Drive feedthrough nulling system

• US 6,621,279 B2
• Filed: 08/30/2002
• Issued: 09/16/2003
• Est. Priority Date: 08/02/2000
• Status: Active Grant

• 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:
  US10/232,277
• Publication Number:
  US 20030006784A1
• Time in Patent Office: 382 Days
• Field of Search: 735/141.6, 735/143.8, 735/040.3, 735/041.2, 735/041.4, 735/044
• 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 method 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 method for nulling error in the output of a sensor caused by feedthrough of one or more drive signals into said sensor output, said sensor including first and second drive electrodes which impart vibratory motion to first and second members in response to said drive signals, said sensor further including one or more sensing electrodes for sensing said vibratory motion and for generating output signals proportional to the amplitude of said vibratory motion, the method comprising the steps of:

(a) measuring a mismatch between a first capacitance and a second capacitance, wherein said first capacitance is defined between said first drive electrode and said first member, and wherein said second capacitance is defined between said second drive electrode and said second member; and

(b) compensating for said capacitance mismatch by preventing the feedthrough of said drive signals into said output, thereby nulling said sensor output error.

2. A method according to claim 1 wherein said measuring step includes adding a separate trace signal to the drive signals.

3. A method according to claim 1 wherein said measuring step includes demodulating an inherent off-frequency artifact of the drive signals.

4. A method according to claim 3 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;

$V_o<br><br>\left(t\right)\approx <br><br>K<br><br>\left(B+A<br><br>\text{\hspace{1em}<br><br>}<br><br>s<br><br>\text{\hspace{1em}<br><br>}<br><br>q<br><br>\left(\mathrm{\omega <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>t\right)\right)<br><br>s<br><br>\text{\hspace{1em}<br><br>}<br><br>q<br><br>\left(\frac{\mathrm{\omega <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>t}{n}\right),$

whereby K is proportional to said capacitance mismatch,

and further wherein the demodulating step further includes passing the first intermediate output through a low pass filter to remove modulation components and provide a signal that is proportional to the capacitance mismatch and that has the form;

5. A method according to claim 1 wherein said step of compensating for said capacitance mismatch includes adjusting a drive signal amplitude.

6. A method according to claim 5 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.

7. A method according to claim 6 wherein said first and second drive signals are a.c. opposite phase signals.

8. A method according to claim 6 wherein the adjusting step includes fixing the first and second drive signals at respective predetermined levels.

9. A method according to claim 6 wherein the adjusting step includes servoing the first and second drive signals so as to adaptively effect an adjustment of respective drive signal amplitudes.