Determining retroreflector orientation by optimizing spatial fit

• US 9,395,174 B2
• Filed: 06/25/2015
• Issued: 07/19/2016
• Est. Priority Date: 06/27/2014
• Status: Active Grant

• Current Assignee: Faro Technologies Incorporated
• Original Assignee: Faro Technologies Incorporated
• Inventors: Bridges, Robert E.
• Primary Examiner(s): Harandi, Siamak
• Assistant Examiner(s): TRAN, MAI H
• Application Number:
  US14/750,149
• Publication Number:
  US 20150377605A1
• Time in Patent Office: 390 Days
• Field of Search: None
• US Class Current: 1/1
• CPC Class Codes: G01B 11/002 for measuring two or more coordinates
• CPC Class Codes: G01B 11/26 for measuring angles or tapers; for testing the alignment of axes
• CPC Class Codes: G01C 15/002 Active optical surveying means optical plumbing G01C15/105
• CPC Class Codes: G01C 15/06 Surveyors' staffs; Movable markers
• 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 5/163 Determination of attitude using inertial means G01C9/00; control of attitude G05D1/49
• CPC Class Codes: G01S 7/481 Constructional features, e.g. arrangements of optical elements
• CPC Class Codes: G02B 5/122 cube corner, trihedral or triple reflector type
• CPC Class Codes: G06T 2207/10004 Still image; Photographic image
• CPC Class Codes: G06T 5/10 using non-spatial domain filtering
• CPC Class Codes: G06T 7/73 using feature-based methods

Abstract

A program storage device having instructions that cause a programmable control device to obtain a two-dimensional (2D) image of markings on a retroreflector, determine a 2D mathematical representation of the markings on the retroreflector, extract a first collection of 2D coordinates from the 2D mathematical representation and a corresponding second collection of 2D coordinates from the 2D image of markings, determine a figure of merit, and adjust guess values for three orientation angles of the retroreflector to improve the figure of merit.

Claims

1. A non-transitory program storage device, readable by a programmable control device and comprising instructions stored thereon to cause the programmable control device to:

obtain a two-dimensional (2D) image of markings on a retroreflector, the retroreflector being a glass prism having a first face, a second face, a third face, and a fourth face, the first face, the second face, and the third face being mutually perpendicular reflecting surfaces, the fourth face being a front face of the prism, the first surface and second surface intersecting in a first intersection line, the second surface and the third surface intersecting in a second intersection line, the third surface and the first surface intersecting in a third intersection line, the first intersection line having a straight first intersection mark, the second intersection line having a straight second intersection mark, the third intersection line having a straight third intersection mark, the front face having a first surface mark;

determine a 2D mathematical representation of the markings on the retroreflector based at least in part on guess values for each of three orientation angles of the retroreflector;

extract a first collection of 2D coordinates from the 2D mathematical representation and a corresponding second collection of 2D coordinates from the 2D image of markings;

determine a figure of merit based at least in part on the first collection of 2D coordinates and the corresponding second collection of 2D coordinates; and

adjust the guess values for the three orientation angles to improve the figure of merit.

2. The non-transitory program storage device of claim 1 wherein the first surface mark is an elliptical line segment.

3. The non-transitory program storage device of claim 1 wherein the first surface mark is a straight line.

4. The non-transitory program storage device of claim 1 wherein the 2D mathematical representation is a mathematical projection of the retroreflector markings in three dimensions onto an image plane in two dimensions.

5. The non-transitory program storage device of claim 1 wherein the first collection of 2D coordinates is obtained for points equally spaced along the 2D mathematical representation of the markings.

6. The non-transitory program storage device of claim 1 wherein the instructions applies image processing to the 2D image of markings before determining the corresponding second collection of 2D coordinates.

7. The non-transitory program storage device of claim 6 wherein the image processing includes spatial filtering.

8. The non-transitory program storage device of claim 7 wherein the image processing includes a filter operation selected from the group consisting of:

Laplacian filter, gradient filter, and Fourier transform filter.

9. The non-transitory program storage device of claim 7 wherein the image processing further includes removing background patterns not extracted by the image processing.

10. The non-transitory program storage device of claim 6 wherein each of the second collection of 2D coordinates are selected to minimize the distance to a corresponding 2D coordinate of the first collection of 2D coordinates.

11. The non-transitory program storage device of claim 1 wherein the figure of merit is based at least in part on a sum of squared residual errors terms.

12. The non-transitory program storage device of claim 1 wherein the guess values for the three orientation angles are based at least in part on a Jacobian matrix.

13. A method for determining three orientational degrees of freedom of a retroreflector comprising:

obtaining a two-dimensional (2D) image of markings on the retroreflector, the retroreflector being a glass prism having a first face, a second face, a third face, and a fourth face, the first face, the second face, and the third face being mutually perpendicular reflecting surfaces, the fourth face being a front face of the prism, the first surface and second surface intersecting in a first intersection line, the second surface and the third surface intersecting in a second intersection line, the third surface and the first surface intersecting in a third intersection line, the first intersection line having a straight first intersection mark, the second intersection line having a straight second intersection mark, the third intersection line having a straight third intersection mark, the front face having a first surface mark;

determining a 2D mathematical representation of the markings on the retroreflector based at least in part on guess values for each of three orientation angles of the retroreflector;

extracting a first collection of 2D coordinates from the 2D mathematical representation and a corresponding second collection of 2D coordinates from the 2D image of markings;

determining a figure of merit based at least in part on the first 2D coordinates and the corresponding second 2D coordinates;

adjusting the guess values for the three orientation angles to improve the figure of merit; and

storing the three orientation angles.

14. A device for measuring three orientational degrees of freedom of a retroreflector comprising:

a memory;

an image sensor;

a light source configured to emit a light to illuminate the retroreflector;

a display communicatively coupled to the memory; and

a programmable control device communicatively coupled to the memory, display, and image sensor, wherein the memory includes instructions for causing the programmable control device to perform the method of claim 13.