Drive feedthrough nulling system

• US 6,445,195 B1
• Filed: 08/02/2000
• Issued: 09/03/2002
• Est. Priority Date: 08/02/2000
• Status: Expired due to Term

• Current Assignee: Charles Stark Draper Laboratory Incorporated
• Original Assignee: Charles Stark Draper Laboratory Incorporated
• Inventors: Ward, Paul
• Primary Examiner(s): Le, N.
• Assistant Examiner(s): Nguyen, Vincent Q.
• Application Number:
  US09/631,249
• Time in Patent Office: 762 Days
• Field of Search: 735/141.6, 735/143.8, 735/040.3, 735/041.2, 735/041.4, 735/040.4, 735/041.6, 702/190, 324/684, 324/687, 324/690, 324/658
• 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, and including a system for nulling drive feedthrough error, 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 drive region;

C. a driver including (i) a first drive electrode opposite said electrically conductive drive region of said first proof mass, (ii) a second drive electrode opposite said electrically conductive drive 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 V₁ and V₂ 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 C₁ being defined between the first drive electrode and the first proof mass, and a second capacitance C₂ 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 a 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 V₁ and V₂ so as to equalize charges Q₁ and Q₂, wherein Q₁=C₁*V₁ and Q₂=C₂*V₂;

whereby the magnitude of the ratio of V₁ and V₂ is proportional to the magnitude of the ratio of C₁ and C₂; 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 V₁ and V₂, by equalizing Q₁ and Q₂, substantially nulls drive feedthrough in said output signal caused by a mismatch in said first and second capacitances C₁ and C₂.

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 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.

8. 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.

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

10. A sensor according to claim 8 wherein said indicative signal comprises a separate tracer signal added to said drive signals.

11. A sensor according to claim 8 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.

12. A sensor according to claim 11 wherein a control signal from the controller is operable to be split into first and second control signals of opposite phase.

13. A sensor according to claim 12 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.

14. A sensor according to claim 8 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.

15. A sensor according to claim 14 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.

16. A sensor according to claim 15 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.