Measuring and testing – Speed – velocity – or acceleration – Angular rate using gyroscopic or coriolis effect
Reexamination Certificate
2000-07-28
2002-04-23
Moller, Richard A. (Department: 2856)
Measuring and testing
Speed, velocity, or acceleration
Angular rate using gyroscopic or coriolis effect
Reexamination Certificate
active
06374672
ABSTRACT:
BACKGROUND
1. Field of the Invention:
The present invention relates to silicon gyros of the type in which rotation rate is measured by the Coriolis effect-induced deflection of a sensor element that includes a paddle that is supported by aligned flexures defining an axis of rotation. In particular, the present invention pertains to such a device that includes wafer elements which integrate driving and sensing functions.
2. Description of the Prior Art
Precision micro-mechanical devices have wide application in the fields of inertial navigation and guidance with respect to both long-range, re-usable vehicles, such as aircraft, and relatively short-range, one-use vehicles, such as munitions. Such devices may be employed to measure acceleration directly and rotation rate indirectly through the Coriolis principle. According to that principle, a body traveling at a velocity {overscore (V)}in a coordinate frame which is subject to rotation {overscore (&OHgr;)}experiences an acceleration {overscore (A)}
c
defined as the cross product {overscore (A)}
c
=2{overscore (&OHgr;)}×{overscore (V)}. By imposing a sinusoidal relative velocity of the form:
{overscore (V)}={overscore (V)}
o
sin &ohgr;t
The corresponding Coriolis acceleration then becomes:
{overscore (A)}
c
=2{overscore (&OHgr;)}×{overscore (V)}
o
sin &ohgr;t
The measurement of rotation rate is obtained by determining the resultant sinusoidal Coriolis force exerted upon a deflectable force sensitive member.
Micromechanical devices are well suited for operation in low cost systems due to the compactness, simplicity and batch processing capabilities that they offer. One type of micromechanical inertial sensor employs a paddle that is rotatable about an axis defined by aligned flexure beams that support it with respect to a counter-oscillating hub (rotation rate sensor element). Pending U.S. patent application Ser. No. 09/127,375 of inventor Stanley F. Wyse entitled “Micromachined Rotation Sensor with Modular Sensor Elements” and 08/903,499 of Robert E. Stewart and Stanlet F. Wyse entitled “Navigation Grade Micromachined Rotation Sensor System” disclose inertial sensors of the foregoing type.
Devices of the above-identified type typically comprise stacks of silicon wafers. One of such wafers, known as a driver wafer, includes a plurality of radially-directed electrodes formed upon one or both of its opposed surfaces. A second wafer includes a plurality of radially-directed electrodes formed upon a facing surface in an assembled device. In some embodiments, this wafer is known as a driven wafer. The electrodes of the driven wafer are offset from those of the driver wafer (which is mounted to the case that surrounds the sensor) so that, upon energization, the driven element will be caused to oscillate in response to an a.c. voltage signal applied to the driver electrodes. A sensor wafer that includes the paddle is fixed to the driven wafer in such a way that the paddle is caused to oscillate at the chosen dither frequency causing out-of-plane oscillations of the paddle with respect to the sensor wafer. Such out-of-plane oscillations of the paddle are detected to provide the Coriolis acceleration that is readily converted to rotation rate.
As an alternative, the above patent applications also teach arrangements employing a pair of driver wafers, each having a set of radially-directed electrodes on facing surfaces offset from one another.
Additional electrodes are provided for torquing and picking-off the rotation of the paddle about the axis formed by aligned central flexure beams. In pending patent application 09/127,375, such electrodes are fixed to cover wafers adjacent opposed surfaces of the sensor wafer.
FIG. 1
is a side elevation view in cross-section of a device in accordance with the above-described prior art. As can be seen, the device comprises a sensor stack
10
comprising a top cover wafer
12
having vias
14
and
16
defined therein for contacting electrodes
18
and
20
respectively of an electrode layer
22
that includes a surrounding guard ring
24
. The wafers
12
and
22
are fusion-bonded to one another at an oxide layer
26
. A bottom cover wafer
28
is configured similar to the top cover wafer
12
and is indirectly fusion-bonded to a lower electrode wafer
30
at an oxide layer
32
to form a like structure.
A sensing element wafer
34
is etched to define a sensing paddle
36
that is supported by aligned flexure beams (one of which is shown at
38
) for joining it to a surrounding frame
40
. Overlying and underlying oxide layers
42
and
44
respectively are provided for fusion-bonding of the opposed surfaces of the sensing element wafer
34
to the above-described structures.
The above-described sensor stack
10
is fusion-bonded to a dither drive stack
46
at an oxide layer
48
. The dither drive stack
46
consists of a driver wafer
50
which, as described above, includes a plurality of radially-arranged electrodes
51
at its top surface, and a driven element wafer
52
that is indirectly fusion-bonded to (a hub of) the driver wafer
50
at an oxide layer
54
. The driven wafer
52
includes a set of radially-arrayed electrodes
55
fixed to its lower surface that faces the set of offset electrodes fixed to the top surface of the driver wafer
50
. The wafer
52
includes a central hub
56
that is fusion bonded to the driver wafer
50
and an outer member
58
that is bonded to the bottom cover wafer
28
and joined to the hub
56
by reduced thickness flexure beams
60
and
62
.
In operation, the sensor stack
10
is dithered at about 2 kHz about a vertical axis
64
. The driver wafer
50
is stationary, as is the hub
56
of the driven wafer
52
. The outer portion
58
of the driven wafer
52
, supported by the flexure beams
60
and
62
, is free to oscillate. Electrostatic torquing is provided by interaction of the sets of offset (by 1/4 cycle) electrodes
51
,
55
.
The device illustrated in
FIG. 1
, which requires a high vacuum environment to run at high Q, is assembled by bonding the sensor stack
10
to the dither stack
46
. This is done by carefully wicking-in EPOXY or like adhesive. Unfortunately, EPOXY outgassing can degrade the quality of the vacuum and, thus, the Q of the device.
As can be seen in
FIG. 1
the gap between the driven wafer
52
and the driver wafer
50
extends to the edges of the chips. Such a structure necessitates the use of special and costly dicing techniques to prevent breakage of the dither beams
60
and
62
during manufacture as well as requiring special techniques to keep particles out of the gap as electrostatic forces make the gap attractive to particles that can interfere with dither motion, generate noise and, in most cases, prevent it altogether.
The prior art device requires five (5) silicon wafers, eighteen (18) different masks and the routing of wires from the bottom of the sensor stack
10
through grooves (not shown) in the driven wafer
52
. Accordingly, assembly is very time consuming, requiring a degree of hand skill unsuitable for large scale production.
SUMMARY OF THE INVENTION
The preceding and other disadvantages of the prior art are addressed by the present invention that provides a rotation sensor. In a first aspect, such sensor includes a first generally-planar wafer that includes a paddle and a plurality of driven elements defined at its opposed sides. A second generally-planar wafer has a plurality of driver electrodes defined on a first surface and a third generally-planar wafer has a plurality of driver electrodes defined on a first surface. The first wafer is arranged relative to the second and third wafers so that the first surfaces of the second and third wafers face the opposed surfaces of said first wafer. A first pair of electrodes is defined on the first surface of the second wafer and is substantially aligned with the paddle. A second pair of electrodes is defined on the first surface of the third wafer and is substantially aligned with the paddle.
In a second aspect, the invention p
Abbink Henry C.
Choi Youngmin A.
Kramsky Elliott N.
Litton Systems Inc.
Moller Richard A.
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