Resonant-bridge two axis microaccelerometer

• US 4,901,570 A
• Filed: 01/30/1989
• Issued: 02/20/1990
• Est. Priority Date: 11/21/1988
• Status: Expired due to Fees

• Current Assignee: General Motors Auto OOO (General Motors Company)
• Original Assignee: General Motors Corporation (Motors Liquidation Co. GUC Trust)
• Inventors: Putty, Michael W., Chang, Shih-Chia, Hicks, David B.
• Primary Examiner(s): NOT, DEFINED
• Assistant Examiner(s): NOT, DEFINED
• Application Number:
  US07/304,057
• Time in Patent Office: 386 Days
• Field of Search: 73/517 AV, 73/DIG. 1
• US Class Current: 73/514.29
• CPC Class Codes: G01P 15/097 by vibratory elements
• CPC Class Codes: G01P 15/18 in two or more dimensions
• CPC Class Codes: G01P 2015/084 the mass being suspended at more than one of its sides, e.g. membrane-type suspension, so as to permit multi-axis movement of the mass
• CPC Class Codes: G01P 2015/0842 the mass being of clover leaf shape
• CPC Class Codes: Y10S 73/01 Vibration

Abstract

A resonant bridge two-axis microaccelerometer is disclosed comprising polysilicon resonant bridges orthogonally attached to a silicon proof mass, such that the silicon proof mass is suspended by the resonant bridges. Acceleration in the plane of the substrate causes differential axial loads on the opposing microbridges in each pair, thereby shifting their resonant frequencies. The acceleration component aligned with a pair is measured by the difference in resonant frequencies.

Claims

1. An accelerometer comprising:

a mass;

a support surrounding said mass with a gap provided substantially everywhere therebetween said support and said mass;

at least two bridges attached at one end to said support from which said mass is suspended, said bridges being attached to opposite sides of said mass such that the longitudinal axis through said bridges form a common axis through said mass;

means for vibrating said bridges at their respective resonant frequencies, their respective resonant frequencies change as a function of extension or compression of said bridges due to acceleration of said mass;

means for detecting a change in the difference of said resonant frequencies between both of said bridges, so as to provide an indication of acceleration in a direction along said common axis; and

wherein said means for detection is provided on said support and said bridges span over said means for detection such that a gap exists between said means for detection and said bridges.

2. An accelerometer as recited in claim 1 wherein said bridges are excited to vibrate at their respective resonant frequencies using electrostatic means.

3. An accelerometer as recited in claim 2 wherein said electrostatic means include a drive electrode provided on said support, said drive electrode is centrally located along the longitudinal axis of its respective bridge and a gap is provided therebetween said drive electrode and said respective bridge.

4. An accelerometer as recited in claim 2 wherein the electrostatic means comprise:

a drive electrode for each of said bridges, each drive electrode positioned across a gap from each of said respective bridges;

at least one sense electrode for each bridge, each of said sense electrodes positioned on said support adjacent to said drive electrode for said respective bridge, thereby a gap is provided therebetween said bridge and said sense and drive electrodes, vibration of said bridge producing a time-varying capacitance across its respective gap; and

a feedback vibration-detection circuit for each of said bridges connected to said drive electrode and said sense electrodes for said respective bridge, the circuit sensing through said sense electrode changes in the time varying capacitance of said respective bridge and thereby sensing changes in the vibration of said bridge and the circuit applying voltage to the drive electrodes as a function of the sensed changes in capacitance.

5. An accelerometer as recited in claim 4 wherein said means for measuring include a frequency measuring circuit for each bridge connected to the feedback vibration detection circuit of the respective bridge.

6. An accelerometer as recited in claim 1 wherein said bridges are resonated by an electromechanical oscillator.

7. An accelerometer as recited in claim 1 wherein said bridges are under compressive stress.

8. An accelerometer as recited in claim 1 further comprising a plurality of stops to prevent deflection of said mass past a predetermined distance in a direction perpendicular to the top surface of said mass.

9. An accelerometer as recited in claim 8 wherein at least one of said stops is attached to said mass and at least one of said stops is attached to said support.

10. An accelerometer as recited in claim 1 further comprising a plurality of constraining arms attached to said mass to suppress the rotation of said mass and to suppress the deflection of said mass in a direction perpendicular to the top surface of said mass.

11. An accelerometer as recited in claim 1 wherein four bridges suspend said mass, two bridges forming a common first axis through said mass, the other two bridges forming a common second axis through said mass perpendicular to said first axis, the change in difference in resonant frequencies between said two bridges of said first axis providing an indication of a component of acceleration along said first axis at the same time the change in difference in resonant frequencies between the other two bridges provide an indication of an orthogonal component of acceleration along said second axis.

12. An accelerometer as recited in claim 11 further comprising a plurality of stops to prevent deflection of said mass past a predetermined distance in a direction perpendicular to the top surface of said mass.

13. An accelerometer as recited in claim 12 wherein at least one of said stops is attached to said mass and at least one of said stops is attached to said support.

14. An accelerometer as recited in claim 1 wherein the means for measuring include a frequency measuring circuit for each bridge.

15. An accelerometer comprising:

a mass centrally positioned within a support with a gap substantially everywhere therebetween said mass and said support,

at least two pairs of bridges for suspending said mass from said support, said bridges spanning over said gap between said mass and said support, each pair of said bridges having a first bridge with one end attached to one side of said mass and a second bridge attached to an opposite side of said mass and respective opposite ends of said first and second bridges attached to said support such that said first and second bridges form a common longitudinal axis through said mass, the formed axes of the two pairs of bridges being perpendicular to each other;

a drive electrode for each bridge provided on said support, each drive electrode positioned such that a gap exists everywhere therebetween said respective bridge which spans over said drive electrode;

a pair of sense electrodes for each bridge provided on said support, one sense electrode positioned adjacent to one side of the drive electrode and the other sense electrode positioned adjacent to an opposite side of the drive electrode, said pair of sense electrodes positioned such that a gap exists everywhere therebetween said respective bridge which spans over said drive and sense electrodes, vibration of said bridges producing a time varying capacitance across said gap;

a feedback vibration detection circuit for each of said bridges connected to said drive electrodes and said pair of sense electrodes for each of said respective bridges, the circuit sensing through the pair of sense electrodes senses changes in the time varying capacitance and thereby the changes in vibration of said respective bridges, and the circuit causes the drive electrode to exert a force on said respective bridge as a function of the sensed changes in vibration to maintain vibration of each respective bridge at the respective resonant frequency;

a plurality of stops to prevent deflection of said mass past a predetermined distance in a direction perpendicular to said common two axes, wherein one of said stops is attached to said mass and one of said stops is attached to said support;

a plurality of constraining bridges attached to said mass to suppress rotation of said mass and to suppress displacement of said mass in a direction perpendicular to said two axes; and

means for simultaneously measuring in each pair of bridges a change in the difference of said resonant frequencies between said first and second bridges of said pairs to provide an indication of acceleration in a direction along the respective common axes formed by said pair of bridges, the simultaneous measuring of said pairs providing an indication of orthogonal components of acceleration, said resonant frequency of each of said bridges changing as a function of extension or compression of said bridge due to acceleration of said mass.

16. A method for fabricating a monolithic accelerometer comprising the steps of:

forming drive and sense electrodes with a first structural layer on one side of a substrate;

forming a body of a predetermined mass in said substrate, symmetrically aligned with said drive and sense electrodes;

depositing a sacrificial layer over said drive and sense electrodes;

forming bridges over said sacrificial layer;

removing said sacrificial layer to form a gap therebetween said bridges and said drive and sense electrodes; and

substantially isolating said mass from said substrate so as to suspend said mass from said bridges, and so as to minimally attach said mass to said substrate with a plurality of constraining bridges.

17. A method for fabricating a monolithic accelerometer as recited in claim 16 further comprising the steps of:

depositing a sacrificial layer over regions of said support and of said body of predetermined mass;

forming stops over said sacrificial layer; and

removing said sacrificial layer to form a stop which is attached to said support and said body of mass and which extends over said body of mass and said support for a predetermined distance.