Note: Descriptions are shown in the official language in which they were submitted.
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FIBER OPTIC COIL AND HUB HAVING MATCHING
COEFFICIENTS OF THERMAL EXPANSION
TECHNICAL FIELD OF THE INVENTION
The present invention relates to fiber optic devices such as fiber optic
rate sensors.
BACKGROUND OF THE INVENTION
A fiber optic rate sensor is frequently used in advanced global
1o positioning and inertial guidance systems to sense rotation. A fiber optic
rate sensor
ordinarily comprises an interferometer which includes a light source, a beam
splitter, a
detector, and an optical path which is mounted on a platform. Light from the
light
source is split by the beam splitter into two light beams which are directed
to opposite
ends of the optical path. The two light beams counterpropagate around the
optical path
15 and, as the light beams exit the optical path, they are recombined. The
recombined light
beams are applied to a detector.
If the optical path rotates, the distance traveled by one of the light beams
is greater than distance traveled by the other light beam, so that there will
be a phase
difference between the two light beams at their optical path exit points. A
sensing
2o circuit connected to the detector determines this phase difference as an
indication of the
extent and direction of rotation.
The optical path of a fiber optic rate sensor is provided by an optical fiber
which is coiled around a spool or hub to form a winding configuration. The
winding
configuration usually has multiple layers where each layer contains multiple
turns.
25 Although many different winding configurations are known, coils used in
fiber optic
rotation sensors are typically wound as quadrupoles.
In order to form a quadrupole, a first end of a continuous optical fiber is
wound onto a first intermediate spool, and a second end of the continuous
optical fiber
is wound onto a second intermediate spool. Then, the optical fiber on the
first
3o intermediate spool is used to wind a first layer of turns in a clockwise
direction around
the hub, the optical fiber on the second intermediate spool is used to wind a
second layer
of turns in a counterclockwise direction over the first layer, the optical
fiber on the
second intermediate spool is used to wind a third layer of turns over the
second layer of
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turns, and the optical fiber on the first intermediate spool is used to wind a
fourth layer
of turns over the third layer of turns.
If "+" and "-" are used to designate the first and second ends of the
optical fiber, respectively, the resulting quadrupole winding pattern has a + -
- +
winding configuration, where + indicates a layer wound from the first end of
the optical
fiber and - indicates a layer wound from the second end of the optical fiber.
Ideally, the
length of optical fiber in the "+" layers is equal to the length of optical
fiber in the "-"
layers. This quadrupole winding pattern may be repeated as often as desired
for a fiber
optic rate sensor. Accordingly, if a second quadrupole is wound with + - - +
layers
1o about the first quadrupole, the resulting two quadrupole arrangement has a
+ - - + + - - +
winding pattern.
It is also known to wind a reverse quadrupole from the "+" and "-" ends
of the optical fiber. In this case, the reverse quadrupole has a + - - + - + +
- winding
pattern and is generally referred to as an octupole. This octupole winding
pattern may
15 be repeated as often as desired for a fiber optic rotation sensor. Indeed,
a reverse
octupole may be wound according to the following winding pattern: + - - + - +
+ - - + +
-+--+.
In order to form a coil having an interleaved winding pattern, one or
more layers of the coil are wound as alternating turns from first and second
ends of an
20 optical fiber. Accordingly, in such a layer, odd numbered turns are wound
from a first
end of the optical fiber, and even numbered turns are wound from a second end
of the
optical fiber. The result of such winding is that each turn (other than the
outer turns) of
an interleaved layer is wound from one end of an optical fiber and is
sandwiched
between two turns wound from the other end of the optical fiber.
25 Not all layers of a coil having an interleaved winding pattern are required
to be wound with the interleaved winding pattern. For example, all of the
turns of the
innermost layer of the coil can be wound from the same end of the optical
fiber, or one
or more groups of adjacent turns of the innermost layer of the coil can be
wound from
the first end of the optical fiber and one or more other groups of adjacent
turns of the
3o innermost layer of the coil can be wound from the second end of the optical
fiber.
The direction of the axis running through the hub and about which the
coil is wound is generally referred to as the axial direction of the coil, and
the direction
perpendicular to the axial direction is generally referred to as the radial
direction of the
coil. Coils of optical fiber typically have a large thermal expansion rate in
the axial
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direction and a much smaller thermal expansion rate in the radial direction.
If these
thermal expansion rates are not matched by the structure, such as the hub,
which
supports the coil, performance of a fiber optic coil may be significantly
degraded.
(D2) discloses a sensor coil with a spool having a coefficient of
thermal expansion approximating that of overlying Fiber, but does not disclose
using
the same material for both. (D4) sets forth negative trimming of a fiber optic
winding. to address axial or radial errors, but does not address modifications
to a hub
or spool to prevent thermal expansion problems.
p0 SUMMARY OF THF INy~TlON
Accordingly, a fiber optic rate sensor includes a support structure
constructed of a fiber and supporting coil wound from an optical fiber having
a first a
second end. A first layer, wound from the first end, overlies a second layer,
wound
from the second end. In an alternative aspect, the support structure is made
of glass
laminations, rather than a fiber. The plurality of glass laminations are
bonded
together by epoxy.
In another aspect, a method of making a fiber optic rate sensor includes
winding an optical fiber into a coil and constructing a supporting of a fiber.
The first
layer is wound from a first end of fiber and overlies a second Layer wound
from a
second end. The hub may be made of a fiber or a plurality of glass
laminations.
In yet another aspect, a fiber optic rate sensor includes a low thermal
expansion rate support structure made of a fiber and supporting a fiber optic
coil, with
a compliant joint situated between.
)OFF D~~PTION OF THE DItA
These and other features and advantages ofthe present invention will become
more apparent from a detailed consideration of the invention when taken in
conj unction with the drawings in which:
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Figure 1 illustrates a first embodiment of the present invention in which a
fiber optic coil is attached externally to a hub having thermal expansion
rates
substantially matching the thermal expansion rates of the fiber optic coil;
Figure 2 illustrates an end view of the first embodiment of the present
invention illustrated in Figure 1;
Figure 3 illustrates a second embodiment of the present invention in
which a fiber optic coil is attached internally to a hub having thermal
expansion rates
substantially matching the thermal expansion rates of the fiber optic coil;
Figure 4 illustrates an end view of the second embodiment of the present
invention illustrated in Figure 3;
Figure 5 illustrates a third embodiment of the present invention in which
a fiber optic coil is attached externally to a hub having thermal expansion
rates
substantially matching the thermal expansion rates of the fiber optic coil;
Figure 6 illustrates a fourth embodiment of the present invention in
which a fiber optic coil is attached to a hub having a glass fiber outer
cylinder bonded
with compliant adhesive to a low expansion inner support structure such that
the hub's
external interface has thermal expansion rates substantially matching the
thermal
expansion rates of the fiber optic coil;
Figure 7 illustrates a first exemplary winding pattern which may be used
2o in connection with the fiber optic coils shown in Figures 1 - 6;
Figure 8 illustrates a second exemplary winding pattern which may be
used in connection with the fiber optic coils shown in Figures 1 - 6; and,
Figure 9 illustrates a third exemplary winding pattern which may be used
in connection with the fiber optic coils shown in Figures 1 - 6.
DETAILED DESCRIPTION
As shown in Figures 1 and 2, a fzber optic rate sensor 10 includes a
sensing coil 12 wound around a hub 14 in any predetermined winding
configuration.
The fiber optic rate sensor 10 has an axial direction 16 and a radial
direction 18. An
optical fiber is used to wind the sensing coil 12 in multiple layers with each
layer having
multiple turns. The coil 12 is suitably attached to the hub 14.
The hub 14 is formed from materials so as to have coefficients of thermal
expansion substantially matching the coefficients of thermal expansion of the
sensing
coil 12 in both the axial direction 16 and the radial direction 18. For
example, the hub
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14 may be a mounting structure wound from a glass fiber which is the same as,
or
substantially similar to, the optical fiber that is used to wind the sensing
coil 12. The
glass fiber used to wind the hub 14 may be coated with a buffer material and
treated
with adhesive so that, after curing, the hub 14 will be a rigid support
structure upon
which the sensing coil 12 can be wound.
Alternatively, the hub 14 may be a mounting structure having a plurality
of glass laminations bonded together by epoxy.
Accordingly, the hub 14 has a thermal expansion rate which is the same
as, or similar to, the thermal expansion rate of the sensing coil 12 in both
the axial
to direction 16 and the radial direction 18. Because the thermal expansion
rates of the
sensing coil 12 and the hub 14 substantially match, performance of the fiber
optic rate
sensor 10 is not significantly degraded due to changing temperature conditions
along the
axial direction 16 and/or the radial direction 18.
An adhesive layer may be provided at the interface
15 between the sensing coil 12 and the hub 14 in order to bond the sensing
coil 12 to the
hub 14.
Figures 3 and 4 show a second embodiment of the present invention in
the form of a fiber optic rate sensor 20. The fiber optic rate sensor 20 has a
sensing coil
22 attached internally to a hub 24. As in the case of the f ber optic rate
sensor 10, the
2o hub 24 may be formed from materials similar to the materials used to form
the hub 14 of
the fiber optic rate sensor 10. Accordingly, the thermal expansion rates of
the sensing
coil 22 and the hub 24 substantially match. An adhesive layer may be provided
at the
interface between the sensing coil 22 and the hub 24 in order to bond the
sensing coil 22
to the hub 24.
25 Figure 5 shows a third embodiment of the present invention in the form
of a fiber optic rate sensor 30. The fiber optic rate sensor 30 includes a
sensing coil 32
wound about a hub 34. In this case, the hub 34 is in the shape of a spool
having end
flanges 36 and 38 and a center cylindrical section 40. An adhesive layer may
be
provided between the sensing coil 32 and the end flanges 36 and 38 in order to
bond the
3o sensing coil 32 to the hub 34. Alternatively, the adhesive layer may be
provided
between the sensing coil 32 and the center cylindrical section 40 of the hub
34 in order
to bond the sensing coil 32 to the hub 34. As a still further alternative, an
adhesive layer
may be provided between the sensing coil 32 and the end flanges 36 and 38 as
well as
the center cylindrical section 40.
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As in the case of the fiber optic rate sensors 10 and 20, the hub 34 may
be formed from materials similar to the materials used to form the hubs 14 and
24.
Accordingly, the sensing coil 32 and the hub 34 have substantially matching
thermal
expansion rates.
Figure 6 shows a fourth embodiment of the present invention in the form
of a fiber optic rate sensor 50. The fiber optic rate sensor 50 includes a
sensing coil 52
wound about a hub having an inner hub 54 and an outer hub 56. The outer hub 56
may
be formed by winding a glass fiber around the inner hub 54. The optical fiber
used to
wind the outer hub 56 is preferably the same as, or substantially similar to,
the optical
to fiber that is used to wind the sensing coil 52. Also, the optical fiber
used to wind the
outer hub 56 may be bonded to the inner hub 54 with a compliant adhesive.
The inner hub 54 may have a low thermal expansion rate and may be
fabricated using (i) a sintered powder such as copper and tungsten, or copper
and
molybdenum, or the like, (ii) an alloy such as MonelTM or stainless steel or
titanium, or
15 the like, or (iii) co-fired ceramics such as MycorTM or the like.
Accordingly, the sensing coil 52 and the hub 54/56 have substantially
matching thermal expansion rates.
Hub and coil configurations other than those shown in Figures 1-6 may
be provided according to the present invention. Moreover, other materials may
be used
20 for the hub of a fiber optic rate sensor in accordance with the present
invention as long
as the thermal expansion rates of the hub substantially matches the thermal
expansion
rates of the coil of such fiber optic rate sensor.
The sensing coils of a fiber optic rate sensor, such as the sensing coils 12,
22, 32, and 52 shown in Figures 1-6, may have various winding configurations.
Three
25 such winding configurations are shown by way of example in Figures 7, 8,
and 9. A
winding conf guration 60 shown in Figure 7 is generally referred to as a
quadrupole
winding arrangement. The winding configuration 60 specifically comprises a
plurality
of quadrupoles wound sequentially about a center line 62. Each layer of the
winding
configuration 60 represents a plurality of turns wound from an optical fiber.
The turns
3o in a layer without x's represent turns wound from one end of the optical
fiber, and the
turns in a layer with x's represent turns wound from the other end of the
optical fiber.
Accordingly, the turns of a first layer 64 are wound from a first end of an
optical fiber,
the turns of a second layer 66 are wound from a second end of the optical
fiber, the turns
of a third layer 68 are wound from the second end of the optical fiber, and
the turns of a
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fourth layer 70 are wound from the first end of the optical fiber to form a
first
quadrupole of the winding configuration 60.
A second quadrupole is wound about the first quadrupole. The second
quadrupole includes layers 72, 74, 76, and 78. As can be seen from Figure 7,
the layers
72, 74, 76, and 78 are wound in the same configuration as the layers 64, 66,
68, and 70.
That is, the turns in the layer 72 are wound from the first end of the optical
fiber, the
turns in the layer 74 are wound from the second end of the optical fiber, the
turns in the
layer 76 are wound from the second end of the optical fiber, and the turns in
the layer 78
are wound from the first end of the optical fiber.
1o A winding configuration 80 is shown in Figure 8 and is an octupole
winding configuration. An octupole winding configuration generally has a first
four
layers wound as a conventional quadrupole, and a second four layer wound as a
reverse
quadrupole. Accordingly, the winding configuration 80 has layers 82, 84, 86,
88, 90,
92, 94, and 96. Each layer comprises a plurality of turns wound from an
optical fiber
15 having first and second ends. As shown in Figure 8, the turns of the layer
82 are wound
from a first end of the optical fiber, the turns of the layer 84 are wound
from a second
end of the optical fiber, the turns of the layer 86 are wound from the second
end of the
optical fiber, the turns of the layer 88 are wound from the first end of the
optical fiber,
the turns of the layer 90 are wound from the second end of the optical fiber,
the turns of
2o the layer 92 are wound from the first end of the optical fiber, the turns
of the layer 94
are wound from the first end of the optical fiber, and the turns of the layer
96 are wound
from the second end of the optical fiber. Additional octupoles may be wound
around
the winding configuration 80 shown in Figure 8. Indeed, as discussed above, a
reverse
octupole may be added to the octupole shown in Figure 8.
25 A winding configuration 130 is shown in Figure 9 and is an interleaved
winding configuration. The winding configuration 130 includes layers 132, 134,
136,
138, 140, 142, 144, 146, and 148. The turns of the layer 132 are wound from a
first
optical fiber, and the turns of the layers 134, 136, 138, 140, 142, 144, 146,
and 148 are
wound from a second optical fiber. Accordingly, the turns in the layer 132 are
not a
3o functional part of the winding configuration 130, although the turns in the
layer 132
could be functional. The first optical fiber that is used to wind the turns of
the layer 132
has an outer diameter that is larger than the outer diameter of the second
optical fiber
which is used to wind the layers 134, 136, 138, 140, 142, 144, 146, and 148.
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As shown in Figure 9, the layers 134-148 include alternate turns wound
from the first and second ends of the second optical fiber. A specific
interleaved
winding pattern for the layers 134-148 is shown in Figure 9, although other
interleaved
winding patterns can be employed. Examples of interleaved winding patterns are
taught
in U.S. Application 08/668,485, which was filed on June 21, 1996, and which
has been
allowed by the U.S. patent and Trademark Office. The disclosure of U.S.
Application
08/668,485 is incorporated by reference herein.
Certain modifications of the present invention have been discussed
above. Other modifications will occur to those practicing in the art of the
present inven-
1o tion. For example, the present invention has been described in terms of a
particular type
of fiber optic device, i.e., a fiber optic rate sensor. However, the present
invention may
be used in connection with other types of fiber optic devices.
Moreover, as discussed above, hub and coil confgurations other than
those shown in Figures 1-6 may be provided according to the present invention.
For
15 example, a sensing coil could be wound onto a low thermal expansion rate
hub using a
compliant joint between the sensing coil and the hub. This compliant joint
reduces
stress on the sensing coil caused by differences in axial thermal expansion
rates between
the sensing coil and the hub. The compliant joint may be a compliant adhesive.
Accordingly, the description of the present invention is to be construed
2o as illustrative only and is for the purpose of teaching those skilled in
the art the best
mode of carrying out the invention. The details may be varied substantially
without
departing from the spirit of the invention, and the exclusive use of all
modifications
which are within the scope of the appended claims is reserved.
