Systems and methods for reducing accumulated systematic errors in image correlation displacement sensing systems

A method for determining accumulated displacement in an image-correlation-based displacement measuring system, comprising:

• determining at least one reference-class displacement between the two images of at least one corresponding reference-class image pair, the two images of a reference-class image pair comprising a first image and a second image, each reference-class displacement being determined to a sub-pixel resolution;
• and
• determining an accumulated displacement based at least partially on the at least one reference-class displacement;
• wherein:
• the at least one reference-class displacement is determined in a manner that at least partially rejects an accumulation of systematic displacement estimate errors that arise from an image-correlation error characteristic having a period corresponding to a pitch of a pixel spacing used to acquire the first and second images;
• and
• the determining the reference-class displacement in a manner that at least partially rejects an accumulation of the systematic displacement estimate errors comprises determining the image that is used as the second image of the at least one corresponding reference-class image pair based on a prescribed displacement for the second image relative to the first image of the at least one corresponding reference-class image pair, wherein the prescribed displacement has a sub-pixel resolution and the second image of at least one reference-class image pair is used as the first image of a spatially-adjacent reference-class image pair.

A method for determining accumulated displacement in an image-correlation-based displacement measuring system, comprising:

• determining at least one reference-class displacement between the two images of at least one corresponding reference-class image pair, the two images of a reference-class image pair comprising a first image and a second image, each reference-class displacement being determined to a sub-pixel resolution;
• and
• determining an accumulated displacement based at least partially on the at least one reference-class displacement;
• wherein:
• the at least one reference-class displacement is determined in a manner that at least partially rejects an accumulation of systematic displacement estimate errors that arise from an image-correlation error characteristic having a period corresponding to a pitch of a pixel spacing used to acquire the first and second images;
• and
• the determining the reference class displacement in a manner that at least partially rejects the accumulation of the systematic displacement estimate errors comprises compensating that portion of the apparent displacement between the two images of the at least one corresponding reference-class image pair that corresponds to a fractional part of the pixel-spacing of the image-correlation based displacement measuring system, the compensation based on a pre-determined periodic error characteristic of the image-correlation-based displacement measuring system.

An image-correlation-based displacement measuring system comprising:

• a readhead usable to acquire an image determined by an image-determining surface on a member movable relative to the readhead;
• a signal generating and processing circuitry element connected to the readhead, the signal generating and processing circuitry comprising a reference image updating portion which determines a reference displacement between a current reference image and a new reference image to a sub-pixel resolution, wherein:
• the reference image updating portion determines a reference displacement between the current reference image and the new reference image to a sub-pixel resolution in a maimer that at least partially rejects an accumulation of systematic displacement estimate errors that arise from an image-correlation error characteristic having a period corresponding to a pitch of a pixel spacing used to acquire the current and new reference images;
• and
• determining the reference displacement between the current reference image and the new reference image to a sub-pixel resolution in a maimer that at least partially rejects the accumulation of the systematic displacement estimate errors comprises at least one of a) compensating the apparent displacement between the current reference image and the new reference image based on a pre-determined error characteristic of the image-correlation-based displacement measuring system, b) determining the image that is used as the new reference image based on a prescribed displacement for the new reference image relative to the current reference image, the prescribed displacement having a sub-pixel resolution.