Vision system for analyzing solid-of-revolution radius profile

• US 5,918,196 A
• Filed: 11/29/1996
• Issued: 06/29/1999
• Est. Priority Date: 11/29/1996
• Status: Expired due to Term

• Current Assignee: Cognex Corporation
• Original Assignee: Cognex Corporation
• Inventors: Jacobson, Lowell D.
• Primary Examiner(s): Barlow, John
• Assistant Examiner(s): MILLER, CRAIG S
• Application Number:
  US08/758,426
• Time in Patent Office: 942 Days
• Field of Search: 117/13-15, 117/20, 117/201, 117/202, 702/94, 702/97, 702/150, 702/152, 702/153, 702/157, 702/158, 702/159-166
• US Class Current: 702/97
• CPC Class Codes: C30B 15/26 using television detectors; using photo or X-ray detectors

Abstract

The invention provides a method for visually monitoring the radius of an item which is rotating about a fixed axis and which has a trackable contour known to lie in a plane normal to the rotation, such as a part turned on a lathe. The present invention can also estimate the cross-sectional diameter of a growing crystal, and the height of the cross-section above the melt surface. In addition, the height and radius of the meniscus at the crystal/melt interface can be tracked by the system of the invention. The present makes it possible to further automate crystal growing processes in a manner that increases manufacturing efficiency, consistency, and overall quality. In general, the invention provides a machine vision method for estimating both a longitudinal position and a radius of a circular cross-sectional feature of a solid of revolution. The method includes the steps of acquiring an image of a circumferential feature of the solid of revolution using a camera having a focal length and disposed at a position characterized by a plurality of position parameters, and then finding a set of points along the image of the circumferential feature. The next step is determining an equation of an ellipse that substantially fits the set of points to provide a plurality of ellipse parameters, and then computing the longitudinal position and the radius of the circular cross-section of the solid of revolution using the focal length, the plurality of position parameters, and the plurality of ellipse parameters.

Claims

1. A machine vision method for estimating both a longitudinal position and a radius of a circular cross-section of a solid of revolution, the method comprising the steps of:

acquiring an image of a circumferential feature of the solid of revolution using a camera having a focal length and disposed at a position characterized by a plurality of position parameters;

finding a set of points along the image of the circumferential feature;

determining an equation of an ellipse that substantially fits the set of points to provide a plurality of ellipse parameters; and

computing the longitudinal position and the radius of the circular cross-section of the solid of revolution using the focal length, the plurality of position parameters, and the plurality of ellipse parameters.

2. The method of claim 1, where the step of finding a set of points along the image of the circumferential feature comprises the steps of:

creating a hypothetical ellipse at a location in the image;

creating a plurality of edge detection windows, each edge detection window located so as to include a portion of the hypothetical ellipse; and

within each edge detection window, locating an edge so as to provide an edge position representing a point along the image of the circumferential feature.

3. The method of claim 1, wherein the step of determining an equation of an ellipse that substantially fits the set of points comprises the step of:

minimizing a fitting error using a numerical optimization technique.

4. The method of claim 3, wherein the numerical optimization technique is a technique that minimizes the square of the fitting error.

5. The method of claim 1, wherein the step of computing the longitudinal position and the radius of the circular cross-section of the solid of revolution further comprises the step of:

using only one of the major axis length and the minor axis length.

6. The method of claim 1, further comprising the step of:

repeating the previous steps to provide a plurality of recent longitudinal position values and radius values;

averaging the plurality of recent longitudinal position values and radius values to provide an average recent longitudinal position value and the average recent radius value;

periodically reporting the average recent longitudinal position value and the average recent radius value of the solid of revolution.

7. The method of claim 1, wherein the step of acquiring an image of a circumferential feature of the solid of revolution further comprises the step of:

first projecting structured light onto the surface of the solid of revolution so as to create the circumferential feature.

9. The method of claim 1, where the step of finding a set of points along the image of the circumferential feature comprises the steps of:

creating a hypothetical ellipse at a location in the image;

creating a plurality of edge detection windows, each edge detection window located so as to include a portion of the hypothetical ellipse; and

within each edge detection window, locating an edge so as to provide an edge position representing a point along the image of the circumferential feature.

8. A machine vision method for estimating both a longitudinal position and a radius of a circular cross-section of a solid of revolution, the method comprising the steps of:

acquiring an image of a circumferential feature of the solid of revolution using a camera having a focal length and disposed at a position characterized by a plurality of position parameters;

finding a set of points along the image of the circumferential feature;

determining an equation of an ellipse that substantially fits the set of points to provide a plurality of ellipse parameters;

computing the longitudinal position and the radius of the circular cross-section of the solid of revolution using the focal length, the plurality of position parameters, and the plurality of ellipse parameters;

repeating the previous steps to provide a plurality of recent longitudinal position values and radius values;

averaging the plurality of recent longitudinal position values and radius values to provide an average recent longitudinal position value and the average recent radius value; and

periodically reporting the average recent longitudinal position value and the average recent radius value of the solid of revolution.

10. A machine vision method for estimating both a melt level and a radius of a circular cross-section of a growing semiconductor crystal, the method comprising the steps of:

acquiring an image of a circumferential feature of the growing semiconductor crystal using a camera having a focal length and disposed at a position characterized by a plurality of position parameters;

finding a set of points along the image of the circumferential feature;

determining an equation of an ellipse that substantially fits the set of points to provide a plurality of ellipse parameters; and

computing the longitudinal position and the radius of the circular cross-section of the semiconductor crystal using the focal length, the plurality of position parameters, and the plurality of ellipse parameters.

11. The method of claim 10, where the step of finding a set of points along the image of the circumferential feature comprises the steps of:

creating a hypothetical ellipse at a location in the image;

creating a plurality of edge detection windows, each edge detection window located so as to include a portion of the hypothetical ellipse; and

within each edge detection window, locating an edge so as to provide an edge position representing a point along the image of the circumferential feature.

12. The method of claim 10, wherein the step of determining an equation of an ellipse that substantially fits the set of points comprises the step of:

minimizing a fitting error using a numerical optimization technique.

13. The method of claim 12, wherein the numerical optimization technique is a technique that minimizes the square of the fitting error.

14. The method of claim 10 wherein the step of computing the longitudinal position and the radius of the circular cross-section of the semiconductor crystal comprises the step of:

using only one of the major axis length and the minor axis length.

15. The method of claim 10, further comprising the step of:

repeating the previous steps to provide a plurality of recent longitudinal position values and radius values;

averaging the plurality of recent longitudinal position values and radius values to provide an average recent longitudinal position value and the average recent radius value;

periodically reporting the average recent longitudinal position value and the average recent radius value of the semiconductor crystal.

16. The method of claim 10, wherein the step of acquiring an image of a circumferential feature of the semiconductor crystal further comprises the step of:

first projecting structured light onto the surface of the semiconductor crystal so as to create the circumferential feature.

17. A machine vision method for estimating both a longitudinal position and a radius of a circular cross-section of a solid of revolution, the method comprising the steps of:

acquiring an image of a circumferential feature of the solid of revolution using a camera having a focal length and disposed at a position characterized by a plurality of position parameters;

finding a set of points along the image of the circumferential feature;

determining an equation of an ellipse that substantially fits the set of points to provide a plurality of ellipse parameters;

computing the longitudinal position and the radius of the circular cross-section of the solid of revolution using the focal length, the plurality of position parameters, and the plurality of ellipse parameters;

repeating the previous steps to provide a plurality of recent longitudinal position values and radius values;

averaging the plurality of recent longitudinal position values and radius values to provide an average recent longitudinal position value and the average recent radius value; and

periodically reporting the average recent longitudinal position value and the average recent radius value of the solid of revolution.

18. The method of claim 17, where the step of finding a set of points along the image of the circumferential feature comprises the steps of:

creating a hypothetical ellipse at a location in the image;

creating a plurality of edge detection windows, each edge detection window located so as to include a portion of the hypothetical ellipse; and

within each edge detection window, locating an edge so as to provide an edge position representing a point along the image of the circumferential feature.