Automatic measurement of dimensional data with a laser tracker

• US 8,724,120 B2
• Filed: 04/10/2013
• Issued: 05/13/2014
• Est. Priority Date: 04/21/2010
• Status: Active Grant

• Current Assignee: Faro Technologies Incorporated
• Original Assignee: Faro Technologies Incorporated
• Inventors: Steffey, Kenneth, Steffensen, Nils P., Bridges, Robert E.
• Primary Examiner(s): NGUYEN, TU T
• Application Number:
  US13/860,010
• Publication Number:
  US 20130222791A1
• Time in Patent Office: 398 Days
• Field of Search: 356/614
• US Class Current: 356/614
• CPC Class Codes: G01B 11/002 for measuring two or more coordinates
• CPC Class Codes: G01B 11/14 for measuring distance or clearance between spaced objects or spaced apertures G01B11/26 takes precedence; rangefinders G01C3/00
• CPC Class Codes: G01B 21/045 Correction of measurements G01B9/02055 takes precedence
• CPC Class Codes: G01C 15/002 Active optical surveying means optical plumbing G01C15/105
• CPC Class Codes: G01S 17/42 Simultaneous measurement of distance and other co-ordinates indirect measurement G01S17/46
• CPC Class Codes: G01S 17/66 Tracking systems using electromagnetic waves other than radio waves
• CPC Class Codes: G01S 17/89 for mapping or imaging
• CPC Class Codes: G01S 7/4808 Evaluating distance, position or velocity data
• CPC Class Codes: G01S 7/481 Constructional features, e.g. arrangements of optical elements
• CPC Class Codes: G01S 7/4811 common to transmitter and receiver
• CPC Class Codes: G01S 7/4813 Housing arrangements
• CPC Class Codes: G01S 7/4815 using multiple transmitters
• CPC Class Codes: G01S 7/4972 Alignment of sensor

Abstract

Measuring with a system having retroreflector targets and a laser tracker includes storing a list of coordinates for three targets and at least one added point; capturing on a photosensitive array a portion of the light emitted by a light beam and reflected off the targets; obtaining spot positions on a photosensitive array of a tracker camera from the reflected light; determining a correspondence between three spot positions on the photosensitive array and the coordinates of the targets; directing a beam of light from the tracker to the targets based at least in part on the coordinates of the first target and the first spot position; measuring 3-D coordinates of the targets with the tracker; determining 3-D coordinates of the at least one added point based at least in part on the measured 3-D coordinates of the targets and the coordinates of the at least one added point.

Claims

1. A method for measuring with a system, the method comprising steps of:

providing the system including a collection of retroreflector targets and a laser tracker, the collection of retroreflector targets including at least three retroreflector targets configured to be arranged in a non-collinear relationship when placed on an object, the at least three non-collinear retroreflector targets including a first target, a second target, and a third target, the laser tracker in a first frame of reference fixed with respect to the laser tracker surroundings, the laser tracker having a structure, a first light source, an absolute distance meter, a first angular transducer, a second angular transducer, a tracking system, a first camera, a second light source, a processor, and a memory, the memory operably coupled to the processor, the structure rotatable about a first axis and a second axis, the first light source producing a first light beam that cooperates with the absolute distance meter, the first angular transducer measuring a first angle of rotation about the first axis, the second angular transducer measuring a second angle of rotation about the second axis, the tracking system configured to move the first light beam to a center of any retroreflector target from among the collection of retroreflector targets, the first camera including a first lens system and a first photosensitive array, the second light source providing a second light beam, and the processor configured to operate the laser tracker;

storing, with the processor into the memory, a list of nominal coordinates for the first target, the second target, the third target, and at least one additional point, the nominal coordinates being three-dimensional coordinates in a second frame of reference;

capturing on the first photosensitive array a portion of the light emitted by the second light beam and reflected off the first target, the second target, and the third target;

obtaining spot positions on the first photosensitive array from the portion of light reflected off each of the first target, second target, and the third target;

determining a correspondence between a first spot position, a second spot position, and a third spot position on the first photosensitive array and the nominal coordinates of the first target, the second target, and the third target, respectively;

directing the first light beam to the first target based at least in part on the nominal coordinates of the first target and the first spot position;

measuring three-dimensional coordinates of the first target using the absolute distance meter, the first angular transducer, and the second angular transducer;

directing the first light beam to the second target based at least in part on the nominal coordinates of the second target and the second spot position;

measuring three-dimensional coordinates of the second target using the absolute distance meter, the first angular transducer, and the second angular transducer;

directing the first light beam to the third target based at least in part on the nominal coordinates of the third target and the third spot position;

measuring three-dimensional coordinates of the third target using the absolute distance meter, the first angular transducer, and the second angular transducer;

directing the first light beam to a plurality of additional points, the plurality of additional points including the at least one additional point, the plurality of additional points indicative of actions to be taken by an operator;

determining three-dimensional coordinates of the at least one additional point in the first frame of reference based at least in part on the measured three-dimensional coordinates of the first target, the second target, the third target, and the nominal coordinates of the at least one additional point; and

storing, with the processor into the memory, the determined three-dimensional coordinates of the at least one additional point.

2. The method of claim 1, further including steps of:

acting by the operator in response to the first light beam being directed to the at least one additional point by placing a selected retroreflector target to intercept at least a portion of the first light beam, the selected retroreflector target selected from among any of the retroreflector targets in the collection of retroreflector targets;

directing the first light beam to a center of the selected retroreflector target; and

measuring three-dimensional coordinates of the selected retroreflector target using the absolute distance meter, the first angular transducer, and the second angular transducer.

3. The method of claim 2, wherein the step of placing the selected retroreflector target to intercept at least a portion of the first light beam includes one step selected from the group consisting of:

moving the selected retroreflector target to intercept a least a portion of the first light beam; and

placing the selected retroreflector target in a fixed nest positioned to intercept at least a portion of the first light beam.

4. The method of claim 2, further including a step of measuring according to an inspection plan.

5. The method of claim 4, wherein the step of measuring according to an inspection plan further includes a step of measuring automatically without the operator intervention.

6. The method of claim 4, wherein the step of measuring according to an inspection plan further includes a step of moving the first light beam to direct the operator to move the selected retroreflector target to measure according to the inspection plan.

7. The method of claim 4, wherein the step of measuring according to an inspection plan further includes detecting when the operator has placed the selected retroreflector target in a wrong location and directing the operator to move the selected retroreflector target to a correct location, the laser tracker moving the first light beam to indicate to the operator the correct location of the selected retroreflector target.

8. The method of claim 4, wherein the step of measuring according to an inspection plan includes steps of:

obtaining nominal coordinates of a target point, the nominal coordinates derived from data provided in a computer aided design (CAD), the CAD data transformed into the first frame of reference;

providing the selected retroreflector target as a spherically mounted retroreflector, the spherically mounted retroreflector having a cube-corner retroreflector embedded within a spherical surface, the spherical surface having a first radius;

placing the selected retroreflector target at the position of the target point, the retroreflector target either held against the target point or placed on a magnetic nest representing the target point;

measuring three-dimensional coordinates of the selected retroreflector target using the absolute distance meter, the first angular transducer, and the second angular transducer;

calculating three-dimensional coordinates of the measured target point based at least in part on the measured three-dimensional coordinates of the selected retroreflector target and on the first radius;

calculating a first difference in three-dimensional coordinates of the measured target point and the nominal coordinates of the target point; and

providing an indication of the first difference.

9. The method of claim 8, wherein the step of providing an indication of the first difference includes providing a warning if the first difference exceeds a first allowable difference.

10. The method of claim 4, wherein the step of measuring according to an inspection plan includes steps of performing a two-face test to obtain a two-face error and providing a warning if the two face error exceeds a maximum allowable two-face error value.

11. The method of claim 4, wherein the step of measuring according to an inspection plan includes steps of measuring three-dimensional coordinates of two target points and providing a warning if relative positions of the two target points exceed a maximum allowable target change value.

12. The method of claim 2, further including steps of:

providing an inspection plan having inspection points to be measured by the laser tracker;

affixing at least one retroreflector target to the object under test;

providing a maximum allowable movement for the three dimensional coordinates of the at least one retroreflector target affixed to the object under test;

measuring three-dimensional coordinates of the at least one retroreflector target affixed to the object under test, the measuring performed at a first time and at a second time;

determining a first change in the three-dimensional coordinates of the at least one retroreflector target from the first time to the second time; and

taking an action when the first change exceeds the maximum allowable movement, the action being one of;

measuring at least three retroreflectors on the object under test to re-establish three-dimensional coordinates of the inspection points, and

notifying the operator that the first change has exceeded the maximum allowable movement.

13. The method of claim 1, further including steps of:

moving the first light beam in a first pattern in space, the first pattern proximate to the at least one additional point;

detecting the presence of the first light beam by the tracking system;

directing the first light beam to a center of a selected retroreflector target, the selected retroreflector target selected from among any of the retroreflector targets in the collection of retroreflector targets; and

measuring three-dimensional coordinates of the selected retroreflector target using the absolute distance meter, the first angular transducer, and the second angular transducer.

14. The method of claim 1, further including the steps of:

providing a third camera and a fourth light source, the third camera including a third lens system and a third photosensitive array, the third camera having a third field of view smaller than a first field of view of the first camera, the fourth light source providing a fourth light beam;

capturing on the third photosensitive array a portion of the light emitted by the fourth light source and reflected off the first target, the second target, and the third target;

obtaining spot positions on the third photosensitive array from the portion of light reflected off each of the first target, second target, and the third target; and

determining a correspondence between a fourth spot position, a fifth spot position, and a sixth spot position on the third photosensitive array and the nominal coordinates of the first target, the second target, and the third target, respectively.

15. The method of claim 1, wherein the step of capturing on the first photosensitive array a portion of the light emitted by the second light beam and reflected off the first target, the second target, and the third target further includes one step selected from the group consisting of:

positioning the laser tracker on a stand so that the first photosensitive array simultaneously obtains the first spot position, the second spot position, and the third spot position;

rotating the first camera and the second light source about the first axis to a plurality of orientations so that the first photosensitive array obtains the first spot position at a first of the plurality of orientations and the second spot position at a second of the plurality of positions; and

moving a selected retroreflector target from among the collection of retroreflector targets to delimit a region of space that contains the first target, the second target, and the third target, the region of space determined by the laser tracker either by tracking the selected retroreflector target or by tracking movement of the selected retroreflector target on the first photosensitive array.

16. The method of claim 1, wherein the step of providing the system including a collection of retroreflector targets and a laser tracker further includes providing a second camera and a third light source, the second camera including a second lens system and a second photosensitive array, the third light source providing a third light beam, the processor further configured to calculate three-dimensional coordinates of any retroreflector targets from among the collection of retroreflector targets, the calculated three-dimensional coordinates based at least in part on first images on the first photosensitive array and on second images on the second photosensitive array, the first images and the second images obtained in response to light emitted by the second light source and the third light source, respectively, the light from the first light source and the second light source reflected by the any retroreflector targets.

17. The method of claim 1, wherein the step of determining a correspondence among the first spot position, the second spot position, and the third spot position on the first photosensitive array and the nominal coordinates of the first target, the second target, and the third target, respectively, includes one step selected from the group consisting of:

determining the correspondence based at least in part on a range of allowable relative orientations between the first frame of reference and the second frame of reference and on the nominal coordinates of the first target, the second target, and the third target;

collecting, at a first time, a first set of images from the first photosensitive array in response to illumination of the first target, the second target, and the third target by light from the second light source;

changing one of the first angle of rotation and the second angle of rotation and collecting, at a second time, a second set of images from the first photosensitive array in response to illumination of the first target, the second target, and the third target by light from the second light source; and

determining the correspondence among the first spot position, the second spot position, and the third spot position on the first photosensitive array and the nominal coordinates of the first target, the second target, and the third target, the determining based at least in part on the first set of images and the second set of images; and

collecting, at a third time, a third set of images from the first photosensitive array in response to illumination of the first target, the second target, and the third target by light from the second light source;

changing the position of the second frame of reference relative to the first frame of reference and collecting, at a fourth time, a fourth set of images from the first photosensitive array in response to illumination of the first target, the second target, and the third target by light from the second light source; and

determining the correspondence among the first spot position, the second spot position, and the third spot position on the photosensitive array and the nominal coordinates of the first target, the second target, and the third target, the determining based at least in part on the third set of images and the fourth set of images.

18. The method of claim 1, wherein the step of storing a list of nominal coordinates for the first target, the second target, the third target, and at least one additional point, the nominal coordinates being three-dimensional coordinates in a second frame of reference, further includes one step selected from the group consisting of:

extracting the nominal coordinates from a computer aided design (CAD) model; and

measuring the nominal coordinates with a 3D measurement instrument.

19. The method of claim 1, wherein the actions to be taken by the operator include an action selected from the group consisting of:

picking up a selected retroreflector target, designated by the plurality of additional points;

moving the selected retroreflector target in a direction indicated by the plurality of additional points, capturing the first light beam with the selected retroreflector target, and measuring three dimensional coordinates of the at least one point using the absolute distance meter, the first angular transducer, and the second angular transducer;

observing a pattern formed by the plurality of additional points, capturing the first light beam with the selected retroreflector target, and measuring three-dimensional points along the pattern; and

observing a pattern formed by the plurality of additional points, interpreting the points as a gestural command, and taking action as indicated by the gestural command.

20. The method of claim 1, wherein the step of determining three-dimensional coordinates of the at least one additional point further includes steps of:

calculating a transformation matrix to transform arbitrary three-dimensional coordinates in the second frame of reference to arbitrary three-dimensional coordinates in the first frame of reference, the calculating based at least in part on the nominal coordinates of the first target, the second target, and the third target; and

calculating three-dimensional coordinates of the at least one additional point based at least in part on the transformation matrix and the nominal coordinates of the at least one additional point.

21. The method of claim 1, wherein the step of providing the system including a collection of retroreflector targets and a laser tracker includes providing at least one of the retroreflector targets from among the collection of retroreflector targets with a cube-corner retroreflector, the cube-corner retroreflector having three reflecting surfaces that are mutually perpendicular.

22. The method of claim 1, wherein the step of providing the system including a collection of retroreflector targets and a laser tracker includes providing at least one of the retroreflector targets from among the collection of retroreflector targets as a spherically mounted retroreflector, the spherically mounted retroreflector having a cube-corner retroreflector embedded within a spherical surface.

23. The method of claim 1, wherein the step of providing the system including a collection of retroreflector targets and a laser tracker includes providing at least one retroreflector target from among the collection of retroreflector targets as a reflective dot centered within a spherical surface.

24. The method of claim 1, further including steps of:

directing the first light beam to the at least one additional point; and

performing an assembly operation at a position of the first light beam.

25. The method of claim 24, wherein the step of performing an assembly operation includes drilling a hole in the object at the position of the first light beam.

26. The method of claim 1, further including steps of:

providing a maximum allowable discrepancy;

providing a coefficient of thermal expansion for the object under test;

providing a reference temperature;

placing a first reference retroreflector and a second reference retroreflector on the object under test, there being a first distance between the first reference retroreflector and the second reference retroreflector at the reference temperature;

measuring a temperature of the object under test;

calculating a first temperature difference by subtracting the reference temperature from the measured temperature of the object under test;

calculating a scale factor by multiplying the first temperature difference by the coefficient of thermal expansion;

measuring three-dimensional coordinates of the first reference retroreflector using the absolute distance meter, the first angular transducer, and the second angular transducer;

measuring three-dimensional coordinates of the second reference retroreflector using the absolute distance meter, the first angular transducer, and the second angular transducer;

calculating a second distance extending from the measured three-dimensional coordinates of the first reference retroreflector to the measured three-dimensional coordinates of the second reference retroreflector;

calculating a third distance by subtracting the first distance from the second distance;

calculating a fourth distance by multiplying the scale factor by the first distance;

calculating a discrepancy value by subtracting the third distance from the fourth distance; and

taking an action when the discrepancy value exceeds the maximum allowable discrepancy, the action being either issuing an alarm or measuring three-dimensional coordinates of at least some retroreflector targets from among the collection of retroreflector targets and, re-establishing a frame of reference for the object under test.