Title: Magnetic shield for a fiber optic gyroscope
Abstract: A magnetic shielding system for a fiber optic gyroscope is disclosed. The fiber optic gyroscope may have a sensing coil with a sensing axis. An exemplary magnetic shield may enclose the sensing coil and have a layer including a plurality of pieces connected at a seam. A portion of the seam may be substantially parallel to the sensing axis of the sensing coil. Also, the pieces may be connected together such that each piece has a portion of each of two sides and a wall of the magnetic shield. Additionally, multiple layers having orthogonal seams may be utilized with the magnetic shield.
Patent Number: 6,952,268 Issued on 10/04/2005 to Olson, et al.
| Inventors:
|
Olson; Matthew A. (Glendale, AZ);
Williams; Wesley H. (Phoenix, AZ);
Vaught; Jesse Allen (Westminster, CO);
McEwen; Matthew Brady (Phoenix, AZ)
|
| Assignee:
|
Honeywell International Inc. (Morristown, NJ)
|
| Appl. No.:
|
224809 |
| Filed:
|
August 21, 2002 |
| Current U.S. Class: |
356/460; 356/483 |
| Intern'l Class: |
G01B 009/02 |
| Field of Search: |
356/459,460,483,465
174/35.MS,35.R
|
References Cited [Referenced By]
U.S. Patent Documents
| 4956868 | Sep., 1990 | Carlson.
| |
| 5486922 | Jan., 1996 | Cordova.
| |
| 5488622 | Jan., 1996 | Mitchell.
| |
| 5517306 | May., 1996 | Yakubovich et al.
| |
| 5545892 | Aug., 1996 | Bilinski et al.
| |
| 5602642 | Feb., 1997 | Bergh et al.
| |
| 5698784 | Dec., 1997 | Hotelling et al.
| |
| 5786895 | Jul., 1998 | Mitchell et al.
| |
| 5822065 | Oct., 1998 | Mark et al.
| |
| 5841932 | Nov., 1998 | Page et al.
| |
| 5896199 | Apr., 1999 | Mark et al.
| |
| 5917983 | Jun., 1999 | Page et al.
| |
| 5939772 | Aug., 1999 | Hurst et al.
| |
| 6058760 | May., 2000 | Van Heyningen.
| |
| 6201923 | Mar., 2001 | Yuhara et al.
| |
| 6242848 | Jun., 2001 | Mori et al.
| |
| 6441366 | Aug., 2002 | Webb.
| |
| 6462824 | Oct., 2002 | McLean et al.
| |
| 6627810 | Sep., 2003 | McEwen et al.
| |
| Foreign Patent Documents |
| WO 01/8186/5 | Nov., 2001 | WO.
| |
| WO 01/8186/5 | Nov., 2001 | WO.
| |
Other References
A.D. King, (1998) Inertial Navigation—Forty Years of Evolution,
"GEC Review" 13(3):140-149.
Marshall Brain, "How Gyroscopes Work", http://www.howstuffworks.com/gyroscope.htm/printable,
pp. 1-5.
Replacement of Gyro #4, "Removal and Replacement of a Gyro", http://einstein.stanford.edu/highlights/gyro4.html,
pp. 1-3.
Copy of International Search Report from International Application PCT/US03/25776.
|
Primary Examiner: Toatley, Jr.; Gregory J.
Assistant Examiner: Connolly; Patrick
Claims
1. A system for magnetically shielding exactly one fiber optic gyroscope, the
system comprising:
exactly one fiber optic gyroscope having a sensing coil with a sensing axis;
and
a magnetic shield including a plurality of layers each fabricated from a magnetic
material and enclosing the exactly one fiber optic gyroscope;
wherein each of the layers includes a first piece and a second piece, the first
piece and second piece are aligned and connected at a seam, and at least a portion
of the seam has an angular separation of less than substantially 10 degrees from
the sensing axis of the sensing coil.
2. The system of claim 1 wherein the magnetic material comprises a nickel-iron
magnetic alloy.
3. The system of claim 1, wherein a flux path is continuous along each layer,
and has an angular separation of less than substantially 10 degrees from the sensing axis.
4. The system of claim 1, wherein the magnetic shield and the sensing coil are
attached to a support structure fabricated from a non-magnetic material.
5. A magnetic shield for a single sensing coil with a sensing axis, the magnetic
shield comprising:
a first layer and a second layer enclosing the single sensing coil;
wherein the first layer includes a first piece and a second piece connected by
a first seam, and the second layer includes a third piece and a fourth piece connected
by a second seam, a portion of the first seam is substantially parallel to the
sensing axis, and a portion of the second seam is substantially perpendicular to
the sensing axis.
6. The magnetic shield of claim 5 wherein the first layer and second layer comprise
a high-permeability magnetic material.
7. The magnetic shield of claim 6, wherein the high-permeability magnetic material
comprises a nickel-iron magnetic alloy.
8. The magnetic shield of claim 5, wherein the first seam and the second seam
are orthogonal.
9. The magnetic shield of claim 5, wherein the first layer has a continuous flux
path that is substantially parallel to the sensing axis, and the second layer has
a continuous flux path that is substantially perpendicular to the sensing axis.
10. The magnetic shield of claim 5, wherein the first layer and second layer
are attached to an intermediate support structure fabricated from a non-magnetic material.
11. The magnetic shield of claim 10, wherein the magnetic shield encloses only
the sensing coil and the intermediate support structure.
12. The magnetic shield of claim 5 further comprising a third layer enclosing
the fiber optic gyroscope.
13. A magnetic shield for a sensing coil, the magnetic shield comprising:
a plurality of layers enclosing the sensing coil, each layer having a first side
and a second side connected by a wall;
wherein each of the plurality of layers includes a plurality of pieces, and each
of the plurality of pieces includes a portion of each of the first side, the second
side, and the wall.
14. The magnetic shield of claim 13 wherein the at least one layer comprises
a high-permeability magnetic material.
15. The magnetic shield of claim 14, wherein the high-permeability magnetic material
comprises a nickel-iron magnetic alloy.
16. The magnetic shield of claim 13 wherein at least one pair of the plurality
of layers is attached to an intermediate support structure fabricated from a non-magnetic material.
17. The magnetic shield of claim 13, wherein the sensing coil is part of a single
fiber optic gyroscope enclosed by the magnetic shield.
18. The magnetic shield of claim 13, wherein a flux path is continuous on each
layer and substantially parallel to the sensing axis.
19. The magnetic shield of claim 1, wherein the seam defines a plane, and the
plane is substantially parallel to the sensing axis.
20. The magnetic shield of claim 13, wherein the magnetic shield comprises a
thin-walled right circular cylinder with closed ends.
21. The magnetic shield of claim 13, wherein the magnetic shield encloses a second
sensing coil having a second sensing axis, wherein the portion of the seam is substantially
parallel to the second sensing axis.
Description
FIELD OF THE INVENTION
The present invention relates to the field of magnetic shields. More specifically,
this invention relates to a mating design of a magnetic shield for a fiber optic gyroscope.
BACKGROUND OF THE INVENTION
Bias sensitivity to magnetic fields is an important performance parameter for
optical gyroscopes, such as fiber optic, ring laser, and other similar optical
gyroscopes used for inertial sensing. External sources of ambient magnetic fields
such as the Earth's magnetic field, electrical machinery, etc., can cause bias
errors in an optical gyroscope. For example, for a fiber optic gyroscope used in
an inertial navigation system, the allowable magnetic bias sensitivity of the instrument
is typically between 0.001 and 0.0001 degrees per hour per gauss (deg/hr/gauss).
In contrast, the inherent sensitivity of an unshielded gyroscope is on the order
of 1 deg/hr/gauss.
An effective method of minimizing bias sensitivity to an external magnetic field
is to reduce the magnitude of the local field by the addition of a magnetic shielding
structure ("magnetic shield") around the gyroscope. Magnetic shields are typically
made from a high-permeability material that functions as a preferred path for an
ambient field. Essentially, the magnetic shield may shunt the ambient magnetic
flux around the gyroscope located inside. Thus, the magnetic shield may reduce
the effect of an ambient magnetic field on the gyroscope.
Fiber optic gyroscopes may include rotation sensitive optical fiber wrapped
into a coil ("sensing coil") in order to detect motion. However, the bias sensitivity
of a sensing coil to an external magnetic field may be orientation dependent. For
example, the magnetic sensitivity of a depolarized fiber optic gyroscope may be
ten times greater in the direction of the rotation sensing axis (e.g., longitudinal
axis of the sensing coil) than in the direction perpendicular to the rotation sensing
axis. Thus, it is advantageous for magnetic shielding to be most effective parallel
to the direction of the sensing axis if the effect of an ambient magnetic field
on the bias sensitivity of the gyroscope is to be significantly reduced.
Currently, magnetic shields are usually manufactured in two pieces for
ease of fabrication and assembly. Prior art magnetic shields are typically assembled
such that the two pieces are connected at a seam perpendicular to a longitudinal
axis of the magnetic shield. Furthermore, fiber optic gyroscopes are usually oriented
within the magnetic shield such that the direction of the sensing axis of the fiber
optic coil is aligned with the longitudinal axis of the magnetic shield. Due to
the orientation of the seam in prior art magnetic shields, this method of assembly
creates a discontinuity in a magnetic flux path parallel to the sensing axis, reducing
the shielding effectiveness in that direction. As a result, the most sensitive
orientation of the depolarized fiber optic gyroscope sensing coil may be parallel
to the least effective orientation of the magnetic shield.
Accordingly, it is desirable to have a magnetic shield for a fiber optic
gyroscope with an improved mating design that overcomes the above deficiencies
associated with the prior art.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of an exemplary fiber optic gyroscope sensing coil.
FIG. 2 is a perspective view of an exemplary magnetic shield for a fiber optic
gyroscope, with the orientation of the sensing coil from FIG. 1 shown.
FIG. 3 shows exemplary magnetic flux paths in the magnetic shield of FIG. 2.
FIG. 4 is a perspective view of another exemplary magnetic shield for a fiber
optic gyroscope having multiple layers, with the orientation of the sensing coil
from FIG. 1 shown.
FIG. 5 shows exemplary magnetic flux paths in the magnetic shield of FIG. 4.
DETAILED DESCRIPTION
FIG. 1 shows an exemplary sensing coil
104 for a fiber optic gyroscope
(not shown) having a sensing axis
101—
101. In the present embodiment,
the sensing coil
104 may include a length of optical fiber
106 wrapped
in multiple layers around a spool
108 in the longitudinal and radial directions
of the coil
104. It should be understood that any number of layers of optical
fiber may be utilized with the sensing coil
104.
Additionally, in the present embodiment, the sensing axis
101—
101
may be the direction of rotation sensitivity of the sensing coil
104. Thus,
the sensing coil
104 may detect rotational movement about the sensing axis
101—
101. Although the sensing axis
101—
101
is shown as substantially parallel to the longitudinal axis of the sensing coil
104 in FIG. 1, it should be understood that the direction of the sensing
axis
101—
101 may vary in alternate embodiments (e.g., along
a radial direction of the sensing coil
104).
In addition, although the exemplary sensing coil
104 shown in FIG. 1 has
a substantially circular cross-section in a plane perpendicular to the sensing
axis
101—
101, and a substantially rectangular cross section
in a plane parallel to the sensing axis
101—
101, alternate
cross-sections may include other shapes (e.g., elliptical or triangular cross-sections).
For more information on sensing coils and fiber optic gyroscopes, one can refer
to U.S. Pat. No. 4,408,882, the contents of which are incorporated in its entirety
herein by reference.
FIG. 2 shows an exemplary magnetic shield
200 forming a layer (e.g.,
container shell) around the sensing coil
104. Although the sensing coil
104 is not shown in FIG. 2, an exemplary orientation of the sensing coil
104 within the magnetic shield
200 is indicated by the direction
of the sensing axis
101—
101.
In the present embodiment, the magnetic shield
200 is a thin-walled right
circular cylinder with closed ends, with a longitudinal and a radial axis, but
other geometries (e.g., a sphere) are also possible. The magnetic shield
200
may have a first side
212 and second side
214 connected to a wall
216. In an alternate embodiment, the corners formed by the sides
212
and
214 and by the wall
216 may include a radius, or fillet, rather
than a sharp corner. Additionally, the magnetic shield
200 may be comprised
of a nickel-iron alloy with high permeability at a low ambient magnetic field,
but other magnetic materials with similar properties may also be suitable.
Due to considerations such as ease of fabrication and assembly, the magnetic
shield
200 may include a first piece
220 and a second piece
230
aligned and connected to each other at a seam
250. Both the first piece
220 and the second piece
230 may include a portion of the first side
212, second side
214, and wall
216, as shown in FIG.
2.
In the present embodiment, a portion of the seam
250 along the wall
216
may be substantially parallel to the sensing axis
101—
101.
Furthermore, the seam
250 may define a plane that is substantially parallel
to the sensing axis
101—
101 of the magnetic shield
200.
The seam
250 may be formed such that the planar surfaces of the two pieces
220,
230 of the magnetic shield
200 abut or overlap.
Each of the pieces
220,
230 of the magnetic shield
200
may be fabricated from a single sheet of material by forming processes such as
deep-drawing or hydroforming. Alternatively, the first piece
220 and second
piece
230 may be fabricated from multiple pieces of material cut and bent
to shape, then welded to one another. Following the fabrication process(es), materials
suitable for magnetic shielding are typically annealed (a heat-treating process)
to develop optimum magnetic properties. Furthermore, both the sensing coil
104
and magnetic shield
200 may be attached to a support structure (not shown)
fabricated from non-magnetic material(s) that maintains their orientation and relative positions.
Although portions of the seam
250 may be substantially parallel to
the sensing axis
101—
101, precise parallelism of the seam
250
and the sensing axis
101—
101 is not necessary for effective
attenuation of an ambient magnetic field. For example, in an alternate embodiment,
the portion of the seam
250 along the wall
216 may have an angular
separation of less than substantially 10 degrees from the sensing axis
101—
101
of the sensing coil
104. Additionally, although the sensing axis
101—
101
is substantially parallel to the longitudinal axis of the magnetic shield
200,
this may vary in alternate embodiments.
FIG. 3 shows exemplary magnetic flux paths through the magnetic shield
200.
In the present embodiment, continuous, low reluctance magnetic flux paths may exist
in the longitudinal direction of the magnetic shield
200, such as along
flux paths
302 and
304. However, flux paths along the radial direction
of the magnetic shield
200 may be discontinuous due to the seam
250.
For example, flux paths
310a and
310b, and flux paths
312a and
312b, respectively, are discontinuous at the
seam
250 where the two pieces
220,
230 of the magnetic shield
200 connect.
Turning now to FIG. 4, another exemplary magnetic shield
400 is shown.
The magnetic shield
400 may include a first (e.g., inner) layer
402
and a second (e.g., outer) layer
404 of high-permeability material enclosing
the sensing coil
104. Although the sensing coil
104 is not shown
in FIG. 4, an exemplary orientation of the sensing coil
104 within the magnetic
shield
400 is indicated by the direction of the sensing axis
101—
101.
In general, multi-layer magnetic shields may provide greater attenuation of an
ambient magnetic field, or the ability to attenuate an ambient magnetic field of
larger magnitude, compared to single-layer designs.
In the present embodiment, the first layer
402 of the magnetic shield
400
may be substantially the same as the single-layer magnetic shield
200 shown
in FIG.
2. The second layer
404 may be a thin-walled right circular
cylinder with closed ends, with a longitudinal axis and a radial axis, but other
geometries (e.g. a sphere) are possible. In the present embodiment, the second
layer
404 may be comprised of a nickel-iron magnetic alloy with high permeability
at a low ambient magnetic field, but other magnetic materials with similar properties
may also be suitable.
Due to considerations such as ease of fabrication and assembly, the second layer
404 of the magnetic shield
400 may include a third piece
420
and a fourth piece
430 aligned and connected to each other at a seam
450.
The seam
450 may define a plane that is perpendicular to the longitudinal
axis of the second layer
404. Further, the seam
450 may be formed
such that the planar surfaces of the two pieces
420,
430 of the second
layer
404 abut or overlap.
Similar to the first layer
402, the third and fourth pieces
420,
430 of the second layer
404 may be fabricated from a single sheet
of material by forming processes such as deep-drawing, or hydroforming. Alternatively,
the pieces
420,
430 may be fabricated from multiple pieces of material
cut and bent to shape, then welded to one another. Following the fabrication process(es),
materials suitable for magnetic shielding are typically annealed (a heat-treating
process) to develop optimum magnetic properties.
Furthermore, both the sensing coil
104 and magnetic shield
400
may be attached to a support structure (not shown) fabricated from non-magnetic
material(s) that maintains the orientation and relative positions of the sensing
coil
104 and magnetic shield
400. In addition, the first and second
layers
402,
404 of the magnetic shield
404 may be attached
to an intermediate support structure (not shown) fabricated from non-magnetic material(s)
that maintains their orientation and relative positions.
FIG. 4 also shows that the sensing axis
101—
101 of the sensing
coil
104 is oriented substantially parallel to the longitudinal axis of
both the first and second layers
402,
404 of the magnetic shield
400. However, precise parallelism of the sensing axis
101—
101
and the longitudinal axes of the first and second layers
402,
404
is not necessary for effective attenuation of an ambient magnetic field.
Additionally, FIG. 4 also shows the seam
450 on the second layer
404 may be substantially perpendicular to the sensing axis
101—
101
of the sensing coil
104, though precise perpendicularity of the seam
450
and the sensing axis
101—
101 is not necessary for effective
attenuation of an ambient magnetic field. For example, in an alternate embodiment,
the second layer
404 may include a seam
450 having an angular separation
of more than substantially 80 degrees from the sensing axis
101—
101
of the sensing coil
104.
It should be understood that alternate embodiments of the multi-layer magnetic
shield
400 may include more than two layers of high-permeability material
(e.g., three layers). Furthermore, in an alternate embodiment, the second layer
404 may enclose the first layer
402 of the magnetic shield
400,
so that seam
250 is along an outer layer and seam
450 is along an
inner layer. Additionally, alternate orthogonal orientations of the seams
250,
450 in each of the layers
402,
404, respectively, are also possible.
FIG. 5 shows exemplary magnetic flux paths on the second (e.g., outer) layer
404 of the magnetic shield
400. Exemplary flux paths along the first
(e.g., inner) layer
402 of the magnetic shield
400 may be substantially
the same as the flux paths for the magnetic shield
200 shown in FIG.
3.
In the present embodiment, continuous, low reluctance magnetic flux paths may
exist in the radial direction of the second layer
404 of the magnetic shield
400, such as along flux paths
502,
504,
506, and
508.
Thus, magnetic flux may travel radially along the second layer
404 without
being interrupted by the seam
450. On the contrary, magnetic flux traveling
along the longitudinal direction of the second layer
404 may encounter a
discontinuity caused by the seam
450, as shown by exemplary flux paths
520a
and
520b. It should be understood that the flux paths described
in the present embodiments are merely exemplary, and any number of different flux
paths may be present within either of the magnetic shields
200 or
400.
Having described the structure and connectivity of the present embodiment,
its method of operation and advantages may now be discussed. For the magnetic shield
200, magnetic flux from an ambient magnetic field may travel continuously
along the direction of the sensing axis
101—
101 along the flux
paths
302,
304. Thus magnetic flux may travel continuously on the
magnetic shield
200 parallel to the direction of the sensing axis
101—
101.
As described earlier, the most sensitive direction for depolarized fiber optic
gyroscope operations is typically parallel to the sensing axis
101—
101
of the sensing coil
104. Thus, it may be advantageous for the magnetic shielding
to be effective in the direction parallel to the sensing axis
101—
101
if the negative effect of an ambient magnetic field is to be significantly reduced.
By enabling magnetic flux to travel continuously along magnetic flux paths
302,
304, which are substantially parallel to the direction of the sensing axis
101—
101, the magnetic shield
200 may protect the sensing
coil
104 from ambient magnetic flux traveling along this direction. Therefore,
in the present embodiment, the most effective orientation of the seam
250
of the magnetic shield
200 may be substantially parallel to the sensing
axis
101—
101.
The exemplary magnetic shield
400 also has various advantages. The first
layer
402 of the magnetic shield
400 may enable magnetic flux to
travel continuously parallel to the direction of the sensing axis
101—
101
(e.g., along flux paths
302,
304). Additionally, the second layer
404 of the magnetic shield
400 may enable magnetic flux to travel
continuously perpendicular to the direction of the sensing axis
101—
101
(e.g., along flux paths
502-
508). Since magnetic flux is less likely
to "leak in" through both orthogonal seams
250,
450, the effectiveness
of magnetic shield
400 may be especially high.
It should be understood that a wide variety of changes and modifications may
be
made to the embodiments of the magnetic shields
200,
400 described
above. For example, in an alternate embodiment, the sensing coil
104, and/or
magnetic shields
200,
400 may be a different geometry (e.g., rectangular
prisms). Furthermore, the magnetic shields
200,
400 may enclose the
entire fiber optic gyroscope, though only the sensing coil
104 may be enclosed
in alternate embodiments. Additionally, in an alternate embodiment, a single piece
may be connected with itself at a seam to form a wall of a magnetic shield, and
the seam may be substantially parallel to the sensing axis
101—
101
of the sensing coil
104. In addition, it should be understood that the present
embodiments may be utilized with any type of optical gyroscope (e.g., ring laser
gyroscope) or device that utilizes sensing coils. It is therefore intended that
the foregoing description illustrates rather than limits this invention, and that
it is the following claims, including all equivalents, which define this invention:
*
