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

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 PSF 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 ψSC 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 ψSC;
• 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.