Drive feedthrough nulling system

• US 20030011385A1
• Filed: 08/30/2002
• Published: 01/16/2003
• Est. Priority Date: 08/02/2000
• Status: Active Grant

• Current Assignee: Paul Ward
• Original Assignee: Paul Ward
• Inventors: Ward, Paul
• Granted Patent: US 6,674,294 B2
• Time in Patent Office: Days
• US Class Current: 324/684
• CPC Class Codes: G01C 19/5719 using planar vibrating masses driven in a translation vibration along an axis

Abstract

The present invention relates to a system for nulling drive feedthrough error in a sensor having first and second drive electrodes which impart vibratory motion to first and second proof masses in response to first and second opposite phase drive signals, and having first and second capacitances defined between the drive electrodes and their associated proof masses. A mismatch between the first and the second capacitance is measured. Drive feedthrough caused by the measured capacitance mismatch is nulled by adjusting the relative amplitudes of the first and second opposite phase drive signals, whereby the ratio of the amplitudes is proportional to the ratio of the first and second capacitances. A servo loop may adaptively effect the ratio of amplitudes.

Claims

1. A sensor for detecting a rotational rate about an input axis of said sensor, comprising:

A. a substrate;

B. a first proof mass and a second proof mass positioned on opposite sides of an input axis, each of said first and second proof masses being flexurally coupled to said substrate so as to permit an in-plane vibratory motion in a nominal plane including said input axis and an out-of-plane vibratory motion in a direction perpendicular to said nominal plane, each of said proof masses including an electrically conductive region;

C. a driver including (i) a first drive electrode opposite said electrically conductive region of said first proof mass, (ii) a second drive electrode opposite said electrically conductive region of said second proof mass, said first and second drive electrodes being adapted to receive an associated one of first and second a.c. opposite phase electrical drive signals for electrostatically driving said first and second proof masses to effect anti-parallel vibration of said proof masses in said nominal plane, a first capacitance being defined between the first drive electrode and the first proof mass, and a second capacitance being defined between the second drive electrode and the second proof mass;

D. a first sensing electrode and a second sensing electrode for detecting said vibratory motion perpendicular to said nominal plane, each sensing electrode being fixedly positioned with respect to said substrate and disposed opposite to said electrically conductive region on a respective one of said first and second proof masses;

E. means for adjusting the relative amplitudes of said first and second a.c. drive signals whereby the ratio of said amplitudes is proportional to the ratio of said first and second capacitances; and

F. a circuit for generating an output signal representative of the separation between said sensing electrodes and said conductive regions of said proof masses opposite thereto, said signal being representative of said rotational rate about said input axis;

wherein said adjustment of relative amplitudes of said first and second a.c. drive signals substantially nulls drive feedthrough in said output signal caused by a mismatch in said first and second capacitances.

2. A sensor according to claim 1 further including a position sensitive pick-off electrode for detecting said in-plane vibratory motion, said in-plane sensing electrode being disposed between said first proof mass and said second proof mass.

3. A sensor according to claim 1 wherein the frequency of said drive signals is substantially equal to a fundamental frequency corresponding to a mechanical resonant frequency of the sensor.

4. A sensor according to claim 1 wherein said drive electrodes, said sensing electrodes, and said proof masses are made from an etched silicon structure.

5. A sensor according to claim 1 wherein said circuit includes an adjustment device responsive to a controller for effecting said ratio of amplitudes.

6. A sensor according to claim 1 wherein said circuit includes a servo loop for adaptively effecting said ratio of amplitudes.

7. A sensor according to claim 1 wherein said circuit comprises first and second circuits, and wherein:

(a) said first circuit is operative to provide a signal indicative of a capacitance mismatch between said first and second capacitances; and

(b) said second circuit is operative to measure said capacitance mismatch and to compensate for said measured capacitance mismatch by adjusting said first and second opposite phase drive signals, and to thereby substantially null drive feedthrough based on said measured capacitance mismatch.

8. A sensor according to claim 7 wherein said first circuit includes a sense preamplifier connected to the proof masses, said sense preamplifier having an output.

9. A sensor according to claim 7 wherein said indicative signal comprises a separate tracer signal added to said drive signals.

10. A sensor according to claim 7 wherein said second circuit comprises:

(a) an adjustment device responsive to a controller for effecting said ratio of amplitudes; and

(b) a servo loop for adaptively effecting said ratio of amplitudes

11. A sensor according to claim 7 wherein the frequency of said drive signals is substantially equal to a fundamental frequency corresponding to a mechanical resonant frequency of the sensor, and wherein said indicative signal comprises a off-frequency artifact in which components of said drive signals at said fundamental frequency have been suppressed.

12. A sensor according to claim 11 wherein said first circuit includes a sense preamplifier connected to the proof masses, said sense preamplifier having an output, and wherein said second circuit includes means for demodulating said off-frequency artifact.

13. A sensor according to claim 12 wherein said means for demodulating said off-frequency artifact comprises a multiplier connected to said output of said preamplifier, and a low pass filter for removing modulation components in said off-frequency artifact.

14. A sensor according to claim 10 wherein a control signal from the controller is operable to be split into first and second control signals of opposite phase.

15. A sensor according to claim 14 wherein said adjustment device includes first and second variable gain amplifiers interposed between said servo loop and said first and second drive electrodes, said variable gain amplifiers being operative to adjust the differential amplitude of the drive signals in response to said first and second control signals.

16. A sensor according to claim 1 wherein the first and second drive electrodes and the pick-off electrode include comb electrodes, further wherein the first proof mass has comb electrodes interleaved with the first drive electrode comb electrodes on a first side and comb electrodes interleaved with the pick-off comb electrodes on a second side, and wherein the second proof mass has comb electrodes interleaved with the second drive electrode comb electrodes on a first side and comb electrodes interleaved with the pick-off comb electrodes on a second side.

18. A system according to claim 17, wherein said means for adjusting the drive signal is responsive to a controller.

19. A system according to claim 18, wherein said means for adjusting the drive signal includes a feedback servo loop.

20. A system according to claim 17, wherein said sensor includes at least two drive electrodes having associated drive capacitances and being adapted to receive equal and opposite-phase drive signals.

21. A system according to claim 20, wherein said feedthrough error is caused at least in part by a mismatch between said at least two drive capacitances.

22. A system according to claim 21, wherein said further signal provides a measure of said capacitance mismatch

24. A method according to claim 23 wherein said measuring step includes adding a separate trace signal to the drive signals.

25. A method according to claim 23 wherein said measuring step includes demodulating an inherent off-frequency artifact of the drive signals.

26. A method according to claim 25 wherein the demodulating step includes multiplying an amplified preamplifier output by a square wave of frequency ω

/n to provide a first intermediate output, said amplified preamplifier output having the form;

27. A method according to claim 23 wherein said step of compensating for said capacitance mismatch includes adjusting a drive signal amplitude.

28. A method according to claim 27 wherein said adjusting step includes adjusting a differential drive signal amplitude in a system having first and second drive signals associated with the first and second drive electrodes, respectively.

29. A method according to claim 28 wherein said first and second drive signals are a.c. opposite phase signals.

30. A method according to claim 28 wherein the adjusting step includes fixing the first and second drive signals at respective predetermined levels.

31. A method according to claim 28 wherein the adjusting step includes servoing the first and second drive signals so as to adaptively effect an adjustment of respective drive signal amplitudes.

17. A system for nulling feedthrough error in a sensor, the system comprising:

(a) a source of a drive signal, and a further signal having characteristics different from the drive signal, the drive signal and the further signal being applied to drive the sensor, wherein the further signal can be detected separately from the drive signal;

(b) means for detecting a response of the sensor to the drive applied thereto; and

(c) means, responsive to components of the further signal in the detected response, for adjusting the drive signal so as to diminish the feedthrough error.

23. A method for nulling drive feedthrough in a sensor having first and second drive electrodes which impart vibratory motion to first and second members in response to drive signals, a first capacitance being defined between the first drive electrode and the first member, and a second capacitance being defined between the second drive electrode and the second member, the method comprising the steps of:

(a) measuring a mismatch between said first capacitance and said second capacitance; and

(b) compensating for said capacitance mismatch, thereby nulling drive feedthrough caused by said mismatch.