Method for correcting a 3D measurement of a spherically mounted retroreflector on a nest

• US 9,239,238 B2
• Filed: 12/11/2013
• Issued: 01/19/2016
• Est. Priority Date: 12/11/2013
• Status: Active Grant

• Current Assignee: Faro Technologies Incorporated
• Original Assignee: Faro Technologies Incorporated
• Inventors: Bridges, Robert E.
• Primary Examiner(s): RATCLIFFE, LUKE D
• Application Number:
  US14/102,901
• Publication Number:
  US 20150308818A1
• Time in Patent Office: 769 Days
• Field of Search: 356 301- 315, 356 401- 41, 356 501- 515, 356 6- 22, 356/28, 356/28.5
• US Class Current: 1/1
• CPC Class Codes: G01B 11/002 for measuring two or more coordinates
• CPC Class Codes: G01C 3/02 Details
• CPC Class Codes: G01C 3/10 using a parallactic triangle with variable angles and a base of fixed length in the observation station, e.g. in the instrument
• 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 for measuring a spherically mounted retroreflector (SMR) with a 3D coordinate measurement device placed on a kinematic nest, the method accounting for the radius of the SMR.

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 body having a first spherical exterior portion that has a first sphere center, 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 SMR having a first sphere radius, the first sphere radius being a radius of the first spherical exterior portion, the first SMR also being provided with a first nominal sphere radius, the first nominal sphere radius being a numerical value provided for the first SMR;

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 a target retroreflector, the two target angles given with respect to the base;

providing a first nest configured to repeatably receive a spherical exterior portion, the first nest having a first support axis, the first support axis coinciding with a locus of sphere centers for spheres of differing diameters placed in the first nest, the first support axis having a first support axis direction, there being a first ideal support position on the first support axis, the first ideal support position being a position coinciding with a center of a first reference sphere when received by the first nest, the first reference sphere having a radius equal to the first nominal sphere radius;

providing a processor and a memory;

providing computer readable media having computer readable instructions which when executed by the processor calculates three-dimensional (3D) coordinates of an ideal support position;

measuring the first sphere radius;

subtracting the first nominal sphere radius from the first sphere radius to get a first radius delta;

storing the first radius delta;

reading by the processor the first radius delta;

placing the first spherical exterior portion in the first nest;

sending the first beam of light from the light source to the first retroreflector and in response receiving at the device a first reflected light;

measuring, from the device to the first retroreflector, 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;

determining the first support axis direction;

executing by the processor the computer readable instructions, the computer readable instructions calculating 3D coordinates of the first ideal support position in the device frame of reference based at least in part on the first target distance, the first set of the two target angles, the first support axis direction, and the first radius delta; and

storing the 3D coordinates of the first ideal support position.

2. The method of claim 1 wherein, in the step of determining with the processor 3D coordinates of the first ideal support position, the determining is further based on a geometry of the first nest.

3. The method of claim 1 wherein, in the step of determining the first support axis direction, the determining is based at least in part on a first surface geometry, wherein the first surface geometry is a geometry of the first surface to which the first nest is affixed.

4. The method of claim 3 wherein, in the step of determining the first support axis direction, the first surface geometry is provided at least in part by CAD drawings.

5. The method of claim 3 wherein, in the step of determining the first support axis direction, the first surface geometry is provided at least in part by 3D measurements of the first surface.

6. The method of claim 1 wherein, in the step of determining the first support axis direction, the first support axis direction is based at least in part on a measurement of the first nest by the device.

7. The method of claim 1 wherein, in the step of determining the first support axis direction, the first support axis direction is provided to application software by an operator.

8. The method of claim 1 wherein, in the step of determining the first support axis direction, the first support axis direction is provided in application software in a default setting.

9. The method of claim 1 wherein, in the step of determining with the processor 3D coordinates of the first ideal support position, the determining includes a step of determining 3D coordinates of the first sphere center based at least in part on the first target distance and the first set of two angles and a step of translating the 3D coordinates of the first sphere center along the first support axis direction by the first radius delta.

10. The method of claim 9 further comprising:

providing a second SMR, the second SMR including a second body and a second retroreflector, the second body having a second spherical exterior portion that has a second sphere center, the second body containing a second cavity, the second cavity sized to hold the second retroreflector, the second cavity open to a region outside the second body, the second retroreflector at least partially disposed in the second cavity, the second SMR having a second sphere radius, the second sphere radius being a radius of the second spherical exterior portion, the second SMR also being provided with a second nominal sphere radius, the second nominal sphere radius being a numerical value provided for the second SMR;

providing a second nest configured to repeatably receive a spherical exterior portion, the second nest having a second support axis, the second support axis coinciding with a locus of sphere centers for spheres of differing diameters placed in the second nest, the second support axis having a second support axis direction, there being a second ideal support position on the second support axis, the second ideal support position being a position coinciding with a center of a second reference sphere when received by the second nest, the second reference sphere having a radius equal to the second nominal sphere radius;

measuring the second sphere radius;

subtracting the second nominal sphere radius from the second sphere radius to get a second radius delta;

storing the second radius delta;

reading by the processor the second radius delta;

placing the second spherical exterior portion in the second nest;

sending the first beam of light from the light source to the second retroreflector and in response receiving at the device a second reflected light;

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

determining the second support axis direction;

determining with the processor 3D coordinates of the second ideal support position in the device frame of reference based at least in part on the second target distance, the second set of the two target angles, the second support axis direction, and the second radius delta; and

storing the 3D coordinates of the second ideal support position.

11. The method of claim 10 wherein, in the step of determining with the processor 3D coordinates of the second ideal support position, the determining includes a step of determining 3D coordinates of the second sphere center based at least in part on the second target distance and the second set of two angles and a step of translating the 3D coordinates of the second sphere center along the second support axis direction by a distance equal to the second radius delta.

12. The method of claim 11 further comprising a step of determining a first distance between the 3D coordinates of the first ideal support position and the 3D coordinates of the second ideal support position.

13. The method of claim 12 wherein, in the step of determining a first distance, at least one of the first nest and the second nest is perpendicular to a line connecting the first ideal support position and the second ideal support position.

14. The method of claim 9 further comprising:

providing a second nest configured to repeatably receive a spherical exterior portion, the second nest having a second support axis, the second support axis coinciding with a locus of sphere centers for spheres of differing diameters placed in the second nest, the second support axis having a second support axis direction, there being a second ideal support position on the second support axis, the second ideal support position being a position coinciding with a center of a second reference sphere when received by the second nest, the second reference sphere having a radius equal to the second nominal sphere radius;

placing the first spherical exterior portion in the second nest;

sending the first beam of light from the light source to the first retroreflector held in the second nest and in response receiving at the device a second reflected light;

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

determining the second support axis direction;

executing by the processor the computer readable instructions, the computer readable instructions calculating 3D coordinates of the second ideal support position in the device frame of reference based at least in part on the second target distance, the second set of the two target angles, the second support axis direction, and the second radius delta; and

storing the 3D coordinates of the second ideal support position.

15. The method of claim 14 wherein, in the step of determining with the processor 3D coordinates of the second ideal support position, the determining includes a step of determining 3D coordinates of the second sphere center based at least in part on the second target distance and the second set of two angles and a step of translating the 3D coordinates of the first sphere center held in the second nest along the second support axis direction by a distance equal to the second radius delta.

16. The method of claim 15 further comprising a step of determining a first distance between the 3D coordinates of the first ideal support position and the 3D coordinates of the second ideal support position.

17. The method of claim 16 wherein, in the step of determining a first distance, at least one of the first nest and the second nest is perpendicular to a line connecting the first ideal support position and the second ideal support position.

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

19. The method of claim 18 further including a step of storing on the information storage element the first radius delta.

20. The method of claim 18 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.

21. The method of claim 20 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.