U.S. patent application number 10/101031 was filed with the patent office on 2003-09-18 for optical displacement sensing device with reduced sensitivity to misalignment.
This patent application is currently assigned to Mitutoyo Corporation. Invention is credited to Atherton, Kim W., Jones, Benjamin K., Masreliez, Karl G..
Application Number | 20030174343 10/101031 |
Document ID | / |
Family ID | 28039950 |
Filed Date | 2003-09-18 |
United States Patent
Application |
20030174343 |
Kind Code |
A1 |
Masreliez, Karl G. ; et
al. |
September 18, 2003 |
Optical displacement sensing device with reduced sensitivity to
misalignment
Abstract
An optical displacement sensing device is provided for
determining the relative displacement of a diffraction grating
scale along a measuring axis. The grating may be reflective and the
grating pitch may be less than the wavelength of the light of the
displacement sensing device. The sensing device includes a split
light beam input portion for inputting two split light beams along
respective light paths, and light beam directing elements for
directing the two split beams to converge proximate to a first zone
on the scale grating to give rise to two diffracted beams which
diverge proximate to the first zone. The sensing device further
includes retroreflector elements for receiving and retroreflecting
the two diffracted beams to converge proximate to a second zone on
the scale grating to give rise to two later-diffracted light beams
which diverge and are directed to a shared zone. An optical
detector detects at least one illumination characteristic arising
from the shared zone, thus sensing displacement of the grating
scale along the measuring axis.
Inventors: |
Masreliez, Karl G.;
(Kirkland, WA) ; Jones, Benjamin K.; (Seattle,
WA) ; Atherton, Kim W.; (Kirkland, WA) |
Correspondence
Address: |
CHRISTENSEN, O'CONNOR, JOHNSON, KINDNESS, PLLC
1420 FIFTH AVENUE
SUITE 2800
SEATTLE
WA
98101-2347
US
|
Assignee: |
Mitutoyo Corporation
|
Family ID: |
28039950 |
Appl. No.: |
10/101031 |
Filed: |
March 18, 2002 |
Current U.S.
Class: |
356/616 |
Current CPC
Class: |
G01D 5/38 20130101 |
Class at
Publication: |
356/616 |
International
Class: |
G01B 011/14 |
Claims
The embodiments of the invention in which an exclusive property or
privilege is claimed are defined as follows:
1. A device for measuring the relative displacement of a scale
along a measuring axis, the scale having a grating formed along the
measuring axis, the device comprising: a split light beam input
portion for inputting two split light beams along respective light
paths; two or more light beam directing elements for directing the
two split light beams along respective light paths which converge
proximate to a first zone on the scale grating to give rise to two
diffracted light beams along respective light paths which diverge
proximate to the first zone; two or more retroreflector elements
for receiving the two diffracted light beams along respective light
paths and retroreflecting the two diffracted light beams along
respective light paths which converge proximate to a second zone on
the scale grating to give rise to two later-diffracted light beams
along respective light paths which diverge proximate to the second
zone; and an optical detector; wherein the two later-diffracted
light beams along respective light paths which diverge proximate to
the second zone are directed to enter a shared zone and the optical
detector detects at least one illumination characteristic arising
from the shared zone, the detected at least one illumination
characteristic usable to determine the relative displacement of the
scale.
2. The device of claim 1, wherein the optical detector is usable to
output at least one output signal which is indicative of the at
least one detected illumination characteristic usable to determine
the relative displacement of the scale.
3. The device of claim 1, wherein the respective light paths which
diverge proximate to the first zone each receive a diffracted light
beam arising from only one of the split beams.
4. The device of claim 1, wherein the respective light paths which
diverge proximate to the second zone each receive a
later-diffracted light beam arising from only one of the
retroreflected light beams.
5. The device of claim 1, wherein the scale grating has a pitch,
the pitch of the scale grating being less than a wavelength of
light input by the split light beam input portion.
6. The device of claim 1, wherein the respective light paths which
converge proximate to the first zone on the scale grating at least
partially overlap at a nominal positioning plane for the scale
grating.
7. The device of claim 1, wherein the respective light paths which
converge proximate to the first zone on the scale grating nominally
fully overlap at a nominal positioning plane for the scale
grating.
8. The device of claim 1, wherein the respective light paths which
converge proximate to the second zone on the scale grating at least
partially overlap at a nominal positioning plane for the scale
grating.
9. The device of claim 1, wherein the respective light paths which
converge proximate to the second zone on the scale grating
nominally fully overlap at a nominal positioning plane for the
scale grating.
10. The device of claim 1, wherein the first and second zones at
least partially overlap, and wherein the respective light paths
which converge proximate to the second zone and the respective
light paths which converge proximate to the first zone at least
partially overlap at a nominal positioning plane for the scale
grating.
11. The device of claim 1, wherein the first and second zones are
separated from one another along a direction perpendicular to the
measuring axis.
12. The device of claim 1, wherein the two split light beams
nominally impinge on the first zone on the scale grating such that
each forms the same respective angle less than or equal to 60
degrees and greater than or equal to 10 degrees relative to a line
extending parallel to the measuring axis and through their
respective impingement points.
13. The device of claim 12, wherein the respective angle is less
than or equal to 60 degrees and greater than or equal to 20
degrees.
14. The device of claim 12, wherein the respective angle is less
than or equal to 50 degrees and greater than or equal to 20
degrees.
15. The device of claim 12, wherein the respective angle is less
than or equal to 40 degrees and greater than or equal to 20
degrees.
16. The device of claim 12, wherein the respective angle is less
than or equal to 30 degrees and greater than or equal to 20
degrees.
17. The device of claim 12, wherein the scale grating has a pitch,
the pitch of the scale grating being less than a wavelength of the
light input by the split light beam input portion.
18. The device of claim 17, wherein the pitch of the scale grating
is less than 0.8 times the wavelength of the light and the
respective angle is less than or equal to 45 degrees and greater
than or equal to 20 degrees.
19. The device of claim 1, wherein at least a portion of the
respective light paths which converge proximate to the first zone
nominally coincide with a plane aligned parallel to the measuring
axis and normal to a nominal positioning plane for the scale
grating, and at least a portion of the respective light paths which
diverge proximate to the first zone nominally coincide with the
plane aligned parallel to the measuring axis and normal to the
nominal positioning plane for the scale grating.
20. The device of claim 1, wherein at least a portion of the
respective light paths which converge proximate to the first zone
are inclined in a first direction away from a plane aligned
parallel to the measuring axis and normal to a nominal positioning
plane for the scale grating, and at least a portion of the
respective light paths which diverge proximate to the first zone
are inclined in an opposite direction away from the plane aligned
parallel to the measuring axis and normal to the nominal
positioning plane for the scale grating.
21. The device of claim 1, wherein the measuring axis comprises a
circular track and the scale grating is formed along at least a
portion of the circular track, the scale is operable by rotation,
and the device is usable to measure the angular displacement of the
scale.
22. The device of claim 1, wherein the two or more light beam
directing elements comprise at least one plane mirror surface for
each respective split light beam path.
23. The device of claim 22, wherein a first portion of each
respective plane mirror surface is used for directing a respective
split light beam and a second portion of each respective plane
mirror surface is used for directing a respective later-diffracted
light beam.
24. The device of claim 22, wherein the two or more light beam
directing elements comprise opposing surfaces of a block of
transparent material.
25. The device of claim 1, wherein the two or more retroreflector
elements comprise respective corner regions of a block of
transparent material.
26. The device of claim 1, wherein the split light beam input
portion comprises a light source and one of a beam splitter and a
grating.
27. The device of claim 26, wherein the light source comprises one
of a laser diode and laser light received through an optical
fiber.
28. The device of claim 26, wherein the split light beam input
portion comprises the beam splitter, and the two split light beams
are input at a first height relative to a nominal positioning plane
for the scale grating, and for each of the two or more
retroreflector elements the reflective portion which is farthest
from nominal positioning plane for the scale grating is positioned
at a height which is not more than 80% of the first height.
29. The device of claim 26, wherein the split light beam input
portion comprises the grating, and the two split light beams are
input at a first height relative to a nominal positioning plane for
the scale grating, and for each of the two or more retroreflector
elements the reflective portion which is farthest from nominal
positioning plane for the scale grating is positioned at a height
which is more than 80% and less than 120% of the first height.
30. The device of claim 1, wherein the optical detector comprises
an optical array and the at least one illumination characteristic
arising from the shared zone arises from an interference fringe
pattern in the shared zone.
31. The device of claim 1, wherein the device further comprises at
least one polarizing element arranged such that the two
later-diffracted light beams along respective light paths which
diverge proximate to the second zone are orthogonally polarized
upon entering the shared zone; and the optical detector comprises
two or more polarization sensitive detector portions.
32. A method for determining the relative displacement of a scale
along a measuring axis, the scale having a diffraction grating
formed along the measuring axis, the method comprising:
transmitting a light beam from a light source onto a light beam
splitting element to generate two split light beams; directing the
two split light beams along respective light paths which converge
proximate to a first zone on the scale grating; diffracting the two
split light beams in the first zone to produce two diffracted light
beams along respective light paths which diverge proximate to the
first zone and which enter respective retroreflectors;
retroreflecting the two diffracted light beams along respective
light paths which converge proximate to a second zone on the scale
grating; diffracting the two diffracted light beams to produce two
later-diffracted light beams along respective light paths which
diverge proximate to the second zone and which enter a shared zone;
and detecting at least one illumination characteristic arising from
the shared zone, the detected at least one illumination
characteristic usable to determine the relative displacement of the
scale.
33. The method of claim 32, further comprising outputting at least
one output signal from the optical detector which is indicative of
the at least one detected illumination characteristic; and
determining the relative displacement of the scale based on the at
least one output signal.
34. The method of claim 32, wherein the respective light paths
which diverge proximate to the first zone each receive a diffracted
light beam arising from only one of the split beams.
35. The method of claim 32, wherein the respective light paths
which diverge proximate to the second zone each receive a
later-diffracted light beam arising from only one of the
retroreflected light beams.
36. The method of claim 32, wherein the scale grating has a pitch,
the pitch of the scale grating being less than a wavelength of the
light transmitted from the light source.
37. The method of claim 32, wherein the respective light paths
which converge proximate to the first zone on the scale grating at
least partially overlap at a nominal positioning plane for the
scale grating.
38. The method of claim 32, wherein the respective light paths
which converge proximate to the first zone on the scale grating
nominally fully overlap at a nominal positioning plane for the
scale grating.
39. The method of claim 32, wherein the respective light paths
which converge proximate to the second zone on the scale grating at
least partially overlap at a nominal positioning plane for the
scale grating.
40. The method of claim 32, wherein the respective light paths
which converge proximate to the second zone on the scale grating
nominally fully overlap at a nominal positioning plane for the
scale grating.
41. The method of claim 32, wherein the first and second zones at
least partially overlap, and wherein the respective light paths
which converge proximate to the second zone and the respective
light paths which converge proximate to the first zone at least
partially overlap at a nominal positioning plane for the scale
grating.
42. The method of claim 32, wherein the first and second zones are
separated from one another along a direction perpendicular to the
measuring axis.
43. The method of claim 32, wherein the two split light beams
nominally impinge on the first zone on the scale grating such that
each forms the same respective angle less than or equal to 60
degrees and greater than or equal to 10 degrees relative to a line
extending parallel to the measuring axis and through their
respective impingement points.
44. The method of claim 43, wherein the respective angle is less
than or equal to 60 degrees and greater than or equal to 20
degrees.
45. The method of claim 43, wherein the respective angle is less
than or equal to 50 degrees and greater than or equal to 20
degrees.
46. The method of claim 43, wherein the respective angle is less
than or equal to 40 degrees and greater than or equal to 20
degrees.
47. The method of claim 43, wherein the respective angle is less
than or equal to 30 degrees and greater than or equal to 20
degrees.
48. The method of claim 43, wherein the scale grating has a pitch,
the pitch of the scale grating being less than a wavelength of the
light transmitted from the light source.
49. The method of claim 48, wherein the scale grating has a pitch,
the pitch of the scale grating being less than 0.8 times the
wavelength of the light transmitted from the light source and the
respective angle is less than or equal to 45 degrees and greater
than or equal to 20 degrees.
50. The method of claim 32, wherein at least a portion of the
respective light paths which converge proximate to the first zone
are inclined in a first direction away from a plane aligned
parallel to the measuring axis and normal to a nominal positioning
plane for the scale grating, and at least a portion of the
respective light paths which diverge proximate to the first zone
are inclined in an opposite direction away from the plane aligned
parallel to the measuring axis and normal to the nominal
positioning plane for the scale grating.
Description
FIELD OF THE INVENTION
[0001] This invention relates to a sensing device and, more
particularly, to an optical displacement sensing device that
utilizes the interference of light beams for detecting relative
displacements of a diffraction grating with reduced sensitivity to
misalignments.
BACKGROUND OF THE INVENTION
[0002] An optical displacement sensing device, or optical encoder,
is described that can overcome several of the most significant
problems faced by designers of these types of devices for practical
high precision measurement. If an optical displacement sensing
device is to be used to make high precision measurements of a
grating surface displacement, e.g., sub-micron resolution and
accuracy, it must effectively eliminate or attenuate any
distortions of such measurements at a very high level. Current
optical displacement sensing devices are incapable of economically
and practically eliminating or significantly reducing the effects
of certain distortions or parameter drifts from their measurements
at the desired levels of accuracy and resolution. Among the
problems frequently encountered while using such devices are those
arising from a change in wavelength of the light source used for
performing measurements, such as associated changes in diffraction
angles, changed lights paths, and changes in the number of
wavelengths occurring in two light paths of differing length--which
affects their relative phase and interference pattern.
[0003] Another problem is associated with very small grating
periods. To achieve high measurement resolution, it is desirable to
use a scale grating with as short a grating period, d, as possible.
The lower limit is set by the wavelength .lambda. of the light
source, according to the formula d>.lambda./2. However, unless
special design measures are taken, an encoder using such a short
scale grating period is too difficult to align with sufficient yaw
accuracy, requiring expensive equipment or excessive time and care
during installation. Yaw misalignment is a rotation of the optical
readhead relative to the grating, in a plane parallel to the
grating.
[0004] With a yaw misalignment, the desired output beams arising
from the scale are no longer parallel and related "distortion"
interference fringes are created. If the distortion interference
fringe period is small relative to the diameter of the detector
area illuminated by the interfering beams, the desired modulation
of the signal due to grating motion will be significantly
attenuated because several distortion fringe periods will fit
within the detector area, and the detector signal will tend toward
the constant average intensity of these distortion fringes. To
avoid this effect when using practical types of optical detectors
and a grating period that approaches the previously discussed
.lambda./2 limit, the yaw misalignment must be smaller than
approximately 0.1 milliradians, in the absence of special design
measures. Such alignment requirements are impractical for many
users and applications. It is known to incorporate retroreflectors
in the light paths of optical encoders in order to overcome such
yaw problems, as shown in U.S. Pat. Nos. 5,079,418 to Dieter and
4,930,895 to Nishimura, each of which is incorporated herein by
reference, in its entirety. However, such arrangements of
retroreflectors have not simultaneously considered compact and
economical optical readhead design and packaging, the versatility
to work with grating periods both greater and lesser than the
wavelength of the light source, and measurement insensitivity to
various parameter drifts--including misalignments other than yaw.
All of these design factors must be considered simultaneously, and
the proper tradeoffs chosen, in order to robustly achieve currently
desired measurement resolutions and accuracies.
[0005] In particular, parameter drifts including dynamic position
misalignments are important error sources in an optical
displacement sensing device. Herein, the term dynamic misalignment
or drift means the change in an alignment component or parameter
that occurs between one displacement position and another
displacement position, or at the same displacement position over a
period of time, for any reason. Among the range of possible dynamic
position misalignments and drift that are introduced in practical
applications are changes in the gap between the readhead and
grating, pitch (rotation about an axis parallel to the grating and
normal to the measuring axis), yaw, roll (rotation about an axis
parallel to the measuring axis), and drift in the wavelength of a
light source. U.S. Pat. No. 5,146,085 to Ishizuka, incorporated
herein by reference, in its entirety, and the '895 patent,
previously incorporated, both disclose optical readhead
configurations which are relatively insensitive to errors
associated with pitch. However, these configurations are not
versatile and robust enough when considering the placement of
retroreflectors in combination with consideration of the other
design factors noted for simultaneous consideration above. Thus,
error sensitivities associated with the relative pitch of the
readhead and grating remain as some of the most difficult error
sources to reduce in practical high resolution encoder design and
application.
[0006] Furthermore, the '085 and '895 configurations may introduce
problems created by the reflection of a light beam into the light
source used for the generation of the light beam, leading to
instability in the wavelength of the light source. Also, these
configurations require polarizers, which attenuate the light
available to the optical detector and which may limit or prohibit
the configuration for use with certain detectors and/or impose
relatively higher system power requirements, which may complicate
or limit their use in certain applications.
[0007] The present invention is directed to optical readhead
configurations which are suitable for compact and economical design
and packaging, versatile enough to apply with grating periods both
greater and lesser than the wavelength of the light source, and
which are substantially simultaneously insensitive to various
parameter drifts including at least dynamic yaw and pitch
misalignments. Some configurations also avoid or limit attenuation
of the light available to the detector and/or avoid reflection of
light beams into the light source.
SUMMARY OF THE INVENTION
[0008] In accordance with this invention, an optical displacement
sensing device or optical encoder readhead is provided for
determining the relative displacement of a diffraction grating
scale along a measuring axis. The grating may be reflective and the
grating pitch may be less than the wavelength of the light of the
encoder readhead. The sensing device includes a split light beam
input portion for inputting two split light beams along respective
light paths, light beam directing elements for directing the two
split beams to converge proximate to a first zone on the scale
grating to give rise to two diffracted beams which diverge
proximate to the first zone, retroreflector elements for receiving
and retroreflecting the two diffracted beams to converge proximate
to a second zone on the scale grating to give rise to two
later-diffracted light beams which diverge and are directed to a
shared zone, and an optical detector. The optical detector detects
at least one illumination characteristic arising from the shared
zone, thus sensing displacement of the grating scale along a
measuring axis.
[0009] In accordance with an aspect of the invention, by directing
the two split beams to converge proximate to the first zone, the
device sensitivity to dynamic pitch misalignments can be
reduced.
[0010] In accordance with a further aspect of the invention, by
retroreflecting the two diffracted beams to converge proximate to
the second zone, yaw sensitivity is substantially eliminated while
the device sensitivity to dynamic pitch misalignments is further
reduced.
[0011] In accordance with a further aspect of the invention, the
sensing device configuration may be aligned with either one
vertical plane perpendicular to the surface of the scale and
parallel to the measuring axis, or with two different inclined
planes inclined away from each and parallel to the measuring axis.
Various inclined configurations can prevent undesirable mixing of
various light beams, while at the same time facilitating compact
design and packaging of the device.
[0012] In accordance with a further aspect of the invention, the
light paths which diverge proximate to the first zone each receive
a diffracted light beam arising from only one of the split beams,
avoiding the need for polarizers in the encoder readhead.
[0013] In accordance with a further aspect of the invention, the
light paths which diverge proximate to the second zone each receive
a later-diffracted light beam arising from only one of the split
beams, further avoiding the need for polarizers in the encoder
readhead.
[0014] In accordance with a further aspect of the invention, the
sensing device configuration may be arranged such that the split
light beams nominally impinge on the first zone within a range of
angles that attenuates the sensitivity of the device to certain
errors associated with at least dynamic pitch changes and
wavelength drift, while at the same time facilitating compact
design and packaging of the device.
[0015] In accordance with another aspect of the invention, the
light beam directing elements comprise opposing surfaces of a block
of transparent material, providing a compact, economical and robust
way to fabricate and precisely position the elements.
[0016] In accordance with another aspect of the invention, the
retroreflector elements comprise respective corners of a block of
transparent material, providing a compact, economical and robust
way to fabricate and precisely position the retroreflector
elements.
[0017] In accordance with another aspect of the invention, a
readhead grating produces the two split beams and the sensing
device configuration may be arranged such that the readhead grating
and retroreflectors are in a configuration that attenuates the
sensitivity of the device to certain errors associated with at
least dynamic pitch changes and wavelength drift.
[0018] In accordance with another aspect of the invention, a
beamsplitter produces the two split beams and the sensing device
configuration may be arranged such that the beamsplitter and
retroreflectors are in a configuration that attenuates the
sensitivity of the device to certain errors associated with at
least dynamic pitch changes and wavelength drift.
[0019] In accordance with a further aspect of the invention, in
various embodiments a polarizing configuration may be used for the
optical detector and in various other embodiments a compact optical
array may be used for the optical detector.
[0020] Hence, the invention overcomes the disadvantages of prior
art optical displacement sensing devices with a compact, economical
and versatile configuration applicable to grating periods both
greater and lesser than the wavelength of the light used, and which
is substantially simultaneously insensitive to various parameter
drifts including at least dynamic yaw and pitch misalignments.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] The foregoing aspects and other 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:
[0022] FIG. 1 is a diagram showing a side view of light beam paths
in a first prior art optical displacement sensing device;
[0023] FIG. 2 is a diagram showing a schematic side view of a
second prior art optical displacement sensing device;
[0024] FIG. 3 is a diagram showing a schematic side view of a third
prior art optical displacement sensing device;
[0025] FIG. 4 is a three-dimensional conceptual view of an optical
displacement sensing device configuration in accordance with a
first exemplary embodiment of the invention;
[0026] FIG. 5A is a three-dimensional conceptual view of an optical
displacement sensing device in accordance with a second exemplary
embodiment of the invention;
[0027] FIG. 5B is a side view of the optical displacement sensing
device shown in FIG. 5A;
[0028] FIG. 5C is a top view of the optical displacement sensing
device shown in FIG. 5A;
[0029] FIG. 5D is an end view of the optical displacement sensing
device shown in FIG. 5A;
[0030] FIG. 6 is a three-dimensional schematic view clarifying the
geometric components and symbols used for describing light path
directions herein;
[0031] FIG. 7 is a three-dimensional schematic view showing a
generic input light ray direction and the resulting conical
distribution of diffracted light rays from a diffraction grating,
using the geometric components and symbols shown in FIG. 6;
[0032] FIG. 8 is an error table showing the errors associated with
various initial and dynamic misalignments and drifts for a prior
art optical displacement sensing device configuration corresponding
to FIG. 1;
[0033] FIG. 9 is an error table showing the errors associated with
various initial and dynamic misalignments and drifts for an optical
displacement sensing device configuration corresponding to FIGS.
5A-5D and FIGS. 12A-12D;
[0034] FIG. 10A is a three-dimensional view clarifying the
operation of a first readhead grating configuration usable in
various exemplary embodiments according to this invention;
[0035] FIG. 10B is a three-dimensional view clarifying the
operation of a second readhead grating configuration usable in
various exemplary embodiments according to this invention;
[0036] FIG. 10C is a three-dimensional view clarifying the
operation of a third readhead grating configuration usable in
various exemplary embodiments according to this invention;
[0037] FIG. 10D is a three-dimensional view clarifying the
operation of a fourth readhead grating configuration usable in
various exemplary embodiments according to this invention;
[0038] FIG. 10E is a three-dimensional view clarifying the
operation of a fifth readhead grating configuration usable in
various exemplary embodiments according to this invention;
[0039] FIG. 11A is a three-dimensional view clarifying the
operation of a first beamsplitter configuration usable in various
exemplary embodiments according to this invention;
[0040] FIG. 11B is a three-dimensional view clarifying the
operation of a second beamsplitter configuration usable in various
exemplary embodiments according to this invention;
[0041] FIG. 11C is a three-dimensional view clarifying the
operation of a third beamsplitter configuration usable in various
exemplary embodiments according to this invention;
[0042] FIG. 11D is a three-dimensional view clarifying the
operation of a fourth beamsplitter configuration usable in various
exemplary embodiments according to this invention;
[0043] FIG. 12A is a three-dimensional conceptual view of an
optical displacement sensing device in accordance with a third
exemplary embodiment of the invention;
[0044] FIG. 12B is a side view of the optical displacement sensing
device shown in FIG. 12A, including a portion of an exemplary
readhead housing;
[0045] FIG. 12C is a top view of the optical displacement sensing
device shown in FIG. 12A;
[0046] FIG. 12D is an end view of the optical displacement sensing
device shown in FIG. 12A;
[0047] FIG. 13 is a graph showing error sensitivity data for
dynamic gap misalignment and wavelength change at various incident
beam angles in an optical displacement sensing device in accordance
with this invention;
[0048] FIG. 14 is a table showing illumination spot length data and
data indicative of readhead size, for various incident beam angles
in an optical displacement sensing device in accordance with this
invention;
[0049] FIG. 15 is a graph showing error sensitivity data for
dynamic gap misalignment at various incident beam angles and at
various inclined plane angles, using a 635 nm wavelength in an
optical displacement sensing device in accordance with this
invention;
[0050] FIG. 16 is a graph showing error sensitivity data for
dynamic gap misalignment at various incident beam angles and at
various inclined plane angles, using a 405 nm wavelength in an
optical displacement sensing device in accordance with this
invention;
[0051] FIG. 17 is a graph showing error sensitivity data for
wavelength drift at various incident beam angles and at various
inclined plane angles, using a 635 nm wavelength in an optical
displacement sensing device in accordance with this invention;
[0052] FIG. 18 is a graph showing error sensitivity data for
wavelength drift at various incident beam angles and at various
inclined plane angles, using a 405 nm wavelength in an optical
displacement sensing device in accordance with this invention;
[0053] FIG. 19 is a three-dimensional conceptual view of an optical
displacement sensing device in accordance with a fourth exemplary
embodiment according to the invention;
[0054] FIG. 20A is a conceptual side view of an optical
displacement sensing device in accordance with a fifth exemplary
embodiment according to the invention;
[0055] FIG. 20B is a top view of the optical displacement sensing
device shown in FIG. 20A;
[0056] FIG. 20C is an end view of the optical displacement sensing
device shown in FIG. 20A;
[0057] FIG. 21 is a schematic side view of an optical detector
including a polarizer configuration usable in various exemplary
embodiments according to the invention;
[0058] FIG. 22A is a conceptual side view of an optical
displacement sensing device in accordance with a sixth exemplary
embodiment according to the invention;
[0059] FIG. 22B is a top view of the optical displacement sensing
device shown in FIG. 22A;
[0060] FIG. 22C is an end view of the optical displacement sensing
device shown in FIG. 22A;
[0061] FIG. 23 is a conceptual side view of an optical displacement
sensing device in accordance with a seventh exemplary embodiment
according to the invention;
[0062] FIG. 24 is a conceptual side view of an optical displacement
sensing device in accordance with an eighth exemplary embodiment
according to the invention;
[0063] FIG. 25 is a three-dimensional view showing a first
exemplary transparent block configuration usable in various
exemplary embodiments according to the invention;
[0064] FIG. 26A is a three-dimensional view showing a second
exemplary transparent block configuration usable in various
exemplary embodiments according to the invention;
[0065] FIG. 26B is a side view of the transparent block
configuration shown in FIG. 26A;
[0066] FIG. 26C is an end view of the transparent block
configuration shown in FIG. 26A;
[0067] FIG. 27A is a three-dimensional view showing a third
exemplary transparent block configuration usable in various
exemplary embodiments according to the invention;
[0068] FIG. 27B is a side view of the transparent block
configuration shown in FIG. 27A;
[0069] FIG. 27C is an end view of the transparent block
configuration shown in FIG. 27A; and
[0070] FIG. 28 is a flow diagram for a method of using an optical
displacement sensing device in accordance with various exemplary
embodiments of the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0071] The invention provides a configuration for a high resolution
displacement sensing device or optical encoder that is compact,
relatively economical to construct, and which is substantially
simultaneously insensitive to various parameter drifts including at
least dynamic yaw and pitch misalignments. The configuration is
versatile enough to be applicable to grating periods both greater
and lesser than the wavelength of the light used.
[0072] FIG. 1 is a diagram showing a side view of light beam paths
in a first prior art optical displacement sensing device. The
separation of light rays in the diagram are exaggerated for
clarity. A relative displacement axis and/or measuring axis
direction 101 is indicated. A beam which is the source of input
rays 100a and 100b is transmitted toward readhead grating 102.
Readhead grating 102 is a transmissive diffraction grating. A ray
tracing process is started at location 110 which is a wavefront at
which input rays 100a and 100b have the same optical phase. The
starting points of the rays on the wavefront are chosen so that,
when traced through the system, both rays arrive at the same output
detection point 106.
[0073] Light ray 100a is diffracted at deflection point X.sub.4 and
produces a light ray N.sub.4, a chosen diffraction order. Light ray
N.sub.4 is diffracted from scale surface 104 at reflection point
X.sub.5 and forms diffracted ray N.sub.5. Diffracted ray N.sub.5 is
transmitted in the gap between scale surface 104 and readhead
grating 102. Light ray N.sub.5 is subsequently transmitted through
readhead grating 102 at transmission point X.sub.6 and produces a
zero order transmitted light ray N.sub.6 which is transmitted to
output detection point 106. In a similar fashion, input ray 100b is
transmitted through readhead grating 102 at transmission point
X.sub.1 and produces a zero order transmitted light ray N.sub.1,
which is subsequently transmitted through the gap between readhead
grating 102 and scale surface 104. Light ray N.sub.1 is diffracted
from scale surface 104 at reflection point X.sub.2, and produces a
light ray N.sub.2, a chosen diffraction order. Light ray N.sub.2 is
diffracted from readhead grating 102 at deflection point X.sub.3
and produces light ray N.sub.3, a chosen diffraction order. Light
ray N.sub.3 is subsequently transmitted in a direction parallel or
nearly parallel to transmitted light ray N.sub.6 to output
detection point 106. It should be appreciated that the deflection
points X.sub.1 and X.sub.4 may be the same point and the deflection
points X.sub.3 and X.sub.6 may be the same point.
[0074] The prior art configuration shown in FIG. 1 is intended to
demonstrate the fundamental problems that exist in the absence of
modifications to overcome the effect of dynamic misalignments. One
primary objective of the claimed invention is to provide accurate
output beam signals and related measurements while rejecting
various combinations of static and dynamic misalignments. Among the
potential sources of dynamic misalignments to be overcome by the
invention are misalignments during the travel of the displacement
sensing device relative to a target object due to bearing
imperfections, and due to misalignments arising from warping and
waviness of the surface of the grating. Warping and waviness, for
example, result from thickness variations in the measuring scale
and/or a bonding layer attaching the measuring scale to the target
object or an intermediate mounting member, as well as from the
measuring scale conforming to surface distortions of a mounting
member. FIG. 1 shows scale surface 104 elevated from an initial or
ideal surface parallel to readhead grating 102 and or measuring
axis direction 101 by an angle 108. Angle 108 represents a dynamic
pitch misalignment of the scale surface 104 relative to an optical
displacement sensing device.
[0075] The dynamic pitch misalignment of angle 108, for example due
to distortion, thermal deformation, or displacement creating a
change in elevation between reflection points X.sub.2 and X.sub.5,
would change the positions of the points X.sub.1-X.sub.6, where the
rays intersect with the gratings. This change of where the rays
intersect the gratings creates a change in the phase difference
between the two light rays, N.sub.3 and N.sub.6, that will be
indistinguishable from any change in phase difference due to the
intended displacement of the scale surface 104 along measuring axis
direction 101, resulting in measurement errors. The errors will be
proportional to the magnitude of the dynamic misalignment
encountered between successive measurements of the displacement of
the scale surface 104. An alternative configuration is required to
overcome such inaccuracies that will be produced from dynamic
misalignments that occur when making measurements with a device
having the prior art configuration shown in FIG. 1.
[0076] FIG. 2 is a diagram showing a schematic side view of a
second prior art optical displacement sensing device disclosed in
the incorporated '085 patent. In FIG. 2, the reference numeral 1
designates a light source comprising a laser diode, the reference
numeral 2 denotes a collimator lens, the reference numeral 9
designates a polarizing beam splitter, the reference numeral 5
denotes a diffraction grating having a pitch P formed on a linear
scale or a rotary scale, the reference numerals 61 and 62 designate
mirrors, the reference numerals 7A and 7B denote quarter-wavelength
plates, the reference numeral 6 designates a non-polarizing beam
splitter, the reference numerals 71 and 72 denote polarizing
elements (such as polarizing plates or polarizing beam splitters),
and the reference numerals 81 and 82 designate light receiving
elements. A laser beam of wavelength .lambda.emitted from the light
source 1 is collimated by the collimator lens 2, and the parallel
light beam is caused to enter the polarizing beam splitter 9,
whereby it is divided into two light beams R1 and R2 whose
polarization azimuths are orthogonal to each other. The light beam
R1 is an S-polarized light reflected by the polarizing beam
splitter 9, and the light beam R2 is a P-polarized light
transmitted through the polarizing beam splitter 9. The light beam
R1 travels along an optical path L1 formed via the mirror 61, and
the light beam R2 travels along an optical path L2 formed via the
mirror 62. The light beams R1 and R2 pass through the quarter
wavelength plates 7A and 7B, whereafter they are incident on a
point P1 on the diffraction grating 5 at an angle of incidence
.theta.o=.theta.b=sin.sup.- -1 (.lambda./2P), and light beams R1
and R2 are obtained by being reflected and diffracted by the
diffraction grating 5. The +1st-order diffracted light (R1+) of the
light beam R1 and the -1st-order diffracted light (R2-) of the
light beam R2 travel toward the original optical paths L1 and L2,
respectively, through the quarter wavelength plates 7A and 7B. The
+1st-order diffracted light traveling reversely along an optical
path L1 and the -1st-order diffracted light traveling reversely
along an optical path L2 are reflected by the mirrors 61 and 62,
respectively, and are directed to the polarizing beam splitter 9,
and are again superposed one upon the other by the polarizing beam
splitter 9. The +1st-order diffracted light is made into a
P-polarized light by the action of the quarter wavelength plate 7B
and the -1st-order diffracted light is made into an S-polarized
light by the action of the quarter wavelength plate 7A and
therefore, these light beams emerge from the polarizing beam
splitter 9 while overlapping with each other, without any loss. The
overlapping two light beams pass through the quarter wavelength
plate 53 and become circularly polarized lights.
[0077] After this the light beam is divided into two light beams
equal in quantity of light by the non-polarizing beam splitter 6.
Only a particular polarized component is separated and taken out
from one of the two light beams by the use of the polarizing
element 72 and is caused to enter the light receiving element 82,
and only a particular polarized component is separated and taken
out from the other light beam by the use of the polarizing element
71 and is caused to enter the light receiving element 81. Periodic
signals are output from the light receiving elements 82 and 81,
respectively in conformity with the displacement of the scale. The
periodic signals are output in "quadrature", according to methods
known to one skilled in the art. The structure and operation of the
configuration shown in FIG. 2 are described in further detail in
the incorporated '085 patent.
[0078] The prior art configuration shown in FIG. 2 substantially
eliminates the dynamic pitch misalignment sensitivity previously
discussed with respect to FIG. 1. However, the configuration has
undesirable limitations. The configuration includes no means for
reducing yaw sensitivity. With yaw misalignment, the output beams
returning from the scale are no longer parallel and the signals
provided by the optical detection scheme will produce large errors
at small yaw misalignments. Thus, the configuration is not robust
and requires a level of care in installation and use that is not
desirable. This is particularly true when the pitch P of
diffraction grating 5 is made small, as previously discussed. In
addition, the zero order beams "crossover" to the opposite optical
path by reflection at the point P1. Thus, "crossover beams" are
undesirably combined with the desired diffracted beams described
above. Light from the crossover beams must be removed from the
respective optical paths by the effects of polarizing beam splitter
9. Thus, the configuration must use polarizers, which impose
unwanted encoder readhead fabrication and/or assembly constraints
in some situations. Optical energy is also wasted. Furthermore, the
disclosed configuration lies in a plane which is normal to the
scale. Thus, a portion of the reflected and diffracted light may be
returned to the light source, causing instability in the light
source. Furthermore, the wavelength .lambda. and the grating pitch
P completely determine the incident angle, which imposes unwanted
encoder readhead packaging and size constraints in some situations.
An alternative configuration is required to overcome such
undesirable limitations.
[0079] FIG. 3 is a diagram showing a schematic side view of a third
prior art optical displacement sensing device that is disclosed in
the incorporated '895 patent. In FIG. 3, the reference numeral 1
designates a semiconductor laser, the reference numeral 2 denotes a
collimator lens, the reference numeral 9 designates a polarizing
beam splitter, the reference numeral 5 denotes an optical scale
having a diffraction grating having a pitch P, the reference
numerals 61 and 62 designate reflecting mirrors, the reference
numerals 8A and 8B denote quarter-wavelength plates, the reference
numeral 6 designates a beam splitter, the reference numerals 71 and
72 denote polarizing plates whose polarization axes form an angle
of 45.degree. with each other. The reference numerals 81 and 82
designate light receiving elements which photoelectrically convert
interference fringes. The reference numeral 11 denotes an index
distribution type stick lens carrying a reflecting film 12, which
together constitute a reflecting element 20 that returns light
substantially along the optical path by which it entered the
reflecting element 20. A coherent light beam from the semiconductor
laser 1 is collimated by the collimator lens 2 and enters the
polarizing beam splitter 9, whereby it is divided into transmitted
and reflected light beams whose polarizations are orthogonal to
each other. The transmitted and reflected light beams are made into
circularly polarized lights through the quarter wavelength plates
8A and 8B, respectively, and are reflected by the reflecting
mirrors 62 and 61, and are caused to obliquely enter the optical
scale 5 at a point P1 so that the mth-order diffracted light from
the optical scale 5 emerges substantially perpendicularly from the
diffraction grating surface of the optical scale 5. That is, each
light beam is caused to enter the optical scale 5 so that
.theta..sub.m.apprxeq.sin.sup.-1(m.lambda./P), where P is the
grating pitch, .lambda. is the wavelength of the light from the
semiconductor laser 1, m is an integer, and .theta..sub.m is the
incident angle, from the light beam to the perpendicular to the
diffraction grating surface. The diffracted lights emerging
substantially perpendicularly from the diffraction grating surface
form a common optical path and enter the reflecting element 20
where they are reflected to return along the original optical path,
are reflected by the reflecting mirrors 61 and 62, are transmitted
through the quarter wavelength plates 8A and 8B and again enter the
polarizing beam splitter 9. The diffracted lights emerge from the
polarizing beam splitter 9 superimposed and are made into
circularly polarized lights opposite in direction to each other
through the quarter wavelength plate 53.
[0080] After this the superimposed lights are divided into two
light beams by the beam splitter 6, and are made into linearly
polarized lights through the polarizing plates 72 and 71 and then
enter the light receiving elements 82 and 81, respectively.
Periodic signals are output from the light receiving elements 82
and 81, respectively in conformity with the displacement of the
scale. The periodic signals are output in "quadrature", according
to methods known to one skilled in the art. Because the received
light is twice diffracted by the optical scale 5 as mth-order
light, once before entering the reflecting element 20 and once
after, when the grating moves by one pitch increment the periodic
signals undergo 4 m cycles. The structure and operation of the
configuration shown in FIG. 3 are described in further detail in
the incorporated '895 patent.
[0081] The prior art configuration shown in FIG. 3 substantially
eliminates the dynamic pitch misalignment sensitivity previously
discussed with respect to FIG. 1. Furthermore, the configuration
substantially eliminates yaw sensitivity through the effects of the
reflecting element 20, which is one type of retroreflector.
Nevertheless, the configuration has all the other undesirable
limitations previously discussed with respect to FIG. 2, as well as
one additional major limitation. The additional limitation is that
the configuration will not operate with a grating pitch P that is
less than the wavelength .lambda. of the light. That is, there is
no solution to the required equation .theta..sub.m
sin.sup.-1(m.lambda./P), in this situation. In addition, even when
the configuration is operable, the zero order beams and "crossover"
to the opposite optical path by reflection at the point P1, and the
diffracted beams are otherwise mixed in the common path to and from
the reflecting element 20. Thus, "crossover beams" are undesirably
combined with the desired diffracted beams. Light from the
crossover beams must be removed from the respective optical paths
by the effects of polarizing beam splitter 9. Thus, the
configuration must use polarizers, which impose unwanted encoder
readhead fabrication and/or assembly constraints in some
situations. Optical energy is also wasted. Furthermore, the
disclosed configuration lies in a plane which is normal to the
scale. Thus, a portion of the reflected and diffracted light may be
returned to the light source, causing instability in the light
source. Furthermore, the wavelength .lambda. and the grating pitch
P completely determine the incident angle, which imposes unwanted
encoder readhead packaging and size constraints in some situations.
An alternative configuration is required to overcome such
undesirable limitations.
[0082] In the following discussions of various exemplary
embodiments according to the principles of this invention, only
operable light beams and/or light paths are shown, as necessary to
explain and clarify the invention. However, it should be
appreciated that various beam splitting elements and gratings shown
and discussed below will give rise to various other split beams
and/or diffracted light beam orders which are "lost" from the
various operable light path configurations according to this
invention. Such "lost light" paths and/or beams are generally not
shown or discussed except herein, except with regard to their
potential for light source wavelength disturbance or crossover
light in a particular embodiment. Accordingly, it should be
appreciated that for simplicity and clarity terms such as "light
beam" or "light path" are used herein to refer to the operable
light beams or operable light paths that contribute light that is
eventually detected in a shared zone according to the principles of
this invention, unless otherwise indicated. The various other "lost
light" paths and/or beams which may be present in various
embodiments according to this invention will be apparent to one of
ordinary skill in the art. It is to be understood that in addition
to the operable light paths and/or beams described and claimed
below, such lost light paths and/or beams are present in various
embodiments according to this invention regardless of whether or
not they are explicitly indicated.
[0083] FIG. 4 is a three-dimensional conceptual view of an optical
displacement sensing device configuration in accordance with a
first exemplary embodiment of this invention. This first exemplary
embodiment is discussed as a generic embodiment, which may include
the specific exemplary elements shown in FIG. 4. It should be
appreciated that the generic portions of the following discussion
are indicative of large number of configurations which vary in
their combination of elements and dimensions from the specific
elements and dimensions indicated in FIG. 4. Thus, FIG. 4 should
not be interpreted as limiting with regard to the meaning and
intent of the generic portions of the following discussion.
[0084] FIG. 4 shows an encoder readhead configuration 400. This
embodiment is a practical configuration that is substantially
insensitive to various misalignments and dynamic misalignments, yet
offers substantial flexibility to locating and fabricating and
assembling various optical components in a compact and economical
way in an encoder readhead. The encoder readhead includes a split
light beam input portion 410, light beam directing elements 420 and
421, retroreflectors 440 and 441, a shared zone 450 and an optical
detector 460 having one or more power and/or signal connections
461. The configuration also includes a scale grating 430, a first
zone 431 on the grating scale, and a second zone 432 on the grating
scale.
[0085] In the exemplary embodiment shown in FIG. 4, the split light
beam input portion 410 includes a light source 412 and a beam
splitting element 415. The light source 412 emits coherent
radiation and in various exemplary embodiments, the light source
412 includes a collimating element and emits collimated light. In
various exemplary embodiments according to this invention, the
light source 412 may be a fiber optic element which conveys light
from a remote light source, or a light emitting diode or a laser
diode included in the encoder readhead. In various exemplary
embodiments, a laser diode is used due to the longer coherence
length of the emitted light. In various other exemplary
embodiments, a vertical cavity surface emitting laser diode is used
due to the greater temperature stability of the emitted wavelength.
The beam splitting element 415 receives a light beam 401 from the
light source 412 and produces two split light beams 401a and 401b
along respective light paths according to known optical principles
and inputs the beams to the remainder of the encoder readhead
configuration 400.
[0086] It should be appreciated that in the exemplary embodiment
shown in FIG. 4, the beam splitting element 415 is conceptually
illustrated as a readhead grating having a first portion impinged
upon by the light beam 401. However, in this and various other
exemplary embodiments herein, the beam splitting element 415
impinged upon by the light beam 401 is more generally intended to
represent any now known or later developed light beam splitting
element or combination of elements, such as a suitably arranged
"half-silvered" beamsplitter or a polarizing beamsplitter or
portions thereof, or portions of a grating or multiple gratings or
the like, including any of the configurations discussed below and
shown in 10A-10E or 11A-11D, which is operable to produce split
light beams according to the principles of this invention.
[0087] Furthermore, it should be appreciated that in various
exemplary embodiments, according to known optical miniaturization
and assembly techniques the split light beam input portion 410 may
incorporate a light source, collimation, beam splitting, and
polarizing functions, into a single integrated optical element or
assembly, such that the light source 412 and the beam splitting
element 415 are difficult or impossible to distinguish as separate
elements.
[0088] The split light beams 401a and 401b are directed along
respective light paths by the light beam directing elements 420 and
421 such that the split light beams 401a and 401b converge
proximate to the first zone 431 on the grating scale. In the
embodiment shown in FIG. 4, and various other embodiments shown
herein, the light beam directing elements 420 and 421 are each
represented as exemplary plane mirrors that reflect the split light
beams 401a and 401b, respectively, once from a first portion of
each plane mirror. However, more generally, each of the light beam
directing elements 420 and 421 impinged upon by the split light
beams 401a and 401b may be any now known or later developed light
beam directing element or combination of elements, such as suitably
arranged mirrors and/or portions of a grating or multiple gratings
or the like, that are operable through one or more reflections or
deflections to direct the light beams 401a and 401b along
respective light paths to converge proximate to the first zone
431.
[0089] The first zone 431 is a nominal zone on the nominal plane of
the scale grating 430 that has a dimension along the measuring axis
direction 101 that bounds the light spots where the light beams
401a and 401b impinge on a nominally aligned scale grating 430.
Generally, the ability of various encoder reader configurations
according to this invention to minimize errors related to pitch
misalignment and dynamic pitch misalignment is improved as the
dimension of the first zone along the measuring axis direction 101
is reduced. Therefore, the dimension of the first zone along the
measuring axis direction 101 generally provides the proper balance
between reducing errors related to pitch misalignment and dynamic
pitch misalignment and achieving the other design objectives of a
particular embodiment according to this invention. In various
exemplary embodiments, the dimension of the first zone along the
measuring axis direction 101 is longer, to provide clearance
between various diffracted and reflected light beams and various
elements of the encoder readhead, as discussed below with reference
to FIGS. 22A-22C, for example. In various other exemplary
embodiments, the dimension of the first zone along the measuring
axis direction 101 is equal to or less than 4 times the nominal
spot length along the measuring axis direction 101. Exemplary
nominal spot lengths are discussed below with reference to FIG. 14.
It should be appreciated that the sensitivity to static and dynamic
pitch misalignments is primarily related to the distance between
the spots in the first zone 431. Thus, it should be appreciated
that due to design choices or various misalignments the light beams
401a and 401b may actually diverge for a small distance before
impinging on the scale grating 430 and after converging proximate
to the first zone, provided that the light beams 401a and 401b
nevertheless impinge on a nominally aligned scale grating 430 in a
first zone 431 according to the principles of this invention.
[0090] The split light beams 401a and 401b enter the first zone
431, giving rise to respective diffracted light beams 402a and
402b, respectively, which are respective diffraction orders
diffracted along respective light paths which diverge proximate to
the first zone 431. The retroreflectors 440 and 441 receive the
diffracted light beams 402a and 402b, respectively, along their
respective light paths. It should be appreciated that if the light
beams 401a and 401b actually diverge for a small distance before
impinging on the scale grating 430 and after converging proximate
to the first zone, then in a complementary way the diffracted light
beams 402a and 402b beams may actually converge for a small
distance after leaving the scale grating 430 and before diverging
proximate to the first zone, provided that the diffracted light
beams 402a and 402b beams nevertheless diverge proximate to the
first zone along respective optical paths to be received by the
retroreflectors 440 and 441 according to the principles of this
invention.
[0091] The retroreflectors 440 and 441 receive the diffracted light
beams 402a and 402b, respectively, along their respective light
paths and reflect them as light beams 402ar and 402br,
respectively. Thus, the light paths of the light beams 402ar and
402br are parallel to the light paths of the diffracted light beams
402a and 402b according to the principles of this invention. As a
result, according to the principles of this invention, the
exemplary encoder readhead configuration 400 is substantially
insensitive to errors related to yaw misalignment and dynamic yaw
misalignment. The retroreflectors 440 and 441 shown in FIG. 4 are
represented by exemplary corner-cube type retroreflectors. However,
a cats-eye type retroreflector or any other now known or later
developed type of retroreflector may be used, provided that it is
operable according to the principles of this invention.
[0092] In various exemplary embodiments, depending on the location
of the retroreflectors 440 and 441 relative to the diffracted light
beams 402a and 402b, respectively, the light paths of the light
beams 402ar and 402br may be offset, that is, separated from the
diffracted light beams 402a and 402b in a direction transverse to
the measuring axis direction 101 as shown in FIG. 4, or along the
measuring axis direction 101, or both. Such an offset prevents
light from reentering the light source and causing light source
instability in various embodiments according to this invention, and
also helps to eliminate "crossover beams" in various other
embodiments according to this invention. Thus, these problems
present in the prior art configurations previously described with
reference to FIGS. 1 and 2 may be avoided in the exemplary encoder
readhead configuration 400, and in various other embodiments
according to this invention which include such a beam separation
offset due to the placement of retroreflectors 440 and 441 relative
to the diffracted light beams 402a and 402b, respectively. It
should be appreciated that such embodiments according to this
invention need not include polarizers, thus avoiding unwanted
encoder readhead fabrication and/or assembly constraints in some
situations. However, it should be further appreciated that
polarizers may be optionally included in such embodiments, as
desired for compatibility with a particular optical detector 460 in
a particular embodiment, for example.
[0093] In any case, the light beams 402ar and 402br are
retroreflected along light paths parallel to the light paths of the
diffracted light beams 402a and 402b, thus converging proximate to
the second zone 432 in a manner similar to the convergence of the
light beams 401a and 401b proximate to the first zone 431.
[0094] The second zone 432, similar to the first zone 431, is a
nominal zone on the nominal plane of the scale grating 430 that has
a dimension along the measuring axis direction 101 that bounds the
light spots where the light beams 402ar and 402br impinge on a
nominally aligned scale grating 430. The design considerations
related to the second zone 432 and the ability of various encoder
reader configurations according to this invention to minimize
errors related to pitch misalignment and dynamic pitch misalignment
are the same as those previously described with reference to the
first zone 431. The dimension of the second zone 432 along the
measuring axis direction 101 is determined similarly to the
dimension of the first zone 431. As previously discussed, in
various exemplary embodiments, depending on the location of the
retroreflectors 440 and 441 relative to the diffracted light beams
402a and 402b, respectively, the light paths of the light beams
402ar and 402br may be offset, that is, separated from the
diffracted light beams 402a and 402b in a direction transverse to
the measuring axis direction 101 as shown in FIG. 4. In such a case
the second zone is dimensioned similarly to the first zone 431, and
is offset from the first zone 431 in a direction transverse to the
measuring axis direction 101 as shown in FIG. 4, corresponding to
the offset of the light paths of the light beams 402ar and 402br.
More generally, in various exemplary embodiments the second zone
432 is nominally offset from the first zone 431 in the same
direction as any offset of the light paths of the light beams 402ar
and 402br relative to the diffracted light beams 402a and 402b.
Furthermore, in various embodiments according to this invention the
light paths of the light beams 402ar and 402br may fully or
partially overlap the light paths of the diffracted light beams
402a and 402b, and in such cases the second zone 432 similarly may
fully or partially overlap the first zone 431.
[0095] The retroreflected light beams 402ar and 402br enter the
second zone 432, giving rise to respective later-diffracted light
beams 403a and 403b, respectively, which are respective diffraction
orders diffracted along respective light paths which diverge
proximate to the second zone 432 in a manner similar to the
divergence of the light beams 402a and 402b proximate to the first
zone 431. The later-diffracted light beams 403a and 403b are then
directed by the light beam directing elements 420 and 421 such that
the light beams later-diffracted light beams 403a and 403b converge
proximate to the beam splitting element 415.
[0096] It should be appreciated that in the embodiment shown in
FIG. 4, and various other embodiments shown herein, the light beam
directing elements 420 and 421 are represented as exemplary plane
mirrors and the later-diffracted light beams 403a and 403b each
impinge upon and are reflected once from a second portion of the
same plane mirrors having a first portion impinged upon by the
split light beams 401a and 401b, respectively. However, as
previously discussed, in various embodiments according to this
invention the light paths of the light beams 402ar and 402br may
fully or partially overlap the light paths of the diffracted light
beams 402a and 402b, and in such cases the light paths of the
later-diffracted light beams 403a and 403b may fully or partially
overlap the light paths of the split light beams 401a and 401b,
respectively. In such cases, previously discussed second portions
and first portions of each plane mirror may similarly fully or
partially overlap. More generally, each of the light beam directing
elements 420 and 421 impinged upon by the later-diffracted light
beams 403a and 403b may be any now known or later developed light
beam directing element or combination of elements, such as suitably
arranged mirrors and/or portions of a grating or multiple gratings
or the like, that are operable through one or more reflections or
deflections to direct the later-diffracted light beams 403a and
403b to converge proximate to the beam splitting element 415.
[0097] The beam splitting element 415 receives the later-diffracted
light beams 403a and 403b and reflects or deflects at least one of
the beams according to known optical principles to bring the
later-diffracted light beams 403a and 403b into alignment or near
alignment in the shared zone 450, as shown in FIG. 4. The aligned
or nearly aligned later-diffracted light beams 403a and 403b then
enter a suitably-chosen optical detector 460.
[0098] As illustrated in FIG. 4, and various other figures shown
herein, the beam splitting element 415 is conceptually represented
as a readhead grating element and the later-diffracted light beams
403a and 403b each impinge upon a second portion of the same
readhead grating element which has a first portion impinged upon by
the light beam 401. Furthermore, in the illustration in FIG. 4, the
later-diffracted light beams 403a and 403b beams are shown
partially aligned in the shared zone 450. The structure and
operation of exemplary readhead grating elements which are
consistent with this illustration are discussed below with
reference to FIG. 10B or 10C. However, in this and various other
exemplary embodiments herein, the beam splitting element 415
impinged upon by the later-diffracted light beams 403a and 403b is
more generally intended to represent any now known or later
developed light beam splitting element or combination of elements,
such as a suitably arranged "half-silvered" beamsplitter or a
polarizing beamsplitter or portions thereof, or portions of a
grating or multiple gratings or the like, including any of the
configurations discussed below and shown in 10A-10E or 11A-11D,
which is operable to produce split light beams according to the
principles of this invention.
[0099] Various alternative ways of bringing the later-diffracted
light beams 403a and 403b into alignment or near alignment in the
shared zone 450, as well as the operation of respective
suitably-chosen optical detectors, will be apparent to one skilled
in the art. Applicable descriptions of various beam alignment and
detection techniques are also included in the discussion of FIGS. 1
and 2 above and in the incorporated '895, '085 and '833 patents, as
well as in related discussion further below. It should be
appreciated that the aligned or partially aligned beams in the
shared zone 450 will give rise to an illumination characteristic in
the shared zone which varies periodically in correspondence with
the relative displacement between the encoder readhead and the
scale grating 430. Because each of the aligned or partially aligned
beams in the shared zone 450 have been twice diffracted by the
scale 430, once before entering the reflecting elements 440 or 441
and once after, when the grating scale 430 moves by one pitch
increment the illumination characteristic in the shared zone will
undergo 4 periodic cycles.
[0100] In various embodiments according to this invention, certain
portions of the light from the later-diffracted light beams 403a
and 403b may be lost due to the operation of the beam splitting
element 415. Such lost light is symbolically indicated by the lost
light 499 shown in FIG. 4. However, other than possible disturbance
of the stability of the light source 412 as discussed herein, such
lost light is not significant to the operation of this invention.
As previously discussed, in general, lost light is not discussed
herein, unless it is relevant to light source disturbance or
crossover light in a particular embodiment.
[0101] In the embodiment shown in FIG. 4, and various other
embodiments shown herein, the exemplary beam paths associated with
the beams 401a, 401b, 403a and 403b are inclined in a respective
first direction away from a plane aligned parallel to the measuring
axis direction 101 and normal to the nominal plane of the scale
grating 430. The exemplary beam paths associated with the beams
402a, 402b, 402ar and 402br are inclined in an opposite direction
away from the plane aligned parallel to the measuring axis
direction 101 and normal to the nominal plane of the scale grating
430. However, it should be appreciated that in various other
embodiments according to the principles of this invention, these
inclinations can be greater or lesser than indicated in the various
exemplary embodiments shown herein and still prevent undesirable
mixing or crossover of various light beams, while at the same time
facilitating compact design and packaging of the device.
[0102] FIG. 5A is a three-dimensional conceptual view of an optical
displacement sensing device configuration in accordance with a
second exemplary embodiment of the invention. FIGS. 5B, 5C and 5D
are, respectively, a side view, a top view and an end view of the
optical displacement sensing device shown in FIG. 5A. In FIGS.
5A-5D, evidently corresponding elements and/or elements having
similar reference numbers are arranged and operate as previously
described with reference to FIG. 4 unless otherwise indicated. The
generic discussions with reference to FIG. 4 apply in a
corresponding manner to FIGS. 5A-5D, unless otherwise indicated.
Furthermore, in FIGS. 5A-5D the reference numbers of several
elements evidently corresponding to previously discussed generic or
specific elements are omitted, since their arrangement and
operation have already been made evident and are not further
described with reference to FIGS. 5A-5D. Conversely, in FIGS. 5A-5D
the reference numbers of elements are generally repeated or added
if there is a related description or if the FIGS. 5A-5D serve to
further clarify such elements.
[0103] The embodiment shown in FIGS. 5A-5D is a practical
configuration that is substantially insensitive to various
misalignments and dynamic misalignments, yet offers substantial
flexibility to locating and fabricating and assembling various
optical components in a compact and economical way in an encoder
readhead. FIG. 5A emphasizes the split light beam input portion 410
including a laser diode light source 412A and a beamsplitter 415X,
the scale grating 430 having a grating pitch P, the shared zone
450, and an optical detector assembly 460A (shown symbolically)
having one or more power and signal connections 461. In various
exemplary embodiments herein the grating pitch P is chosen to be
0.4 .mu.m. However, in various other exemplary embodiments the
pitch P may be chosen in a range from less than 0.4 .mu.m up to
many microns.
[0104] The laser diode light source 412A receives power, emits
coherent radiation, and in various exemplary embodiments includes
an integrated collimating element and emits collimated light. In
various exemplary embodiments the laser diode light source 412A is
chosen to emit light having a wavelength of approximately 635 nm
and in various other embodiments to emit light having a wavelength
of 405 nm, but any convenient wavelength may be used. In various
exemplary embodiments, a vertical cavity surface emitting laser
diode is used for the laser diode light source 412A. The
beamsplitter element 415X receives the light beam 401 from the
laser diode light source 412A and produces two split light beams
401a and 401b according to known optical principles.
[0105] It should be appreciated that in the embodiment shown in
FIGS. 5A-5D, and in various other embodiments shown herein, the
exemplary beamsplitter 415X is a beamsplitter such as one of the
beamsplitters described below with reference to FIG. 11C or 11D
having a first portion impinged upon by the light beam 401, or any
now known or later developed polarizing beamsplitter which provides
similar functions. However, in various other exemplary embodiments,
the beamsplitter 415X impinged upon by the light beam 401 may be
any now known or later developed beamsplitter element or
combination of elements, such as a suitably arranged
"half-silvered" beamsplitter or a polarizing beamsplitter or
portions thereof, or the like, including any of the configurations
discussed below and shown in FIGS. 11A-11D, which is operable to
produce split light beams according to the principles of this
invention, and which is also usable in conjunction with the
particular optical detector type chosen for a particular
embodiment.
[0106] The beamsplitter element 415X inputs the two split light
beams 401a and 401b to the remainder of the encoder readhead 500,
as previously described. FIGS. 5B-5D further clarify one exemplary
path configuration for the previously described series of light
beams 401, 401a, 401b, 402a, 402b, 402ar, 402br, 403a, and
403b.
[0107] As best shown in FIG. 5B, the later-diffracted light beams
403a and 403b converge proximate to the beamsplitter 415X. The
beamsplitter 415X receives the later-diffracted light beams 403a
and 403b, and reflects and transmits them, respectively, into
alignment in the shared zone 450. The beamsplitter 415X further
operates such that the reflected and transmitted light beams
aligned in the shared zone are orthogonally polarized relative to
each other. The aligned orthogonally polarized beams then enter the
optical detector assembly 460A.
[0108] It should be appreciated that in the embodiment shown in
FIGS. 5A-5D, and in various other embodiments shown herein, the
later-diffracted light beams 403a and 403b each impinge upon a
second portion of the same exemplary beamsplitter 415X which has a
first portion impinged upon by the light beam 401. However, in
various other exemplary embodiments, the beamsplitter/combiner
impinged upon by the later-diffracted light beams 403a and 403b may
be any now known or later developed light beam splitting/combining
element or combination of elements, such as a suitably arranged
"half-silvered" beamsplitter or a polarizing beamsplitter or
portions thereof, or the like, including any of the configurations
discussed below and shown in FIGS. 11A-11D, which is operable in
conjunction with the particular type of optical detector selected
for a particular embodiment.
[0109] The aligned orthogonally polarized beams then enter the
optical detector assembly 460A, which in the embodiment shown in
FIGS. 5A-5D is arranged and operates similarly to the optical
detector 460P described below with reference to FIG. 21. The
optical detector assembly 460A then outputs one or more signals on
the one or more power and signal connections 461, the signals
usable to determine the displacement of the scale grating 430
relative to the encoder readhead. More generally, the optical
detector assembly 460A can be any now known or later developed
optical detector which provides signals that are usable to
determine the relative phase between the lights of the orthogonally
polarized beams entering the optical detector. Various alternative
detector schemes will be apparent to one skilled in the art.
Instructive descriptions of detector schemes are also included in
the discussion of FIGS. 1 and 2 above and in the incorporated '895,
'085 and '833 patents.
[0110] As previously described, in various embodiments shown
herein, the exemplary beam paths associated with the beams 401a,
401b, 403a and 403b are inclined in a respective first direction
away from a plane aligned parallel to the measuring axis direction
101 and normal to the nominal plane of the scale grating 430, and
the exemplary beam paths associated with the beams 402a, 402b,
402ar and 402br are inclined in an opposite direction away from the
plane aligned parallel to the measuring axis direction 101 and
normal to the nominal plane of the scale grating 430. For purposes
of clarification, FIG. 5D shows an exemplary normal plane 475
aligned parallel to the measuring axis direction 101 and normal to
the nominal plane of the scale grating 430, an exemplary
inclination plane 474 inclined in a respective first direction away
from the normal plane 475, and the inclination angle 473 between
them. The inclination angle 473 is also referred to as the angle
"delta" or ".delta." herein. It should appreciated that the
inclination angle 473 can be greater or lesser than indicated in
the exemplary embodiment shown in FIG. 5D and still prevent
undesirable mixing or crossover of various light beams, while at
the same time facilitating compact design and packaging of the
device.
[0111] Also shown in FIG. 5D is a nominal beam splitting height
dimension 471, corresponding to the height of the beam splitting
portion of any beam splitting element, such as the beamsplitter
415X, above the nominal plane of the scale grating 430. Also shown
in FIG. 5D is a nominal retroreflector height dimension 472,
corresponding to the maximum height of any reflective surface of
any retroreflector 440 or 441 above the nominal plane of the scale
grating 430. It has been found that in embodiments according to
this invention where the beam splitting element is a beamsplitter
such as the beamsplitter 415X, or any of the beamsplitters shown in
FIGS. 11A-11D, or the like, that the errors in such embodiments
related to pitch misalignment and dynamic pitch misalignment and
the like tend to be relatively reduced when the retroreflector
height dimension 472 is made smaller. It has also been found that
in embodiments according to this invention where the beam splitting
element is a readhead grating such as the readhead gratings shown
in FIGS. 10A-10E, or the like, that the errors in such embodiments
related to pitch misalignment and dynamic pitch misalignment and
the like tend to be relatively reduced as the beam splitting height
dimension 471 is made smaller and/or as the retroreflector height
dimension 472 approaches equality with the beam splitting height
dimension 471.
[0112] It should be appreciated that the planes 474 and 475, the
angle 473 and the heights 471 and 472 are intended to be
generically illustrative of similarly numbered or evidently
analogous planes, angles and heights in various other embodiments,
thus, their locations and values are not limited by the exemplary
embodiment shown in FIG. 5D. More generally, it should be
appreciated that any new generic discussions included in the
preceding discussion of FIGS. 5A-5D are indicative of a number of
configurations which vary in their combination of elements and
dimensions from the specific elements and dimensions indicated in
FIGS. 5A-5D. Thus, FIGS. 5A-5D should not be interpreted as
limiting with regard to the meaning and intent of the generic
portions of the preceding discussion.
[0113] FIG. 6 is a three-dimensional schematic view clarifying
conventional geometric components and symbols used for describing
light path directions herein. FIG. 6 shows a set of orthogonal X, Y
and Z axes. The X axis is aligned parallel to the measuring axis
direction 101. The Z axis is aligned normal to the grating surface
of a nominally aligned scale grating, and the Y axis is aligned
orthogonal to the X and Z axes at the grating surface of a
nominally aligned scale grating. Three angles are used to indicate
the orthogonal components of unit vector 601, each of which are
measured from a respective principal axis as shown in FIG. 6. The
angle formed between the unit vector 601 and the x axis is .alpha.,
the angle formed between the unit vector 601 and the y axis is
.beta., and the angle formed between the unit vector 601 and the z
axis is .gamma.. The respective x axis, y axis and z axis
components of the unit vector 601 are thus, cos .alpha., cos .beta.
and cos .gamma., respectively, as shown in FIG. 6. This same
terminology is useful for describing the orientation and
relationship of various light beam paths in various exemplary
embodiments according to this invention.
[0114] FIG. 7 is a three-dimensional schematic view showing a
generic input light ray direction and the resulting conical
distribution of diffracted light rays from a grating analogous to
the scale grating 430 described herein, using the terminology of
FIG. 6. In FIG. 7 an input light ray 701 impinges on grating 702
with input angles (.alpha., .beta., .gamma.) and various diffracted
output beams each having a respective set of output angles
(.alpha..sub.1, .beta..sub.1, .gamma..sub.1) result. The various
diffracted output beams, shown as arrows in FIG. 7, together define
a cone 703. The relationship between the input angles for a beam
incident on the grating 702, and the output angles for the nth
diffraction order are as follows: 1 Input Workspace Date : 09 / 18
/ 2003 Number : 10101031 Folder : 03
[0115] where n is the diffraction order, .lambda. is the wavelength
of the light, and d is the grating pitch. The grating pitch is also
referred to as P herein. One skilled in the art will readily
understand that these equations may be used to determine an
operable arrangement of encoder readhead components for any
embodiment of an encoder readhead configuration according to the
principles of this invention, including configurations where the
inclination angle 473, best shown in FIG. 5D, is not equal to
zero.
[0116] FIG. 8 shows an error table 801, which lists as entries the
error magnitudes associated with various dynamic misalignments and
drifts for a prior art optical displacement sensing device
configuration corresponding to FIG. 1. The individual dynamic
misalignments and drifts listed across the tops of the columns
occur in various combinations with the individual initial
misaligmnents or deviations from nominal (also referred to as
initial "static" misalignments or deviations, herein) listed down
the left end of the rows. "Gap" refers to the separation between an
encoder readhead and a scale grating surface along a direction
normal to the nominal scale grating surface. A scale grating
surface in its intended nominal design plane in an encoder readhead
configuration according to the principles of this invention has a
gap misalignment of zero.
[0117] For all entries in table 801, the nominal wavelength
.lambda. is 0.635 .mu.m, the nominal angle .alpha. is 38 degrees,
the nominal angle .beta. is 80 degrees, and the grating period of
the scale is 0.4 .mu.m. Additionally, the results are for an
encoder readhead configuration approximately as shown in FIG. 1,
where the element corresponding to element 102 shown in FIG. 1 is a
readhead grating positioned at a beam splitting height dimension of
10 mm relative the to the scale grating surface and having a
grating pitch of 0.8 .mu.m. Each entry in the table corresponds to
the displacement measurement error, in nanometers, that arises when
the individual initial installation misalignment listed at the left
of the entry row is combined with the individual dynamic
misalignments listed at the top of the entry column. Each dynamic
misalignment represents a change in alignment that occurs between
the time a reference position measurement is made, and the time the
displacement measurement is made. Accordingly, each entry is the
measurement error that is included in the apparent displacement
indicated by the encoder readhead signals, between the reference
position and the displacement position, due to the dynamic
misalignment or deviation.
[0118] As a clarifying example, in table 801, given an installation
pitch misalignment of 30 minutes, the error associated with either
a dynamic roll, or yaw misalignment of 2 seconds is 0.0 nm. For the
same installation pitch misalignment, a dynamic wavelength drift of
0.00025 .mu.m produces an error of 1.0 nm magnitude and a dynamic
gap change of 0.01 mm produces an error of 87.3 nm. It should be
appreciated that this 87.3 nm error is the result of an "apparent"
geometric translation of the encoder readhead relative to the scale
grating along the measuring axis, and is thus an error that will
appear for a wide variety of encoder readhead configurations,
regardless of their design. More importantly, for purposes of
comparison with various encoder readhead embodiments according to
this invention, the significant aspect of table 801 is that a
dynamic pitch of only 2 seconds of arc in this prior art
configuration produces 97.0 nm displacement measuring error for all
initial alignment conditions. As previously discussed, dynamic
pitch misalignment is one of the most prevalent and/or difficult to
eliminate conditions in many practical encoder applications, and is
thus of particular significance for encoder readhead design. As
previously discussed, dynamic pitch errors are substantially
reduced by the prior art configurations shown in FIGS. 2 and 3, but
those configurations have the other limitations previously
discussed.
[0119] FIG. 9 shows an error table 901, which lists as entries the
error magnitudes associated with various dynamic misalignments and
drifts for a optical displacement sensing device configuration
according to this invention which corresponds to the basic
configuration shown in FIGS. 5A-5D, and more closely to basic
configuration described with reference to FIGS. 12A-12D below,
having the beamsplitter 415X positioned at a beam splitting height
dimension 471 of 10 mm and the retroreflectors 440 and 441
positioned at retroreflector height dimension 472 of 5 mm. For all
entries in table 901, the nominal wavelength .lambda. is 0.635
.mu.m, the nominal angle .alpha. is 38 degrees, the nominal angle
.beta. is 80 degrees, and the grating period of the scale is 0.4
.mu.m. The significant aspect of table 901 is that the underlying
encoder readhead embodiment according to this invention virtually
eliminates dynamic pitch error. It should be further appreciated
that the underlying encoder readhead embodiment according to this
invention achieves this performance while at the same time
overcoming the various previously discussed limitations of the
prior art configurations shown in FIGS. 2 and 3. It has been
determined that various other encoder readhead embodiments
according to this invention, including but not limited to those
corresponding to FIGS. 4, 5A-5D, 12A-12D, 19, 20A-20C, 22A-22C, 23
and 24 and their discussed variations, also substantially reduce
dynamic pitch errors while overcoming at least one of the
previously discussed limitations of the prior art configurations
shown in FIGS. 2 and 3.
[0120] The most significant errors shown in table 901 are for
dynamic wavelength deviation and dynamic gap change. The errors
associated with dynamic gap change have the same magnitude in table
901 and table 801, and have been previously discussed. The error
associated with dynamic wavelength deviation may be overcome by
using more stable light sources and/or improved temperature control
in various exemplary embodiments. However, it has also been
determined that various encoder readhead configurations according
to the principles of this invention also influence the magnitude of
the errors associated with dynamic wavelength deviation, which is
therefore discussed further with reference to FIGS. 13-18
below.
[0121] FIGS. 10A-10E are three-dimensional views clarifying the
operation of respective first through fifth readhead grating
configurations usable in the beam splitting portion of various
exemplary encoder readhead embodiments according to this invention.
Each view shows hypothetical apertures on the surfaces of the
readhead gratings. These apertures are not physical elements, but
are illustrated only to clarify the paths of the light beams in the
figures.
[0122] FIG. 10A shows readhead grating element 415A having a
configuration wherein the input beam 401 impinges on a first
portion 416 having a first grating pitch to produce the split beams
401a and 401b by transmission of a zero order beam and by
transmission of a first or higher order diffracted beam. The paths
of the beams 401, 401a, 401b, 403a and 403b shown in FIGS. 10A-10E
have been generally shown and described previously, with reference
to FIGS. 4 and 5A-5D. The later-diffracted light beams 403a and
403b impinge on a second portion 417 having the first grating
pitch, which transmits the beam 403b as a zero order beam and the
beam 403a as a first or higher order diffracted beam, to bring the
beams 403a and 403b into alignment in the shared zone 450, as
shown. In various exemplary embodiments, the first readhead grating
pitch is the same as the pitch of the scale grating used with that
embodiment. The readhead grating element 415A may be used in
encoder readhead embodiments which prevent "crossover beams" as
previously described, that is, where lights arising from the split
beams 401a and 401b do not share a common path before reaching the
shared zone 450. However, the resulting illumination characteristic
in the shared zone may not, by itself, indicate the direction of
the relative displacement and thus has limited application. This
problem may be remedied by polarizing the encoder readhead beams in
the manner describe below with reference to FIG. 10D or 10E, but
inserting the polarizers into the beam path remotely from the
readhead grating element, and using a polarization sensitive
detector such as the optical detector described with reference to
FIG. 21 below.
[0123] FIG. 10B shows readhead grating element 415B having a
configuration which operates identically to readhead grating
element 415A, except that the second portion 417 has a second
grating pitch varying from the first grating pitch of the first
portion 416. Thus, the beam 403a is diffracted as a first or higher
order diffracted beam that is slightly out of alignment with the
transmitted beam 403b in the shared zone 450, which gives rise to
an interference pattern in the shared zone 450, represented
conceptually by the interference pattern 451. The interference
pattern 451 translates spatially in correspondence to the scale
grating displacement. The translation of the interference pattern
451 can be quantitatively detected by various optical detector
array techniques known in the art and/or currently commercially
available, including the use of quadrature arrays which directly
produce quadrature signals, or arrays which allow the interference
pattern translation to be imaged and digitally analyzed at a higher
resolution.
[0124] FIG. 10C shows readhead grating element 415C having a
configuration which produces the same result as readhead grating
element 415B. In the readhead grating element 415C, the first
portion 416 and the second portion 417 have the same grating pitch,
but an optical wedge element 491 is added. Thus, the beam 403b is
transmitted as zero order beam that is slightly out of alignment
with the diffracted order beam 403a in the shared zone 450, which
gives rise to an interference pattern in the shared zone 450,
represented conceptually by the interference pattern 451.
[0125] The readhead grating elements 415B or 415C have
configurations that may be used in any encoder readhead embodiments
which prevent "crossover beams" as previously described, that is,
where lights arising from the split beams 401a and 401b do not
share a common path before reaching the shared zone 450.
[0126] FIG. 10D shows a readhead grating element 415D having a
configuration which produces the same result as readhead grating
element 415A, except the beams aligned in the shared zone are, in
addition, mutually orthogonally polarized. In the readhead grating
element 415D, the first portion 416 and the second portion 417 have
the same grating pitch, but a first polarizer 492 and a second
polarizer 493 are arranged to insure that the beams 403a and 403b
are mutually orthogonally polarized before entering the shared zone
450. The readhead grating element 415A may be used in any encoder
readhead embodiments which prevent "crossover beams" as previously
described, that is, where lights arising from the split beams 401a
and 401b do not share a common path before reaching the polarizers
493 and 492, respectively. The illumination characteristic in the
shared zone 450 may be sensed using a polarization sensitive
detector such as the optical detector described with reference to
FIG. 21 below.
[0127] FIG. 10E shows a readhead grating element 415E having a
configuration which produces the same result as readhead grating
element 415A, except the split beams 401a and 401b are, in
addition, mutually orthogonally polarized. In the readhead grating
element 415E, the first portion 416 and the second portion 417 have
the same grating pitch. A first polarizer 494 and a second
polarizer 495 are arranged to insure that the beams 401a and 401b
are mutually orthogonally polarized before any of the resulting
beams "cross over" or share a common beam path. Furthermore, after
any beams "cross over" or share a common beam path in the encoder
readhead, the first polarizer 494 and the second polarizer 495 are
also arranged to filter the beams 403a and 403b such that they are
mutually orthogonally polarized in the same orientation as the
beams 401a and 401b, respectively. Because of the initial
polarization of the split beams 401a and 401b, any crossover beams
and/or beams sharing common beam paths can be removed by this final
filtering of the beams 403a and 403b. Thus, the configuration of
readhead grating element 415E may be used in any encoder readhead
embodiments according to this invention, including configurations
which do not include the beam offsets, as previously described,
and/or which do not include an appreciable inclination angle, as
previously described. The illumination characteristic in the shared
zone 450 may be sensed using a polarization sensitive detector such
as the optical detector described with reference to FIG. 21
below.
[0128] FIGS. 11A-11D are three-dimensional views clarifying the
operation of respective first through fourth beamsplitter
configurations usable in the beam splitting portion of various
exemplary encoder readhead embodiments according to this invention.
Each view shows hypothetical apertures on the surfaces of the
beamsplitters. These apertures are not physical elements, but are
illustrated only to clarify the paths of the light beams in the
figures. The beamsplitter configurations shown in FIGS. 11A-11D are
described by analogy with corresponding readhead grating
configurations shown in the FIGS. 10A and 10C-10E. However, in each
FIGS. 11A-11D, at the first portion 416 and the second portion 417,
the beams 401b and 403a, respectively, are reflected at a partially
reflective beamsplitter interface 418 arranged vertically, rather
than diffracted at a horizontal grating surface as in the FIGS. 10A
and 10C-10E. Each beamsplitter configuration has the same result in
the shared zone 450 as the indicated analogous readhead grating
configuration, and may be similarly used with similar encoder
readhead embodiments according to this invention. Their operation
will be apparent to one skilled in the art.
[0129] FIG. 1A shows beamsplitter element 415F which has a
configuration analogous to the configuration of readhead grating
element 415A shown in FIG. 10A. FIG. 1B shows beamsplitter element
415G which has an optical wedge element 491 added in a
configuration analogous to the configuration of readhead grating
element 415C shown in FIG. 10C. FIG. 11C shows beamsplitter element
415H which has a first polarizer 492 and a second polarizer 493
arranged to insure that the beams 403a and 403b are mutually
orthogonally polarized before entering the shared zone 450 in a
configuration analogous to the configuration of readhead grating
element 415D shown in FIG. 10D. FIG. 11D shows a beamsplitter
element 415J which has a first polarizer 494 and a second polarizer
495 are arranged in a configuration analogous to the configuration
of readhead grating element 415E shown in FIG. 10D to insure that
the beams 401a and 401b are mutually orthogonally polarized before
any of the resulting beams "cross over" or share a common beam
path, and that after any beams "cross over" or share a common beam
path in the encoder readhead, the first polarizer 494 and the
second polarizer 495 filter the beams 403a and 403b such that they
are mutually orthogonally polarized in the same orientation as the
beams 401a and 401b, respectively.
[0130] It should be appreciated for all of the readhead grating
configurations shown in FIGS. 10A-10E and 11A-11D, the operable
portions of each element are the portions struck by the light
beams. Thus, in various exemplary embodiments, these portions may
be more separated, less separated, provided as integrated
assemblies or as separate elements, or the like, so long as they
are positioned with respect to operable encoder readhead beam paths
in accordance with the principles of this invention.
[0131] FIG. 12A is a three-dimensional conceptual view of an
optical displacement sensing device in accordance with a third
exemplary embodiment of the invention. FIGS. 12B, 12C and 12D are,
respectively, a side view, a top view and an end view of the
optical displacement sensing device in shown in FIG. 12A. The
embodiment shown in FIGS. 12A-12D is analogous to the embodiment
shown in FIGS. 5A-5D, except for the location of the
retroreflectors 440 and 441. The generic discussions with reference
to FIGS. 5A-5D apply in a corresponding manner to FIGS. 12A-12D,
unless otherwise indicated. Thus, in FIGS. 12A-12D the reference
numbers of several elements evidently corresponding to previously
discussed generic or specific elements are omitted, since their
arrangement and operation have already been made evident.
Conversely, in FIGS. 12A-12D the reference numbers of elements are
generally repeated or added if there is a related description or if
the FIGS. 12A-12D serve to further clarify such elements.
[0132] The embodiment shown in FIGS. 12A-12D is a practical
configuration that is substantially insensitive to various
misalignments and dynamic misalignments, yet exhibits substantial
flexibility for fabricating, locating and assembling various
optical components in a compact and economical way in an encoder
readhead. FIG. 12A emphasizes the angle a formed between the split
light beams 401a and 401b that impinge on the scale grating 430 in
the first zone 431 and a hypothetical line 1201 which extends
parallel to the measuring axis direction 101 through the respective
points of impingement of the light beams 401a and 401b. The angle a
shown in FIG. 12A corresponds to the angle .alpha. defined with
reference for FIG. 6. Thus, the angle .alpha. lies in the plane
474, best seen in FIG. 12D. Various errors related to dynamic
misalignment or drift are discussed further below in relation to
various design values for the angle a.
[0133] FIGS. 12A-12D also emphasize a configuration and/or height
for the retroreflectors 440 and 441 that results in relatively
short optical path lengths for the light beams 402a, 402ar, 402b
and 402br, respectively. As previously discussed, it has been found
that in embodiments according to this invention where the beam
splitting element is a beamsplitter, such as any of the
beamsplitters shown in FIGS. 11A-11D, or the like, the errors in
such embodiments related to pitch misalignment and dynamic pitch
misalignment and the like tend to be reduced as the retroreflector
height dimension 472 is decreased. Furthermore, shorter optical
path lengths tend to reduce the sensitivity of the encoder readhead
to dynamic variations in the wavelength of the light source. It
also generally permits the encoder readhead to be made more
compact. The beam splitting height dimension 471 and retroreflector
height dimension 472 for this exemplary configuration are best seen
in FIGS. 12B and 12D.
[0134] FIG. 12B also shows portions of an exemplary readhead
component housing 1225 and a gap dimension 1220 between the bottom
of the readhead component housing 1225 and the surface of the scale
grating 430. The portions of the readhead component housing 1225
are not shown in FIGS. 12A, 12C and 12D so that the optical paths
may be seen more clearly. It should be appreciated that factors
limiting how close the light beam directing elements 420 and 421,
and the retroreflectors 440 and 441 can be placed to the scale, in
order to shorten the related optical paths, are the desired
operating gap dimension 1220 and practical wall thicknesses and
mounting arrangements within the readhead component housing 1225.
In various exemplary embodiments, to achieve short optical path
lengths and a compact encoder readhead, the bottom edges of light
beam directing elements 420 and 421, and/or the retroreflectors 440
and 441 are operatively positioned proximate to the bottom of the
readhead component housing 1225 with due consideration to any
design constraints imposed by optical path clearance considerations
and the desired operating gap dimension 1220. Considering that a
smaller gap dimension shortens various optical paths to reduce
errors and a larger gap dimension simplifies installation, in
various exemplary embodiments the desired gap dimension 1220 may be
on the order of 1-2 mm, for example.
[0135] As shown in FIGS. 12A-12D, the light beam paths 402a, 402ar,
402b and 402br are offset from the light beam paths 401a, 403a,
401b and 403b, respectively, by both a vertical separation angle
component 1221, best seen in FIG. 12B, and by the inclination angle
473, "delta", best seen in FIG. 12D. It should be appreciated that
the vertical separation angle component 1221 and the inclination
angle 473 may be chosen in combination to reduce the height and/or
the width of the associated encoder readhead, as emphasized by the
hypothetical dimension box 1230 shown in FIG. 6D. It can be seen
that if the inclination angle 473 is decreased, to maintain light
beam clearance between the light beam directing elements 420, 421
and the retroreflectors 440, 441 the height of the dimension box
630 must increase, corresponding to increased encoder readhead
height. Similarly, if the vertical separation angle component 1221
is decreased, to maintain light beam clearance the width of the
dimension box 630 must increase, corresponding to increased encoder
readhead width. It should be appreciated that the planes 474 and
475, the angles 1221 and 473 and the heights 471 and 472 are
intended to be generically illustrative of analogous planes, angles
and heights in various other embodiments, thus, their locations and
values are not limited by the exemplary embodiment shown in FIGS.
12A-12D, but are to be chosen to achieve the overall encoder
readhead dimension desired in a particular application.
Furthermore, FIGS. 12A-12D are indicative of a number of additional
embodiments according to this invention which vary in their
combination of elements and dimensions from the specific elements
and dimensions indicated. Thus, FIGS. 12A-12D should not be
interpreted as limiting with regard to the teachings discussed
above.
[0136] FIGS. 13-18 show information useful for determining
desirable design dimensions for the angle a, previously discussed
with reference to FIGS. 6, 7 and 12A, and for the inclination angle
"delta", previously discussed with reference to FIGS. 5D and
12D.
[0137] FIG. 13 is a graph showing error sensitivity data for
dynamic gap misalignment and wavelength change at various incident
beam angles .alpha. in an optical displacement sensing device in
accordance with this invention where the scale grating pitch is 0.4
.mu.m, the light source has a nominal 635 nm wavelength, a beam
splitter is positioned at a height 10 mm above the scale grating
surface, the retroreflectors are positioned such that the path
length from the scale to the corner of the retroreflectors is 10
mm, the angle .alpha. lies in a plane of incidence which is
perpendicular to the scale grating grooves and to the scale (that
is, the angle delta is 0.degree.), and the static pitch
misalignment is set at 0.5.degree.. A curve 1301 shows the dynamic
errors resulting at various values of the angle .alpha. when the
light source wavelength dynamically changes by 0.25 nm. A curve
1302 shows the dynamic errors resulting at various values of the
angle .alpha. when there is a dynamic gap change of 10 .mu.m. As
previously discussed with reference to FIG. 9, these tend to be the
largest remaining dynamic errors in various exemplary embodiments
according to this invention, therefore, it is especially useful to
choose design values which further reduce these dynamic errors.
[0138] With the stated combination of wavelength and scale grating
pitch, the associated encoder readhead is operable when .alpha.
ranges from approximately 50 degrees to 10 degrees. However, as
shown by the curves 1301 and 1302, for the stated combination of
readhead parameters, the errors associated with dynamic wavelength
change and gap change increase rapidly when .alpha. is greater than
approximately 40 degrees. Therefore, in various exemplary
embodiments according to this invention, the components of the
encoder readhead are configured such that .alpha. is less than or
equal to 40 degrees.
[0139] FIG. 14 shows additional design considerations for selecting
a design value for the angle .alpha.. FIG. 14 assumes a beam having
a nominal diameter of 1.0 mm and a wavelength of 635 nm is directed
onto a scale grating having a grating pitch of 0.4 .mu.m from a
height 5.0 mm above the surface of the scale grating, at various
values of the angle .alpha.. The second column of the table shown
in FIG. 14 shows data comparing the resulting maximum beam cross
section dimension at the retroreflectors versus the angle a shown
in the first column. The third column of the table shown in FIG. 14
shows data comparing the distance between mirrors used as beam
directing elements at the height 5.0 mm above the surface of the
scale grating versus the angle .alpha. shown in the first column.
The resulting maximum beam cross section dimension varies from 4.6
mm when the angle .alpha. is 10 degrees to 0.9 mm when the angle
.alpha. is 40 degrees. The resulting distance between beam
directing mirrors varies from 56.7 mm when in the angle .alpha. is
10 degrees to 11.9 mm when in the angle .alpha. is 40 degrees.
[0140] One design consideration in various exemplary embodiments
according to this invention is that the spot on a retroreflector
becomes elongated and overfills the aperture of the retroreflection
element, thereby wasting light, and/or falling on imperfect edges
of the retroreflector. Another design consideration is that the
distance between beam directing mirrors may determine the overall
length of the encoder readhead. A beam cross section of 4.6 mm,
requires relatively larger retroreflector elements, leading to a
relatively larger encoder readhead. A distance between mirrors of
56.7 mm likewise leads to a relatively larger encoder readhead.
Therefore, even though lesser angles a generally reduce the
sensitivity to various dynamic errors as shown in FIG. 13, in
various exemplary embodiments, the angle .alpha. is made greater
than or equal to 20 degrees, to enable a smaller overall size for
the encoder readhead.
[0141] FIG. 15 is a graph showing error sensitivity data for
dynamic gap misalignment at various incident beam angles a and at
various inclination angles (delta), for the encoder parameters
previously described with reference to FIG. 13, unless otherwise
indicated. The dynamic gap change is 10 .mu.m. The curves
1501-1503, corresponding to inclination angles of delta=0 degrees,
delta=15 degrees and delta=45 degrees, respectively, are nearly
indistinguishable. However, as best shown by the curves 1501 and
1502, for the stated combination of readhead parameters, the errors
associated with dynamic gap change increase more rapidly when
.alpha. is greater than approximately 40 degrees. Therefore, in
various exemplary embodiments according to this invention, the
components of the encoder readhead are configured such that .alpha.
is less than or equal to 40 degrees.
[0142] FIG. 16 is a graph identical to the graph shown in FIG. 15,
except that the light source wavelength in nominally 405 nm,
instead of 635 nm. The curves 1601-1603, corresponding to
inclination angles of delta=0 degrees, delta=15 degrees and
delta-45 degrees, respectively, are indistinguishable. Compared to
the results shown in FIG. 15, the reduced light source wavelength
reduces the dynamic errors due to gap change, for the stated
combination of readhead parameters. Furthermore, the errors
associated with dynamic gap change increase relatively slowly up to
an operating range of 60 degrees for the angle .alpha.. Therefore,
in various exemplary embodiments according to this invention, the
components of the encoder readhead are configured such that .alpha.
is less than or equal to 60 degrees. However, since the errors
associated with dynamic gap change are restricted to values below
approximately 50 nm when the angle .alpha. is less than or equal to
45 degrees and the wavelength and grating pitch are as stated, in
various exemplary embodiments according to this invention, the
components of the encoder readhead are configured such that .alpha.
is less than or equal to 45 degrees.
[0143] FIG. 17 is a graph showing error sensitivity data for
dynamic wavelength drift at various incident beam angles and at
various inclined plane angles, for the encoder parameters
previously described with reference to FIG. 13, unless otherwise
indicated. The curves 1701-1703 correspond to inclination angles of
delta=0 degrees, delta=15 degrees and delta=45 degrees,
respectively. The light source wavelength dynamically changes by
0.25 nm. This dynamic light source wavelength change produces
significantly larger dynamic errors than those due to the due to
gap change previously discussed with reference to FIGS. 15 and 16.
Thus, if the light source of an encoder readhead exhibits this type
of instability, for high accuracy it is particularly important to
choose an angle a that reduces this sensitivity. For the curves
1701 and 1702, the errors associated with dynamic wavelength change
increase relatively slowly up to an operating range of 40 degrees
for the angle .alpha.. Therefore, in various exemplary embodiments
according to this invention, the components of the encoder readhead
are configured such that .alpha. is less than or equal to 40
degrees. However, for the curve 1703, corresponding the angle
delta=45 degrees, the errors increase more rapidly versus the angel
.alpha.. Therefore, in various other exemplary embodiments
according to this invention, the components of the encoder readhead
are configured such that .alpha. is less than or equal to 30
degrees.
[0144] FIG. 18 is a graph identical to the graph shown in FIG. 17,
except that the light source wavelength in nominally 405 nm,
instead of 635 nm. The curves 1801-1803, corresponding to
inclination angles of delta=0 degrees, delta=15 degrees and
delta=45 degrees, respectively, are nearly indistinguishable.
Compared to the results shown in FIG. 17, the reduced light source
wavelength reduces the dynamic errors due to wavelength change, for
the stated combination of readhead parameters. Furthermore, the
errors associated with dynamic wavelength change increase
relatively slowly up to an operating range of 60 degrees for the
angle .alpha.. Therefore, in various exemplary embodiments
according to this invention, the components of the encoder readhead
are configured such that .alpha. is less than or equal to 60
degrees. However, since the errors associated with dynamic
wavelength change are restricted to values below approximately 100
nm when the angle that a is less than or equal to 45 degrees and
the wavelength and grating pitch are as stated, in various
exemplary embodiments according to this invention, the components
of the encoder readhead are configured such that .alpha. is less
than or equal to 45 degrees.
[0145] FIG. 19 is a three-dimensional conceptual view of an optical
displacement sensing device in accordance with a fourth exemplary
embodiment within the scope of the invention. The configuration
shown in FIG. 19 is analogous to the configuration shown in FIGS.
5A-5D, except the inclination angle 473 shown in FIG. 5D is chosen
to be zero in the configuration shown in FIG. 19. Because the
inclination angle is zero, the embodiment shown in FIG. 19 is a
practical configuration that offers the possibility to fabricate a
relatively narrow encoder readhead. However, when the inclination
angle is zero, the zero-order reflection beams arising from the
light beams 401a, 401b, 402a and 402b in the first zone 431, and
from the light beams 402ar, 402br, 403a and 403b in the second zone
432, become cross-over beams, as previously discussed. Thus, for
the configuration shown in FIG. 19, the beam splitting element 415
of the split light beam input portion 410 should incorporate
polarizing elements to separate or filter out the crossover beams,
such as the readhead grating shown in FIG. 10E, or the beamsplitter
shown in FIG. 11D, or the like. Furthermore, the optical detector
assembly 460 should include further include polarizing elements,
such as the optical detector 460P described below with reference to
FIG. 21, or the like. More generally, the optical detector assembly
460 can be any now known or later developed optical detector which
provides signals that are usable to determine the relative phase
between the lights of orthogonally polarized beams entering the
optical detector. Various alternative detector schemes will be
apparent to one skilled in the art. Instructive descriptions of
detector schemes are also included in the discussion of FIGS. 1 and
2 above and in the incorporated '895, '085 and '833 patents.
[0146] FIG. 20A is a conceptual side view of an optical
displacement sensing device in accordance with a fifth exemplary
embodiment within the scope of the invention. FIG. 20B is a top
view of the optical displacement sensing device shown in FIG. 20A
and FIG. 20C is an end view of the optical displacement sensing
device shown in FIG. 20A. The configuration shown in FIGS. 20A-20C
is analogous to the configuration shown in FIGS. 12A-12D, except
the components of the encoder readhead are arranged such that the
portion of each of the light beams that impinges upon or is
diffracted from the scale grating 430, as best shown by the
numbered light beam portions shown in FIG. 20B, forms the same
angle with respect to a hypothetical line which extends parallel to
the measuring axis direction 101 and through the respective points
of impingement or diffraction. Such a configuration can be designed
in accordance with EQUATIONS 1-3, previously discussed.
[0147] In such a configuration, using a suitable inclination angle
the light beam directing elements 420, 421 and the retroreflectors
440, 441 may be positioned at approximately the same height, as
best shown in FIG. 20C, which in various exemplary embodiments
results in a readhead having a relatively small overall height.
However, because each of the light beams that impinges upon or is
diffracted from the scale grating 430 forms the same angle with
respect to the hypothetical line which extends parallel to the
measuring axis direction 101, the zero-order reflection beams
arising from the light beams 401a, 401b, 402a and 402b in the first
zone 431, and from the light beams 402ar, 402br, 403a and 403b in
the second zone 432, become cross-over beams, as previously
discussed. Thus, for the configuration shown in FIGS. 20A-20C, the
beam splitting element 415 of the split light beam input portion
410 should incorporate polarizing elements to separate or filter
out the crossover beams, such as the readhead grating shown in FIG.
10E, or the beamsplitter shown in FIG. 11D, or the like.
Furthermore, the optical detector assembly 460 should include
polarizing elements, such as the optical detector 460P described
below with reference to FIG. 21, or the like, as previously
discussed with reference to FIG. 19.
[0148] FIG. 21 is a schematic side view of an optical detector 460P
including a polarizer configuration. The optical detector 460P is,
thus, useful in various exemplary embodiments according to the
invention that include crossover beams. More generally, the optical
detector 460P is useful in various exemplary embodiments wherein
the light beams 403a and 403b are orthogonally polarized in the
shared zone 450 for any reason. In operation, orthogonally
polarized light beams 403a and 403b are emitted from a beam
splitting element 415 into the shared zone 450 along aligned
optical paths to optical detector 460P. Beam splitter 4606 splits
the orthogonally polarized light beams 403a and 403b into two split
detection beams. A first split detection beam passes through a
quarter wave plate 4653 and a polarizer 4671 to a photodetector
element 4681. A second split detection beam passes through a
polarizer 4672 to a photodetector element 4682. The polarizers 4671
and 4672 are each nominally oriented at a 45 degree angle between
the orthogonally polarized beams that reach them, and are further
arranged to pass mutually orthogonally polarized light. The quarter
wave plate 4653 delays one of the output light beams a quarter of a
wave or 90 degrees. Hence, detector 4681 detects a signal that is
90 degrees phase shifted relative to the signal detected by
detector 4682, providing a well known quadrature signal format on
one or more signal lines 461. In this embodiment of the invention,
the optical detector 460P is shown as one integrated unit. However,
other configurations of a detector may be used to achieve the same
objective as the configuration shown in FIG. 21, as will be
apparent to one skilled in the art.
[0149] FIG. 22A is a conceptual side view of an optical
displacement sensing device in accordance with a sixth exemplary
embodiment within the scope of the invention. FIG. 22B is a top
view of the optical displacement sensing device shown in FIG. 22A,
and FIG. 22C is an end view of the optical displacement sensing
device in shown in FIG. 22A. The configuration shown in FIGS.
22A-22C is analogous to the configuration shown in FIGS. 20A-20C.
The components of the encoder readhead are arranged such that the
portion of each of the light beams that impinges upon or is
diffracted from the scale grating 430 forms the same angle with
respect to a hypothetical line which extends parallel to the
measuring axis direction 101 and through the respective points of
impingement or diffraction. However, in contrast to the
configuration shown in FIGS. 20A-20C, the impingement points of the
light beams in the first zone 431 and the second zone 432,
respectively, are separated along the measuring axis direction, as
best shown in FIG. 22B. As a result, the zero order reflection
paths represented by the lines 2201 and 2202 shown in FIG. 22A,
fall outside the effective apertures of the light beam directing
elements 420, 421 and the retroreflectors 440, 441, eliminating
potential crossover beams. Thus, the configuration shown in FIGS.
22A-22C need not include polarizers in the light beam paths or in
the optical detector. However, similarly to the configuration shown
in FIGS. 20A-20C, in such a configuration the light beam directing
elements 420, 421 and the retroreflectors 440, 441 may be
positioned at approximately the same height, which in various
exemplary embodiments results in a readhead having a relatively
small overall height.
[0150] FIG. 23 is a conceptual side view of an optical displacement
sensing device in accordance with a seventh exemplary embodiment
within the scope of the invention. In the configuration shown in
FIG. 23, the optical readhead components are positioned such that
the light beam paths 402a, 402ar, 402b and 402br lie nearer to the
scale grating than the light beam paths 401a, 403a, 401b and 403b.
This configuration may be used with an inclination angle which
eliminates crossovers beams, and thus eliminates the need for
polarizers in the configuration. Alternatively, for inclination
angles which do not eliminate crossover beams, the beam splitting
element 415 of the split light beam portion 410 may include
polarizers to separate out the crossover beams, as previously
discussed.
[0151] FIG. 24 is a conceptual side view of an optical displacement
sensing device in accordance with an eighth exemplary embodiment
within the scope of the invention. In the configuration shown in
FIG. 24, the scale grating 430 is a transmission grating and the
retroreflectors 440, 441 are positioned to receive transmitted
diffracted orders. If the configuration is arranged such that
transmitted zero-order beams fall outside the effective apertures
of the light beam directing elements 420, 421, and the
retroreflectors 440, 441, as represented by the lines 2401-2404,
then crossover beams will be eliminated from the configuration,
thus eliminating the need for polarizers in the configuration,
regardless of the inclination angle used. Alternatively, for
configurations which do not eliminate crossover beams, the beam
splitting element 415 of the split light beam portion 410 may
include polarizers to separate out the crossover beams, as
previously discussed.
[0152] FIG. 25 is a three-dimensional view showing a first
exemplary transparent block configuration usable in various
exemplary embodiments according to the invention. Refractive
effects on the various light beam paths are ignored in FIG. 25, but
the adjustments needed for such effects will be readily understood
by one skilled in the art. FIG. 25 shows a transparent block 2500
including a first block portion 2510 and a second block portion
2515 which are joined at an interface which is fabricated to
include a half-silvered mirror or the like in order to provide a
beam splitting element 415. The first block portion 2510 includes
an end surface 2502 and orthogonal retroreflector surfaces 2504 and
2506 that are all coated to be reflective. The second block portion
2515 includes an end surface 2503 and orthogonal retroreflector
surfaces 2505 and 2507 that are all coated to be reflective. The
transparent block 2500 is usable for providing the beam path
configuration and encoder readhead operating characteristics
previously described with reference to FIG. 19. Thus, the
transparent block 2500 must also provide a polarizing function
equivalent to the beamsplitter shown in FIG. 1D, or the like, in
order to separate out crossover beams, as previously described with
reference to FIG. 19. In various exemplary embodiments, this is
achieved by incorporating mutually orthogonal polarizers on the
portions of the end surfaces 2502 and 2503 where the beams 401a,
403a, 401b and 403b, are reflected.
[0153] The transparent block 2500 is usable to provide a very
compact and dimensionally stable encoder readhead assembly. It
should be appreciated that the transparent block 2500 may be
fabricated by bonding together, or fabricating together, various
combinations of individual optical elements. The optical pieces may
comprise a fewer number of more complex individual optical elements
or a greater number of less complex individual optical elements.
The individual elements may be joined along any combination of
bonding planes (not shown) which allow practical, compact and
accurate fabrication of the optical pieces. The optical pieces may
further incorporate mounting provisions for being joined to various
light sources, photodetector arrangements and/or encoder readhead
mounting elements. Thus, the configuration shown in FIG. 25 is an
exemplary configuration only, and is not intended to be
limiting.
[0154] FIG. 26A is a three-dimensional view showing a second
exemplary transparent block configuration usable in various
exemplary embodiments according to the invention. FIGS. 26B and 26C
are side and end views, respectively, clarifying the beam paths
provided by the transparent block configuration shown in FIG. 26A.
Refractive effects on the various light beam paths are included in
all figures. The nominal light beam diameters are reduced only in
FIG. 26A, to better distinguish the various beam paths. FIG. 26A
shows a light source 412, including a collimating lens 413, which
inputs a light beam 401 to a transparent block 2600. An optical
detector 460 is also shown. The transparent block 2600 includes a
left half block portion 2510 and a right half block portion 2515.
An upper left half portion 2612 and an upper right half portion
2617 are joined at an interface which is fabricated to include a
half-silvered mirror or the like in order to provide a beam
splitting element 415. The left half block portion 2610 includes an
end surface 2602 and the left half of orthogonal retroreflector
surfaces 2604 and 2605, which are all coated to be reflective, and
which together comprise the retroreflector 440, as best shown in
FIGS. 26B and 26C. The right half block portion 2615 includes an
end surface 2603 and the right half of orthogonal retroreflector
surfaces 2604 and 2605, which are all coated to be reflective, and
which together comprise the retroreflector 441, as best shown in
FIGS. 26B and 26C. The transparent block 2600 is usable for
providing the beam path configuration and encoder readhead
operating characteristics previously described with reference to
FIGS. 5A-5D or 12A-12D, for example, when the nominal
retroreflector height dimension 472 is suitably chosen. The
transparent block 2600 provides for suitable non-zero inclination
angles 473. Thus, in contrast to the block configuration shown in
FIG. 25, crossover beams may be eliminated and polarizers are not
needed.
[0155] In various exemplary embodiments, the transparent block 2600
incorporates an optical deflection element (not shown), which
provides the same function as the optical wedge element 491
previously discussed with reference FIG. 11B. The optical
deflection element is incorporated on the portion of the end
surface 2602 or 2603 where the beam 403a or 403b, respectively, is
reflected, solely to enable the use of an optical detector scheme
which uses an array detector to detect an interference fringe
pattern such as the interference fringe pattern 451 discussed with
reference to FIG. 1B. The resulting interference pattern translates
spatially in correspondence to the scale grating displacement. The
translation of the interference pattern can be quantitatively
detected by various optical detector array techniques known in the
art and/or currently commercially available. In various alternative
embodiments the optical detector 460 used with the transparent
block 2600 includes the use of a quadrature array which directly
produces quadrature signals, and in various other embodiments the
optical detector 460 includes the use of a linear or 2-dimensions
CCD arrays or the like, allows the interference pattern translation
to be imaged and digitally analyzed at a higher resolution. Any of
these arrays may be assembled to the transparent block 2600 at a
desired position, orientation and spacing, by means of adhesives
for example, to provide a precise, stable, economical and compact
encoder readhead.
[0156] In various other exemplary embodiments, mutually orthogonal
polarizers are incorporated on the portions of the end surfaces
2602 and 2603 where the beams 403a and 403b and/or 401a and 401b,
are reflected, solely to enable the use of an optical detector
scheme which uses polarizers.
[0157] The transparent block 2600 is usable to provide a very
compact and dimensionally stable encoder readhead assembly. It
should be appreciated that the transparent block 2600 may be
fabricated by bonding together, or fabricating together, various
combinations of individual optical elements. It should further be
appreciated that although the beams 402a and/or 402b appear to
impinge on corner of the surfaces 2602 and 2604, and 2603 and 2604,
respectively, before impinging on the surface 2605 to complete the
retroreflection, in various exemplary embodiments the beam 402a
avoids the corner and impinges on the surfaces 2602 and 2604 in
succession. Likewise, the beam 402b avoids the corner and impinges
on the surfaces 2603 and 2604 in succession, in order to avoid
wavefront aberrations induced by imperfect corners. Thus, the
configuration shown in FIGS. 26A-26C is an exemplary configuration
only, and is not intended to be limiting.
[0158] FIG. 27A is a three-dimensional view showing a third
exemplary transparent block configuration usable in various
exemplary embodiments according to the invention. FIGS. 27B and 27C
are side and end views, respectively, clarifying the beam paths
provided by the transparent block configuration shown in FIG. 27A.
The transparent block configuration shown in FIGS. 27A-27C is
usable for providing the beam path configuration and encoder
readhead operating characteristics previously described with
reference to FIG. 4, for example. Refractive effects on the various
light beam paths are included in all figures. The nominal light
beam diameters are reduced only in FIG. 27A, to better distinguish
the various beam paths.
[0159] FIG. 27A shows a light source 412, including an input beam
directing element 414, which inputs a light beam 401 to a
transparent block 2700. An optical detector 460 is also shown. The
transparent block 2700 includes a left half block portion 2710 and
a right half block portion 2715. The left half block portion 2710
includes a rear end surface 2702, a forward end surface 2704 and
the left half of orthogonal retroreflector surfaces 2706 and 2707,
which are all coated to be reflective. The forward end surface 2704
and the left half of orthogonal retroreflector surfaces 2706 and
2707 together comprise the retroreflector 440, as best shown in
FIGS. 27B and 27C. The right half block portion 2715 includes a
rear end surface 2703, a forward end surface 2705 and the right
half of orthogonal retroreflector surfaces 2706 and 2707, which are
all coated to be reflective. The forward end surface 2705 and the
right half of orthogonal retroreflector surfaces 2706 and 2707
together comprise the retroreflector 441, as best shown in FIGS.
27B and 27C. A top rear surface of the transparent block 2700
includes a transmissive readhead grating, which in various
embodiments may be any of the readhead gratings previously
discussed with reference to FIGS. 10A-10E, and which may be
fabricated as a separate element or integrated to the surface in
order to provide a beam splitting element 415. As previously
discussed, the inventor has found that in embodiments according to
this invention where the beam splitting element is a readhead
grating such as the readhead gratings shown in FIGS. 10A-10E, or
the like, that the errors in such embodiments related to pitch
misalignment and dynamic pitch misalignment and the like tend to be
relatively reduced as the beam splitting height dimension 471 is
made smaller, and/or the retroreflector height dimension 472
approaches equality with the beam splitting height dimension 471,
as provided by the exemplary transparent block 2700.
[0160] The transparent block 2700 provides for non-zero inclination
angles 473. Thus, similar to the block configuration shown in FIGS.
26A-26C, crossover beams may be eliminated and polarizers are not
needed. In various exemplary embodiments, the transparent block
2700 incorporates an optical deflection element (not shown), which
provides the same function as the optical wedge element 491
previously discussed with reference FIG. 10C. The optical
deflection element is incorporated on the portion of the rear end
surface 2702 or 2703 where the beam 403a or 403b, respectively, is
reflected, solely to enable the use of an optical detector scheme
which uses an array detector to detect an interference fringe
pattern, as previously discussed with reference to FIGS. 26A-26C.
Alternatively, the readhead grating shown in FIG. 10B may be used,
to achieve the same effect. In such cases, the optical detector 460
includes an array detector according to previously described
interference fringe detection schemes, and may be assembled to the
transparent block 2600 at a desired position, orientation and
spacing, by means of adhesives for example, to provide a precise,
stable, economical and compact encoder readhead.
[0161] In various other exemplary embodiments, mutually orthogonal
polarizers are incorporated on the portions of the rear end
surfaces 2702 and 2703 where the beams 403a and 403b and/or 401a
and 401b, are reflected, solely to enable the use of an optical
detector scheme which uses polarizers.
[0162] The transparent block 2700 is usable to provide a very
compact and dimensionally stable encoder readhead assembly. It
should be appreciated that the transparent block 2700 may be
fabricated by bonding together, or fabricating together, various
combinations of individual optical elements. It should further be
appreciated that although the beams 402a and/or 402b appear to
impinge on corner of the surfaces 2704 and 2706, and 2705 and 2706,
respectively, before impinging on the surface 2707 to complete the
retroreflection, in various exemplary embodiments the beam 402a
avoids the corner and impinges on the surfaces 2704 and 2706 in
succession. Likewise, the beam 402b avoids the corner and impinges
on the surfaces 2705 and 2706 in succession, in order to avoid
wavefront aberrations induced by imperfect corners. Thus, the
configuration shown in FIGS. 26A-26C is an exemplary configuration
only, and is not limiting.
[0163] FIG. 28 shows a flow diagram for a method of using an
optical displacement sensing device in accordance with various
exemplary embodiments of the invention. The method begins at a
block S2810, by inputting split light beams to be received by
respective light directing elements of the optical displacement
sensing device. The method then continues to a block S2820, by
directing the split light beams to a first zone on a scale grating
using various light directing elements. In a preferred embodiment,
the split light beams area directed along nominally symmetrical
paths to the first zone. The method then continues to a block
S2830.
[0164] At block S2830, the split light beams are each diffracted
from the first zone on the scale grating, and two diffracted beams
are directed along divergent paths and enter respective
retroreflectors. The method then continues to a block S2840, by
retroreflecting the beams entering the respective retroreflectors
back to a second zone on the scale grating. The method then
continues to a block S2850, where the beams retroreflected back to
the second zone on the scale are each diffracted from the second
zone on the scale grating, and two diffracted beams are directed
along divergent paths after leaving the second zone. The method
then continues to a block S2860.
[0165] At block S2860, the two diffracted beams along divergent
paths after leaving the second zone are directed along respective
paths to a shared zone, using various light directing elements. In
a preferred embodiment, the beams directed to the shared zone are
directed along nominally symmetrical paths. The method then
continues to a block S2870, where the method ends by detecting at
least one illumination characteristic arising from the shared zone
and determining a relative displacement of the scale grating
relative to the optical displacement sensing device based on the
detection result.
[0166] While this invention has been described in conjunction with
the specific embodiments above, it should be appreciated that these
specific embodiments offer many alternatives, combinations,
modifications, and variations. As one example, although the various
embodiments according to the invention are shown herein as linear
transducers, the designs may be used in or adapted to cylindrical
and circular rotary applications by one of ordinary skill in the
art. As a separate example, this invention may use light
wavelengths outside the visible spectrum, provided that such
wavelengths are operable with scale gratings and optical detectors
according to the principles of this invention. Accordingly, the
preferred embodiments of this invention, as set forth above, are
intended to be illustrative and not limiting. Various changes can
be made without departing from the spirit and scope of this
invention.
* * * * *