Method and apparatus for automatically determining sheet metal strain

• US 4,288,852 A
• Filed: 11/28/1979
• Issued: 09/08/1981
• Est. Priority Date: 11/28/1979
• Status: Expired due to Term

• Current Assignee: General Motors Corporation (Motors Liquidation Co. GUC Trust)
• Original Assignee: General Motors Corporation (Motors Liquidation Co. GUC Trust)
• Inventors: Holland, Steven W.
• Primary Examiner(s): Wise, Edward J.
• Application Number:
  US06/098,260
• Time in Patent Office: 650 Days
• Field of Search: 364/508, 364/507, 364/515, 364/570, 364/300, 364/490, 364/488, 364/489, 364/491, 73/760, 73/783, 340/146.3 AH, 340/146.3 AC
• US Class Current: 382/152
• CPC Class Codes: G01B 11/024 by means of diode-array scanning
• CPC Class Codes: G01B 11/165 by means of a grating deformed by the object
• CPC Class Codes: G06T 7/60 Analysis of geometric attributes

Abstract

Disclosed is a method and apparatus for automatic measurement of strain in a formed sheet metal sample. Prior to the forming operation, a grid of circles is imprinted on the sample and during the forming operation the circles are stretched to ellipses. The formed sample is exposed to an instrumentation camera and a digitized image of the pattern on the sample is stored in a digital computer. The computer is programmed to segment the image to distinguish the elliptical patterns from the background; to extract boundary points on an elliptical pattern and fit these points to an ellipse. Strain is determined as a function of the diameters of the fitted ellipse as well as the chord lengths of the actual pattern along the axes of the fitted ellipse.

Claims

1. A method of automatically determining the strain induced in a sheet metal sample by a forming operation, said sample having a grid of circles of predeterming diameter printed thereon prior to said operation, which circles are stretched into patterns of generally elliptical shape as a result of said operation, said method comprising the steps of:

obtaining a digital representation of the image of a portion of the sample including at least one of said patterns, which digital representation is a sequence of integers, each integer corresponding to the average light intensity of the image at discrete image element locations,

storing said integers in addressable locations in the memory of a digital computer, said computer being programmed to perform the steps of;

(a) segmenting the image into binary regions, one region type comprising contiguous image elements whose light intensity levels are less than a threshold level and the other region type comprising contiguous image elements whose light intensity levels are equal to or greater than said threshold level;

(b) identifying a plurality of points residing on the boundary of one region type which is totally within the field or view defined by the image and which is greater than a predetermined size;

(c) fitting an ellipse to said points;

(d) calculating a diameter of the fitted ellipse and;

(e) calculating the strain induced in the sample as a function of said calculated diameter and the predetermined diameter of said circles.

2. A method of automatically determining the strain induced in a sheet metal sample by a forming operation, said sample having a grid of circles of predetermining diameter printed thereon prior to said operation, which circles are stretched into patterns of generally elliptical shape as a result of said operation, said method comprising the steps of:

obtaining a digital representation of the image of a portion of the sample including at least one of said patterns, which digital representation is a sequence of integers, each integer corresponding to the average light intensity of the image at discrete image element locations,

storing said integers in addressable locations in the memory of a digital computer, said computer being programmed to perform the steps of;

(a) segmenting the image into binary regions, one region type comprising contiguous image elements whose light intensity levels are less than a threshold level and the other region type comprising contiguous image elements whose light intensity levels are equal to or greater than said threshold level;

(b) identifying a plurality of points residing on the boundary of one region type which is totally within the field of view defined by the image and which is greater than a predetermined size;

(c) fitting an ellipse to said points;

(d) calculating the distance between two of said points lying on one of the axes of the fitted ellipse and;

(e) calculating the strain induced in the sample as a function of said distance and the predetermined diameter of said circles.

3. A method of automatically determining the strain induced in a sheet metal sample by a forming operation, said sample having a grid of circles of predetermining diameter printed thereon prior to said operation, which circles are stretched into patterns of generally elliptical shape as a result of said operation, said method comprising the steps of:

obtaining a digital representation of the image of a portion of the sample including at least one of said patterns, which digital representation is a sequence of integers, each integer corresponding to the average light intensity of the image at discrete identified image element locations,

storing said integers in addressable locations in the memory of a digital computer, said computer being programmed to perform the steps of;

(a) determining from selected groups of said integer values a threshold level of light intensity which is a gradient-weighted average light intensity in transition areas of the image where the gradient of light intensity exceeds a predetermined value;

(b) segmenting the image into binary regions, one region type comprising contiguous image elements whose light intensity levels are less than said weighted average and the other region type comprising contiguous image elements whose light intensity levels are equal to or greater than said weighted average;

(c) identifying the location of a plurality of points residing on the boundary of one region which is totally within the field of view defined by the image and is greater than a predetermined size;

(d) fitting an ellipse to said points;

(e) identifying those points within a predetermined tolerance distance from the fitted ellipse and refitting an ellipse to the points so identified, reducing said tolerance distance and repeating this step (e) a predetermined number of times or until a predetermined root-mean-square percent error is attained;

(f) calculating at least one of the major and minor diameters of the ellipse fitted in step (e) and;

(g) calculating the strain induced in the sample as a function of the calculated diameter and the predetermined diameter of said circles.

4. Apparatus for automatically determining the strain induced in a sheet metal sample by a forming operation, said sample having a grid of circles of predetermining diameter printed thereon prior to said operation, which circles are stretched into patterns of generally elliptical shape as a result of said operation, said apparatus comprising:

means for obtaining a digital representation of the average light intensity of discrete elements of the image of a portion of the sample including at least one of said patterns;

digital computer means for storing said digital representation in addressable memory locations, said computer being programmed to;

(a) determine a threshold level of light intensity which is a gradient-weighted average light intensity in transition areas of the image where the gradient of light intensity exceeds a predetermined value;

(b) segment the image into binary regions, one region type comprising contiguous image elements whose light intensity levels are less than said threshold and the other region type comprising contiguous image elements whose light intensity levels are equal to or greater than said threshold;

(c) identify a plurality of points residing on the boundary of one region type which is totally within the field of view defined by the image and is greater than a predetermined size;

(d) fit an ellipse to said points;

(e) identify those points within a predetermined tolerance distance from the fitted ellipse and refit an ellipse to the points so identified, reduce said tolerance distance and repeat this step (e) a predetermined number of times or until a predetermined root-mean-square percent error is attained;

(f) calculate at least one of the diameters of the fitted ellipse and;

(g) calculate the strain induced in the sample as a function of the calculated diameter and the predetermined diameter of said circles.

5. Apparatus for automatically determining the strain induced in a sheet metal sample by a forming operation, said sample having a grid of circles of predetermining diameter printed thereon prior to said operation, which circles are stretched into patterns of generally elliptical shape as a result of said operation, said apparatus comprising:

means for obtaining a digital representation of the average light intensity of discrete elements of the image of a portion of the sample including at least one of said patterns;

digital computer means for storing said digital representation in addressable memory locations, said computer being programmed to;

(a) segment the image into binary regions, one region type comprising contiguous image elements whose light intensity levels are less than a threshold and the other region type comprising contiguous image elements whose light intensity levels are equal to or greater than a threshold;

(b) identify a plurality of points residing on the boundary of one region type which is totally within the field of view defined by the image and is greater than a predetermined size;

(c) fit an ellipse to said points;

(d) calculate at least one of the diameters of the fitted ellipse and;

(e) calculate the strain induced in the sample as a function of the calculated diameter and the predetermined diameter of said circles.