Contamination and defect resistant optical encoder configuration including a normal of readhead plane at a non-zero pitch angle relative to measuring axis for providing displacement signals
An optical encoder configuration comprises a scale, an illumination source, and a photodetector configuration. The illumination source is configured to output structured illumination to the scale. The scale extends along a measuring axis direction and is configured to output scale light that forms a detector fringe pattern comprising periodic high and low intensity bands that extend over a relatively longer dimension along the measuring axis direction and are relatively narrow and periodic along a detected fringe motion direction transverse to the measuring axis direction. The high and low intensity bands move along the detected fringe motion direction transverse to the measuring axis direction as the scale grating displaces along the measuring axis direction. The photodetector configuration is configured to detect a displacement of the high and low intensity bands along the detected fringe motion direction and provide respective spatial phase displacement signals that are indicative of the scale displacement.
1. A contamination and defect resistant optical encoder configuration for providing displacement signals, comprising:
a scale that extends along a measuring axis direction, the scale comprising a scale grating comprising scale grating bars arranged in a scale plane that is nominally parallel to the measuring axis direction, wherein the scale grating bars are narrow along the measuring axis direction and elongated along a scale grating bar direction transverse to the measuring axis direction, and are arranged periodically at a scale pitch P SF along the measuring axis direction;
an illumination source comprising a light source that outputs light, and a structured illumination generating portion configured to input the light and output structured illumination along a source light path SOLP to an illumination region at the scale plane where the structured illumination comprises an illumination fringe pattern comprising fringes that are narrow along the measuring axis direction and elongated along an illumination fringe direction oriented transverse to the measuring axis direction; and
a photodetector configuration comprising a set of N spatial phase detectors arranged periodically at a detector pitch PD along a detected fringe motion direction transverse to the measuring axis direction, wherein each spatial phase detector is configured to provide a respective spatial phase detector signal and at least a majority of the respective spatial phase detectors extend over a relatively longer dimension along the measuring axis direction and are relatively narrow along the detected fringe motion direction transverse to the measuring axis direction, and the set of N spatial phase detectors are arranged in a spatial phase sequence along the detected fringe motion direction;
wherein:
the scale grating is configured to input the illumination fringe pattern at the illumination region and output scale light along a scale light path SCLP that forms a fringe pattern at the photodetector configuration, the fringe pattern comprising periodic high and low intensity bands that extend over a relatively longer dimension along the measuring axis direction and are relatively narrow and periodic with a detected fringe period PDF along the detected fringe motion direction transverse to the measuring axis direction;
the scale grating bar direction is oriented at a nonzero yaw angle ψ SC relative to a readhead plane RHP defined by the source light path SOLP and the scale light path SCLP;
the detected fringe period PDF and the detected fringe motion direction transverse to the measuring axis direction depend at least partially on the nonzero yaw angle ψ SC ;
the high and low intensity bands move along the detected fringe motion direction transverse to the measuring axis direction as the scale grating displaces along the measuring axis direction;
a normal of the readhead plane RHP is oriented with a non-zero pitch angle ϕ relative to the measuring axis direction; and
the photodetector configuration is configured to detect a displacement of the high and low intensity bands along the detected fringe motion direction transverse to the measuring axis direction and provide respective spatial phase displacement signals that are indicative of the scale displacement.
2. The contamination and defect resistant optical encoder configuration of claim 1 , wherein ϕ is greater than 0.3 degrees and less than 2.0 degrees.
3. The contamination and defect resistant optical encoder configuration of claim 1 , wherein each of the N spatial phase detectors comprises an even number of scale light receptor areas.
4. The contamination and defect resistant optical encoder configuration of claim 1 , wherein:
the structured illumination generating portion comprises:
a first illumination source light diffraction grating comprising first illumination source grating bars arranged periodically at a first index pitch P 1 in a first index plane, wherein the first index grating bars are narrow along the measuring axis direction, and elongated along a first grating bar direction which is transverse to the measuring axis direction and rotated by an angle ψ 1 with respect to the readhead plane RHP; and
a second illumination source light diffraction grating comprising second illumination source grating bars arranged periodically at a second index pitch P 2 in a second index plane which is parallel to the first index plane, wherein the second index grating bars are narrow along the measuring axis direction, and elongated along a second index grating bar direction which is transverse to the measuring axis direction and rotated by an angle ψ 2 with respect to the readhead plane RHP.
5. The contamination and defect resistant optical encoder configuration of claim 4 , wherein:
the scale grating is a reflective grating;
the source light path SOLP is oriented at an angle V with respect to a direction normal to the scale plane; and
the yaw angle ψ SC satisfies the relation:
ψ
sc
=
sin
-
1
[
P
SF
(
1
2
-
sin
(
ψ
1
)
P
1
+
sin
(
ψ
2
)
P
2
)
]
.
6. The contamination and defect resistant optical encoder configuration of claim 4 , wherein:
the light output by the light source has a wavelength λ;
a factor Ω is defined by the expression:
Ω
=
4
π
λ
P
SF
2
sin
(
V
)
(
(
cos
2
(
V
)
-
λ
2
P
SF
2
)
-
3
/
2
-
cos
-
3
(
V
)
)
+
8
π
λ
tan
(
V
)
;
and
the angle ψ 1 and the angle ψ 2 satisfy the expression:
-
sin
(
ψ
1
)
d
1
+
sin
(
ψ
2
)
d
2
=
2
π
tan
(
V
)
cos
(
V
)
(
Ω
λ
-
4
π
tan
V
)
.
7. The contamination and defect resistant optical encoder configuration of claim 4 , wherein the first illumination source light diffraction grating and the second illumination source light diffraction grating are phase gratings.
8. The contamination and defect resistant optical encoder configuration of claim 4 , wherein the detected fringe period PDF is at least 40 micrometers.
9. The contamination and defect resistant optical encoder configuration of claim 4 , wherein:
the scale grating is a reflective grating;
the source light path SOLP is oriented at an angle V with respect to a direction normal to the scale plane;
the light output by the light source has a wavelength λ;
a factor Ω is defined by the expression:
Ω
=
4
π
λ
P
SF
2
sin
(
V
)
(
(
cos
2
(
V
)
-
λ
2
P
SF
2
)
-
3
/
2
-
cos
-
3
(
V
)
)
+
8
π
λ
tan
(
V
)
;
and
the angle ψ 1 and the angle ψ 2 satisfy the expression:
-
sin
(
ψ
1
)
d
1
+
sin
(
ψ
2
)
d
2
=
2
π
tan
(
V
)
cos
(
V
)
(
Ω
λ
-
4
π
tan
V
)
.