U.S. patent application number 10/423456 was filed with the patent office on 2004-11-18 for electrostatic spring softening in redundant degree of freedom resonators.
This patent application is currently assigned to California Institute of Technology. Invention is credited to Challoner, A. Dorian, Hayworth, Ken J., Humphreys, Todd E., Shcheglov, Kirill V..
Application Number | 20040226370 10/423456 |
Document ID | / |
Family ID | 33422749 |
Filed Date | 2004-11-18 |
United States Patent
Application |
20040226370 |
Kind Code |
A1 |
Hayworth, Ken J. ; et
al. |
November 18, 2004 |
ELECTROSTATIC SPRING SOFTENING IN REDUNDANT DEGREE OF FREEDOM
RESONATORS
Abstract
The present invention discloses an isolated electrostatic biased
resonator gyroscope. The gyroscope includes an isolated resonator
having a first and a second differential vibration mode, a
baseplate supporting the isolated resonator, a plurality of
excitation affixed to the baseplate for exciting the first
differential vibration mode, a plurality of sensing electrodes
affixed to the baseplate for sensing movement of the gyroscope
through the second differential vibration mode and a plurality of
bias electrodes affixed to the baseplate for trimming isolation of
the resonator and substantially minimizing frequency split between
the first and second differential vibration modes. Typically, the
isolated resonator comprises a proof mass and a counterbalancing
plate with the bias electrodes disposed on the baseplate below.
Inventors: |
Hayworth, Ken J.;
(Northridge, CA) ; Shcheglov, Kirill V.; (Los
Angeles, CA) ; Humphreys, Todd E.; (Ogden, UT)
; Challoner, A. Dorian; (Manhattan Beach, CA) |
Correspondence
Address: |
PATENT VENTURE GROUP
1042 N. MOUNTAIN AVE., #333
UPLAND
CA
91786
US
|
Assignee: |
California Institute of
Technology
The Boeing Company
|
Family ID: |
33422749 |
Appl. No.: |
10/423456 |
Filed: |
April 25, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60375749 |
Apr 26, 2002 |
|
|
|
Current U.S.
Class: |
73/504.12 ;
74/5.6D |
Current CPC
Class: |
G01C 19/56 20130101;
B81B 3/0062 20130101; B81B 2201/0242 20130101; Y10T 74/1275
20150115 |
Class at
Publication: |
073/504.12 ;
074/005.60D |
International
Class: |
G01C 019/56; G01P
009/04 |
Goverment Interests
[0008] This invention was made with Government support awarded by
the National Aeronautics and Space Administration (NASA). The
Government has certain rights in this invention.
Claims
1. A resonator gyroscope, comprising: an isolated resonator having
a first and a second differential vibration mode; a baseplate
supporting the isolated resonator; a plurality of excitation
electrodes affixed to the baseplate for exciting the first
differential vibration mode; a plurality of sensing electrodes
affixed to the baseplate for sensing movement of the gyroscope
through the second differential vibration mode; and a plurality of
bias electrodes affixed to the baseplate for trimming isolation of
the resonator and substantially minimizing frequency split between
the first and second differential vibration modes.
2. The resonator gyroscope of claim 1, wherein the isolated
resonator provides redundant degrees of freedom in a structure
comprising: an inner resonant post; an outer resonant plate; the
baseplate supporting the isolated resonator; and baseplate
frame.
3. The resonator gyroscope of claim 1, further comprising a
mounting frame attached to the baseplate through one or more
isolation flexures to provide redundant degrees of freedom to the
isolated resonator.
4. The resonator gyroscope of claim 1, wherein the isolated
resonator comprises two bodies with transverse inertia symmetry and
centers of mass about a substantially coincident axis and the first
and second differential vibration modes are transverse to the
substantially coincident axis with substantially equal
frequencies.
5. The resonator gyroscope of claim 4, wherein the two bodies of
the isolated resonator comprise a proof mass and a counterbalancing
plate.
6. The resonator gyroscope of claim 5, wherein the sense and
excitation electrodes and at least one of the bias electrodes are
disposed to react with the counterbalancing plate.
7. The resonator gyroscope of claim 5, further comprising one or
more flexures joining the proof mass and the counterbalancing plate
and attached to the baseplate.
8. The resonator gyroscope of claim 7, wherein the one or more
flexures are arranged around a perimeter of the proof mass, each
with a first end attached to the counterbalancing plate and a
second end attached to the proof mass.
9. The resonator gyroscope of claim 5, wherein the proof mass
comprises a vertical portion attached to a support plate portion
elastically connected to the counterbalancing plate.
10. The resonator gyroscope of claim 9, wherein the vertical
portion is longer than required to isolate the resonator in the
absence of trimming isolation with the plurality of bias
electrodes.
11. The resonator gyroscope of claim 9, wherein the vertical
portion comprises a post attached to the support plate portion.
12. The resonator gyroscope of claim 9, wherein: a length of the
vertical portion is imbalanced; and spring softening compensates
for the imbalance.
13. The resonator gyroscope of claim 9, wherein at least one of the
bias electrodes reacts with the support plate portion.
14. A method of producing a resonator gyroscope, comprising:
providing an isolated resonator having a first and a second
differential vibration mode; supporting the isolated resonator with
a baseplate; affixing a plurality of excitation electrodes to the
baseplate for exciting the first differential vibration mode;
affixing a plurality of sensing electrodes to the baseplate for
sensing movement of the gyroscope through the second differential
vibration mode; and affixing a plurality of bias electrodes to the
baseplate for trimming isolation of the resonator and substantially
minimizing frequency split between the first and second
differential vibration modes.
15. The method of claim 14, wherein the isolated resonator provides
redundant degrees of freedom in a structure comprising: an inner
resonant post; an outer resonant plate; the baseplate supporting
the isolated resonator; and baseplate frame.
16. The method of claim 14, further comprising a mounting frame
attached to the baseplate through one or more isolation flexures to
provide redundant degrees of freedom to the isolated resonator.
17. The method of claim 14, wherein the isolated resonator comprise
two bodies with transverse inertia symmetry and centers of mass
about a substantially coincident axis and the first and second
differential vibration modes are transverse to the substantially
coincident axis with substantially equal frequencies.
18. The method of claim 17, wherein the two bodies of the isolated
resonator comprise a proof mass and a counterbalancing plate.
19. The method of claim 18, wherein the sense and excitation
electrodes and at least one of the bias electrodes are disposed to
react with the counterbalancing plate.
20. The method of claim 18, further comprising one or more flexures
joining the proof mass and the counterbalancing plate and attached
to the baseplate.
21. The method of claim 20, wherein the one or more flexures are
arranged around a perimeter of the proof mass, each with a first
end attached to the counterbalancing plate and a second end
attached to the proof mass.
22. The method of claim 18, wherein the proof mass comprises a
vertical portion attached to a support plate portion elastically
connected to the counterbalancing plate.
23. The method of claim 22, wherein the vertical portion is longer
than required to isolate the resonator in the absence of trimming
isolation with the plurality of bias electrodes.
24. The method of claim 22, wherein the vertical portion comprises
a post attached to the support plate portion.
25. The method of claim 22, wherein the resonator isolation is
tuned by changing a length of the vertical portion.
26. The method of claim 22, wherein at least one of the bias
electrodes reacts with the support plate portion.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit under 35 U.S.C.
.sctn.119 of the following co-pending U.S. provisional patent
application, which is incorporated by reference herein:
[0002] U.S. Provisional Patent Application No. 60/375,749, by Ken
Hayworth et al., filed Apr. 26, 2002, and entitled "ELECTROSTATIC
SPRING SOFTENING APPLIED TO ADJUSTING MECHANICAL MODAL FREQUENCIES
(AND AXES) AND MECHANICAL QUALITY (Q) FACTOR (AND DAMPENING AXES)
IN REDUNDANT DEGREE OF FREEDOM RESONATORS".
[0003] This application is related to the following co-pending U.S.
utility patent applications, which are all incorporated by
reference herein:
[0004] U.S. patent application Ser. No. 09/928,279, by A. Dorian
Challoner, filed Aug. 10, 2001, and entitled "ISOLATED RESONATOR
GYROSCOPE";
[0005] U.S. patent application Ser. No. 10/370,953 (Attorney Docket
No. PD-01-584), by A. Dorian Challoner et al., filed Feb. 20, 2003,
and entitled "ISOLATED RESONATOR GYROSCOPE WITH A DRIVE AND SENSE
PLATE";
[0006] U.S. patent application Ser. No. XX/XXX,XXX (Attorney Docket
No. PD-01-586), by A. Dorian Challoner et al., filed Apr. 10, 2003,
and entitled "ISOLATED RESONATOR GYROSCOPE WITH COMPACT FLEXURES";
and
[0007] U.S. patent application Ser. No. XX/XXX,XXX (Attorney Docket
No. PD-01-585), by A. Dorian Challoner et al., filed on this same
day herewith, and entitled "ISOLATED RESONATOR GYROSCOPE WITH
ISOLATION TRIMMING USING A SECONDARY ELEMENT".
BACKGROUND OF THE INVENTION
[0009] 1. Field of the Invention
[0010] The present invention relates to electrostatic spring
softening applied to mechanical resonators, and in particular to
improved microgyroscope resonators and their manufacture. More
particularly, this invention relates to producing and tuning
microgyroscopes, and particularly, isolated resonator
gyroscopes.
[0011] 2. Description of the Related Art
[0012] Gyroscopes are used to determine direction based upon the
sensed inertial reaction of a moving mass. In various forms they
are often employed as a critical sensor for vehicles such as
aircraft and spacecraft. They are generally useful for navigation
or whenever it is necessary to autonomously determine the
orientation of a free object. A number of recent technologies have
brought new forms of gyroscopes, including optical gyroscopes such
as laser gyroscopes and fiberoptic gyroscopes as well as vibratory
gyroscopes. Such vibratory gyroscopes generally operate using
electrostatic actuation and sensing, employing controlled
capacitance gaps between a moving resonator and a supporting
structure, e.g. a baseplate or package. These and other new
gyroscopes greatly widen the range of gyroscope applications
because they offer increasing performance levels at lower
costs.
[0013] Some prior vibratory gyroscopes with symmetric designs have
been produced, however their vibratory momentum is transferred
directly to their baseplates or packages. This coupling admits
external disturbances and energy loss indistinguishable from
inertial rate input and hence leads to sensing errors and drift.
Other planar tuning fork gyroscopes may achieve a degree of
isolation of the vibration from the baseplate, however these
gyroscopes lack the vibrational symmetry desirable for tuned
operation. One example of such a vibratory gyroscope may be found
in U.S. Pat. No. 5,894,090 to Tang et al. which describes a
symmetric cloverleaf vibratory gyroscope design and is hereby
incorporated by reference herein.
[0014] Recently, a variety of new resonator gyroscopes have been
developed which include a resonator that is isolated from the
baseplate or supporting structure. Some of these vibratory
gyroscopes include various post resonator gyroscopes such as
described in U.S. application Ser. Nos. 09/928,279, 10/370,953,
XX/XXX,XXX (Attorney Docket No. PD-01-586), by A. Dorian Challoner
et al., filed Apr. 10, 2003, and entitled "ISOLATED RESONATOR
GYROSCOPE WITH COMPACT FLEXURES" and XX/XXX,XXX (Attorney Docket
No. PD-01-585), by A. Dorian Challoner et al., filed on this same
day herewith, and entitled "ISOLATED RESONATOR GYROSCOPE WITH
ISOLATION TRIMMING USING A SECONDARY ELEMENT".
[0015] Vibration isolation using a low-frequency seismic support is
also known (e.g., U.S. Pat. No. 6,009,751, which is incorporated by
reference herein), however such increased isolation comes at the
expense of proportionately heavier seismic mass and/or lower
support frequency. Both effects are undesirable for compact
tactical inertial measurement unit (IMU) applications.
[0016] In the case of many of the recent resonator gyroscope which
use electrostatic excitation and sensing such as some of those
mentioned above, mechanical tuning of the gyroscope modes is often
necessary to obtain optimum performance, e.g. navigation grade
performance. Mechanical tuning, e.g. laser ablation or focused ion
beam machining, of such resonator gyroscopes is very expensive to
perform after the manufacturing process. In addition, it is
difficult to correct for perturbations which will result from the
gyroscope packaging as the mechanical tuning operations are
performed prior to final packaging. Such mechanical tuning cannot
be used to recalibrate the gyroscope in the field after final
packaging.
[0017] There is a need in the art for small gyroscopes with greatly
improved performance for navigation and spacecraft payload
pointing. Further, there is a need for such gyroscopes to be easily
tunable and capable of selective control of differential rocking
modes and balance control. Still further, there is a need for
techniques which allow tuning of such gyroscopes without machining.
Finally, there is also a need for gyroscopes and tuning techniques
that allow for recalibration in the field after final packaging.
These and other needs are met by the present invention.
SUMMARY OF THE INVENTION
[0018] The present invention discloses the use of electrostatic
spring softening to induce a balanced condition while
simultaneously minimizing frequency split. This invention has
application to various of vibratory Coriolis force sensing
gyroscopes using electrostatic driving and sensing, but
particularly those with isolated or balanced resonators. For
example, embodiments of the invention can be employed in various
isolated resonator gyroscopes such as described in U.S. application
Ser. Nos. 09/928,279, 10/370,953, XX/XXX,XXX (Attorney Docket No.
PD-01-586), by A. Dorian Challoner et al., filed Apr. 10, 2003, and
entitled "ISOLATED RESONATOR GYROSCOPE WITH COMPACT FLEXURES" and
XX/YXX,XXX (Attorney Docket No. PD-01-585), by A. Dorian Challoner
et al., filed XXXXXXXX, and entitled "ISOLATED RESONATOR GYROSCOPE
WITH ISOLATION TRIMMING USING A SECONDARY ELEMENT". More generally,
the invention can be used in any redundant degree of freedom
mechanical resonator in any application which may require one.
Typical embodiments of the invention include a balanced redundant
degrees-of-freedom resonator structure combined with electrostatic
spring softening to tune both mechanical modes (and axes) and
mechanical quality (Q) factor (and dampening axes).
[0019] One significant advantage of the present invention is that
it allows post-manufacture tuning using simple electronics on a
fully vacuum packaged gyroscope and can be performed in the field
as a calibration routine to compensate for aging, radiation damage
and other effects which result in a gradual decay in performance.
Thus, the invention can yield an affordable vibratory gyroscope
with navigation grade performance by means of a precision isolated
symmetric planar resonator of optimum scale that can be fabricated
with silicon photolithography from commercial double-side polished
silicon wafers with low total thickness variation.
[0020] In one exemplary embodiment, the present invention can be
employed with an isolated resonator comprising two bodies with
transverse inertia symmetry about an axis aligned with an input
axis and elastically supported so that their axes of symmetry and
centers of mass coincide and together form two differential rocking
modes of vibration transverse to the axis of symmetry. The two
bodies are supported on a baseplate having an inertial rate input
axis and exhibit substantially equal frequencies distinct from
other modes of vibration, mutually orthogonal and imparting
substantially zero net momentum to the baseplate. Excitation and
sense electrodes are disposed below the resonator on the supporting
baseplate structure to excite and sense movement of the
resonator.
[0021] In the exemplary embodiments which follow, a first one of
the bodies is a proof mass, a second one of the bodies is a
counterbalancing plate. The counterbalancing plate is for reacting
with the excitation and sense electrodes. However, other structures
and arrangements which can employ the inventive principle of
electrostatic spring softening will be apparent to those skilled in
the art.
[0022] In general, the excitation and sense electrodes are disposed
below the counterbalancing plate. The excitation electrodes are
aligned to drive a first one of the differential rocking modes to
vibrate. The sense electrodes are aligned to sense the motion of
the second differential rocking mode induced by Coriolis
accelerations resulting from the inertial rate input and internally
driven differential rocking motion about the first mode axis.
[0023] A key element of the present invention is the incorporation
of a plurality of bias electrodes which are used to tune isolation
of the resonator from the baseplate and minimize frequency split
between the excitation and sensing vibration modes. Typically, the
bias electrodes are disposed on the baseplate beneath the two
bodies of the resonator to provide a bias against it. The addition
of these bias electrodes permits selective control of the
differential rocking mode and electrostatic balance control. In
addition to isolated resonator gyros, the secondary element is also
applicable to other types of vibratory gyroscopes, such as
hemispherical resonator gyroscopes.
[0024] The present invention discloses an isolated electrostatic
biased resonator gyroscope. The gyroscope includes an isolated
resonator having a first and a second differential vibration mode,
a baseplate supporting the isolated resonator, a plurality of
excitation affixed to the baseplate for exciting the first
differential vibration mode, a plurality of sensing electrodes
affixed to the baseplate for sensing movement of the gyroscope
through the second differential vibration mode and a plurality of
bias electrodes affixed to the baseplate for trimming isolation of
the resonator and substantially minimizing frequency split between
the first and second differential vibration modes. Typically, the
isolated resonator comprises a proof mass and a counterbalancing
plate joined and supported by flexures with the bias electrodes
disposed on the baseplate below.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] Referring now to the drawings in which like reference
numbers represent corresponding parts throughout:
[0026] FIG. 1 depicts a top view of an exemplary reactionless
resonator gyroscope of the present invention;
[0027] FIG. 2 depicts a side view of the exemplary reactionless
resonator gyroscope of the present invention in an exaggerated
displaced position; and
[0028] FIG. 3 is a flowchart of a typical method of using the
invention.
DETAILED DESCRIPTION INCLUDING PREFERRED EMBODIMENTS
[0029] In the following description, reference is made to the
accompanying drawings which form a part hereof, and which is shown,
by way of illustration, several embodiments of the present
invention. It is understood that other embodiments may be utilized
and structural changes may be made without departing from the scope
of the present invention.
[0030] 1.0 Overview
[0031] Gyroscope embodiments of the invention include a plurality
of bias electrodes for electrostatic balancing or trimming of the
isolation of the differential rocking mode from the baseplate and
outer mounting frame which support the resonator. In a typical
embodiment, the a plurality of bias electrodes are affixed to the
baseplate disposed beneath the resonator. The plurality of bias
electrodes, in conjunction with the excitation and sense
electrodes, permit selective electrostatic control of the
differential rocking mode of the gyroscope. In further embodiments,
the plurality of bias electrodes can be applied to isolated
resonator gyroscopes beyond those employing a central proof mass
and counterbalancing plate as described herein.
[0032] In one exemplary embodiment of the invention, static
voltages are applied to multiple bias electrodes beneath both an
inner resonant post and outer resonant plate body. This enables
changing the restoring spring matrices associated with each of
these two bodies. These static voltages can fully manipulate
(within voltage range bounds) the spring axes and spring values of
both the inner resonant post and outer resonant plate, a total of
six parameters. Models can predict that these are enough parameters
to make up for any manufacturing induced inertia or mechanical
spring mismatches and to produce a resonant structure with the
desired modes having no inner baseplate motion. The split can
theoretically be brought to zero this way, and the resonator
mechanical quality (Q) can be brought to a value determined solely
by the internal losses due to the materials of the post and plate.
Of equal importance, the intrinsic Q (due to such material losses)
is more easily modeled over temperature and therefore results in
greater overall bias stability for the gyroscope.
[0033] Embodiments of the present invention can operate within an
isolated planar vibratory gyroscope that provides a desired
differential rocking mode vibration of a counterbalancing plate
versus a central elongated proof mass, while ensuring that the
undesirable common rocking mode frequency is practically separated
above the desirable differential mode frequency. Embodiments of the
invention include isolation tuning that can be adjusted by altering
the elongated proof mass length. The inertias of the proof mass and
counterbalancing plate do not have to be as precisely equal.
[0034] Further embodiments of the invention also provide integral
vibration isolation to the baseplate through additional baseplate
isolation flexures, attaching the baseplate to a mounting frame to
further improve resonator isolation.
[0035] As electrostatic reactions play an important role, typical
embodiments of the present invention also use the counterbalancing
plate to react with the sense and excitation and bias electrodes,
disposed proximate to the counterbalancing plate beneath it. Thus,
the counterbalancing plate serves a dual role, forming an isolated
resonator with the proof mass and providing a surface for reaction
with the various electrodes. In addition, to enhance the operation
of the gyroscope, the counterbalancing plate can be structured with
extensive planar surface regions to better support electrostatic
operation with the electrodes.
[0036] Embodiments of the invention generally comprise an
all-silicon, isolated symmetric vibratory gyroscope that is
inexpensive to produce using photolithography and that can be
scaled large enough (e.g., approximately 20 mm resonator) to
achieve the required performance. Combined with low-cost,
closed-loop, analog control electronics, a complete redundant
inertial reference unit can be inexpensively manufactured, even
when produced in small quantities. Further, when combined with a
low-power digital control electronics application-specific
integrated circuit (ASIC) for much larger quantities, a very small
inertial navigation unit can be produced. Such a compact,
lightweight and inexpensive precision inertial reference unit can
find a wide range of applications in military as well as commercial
products.
[0037] 2.0 Exemplary Resonator Gyroscope Embodiment
[0038] FIG. 1 depicts a top view of an exemplary isolated resonator
gyroscope 100 embodiment of the present invention. The gyroscope
100 comprises a baseplate 106 and a unique resonator 124 which
includes a post inertial proof mass 102 and a counterbalancing
plate 104. The counterbalancing plate 104 has a rocking inertia
substantially comparable to that of the inertial proof mass 102 and
these two bodies are interconnected and interact as described
above. The counterbalancing plate 104 and central proof mass 102
are coupled to the baseplate 106 at four mounting points 134 and
interconnected to each other by way of flexures 108. The principal
axes of concern which will be referenced throughout the
specification are the X axis 110, the Y axis 112 and the Z axis 114
(which is directed out of the page of FIG. 1). Alternately, the
counterbalancing plate 104 can also be designed in any other shape,
such as a circular ring or other arbitrary shape so long as the two
bodies (inertial proof mass 102 and counterbalancing plate 104)
interact as previously described.
[0039] FIG. 2 depicts a side view of an isolated resonator
gyroscope 100 of the present invention in a displaced position. The
gyroscope is shown displaced about the X axis 110. The mechanical
assembly comprises a central inertial proof mass 102 element
interconnected to an outer counterbalancing plate 104 and affixed
to a baseplate 106 via elastic beam flexures 108 attached at the
four mounting points 126. In one embodiment, the counterbalancing
plate 104, flexures 108 and support plate 118 for the central
inertial proof mass 102 can be photolithographically etched-through
from the same double-side polished crystal silicon wafer to produce
a precision planar resonator gyroscope.
[0040] The axisymmetric resonator 124 is coupled to a baseplate 106
such that the axisymmetric counterbalancing plate 104 can freely
vibrate against the axisymmetric central proof mass 102 with
counterbalanced oscillatory rocking motion and results in a
distinct differential rocking mode with substantially no momentum
transfer to or net reaction on the baseplate 106.
[0041] The proof mass 102 can be constructed in various forms,
however the inertial distribution of the central proof mass is
designed to have significantly more mass out of plane than in plane
and hence high angular gain, or Coriolis reaction to inertial rate
input with driven rocking motion of the resonator 124. To achieve
this, the proof mass 102 comprises a vertical post portion 116
(elongated along the Z axis 114). The post portion 116 is disposed
on a small central support plate portion 118 (in the X-Y plane).
The post portion 116 and support plate portion 118 can be formed
together from a thick silicon wafer for manufacturing ease as
previously mentioned. Alternately, the proof mass 102 can be bonded
as a separable central post portion 116 to the support plate
portion 118 of the counterbalancing plate 104 formed from a thin
silicon wafer.
[0042] As further detailed hereafter, electrostatic excitation and
sensing can be implemented with the excitation and sense electrodes
120A, 120B (collectively referred to as electrodes 120) affixed to
the baseplate 106 beneath the large planar surfaces of the
counterbalancing plate 104. See FIG. 1. The large surface area of
the counterbalancing plate 104 is used to react with the driving
and sensing electrodes 120. In general, the extensive planar
electrode 120 regions can be formed on the baseplate 106 beneath
the counterbalancing plate 104. The counterbalancing plate 104
structure extends toward the central proof mass 102 with the
flexures 108 arranged around a perimeter of the central proof mass
102 as shown in FIG. 1 to maximize the useful electrostatic
reacting area. Thus, the gap between the proof mass 102 and the
counterbalancing plate 104 is reduced and the counterbalancing
plate 104 obtains a more plate-like configuration.
[0043] Also as shown in FIG. 1, a typical arrangement of the
excitation and sense electrodes 120A, 120B is for the excitation
electrodes 120A to be disposed nearer to the proof mass 102 than
the sense electrodes 120B; one excitation electrode 120A and one
sense electrode 120B each are disposed on the baseplate 106 under
each quarter segment of the counterbalancing plate 104. This can
improve the overall sensitivity of the microgyro 100 as the sense
electrodes 120B obtain a larger surface area and the gaps around
the periphery of the counterbalancing plate 104 undergo larger
displacements relative to the baseplate 106. Other electrode 120
patterns can also be used as well, however. For example, the
electrodes 120 can be interwoven with each other as well as the
bias electrodes 136.
[0044] 2.1 Isolation Trimming Using a Plurality of Bias
Electrodes
[0045] All vibratory gyroscopes employ a rotation sensing
mechanical element which is excited to oscillate in a first mode,
i.e. the input or drive mode. A Coriolis acceleration of the
element under rotation induces energy transfer from the input mode
to a second mode, i.e. the output or sense mode. The second mode
produces an excitation in the sense element which is then detected.
Optimum performance of a vibratory gyroscope is obtained when the
excitation and sense modes have substantially the same resonant
frequency and a high Q factor. The response to the Coriolis
acceleration is then mechanically amplified by the Q factor of the
resonance and provides improved sensor performance. Closed loop
control of the resonator sense mode (e.g. co-pending U.S.
application Ser. No. 09/488,425, which is hereby incorporated by
reference herein) provides Coriolis force-rebalance, sense mode
damping and wide gyroscope bandwidth.
[0046] Embodiments of the present invention employ bias trimming to
modify the overall modal stiffness with a plurality of bias
electrodes 126, 136 affixed to the baseplate 106 beneath both the
central plate portion 118 and the counterbalancing plate 104 of the
resonator 124 structure as shown in FIGS. 1 and 2. The bias
electrodes 126, 136 are employed to apply bias voltages to the
resonator 124 separately from the excitation and sense electrodes
120A, 120B during operation of the gyroscope. Thus, the bias
electrodes 126, 136 can be used to maintain this isolation and
tuning after vacuum packaging and at varying temperatures
throughout the life of the gyroscope. Although FIG. 1 shows a
single bias electrode 126 disposed beneath the central plate
portion 118 and four separate bias electrodes 136 disposed beneath
the counter balancing plate 104 (one under the middle of each
side), this arrangement is only illustrative. Many other patterns
and arrangements of the plurality of bias electrodes 126, 136 are
possible. For example, multiple separate electrodes can be disposed
under the central plate portion 118 (e.g., under the middle of each
side) and/or other patterns for the bias electrodes can be employed
(e.g., the bias electrodes 136 can be disposed between the
excitation and sense electrodes 120).
[0047] Embodiments of the invention can be applied to a class of
vibratory Coriolis force sensing gyroscopes with isolated or
balanced resonator designs such as described in the exemplary
embodiment above. The rate sensing element comprises a resonant
structure with two vibratory modes offset by approximately 90
degrees. Motion is induced in the "excitation" mode and this motion
is transferred to the "sense" mode only under rotation of the
gyroscope around its principle axis. The performance of such
gyroscopes is highly dependent upon the frequency split between the
excitation and sense modes as well as on the Q of the resonant
structure; a small frequency split and large Q are desirable.
[0048] The exemplary isolated resonator gyroscope 100 described
above includes redundant degrees of freedom (i.e., more than the
two major rocking degrees of freedom needed to give two modes). Its
structure consists of an inner resonant post 116, and outer
resonant plate 104, an inner baseplate 106 (also part of the
"resonant modes"), and a baseplate frame 128 for mounting to the
external world. This provides a total of six major degrees of
freedom. The inner resonator post 116 and outer resonant plate 104
inertias and spring constants are designed so that under perfect
manufacturing there are two orthogonal modes where the inner
baseplate 106 does not move. This means that for those modes the
mounting to the external world does not introduce any sensor error
sources (i.e., no changes in frequency split, no external dampening
losses to lower the Q, and lower sensitivity to external vibration
noise).
[0049] The manufacturing process discussed in various isolated
resonator gyroscopes approaches this optimum condition, but for a
navigation quality gyroscope post-manufacturing tuning of the key
parameters is necessary. As discussed previously, other gyroscopes
have used laser ablation tuning to refine these parameters after
manufacture and focused ion beam trimming has been proposed for the
same purposes.
[0050] Embodiments of the present invention employ the use of
electrostatic spring softening to induce the isolation or balance
condition while simultaneously minimizing frequency split. This
electrostatic biasing technique has great advantage in that the
post-manufacture tuning can now be done using simple electronics on
a fully vacuum packaged gyro and can even be redone in the field as
a calibration routine to account for aging, radiation damage, and
other performance degrading effects which occur throughout the life
of the resonator.
[0051] Electrostatic biasing with the present invention employs
static voltages applied to multiple bias electrodes 126, 136
beneath both the inner resonant post 102 and outer resonant plate
104 structures. This allows for changing the restoring spring
matrices associated with each of these structures. These static
voltages can fully manipulate (within voltage range bounds) the
spring axes and spring values of both the inner resonant post 102
and outer resonant plate 104 structures, a total of six
parameters.
[0052] These are enough parameters to compensate for manufacturing
induced inertia or mechanical spring mismatches, and to produce a
resonant structure with the desired modes having no inner baseplate
motion. The differential frequency split can be theoretically
brought to zero this manner and the resonator mechanical Q can be
brought to a value determined solely by the internal losses due to
the materials of the post and plate. Another consequence of
electrostatic tuning is that the Q is more easily modeled over
temperature and will result in greater overall bias stability for
the gyroscope.
[0053] In a further embodiment of the present invention, the
mechanical resonator can be intentionally designed to build in a
slight imbalance into the physical structure so that the spring
softening can compensate for both over and under manufacturing
tolerance errors. For example, the resonator gyroscope 100
described above, the vertical post portion 116 would be slightly
too long so that the spring softening (which has greater effect on
the outer resonant plate 104) will still be able to compensate for
manufacturing errors.
[0054] In typical embodiments, the secondary element 126 applies
electrostatic balancing or trimming of the isolation of the
differential rocking mode from the baseplate 106 (and outer
mounting frame 128, discussed hereafter). In a typical embodiment,
the secondary element 126 is disposed beneath the support plate
portion 118 of a central proof mass 102. Thus, the secondary
element also affords independent control of one body (the proof
mass 102) from the other body (the counterbalancing plate 104). The
secondary element 126, operated in conjunction with the primary
element (i.e., the excitation and sense electrodes 120A, 120B
beneath the counterbalancing plate 104), permits selective
electrostatic control of the differential rocking mode of the
gyroscope 100. It should be noted that the use of the secondary
element 126 can be applied to other isolated resonator gyroscopes,
beyond those employing a central proof mass 102 and
counterbalancing plate 104 as detailed herein.
[0055] 2.2 Integral Baseplate Isolation
[0056] In some embodiments of the invention, the baseplate 106 can
be a relatively thick silicon plate of rigid material. Such a thick
rigid baseplate 106 can be directly bonded to the remainder of the
gyroscope in a vacuum package. Alternatively, a more flexible thin
baseplate 106 may be used to reduce cost and ease assembly with
standard wafer processing equipment. Common elasticity in the
resonator flexures 108 such as in the attachment to the baseplate
106 and finite inertia of the baseplate provides inherent
separation of the differential rocking mode frequency from the
common rocking mode of vibration. The singular attribute of any of
these arrangements is that any external motion of the gyroscope
package cannot excite differential rocking motion of the resonator,
unless such motion is first internally driven and only then by
virtue of Coriolis accelerations due to rotation of the gyroscope
about the input axis or axis of inertial symmetry.
[0057] However, preferably embodiments of the present invention
incorporate integral vibration isolation to the baseplate 106 to
further improve differential and common rocking mode frequency
separation and vibration isolation of the resonator 124. As
discussed above, this technique can provide redundant degrees of
freedom which assist in electrostatic tuning of the resonator 124.
In these embodiments, a mounting frame 128 is attached to the
baseplate 106 through one or more isolation flexures 130. In this
case, the baseplate 106 isolation of the resonator 124 is primarily
considered with respect to the mounting frame 128 with the
baseplate 106 serving as an additional isolating element. For
example, the baseplate 106 isolation flexure 130 width and/or
length can be set to attenuate axial or rocking vibrations above
500 Hz from the case. In the embodiment shown in FIGS. 1 and 2 the
isolating flexures 130 extend around a perimeter of the baseplate
106, attached to the baseplate 106 at a first end and the mounting
frame 128 at a second end (illustrating flexures extending around a
perimeter as described above). As previously discussed with respect
to the resonator flexures 108, radial flexure layouts and hybrids
can similarly be used for the isolation flexures 130.
[0058] 3.0 Producing an Isolated Resonator Gyroscope
[0059] FIG. 3 is a flowchart of a typical method 300 of producing
an isolated resonator gyroscope of the invention. The method
comprises providing an isolated resonator 124 having a first and a
second differential vibration mode at block 302. The isolated
resonator 124 transfers substantially no net momentum to a
baseplate 106 when the resonator 124 is excited. Next at block 304,
the isolated resonator 124 is supported with a baseplate 106. A
plurality of excitation electrodes 120A for exciting the first
differential vibration mode of the resonator are affixed to the
baseplate 106 at block 306. A plurality of sensing electrodes are
affixed to the baseplate 106 for sensing movement of the gyroscope
100 through the second differential vibration mode at block 308. At
block 310, a plurality of bias electrodes 126, 136 are affixed to
the baseplate 106 for trimming isolation of the resonator 124 and
substantially minimizing frequency split between the first and
second differential vibration modes.
[0060] Providing the isolated resonator 124 may comprise etching
the entire proof mass 102 and counterbalancing plate 104 from a
single double-side polished crystal silicon wafer or etching only a
plate portion 118 and the counterbalancing plate 104 from the
silicon and bonding on a separate vertical portion 116 of the proof
mass 102. A gold-gold thermo-compression bond for a silicon post or
anodic bond for a pyrex post can be used with critical precision
bonding surfaces and dimensions defined by planar polishing.
[0061] The baseplate 106 is also preferably etched from a silicon
wafer with standard semiconductor processing. Since the baseplate
106 does not move substantially, by virtue of the disclosed
resonator isolation technique, its mechanical precision is of less
importance. Electrodes 120, 126, 136 can patterned onto the
baseplate 106 using known plating and etching techniques.
[0062] The foregoing description of the preferred embodiment of the
invention has been presented for the purposes of illustration and
description. It is not intended to be exhaustive or to limit the
invention to the precise form disclosed. Many modifications and
variations are possible in light of the above teaching. It is
intended that the scope of the invention be limited not by this
detailed description, but rather by the claims appended hereto. The
above specification, examples and data provide a complete
description of the manufacture and use of the invention. Since many
embodiments of the invention can be made without departing from the
scope of the invention, the invention resides in the claims
hereinafter appended.
* * * * *