relative to the second electrode when the proof mass is deflected from the null position;

wherein the deflecting means counteracts the electrostatic force between the proof mass and the second electrode in the energized state, such that the proof mass to be maintained at the null position in the energized state.

2. A motion sensor as recited in claim 1 further comprising a silicon chip from which the proof mass is cantilevered.

3. A motion sensor as recited in claim 1 further comprising a silicon chip with which the second electrode is supported.

4. A motion sensor as recited in claim 1 further comprising a bridge by which the proof mass is cantilevered, wherein the deflecting means is disposed between the first electrode and the bridge.

5. A motion sensor as recited in claim 1 further comprising a bridge by which the proof mass is cantilevered, wherein the deflecting means is a thin film material on a surface of the bridge.

6. A motion sensor as recited in claim 1 wherein the null position of the proof mass is characterized by the proof mass being substantially parallel with the second electrode.

7. A motion sensor as recited in claim 1 wherein the strain sensing means comprises piezoresistors.

8. A motion sensor as recited in claim 1 wherein the gap between the first electrode of the proof mass and the second electrode is vented to atmosphere.

9. A motion sensor comprising:

a first chip comprising a support frame, a bridge projecting from the support frame, and a proof mass suspended from the support frame by the bridge such that the proof mass is subject to input forces imposed on the motion sensor;

a first electrode disposed on the proof mass;

a second chip secured to the first chip, the second chip having a second electrode disposed adjacent the first electrode of the proof mass so as to form a gap therebetween, the second electrode being adapted to generate an electrostatic force that electrostatically attracts the proof mass toward the second chip in an energized state of the second electrode;

means mounted on the bridge for deflecting the proof mass in a direction away from the electrode such that the proof mass is deflected away from the second electrode in absence of an electrostatic force on the proof mass;

strain sensing means disposed on the bridge for sensing a deflection in the proof mass relative to the second electrode and producing an output signal therefrom; and

means for controlling the electrostatic force generated by the second electrode based on the output signal of the strain sensing means, the controlling means serving to return the proof mass to a null position relative to the second chip when the proof mass is deflected from the null position;

wherein the second electrode electrostatically attracts the proof mass toward the second chip and the deflecting means repels the proof mass away from the second chip in the energized state, such that the proof mass is maintained at the null position in the energized state.

10. A motion sensor as recited in claim 9 wherein the proof mass is cantilevered from the support frame.

11. A motion sensor as recited in claim 9 wherein the deflecting means is a thin film material on a surface defined by the bridge and the proof mass.

12. A motion sensor as recited in claim 9 wherein the null position of the proof mass is characterized by the proof mass being substantially coplanar with the first chip.

13. A motion sensor as recited in claim 9 wherein the strain sensing means comprises implanted piezoresistors in the bridge.

14. A motion sensor as recited in claim 9 wherein the gap between the first electrode of the proof mass and the second electrode of the second chip is vented to atmosphere.

15. A motion sensor as recited in claim 9 wherein the deflecting means is disposed between the first electrode and at least a portion of a surface defined by the bridge and the proof mass.

16. A piezoresistive force rebalance accelerometer comprising:

a bulk-micromachined first chip comprising a support frame defining an aperture, a bridge projecting from the support frame, and a proof mass cantilevered within the aperture by the bridge such that the proof mass is subject to input forces imposed on the accelerometer;

a first electrode disposed on a first surface of the proof mass;

a second chip secured to the first chip so as to form with the first chip a cavity vented to atmosphere, the second chip having a second electrode disposed adjacent the first electrode of the proof mass so as to form a gap therebetween, the second electrode being adapted to generate an electrostatic force that electrostatically attracts the proof mass toward the second chip in an energized state of the second electrode;

a thin film on the bridge and proof mass for deflecting the proof mass in a direction away from the second electrode such that the proof mass is deflected away from the second electrode in a absence of an electrostatic force on the proof mass;

implanted piezoresistors in the bridge for sensing a deflection in the proof mass relative to the second electrode and producing an output signal therefrom; and

means for controlling the electrostatic force generated by the second electrode based on the output signal of the piezoresistors, the controlling means serving to maintain the proof mass in a null position characterized by the proof mass being substantially coplanar with the first chip in the energized state;

whereby acceleration forces on the proof mass cause a change in the output signal of the piezoresistors, such that the controlling means causes a corresponding change in the electrostatic force generated by the second electrode and thereby returns the proof mass to the null position, such that the proof mass remains substantially at the null position while the second electrode is in the energized state.

17. A motion sensor as recited in claim 16 wherein the thin film is disposed between the first electrode and at least a portion of a surface defined by the bridge and the proof mass.

18. A motion sensor as recited in claim 16 wherein multiple thin films are provided on the bridge and the proof mass for deflecting the proof mass in a direction away from the second electrode.