Three-dimensional optical measurement method for ropes or cables and system

A method for three-dimensional measurement of geometric parameters of a rope or a cable having an outer surface, the method comprising the steps of:

• a1) capturing a first digital image of a first region of the outer surface of the rope or cable;
• b1) capturing a second digital image of a second region of the outer surface of the rope or cable, said second region being at least partially distinct from said first region;
• c1) determining a first and a second series of contour lines respectively on said first digital image and said second digital image of said first region and said second region of the outer surface of the rope or cable, wherein said first and second series of contour lines comprise respectively a first plurality of image contour points and a second plurality of image contour points, wherein at least a part of the first series and a part of the second series of contour lines respectively delimit a first area and a second area of digital image of said first digital image and said second digital image, the method further comprising:
• c2) calculating a first mid-axis and a second mid-axis in each first and second series of contour lines, wherein said first mid-axis and said second mid-axis are obtained as a regression that best approximates respectively at least a part of the first plurality of image contour points and at least a part of the second plurality of image contour points, and wherein said first mid-axis and said second mid-axis subdivide respectively said first area and said second area of digital image in a respective first sub-area and second sub-area, and the following steps:
• a4) defining lines of separation, on said first digital image and said second digital image of said first region and said second region of the outer surface of the rope or cable, wherein said lines of separation delimit contiguous areas of said first digital image and said second digital image that follow one another along a direction substantially parallel with respect to the first mid-axis or the second mid-axis and cross said first mid-axis or second mid-axis from the first sub-area to the second sub-area;
• b4) identifying an intersection point between the lines of separation and the first mid-axis and/or the second mid-axis;
• c4) searching for a homologous intersection point in such a way that said homologous intersection point represents a homologous point of said intersection point and that said intersection point and homologous intersection point each represent an image of a point common to the first region and second region of the outer surface of the rope or cable;
• d4) back-projecting photogrammetrically the intersection point and the homologous intersection point in a three-dimensional space so as to obtain a 3D intersection point referring to the three-dimensional space;
• e4) repeating steps a4) to d4) a plurality of times until a three-dimensional representation of a plurality of 3D intersection points belonging to the first region and second region of the outer surface of the rope of cable is obtained;
• and
• f4) calculating a distance between at least one first 3D intersection point and at least one second 3D intersection point, said at least one second 3D intersection point being adjacent to the at least one first 3D intersection point.

A calibrated three-dimensional optical measurement system, for measurement of geometric parameters of a rope or a cable, said calibrated three-dimensional optical measurement system comprising:

• a plurality of digital image capture devices suitable for capturing a multiplicity of digital images of at least one region of an external surface of the rope or cable, and
• a digital image processing device configured to perform the steps of
• a method for three-dimensional measurement of geometric parameters of a rope or a cable having an outer surface, the method comprising:
• a1) capturing a first digital image of a first region of the outer surface of the rope or cable;
• b1) capturing a second digital image of a second region of the outer surface of the rope or cable, said second region being at least partially distinct from said first region;
• c1) determining a first and a second series of contour lines respectively on said first digital image and said second digital image of said first region and said second region of the outer surface of the rope or cable, wherein said first and second series of contour lines comprise respectively a first plurality of image contour points and a second plurality of image contour points;
• d1) searching for a first contour point and a second contour point belonging respectively to said first plurality of image contour points and said second plurality of image contour points, so that the first contour point and the second contour point are homologous points or points belonging to a same epipolar line, each contour point representing an image of a surface point, said surface point being a point in common with the first region and the second region of the outer surface of the rope or cable;
• e1) back-projecting photogrammetrically the first contour point and the second contour point in a three-dimensional space, so as to obtain a 3D contour point referring to said three-dimensional space;
• f1) repeating steps a1) to e1) a plurality of times until a three-dimensional representation of at least one first plurality of 3D contour points and at least one second plurality of 3D contour points referring to said three-dimensional space is obtained;
• g1) calculating by the at least one first plurality of 3D contour points and/or the at least one second plurality of 3D contour points at least one geometric parameter of the rope or cable selected from:
• diameter of the rope or cable, roundness of the rope or cable, and axis of the rope or cable;
• wherein at least a part of the first series of contour lines and a part of the second series of contour lines respectively delimit a first area and a second area of digital image of said first and second digital images, said method further comprising the following steps for calculating said axis of the rope or cable:
• c2) calculating a first mid-axis and a second mid-axis in each first and second series of contour lines, wherein said first mid-axis and said second mid-axis are obtained as a regression that best approximates respectively at least a part of the first plurality of image contour points and at least a part of the second plurality of image contour points, and wherein said first mid-axis and said second mid-axis subdivide respectively said first area and said second area of digital image in a respective first sub-area and second sub-area;
• d2) searching for a first midpoint and a second midpoint belonging to said first mid-axis and said second mid-axis respectively, in such a way that the first midpoint belongs to the same epipolar line as the second midpoint and that the first and second midpoints represent a virtual image of a point belonging to a 3D mid-axis of the rope or cable;
• e2) back-projecting photogrammetrically the first and the second midpoints in the three-dimensional space, in such a way to obtain a 3D midpoint in said three-dimensional space;
• and
• f2) repeating steps c2) to e2) a plurality of times until the three-dimensional representation of a plurality of 3D midpoints representing the points of the axis of the rope or cable is obtained;
• or of a method for three-dimensional measurement of geometric parameters of a rope or a cable having an outer surface, the method comprising:
• a1) capturing a first digital image of a first region of the outer surface of the rope or cable;
• b1) capturing a second digital image of a second region of the outer surface of the rope or cable, said second region being at least partially distinct from said first region;
• c1) determining a first and a second series of contour lines respectively on said first digital image and said second digital image of said first region and said second region of the outer surface of the rope or cable, wherein said first and second series of contour lines comprise respectively a first plurality of image contour points and a second plurality of image contour points, wherein at least a part of the first series and a part of the second series of contour lines respectively delimit a first area and a second area of digital image of said first digital image and said second digital image, the method further comprising:
• c2) calculating a first mid-axis and a second mid-axis in each first and second series of contour lines, wherein said first mid-axis and said second mid-axis are obtained as a regression that best approximates respectively at least a part of the first plurality of image contour points and at least a part of the second plurality of image contour points, and wherein said first mid-axis and said second mid-axis subdivide respectively said first area and said second area of digital image in a respective first sub-area and second sub-area, and the following steps:
• a4) defining lines of separation, on said first digital image and said second digital image of said first region and said second region of the outer surface of the rope or cable, wherein said lines of separation delimit contiguous areas of said first digital image and said second digital image that follow one another along a direction substantially parallel with respect to the first mid-axis or the second mid-axis and cross said first mid-axis or second mid-axis from the first sub-area to the second sub-area;
• b4) identifying an intersection point between the lines of separation and the first mid-axis and/or the second mid-axis;
• c4) searching for a homologous intersection point in such a way that said homologous intersection point represents a homologous point of said intersection point and that said intersection point and homologous intersection point each represent an image of a point common to the first region and second region of the outer surface of the rope or cable;
• d4) back-projecting photogrammetrically the intersection point and the homologous intersection point in a three-dimensional space so as to obtain a 3D intersection point referring to the three-dimensional space;
• e4) repeating steps a4) to d4) a plurality of times until a three-dimensional representation of a plurality of 3D intersection points belonging to the first region and second region of the outer surface of the rope of cable is obtained;
• and
• f4) calculating a distance between at least one first 3D intersection point and at least one second 3D intersection point, said at least one second 3D intersection point being adjacent to the at least one first 3D intersection point.