Non-degenerate mode MEMS gyroscope

• US 8,794,068 B2
• Filed: 12/01/2011
• Issued: 08/05/2014
• Est. Priority Date: 12/01/2010
• Status: Active Grant

• Current Assignee: Analog Devices, Inc.
• Original Assignee: Analog Devices, Inc.
• Inventors: Judy, Michael W., Geen, John A., Johari-Galle, Houri
• Primary Examiner(s): CHAPMAN JR, JOHN E
• Application Number:
  US13/309,511
• Publication Number:
  US 20120137774A1
• Time in Patent Office: 978 Days
• Field of Search: 735/040.1, 735/041.2, 735/041.3
• US Class Current: 73/504.12
• CPC Class Codes: G01C 19/5698 using acoustic waves, e.g. surface acoustic wave gyros

Abstract

Bulk acoustic wave (BAW) gyroscopes purposefully operate using non-degenerate modes, i.e., resonant frequencies of drive and sense modes are controlled so they are not identical. The resonant frequencies differ by a small controlled amount (Δf). The difference (Δf) is selected such that the loss of sensitivity, as a result of using non-degenerate modes, is modest. Non-degenerate operation can yield better bandwidth and improves signal-to-noise ratio (SNR) over comparable degenerate mode operation. Increasing Q of a BAW resonator facilitates trading bandwidth for increased SNR, thereby providing a combination of bandwidth and SNR that is better than that achievable from degenerate mode devices. In addition, a split electrode configuration facilitates minimizing quadrature errors in BAW resonators.

Claims

1. A microelectromechanical systems (MEMS) gyroscope, comprising:

a substrate;

a primary member attached to the substrate and configured to vibrate in a first bulk acoustic mode at a drive frequency in response to a varying electrostatic signal and to vibrate in a second bulk acoustic mode at the drive frequency in response to the primary member being rotated about an axis, wherein the first bulk acoustic mode and the second bulk acoustic mode are non-degenerate;

a circuit coupled to the primary member and configured to maintain a difference between the first bulk acoustic mode and the second bulk acoustic mode.

2. A microelectromechanical systems (MEMS) gyroscope according to claim 1, wherein:

the circuit comprises;

a drive circuit configured to generate the varying electrostatic signal; and

a servo circuit coupled to the drive circuit;

the second bulk acoustic mode is characterized by a resonant frequency different than the drive frequency; and

the servo is configured to sense phase response of the second bulk acoustic mode and to automatically adjust, based on the sensed phase response, operation of the drive circuit, so as to maintain the difference between the resonant frequency and the drive frequency.

3. A microelectromechanical systems (MEMS) gyroscope according to claim 1, wherein the second bulk acoustic mode is characterized by a resonant frequency, and the drive frequency differs from the resonant frequency of the second bulk acoustic mode by Δ

f, where Δ

f is between about f₀/24Q and about 6f₀/Q, and where f₀ is the drive frequency.

4. A microelectromechanical systems (MEMS) gyroscope according to claim 1, wherein the second bulk acoustic mode is characterized by a resonant frequency, and the drive frequency differs from the resonant frequency of the second bulk acoustic mode by Δ

f, where Δ

f is between about f₀/12Q and about 3f₀/Q, and where f₀ is the drive frequency.

5. A microelectromechanical systems (MEMS) gyroscope according to claim 1, wherein the second bulk acoustic mode is characterized by a resonant frequency, and the drive frequency differs from the resonant frequency of the second bulk acoustic mode by Δ

f, where Δ

f is about f₀/2Q, and where f₀ is the drive frequency.

6. A microelectromechanical systems (MEMS) gyroscope according to claim 1, wherein the second bulk acoustic mode is characterized by a resonant frequency, and the drive frequency differs from the resonant frequency of the second bulk acoustic mode by Δ

f, where Δ

f is greater than 0 Hz and up to about 10 KHz.

7. A microelectromechanical systems (MEMS) gyroscope according to claim 1, wherein the second bulk acoustic mode is characterized by a resonant frequency, and the drive frequency differs from the resonant frequency of the second bulk acoustic mode by Δ

f, where Δ

f corresponds to a second bulk acoustic mode displacement phase angle of between about −

5° and

about −

85°

.

8. A microelectromechanical systems (MEMS) gyroscope according to claim 1, wherein the second bulk acoustic mode is characterized by a resonant frequency, and the drive frequency differs from the resonant frequency of the second bulk acoustic mode by Δ

f, where Δ

f corresponds to a second bulk acoustic mode displacement phase angle of between about −

20° and

about −

60°

.

9. A microelectromechanical systems (MEMS) gyroscope according to claim 1, wherein the second bulk acoustic mode is characterized by a resonant frequency, and the drive frequency differs from the resonant frequency of the second bulk acoustic mode by Δ

f, where Δ

f corresponds to a second bulk acoustic mode displacement phase angle of about −

45°

.

10. A microelectromechanical systems (MEMS) gyroscope according to claim 1, wherein:

the circuit comprises a drive circuit configured to generate the varying electrostatic signal;

the microelectromechanical systems (MEMS) gyroscope further comprising;

at least one split drive electrode having a surface counterfacing a side of the primary member and coupled to the drive circuit to receive the varying electrostatic signal, wherein each of the at least one split electrode comprises a first sub-electrode and a second-sub-electrode, the counterfacing surface being approximately equally divided between the first and second sub-electrodes, the first sub-electrode spaced apart from the second sub-electrode by distance less than about 0.1 times a width of the surface of the split electrode.

11. A microelectromechanical systems (MEMS) gyroscope according to claim 1, further comprising:

a split electrode disposed proximate a desired location of an anti-node of the first bulk acoustic mode, the split electrode comprising at least two electrodes disposed symmetrically about the desired location of the anti-node;

a first gain-adjustable amplifier, an input of which is coupled to one of the two electrodes, a second gain-adjustable amplifier, an input of which is couple to the other of the two electrodes, and a summer coupled to outputs of the first and second gain-adjustable amplifiers; and

a servo circuit configured to automatically adjust gains of the first and second gain-adjustable amplifiers, based on at least one phase-shifted reference signal, so as to reduce a quadrature error.

12. A microelectromechanical systems (MEMS) gyroscope according to claim 1, further comprising:

a split electrode disposed proximate a desired location of an anti-node of the first bulk acoustic mode, the split electrode comprising at least two electrodes disposed symmetrically about the desired location of the anti-node;

a first gain-adjustable amplifier, an input of which is coupled to one of the two electrodes, a second gain-adjustable amplifier, an input of which is couple to the other of the two electrodes, and a summer coupled to outputs of the first and second gain-adjustable amplifiers; and

a servo circuit configured to automatically adjust gains of the first and second gain-adjustable amplifiers, based on at least one phase-shifted reference signal, so as to maximize output of the summer, in response to application of a signal to the bulk acoustic resonator.

13. A microelectromechanical systems (MEMS) gyroscope according to claim 1, further comprising:

a split electrode disposed proximate a desired location of an anti-node of the first bulk acoustic mode, the split electrode comprising at least two electrodes disposed symmetrically about the desired location of the anti-node;

wherein the circuit comprises;

a drive circuit configured to generate the varying electrostatic signal, coupled to the at least two electrodes and configured to differentially drive the at least two electrodes; and

a servo circuit coupled to the drive circuit and configured to automatically adjust how the drive circuit differentially drives the at least two electrodes, based on at least one phase-shifted reference so as to reduce a quadrature error.

14. A microelectromechanical systems (MEMS) gyroscope according to claim 1, wherein the circuit comprises:

a drive circuit configured to generate the varying electrostatic signal; and

a servo circuit coupled to the drive circuit and configured to maintain the difference between the first bulk acoustic mode and the second bulk acoustic mode.

15. A microelectromechanical systems (MEMS) gyroscope, comprising:

a bulk acoustic resonator;

at least one first electrode coupled to the bulk acoustic resonator, the at least one first electrode being positioned to excite vibration of the bulk acoustic resonator in a first bulk acoustic wave mode;

at least one second electrode coupled to the bulk acoustic resonator, the at least one second electrode being positioned to detect vibration of the bulk acoustic resonator in a second bulk acoustic mode, the first and second bulk acoustic wave modes being non-degenerate;

a drive circuit configured to deliver a drive signal to the at least one first electrode; and

a servo circuit coupled to the drive circuit and configured to adjust operation of the drive circuit to maintain a difference between the first bulk acoustic wave mode and the second bulk acoustic mode.

16. A method for sensing angular rotation, the method comprising:

applying a varying electrostatic signal to a primary member to cause the primary member to vibrate in a first bulk acoustic mode at a drive frequency;

rotating the primary member;

sensing a second bulk acoustic mode vibration of the primary member at the drive frequency, the first bulk acoustic mode and the second bulk acoustic mode being non-degenerate; and

maintaining a difference between the first bulk acoustic mode and the second bulk acoustic mode.

17. A method according to claim 16, wherein:

the second bulk acoustic mode is characterized by a resonant frequency different than the drive frequency;

the method further comprising;

sensing phase response of the second bulk acoustic mode; and

based on the sensed phase response, automatically maintaining the difference between the resonant frequency and the drive frequency.

18. A method according to claim 16, wherein the second bulk acoustic mode is characterized by a resonant frequency, and the drive frequency differs from the resonant frequency of the second bulk acoustic mode by Δ

f, where Δ

f is between about f₀/24Q and about 6f₀/Q, and where f₀ is the drive frequency.

19. A method according to claim 16, wherein the second bulk acoustic mode is characterized by a resonant frequency, and the drive frequency differs from the resonant frequency of the second bulk acoustic mode by Δ

f, where Δ

f is between about f₀/12Q and about 3f₀/Q, and where f₀ is the drive frequency.

20. A method according to claim 16, wherein the second bulk acoustic mode is characterized by a resonant frequency, and the drive frequency differs from the resonant frequency of the second bulk acoustic mode by Δ

f, where Δ

f is about f₀/2Q, and where f₀ is the drive frequency.

21. A method according to claim 16, wherein the second bulk acoustic mode is characterized by a resonant frequency, and the drive frequency differs from the resonant frequency of the second bulk acoustic mode by Δ

f, where Δ

f is greater than 0 Hz and up to about 10 KHz.

22. A method according to claim 16, wherein the second bulk acoustic mode is characterized by a resonant frequency, and the drive frequency differs from the resonant frequency of the second bulk acoustic mode by Δ

f, where Δ

f corresponds to a second bulk acoustic mode displacement phase angle of between about −

5° and

about −

85°

.

23. A method according to claim 16, wherein the second bulk acoustic mode is characterized by a resonant frequency, and the drive frequency differs from the resonant frequency of the second bulk acoustic mode by Δ

f, where Δ

f corresponds to a second bulk acoustic mode displacement phase angle of between about −

20° and

about −

60°

.

24. A method according to claim 16, wherein the second bulk acoustic mode is characterized by a resonant frequency, and the drive frequency differs from the resonant frequency of the second bulk acoustic mode by Δ

f, where Δ

f corresponds to a second bulk acoustic mode displacement phase angle of about −

45°

.