Systems and methods for absolute positioning using repeated quasi-random pattern

• US 6,937,349 B2
• Filed: 05/02/2003
• Issued: 08/30/2005
• Est. Priority Date: 05/02/2003
• Status: Active Grant

• Current Assignee: Mitutoyo Corporation
• Original Assignee: Mitutoyo Corporation
• Inventors: Nahum, Michael, Jones, Benjamin K.
• Primary Examiner(s): LAUCHMAN, LAYLA G
• Application Number:
  US10/427,921
• Publication Number:
  US 20040218181A1
• Time in Patent Office: 851 Days
• Field of Search: 356/401, 356614-617, 250/231.13, 250/231.14, 250/231.16, 382/278, 382/295, 345/428
• US Class Current: 356/617
• CPC Class Codes: G01D 2205/90 Two-dimensional encoders, i.e. having one or two codes extending in two directions
• CPC Class Codes: G01D 5/2455 with incremental and absolute tracks on the same encoder

Abstract

An absolute 2D position-sensing device is usable to measure the position of a first element with respect to a second element. A 2D absolute scale includes an integrated 2D absolute scale pattern extending over the 2D scale area along each measuring axis of the scale. The integrated 2D absolute scale pattern includes a predetermined quasi-random pattern repeatedly interleaved with a plurality of code portions along each axis. Each code portion includes a plurality of code elements indicative of an absolute measurement value. The offset of the quasi-random pattern relative to a readhead of the device is combined with the absolute measurement value to determine an absolute position to a very high resolution over a relatively large 2D range.

Claims

1. An absolute two-dimensional position sensing device usable to measure a position of a first member with respect to a second member along first and second measuring axes, the absolute two-dimensional position sensing device including a readhead and a scale, the scale including a two-dimensional integrated scale pattern extending along the first and second measuring axes, the two-dimensional integrated scale pattern comprising:

a plurality of quasi-random pattern portions, each quasi-random pattern portion extending along the first and second measuring axes and having first and second dimensions, and each quasi-random pattern portion comprising at least a portion of at least one quasi-random pattern; and

a plurality of code portions distributed within the two-dimensional integrated scale pattern and spaced apart along a first direction by substantially the first dimension and spaced apart along a second direction by substantially the second dimension, each code portion comprising a plurality of code elements defining at least one code word, the at least one code word usable to identify a measurement value of a local datum along each of the first and second measuring axes;

wherein the plurality of quasi-random pattern portions and the plurality of code portions are configured such that a detection window of the readhead that extends along each of the first and second directions includes a number of code elements sufficient to define at least one code word, the defined at least one code word usable to identify the measurement value of a corresponding local datum along the first and second measuring axes, regardless of the position of the detection window within the two-dimensional integrated scale pattern.

2. The two-dimensional absolute position sensing device of claim 1, wherein a predetermined portion of each of the plurality of code portions comprises a pattern that is the same for each of the plurality of code portions.

3. The two-dimensional absolute position sensing device of claim 1, wherein the at least a portion of at least one quasi-random pattern comprises one of a) a portion of single quasi-random random pattern that is larger than random pattern portion, b) a plurality of similar quasi-random random patterns that are smaller than the random pattern portion, c) a plurality of different quasi-random random patterns that are smaller than the random pattern portion and d) a single entire quasi-random pattern that is the same size as the random pattern portion.

4. The two-dimensional absolute position sensing device of claim 1, wherein the at least a portion of at least one quasi-random pattern comprises a pattern that is the same for a plurality of the quasi-random pattern portions.

5. The two-dimensional absolute position sensing device of claim 4, wherein the at least a portion of at least one quasi-random pattern comprises a pattern that is the same for all of the quasi-random pattern portions.

6. The two-dimensional absolute position sensing device of claim 1, wherein a percentage of an area of the detection window occupied by the code portions is at most 20%, regardless of the position of the detection window along the first and second measuring axes relative to the at least one quasi-random pattern and the plurality of code portions.

7. The two-dimensional absolute position sensing device of claim 6, wherein the percentage is at most 10%.

8. The two-dimensional absolute position sensing device of claim 1, wherein the two-dimensional integrated scale pattern comprises at least one diffusely reflecting surface.

9. The two-dimensional absolute position sensing device of claim 8, wherein:

the scale comprises a transparent substrate, the two-dimensional integrated scale pattern provided on a surface of the substrate that is farthest from the readhead; and

the at least one diffusely reflecting surface comprises at least one portion of at least one diffuse backing provided on or over the two-dimensional integrated scale pattern on the surface that is farthest from the readhead.

10. The absolute position sensing device of claim 1, wherein a first part of the number of code elements sufficient to define the defined at least one code word are included in a first one of a plurality of code portions included at least partly in the detection window and a second part of the number of code elements sufficient to define the defined at least one code word are included in a second one of the plurality of code portions included at least partly in the detection window.

11. The absolute position sensing device of claim 1, wherein the detection window of the readhead includes at least one complete code portion which includes the number of code elements sufficient to define the defined at least one code word, regardless of the position of the detection window within the two-dimensional integrated scale pattern.

12. The absolute position sensing device of claim 1, wherein:

the readhead comprises a light source and a two-dimensional detector array usable to detect an image of the scale coextensive with the detection window; and

the pluralities of code elements are arranged along the integrated scale pattern such that, when the readhead is operably positioned relative to the scale, the detected image of the scale includes a feature usable as the local datum point, regardless of the readhead position along the two-dimensional integrated scale pattern.

13. The absolute position sensing device of claim 12, wherein the detected image is usable to determine a measurement value of an offset of the readhead relative to the local datum point along each of two directions corresponding to the first and second measuring axes, respectively.

14. The absolute position sensing device of claim 1, wherein each at least one code word is directly usable to determine a measurement value of a local datum along each of the first and second measuring axes without reference to a lookup table.

15. The absolute position sensing device of claim 1, further comprising a signal processing unit, wherein, when an operably positioned readhead detects an image of the scale, the signal processing unit is usable to determine a first resolution absolute measurement value of a local datum along each of the first and second measuring axes based on the sufficient number of code elements included in the detected image.

16. The absolute position sensing device of claim 15, wherein the signal processing unit is further usable to determine a measurement value of an offset of the readhead relative to the local datum along each of two directions corresponding to the first and second measuring axes based on at least one of a) the location of the local datum in the detected image, and b) the location of the at least a portion of at least one quasi-random pattern in the detected image, the measurement value of the offset of the readhead relative to the local datum point having a measurement resolution finer than the first resolution absolute measurement value along each of the first and second measuring axes.

17. The absolute position sensing device of claim 16, wherein the signal processing unit is further usable to combine the first resolution absolute measurement value of the local datum point along the first measuring axis and the measurement value of the offset of the readhead relative to the local datum point along the first measuring axis to determine an absolute position measurement along the first measuring axis with a measurement resolution finer than the first resolution absolute position measurement.

18. The absolute position sensing device of claim 17, wherein the signal processing unit is further usable to combine the first resolution absolute measurement value of the local datum point along the second measuring axis and the measurement value of the offset of the readhead relative to the local datum point along the second measuring axis to determine an absolute position measurement along the second measuring axis with a measurement resolution finer than the first resolution absolute position measurement.

19. The absolute position sensing device of claim 16, wherein the signal processing unit comprises a memory portion including data usable to form at least one reference image representation such that at least one reference image representation is correlatable with the at least a portion of at least one quasi-random pattern in the detected image in order to determine the measurement value of an offset of the readhead relative to the local datum along each of two directions corresponding to the first and second measuring axes.

20. The absolute position sensing device of claim 1, wherein the at least a portion of at least one quasi-random pattern includes at least a portion based on a real random pattern.

21. The absolute position sensing device of claim 20, wherein the real random pattern comprises a speckle pattern.

22. The absolute position sensing device of claim 1, wherein the at least a portion of at least one quasi-random pattern includes at least a portion based on a synthetically-generated random pattern.

23. The absolute position sensing device of claim 1, wherein the plurality of code portions comprise a relatively narrow configuration of code elements that extend along at least one of the first and second directions, the relatively narrower configuration having a relatively longer dimension and a relatively narrower dimension wherein the relatively longer dimension is at least 8 times the relatively narrower dimension.

24. The absolute position sensing device of claim 23, wherein along at least one direction the relatively longer dimension extends completely across the corresponding quasi-random pattern portion.

25. A method for determining a high resolution absolute position of a two-dimensional detector array along a two-dimensional absolute scale, wherein the two-dimensional absolute scale includes a two-dimensional integrated scale pattern extending along first and second measuring axes, the two-dimensional integrated scale pattern comprising:

a plurality of quasi-random pattern portions, each quasi-random pattern extending along the first and second measuring axes and having first and second dimensions, and each quasi-random pattern portion comprising at least a portion of at least one quasi-random pattern; and

a plurality of code groups distributed within the two-dimensional integrated scale pattern and spaced apart along a first direction by substantially the first dimension and spaced apart along a second direction by substantially the second dimension, each code group comprising a plurality of code elements defining at least one code word, the at least one code word usable to identify a measurement value of a local datum along each of the first and second measuring axes;

the method comprising;

detecting an image of a portion of the two-dimensional integrated scale pattern corresponding to a current position using the detector array;

determining an absolute measurement value of a local datum along each of the first and second measuring axes based on at least one code group included in the detected image;

determining a measurement value of an offset of the detector array relative to the local datum along each of the first and second measuring axes based on at least one of a) the location of the local datum in the detected image, and b) the location of the at least a portion of at least one quasi-random pattern in the detected image; and

combining the absolute measurement value of the local datum along each of the first and second measuring axes and the measurement value of the offset of the detector array relative to the local datum along each of the first and second measuring axes to determine a high resolution absolute position of the detector array along the first and second measuring axes of the two-dimensional absolute scale.

26. The method of claim 25, wherein:

each code group includes a predetermined portion; and

determining the absolute measurement value of the local datum along each of the first and second measuring axes based on at least one code group comprises;

determining a location of the predetermined portion included in at least one of the at least one code group,

determining a location of the code elements in the detected image based on the determined location of the predetermined portion, and

processing the detected image corresponding to the location of the code elements to determine code element values of the code elements appearing in the detected image; and

determining the measurement value of the local datum comprises determining the measurement value of the local datum along each of the first and second measuring axes based on the determined code element values.

27. The method of claim 25, wherein:

the detector array comprises a plurality of detector elements extending in at least one row along a direction corresponding to the first measuring axis direction, the detector elements spaced apart along the at least one row according to a detector element first pitch;

the detector array comprises a plurality of detector elements extending in at least one column along a direction corresponding to the second measuring axis direction, the detector elements spaced apart along the at least one column according to a detector element second pitch; and

determining the measurement value of the offset of the detector array relative to the local datum along each of the first and second measuring axes comprises;

estimating an offset of a local datum characteristic relative to the detector array along the first measuring axis direction,

estimating an offset of at least a portion of at least one quasi-random pattern in the detected image relative to the detector array along the first measuring axis direction to a resolution that is at least as fine in the detected image as the detector element first pitch,

estimating an offset of a local datum characteristic relative to the detector array along the second measuring axis direction,

estimating an offset of at least a portion of at least one quasi-random pattern in the detected image relative to the detector array along the second measuring axis direction to a resolution that is at least as fine in the detected image as the detector element second pitch,

determining the measurement value of the offset of the detector array relative to the local datum along the first measuring axis based at least on the estimate of the offset of the at least a portion of at least one quasi-random pattern relative to the detector array along the first measuring axis, and

determining the measurement value of the offset of the detector array relative to the local datum along the second measuring axis based at least on the estimate of the offset of the at least a portion of at least one quasi-random pattern relative to the detector array along the second measuring axis.