WO2002010678A1 - Angular sensor and drive feedback for adjusting error - Google Patents

Angular sensor and drive feedback for adjusting error Download PDF

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Publication number
WO2002010678A1
WO2002010678A1 PCT/US2001/041490 US0141490W WO0210678A1 WO 2002010678 A1 WO2002010678 A1 WO 2002010678A1 US 0141490 W US0141490 W US 0141490W WO 0210678 A1 WO0210678 A1 WO 0210678A1
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drive
signal
sensor
ofthe
electrodes
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PCT/US2001/041490
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French (fr)
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WO2002010678A8 (en
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Paul A. Ward
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The Charles Stark Draper Laboratory, Inc.
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Priority to AU2001285387A priority Critical patent/AU2001285387A1/en
Publication of WO2002010678A1 publication Critical patent/WO2002010678A1/en
Publication of WO2002010678A8 publication Critical patent/WO2002010678A8/en

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    • GPHYSICS
    • G01MEASURING; TESTING
    • G01CMEASURING DISTANCES, LEVELS OR BEARINGS; SURVEYING; NAVIGATION; GYROSCOPIC INSTRUMENTS; PHOTOGRAMMETRY OR VIDEOGRAMMETRY
    • G01C19/00Gyroscopes; Turn-sensitive devices using vibrating masses; Turn-sensitive devices without moving masses; Measuring angular rate using gyroscopic effects
    • G01C19/56Turn-sensitive devices using vibrating masses, e.g. vibratory angular rate sensors based on Coriolis forces
    • G01C19/5719Turn-sensitive devices using vibrating masses, e.g. vibratory angular rate sensors based on Coriolis forces using planar vibrating masses driven in a translation vibration along an axis

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  • Physics & Mathematics (AREA)
  • Engineering & Computer Science (AREA)
  • General Physics & Mathematics (AREA)
  • Radar, Positioning & Navigation (AREA)
  • Remote Sensing (AREA)
  • Gyroscopes (AREA)

Abstract

The present invention relates to a system (36, 38) for nulling drive feedthrough error in a sensor (10) having first and second drive electrodes (15, 16) which impart vibratory motion to first and second proof masses (20, 22) in response to first and second opposite phase drive signals (V1, V2), and having first and second capacitances defined between the drive electrodes (14, 16) and their associated proof masses (20, 22). 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 (V1, V2), whereby the ration of the amplitudes is proportional to the ratio of the first and second capacitances. A servo loop may adaptively effect the ratio of the amplitudes.

Description

Angular Sensor and Drive Feedback for Adjusting Error
Field of the Invention
This invention relates generally to feedthrough nulling systems, and more particularly to systems for nulling drive signal feedthrough in sensors.
Background of the Invention
Coriolis force sensors such as tuning-fork gyroscopes are known in the art. These sensors can be fabricated using MEMS (microelectromechanical systems) techniques, by way of example. Micromachined Coriolis force sensors generally include a pair of proof masses which are positioned on opposite sides of an input axis, and which are connected to a substrate by support flexures. The flexures support the proof masses so as to allow the proof masses to vibrate in a nominal plane that includes the input axis. Time varying drive signals are provided by way of drive electrodes to impart an in-plane, anti-parallel vibratory motion to the proof masses. Typically, the frequency of the drive signals is set at or near the mechanical resonant frequency of the sensor assembly.
Position sensitive in-plane pick-off electrodes are usually provided for sensing in-plane displacements of the proof masses caused by the vibratory motion. The pick-off electrodes are coupled to a feedback gain control circuit, which controls the amplitude of the drive signals, and thus the amplitude of vibration of the proof masses. For stable and predictable gyroscope performance, the amplitude of vibration of the proof masses is preferably maintained at a predetermined, constant level.
Upon rotation of the sensor assembly about the input axis, the vibrating proof masses produce anti-parallel, out-of-plane deflections, caused by Coriolis forces which arise in response to the rotation. Out-of-plane sensing electrodes are provided in order to detect the out-of-plane deflections. Due to the nature of the Coriolis force, whose magnitude depends on the rotational rate of the sensor assembly, the amplitudes of the Coriolis deflections are proportional to the rotational rate of the sensor about the input axis. The sensing electrodes thus generate a signal indicative of the rotational rate of the gyroscope. The performance of sensors such as the tuning fork gyroscope discussed above is adversely affected by a feedthrough of the drive signals into the sensor output, resulting from a coupling of signals from the drive electrodes into the out-of-plane sensing electrodes. Drive feedthrough affects measurements by both the out-of-plane sensing electrodes and the in-plane pick-off electrodes, thus limiting gyroscope performance. It is an object of this invention to provide a system for significantly reducing drive feedthrough, thereby improving sensor performance. Summary of the Invention
The present invention relates to a system for nulling drive feedthrough error in sensors such as tuning fork gyroscopes. By reducing the undesirable effects of drive feedthrough, the present invention reduces one of the largest electronic sources of error in tuning fork gyroscopes. Further, the technique of the present invention has general applicability, and can be used to null other forms of drive feedthrough.
In one embodiment, the present invention relates to a system for nulling 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 capacitances defined between the drive electrodes and their associated proof masses. The present invention is predicated in part on the recognition that capacitance mismatch between the drive electrodes and the associated proof masses is a dominant cause of drive feedthrough in Coriolis force sensors.
In accordance with the present invention, a differential drive amplitude is adjusted in order to compensate for drive capacitance mismatch. Compensation is accomplished by measuring the capacitance mismatch between the first and second drive electrodes and their associated proof masses, and nulling the measured capacitance mismatch by adjusting the drive signals. The capacitance mismatch can be measured by detecting signals that are proportional to the capacitance mismatch. In one embodiment, an off-frequency artifact is detected in order to obtain a measure of the drive capacitance mismatch.
In accordance with the present invention, a sensor for detecting a rotational rate about an input axis includes a substrate, and first and second proof masses positioned on opposite sides of an input axis. The proof masses are flexurally coupled to the substrate so as to permit an in- plane vibratory motion of the masses in a nominal plane including the input axis, and an out-of- plane vibratory motion in a direction perpendicular to the nominal plane. Each proof mass includes a electronically conductive region.
A driver includes first and second drive electrodes opposite the electrically conductive regions of the first and second proof masses. First and second capacitances are defined between the first drive electrode and the first proof mass, and between the second drive electrode and the second proof mass, respectively. The first and second drive electrodes are adapted to receive first and second a.c. opposite phase electrical drive signals for electrostatically driving the first and second proof masses to effect the anti-parallel vibration in the nominal plane. The frequency of the drive signals is preferably set to be substantially equal to a fundamental frequency corresponding to a mechanical resonant frequency of the sensor. First and second sensing electrodes are provided for detecting the out of plane vibratory motion perpendicular to the nominal plane, arising from the Coriolis force that acts upon the proof masses when the sensor assembly rotates about the input axis. Each sensing electrode is fixedly positioned with respect to the substrate, and is disposed opposite the conductive region on a respective one ofthe first and second proof masses. The sensor is preferably micromachined, i.e. the proof masses, the drive electrodes, and the sensing electrodes are made from an etched silicon structure.
A circuit generates an output signal corresponding to the separation between the sensing electrodes and the conductive regions ofthe proof masses opposite. The output signal is representative ofthe rotational rate ofthe sensor about the input axis. Means are provided for adjusting the relative amplitudes ofthe first and second a.c. drive signals, whereby the ratio of the amplitudes is proportional to the ratio ofthe first and second capacitances. The adjustment of relative amplitudes ofthe drive signals substantially nulls drive feedthrough in the output signal caused by a mismatch in the first and second capacitances. In particular, when the drive signals have opposite polarities, and have a magnitude ratio equal to the reciprocal ofthe ratio of the corresponding capacitances, drive feedthrough from drive capacitance mismatch is substantially nulled.
The circuit may include first and second circuits. The first circuit is operative to provide an indicative signal proportional to a capacitance mismatch between the first and second capacitances. The second circuit is operative to measure the capacitance mismatch using the indicative signal, and to compensate for the measured capacitance mismatch by adjusting the first and second drive signals so as to substantially null drive feedthrough.
The first circuit may include a sense preamplifier connected to the proof masses. The output from the sense preamplifier contains a signal that is indicative ofthe mismatch between the first and second capacitances. The indicative signal can be demodulated in the second circuit to obtain a measure ofthe drive capacitance mismatch. The indicative signal may be a separate tracer signal that has been added to the drive signals. Alternatively, the indicative signal may be an inherent off-frequency component ofthe drive signal, in which the sensor resonant frequency and its harmonics have been suppressed. The second circuit may include an adjustment device responsive to a controller for effecting the ratio of amplitudes, and a servo loop for adaptively effecting the ratio of amplitudes, h one embodiment, the adjustment device includes first and second variable gain amplifiers (VGAs) interposed between the servo loop and the first and second drive electrodes. The VGAs are operative to adjust the differential amplitude ofthe drive signals in response to the controller.
The second circuit may include means for demodulating the indicative signal from the first circuit so as to obtain a DC signal proportional to the capacitance mismatch. In one embodiment, the means for demodulating the indicative signal may include a multiplier, a low pass filter for removing modulation components from the indicative signal, and a controller. The demodulated signal may be passed through the controller to provide a control signal. The control signal may be split into first and second control signals of opposite phase, which control the first and second VGAs, respectively.
The present invention also relates to 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 first and second drive signals, and having first and second capacitances defined between the first and second drive electrodes and their associated proof masses. A mismatch between the first capacitance and the second capacitance is measured. Drive feedthrough caused by the mismatch is nulled by compensating for the measured capacitance mismatch. The relative amplitudes ofthe first and second drive signals is adjusted in order to compensate for the measured capacitance mismatch.
Brief Description of the Drawing
Other features and advantages ofthe present invention will be apparent from the following detailed description ofthe drawing in which:
FIG. 1 is a perspective view of a prior art Coriolis sensor.
FIG.s 2(a) and 2(b) illustrate a drive coupling model for the sensor of FIG. 1.
FIG. 3 is a diagram of an automatic drive feedthrough nulling system, in accordance with the present invention, for the sensor of FIG. 1.
Detailed Description of the Preferred Embodiment
FIG. 1 illustrates a prior art sensor 10, such as a micromechanical tuning-fork gyroscope, for detecting a rotational rate about an input axis 11 ofthe sensor 10. The sensor 10 includes a substrate 32, and first 20 and second 22 proof masses positioned on opposite sides ofthe input axis 11. The proof masses 20 and 22 are flexurally coupled to the substrate 32 by means of support flexures 30. In one embodiment, the proof masses 20 and 22 are suspended by the flexures 30 above the substrate 32, so as to permit vibratory motion ofthe proof masses in-plane in a nominal plane including the input axis, as well as out-of-plane in a direction perpendicular to the nominal plane. The nominal plane may be parallel to the surface ofthe substrate 32. Each proof mass includes an electrically conductive region, shown as 21a and 21b, respectively. The prior art sensor 10 includes a driver having first 14 and second 16 drive electrodes disposed opposite the electrically conductive regions 21a, 21b ofthe first 20 and second 22 proof masses. The sensor 10 also includes first 24 and second 26 out-of-plane sensing electrodes, and an in-plane position-sensitive pickoff electrode 28. In one embodiment, the in-plane pickoff electrode 28 is also suspended above the substrate 32 by the flexures 30, while the out-of-plane sensing electrodes 24 and 26 are disposed on the substrate 32.
In one embodiment, each proof mass has comb electrodes 18 extending laterally therefrom and toward an adjacent one ofthe drive electrodes. The drive electrodes 14, 16 have complementary comb electrodes 18 extending toward, and interleaved with, the comb electrodes 18 of the adjacent one ofthe proof masses 20 and 22. The drive electrodes 14 and 16 are electrostatically coupled to the proof masses 20 and 22, respectively, by the sets of interleaved comb electrodes 18. First and second capacitances Ci and C are thus defined between the first 14 and second 16 drive electrodes, and the first 20 and second 22 proof masses, respectively.
The first and second drive electrodes 14 and 16 are adapted to receive first and second a.c. opposite phase electrical drive signals which induce electrostatic forces between the sets of interleaved comb electrodes 18 between electrode 14 and proof mass 20 on the one hand and electrode 16 and proof mass 22 on the other. The drive signals typically are square waves. The drive electrodes convert the drive signal into an electrostatic Coulomb force signal proportional to the square ofthe drive signal, thereby electrostatically driving the proof masses to impart in- plane vibration to the proof masses in the nominal plane (in the directions shown by arrows. 15).
In response to a rotation ofthe sensor 10 about the input axis 11, the proof masses deflect out ofthe plane of vibration. Such out-of-plane deflections ofthe proof masses 20, 22 occur because ofthe Coriolis forces that arise from the rotation around the input axis and act on the moving proof masses. The Coriolis force is given by:
F = -2mΩ x V Eq. 1
where m = mass ofthe proof mass, Ω == angular velocity of the proof mass about the input axis,
V = velocity of in-plane vibration ofthe proof mass, and x denotes the vector cross-product.
Since the in-plane vibrations occur at a drive frequency, from Eq. 1 it follows that the out-of- plane deflections ofthe proof masses also occur at the drive frequency. The drive frequency is typically set to be substantially equal to the mechanical resonant frequency ofthe sensor, generally referred to as the fundamental frequency.
In order to maintain stable gyroscope performance, the vibratory amplitudes ofthe proof masses 20, 22 should be kept relatively constant. This is accomplished by monitoring the in- plane proof mass deflection with the position sensitive pick-off electrode 28. The pick-off electrode 28 produces an output signal which is indicative ofthe amplitude ofthe in-plane motion ofthe proof masses. The output signal from the pick-off electrode 28 is fed to a feedback gain control loop to control the amplitudes ofthe drive signals, and thus the amplitudes ofthe vibratory motion ofthe proof masses. The detection of in-plane deflection is susceptible, however, to errors from drive feedthrough.
First and second sensing electrodes 24, 26 are disposed on the substrate 32 for detecting the anti-parallel, out-of-plane Coriolis deflections ofthe proof masses 20 and 22. The amplitudes ofthe out-of-plane deflections are proportional to the input rotational rate, according to Eq. 1. The sensor output signals therefore provide a measure ofthe rotational rate Ω . In one embodiment, capacitive sensing is used. In this embodiment, each out-of-plane sensing electrode is fixedly positioned with respect to the substrate 32, and is disposed below a conductive region (21a or 21b) of a respective one ofthe first 20 and second 22 proof masses. The capacitances between the sensing electrodes 24, 26 and the proof masses 20, 22 thus oscillate in accordance with the amplitudes ofthe out-of-plane Coriolis deflections. The sensing electrodes generate output signals that are proportional to the separation between the conductive regions 21a, 21b, and the associated sensing electrodes 24, 26, and hence are proportional to the amplitudes ofthe Coriolis deflections ofthe proof masses. The sensor output signals thus reflect the changes in the capacitances between the sensing electrodes and their associated proof masses, and vary according to the frequency ofthe out-of-plane oscillations. A DC bias voltage V is applied to the sensing electrodes 24, 26 to establish a potential difference. Because ofthe capacitances between the conductive regions 21a, 21b and the associated sensing electrodes, the sensing electrodes 24, 26 induce on each proof mass a charge that is proportional to the separation between the proof mass and its associated sense electrode. The changes in capacitance between the proof masses 20, 22 and their associated sensing electrodes 24, 26 thus result in changes in the charge on the proof masses 20, and 22, which are reflected in the sensor output signals.
The detection ofthe out-of-plane deflections is susceptible, however, to errors resulting from drive feedthrough into the sensor output signal. In particular, the sensor output signal is contaminated by capacitive coupling from the drive electrodes. Such capacitive coupling is caused by parasitic capacitances related to the fabrication and packaging ofthe sensor system. Drive feedthrough includes synchronous and asynchronous components. Synchronous drive feedthrough includes signals at the fundamental frequency and its harmonics. Asynchronous drive feedthrough includes all other signals originating at the drive electrodes. Synchronous feedthrough causes a direct instrument bias. Asynchronous feedthrough imposes gain limitations and limits desensitization ofthe sensor to DC offsets.
An off-frequency drive scheme for eliminating synchronous feedthrough is disclosed in prior art U.S. Pat. Nos. 5,481,914 and 5,703,292, both of which are incorporated herein by reference. In this scheme, the sensor 10 is connected in a feedback relationship with a frequency translation circuit. The input drive signal is converted into a feedback signal which is further processed to control the drive signal in a closed loop. The frequency translation circuit suppresses components ofthe feedback signal at the resonant frequency ofthe sensor, by effectively multiplying the feedback signal by a commutation signal. Any coupling ofthe drive signal into the sensor output signal is then readily removed by using conventional filtering techniques, since the drive signal now has a frequency different from the frequency ofthe sensor output signal. The frequency translation circuit thus substantially reduces synchronous feedthrough, i.e. in-band coupling ofthe drive signal into the feedback signal and into the sensor output signal. The off-frequency drive scheme summarized above still does not remove, however, asynchronous feedthrough. The present invention provides a system for substantially reducing both synchronous and asynchronous feedthrough. It has been found that the dominant cause of drive feedthrough is a mismatch in the capacitances and C2 between the drive electrodes 14, 16 and their associated proof masses 20, 22. The difference between Ci and C2, which should ideally be zero, is the main cause of drive feedthrough. The present invention compensates for variations in C1 and C2 by varying the drive voltages V\ and V2 so as to equalize the magnitudes ofthe charges Q and Q2 induced on the first and second proof masses by way ofthe drive voltages. The charges are related to the drive voltages by the relation Q=CV, where C is the associated capacitance, C\ or C2. By balancing out the charges induced on the proof masses, i.e. by rendering the charges equal and opposite, feedthrough ofthe drive signals into the sensor output signals is nulled. In the present invention, compensation for capacitance mismatch is accomplished by measuring one or more ofthe drive feedthrough components, and then nulling the measured components by adjusting the relative amplitudes ofthe drive signals V1} V2j as explained further below.
FIG.s 2(a) and 2(b) provides a circuit diagram of a drive coupling model for the sensor discussed above. In FIG. 2(a), a sense preamplifier 31 is shown connected to the proof masses 20 and 22. The proof masses are placed at a virtual ground ofthe sense preamplifier 31. A first C\ and a second C2 capacitance is defined between each drive electrode and the summing node ofthe sense preamplifier 31.
Referring to FIG. 2(b), the preamplifier output V0 is given by:
v0 = -c- [vlcx - v2c1]+ ve c1 + c2 + cN λ
1 + Eq. 2 c.
where Ve is the preamplifier input error voltage, which is generally small unless the input changes very rapidly, and the voltages and capacitances are as shown in FIG. 2(b). If the preamplifier bandwidth is made arbitrarily large, the error term becomes arbitrarily small, and the preamplifier output is simplified, becoming:
V0 = -C/- [VlCl + V2C2] = -C/- [Q1 + Q2] Eq. 3
where Q and Q2 represent the charge transferred to the summing node by and C2, respectively. Drive feedthrough in the preamplifier output can thus be nulled by effecting a charge balance condition:
a =-β 2 Eq- 4
The charge balance condition can be alternatively expressed as:
V, C,
Eq. 5 v.
Therefore, if V] and V have opposite polarities and a magnitude ratio equal to the. reciprocal of the ratio ofthe corresponding capacitors, drive feedthrough fromstatic capacitance mismatch is nulled.
In the present invention, drive feedthrough can be measured and nulled in one of several embodiments. In order to measure drive feedthrough, a signal that is proportional to the drive capacitance mismatch must first be obtained. In one embodiment, a separate trace signal having a characteristic such as frequency different from the drive signals, can be obtained from a trace source and added to the drive signals. The trace signal is separately detected, and used to measure drive capacitance mismatch. In another embodiment in which the drive signal inherently contains components at off-frequencies, as in the off-frequency drive scheme discussed above, one or more ofthe inherent off-frequency components ofthe drive signal can be detected that are proportional to the capacitance mismatch. The detected off-frequency components are then demodulated in order to measure the capacitance mismatch. The measured capacitance mismatch is nulled by making a fixed adjustment to the differential drive voltages, or with an automatic feedback servo loop.
FIG. 3 illustrates one embodiment ofthe present invention in which an off- frequency artifact ofthe drive signal, i.e. an inherent off-frequency component of the drive signal, is used to measure the capacitance mismatch. A nulling compensator 36 is used in conjunction with a feedback servo loop 38, to compensate for the measured capacitance mismatch. The nulling compensator 36 demodulates the off-frequency artifact to provide a measure of capacitance mismatch. Based on the measured capacitance mismatch, the differential amplitude ofthe drive signals Vl5 V2 is modified so as to effect the charge balance condition of Eq. 5, thereby substantially reducing drive feedthrough into the sensor output signal.
The capacitance mismatch between the drive electrodes 14, 16 and their associated proof masses 20, 22 is measured by detecting an off-frequency artifact ofthe drive signals V1} V at the output ofthe sense preamplifier 31, and demodulating the detected artifact. In one embodiment ofthe present invention, the form ofthe drive signals is described by the following equation:
Figure imgf000010_0001
where ω is the fundamental frequency, and sq(ωt) is a zero-mean square-wave with amplitude = 1 volt and period 2π/ω. Because of mismatch in the time-average drive capacitance, a smaller version ofthe signal of Eq. 5 exists, along with other signals, at the preamplifier output. That is, the amplified preamplifier output is:
V0 it) » K{B + Asq(ωt))sq — + other signals, Eq. 7
where the value of K is given by:
Figure imgf000011_0001
in which GAC is the gain following the preamplification, and ΔCD is the mismatch in the time- average drive capacitors.
The signal of Eq. 6 includes a significant term at frequency ω/n. This term can be synchronously demodulated to yield a DC signal which is proportional to K, and thus proportional to the capacitance mismatch. The sense preamplifier output is multiplied by a square wave 93 of frequency ω/n, in order to eliminate even harmonics ofthe basic subharmonic frequency ω/n. The multiplied output is given by:
nitλ Vm (t) = K(B + Asq(ωt))sq2 — = K(β + Asq(ωi)) Eq. 9
The modulation components can be filtered out by passing the signal of Eq. 9 through the low pass filter 52. The result is:
VLPF {t) = KB= GAC ( ΔCD / Cf) B Eq. 10
which is the desired DC signal that provides a measure ofthe capacitance mismatch. The capacitance mismatch is thus given by:
ΔCD = ( Cf Vi t) / B GAc ) Eq. 11
In the illustrated embodiment, the feedthrough nulling compensator 36 is used in conjunction with a feedback gain control loop 38. The feedback gain control loop is operative to receive a signal 90 from the position sensitive pick off 28. The signal 90 is indicative ofthe amplitude ofthe in-plane displacement ofthe first 20 and the second 22 proof masses. The signal 90 is fed into a frequency translation circuit 48, which suppresses components at the fundamental frequency ofthe sensor 10. The frequency translation circuit outputs drive signals 92a and 92b for driving the proof masses ofthe sensor 10. In one embodiment, the feedback control loop 38 includes a motor preamplifier 42, a shifter 44, a summer 46, and the frequency translation circuit 48. In one embodiment, the signal
90 is fed to the motor preamplifier 42, the output of which is shifted by ninety degrees in the shifter 44. The output signal from the shifter 44 is summed in the summer 46 with an amplitude control voltage 91. The output signal 77 from the summer 46 is fed to the frequency translation circuit, and multiplied by a commutation clock signal 79 in order to suppress components in the signal 77 at the resonant frequency ofthe sensor 10. The output 99 from the frequency translation circuit 48 is then split into first and second opposite phase drive signals 92a and 92b for driving the proof masses 20 and 22.
The output from the LPF 52 (Eq. 10), which is proportional to the capacitance mismatch, is integrated in the integral controller 54. The output signal from the integrated controller 54 is split into two signals, one of which is inverted in the amplifier 56. The split signals provide opposite-phase control signals 94a and 94b, which are used to control the VGAs 58a and 58b respectively, thus closing the automatic feedback servo loop. The common-mode gain ofthe VGAs, controlled by the drive signals 92a and 92b from the frequency translation circuit 48, remains constant.
The differential gain ofthe VGAs 58a and 58b is adjusted by the control signals 94a and 94b according to the measured capacitance mismatch, h response to the control signals 94a and 94b, the VGAs 58a and 58b adjust the ratio ofthe amplitudes ofthe first and second drive signals to be equal to the reciprocal ofthe ratio ofthe first and second C2 capacitances. The charge balance condition of Eq. 4 and Eq. 5 is thus achieved, so that drive feedthrough in the sense preamplifier output is automatically and continuously nulled. The magnitude adjusted drive signals
Figure imgf000012_0001
and V2, shown in FIG. 3, are delivered to the drive electrodes 14 and 16 through the VGAs 58a and 58b.
It will be noted that the techniques described above have general applicability. Many forms of drive feedthrough can be nulled using the techniques described above. It should therefore be understood that the invention is not limited to the particular embodiment shown and described herein, and that various changes and modifications may be made without departing from the spirit and scope of this concept as defined by the following claims.

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 ofthe 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 ofthe 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 ofthe 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 ofthe 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.
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 ofthe sensor to the drive applied thereto; and (c) means, responsive to components ofthe further signal in the detected response, for adjusting the drive signal so as to diminish the feedthrough error.
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
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.
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 ofthe 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: fωX
V0 {f) « K(B + Asq(ωt))sq n J 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:
VLPF (ή = KB .
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 serving the first and second drive signals so as to adaptively effect an adjustment of respective drive signal amplitudes.
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US6621279B2 (en) 2003-09-16
US20030011385A1 (en) 2003-01-16

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