CONTAMINATION AND DEFECT RESISTANT ROTARY OPTICAL ENCODER CONFIGURATION FOR PROVIDING DISPLACEMENT SIGNALS
An optical encoder configuration comprises a rotary scale, an illumination source, and a photodetector configuration. The illumination source is configured to output collimated light to the scale at a first illumination region, which is then output to the scale at a second illumination region from which the scale outputs scale light that forms a detector fringe pattern comprising periodic high and low intensity bands that extend over a relatively longer dimension along the rotary measuring direction and are relatively narrow and periodic along a detected fringe motion direction transverse to the rotary measuring direction. The high and low intensity bands move along the detected fringe motion direction as the scale grating displaces along the rotary measuring direction. The photodetector configuration is configured to detect a displacement of the high and low intensity bands and provide respective spatial phase displacement signals that are indicative of the rotary scale displacement.
1 . A contamination and defect resistant rotary optical encoder configuration for providing displacement signals, comprising:
a rotary scale that extends along a rotary measuring direction about a rotary axis, the rotary scale comprising a rotary scale grating comprising scale grating bars arranged in a rotary surface along the rotary measuring direction, wherein the scale grating bars are narrow along the rotary measuring direction and elongated along a rotary scale grating bar direction transverse to the rotary measuring direction, and are arranged periodically at a scale pitch P SF along the rotary measuring direction;
an illumination source comprising a light source that outputs collimated to a first illumination region on the rotary scale which is configured to input the light and output structured illumination along a light path LP to a second illumination region on the rotary scale where the structured illumination comprises an illumination fringe pattern comprising fringes that are narrow along the rotary measuring direction and elongated along an illumination fringe direction oriented transverse to the rotary measuring 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 rotary measuring 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 rotary measuring direction and are relatively narrow along the detected fringe motion direction transverse to the rotary measuring direction, and the set of N spatial phase detectors are arranged in a spatial phase sequence along the detected fringe motion direction;
wherein:
the rotary scale grating is configured to input the illumination fringe pattern at the second illumination region and output scale light 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 rotary measuring direction and are relatively narrow and periodic with a detected fringe period PDF along the detected fringe motion direction transverse to the rotary measuring direction;
the rotary scale grating bar direction is oriented at a nonzero yaw angle ψ relative to the rotary axis;
the detected fringe period PDF and the detected fringe motion direction are transverse to the rotary measuring direction and depend at least partially on the nonzero yaw angle ψ;
the high and low intensity bands move along the detected fringe motion direction transverse to the rotary measuring direction as the scale grating rotates about the rotary axis; 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 rotary measuring direction and provide respective spatial phase displacement signals that are indicative of the rotary scale displacement.
2 . The contamination and defect resistant rotary optical encoder configuration of claim 1 , wherein each of the N spatial phase detectors comprises an even number of scale light receptor areas.
3 . The contamination and defect resistant optical encoder configuration of claim 1 , wherein the detected fringe period PDF is at least 40 micrometers.
4 . The contamination and defect resistant optical encoder configuration of claim 1 , wherein the rotary scale grating is a transmissive grating.
5 . The contamination and defect resistant optical encoder configuration of claim 1 , wherein the rotary scale grating is a reflective grating.
6 . The contamination and defect resistant optical encoder configuration of claim 1 , wherein the yaw angle ψ satisfies the relation:
ψ
=
sin
-
1
(
P
SF
4
*
)
7 . The contamination and defect resistant optical encoder configuration of claim 1 , further comprising a first mirror and a second mirror to direct the structured illumination to the second illumination region.
8 . The contamination and defect resistant optical encoder configuration of claim 7 , wherein the first mirror and second mirror are surfaces of a monolithic optical material.
9 . The contamination and defect resistant optical encoder configuration of claim 1 , further comprising a first grating and a second grating to direct the structured illumination to the second illumination region.