Finding fiducials on printed circuit boards to sub pixel accuracy

Claim 1

A method for locating a circular or symmetrical convex polygonal fiducial of known dimensions on a circuitized substrate comprising the steps of: determining fiducial presence by; acquiring a two dimensional digitized image array; selecting a subarray representing a predetermined area within which a fiducial is expected; calculating summation profiles of rows and columns in said selected subarray; constructing a first difference profile corresponding to each calculated summation profile; and screening said first difference profiles for choosing presumptive leading and trailing edges of the fiducial; finding approximate fiducial center by taking midpoints between presumptive leading edges and trailing edges; and calculating fiducial center to sub pixel accuracy by selecting from said two dimensional array of digitized image data, four two dimensional subarrays located a predetermined distance on either side of the X and Y coordinates found in said fining step; determining first differences for each of said four subarrays; choosing maximum and minimum values for X from said firt differences from subarrays located on either side of the X value found in said finding step; choosing maximum and minimum values for Y first differences from said subarrays on either side of the Y value found in said finding step; and averaging chosen values for X and Y, respectively, to find exact X and Y values.

Claim 2

A method for locating a circular or symmetrical convex polygonal fiducial of known dimensions on a circuitized substrate comprising the steps of: determining fiducial presence by; acquiring a two dimensional digitized image array; selecting a subarray representing a predetermined area within which a fiducial is expected; calculating summation profiles of rows and columns in said selected subarray; constructing a first difference profile corresponding to each calculated summation profile; and screening said first difference profiles for choosing presumptive leading and trailing edges of the fiducial; finding approximate fiducial center by taking midpoints between presumptive leading edges and trailing edges; and calculating fiducial center to sub pixel accuracy by selecting from said two dimensional array of digitized image data, four two dimensional subarrays located a predetermined distance on either side of the X and Y coordinates found in said fining step; determining first differences for each of said four subarrays; choosing maximum and minimum values for X from said firt differences from subarrays located on either side of the X value found in said finding step; choosing maximum and minimum values for Y first differences from said subarrays on either side of the Y value found in said finding step; and averaging chosen values for X and Y, respectively, to find exact X and Y values.

Claim 5

A method of locating a fiducial center to sub pixel accuracy comprising the steps of: finding leading and trailing edges and apprxoimate center coordinates through analysis of one dimensional first differences of summation profile arrays, said finding step including; obtaining a digitized image array of whole scene; examining a predetermined two dimensional subarray wherein object is expected; forming one dimensional summation profiles for rows and columns of pixels in said subarray; calculating row summations of pixel values in said subarray for structuring a one dimensional profile array; calculating column summations of pixel values in said subarray for structuring a one dimensional profile array; assuming leading edge of the object to correspond to a high positive difference and trailing edge to correspond to a low negative difference in each on dimensional profile array; establishing approximate X, Y center coordinates, at points midway between said high and low differences; determining exact center coordinates through analysis of four narrowly constrained, two dimensional pixel arrays whose locations are chosen as a function of distance from approximate center coordinates, said determining step including; examining four subarrays of pixel data selected as a function of their location in relation to established apprxoimate X, Y center coordinates; finding first differences in each subarray; and averaging maximum and minimum values in each subarray.

Claim 6

A method of locating a fiducial center to sub pixel accuracy comprising the steps of: finding leading and trailing edges and apprxoimate center coordinates through analysis of one dimensional first differences of summation profile arrays, said finding step including; obtaining a digitized image array of whole scene; examining a predetermined two dimensional subarray wherein object is expected; forming one dimensional summation profiles for rows and columns of pixels in said subarray; calculating row summations of pixel values in said subarray for structuring a one dimensional profile array; calculating column summations of pixel values in said subarray for structuring a one dimensional profile array; assuming leading edge of the object to correspond to a high positive difference and trailing edge to correspond to a low negative difference in each on dimensional profile array; establishing approximate X, Y center coordinates, at points midway between said high and low differences; determining exact center coordinates through analysis of four narrowly constrained, two dimensional pixel arrays whose locations are chosen as a function of distance from approximate center coordinates, said determining step including; examining four subarrays of pixel data selected as a function of their location in relation to established apprxoimate X, Y center coordinates; finding first differences in each subarray; and averaging maximum and minimum values in each subarray.

Claim 7

A method of precisely locating an object in a field of view comprising the steps of: obtaining a digitized image array of a whole scene in the field of view; examining a predetermined two dimensional subarray of said digitized image array wherein an object is expected; forming one dimensional summation profiles for rows and columns of pixels in said subarray by calculating row summations of pixel values in said subarray for structuring a one dimensional profile array, and calculating column summations of pixel values in said subarray for structuring a one dimensional profile array; assuming the object's leading edge to correspond to a high positive difference and its trailing edge to correspond to a low negative difference in each one dimensional profile array; establishing approximate X, Y center coordinates, at points midway between said high and low differences; examining four subarrays of pixel data selected as a function of their location in relation to established approximate X, Y center coordinates; finding first differences in each subarray; and averaging maximum and minimum values in each subarray.

Claim 8

A method of precisely locating an object in a field of view comprising the steps of: obtaining a digitized image array of a whole scene in the field of view; examining a predetermined two dimensional subarray of said digitized image array wherein an object is expected; forming one dimensional summation profiles for rows and columns of pixels in said subarray by calculating row summations of pixel values in said subarray for structuring a one dimensional profile array, and calculating column summations of pixel values in said subarray for structuring a one dimensional profile array; assuming the object's leading edge to correspond to a high positive difference and its trailing edge to correspond to a low negative difference in each one dimensional profile array; establishing approximate X, Y center coordinates, at points midway between said high and low differences; examining four subarrays of pixel data selected as a function of their location in relation to established approximate X, Y center coordinates; finding first differences in each subarray; and averaging maximum and minimum values in each subarray.