Non-contact apparatus and method for measuring surface profile

• US 20060103854A1
• Filed: 12/22/2005
• Published: 05/18/2006
• Est. Priority Date: 06/27/2001
• Status: Active Grant

• Current Assignee: Southwest Research Institute
• Original Assignee: Southwest Research Institute
• Inventors: Mitchell, Joseph N., Beeson, Robert J., Franke, Ernest A., Rigney, Michael P., Magee, Michael J.
• Granted Patent: US 7,382,471 B2
• Time in Patent Office: Days
• US Class Current: 356/603
• CPC Class Codes: G01B 11/25 by projecting a pattern, e.g. one or more lines, moiré fringes on the object G01B11/255 takes precedence ; image analysis for depth or shape recovery G06T7/50
• CPC Class Codes: G01B 11/2504 Calibration devices
• CPC Class Codes: G06T 7/521 from laser ranging, e.g. using interferometry; from the projection of structured light

Abstract

Embodiments of the invention provide a non-contact method for measuring the surface profile of an object that can include generating a point-type optical signal and projecting it on a rotatable precision optical grating, generating a rotating pattern of light and dark lines onto the object, recording a series of images of the rotating pattern moving across the object with an image receiving device and calculating the surface profile of the object. Other embodiments can include a method to calibrate the system and a non-contact apparatus that generally includes a point-type light source, a rotatably mounted optical grating being configured to project a moving grating image on the object, a processor in communication with the image capturing device and configured to receive image input from the image capturing device and generate a surface profile representation of the object therefrom.

Claims

1. A method for calibrating a dynamic structured light measuring system, comprising:

determining a focal length of an imaging device;

determining a transformation from an imaging device coordinate system to an absolute coordinate system;

determining absolute coordinates of a point in a plane, wherein a Z coordinate of the plane is know; and

determining at least one equation for at least one quadric surface containing projected grid line tracks.

2. The method of claim 1, wherein determining the focal length comprises:

acquiring a first image of a calibration object;

calculating a first calibration parameter for the calibration object from the first image;

adjusting the position of the object a first know distance;

acquiring a second image of the calibration object;

calculating a second calibration parameter of the calibration object from the second image; and

determining the focal length from the first and second calibration parameters.

3. The method of claim 2, wherein the calibration object comprises at least 3 circular markings having collinear centers such that a line joining the collinear centers such that a line joining the collinear centers is parallel with X axis of the imaging device.

4. The method of claim 3, wherein the first and second calibration parameters comprise a distance between outer centers of the at least three circular markings.

5. The method of claim 2, further comprising:

adjusting the position of the object a second known distance;

acquiring a third image of the calibration object;

calculating a third calibration parameter for the calibration object from the third image; and

determining the focal length from the first, second, and third calibration parameters.

6. The method of claim 1, wherein determining a transformation from an imaging device coordinate system to an absolute coordinate system comprises:

positioning a calibration object such that a plane containing the calibration object is coplanar with an absolute coordinate system and contains an origin of the absolute coordinate system, and such that a line joining circles of the calibration object is collinear with an X axis of the absolute coordinate system; and

determining a distance between an optical center of the imaging device and the plane containing the calibration object using the focal length.

7. The method of claim 1, wherein each individual quadric surface of the at least one quadric surface is derived through curve fitting to the projected gridline tracks in three or more reference planes.

8. The method of claim 1, wherein each individual quadric surface of the at least one quadric surface is derived through a least squares fit method.

9. The method of claim 8, wherein the least squares fit method further comprises solving the equation C=U M^T (M M^T)⁻

1, wherein C represents a nine element row vector of unknown coefficients, M represents a matrix having nine rows and a number of columns that corresponds to a number of projected points, and U represents a row vector of ones having a number of vector elements that corresponds to the number of projected points.

10. The method of claim 9, further comprising determining a set of coefficients associated with each projected grid line.

11. The method of claim 1, wherein determining at least one equation for at least one quadric surface containing projected grid line tracks further comprises independently determining an equation for each individual quadric surface.

12. A method for calibrating a dynamic structured light measuring system, comprising:

determining a focal length of an optical imaging device positioned above a reference measurement surface;

calculating a transformation from a camera coordinate system to an absolute coordinate system;

determining absolute coordinates of a point in a first plane above the reference measurement surface using the transformation, wherein a distance from the reference measurement surface to the first plane is known; and

determining at least one representative equation for at least one quadric surface containing projected grid line tracks.

13. The method of claim 12, wherein determining the focal length comprises:

orienting the optical imaging device;

positioning a planar calibration object having at least three markings having collinear centers above the reference measurement surface a first distance in a manner such that a plane of the calibration object is perpendicular to an optical axis of the optical imaging device and so that a line joining the centers is parallel with an X-axis of the optical imaging device;

calculating a first image plane distance between centroids of outer markings of the at least three markings of the first distance;

positioning the calibration object a second distance above the reference measurement surface, where the second distance is not equal to the first distance;

calculating a second image plane distance between centroids of outer markings of the at least three markings for the second distance; and

calculating the focal length from the first and second image plane distances.

14. The method of claim 12, wherein calculating a transformation from a camera coordinate system to an absolute coordinate system comprises:

positioning calibration markings such that a plane containing the calibration markings is coplanar with an absolute system, the plane containing the calibration markings contains an origin of the absolute coordinate system, and such that a line joining the calibration markings is collinear with an X-axis of the absolute coordinate system;

determining the distance between an optical center of the optical imaging device and a plane of a calibration object from the equation

$Z_0=f<br><br>\left(\frac{D}{d_0}\right),$

where Z₀ is the distance between an optical center of the optical imaging device and the plane of a calibration object, f is a focal length of the optical imaging device, D is a distance between centers of calibration markings, and d₀ is the distance between projections of the centers of the calibration markings; and

calculating the transformation from the optical imaging device coordinate system to the absolute coordinate system from the equation Z_Camera=A_bsolute+Z₀.

15. The method of claim 12, wherein determining at least one representative equation for at least one surface containing projected grid line tracks comprises:

providing a point light source;

rotating a grid about a central point at a location between the point light source and the reference measurement surface such that the a line joining the point light source and a point on each grid line that is closest to the central point traces out a quadric cone; and

independently determining an equation for the quadric surface.

16. The method of claim 12, wherein each of the at least one quadric surfaces is through curve fitting to the projected gridline tracks in three or more reference planes.

17. The method of claim 12, wherein each of the at least one quadric surfaces is derived through a least squares fit method.

18. The method of claim 17 wherein the least squares fit method further comprises solving the equation C=U M^T (M M^T)⁻

1, wherein, C represents a nine element row vector of unknown coefficients, M represents a matrix having nine rows and a number of columns that corresponds to a number of projected points, and U represents a row vector of ones having a number of vector elements that corresponds to the number of projected points.

19. The method of claim 18, further comprising determining a set of coefficients associated with each projected grid line.

20-55. -55. (canceled)