Method and apparatus for automatically determining sheet metal strain

Claim 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.

Claim 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 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.

Claim 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 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.

Claim 4

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.

Claim 5

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.

Claim 6

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.