DEVICE AND METHOD FOR MEASURING SIX DEGREES OF FREEDOM

• US 20100128259A1
• Filed: 11/17/2009
• Published: 05/27/2010
• Est. Priority Date: 11/17/2008
• Status: Active Grant

• Current Assignee: Faro Technologies Incorporated
• Original Assignee: Faro Technologies Incorporated
• Inventors: Ackley, Kevin R., Brown, Lawrence B., Hoffer, John M. JR., Bridges, Robert E.
• Granted Patent: US 8,525,983 B2
• Time in Patent Office: Days
• US Class Current: 356/138
• CPC Class Codes: G01B 11/03 by measuring coordinates of points
• CPC Class Codes: G01C 15/002 Active optical surveying means optical plumbing G01C15/105
• CPC Class Codes: G01S 1/70 using electromagnetic waves other than radio waves
• CPC Class Codes: G01S 17/66 Tracking systems using electromagnetic waves other than radio waves
• CPC Class Codes: G01S 5/163 Determination of attitude using inertial means G01C9/00; control of attitude G05D1/49

Abstract

A laser tracker system for measuring six degrees of freedom may include a main optics assembly structured to emit a first laser beam, a pattern projector assembly structured to emit a second laser beam shaped into a two-dimensional pattern, and a target. The target may include a retroreflector and a position sensor assembly. A center of symmetry of the retroreflector may be provided on a different plane than a plane of the position sensor assembly. A method of measuring orientation of a target may include illuminating the target with a laser beam comprising a two-dimensional pattern, recording a position of the two-dimensional pattern on a position sensor assembly to create a measured signature value of the two-dimensional pattern, and calculating an orientation of the target based on the measured signature value.

Claims

1. A laser tracker system for measuring six degrees of freedom, the system comprising:

a tracking unit comprising;

a payload assembly rotatable around at least one axis, the payload assembly comprising;

a main optics assembly structured to emit a first laser beam; and

a pattern projector assembly structured to emit a second laser beam shaped into a two-dimensional pattern; and

a target comprising;

a retroreflector; and

a position sensor assembly provided proximate to the retroreflector;

wherein a center of symmetry of the retroreflector is provided on a different plane than a plane of the position sensor assembly.

2. The laser tracker system of claim 1, wherein the main optics assembly further comprises:

a first laser structured to emit the first laser beam;

distance processing electronics;

a first beam splitter structured to direct at least a portion of a retroreflected laser beam to the distance processing electronics;

a position detector; and

a second beam splitter structured to direct at least a portion of the retroreflected laser beam to the position detector.

3. The laser tracker system of claim 1, wherein the pattern projector assembly further comprises:

a second laser structured to emit the second laser beam;

a beam expander structured to expand the second laser beam; and

a shaping element structured to shape the expanded second laser beam into the two-dimensional pattern.

4. The laser tracker system of claim 3, wherein the shaping element comprises an apodizer.

5. The laser tracker system of claim 4, wherein the apodizer comprises a continuous tone film transparency attached with optical cement between two glass plates.

6. The laser tracker system of claim 3, wherein the shaping element comprises a diffractive element.

7. The laser tracker system of claim 3, wherein the shaping element comprises a plurality of lenses, a plurality of beam splitters, and a plurality of prisms.

8. The laser tracker system of claim 3, wherein the two-dimensional pattern comprises a pattern of stripes.

9. The laser tracker system of claim 1, wherein the tracking unit further comprises:

an azimuth assembly mounted on a stand; and

a zenith assembly mounted on the azimuth assembly and rotatable about an azimuth axis;

wherein the payload assembly is mounted on the zenith assembly and is rotatable about a zenith axis;

the azimuth assembly comprises an azimuth motor assembly structured to rotate the zenith assembly about the azimuth axis and a azimuth encoder assembly structured to measure an angle of rotation of the zenith assembly; and

the zenith motor assembly comprises a zenith motor assembly structured to rotate the payload assembly about the zenith axis and a zenith encoder assembly structured to measure an angle of rotation of the payload assembly.

10. The laser tracker system of claim 9, wherein the tracking unit further comprises a position detector structured such that when a retroreflected laser beam strikes the position detector at a position away from a predetermined position, an error signal is generated to direct the azimuth motor assembly and the zenith motor assembly to rotate the payload assembly such that the first laser beam strikes a center of the retroreflector.

11. The laser tracker system of claim 3, wherein the pattern projector assembly further comprises a camera assembly comprising a camera and at least one light-emitting diode, the camera assembly being provided on an optical axis of the pattern projector assembly.

12. The laser tracker system of claim 3, wherein the pattern projector assembly further comprises a plurality of camera assemblies, each camera assembly comprising a camera and at least one light-emitting diode.

13. The laser tracker assembly of claim 11, wherein the target further comprises a locator spot.

14. The laser tracker assembly of claim 1, wherein the retroreflector comprises a cube-corner retroreflector.

15. The laser tracker assembly of claim 1, wherein the retroreflector and the position detector assembly are rigidly connected to a common structural component.

16. The laser tracker assembly of claim 1, wherein the target further comprises a probe.

33. The laser tracker system of claim 1, wherein the position sensor assembly comprises:

a position sensitive detector or a photosensitive array; and

an optical filter provided on the photosensitive array.

34. The laser tracker system of claim 33, wherein the position sensor assembly comprises:

at least two position sensitive detectors or photosensitive arrays.

35. The laser tracker system of claim 33, wherein the photosensitive array is a CCD or CMOS array.

36. The laser tracker system of claim 34, wherein the at least two position sensitive detectors or photosensitive arrays are linear arrays.

37. The laser tracker assembly of claim 33, wherein the optical filter comprises an optical bandpass filter and a neutral density filter.

43. The laser tracker system of claim 1, further comprising electronics structured to:

record a position of the two-dimensional pattern on the position sensor assembly to create a measured signature value of the pattern orientation;

iteratively compare the measured signature value with a theoretical signature value;

calculate an orientation of the target from the measured signature value when a difference between the measured signature value and the theoretical signature value satisfies a convergence criteria.

44. The laser tracker system of claim 43, wherein the two-dimensional pattern is a pattern of stripes radiating from a central point;

and

the position sensor assembly comprises at least two linear position sensitive detectors or photosensitive arrays.

45. The laser tracker system of claim 44, wherein the at least two photosensitive arrays comprise a plurality of pixels;

and the electronics are further structured to;

accumulate a set of pixel samples by repeatedly collecting a set of pixel values and averaging the pixel values together for each of the plurality of pixels;

filter the accumulated set of pixel samples;

decimate the accumulated set of pixel samples;

calculate a derivative between adjacent pixels;

identify zero crossings in the derivatives to identify potential peaks and valleys;

remove from consideration peaks and valleys that are smaller than a predetermined threshold; and

fit a parabola to data points near remaining peaks and valleys.

17. A pattern projector assembly for use in a laser tracker system for measuring six degrees of freedom, the pattern projector assembly comprising:

a laser structured to emit a laser beam;

a beam expander structured to expand the second laser beam; and

a shaping element structured to shape the expanded second laser beam into a two-dimensional pattern.

18. The pattern projector assembly of claim 17, wherein the shaping element comprises an apodizer.

19. The pattern projector assembly of claim 18, wherein the apodizer comprises a continuous tone film transparency attached with optical cement between two glass plates.

20. The pattern projector assembly of claim 17, wherein the shaping element comprises a diffractive element.

21. The pattern projector assembly of claim 17, wherein the shaping element comprises a plurality of lenses, a plurality of beam splitters, and a plurality of prisms.

22. The pattern projector assembly of claim 17, wherein the two-dimensional pattern comprises a pattern of stripes.

23. The pattern projector assembly of claim 17, wherein the pattern projector assembly further comprises a camera assembly comprising a camera and at least one light-emitting diode, the camera assembly being provided on an optical axis of the pattern projector assembly.

24. The pattern projector assembly of claim 17, wherein the pattern project assembly further comprises a plurality of camera assemblies, each camera assembly comprising a camera and at least one light-emitting diode.

25. A target for use with a laser tracker system for measuring six degrees of freedom, the target comprising:

a retroreflector; and

a position sensor assembly provided proximate to the retroreflector;

wherein a center of symmetry of the retroreflector is provided on a different plane than a plane of the position sensor assembly.

26. The target of claim 25, further comprising a locator spot.

27. The target of claim 25, wherein the retroreflector comprises a cube-corner retroreflector.

28. The target of claim 25, wherein the retroreflector and the position detector assembly are rigidly connected to a common structural component.

29. The target of claim 25, wherein the target further comprises a probe.

38. The target of claim 25, wherein the position sensor assembly comprises:

a position sensitive detector or a photosensitive array; and

an optical filter provided on the photosensitive array.

39. The target of claim 38, wherein the position sensor assembly comprises:

at least two position sensitive detectors or photosensitive arrays.

40. The target of claim 38, wherein the photosensitive array is a CCD or CMOS array.

41. The target of claim 39, wherein the at least two position sensitive detectors or photosensitive arrays are linear arrays.

42. The target of claim 38, wherein the optical filter comprises an optical bandpass filter and a neutral density filter.

30. A method of measuring orientation of a target, the method comprising:

providing the target comprising;

a retroreflector; and

a position sensor assembly provided proximate to the retroreflector;

wherein a center of symmetry of the retroreflector is provided on a different plane than a plane of the position sensor assembly;

illuminating the target with a laser beam shaped a two-dimensional pattern;

recording a position of the two-dimensional pattern on the position sensor assembly to create a measured signature value of the pattern orientation;

iteratively comparing the measured signature value with a theoretical signature value;

calculating an orientation of the target from the measured signature value when a difference between the measured signature value and the theoretical signature value satisfies a convergence criteria.

31. The method of claim 30, wherein the two-dimensional pattern is a pattern of stripes radiating from a central point;

and

the position sensor assembly comprises at least two linear position sensitive detectors or photosensitive arrays.

32. The method of claim 31, wherein the at least two photosensitive arrays comprise a plurality of pixels;

and the recording a position of the two-dimensional pattern comprises;

accumulating a set of pixel samples by repeatedly collecting a set of pixel values and averaging the pixel values together for each of the plurality of pixels;

filtering the accumulated set of pixel samples;

decimating the accumulated set of pixel samples;

calculating a derivative between adjacent pixels;

identifying zero crossings in the derivatives to identify potential peaks and valleys;

removing from consideration peaks and valleys that are smaller than a predetermined threshold; and

fitting a parabola to data points near remaining peaks and valleys.

46. A laser tracker system comprising:

a tracking unit comprising;

a payload assembly rotatable around at least one axis, the payload assembly comprising;

a main optics assembly structured to emit a first laser beam; and

at least two camera assemblies, each camera assembly comprising a camera and at least one light-emitting diode.

47. The laser tracker system of claim 46, wherein the at least two cameras are placed symmetrically about an optical axis of the payload assembly.

48. The laser tracker system of claim 46, wherein the at least two cameras are placed proximate to an output window of the payload assembly.

49. The laser tracker system of claim 46, further comprising:

a target comprising a retroreflector;

wherein the at least two cameras are structured to provide stereoscopic viewing of the target to provide an estimate of distance and angles to the target.