Method for correcting three-dimensional measurements of a spherically mounted retroreflector

• US 8,947,678 B2
• Filed: 12/11/2013
• Issued: 02/03/2015
• Est. Priority Date: 12/11/2013
• Status: Expired due to Fees

• Current Assignee: Faro Technologies Incorporated
• Original Assignee: Faro Technologies Incorporated
• Inventors: Bridges, Robert E.
• Primary Examiner(s): Geisel, Kara E
• Assistant Examiner(s): Underwood, Jarreas C
• Application Number:
  US14/102,869
• Publication Number:
  US 20140098381A1
• Time in Patent Office: 419 Days
• Field of Search: 356600-640
• US Class Current: 356/614
• CPC Class Codes: G01B 11/00 Measuring arrangements characterised by the use of optical techniques
• CPC Class Codes: G01B 11/002 for measuring two or more coordinates
• CPC Class Codes: G01B 11/005 coordinate measuring machines
• CPC Class Codes: G01B 11/02 for measuring length, width or thickness G01B11/08 takes precedence
• CPC Class Codes: G01C 15/002 Active optical surveying means optical plumbing G01C15/105
• CPC Class Codes: G01S 17/66 Tracking systems using electromagnetic waves other than radio waves
• CPC Class Codes: G01S 7/497 Means for monitoring or calibrating

Abstract

A method of measuring spherically mounted retroreflector (SMR) with a 3D coordinate measurement device such as a laser tracker. The SMR includes an open-air cube corner retroreflector having a vertex point located near a sphere center of the SMR. Measurements of the SMR to the vertex point are moved to the SMR sphere center by accounting for SMR depth error and SMR runout error.

Claims

1. A method of measuring a spherically mounted retroreflector (SMR) with a device, the method comprising the steps of:

providing a first SMR, the first SMR including a first body and a first retroreflector, the first SMR including a first reference point, the first body having a first spherical exterior portion that has a first sphere center and a first sphere radius, the first body containing a first cavity, the first cavity sized to hold the first retroreflector, the first cavity open to a region outside the first body, the first retroreflector at least partially disposed in the first cavity, the first retroreflector being an open-air cube-corner retroreflector, the first retroreflector having a first set of three mutually perpendicular planar reflectors that intersect in a first set of three lines and in a common first vertex point, the first cavity including an air-filled region interior to reflecting surfaces of the first set of three planar reflectors, the first retroreflector having a first axis of symmetry relative to the first set of three lines, the first SMR having a first SMR runout plane perpendicular to the first axis of symmetry and passing through the first sphere center, the first SMR having a first SMR intersection point, the first SMR intersection point being a point of intersection of the first axis of symmetry with the first SMR runout plane, the first SMR having a first error vector extending from the first vertex point to the first sphere center, the first SMR error vector having a first SMR depth error vector component and a first SMR runout error vector component, the first SMR depth error vector component being a vector that extends from the first vertex point to the first SMR intersection point, the first SMR runout error vector component being a vector that extends from the first SMR intersection point to the first sphere center, the first SMR depth error vector component having a magnitude equal to a first SMR depth error;

providing the device, wherein the device has a device frame of reference fixed with respect to a base of the device and a light source that emits a first beam of light, the device being configured to measure a target distance and two target angles from the device to the first vertex point, the two target angles given with respect to the base;

providing a processor and a memory;

providing computer readable media having computer readable instructions which when executed by the processor calculates first three-dimensional (3D) coordinates of the first sphere center in the device frame of reference;

determining the first SMR depth error and the first SMR runout error vector component;

storing as first stored values the first SMR depth error and a numerical representation of the first SMR runout error vector component;

reading by the processor the first stored values; and

performing a first measurement, the first measurement including steps A through J;

A) providing a rule for orienting the first SMR to obtain a first preferred orientation of the first SMR;

B) determining, by the operator, the first preferred orientation of the first SMR based at least in part on the rule;

C) orienting, by the operator, the first SMR in the first preferred orientation, the first preferred orientation based at least in part on the first reference point and on the axis of symmetry;

D) determining by the processor the first preferred orientation in the device frame of reference based at least in part on the rule;

E) sending the first beam of light from the light source to the first vertex point in a first beam direction and in response receiving at the device a first reflected light;

F) measuring, from the device to the first vertex point, a first target distance and a first set of the two target angles based at least in part on the first reflected light, the first target distance further based at least in part on a speed of light over a path traveled by the first beam of light;

G) determining the first beam direction in the device frame of reference based at least in part on the first set of the two target angles;

H) determining with the processor 3D coordinates of the first vertex point in the device frame of reference based at least in part on the first target distance and the first set of the two target angles;

I) executing by the processor the computer readable instructions, the computer readable instructions performing a mathematical transformation of the 3D coordinates of the first vertex point to obtain the 3D coordinates of the first sphere center in the device frame of reference, the mathematical transformation including translating the 3D coordinates of the first vertex point along the first beam direction by the first SMR depth error and along a direction perpendicular to the first beam direction, the translating along the direction perpendicular to the first beam direction being based at least in part on the first beam direction, the numerical representation of the first SMR runout error vector component, and the first preferred orientation of the first SMR; and

J) storing the 3D coordinates of the first sphere center.

2. The method of claim 1 wherein, in the step of providing a processor and a memory, the processor is included in the device.

3. The method of claim 1 wherein, in the step of providing a processor and a memory, the processor is included in a computing instrument external to the device.

4. The method of claim 1 wherein:

in the step of providing a first SMR, the first SMR has a first SMR reference plane that includes the first reference point and the first axis of symmetry, there being a first SMR reference ray coincident with a line of intersection between the first SMR reference plane and the first SMR runout plane, the first SMR reference ray beginning at the first SMR intersection point and lying in a half of the first SMR reference plane that includes the first reference point, the first SMR runout error vector component having a first SMR runout reference angle, the first SMR runout reference angle being an angle between the first SMR reference ray and the first SMR runout error vector component, a length of the first SMR runout error vector component being equal to a first SMR runout error; and

in the step of storing as first stored values the first SMR depth error and a numerical representation of the first SMR runout error vector component, the numerical representation of the first SMR runout error vector component includes the first SMR runout error and the first SMR runout reference angle.

5. The method of claim 4 wherein, in the step of executing by the processor the computer readable instructions, the translating along a direction perpendicular to the first beam direction includes translating in a plane perpendicular to the first beam direction by an amount equal to the first SMR runout error in a direction given by a first beam runout reference angle relative to a first beam reference ray, the first beam reference ray coinciding with a line of intersection between a first beam reference plane and a first beam runout plane, the first beam reference ray beginning at a first beam intersection point and lying in a half of the first beam reference plane that includes the first reference point, the first beam reference plane being a plane that includes the first reference point and the first beam direction, the first beam runout plane being a plane perpendicular to the first beam direction and that includes a first beam intersection point, the first beam intersection point being a point translated from the first vertex point along the first beam direction by the first SMR depth error, the first beam runout plane being a plane perpendicular to the first beam direction and passing through the first beam intersection point.

6. The method of claim 1 wherein, in the step of providing a first SMR, the first SMR is provided with a first collar, the first collar configured to be gripped by an operator.

7. The method of claim 6 wherein, in the step of providing a first SMR, the first reference point is included on the first collar.

8. The method of claim 7 wherein, in the step of providing a first SMR, the first reference point is located on one of the group consisting of a reference mark, a serial number, a one-dimensional barcode, a two-dimensional barcode, a radio frequency identification tag, and a reflective region.

9. The method of claim 1 wherein, in the step of performing a first measurement, the first measurement includes aligning by the operator the axis of symmetry to the first beam direction.

10. The method of claim 9 wherein, in the step of providing a rule for orienting the first SMR to obtain a first preferred orientation of the first SMR, the rule is to rotate the SMR about the first axis of symmetry until the first SMR reference plane is aligned to a plane that includes a gravity vector and the first beam of light.

11. The method of claim 9 wherein, in the step of providing a rule for orienting the first SMR to obtain a first preferred orientation of the first SMR, the rule is to rotate the SMR about the first axis of symmetry until the first SMR reference plane is aligned to a plane that includes an axis of rotation of the device and the first beam of light.

12. The method of claim 11 wherein, in the step of providing a rule for orienting the first SMR to obtain a first preferred orientation of the first SMR, the axis of rotation is an azimuth axis.

13. The method of claim 11 wherein, in the step of providing a rule for orienting the first SMR to obtain a first preferred orientation of the first SMR, the axis of rotation is a zenith axis.

14. The method of claim 9 wherein, in the step of providing a rule for orienting the first SMR to obtain a first preferred orientation of the first SMR, the rule is further based on a first dimensional quantity of interest, wherein the first dimensional quantity of interest is given with respect to a first measurement line.

15. The method of claim 14 wherein, in the step of providing a rule for orienting the first SMR to obtain a first preferred orientation of the first SMR, the first dimensional quantity of interest is a first length between two points on the first measurement line, and the rule is to rotate the SMR about the first axis of symmetry until the first SMR reference ray is perpendicular to the first measurement line.

16. The method of claim 1 wherein, in the step of providing a first SMR, the first SMR further includes an information storage element.

17. The method of claim 16 further including a step of storing on the information storage element the first stored values.

18. The method of claim 17 wherein, in the step of providing a first SMR, the first SMR is provided with a first collar, the first collar configured to be gripped by an operator, the information storage element being affixed to or integrated into the collar.

19. The method of claim 16 wherein the information storage element is selected from the group consisting of a one-dimensional barcode, a two-dimensional barcode, and a radio frequency identification tag.

20. The method of claim 19 wherein, in the step of providing the device, the device further includes a storage element reader, the storage element reader configured to read the information stored in the information storage element.

21. The method of claim 17 wherein, in the step of storing on the information storage element, the information storage element is a one-dimensional or a two-dimensional barcode, and wherein, in the step of providing a device, the device further includes a camera configured to read the information storage element when the first SMR is placed in a kinematic support affixed to the device.

22. The method of claim 17 wherein the step of storing on the information storage element the first stored values further includes a step of storing an SMR serial number.

23. The method of claim 9 wherein, in the step of performing a plurality of first measurements, the operator uses an alignment tool to assist in the aligning.

24. The method of claim 23 wherein, in the step of performing a plurality of first measurements, the alignment tool includes an opaque cover that fits over the first collar, the cover including a visible mark, the visible mark being a circle or a crosshair.

25. The method of claim 23 wherein, in the step of performing a plurality of first measurements, the alignment tool includes a mirror that fits over the first collar.

26. The method of claim 24 wherein, in the step of performing a plurality of first measurements, the mirror includes a visible mark, the visible mark being a circle or a crosshair.

27. The method of claim 22 wherein, in the step of performing a plurality of first measurements, each of the first measurements include steps of sending the first beam of light from the device in a circular pattern and aligning by the operator the axis of symmetry to center the SMR on the circular pattern.

28. The method of claim 23 wherein, in the step of performing a plurality of first measurements, the alignment tool is selected from the group consisting of a reflective strip, a finger, a window onto which a circle or crosshair has been marked.

29. The method of claim 1 wherein:

the step of providing a first SMR further includes providing a first temperature sensor, the first temperature sensor affixed to the first SMR;

the step of performing a plurality of first measurements further includes measuring a first temperature of the first temperature sensor; and

in the step of executing by the processor the computer readable instructions, the 3D coordinates of the first sphere center are further based the first temperature measured by the first temperature sensor.

30. The method of claim 1 wherein, in the step of measuring, from the device to the first vertex point, a first target distance and a first set of the two target angles, the first SMR is held in contact with a first surface at a first contact point.

31. The method of claim 30 wherein the step of a making a first measurement is repeated multiple times to obtain a plurality of first measurements, the plurality of first measurements resulting in a plurality of first contact points and a corresponding plurality of first sphere centers, the plurality of first sphere centers having a corresponding plurality of 3D coordinates of the first sphere centers.

32. The method of claim 31 further including a step of selecting a first chosen contact point from among the plurality of contact points, there being a correspondence between the first chosen contact point and a first chosen sphere center point, the method including:

selecting a first subset of the plurality of first contact points;

selecting a second subset of the stored 3D coordinates of the first sphere center, each element of the second subset corresponding to an element of the first subset;

determining a first surface normal vector based at least in part on the second subset; and

determining the 3D coordinates of the first chosen contact point based at least in part on the 3D coordinates of the first chosen sphere center point, the first surface normal vector, and a first radius, the first radius being a value of the first sphere radius.

33. The method of claim 32 further including steps of:

measuring a first radius;

storing a first radius parameter, the radius parameter being the first radius or a first radius delta, the first radius delta being the first radius minus a reference value of the first radius; and

reading by the processor the first radius parameter.

34. The method of claim 15 further including storing on the information storage element the first radius parameter.