Two-dimensional scale structures and method usable in an absolute position transducer

• US 6,781,694 B2
• Filed: 09/13/2002
• Issued: 08/24/2004
• Est. Priority Date: 07/16/2002
• Status: Expired due to Fees

• Current Assignee: Mitutoyo Corporation
• Original Assignee: Mitutoyo Corporation
• Inventors: Nahum, Michael, Zwilling, Avron M., Renalds, Patrick A., Atherton, Kim W., Jones, Benjamin K.
• Primary Examiner(s): Font, Frank G.
• Assistant Examiner(s): Nguyen, Sang H.
• Application Number:
  US10/242,351
• Publication Number:
  US 20040012794A1
• Time in Patent Office: 711 Days
• Field of Search: 356/402-425, 250/226, 250/559.01, 382/312-313, 235/460, 235/462.1, 235/494
• US Class Current: 356/420
• CPC Class Codes: G01D 2205/90 Two-dimensional encoders, i.e. having one or two codes extending in two directions
• CPC Class Codes: G01D 5/34707 Scales; Discs, e.g. fixation, fabrication, compensation
• CPC Class Codes: G01D 5/34776 Absolute encoders with analogue or digital scales
• CPC Class Codes: G01D 5/34792 with only digital scales or both digital and incremental scales

Abstract

An absolute 2D position-sensing device is usable to measure the relative position of two elements. 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 plurality of periodic portions interleaved with a plurality of non-periodic portions along each axis. Each periodic portion includes a plurality of periodically-placed scale elements. Each non-periodic portion includes a plurality of code elements indicative of an absolute measurement value. The code elements may have a length that is narrower along each measuring axis is than the length of the periodic scale elements along each measuring axis. The offset of the periodically-placed elements relative to a readhead of the device is combined with the absolute measurement value to determine an absolute position.

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:

at least one periodic portion, each periodic portion extending along the first and second measuring axes, each periodic portion comprising;

a plurality of strip-like first periodic scale element zones arranged periodically along a first direction over at least a first range and extending along a second direction over at least a second range, a plurality of the first periodic scale element zones including a characteristic first arrangement of incremental scale features along the second direction, and

a plurality of strip-like second periodic scale element zones arranged periodically along the second direction over at least the second range and extending along the first direction over at least the first range, a plurality of the second periodic scale element zones including a characteristic second arrangement of incremental scale features along the first direction; and

a plurality of non-periodic portions distributed within the two-dimensional integrated scale pattern and spaced apart along the first direction by at least a first distance that spans at least two first periodic scale element zones including the characteristic first arrangement of incremental scale features along the second direction and spaced apart along the second direction by at least a second distance that spans at least two second periodic scale element zones including the characteristic second arrangement of incremental scale features along the first direction, each non-periodic 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 at least one periodic portion and the plurality of non-periodic 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 the 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 the first distance spans at least four first periodic scale element zones including the characteristic first arrangement of incremental scale features along the second direction.

3. The two-dimensional absolute position sensing device of claim 2, wherein the second distance spans at least four second periodic scale element zones including the characteristic second arrangement of incremental scale features along the first direction.

4. The two-dimensional absolute position sensing device of claim 1, wherein a length along the first direction of at least some of the code elements is narrower than a length of the first periodic scale element zones along the first direction.

5. The two-dimensional absolute position sensing device of claim 4, wherein a length along the second direction of at least some of the code elements is narrower than a length of the second periodic scale element zones along the second direction.

6. The two-dimensional absolute position sensing device of claim 4, wherein the length along the first direction of at least some of the code elements is about one-half the length of the first periodic scale element zones along the first direction.

7. The two-dimensional absolute position sensing device of claim 1, wherein the characteristic first arrangement of scale features along the second direction comprises at least one of a) a scale element extending along the second direction such that it crosses at least two second periodic scale element zones, b) a scale space extending along the second direction such that it crosses at least two second periodic scale element zones, and c) an alternating pattern of scale elements and scale spaces arranged periodically along the second direction at a pitch which is the same as the pitch of the second periodic scale element zones along the second direction.

8. The two-dimensional absolute position sensing device of claim 7, wherein the characteristic second arrangement of scale features along the second direction comprises at least one of a) a scale element extending along the first direction such that it crosses at least two first periodic scale element zones, b) a scale space extending along the first direction such that it crosses at least two first periodic scale element zones, and c) an alternating pattern of scale elements and scale spaces arranged periodically along the first direction at a pitch which is the same as the pitch of the first periodic scale element zones along the first direction.

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

10. The two-dimensional absolute position sensing device of claim 1, wherein a percentage of an area of the detection window occupied by the non-periodic 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 periodic portion and the plurality of non-periodic portions.

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

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

13. The two-dimensional absolute position sensing device of claim 12, 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.

14. 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 non-periodic 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 non-periodic portions included at least partly in the detection window.

15. The absolute position sensing device of claim 1, wherein the detection window of the readhead includes at least one complete non-periodic 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.

16. The absolute position sensing device of claim 1, wherein each of the plurality of strip-like first periodic scale element zones arranged periodically along the first direction are arranged according to a first underlying incremental pitch that is continuously periodic along the first direction for the first range.

17. The absolute position sensing device of claim 16, wherein each of the plurality of strip-like second periodic scale element zones arranged periodically along the second direction are arranged according to a second underlying incremental pitch that is continuously periodic along the second direction for the second range.

18. The absolute position sensing device of claim 17, wherein the first underlying incremental pitch and the second underlying incremental pitch are a same length.

19. The absolute position sensing device of claim 16, wherein the second range is at least 10 times the length of the first underlying incremental pitch.

20. The absolute position sensing device of claim 16, wherein the plurality of the first periodic scale element zones includes all the first periodic scale element zones except those in the vicinity of the non-periodic portions.

21. The absolute position sensing device of claim 20, wherein the plurality of the second periodic scale element zones includes all the second periodic scale element zones except those in the vicinity of the non-periodic portions.

22. 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 and the incremental scale features 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.

23. The absolute position sensing device of claim 22, 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.

24. The absolute position sensing device of claim 22, wherein:

the two-dimensional detector array comprises a plurality of detector elements arrayed according to a detector element first pitch along a direction corresponding to the first measuring axis;

the readhead further comprises at least one optical element, light from the scale passing through the at least one optical element such that the image of the scale detected by the detector array is magnified, thus imparting a readhead magnification to the detected image, and

each code element has a length L_c along the first measuring axis such that the length L_c times the readhead magnification is at least 3 times the detector element first pitch.

25. 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.

26. 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 the 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.

27. The absolute position sensing device of claim 26, wherein the signal processing unit is further 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, based at least partly on a plurality of incremental scale features included in the detected image.

28. The absolute position sensing device of claim 27, wherein the signal processing unit is further usable to combine the 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.

29. The absolute position sensing device of claim 28, wherein the signal processing unit is further usable to combine the 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.

30. The absolute position sensing device of claim 1, wherein the first direction is parallel to the first measuring axis, the second direction is parallel to the second measuring axis, and the first and second measuring axes are orthogonal to one another.

31. The absolute position sensing device of claim 30, wherein the scale is cylindrically shaped and one of the first and second measuring axes is parallel to the axis of a cylinder and the other of the first and second measuring axes lies along the circumference of the cylinder.

32. The absolute position sensing device of claim 1, wherein the two-dimensional integrated scale pattern further comprises a plurality of strip-like regions extending along the first direction for at least the first range and spaced apart along the second direction such that any line along the second direction in the detection window extends across at least one of the plurality of strip-like regions, each strip-like region comprising scale features such that any line along the second direction includes a pattern of scale features usable to locate that strip-like region relative to the detection window along the second direction, each strip-like region further comprising a plurality of indicator scale features located along a line extending along the first direction, each indicator scale feature indicative of the location of at least one code element of a non-periodic portion associated with that indicator scale feature.

33. The absolute position sensing device of claim 32, wherein the pattern of scale features usable to locate the strip-like region relative to the detection window along the second direction includes a uniquely placed scale space.

34. A method for determining a high resolution 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:

at least one periodic portion, each periodic portion extending along the first and second measuring axes, each periodic portion comprising;

a plurality of strip-like first periodic scale element zones arranged periodically along a first direction according to an underlying first incremental pitch for at least a first range and extending along a second direction for at least a second range, a plurality of the first periodic scale element zones including a characteristic first arrangement of incremental scale features along the second direction, and

a plurality of strip-like second periodic scale element zones arranged periodically along the second direction according to an underlying second incremental pitch for at least the second range and extending along the first direction for at least the first range, a plurality of the second periodic scale element zones including a characteristic second arrangement of incremental scale features along the first direction; and

a plurality of code groups distributed within the two-dimensional integrated scale pattern and spaced apart along the first direction by at least a first distance that spans at least two first periodic scale element zones including the characteristic first arrangement of incremental scale features along the second direction and spaced apart along the second direction by at least a second distance that spans at least two second periodic scale element zones including the characteristic second arrangement of incremental scale features along the first direction, 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 a 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 at least partly on a plurality of incremental scale features included in the detected image; and

combining the 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 the high resolution position of the detector array along the first and second measuring axes of the two-dimensional absolute scale.

35. The method of claim 34, wherein:

each code group includes a predetermined portion; and

determining the 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.

36. The method of claim 34, 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 to a resolution that is finer in the detected image than a distance corresponding to one half of the underlying first incremental pitch in the detected image,

estimating an offset of a plurality incremental scale features 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 to a resolution that is finer in the detected image than a distance corresponding to one half of the underlying second incremental pitch in the detected image,

estimating an offset of a plurality incremental scale features 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 partly on the estimate of the offset of the local datum characteristic relative to the detector array along the first measuring axis and at least partly on the estimate of the offset of the plurality incremental scale elements 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 partly on the estimate of the offset of the local datum characteristic relative to the detector array along the second measuring axis and at least partly on the estimate of the offset of the plurality incremental scale elements relative to the detector array along the second measuring axis.