Title: High accuracy miniature grating encoder readhead using fiber optic receiver channels
Abstract: A fiber optic encoder readhead for sensing the displacement of a scale grating is disclosed. The detector channels of the readhead are fiber optic detector channels having respective phase grating masks. The fiber optic encoder readhead is configured to detect the displacement of a self-image of the scale grating. In various exemplary embodiments, the fiber optic readhead is constructed according to various design relationships that insure a robust signal-to-noise ratio. Accordingly, high levels of displacement signal interpolation may be achieved, allowing sub-micrometer displacement measurements. The fiber optic encoder readhead may be assembled in a particularly accurate and economical manner and may be provided in a package with dimensions on the order of 1-2 millimeters.
Patent Number: 6,906,315 Issued on 06/14/2005 to Tobiason
| Inventors:
|
Tobiason; Joseph D. (Woodinville, WA)
|
| Assignee:
|
Mitutoyo Corporation (Kawasaki, JP)
|
| Appl. No.:
|
298312 |
| Filed:
|
November 15, 2002 |
| Current U.S. Class: |
250/237R; 250/231.16; 356/499 |
| Intern'l Class: |
G01B 009/02; G01D005/38 |
| Field of Search: |
250/23113,231.16,237.R,237.G,227.11
356/498,499,600,616,617
341/11,13,31
|
References Cited [Referenced By]
U.S. Patent Documents
| 3483389 | Dec., 1969 | Cronin.
| |
| 4733071 | Mar., 1988 | Tokunaga.
| |
| 4774494 | Sep., 1988 | Extance et al.
| |
| 5808730 | Sep., 1998 | Danielian et al.
| |
| 5909283 | Jun., 1999 | Eselun.
| |
| Foreign Patent Documents |
| 1 382 941 | Jan., 2004 | EP.
| |
| 59173713 | Oct., 1984 | JP.
| |
| 1272917 | Oct., 1989 | JP.
| |
Other References
Cowley, J.M., and Moodie, A.F., "Fourier Images: I—The Point Source,"
May 1, 1957, Proc. Phys. Soc. B 70:486-496.
Patorski, K., "The Self-Imaging Phenomenon and Its Applications," Progress
in Optics, ed. E. Wolf, 27, 3-108, North Holland, Amsterdam 1989.
|
Primary Examiner: Allen; Stephone B.
Attorney, Agent or Firm: Christensen O'Connor Johnson Kindness PLLC
Parent Case Text
CROSS-REFERENCE(S) TO RELATED APPLICATION(S)
This application claims the benefit of U.S. Provisional Application No. 60/396,659,
filed Jul. 16, 2002, under the provisions of 35 U.S.C. § 119, the disclosure
and drawings of which are incorporated herein by reference.
Claims
1. A device for measuring the relative displacement between two members, the
device comprising:
a scale having a scale grating formed along a measuring axis direction; and
a readhead operable to provide an operable self-image of the scale, the readhead
comprising:
a light source portion comprising at least one respective light source element;
and
a plurality of fiber-optic receiver channels, each respective fiber-optic receiver
channel comprising:
a respective receiver channel spatial phase mask portion having a respective
spatial phase and having its light-blocking elements arranged at a pitch that is
operable for spatially filtering the operable self-image of the scale, and generally
being located at a nominal spatial phase mask plane that is operable for spatially
filtering the operable self-image of the scale; and
at least one respective receiver channel optical fiber having an input end that
receives a respective receiver channel optical signal light;
wherein:
the respective receiver channel optical signal light received by the at least
one respective receiver channel optical fiber comprises optical signal light collected
through the respective receiver channel spatial phase mask portion over a respective
collected light area having a collected light area dimension along the measuring
axis direction that is at least three full periods of the respective receiver channel
spatial phase mask portion;
when the readhead is operably positioned relative to the scale grating at least
first and second respective channels of the plurality of fiber-optic receiver channels
spatially filter their respective portions of the operable self-image of the scale
at the nominal spatial phase mask plane to provide at least first and second respective
receiver channel optical signals having at least first and second respective signal
phases; and
the device outputs the at least first and second respective receiver channel
optical signals along respective optical fibers to provide relative displacement
measurement information in the form of a plurality of respective optical output
signals, the respective optical output signals produced without the use of an electronic
photodetector element.
2. The device of claim 1 wherein:
at least one respective light source element emits respective radiation distributed
as a respective source light that is directed towards the scale grating, the respective
source light having a respective source light central axis, the respective source
light diverging about that source light central axis at least proximate to the
scale grating to give rise to a respective scale light that is directed towards
the nominal spatial phase mask plane, the respective scale light having a respective
scale light central axis, the respective scale light diverging about that scale
light central axis at least proximate to the scale grating;
each fiber-optic receiver channel has a respective nominal light-carrying area
corresponding to an aggregate light-carrying core area of the at least one respective
receiver channel optical fiber and the nominal light-carrying area corresponds
to the area of a circle having a circle diameter of at most 2 millimeters; and
the nominal light-carrying area of each fiber-optic receiver channel has a nominal
centroid and at least proximate to the input end of the at least one respective
receiver channel optical fiber the nominal centroid is separated from at least
one respective scale light central axis by a nominal respective location radius
that is at most 8 times the circle diameter corresponding to that nominal light-carrying
area.
3. The device of claim 2 wherein the circle diameter of the circle corresponding
to the nominal light carrying area is at most 1 millimeter.
4. The device of claim 2 wherein the circle diameter of the circle corresponding
to the nominal light carrying area is at most 0.5 millimeters.
5. The device of claim 2 wherein:
when the readhead is operably positioned relative to the scale grating to provide
an operable self-image, for at least one respective scale light a total illumination
circle may be defined at the nominal spatial phase mask plane such that at least
95% of the optical power due to that respective scale light is included in that
total illumination circle, that total illumination circle has a corresponding total
illumination radius R
tot, and a corresponding illumination field radius
R may be defined as R=(R
tot/2.55); and
when the nominal respective location radius is at least 5 times the circle diameter
corresponding to the nominal light-carrying area, the light source portion is configured
to distribute a respective source light corresponding to a respective scale light
such that the illumination field radius R for that respective scale light at the
nominal spatial phase mask plane satisfies the condition that R is at least 0.5
times the nominal respective location radius and less than 1.05 times the nominal
respective location radius when the readhead is operably positioned relative to
the scale grating according to at least one operable configuration that is specified
for the device and that provides an operable self-image.
6. The device of claim 5 wherein when the nominal respective location radius
is at least 3 times the circle diameter corresponding to the nominal light-carrying
area and less than 5 times that circle diameter, the illumination field radius
R at the nominal spatial phase mask plane satisfies the condition that R is at
least 0.35 times the nominal respective location radius and less than 2.2 times
the nominal respective location radius.
7. The device of claim 6 wherein when the nominal respective location radius
is at least 1 times the circle diameter corresponding to the nominal light-carrying
area and less than 3 times that circle diameter, the illumination field radius
R at the nominal spatial phase mask plane satisfies the condition that R is at
least 0.21 times the nominal respective location radius and less than 3.9 times
the nominal respective location radius.
8. The device of claim 7 wherein when the nominal respective location radius
is less than 1 times the circle diameter corresponding to the nominal light-carrying
area, the illumination field radius R at the nominal spatial phase mask plane satisfies
the condition that R is at least 0.21 times the nominal respective location radius
and less than 10.25 times the nominal respective location radius.
9. The device of claim 2 wherein:
when the readhead is operably positioned relative to the scale grating to provide
an operable self-image, for at least one respective scale light a total illumination
circle may be defined at the nominal spatial phase mask plane such that at least
95% of the optical power due to that respective scale light is included in that
total illumination circle, that total illumination circle has a corresponding total
illumination radius R
tot, and a corresponding illumination field radius
R may be defined as R=(R
tot/2.55); and
the light source portion is configured to distribute a respective source light
corresponding to a respective scale light such that the illumination field radius
R for that respective scale light at the nominal spatial phase mask plane satisfies
the condition that R is at least 0.5 times the nominal respective location radius
and less than 1.05 times the nominal respective location radius when the readhead
is operably positioned relative to the scale grating according to at least one
operable configuration that is specified for the device and that provides an operable
self-image, regardless of the ratio between the nominal respective location radius
and the circle diameter corresponding to the nominal light-carrying area.
10. The device of claim 2 wherein at least three respective fiber-optic receiver
channels of the plurality of fiber-optic receiver channels each have a respective
nominal centroid that is separated from a same respective scale light central axis
by a nominal respective location radius that is approximately the same for each
of the at least three respective fiber-optic receiver channels.
11. The device of claim 2 wherein each light source element comprises a source
optical fiber connectable to a remote radiation source that provides radiation
operable to produce self-images, the source optical fiber having an output end,
at least a portion of the output end emitting the respective radiation.
12. The device of claim 11 wherein each source optical fiber comprises a single-mode
optical fiber and the at least a portion of the of the output end that emits the
respective radiation comprises a light-carrying core area of the single-mode optical fiber.
13. The device of claim 2 wherein the readhead is configured such that each respective
source light central axis is approximately collinear with its corresponding respective
scale light central axis when the readhead is nominally aligned relative to the
scale grating.
14. The device of claim 1 wherein the readhead is located entirely on a first
side of the scale grating, the scale grating includes reflective elements, and
respective scale light that is directed towards the nominal spatial phase mask
plane comprises light reflected from the scale grating.
15. The device of claim 1 wherein the readhead comprises a transparent mask substrate
and each respective receiver channel spatial phase mask portion is fabricated on
a surface of the transparent mask substrate with its light-blocking elements positioned
along the measuring axis direction with respect to the light-blocking elements
of the other receiver channel spatial phase mask portions in a manner that establishes
desired relationships between the respective spatial phases of the respective receiver
channel spatial phase mask portions.
16. The device of claim 15 wherein the input end of each respective receiver
channel optical fiber is nominally positioned against the corresponding respective
receiver channel spatial phase mask portion on the surface of the transparent mask substrate.
17. The device of claim 1 wherein the respective collected light area is at least
partially determined by at least one of a) an aggregate light-carrying core area
proximate to the input end of the corresponding at least one respective receiver
channel optical fiber, b) a light receiving area of a miniature lens positioned
proximate to the respective receiver channel spatial phase mask portion and proximate
to the input end of the at least one respective receiver channel optical fiber
and c) a limiting aperture feature of the respective receiver channel spatial phase
mask portion.
18. The device of claim 1 wherein the readhead is configured such that the respective
receiver channel optical signal light downstream of one respective receiver channel
spatial phase mask portion does not intersect with the respective receiver channel
optical signal light downstream of a different respective receiver channel spatial
phase mask portion prior to being received by the input end of the at least one
respective receiver channel optical fiber.
19. The device of claim 1 wherein at least each collected light area and each
input end are positioned entirely within a cylindrical volume having an axis perpendicular
to the nominal spatial phase mask plane and having a cylinder radius that is at
most 3 millimeters.
20. The device of claim 19 wherein the cylinder radius containing at least each
collected light area and input end is at most 2.0 millimeters.
21. The device of claim 20 wherein the cylinder radius containing the collected
light area and input end is at most 1.25 millimeters.
22. The device of claim 21 wherein the cylinder radius containing the collected
light area and input end is at most 0.5 millimeters.
23. The device of claim 19 wherein each respective light source element comprises
one of a) an electronic solid-state laser element, at least a portion of the solid-state
laser element emitting the respective radiation, b) an electronic solid-state light
emitting diode element, at least a portion of the solid-state light emitting diode
element emitting the respective radiation and c) a source optical fiber connectable
to a remote radiation source that provides radiation operable to produce self-images,
the source optical fiber having an output end, at least a portion of the output
end emitting the respective radiation.
24. The device of claim 23 wherein:
the readhead is located entirely on a first side of the scale grating;
the scale grating comprises reflective elements; and
at least one respective light source element emits respective radiation distributed
as a respective source light that is directed towards the scale grating, the respective
source light having a respective source light central axis, the respective source
light diverging about that source light central axis at least proximate to the
scale grating to give rise to a respective scale light that is reflected towards
the nominal spatial phase mask plane, the respective scale light having a respective
scale light central axis, the respective scale light diverging about that scale
light central axis at least proximate to the scale grating.
25. The device of claim 24 wherein:
the respective source light is distributed toward the scale grating from at least
one of a) at least one nominal point source and b) at least one nominal line source
comprising a line source slit in a source grating;
each nominal point source and each nominal line source are located proximate
to the nominal spatial phase mask plane and the nominal spatial phase mask plane
coincides with a surface of a transparent mask substrate, the transparent mask
substrate carrying at least each respective receiver channel spatial phase mask
portion.
26. The device of claim 24 wherein the respective radiation is emitted from a
portion of the light source element that is positioned entirely within the cylindrical volume.
27. The device of claim 26 wherein the readhead is configured such that when
the readhead is nominally aligned relative to the scale grating at least one respective
source light central axis is oriented to intersect with the scale grating along
a direction which is approximately normal to the scale grating plane at the point
of intersection such that the at least one respective scale light is reflected
along a respective scale light central axis that is nominally collinear with its
respective source light central axis.
28. The device of claim 27 wherein:
each respective light source element comprises a source optical fiber, the portion
of the output end of the source optical fiber that emits the respective radiation
comprising a light-carrying core area at the output end of the source optical fiber;
each fiber-optic receiver channel has a respective nominal light-carrying area
corresponding to an aggregate light-carrying core area of the at least one respective
receiver channel optical fiber, the respective nominal light-carrying area proximate
to the input end of the at least one respective receiver channel optical fiber
having a respective nominal centroid; and
at least three respective fiber-optic receiver channels of the plurality of fiber-optic
receiver channels each have a respective nominal centroid that is separated from
a same light-carrying core area at the output end of shared source optical fiber
by a nominal respective location radius that is approximately the same for each
of the at least three respective fiber-optic receiver channels.
29. The device of claim 28 wherein the respective receiver channel optical fibers
corresponding to the at least three respective fiber-optic receiver channels are
positioned substantially against the shared source optical fiber at least proximate
to the input ends of the respective receiver channel optical fibers and proximate
to the output end of the source optical fiber.
30. The device of claim 29 wherein the shared source optical fiber comprises
a single mode optical fiber, the portion of the output end of the source optical
fiber that emits the respective radiation comprises a single mode core area that
operates to provide respective radiation distributed from a nominal point source,
and the single mode core area is surrounded by optical fiber material that provides
outer dimensions for the source optical fiber that nominally fit in a close-pack
fashion with the respective receiver channel optical fibers corresponding to the
at least three respective fiber-optic receiver channels that are positioned substantially
against the shared source optical fiber.
31. The device of claim 28 wherein:
the at least one respective light source element consists of one source optical
fiber; and
the at least three respective fiber-optic receiver channels that each have a
respective nominal centroid that is separated from a same light-carrying core area
at the output end of the source optical fiber by a nominal respective location
radius that is approximately the same for each of the at least three respective
fiber-optic receiver channels comprise all of the plurality fiber-optic receiver
channels.
32. The device of claim 31 wherein the plurality fiber-optic receiver channels
comprise at least 2N respective fiber-optic receiver channels arranged in an arrangement
of N operable pairs, where N is an integer equal to at least 2, each operable pair
comprising two respective fiber-optic receiver channels arranged on opposite sides
of a center of the arrangement of N operable pairs, wherein the two respective
spatial phase mask portions corresponding to those two respective fiber-optic receiver
channels have one of a) the same spatial phase and b) spatial phases that nominally
differ by 180 degrees.
33. The device of claim 27 further comprising a reflective surface, wherein:
the reflective surface is arranged to deflect each respective source light central
axis and each respective scale light central axis by approximately 90 degrees at
a location along the axes between the readhead and the scale grating; and
the readhead and reflective surface are arranged relative to the scale grating
such that the nominal spatial phase mask plane and the operable self-image of the
scale grating are nominally perpendicular to the plane of the scale grating.
34. The device of claim 24 wherein the readhead further comprises a readhead
housing element that surrounds at least all of the optical fibers included in the
readhead, the readhead housing element having a relatively longer outer dimension
in a length direction parallel to the axis of the optical fibers and relatively
narrower outer dimensions in directions perpendicular to the axis of the optical
fibers over at least a portion of its length, and the readhead is constructed such
that at least a portion of the length of the readhead can be inserted into a bore
having a dimension perpendicular to its central axis that is at least as small
as 2.5 millimeters.
35. The device of claim 34 wherein the readhead is assembled into an orientation-maintaining
connector that is mechanically interchangeable with at least one standard commercially-available
polarization-maintaining optical fiber connector.
36. The device of claim 1 wherein, when there is relative displacement between
the readhead and scale grating along the measuring axis direction, each respective
optical output signal comprises a sinusoidal function of the relative displacement,
and each such sinusoidal function varies from an ideal sinusoidal function by at
most 1/32 of the peak-to-peak variation of each such sinusoidal function.
37. The device of claim 36 wherein each such sinusoidal function varies from
an ideal sinusoidal function by at most 1/64 of the peak-to-peak variation of each
such sinusoidal function.
38. A device for measuring the relative displacement between two members, the
device comprising:
a scale having a scale grating formed along a measuring axis direction, the scale
grating comprising reflective elements; and
a readhead located entirely on a first side of the scale grating, the readhead
operable to provide an operable self-image of the scale, the readhead comprising:
a light source portion comprising at least one respective light source element;
and
a plurality of fiber-optic receiver channels, each respective fiber-optic receiver
channel comprising:
a respective receiver channel spatial phase mask portion having a respective
spatial phase and having its light-blocking elements arranged at a pitch that is
operable for spatially filtering the operable self-image of the scale, and generally
being located at a nominal spatial phase mask plane that is operable for spatially
filtering the operable self-image of the scale; and
at least one respective receiver channel optical fiber having an input end that
receives a respective receiver channel optical signal light;
wherein:
the respective receiver channel optical signal light received by the at least
one respective receiver channel optical fiber comprises light reflected from the
scale grating and collected through the respective receiver channel spatial phase
mask portion over a respective collected light area having a collected light area
dimension along the measuring axis direction that is at least one full period of
the respective receiver channel spatial phase mask portion, such that a respective
signal phase corresponding to the respective spatial phase is relatively insensitive
to the position of the respective collected light area relative to the light-blocking
elements of the receiver channel spatial phase mask portion;
at least each collected light area and each input end are positioned entirely
within a cylindrical volume having an axis perpendicular to the nominal spatial
phase mask plane and having a cylinder radius that is at most 3 millimeters;
when the readhead is operably positioned relative to the scale grating at least
first and second respective channels of the plurality of fiber-optic receiver channels
spatially filter their respective portions of the operable self-image of the scale
at the nominal spatial phase mask plane to provide at least first and second respective
receiver channel optical signals having at least first and second respective signal
phases; and
the device outputs the at least first and second respective receiver channel
optical signals along respective optical fibers to provide relative displacement
measurement information in the form of a plurality of respective optical output
signals, the respective optical output signals arising from spatially filtered
scale light without the use of an electronic photodetector element.
39. The device of claim 38 wherein the readhead comprises a transparent mask
substrate and each respective receiver channel spatial phase mask portion is fabricated
on a surface of the transparent mask substrate with its light-blocking elements
positioned along the measuring axis direction with respect to the light-blocking
elements of the other receiver channel spatial phase mask portions in a manner
that establishes desired relationships between the respective spatial phases of
the respective receiver channel spatial phase mask portions.
40. The device of claim 39 wherein:
each respective light source element comprises a source optical fiber;
a light-carrying core area at the output end of the source optical fiber emits
a radiation that is operable to provide the operable self-image of the scale; and
a light-carrying core area at the output end of the source optical fiber is positioned
entirely within the cylindrical volume.
41. A method for operating a device for measuring the relative displacement between
two members, the device comprising:
a scale having a scale grating formed along a measuring axis direction, the scale
grating comprising reflective elements; and
a readhead located entirely on a first side of the scale grating, the readhead
operable to provide an operable self-image of the scale, the readhead comprising:
a light source portion comprising at least one respective light source element;
a transparent mask substrate; and
a plurality of fiber-optic receiver channels, each respective fiber-optic receiver
channel comprising:
a respective receiver channel spatial phase mask portion having a respective
spatial phase and having its light-blocking elements arranged at a pitch that is
operable for spatially filtering the operable self-image of the scale, and generally
being located at a nominal spatial phase mask plane that is operable for spatially
filtering the operable self-image of the scale; and
at least one respective receiver channel optical fiber having an input end that
receives a respective receiver channel optical signal light;
wherein:
each respective receiver channel spatial phase mask portion is fabricated on
a surface of the transparent mask substrate with its light-blocking elements positioned
along the measuring axis direction with respect to the light-blocking elements
of the other receiver channel spatial phase mask portions in a manner that establishes
desired relationships between the respective spatial phases of the respective receiver
channel spatial phase mask portions;
the respective receiver channel optical signal light received by the at least
one respective receiver channel optical fiber comprises light reflected from the
scale grating and collected through the respective receiver channel spatial phase
mask portion over a respective collected light area having a collected light area
dimension along the measuring axis direction that is at least three full periods
of the respective receiver channel spatial phase mask portion; and
each respective fiber-optic reciver channel has a respective nomial light-carrying
area corresponding to an aggregate light-carrying core area of the at least one
respective receiver channel optical fiber and the nominal light-carrying area corresponds
to the area of a circle having a circle diameter of at most 2 millimeters,
the method comprising:
operably positioning the readhead relative to the scale grating;
operating the readhead such that at least one respective light source element
emits a respective source light directed towards the scale grating to give rise
to at least one respective scale light reflected towards the nominal spatial phase
mask plane, the respective scale light including the operable self-image of the
scale grating that coincides with a nominal spatial phase mask plane;
receiving the respective scale light including the operable self-image with at
least first and second respective channels of the plurality of fiber-optic receiver
channels and spatially filtering respective portions of the scale light including
the operable self-image at the nominal spatial phase mask plane to provide at least
first and second respective receiver channel optical signals having at least first
and second respective signal phases; and
outputting the at least first and second respective receiver channel optical
signals along respective optical fibers to provide relative displacement measurement
information in the form of a plurality of respective optical output signals, the
respective optical output signals arising from spatially filtered scale light without
the use of an electronic photodetector element.
42. The method of claim 41 wherein:
the respective nominal light-carrying area has a respective nominal centroid;
when the readhead is operated to give rise to the at least one respective scale
light including the operable self-image of the scale grating:
for at least one respective scale light a total illumination circle may be defined
at the nominal spatial phase mask plane such that at least 95% of the optical power
due to that respective scale light is included in that total illumination circle,
that total illumination circle has a corresponding total illumination radius R
tot,
and a corresponding illumination field radius R may be defined as R=(R
tot/2.55);
and
at least proximate to the input end of at least one respective receiver channel
optical fiber the respective nominal centroid is separated from a nominal central
axis of at least one respective scale light by a nominal respective location radius,
and
when the nominal respective location radius is at least 5 times the circle diameter
corresponding to the nominal light-carrying area, the step of operably positioning
the readhead relative to the scale grating comprises positioning the readhead such
that the illumination field radius R for that respective scale light at the nominal
spatial phase mask plane satisfies the condition that R is at least 0.5 times the
nominal respective location radius and less than 1.05 times the nominal respective
location radius.
43. The method of claim 42 wherein when the nominal respective location radius
is at least 3 times the circle diameter corresponding to the nominal light-carrying
area and less than 5 times that circle diameter, the step of operably positioning
the readhead relative to the scale grating further comprises positioning the readhead
such that the illumination field radius R at the nominal spatial phase mask plane
satisfies the condition that R is at least 0.35 times the nominal respective location
radius and less than 2.2 times the nominal respective location radius.
44. The method of claim 43 wherein when the nominal respective location radius
is at least 1 times the circle diameter corresponding to the nominal light-carrying
area and less than 3 times that circle diameter, the step of operably positioning
the readhead relative to the scale grating further comprises positioning the readhead
such that the illumination field radius R at the nominal spatial phase mask plane
satisfies the condition that R is at least 0.21 times the nominal respective location
radius and less than 3.9 times the nominal respective location radius.
45. The method of claim 44 wherein when the nominal respective location radius
is less than 1 times the circle diameter corresponding to the nominal light-carrying
area, the step of operably positioning the readhead relative to the scale grating
further comprises positioning the readhead such that the illumination field radius
R at the nominal spatial phase mask plane satisfies the condition that R is at
least 0.21 times the nominal respective location radius and less than 10.25 times
the nominal respective location radius.
46. The method of claim 41 wherein:
the respective nominal light-carrying area has a respective nominal centroid;
when the readhead is operated to give rise to the at least one respective scale
light including the operable self-image of the scale grating:
for at least one respective scale light a total illumination circle may be defined
at the nominal spatial phase mask plane such that at least 95% of the optical power
due to that respective scale light is included in that total illumination circle,
that total illumination circle has a corresponding total illumination radius R
tot,
and a corresponding illumination field radius R may be defined as R=(R
tot/2.55);
and
at least proximate to the input end of at least one respective receiver channel
optical fiber the respective nominal centroid is separated from a nominal central
axis of at least one respective scale light by a nominal respective location radius,
and
the step of operably positioning the readhead relative to the scale grating comprises
positioning the readhead such that the illumination field radius R for that respective
scale light at the nominal spatial phase mask plane satisfies the condition that
R is at least 0.5 times the nominal respective location radius and less than 1.05
times the nominal respective location radius, regardless of the ratio between the
nominal respective location radius and the circle diameter corresponding to the
nominal light-carrying area.
Description
FIELD OF THE INVENTION
This invention relates generally to displacement sensing optical encoders, and
more particularly to an optical encoder utilizing optical fibers as receiver elements
to provide an ultra-compact high accuracy system.
BACKGROUND OF THE INVENTION
Various movement or position encoders for sensing linear, rotary or angular
movement are currently available. These encoders are generally based on either
optical systems, magnetic scales, inductive transducers, or capacitive transducers.
For optical encoders, a number of systems have been developed. One recent system
utilizing fewer parts than most previous systems is disclosed in U.S. Pat. No.
5,909,283, to Eselun. The system described in the '283 patent has a grating scale
and readhead including a point source (laser diode in readhead), a Ronchi grating
or holographic element, and a photodetector array. As described, the point source
results in interference fringes having a spacing equal to that of the scale. The
interference fringe light is transmitted through the Ronchi grating or holographic
element to the photodetector array. The photodetector array is arranged to derive
four channels of quadrature signals from the transmitted fringe light. However,
the resulting encoder is still of a size that is relatively large or prohibitive
for a number of applications.
One system utilizing optical fibers as receivers is disclosed in U.S. Pat. No.
4,733,071, to Tokunaga. The system described in the '071 patent has a code member
scale, and an optical sensor head comprising an optical fiber tip light emitter
and two optical fiber tip receptors closely arranged along the code member measuring
axis. The optical sensor head is rotated (yawed) to adjust phase difference between
the two optical fiber tip receptors. However, the accuracy of the resulting encoder
is relatively crude.
SUMMARY OF THE INVENTION
The present invention is directed to providing an encoder that overcomes the
foregoing and other disadvantages. More specifically, the present invention is
directed to an optical encoder that is of extremely small size while providing
very high accuracy, in addition to having a number of other desirable features.
A fiber optic encoder readhead for sensing the displacement of a scale grating
is disclosed. The readhead includes a light source for transmitting light to the
scale grating and detector channels for receiving light from the scale grating.
In accordance with one aspect of the invention, the detector channels of the encoder
readhead are fiber optic detector channels. Electronic readhead receivers (photodetectors)
such as disclosed in the '283 patent suffer limitations in converting the high
frequency detector signals associated with high speed scale motion and transmitting
those signals over long cables without significant signal loss or interference.
In addition, electronic photodetectors and the associated circuit connections contribute
to readheads that are too large for many potential encoder applications. It will
be appreciated that the fiber optic detector channels of the present invention
overcome these limitations.
In accordance with another aspect of the invention, the fiber optic encoder readhead
detects the location of a scale grating image using multiple fiber optic detector
channels having respective phase grating masks. Optical fiber tip receptors such
as those disclosed in the '071 patent have insufficient spatial resolution for
fine phase signal discrimination if they have a large diameter, and gather too
little light to provide a good signal if they have a small diameter. Thus, their
accuracy is limited. It will be appreciated that the fiber optic detector channels
of the present invention overcome these and other limitations to provide high accuracy.
In accordance with another aspect of the invention, the scale grating image detected
by the multiple fiber optic detector channels is a self-image, also known by other
names such as a Talbot image, which provides for relatively robust alignment tolerances
and high resolution.
In accordance with another aspect of the invention, the fiber optic encoder readhead
is constructed according to a design relationship based on an input aperture size
of the fiber optic detector channels, to insure reliable signals and enhanced accuracy.
In accordance with a separate aspect of the invention, the fiber optic detector
channels are arranged in balanced pairs, to provide enhanced accuracy.
In accordance with a further aspect of the invention, 3 balanced pairs of fiber
optic detector channels are signal processed in a manner that provides enhanced accuracy.
In accordance with a separate aspect of the invention, the light source is provided
by an optical fiber, to provide an all-optical readhead, free of all limitations
and costs associated with electronic assembly and electronic signals in an encoder readhead.
In accordance with a separate aspect of the invention, the various optical fibers
of the fiber optic encoder are selected from various types such that the encoder
measurement accuracy is relatively unaffected by bending of the fiber optic readhead cable.
In accordance with a separate aspect of the invention, various embodiments of
the fiber optic encoder readhead are constructed in a particularly economical,
accurate and compact manner.
In accordance with a separate aspect of the invention, the fiber optic encoder
readhead is constructed such that it may be inserted into a standard commercially
available fiber optic connector configuration.
In accordance with a separate aspect of the invention, a light deflecting element
is provided to deflect the readhead light path between the basic readhead elements
and the scale grating, such that the operable mounting orientation of the readhead
relative to the scale is changed.
In accordance with separate aspect of the invention, in one embodiment a remote
interface box is utilized that contains appropriate electronic light sources and
photodetectors that interface with the fiber optics to and from one or more fiber
optic readheads according to this invention, and converts received optical signals
to a form suitable for further signal processing and readhead position determination.
Hence, the invention overcomes the disadvantages of prior art optical-displacement
sensing devices and provides new application possibilities with an ultra-compact,
highly accurate, economical and high speed configuration.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing aspects and many of the attendant advantages of this invention
will become more readily appreciated as the same become better understood by reference
to the following detailed description, when taken in conjunction with the accompanying
drawings, wherein:
FIG. 1 is an isometric view of a first generic embodiment of a fiber-optic receiver
channel arrangement according to this invention;
FIG. 2 is an isometric view of a first generic embodiment of a self-imaging
arrangement usable in various exemplary fiber optic readheads according to this invention;
FIG. 3 is an isometric view of a first generic embodiment of a fiber-optic readhead
arrangement according to this invention;
FIG. 4 is an isometric view of a second generic embodiment of a fiber-optic
readhead arrangement according to this invention;
FIG. 5 is a partly orthographic, partly isometric view of a third embodiment
of a fiber-optic readhead arrangement according to this invention;
FIG. 6 shows an exemplary a receiver channel optical fiber usable according
to this invention;
FIG. 7 shows an exemplary a light source optical fiber usable according to this invention;
FIG. 8 shows a block diagram including a remote electronic interface unit usable
in conjunction with a fiber-optic readhead according to this invention;
FIG. 9 is a diagram showing representative signal to noise ratios that result
for various receiver aperture diameters when the receiver aperture of a fiber optic
detector channel is positioned at various radii from the center of an illumination
field, for a fiber-optic readhead arrangement approximately corresponding to FIG. 3;
FIG. 10 is a partly orthographic, partly isometric view of a fourth embodiment
of a fiber-optic readhead arrangement according to this invention;
FIG. 11 shows an exemplary phase mask element usable in various fiber-optic
readhead arrangements according to this invention;
FIG. 12 shows a first exemplary embodiment of a fiber-optic readhead and cable
according to this invention;
FIG. 13 is a diagram illustrating a yaw misalignment consideration relevant
to various exemplary embodiments according to this invention;
FIG. 14 shows a fifth exemplary embodiment of a fiber-optic readhead arrangement
according to this invention;
FIG. 15 shows a sixth exemplary embodiment of a fiber-optic readhead arrangement
according to this invention;
FIG. 16 shows a seventh exemplary embodiment of a fiber-optic readhead arrangement
according to this invention;
FIG. 17 shows an eighth exemplary embodiment of a fiber-optic readhead arrangement
according to this invention;
FIG. 18 shows a ninth exemplary embodiment of a fiber-optic readhead arrangement
according to this invention, which uses a source grating;
FIG. 19A shows an optical deflector usable in conjunction with various fiber-optic
readheads according to this invention in a first orientation relative to a scale grating;
FIG. 19B shows an optical deflector usable in conjunction with various fiber-optic
readheads according to this invention in a second orientation relative to exemplary
scale gratings; and
FIG. 20 shows a mounting bracket and optical deflector usable in conjunction
with various fiber-optic readheads according to this invention, arranged in a first
orientation relative to a rotary scale grating.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 1 shows a first generic embodiment of a fiber-optic receiver channel arrangement
100 according to this invention. As shown in FIG. 1, the fiber-optic receiver
channel arrangement
100 includes three fiber-optic receiver channels
190A,
190B and
190C. The fiber-optic receiver channel
190A includes
a receiver channel aperture
110A, a phase mask
120A, and a receiver
optical fiber
130A. Similarly, The fiber-optic receiver channel
190B
includes a receiver channel aperture
110B, a phase mask
120B, and
a receiver optical fiber
130. Similarly, The fiber-optic receiver channel
190C includes a receiver channel aperture
110C, a phase mask
120C,
and a receiver optical fiber
130C.
For each fiber-optic receiver channel
190, the phase mask
120 includes
a grating that completely covers the receiver channel aperture
110, acting
as a spatial filter for incoming illumination. The receiver optical fiber
130
is aligned with the receiver channel aperture
110 such that nominally all
illumination received by the receiver channel aperture
110 is channeled
down the optical fiber
130 to provide an optical signal
191. In various
exemplary embodiments the receiver channel aperture
110 is simply a flat
end of the receiver optical fiber
130. In various other exemplary embodiments
the receiver channel aperture
110 is a shaped end of the receiver optical
fiber
130. In various other exemplary embodiments the receiver channel aperture
110 is a compact refractive or diffractive lens, which gathers the incoming
illumination through the phase mask
120, concentrates the light, and directs
the light to the end of the receiver optical fiber
130, which is aligned
to receive the light efficiently. The receiver channel aperture
110, the
phase mask
120 and the end of the receiver optical fiber
130 of each
fiber-optic receiver channel
190 are fastened in a fixed relationship to
each other by adhesives or other suitable methods.
In various exemplary embodiments according to this invention, the phase masks
120 are arranged in a coplanar arrangement which defines and/or coincides
with a nominal receiving plane
160. Various exemplary embodiments of the
phase masks
120, as well as their specific orientation and individual phase
positions are described in detail further below. The location of the receiver channel
apertures
110 is conveniently described with reference to a channel arrangement
center
157 of the fiber-optic receiver channel arrangement
100. In
various high accuracy optical fiber readhead embodiments according to this invention,
the channel arrangement center
157 is positioned to coincide with the nominal
center of any illumination field presented to the fiber-optic receiver channel
arrangement
100, as described further below. The effective center of each
respective receiver channel aperture
110A-
110C is located at a respective
location radius
140A-
140C from the channel arrangement center
157,
as shown in FIG.
1. The receiver aperture location radius is generically
indicated as R
AL herein. For purposes of this invention, in various
embodiments where a receiver channel aperture
110 does not have an obvious
geometric center, the effective center may be taken as the centroid of the aperture area.
Useful receiver aperture location radii
140, and aperture areas, may
be determined according to the principles of this invention as discussed in detail
with reference to FIGS. 9-12, below. In various exemplary embodiments the receiver
channel apertures
110 are identical and their respective location radii
140 are identical. Generally, using identical fiber-optic receiver channels
190 in a fiber optic readhead according to this invention allows simpler
construction, simpler signal processing and relatively higher measurement accuracy.
However, more generally, the receiver channel apertures
110 and/or their
respective location radii
140 need not be identical in various exemplary
embodiments according to this invention.
The fiber-optic receiver channels
190 are generally arranged in a fixed
relationship to each other. In particular, the gratings of the phase masks
120
of each fiber-optic receiver channel
190 are nominally coplanar and are
fixed in a particular spatial phase relationship with respect to one another in
the receiving plane
160. In various exemplary embodiments the phase masks
120 are fixed in a particular spatial phase relationship by fabricating
them on a single mask substrate, as described further below. Exemplary assembly
pieces and methods are discussed in detail further below.
FIG. 2 shows a first generic embodiment of a self-imaging arrangement
200
usable in various exemplary fiber optic readheads according to this invention.
The basic principle of self-images, also known as Talbot images, is well known
and is not described in detail here. One classic analysis is presented in the paper
by Cowley, J. M., and Moodie, A. F., 1957
, Proc. Phys. Soc. B, 70, 486,
which is incorporated herein by reference. The self-imaging arrangement
200
includes a light source
280 and a scale grating
80, separated by
a source gap
284. The dimension of the source gap is generally indicated
as either z
s or, if the source gap
284 and an image gap
285
are the same, as z herein. The scale grating
80 is aligned along a measuring
axis
82 and includes grating elements or bars extending perpendicular to
the measuring axis
82, as indicated by vertical lines in an illumination
spot
253. The grating elements or bars are arranged periodically along the
measuring axis
82 according to a grating period
81, generally indicated
herein as the grating period or grating pitch P
g.
The X, Y and Z axes shown in FIG. 2 may be defined with reference to the plane
of the scale grating
80. The X axis is parallel to the plane of the scale
grating
80 and to the measuring axis
82. The X-Y plane is parallel
to the plane of the scale grating
80 and the Z axis is perpendicular to
that plane.
In the generic self-imaging arrangement
200 the light source
280
emits a source light
250 generally along a source light axis
251.
The source light is generally monochromatic or quasi-monochromatic and has a nominal
wavelength λ. The source light
250 generally diverges at a divergence
half-angle
252. The source light
250 travels over a distance equal
to a source gap
284 and illuminates the scale grating
80 at an illumination
spot
253 and is reflected as scale light
254 generally along a scale
light axis
255. In the embodiment shown in FIG. 2, the source light axis
251 and the scale light axis
255 are parallel to the Z axis and mutually
coincide. The scale light
254 travels over a distance equal to the image
gap
285 to a self-image plane
265. The dimension of the image gap
is generally indicated as z herein. In a self image plane
265, the scale
light
254 provides an illumination field
256 including a self-image
266. The illumination field
256 has an illumination field center
257 and a nominal illumination field radius
258. The self-image
266
is an image consisting of light and dark stripes, each extending perpendicular
to the measuring axis
82. The light and dark stripes are periodic in the
direction parallel to the measuring axis
82 according to a self-image period
83, generally indicated herein as the self-image period or self-image pitch P
si.
In the self-imaging arrangement
200, the self-image plane
226 is
parallel to the plane of the scale grating
80. It should be appreciated
that self-images are localized in space at a particular set of self-image planes.
When the light source
280 is effectively a point source, and the arrangement
is approximately as shown in FIG. 2, the self-image conditions for usable the self-image
planes, including both "in phase" images and "reverse images" are:
##EQU1##
and for the magnification of the image pitch P
si relative to the grating
pitch P
g:
##EQU2##
where:
ν=0, 1, 2, . . .
z
s is the source gap;
z is the image gap; and
λ is the wavelength of the source light.
Thus, for the configuration shown in FIG. 2, with z=z
s usable self-image
planes are located at integer multiples of 2P
g2/λ and
the pitch P
si will be twice the grating pitch P
g.
It should be appreciated that there are also images generally known as Fresnel
images located at planes between the self-image planes. So long as the pitch of
the phase masks
120 are adjusted to match the pitch of a chosen Fresnel
image, Fresnel images may be used as self-images according to the principles of
this invention and are encompassed within the term self-image as used herein. The
characteristics of Fresnel images can be understood and applied with reference
the article by Krzysztof Patorski, The Self-Imaging Phenomenon and Its Applications,
Progress in Optics, ed. E. Wolf, 27, 3-108, North Holland, Amsterdam 1989.
In various other embodiments according to this invention, the scale grating
80
is a reflective phase grating type scale specifically constructed such that the
0
th order reflection from the scale is suppressed. While the self-images
of a phase grating are not usable for an encoder, other usable images are available
that give stronger signal than available with an amplitude grating such as that
in the analysis above. It should be appreciated that for such embodiments, the
location of the usable images deviates from the location of the self-images in
the analysis above. The distance between the best usable image planes will remain
the same as analyzed above, except there will be a certain additional offset in
the gap between the scale and the first usable image plane of half the distance
between usable image planes. For instance, a phase grating of 20 micron period
with source wavelength 780 nm in a reflective configuration with z=z
s
will have usable image planes (with successively opposing phases) at nominal gaps
of z=0.513+ν*1.026 mm, ν=1,2,3 . . . , neglecting possible offsets
from mask and scale substrate thicknesses. The offset required to adjust the gap
for best operation may easily determined experimentally by observing the fiber
optic receiver channel signals at various operating gaps. Alternatively, appropriate
analysis or simulation may be used to determ
