Inertial sensor and method of use

• US 6,584,845 B1
• Filed: 02/10/2000
• Issued: 07/01/2003
• Est. Priority Date: 02/10/1999
• Status: Expired due to Term

• Current Assignee: California Institute of Technology
• Original Assignee: California Institute of Technology
• Inventors: Gutierrez, Roman C., Tang, Tony K.
• Primary Examiner(s): CHAPMAN JR, JOHN E
• Application Number:
  US09/502,403
• Time in Patent Office: 1,237 Days
• Field of Search: 735/141.5, 735/142.9, 735/040.4, 735/041.2, 735/10
• US Class Current: 73/514.15
• CPC Class Codes: G01C 19/56 Turn-sensitive devices using vibrating masses, e.g. vibratory angular rate sensors based on Coriolis forces
• CPC Class Codes: G01P 15/097 by vibratory elements
• CPC Class Codes: G01P 15/131 with electrostatic counterbalancing means
• 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

Abstract

The inertial sensor of the present invention utilizes a proof mass suspended from spring structures forming a nearly degenerate resonant structure into which a perturbation is introduced, causing a split in frequency of the two modes so that the mode shape become uniquely defined, and to the first order, remains orthogonal. The resonator is provided with a mass or inertia tensor with off-diagonal elements. These off-diagonal elements are large enough to change the mode shape of the two nearly degenerate modes from the original coordinate frame. The spring tensor is then provided with a compensating off-diagonal element, such that the mode shape is again defined in the original coordinate frame. The compensating off-diagonal element in the spring tensor is provided by a biasing voltage that softens certain elements in the spring tensor. Acceleration disturbs the compensation and the mode shape again changes from the original coordinate frame. By measuring the change in the mode shape, the acceleration is measured.

Claims

1. An inertial sensor comprising:

a proof mass suspended by spring structures;

a plurality of electrodes for detecting resonant motion of the proof mass;

drive circuitry coupled to the electrodes for exciting a resonant motion of the proof mass and spring structures;

bias circuitry for supplying a voltage to modify the resonant motion of the proof mass and spring structures;

sensing circuitry for measuring acceleration by detecting a sinusoidal output representative of a change of said resonant motion;

output circuitry for outputting a signal indicating the acceleration;

wherein the proof mass has a defined center and a mass imbalance such that the center of mass of the structure is spaced from the defined center, the proof mass suspended from a frame by the spring structures;

the drive circuitry rocks the proof mass about a first rocking axis passing through the defined center; and

the bias circuitry supplies the voltage to modify the resonant motion of the proof mass and spring structures to compensate for the mass imbalance.

2. The inertial sensor of claim 1, wherein:

the resonant motion of the proof mass and spring structures includes the first rocking axis and a second rocking axis substantially orthogonal to the first rocking axis; and

the bias circuitry exerts a force on a portion of the proof mass to rotate the first and second rocking axes of the resonant motion of the proof mass and spring structures into alignment with substantially orthogonal first and second spring axes passing through the spring structures.

3. The inertial sensor of claim 2, further comprising:

a first signal generated by the rocking motion of the proof mass about the first spring axis;

a second signal generated by the rocking motion of the proof mass about the second spring axis, wherein the sensing circuitry detects the second signal as an indication of the change of the resonant motion of the proof mass and the spring structures;

a feedback circuit for providing the first signal to the drive circuitry to rock the proof mass about the first spring axis; and

a feedback circuit for providing the second signal to the drive circuitry to dampen the rocking motion of the proof mass about the second sprig axis.

4. The inertial sensor of claim 2, wherein:

the sensing circuitry measures the acceleration by determining the rotation of the first and second rocking axes away from the first and second spring axes.

5. The inertial sensor of claim 3, wherein the output circuitry further comprises:

a first demodulator for demodulating the first and second signals to output a signal indicating the acceleration.

6. The inertial sensor of claim 3, wherein the output circuitry further comprises:

a mass rigidly attached to the defined center;

a first demodulator for demodulating the first and second signals in quadrature to generate a signal indicating rotation of the proof mass; and

a second demodulator for demodulating the first and second signals in phase to generate a signal indicating the acceleration.

7. The inertial sensor of claim 1, further comprising:

a base structure having eight electrodes patterned on its surface, the base structure being positioned adjacent the proof mass; and

wherein the drive circuitry, bias circuitry and sensing circuitry are electrically connected to the electrodes.

8. The inertial sensor of claim 1, further comprising:

a base structure having four electrodes patterned on its surface, the base structure being positioned adjacent the proof mass; and

wherein the drive circuitry, bias circuitry and sensing circuitry are electrically connected to the electrodes.

9. The inertial sensor of claim 1, further comprising

a base structure having electrodes patterned on its surface, the base structure positioned adjacent the proof mass;

wherein the drive circuitry, bias circuitry and sensing circuitry are electrically connected to the electrodes;

and wherein the electrodes electrically connected to the sensing circuitry produce a fringing electric field for detecting movement of the proof mass.

10. The inertial sensor of claim 1, wherein:

the proof mass is a micromachined symmetric leaf structure substantially defined in a plane.

11. The inertial sensor of claim 10, further comprising:

a post rigidly attached through the defined center and substantially perpendicular to the plane of the symmetric leaf structure.

12. A method for measuring acceleration comprising the steps of:

exciting a resonant motion of a proof mass and spring structures by rocking the proof mass about a first rocking axis passing through a defined center;

supplying a voltage to modify the resonant motion to compensate for a mass imbalance of the proof mass;

measuring acceleration by detecting a sinusoidal output representative of a change of the resonant motion; and

outputting a signal indicating the acceleration.

13. The method for measuring acceleration of claim 12, further comprising the step of:

supplying a bias voltage to compensate for a mass imbalance of the proof mass so that the bias voltage exerts a force on a portion of the proof mass to rotate first and second substantially orthogonal rocking axes into alignment with substantially orthogonal first and second spring axes passing through the spring structures.

14. The method for measuring acceleration of claim 13, further comprising the steps of:

generating a first signal from the rocking motion of the proof mass about the first spring axis;

generating a second signal from the rocking motion of the proof mass about the second spring axis, wherein the second signal indicates the change of the resonant motion;

providing the first signal to rock the proof mass about the first spring axis; and

providing the second signal to dampen the rocking motion of the proof mass about the second spring axis.

15. The method for measuring acceleration of claim 14, further comprising the step of:

measuring the acceleration by determining the rotation of the first and second rocking axes away from the first and second spring axes.

16. The method for measuring acceleration of claim 15, further comprising the step of:

demodulating the first and second signals to output a signal indicating the acceleration.

17. The method for measuring acceleration of claim 14, further comprising the steps of:

rigidly attaching a mass to the defined center;

demodulating the first and second signals in quadrature to generate a signal indicating rotation of the proof mass; and

demodulating the first and second signals in phase to generate a signal indicating the acceleration.

18. The method for measuring acceleration of claim 12, further comprising the step of:

capacitively interacting with the proof mass with a base structure having eight electrodes patterned on its surface and positioned adjacent the proof mass.

19. The method for measuring acceleration of claim 12, further comprising the step of:

capacitively interacting with the proof mass with a base structure having four electrodes patterned on its surface and positioned adjacent the proof mass.

20. The method for measuring acceleration of claim 12, further comprising the steps of:

capacitively interacting with the proof mass with a base structure having electrodes patterned on its surface and positioned adjacent the proof mass; and

detecting movement of the proof mass utilizing a fringing electric field produced by at least two of the electrodes.

21. The method for measuring acceleration of claim 12, wherein:

the proof mass is a micromachined symmetric leaf structure substantially defined in a plane.

22. The method for measuring acceleration of claim 21, further comprising the step of:

inserting a node at the defined center by rigidly attaching a post through the defined center and substantially perpendicular to the plane of the symmetric leaf structure.