Mechanical resonance, silicon accelerometer

• US 5,780,742 A
• Filed: 02/10/1997
• Issued: 07/14/1998
• Est. Priority Date: 04/15/1993
• Status: Expired due to Term

• Current Assignee: Honeywell Incorporated (Honeywell International Inc.)
• Original Assignee: Honeywell Incorporated (Honeywell International Inc.)
• Inventors: Frische, Richard H., Burns, David W.
• Primary Examiner(s): Oda, Christine K.
• Application Number:
  US08/799,587
• Time in Patent Office: 519 Days
• Field of Search: 73/514.29, 73/514.15, 73/514.32, 73/514.18, 73/514.34, 73/514.38, 73/862.59
• US Class Current: 73/514.29
• CPC Class Codes: G01P 15/097 by vibratory elements
• CPC Class Codes: G01P 2015/0828 the mass being of the paddle type being suspended at one of its longitudinal ends

Abstract

A silicon accelerometer includes a plurality of silicon layers. The silicon layers form first and second silicon beams supported by flexure members. An acceleration responsive silicon mass is arranged to bend the flexure members in response to accelerations. The silicon beams are vibrated in vacuum chambers and gas damping is provided for the acceleration responsive mass and the flexure members. A sensor is provided for sensing vibration of the first and second silicon beams.

Claims

1. A sensor comprising:

a supporting frame having an opening therein;

a mass disposed in the opening and moveable in directions approximately parallel to a first axis; and

wherein;

the first axis is perpendicular to a plane parallel to the perimeter of said supporting frame; and

a second axis is parallel to the plane;

a flexure coupled between the frame and the mass; and

wherein;

said flexure supports said mass relative to said supporting frame;

said flexure bends a first way upon movement of said mass in a first direction approximately parallel to the first axis; and

said flexure bends a second way upon movement of said mass in a second direction approximately parallel to the first axis; and

a vibratory beam formed on said flexure; and

wherein;

said vibratory beam is stretched in a direction approximately parallel to the second axis when said flexure bends the first way;

said vibratory beam is compressed in a direction approximately parallel to the second axis when said flexure bends the second way;

said vibratory beam has a resonant frequency;

the resonant frequency changes when said vibratory beam is stretched;

the resonant frequency changes when said vibratory beam is compressed; and

the resonant frequency is indicative of a magnitude and direction of acceleration affecting said mass in a direction approximately parallel to the first axis.

2. The sensor of claim 1 wherein said sensor is constructed from at least one micromachined wafer.

3. A sensor comprising:

a supporting frame having an opening therein;

a mass disposed in the opening and moveable in directions approximately parallel to a first axis;

wherein;

the first axis is perpendicular to a plane approximately parallel to the perimeter of said supporting frame; and

a second axis and a third axis are parallel to the plane;

a plurality of flexures coupled between said frame and said mass; and

wherein;

said plurality of flexures support said mass relative to said supporting frame;

each flexure of a first portion of said plurality of flexures, bends a first way upon movement of said mass in a first direction approximately parallel to the first axis;

each flexure of the first portion of said plurality of flexures, bends a second way upon movement of said mass in a second direction approximately parallel to the first axis;

each flexure of a second portion of said plurality of flexures, bends a first way upon movement of said mass in a first direction approximately parallel to the first axis; and

each flexure of the second portion of said plurality of flexures, bends a second way upon movement of said mass in a second direction approximately parallel to the first axis;

a first vibratory beam formed on each flexure of the first portion of said plurality of flexures; and

wherein;

said first vibratory beam is stretched in a direction approximately parallel to a plane containing the second and third axes, when said first portion flexure bends the first way;

said first vibratory beam is compressed in a direction approximately parallel to the plane containing the second and third axes, when said first portion flexure bends the second way;

said first vibratory beam has a first resonant frequency;

the first resonant frequency increases when said first vibratory beam is stretched; and

the first resonant frequency decreases when said first vibratory beam is compressed; and

a second vibratory beam formed on each flexure of the second portion of said plurality of flexures; and

wherein;

said second vibratory beam is compressed in a direction approximately parallel to the plane containing the second and third axes, when said second portion flexure bends the first way;

said second vibratory beam is stretched in a direction approximately parallel to the plane containing the second and third axes, when said second portion flexure bends the second way;

said second vibratory beam has a second resonant frequency;

the second resonant frequency changes when said second vibratory beam is stretched; and

the second resonant frequency changes when said second vibratory beam is compressed; and

the first and second resonant frequencies are indicative of a magnitude and direction of an acceleration force affecting said mass in a direction approximately parallel to the first axis.

4. The sensor of claim 3 wherein said sensor is constructed from at least one micromachined wafer.

5. An accelerometer comprising:

a first wafer having a thickness, having a first surface, having a second surface approximately parallel to and at a distance equal to the thickness from the first surface, having a recess formed in said first wafer resulting in a proof mass having first and second surfaces, surrounded by a perimeter support structure having first and second surfaces, the proof mass supported by a first plurality of flexures connected to the perimeter support structure and to the proof mass at the first surface of said first wafer, and the proof mass supported by a second plurality of flexures connected to the perimeter support structure and to the proof mass at the second surface of said first wafer, the first and second pluralities of flexures formed from said first wafer;

a resonant vibratory microbeam formed on at least one flexure of the first plurality of flexures;

a first insulating layer situated next to the first surface of the perimeter support structure;

a second insulating layer situated next to the second surface of the perimeter support structure;

a second wafer, formed over the first surface of said first wafer and situated next to said first insulating layer, having a recess for permitting movement of the proof mass;

a third wafer, formed under the second surface of said first wafer and situated next to said second insulating layer, having a recess for permitting movement of the proof mass;

a sensing means, proximate to said resonant vibratory microbeam, for detecting a vibration of said resonant vibratory microbeam; and

a force rebalance circuit connected to said first wafer, said second wafer, said third wafer and to said sensing means; and

wherein;

the proof mass is moveable in first and second directions perpendicular to a plane parallel to the first surface of said first wafer;

the proof mass moves in the first direction when said accelerometer is subject to an acceleration force in the first direction, the proof mass moves in the second direction when said accelerometer is subject to an acceleration force in the second direction, and the proof mass is situated in a null position when not subject to an acceleration force;

said force rebalance circuit provides an electrostatic force on the proof mass, which has a magnitude equal to a magnitude of an acceleration force simultaneously affecting the proof mass, the electrostatic force having a direction opposite of that of the acceleration force, and the electrostatic force maintaining the proof mass in the null position;

an amount of distance that the proof mass moves is proportional to a difference of the magnitudes of the acceleration and electrostatic forces;

an amount of bending of the at least one flexure is proportional to the amount of distance that the proof mass moves;

said resonant vibratory microbeam has a resonant frequency that increases when the proof mass moves in the first direction and decreases when the proof mass moves in the second direction, wherein a variation of the resonant frequency is due to a change of length of said resonant vibratory microbeam which in turn is due to a bending of the at least one flexure;

said first and second wafers form electrodes of a first capacitor and said first and third wafers form electrodes of a second capacitor, such that a first signal on the electrodes of the first capacitor provides an electrostatic force to attract the proof mass towards said second wafer and a second signal on the electrodes of the second capacitor provides an electrostatic force to attract the proof mass towards said third wafer;

the resonant frequency of said resonant vibratory microbeam is at a first frequency when the magnitude of the acceleration force is zero, is initially slightly greater than the first frequency when the magnitude of the acceleration force is greater than zero in the first direction, and is initially slightly less than the first frequency when the magnitude of the acceleration force is greater than zero in the second direction; and

said force rebalance circuit provides first and second signals to the electrodes of the first and second capacitors, respectively, for maintaining the proof mass in the null position, the first and second signals indicative of magnitude and direction of the acceleration force in the first and second direction.

6. The accelerometer of claim 5 further comprising a temperature sensor, formed on the perimeter support structure and connected to said force rebalance circuit, wherein said temperature sensor comprises a resonant vibratory microbeam having a variable resonant frequency indicative of temperature, and for sending a temperature-indicative signal to said force rebalance circuit to compensate for temperature-related error in the first and second signals indicative of magnitude and direction of the acceleration force.

7. A sensor comprising:

a supporting frame having an opening therein;

a mass disposed in the opening and moveable in directions approximately parallel to a first axis; and

wherein;

the first axis is perpendicular to a plane parallel to the perimeter of said supporting frame; and

a second axis is parallel to the plane;

at least one flexure coupled between the frame and the mass; and

wherein;

said flexure supports said mass relative to said supporting frame;

said flexure bends a first way upon movement of said mass in a first direction approximately parallel to the first axis; and

said flexure bends a second way upon movement of said mass in a second direction approximately parallel to the first axis; and

a vibratory beam formed on said flexure; and

wherein;

said vibratory beam is stretched in a direction approximately parallel to the second axis when said flexure bends the first way;

said vibratory beam is compressed in a direction approximately parallel to the second axis when said flexure bends the second way;

said vibratory beam has a resonant frequency;

the resonant frequency changes when said vibratory beam is stretched;

the resonant frequency changes when said vibratory beam is compressed; and

the resonant frequency is indicative of a magnitude of acceleration affecting said mass in a direction approximately parallel to the first axis.

8. A sensor comprising:

first, second, and third layers, each layer having first and second surfaces wherein all of the first and second surfaces said first, second and third layers are approximately parallel to one another;

said second layer fabricated to form an acceleration responsive mass, a supportive structure, a first plurality of flexure members at the first surface of said second layer and a second plurality of flexure members at the second surface of said second layer, wherein the first and second pluralities of flexure members support the acceleration responsive mass relative to the supportive structure, and the acceleration responsive mass is moveable relative to the supportive structure, along an axis approximately perpendicular to the first and second surfaces of said first, second and third layers;

said first layer having the second surface proximate to the first surface of said second layer, and having a first recess formed in the second surface of said first layer to permit movement of the acceleration responsive mass along the axis towards said first layer;

said third layer having the first surface proximate to the second surface of said second layer, and having a second recess formed in the first surface of said third layer to permit movement of the acceleration responsive mass along the axis towards said third layer;

first means, situated on a first flexure member of the first plurality of flexure members, for sensing bending of the first flexure member of the first plurality of flexure members; and

wherein;

the acceleration responsive mass, when affected by an acceleration force that is in a direction parallel to the axis, results in a bending of the first flexure member of the first plurality of flexure members at an amount indicative of an amount of the acceleration force;

said first means for sensing, formed on the first flexure member of the first plurality of flexure members, comprises a vibratory beam, having a first variable resonant frequency that is indicative of an amount of bending of the first flexure member of the first plurality of flexure members and that the bending subjects the vibratory beam to compression or tension and thus results in a change of the first variable resonant frequency; and

the first resonant frequency is indicative of the amount of acceleration force affecting the acceleration responsive mass in the direction parallel to the axis.

9. The accelerometer of claim 8 further comprising:

second means, situated on a first flexure member of the second plurality of flexure members, for sensing bending of the first flexure member of the second plurality of flexure members; and

wherein;

said second means for sensing, formed on the first flexure member of the second plurality of flexure members, comprises a vibratory beam, having a second variable resonant frequency that is indicative of an amount of bending of the first flexure member of the second plurality of flexure members and that the bending subjects the vibratory beam to compression or tension and thus results in a change of the second variable resonant frequency;

and

the second resonant frequency is indicative of acceleration force affecting the acceleration responsive mass in the direction parallel to the member axis.

10. The accelerometer of claim 9 wherein:

said first means for sensing is situated on the first flexure member of the first plurality of flexure members, closer to said supportive structure than to said acceleration responsive mass;

said second means for sensing is situated on the first flexure member of the second plurality of flexure members, closer to said supportive structure than to said acceleration responsive mass; and

the first flexure member of the first plurality of flexure members and the first flexure member of the second plurality of flexure members, are located next to each other.

11. The accelerometer of claim 10 further comprising third means for sensing temperature, formed on the supportive structure, wherein said third means for sensing comprises a vibratory beam, having a third variable resonant frequency that is indicative of the temperature on the supportive structure, for compensating errors of said first and second means for sensing due to temperature.

12. The accelerometer of claim 11 wherein said first, second and third layers are micromachined silicon.

13. The accelerometer of claim 8 further comprising:

second means, situated on a second flexure member of the first plurality of flexure members, for sensing bending of the second flexure member of the first plurality of flexure members; and

wherein;

said second means for sensing comprises a vibratory beam, having a second variable resonant frequency that is indicative of an amount of bending of the second flexure member of the first plurality of flexure members, formed on the second flexure member of the first plurality of flexure members; and

the first and second resonant frequencies are indicative of the amount of the acceleration force affecting the acceleration responsive mass.

14. The accelerometer of claim 13 wherein:

said first means for sensing is situated closer to said supportive structure than to said acceleration responsive mass;

the second means for sensing is situated closer to said acceleration responsive mass than to said supportive structure; and

the first flexure member and the second flexure member of the first plurality of flexure members, are located diametrically opposite from each other relative to said acceleration responsive mass.

15. The accelerometer of claim 14 further comprising third means for sensing temperature, formed on the supportive structure, wherein said third means for sensing comprises a vibratory beam having a third variable resonant frequency that is indicative of the temperature on the supportive structure, for providing information for compensating errors of said first and second means for sensing due to temperature.

16. The accelerometer of claim 15 wherein:

said first, second and third layers are micromachined silicon; and

the vibratory beams of said first, second and third means for sensing are first, second and third silicon vibratory beams, respectively.

17. The accelerometer of claim 16 wherein:

said first means for sensing further comprises first vibrating means for vibrating the first silicon vibratory beam; and

said second means for sensing further comprises second vibrating means for vibrating the second silicon vibratory beam.

18. The accelerometer of claim 17 wherein:

the first vibrating means comprises a first capacitor having at least one electrode on the first silicon vibratory beam; and

the second vibrating means comprises a second capacitor having at least one electrode on the second silicon vibratory beam.

19. The accelerometer of claim 18 wherein:

the at least one electrode of the first capacitor is a doped region of the first silicon vibratory beam; and

the at least one electrode of the second capacitor is a doped region of the second silicon vibratory beam.

20. The accelerometer of claim 19 wherein:

the first means for sensing further comprises first vibration sensing means for sensing the vibration of the first silicon vibratory beam; and

the second means for sensing further comprises second vibration sensing means for sensing vibration of the second silicon vibratory beam.

21. The accelerometer of claim 20 wherein:

the first means for sensing further comprises a first amplifier having an input coupled to the first vibration sensing means and an output coupled to the first capacitor, the first amplifier being arranged for controlling the first capacitor to vibrate the first silicon vibratory beam at a resonant frequency dependent upon the first vibration sensing means; and

the second means for sensing further comprises a second amplifier having an input coupled to the second vibration sensing means and an output coupled to the second capacitor, the second amplifier being arranged for controlling the second capacitor to vibrate the second silicon vibratory beam at a resonant frequency dependent upon the second vibration sensing means.

22. The accelerometer of claim 21 wherein:

the first means for sensing further comprises a first vacuum chamber surrounding the first silicon vibratory beam; and

the second means for sensing further comprises a second vacuum chamber surrounding the second silicon vibratory beam.

23. The accelerometer of claim 22 wherein the accelerometer comprises a chamber that includes first and second recesses of first and third layers, respectively, and any volume proximate to the first and second pluralities of flexure members, and a third recess in said second layer about the acceleration responsive mass.

24. The accelerometer of claim 23 wherein:

said chamber contains a gas to dampen movement of the acceleration responsive mass and the first and second pluralities of flexure members; and

the first and second silicon vibratory beams are in a vacuum, free of the gas thereby permitting the first and second silicon vibratory beams to vibrate with a high Q.

25. The accelerometer of claim 17 wherein:

the first vibrating means comprises a first piezoelectric device; and

the second vibrating means comprises a second piezoelectric device.

26. The accelerometer of claim 25 wherein:

the first means for sensing further comprises first vibration sensing means for sensing vibration of the first silicon vibratory beam;

the second means for sensing further comprises second vibration sensing means for sensing vibration of the second silicon vibratory beam.

27. The accelerometer of claim 26 wherein:

the first means for sensing further comprises a first amplifier having an input coupled to the first vibration sensing means and an output coupled to the first piezoelectric device, the first amplifier being arranged for controlling the first piezoelectric device to vibrate the first silicon vibratory beam at a resonant frequency dependent upon the first vibration sensing means; and

the second means for sensing further comprises a second amplifier having an input coupled to the second vibration sensing means and an output coupled to the second piezoelectric device, the second amplifier being arranged for controlling the second piezoelectric device to vibrate the second silicon vibratory beam at a resonant frequency dependent upon the second vibration sensing means.

28. The accelerometer of claim 27 wherein:

the first vibration sensing means is a third piezoelectric device; and

the second vibration sensing means is a fourth piezoelectric device.