Abstract:
A method and apparatus for automating the measurement of straightness of linear stock material produced by the operation of a mill. As the linear stock material exits from the mill, sensors acquire a sequence of image or distance measurement pairs associated with the material at discrete longitudinal points over a segment of the material. A processing system utilizes the data pairs to determine a set of centroids for a segment of the material. A virtual axis is identified between centroids associated with the segment, and the processing system determines a measure of deviation of each remaining centroid in a segment from the virtual axis to identify a degree of concavity of the material within the segment, as well as to identify a measure of the angular orientation of the concavity about the virtual axis.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   Not Applicable. 
   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
   Not Applicable. 
   BACKGROUND OF THE INVENTION 
   This invention relates in general to the production of linear stock material such as tube stock material and round bar stock material, and more particularly to an machine vision method and apparatus for the automated determination of the straightness of linear stock material during production. 
   Ideally, tube stock and round bar stock materials, generally referred to herein as linear stock material, should emerge from the mills in which they are produced in a perfectly straight configuration for the entire production length. But typically some deviation from perfectly straight exists in the linear stock material, which is acceptable for most purposes so long as the deviation is within prescribed tolerances. Currently mill operators conduct visual inspections to determine if sections of the linear stock material being produced by a mill falls within the prescribed tolerances. But this leaves much of the produced material without a quantitative inspection, and fails to detect deviation trends that could be corrected by making adjustment in the operation of the mill. 
   The typical inspection of a length of linear stock material, which may be between 4.5 meters to 12.0 meters in length, involves removing it from the mill in which it is produced and placing it on a flat table or level rails. There, an inspector using the flat surface of the table, determines the direction of any bow present in the linear stock material, and on which side of the bow is most pronounced—that is to say, the inspector determines the angular orientation, about the circumference of the linear stock material, the bow creates the greatest concavity. 
   Having located the side with the greatest concavity, the inspector places a precision straight edge, which can be typically one meter in length, against that side of the linear stock material. Any deviation from straight appears as a gap between the surface of the linear stock material and the straight edge, and is usually measured with a feeler gauge between the ends of the precision straight edge. The procedure is repeated for several segments along the length of the tube stock. Tube stock material and round bar stock material undergo similar inspection procedures for determining the presence of any deviation from an ideal straightness. 
   If quantitative inspection results are required, the typical inspecting procedure consumes a considerable amount of time, and without incurring inordinate expense, cannot be performed on all points of the material from a full production run at a mill. Thus, quantitative inspections are performed only on linear stock material selected using a screening method from a production run of a mill. 
   BRIEF SUMMARY OF THE INVENTION 
   Briefly stated, an embodiment of the present invention provides a method for automating the measurement of straightness of linear stock material produced by the operation of a mill. As the linear stock material exits from the mill, a sequence of image pairs of the material is acquired at discrete longitudinal points over a segment of the material. Each image pair includes images acquired along fields of view aligned perpendicularly to the material and which are preferably orthogonal to each other. Using the image pairs, a centroid for the material is determined for each segment end point and at least one intermediate segment point. A virtual axis is identified between each segment end point centroid. A measure of deviation of each intermediate centroid in a segment from the virtual axis identifies a degree of concavity of the material within the segment, as well as a measure of the angular orientation of the concavity about the virtual axis. 
   An alternate embodiment of the present invention provides a method for automating the measurement of straightness of linear stock material produced by the operation of a mill. As the linear stock material exits from the mill, measurement pairs of distances to the material are acquired at discrete longitudinal points over a segment of the material. Each measurement pair includes two measures of distance to the surface of the material acquired along fields of view aligned perpendicularly to the exterior surface of the material and which are angled relative to each other. Using the measured distances, the known diameter of the material, and the known configuration of the measurement system, a centroid for the material is determined for each segment end point and at least one intermediate segment point. A virtual axis is identified between each segment end point centroid. A measure of deviation of each intermediate centroid in a segment from the virtual axis identifies a degree of concavity of the material within the segment, as well as a measure of the angular orientation of the concavity about the virtual axis. 
   An alternate embodiment of the present invention provides an apparatus for automating the measurement of straightness of linear stock material produced by the operation of a mill. A pair of imaging sensors are disposed to acquire images of the linear stock material as it exits from a mill. The imaging sensors are disposed to have fields of view which are perpendicular to the material, and which are preferably orthogonal to each other within a common cross-sectional plane of the linear stock material. A processing system is configured to evaluate an image pair comprising images of the material acquired by each imaging sensor for a discrete point on the material, and to determine a centroid of the material at that discrete point. The processing system is further configured to establish a virtual longitudinal axis for a segment of material from the locations of determined centroids at each end of the segment, and to identify a degree of concavity of the linear stock material within the segment, as well as a measure of the angular orientation of the concavity about the virtual longitudinal axis, from at least one determined centroid disposed within the segment of material. 
   An alternate embodiment of the present invention provides an apparatus for automating the measurement of straightness of linear stock material produced by the operation of a mill. A pair of distance measurement sensors are disposed to acquire images of the linear stock material as it exits from a mill via a longitudinal conveyance device. The distance measurement sensors are disposed to acquire distance measurements along axis which are perpendicular to the material, and which are aligned at an angle relative to each other within a common cross-sectional plane of the linear stock material. A processing system is configured to evaluate an distance measurements to the exterior surface of the material acquired by each distance measurement sensor for a discrete cross-sectional plane of the material, and to determine a centroid of the material at that discrete point. The processing system is further configured to establish a virtual longitudinal axis for a segment of material from the locations of determined centroids at each end of the segment, and to identify a degree of concavity of the linear stock material within the segment, as well as a measure of the angular orientation of the concavity about the virtual longitudinal axis, from at least one determined centroid disposed within the segment of material. 
   The foregoing and other objects, features, and advantages of the invention as well as presently preferred embodiments thereof will become more apparent from the reading of the following description in connection with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       FIG. 1  is a perspective view of an embodiment of the measurement system of the present invention disposed to measure linear stock material from a mill as it is conveyed through the measurement system; 
       FIG. 2  is a perspective view illustrating imaging components of an embodiment present invention and associated fields of view in relationship to a segment of linear stock material; 
       FIG. 3A  is a exemplary illustration of an image and field-of-view of the linear stock material acquired by a Y-axis imaging component of  FIG. 2 ; 
       FIG. 3B  is a exemplary illustration of an image and field-of-view of the linear stock material acquired by an X-axis imaging component of  FIG. 2 ; 
       FIG. 4  is a perspective view illustrating exemplary placement of backlights relative to an imaging component and a segment of linear stock material; 
       FIG. 5  is an illustration of a minimal set of calculated centroids for a segment of linear stock material; 
       FIG. 6  is an illustration of a set of calculated centroids for the segment of  FIG. 5 ; and 
       FIG. 7  is a cross-sectional view of a segment of linear stock material illustrating an alternate embodiment distance measurement system of the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Turning to  FIG. 1 , a length of linear stock material  10  exiting a mill production line  12  is passed through a non-contact straightness measurement system of the present invention, shown generally at  100 . In one embodiment, the measurement system  100  comprises at least three imaging modules, shown generally at  102 , which are disposed in a linearly spaced configuration along a common longitudinal axis corresponding to the direction of travel for the linear stock material  10 . Preferably, each imaging module  102  is spaced apart by approximately ½ the straightness reference length. 
   Each imaging module  102  includes a pair of orthogonally disposed imaging sensors  104 X and  104 Y, each having a field of view aligned with a cross-sectional plane P of the linear stock material  10  passing through the imaging module  102 , as best seen in  FIG. 2 . The specific angular orientation of the pair of imaging sensors  104 X and  104 Y about the longitudinal axis of the linear stock material  10  is not limited to that shown in  FIG. 2 , and those of ordinary skill in the art will recognize that the pair of imaging sensors may have a different angular orientation, and that the relationship between each imaging sensor  104 X and  104 Y does not have to be orthogonal. 
   Each imaging sensor  104 X and  104 Y includes conventional optics, imaging components, and image processing components. For example, a pin-hole lens or optical lens, a CMOS or CCD image array, and associated electronic circuits. Each imaging sensor  104 X and  104 Y is capable of acquiring an image of a portion of an exterior surface of the linear stock material  10 , including visible edges, within an associated field of view. For example, as shown in  FIGS. 2 and 3A , imaging sensor  104 X is disposed within the imaging module  102  to acquire an image of an exterior surface of the linear stock material  10  visible along the X-axis within a field of view FOV(X), while the imaging sensor  104 Y is disposed within the imaging module  102  to acquire an image of an exterior surface of the linear stock material  10  visible along the Y-axis within a field of view FOV(Y). 
   To facilitate the acquisition of the images of the linear stock material  10  by each imaging module  102 , a variety of illumination techniques may be utilized. Preferably, backlighting illumination is utilized, such that the exterior surface of the linear stock material  10  appears dark relative to a highly illuminated background, providing a sharp contrast between the observed light background and dark exterior surface of the linear stock material  10  in each image. For example, as shown in  FIG. 4 , a pair of fluorescent bulbs  105 , having a length which is greater than the width of the linear stock material  10  may be disposed on the opposite side of the linear stock material  10  from an individual imaging sensor  104 X or  104 Y. Preferably, the fluorescent bulbs  105  are offset from the optical axis of the imaging sensors, providing illumination for multiple imaging sensors, and to reduce the occurrence of shadow effects in the illumination of the linear stock material peripheral edges. Those of ordinary skill in the art will recognize that other illumination techniques, including front-lighting techniques, may be utilized which facilitate the identification of the peripheral edges of the linear stock material  10  in images acquired by each imaging module  102 , and that for some applications, supplemental illumination may not be required. 
   Images acquired simultaneously by each imaging sensor  104 X and  104 Y in each imaging module  102  are communicated to a processing system  106 , wherein the edges of the linear stock material  10  in each image are identified. The processing system  106  preferably include at least one central processing unit having sufficient computation capacity, and configured with a software application, to carry out the functions of the present invention. Utilizing the identified edges in each image, together with known parameters of the measurement system, the processing system  106  calculates one or more dimensions L(x) and L(y) of the stock material on the X and Y axis, respectively. The calculated dimensions are further utilized by the processing system  106  to determine at least a three-dimensional coordinate of a centroid C associated with each cross-sectional plane P of the linear stock material  10  imaged by an associated imaging module  102 . Additional characteristics or features of the linear stock material  10 , such as an outer diameter or a degree of roundness, may be calculated by the central processing system  106 . 
   Once a set of centroids C, including two end centroids and at least one intermediate centroid, have been calculated by the central processing system  106  for a segment  10 A of the linear stock material  10 , the central processing system  106  establishes at least virtual longitudinal axis VA for the segment  10 A between two of the calculated centroids C. Preferably, as shown in  FIGS. 5 and 6 , the virtual longitudinal axis VA is established between the outermost or end centroids C in the segment  10 A, however, those of ordinary skill in the art will recognize that a virtual longitudinal axis VA may be established using any two calculated centroids C. Similarly, the virtual longitudinal axis VA may be established by the central processing system  106  utilizing a linear best-fit algorithm and more than two of the calculated centroids C. 
   Once the virtual longitudinal axis VA is established for a segment  10 A of the linear stock material  10 , the central processing system  106  calculates a displacement of each remaining centroid C in the segment  10 A from the virtual longitudinal axis VA. The calculated displacement represents a measure of the straightness or curvature (concavity) of the linear stock material  10  at that point within the segment  10 A. Those of ordinary skill in the art will recognize that sensitivity of the measurement system  100  to curvature in a segment  10 A of linear stock material  10  can be increased by determining an increased number of centroids C over the length of the segment  10 A, as shown in  FIG. 6 . 
   Once the measure of curvature or straightness of a segment  10 A of the linear stock material  10  is determined by the central processing system  106 , the measure of curvature or straightness profile may be utilized in a variety of ways. For example, the straightness or curvature of a single segment  10 A of the linear stock material may be compared with a predetermined tolerance to identify linear stock material  10  which has at least one identified segment  10 A exceeding the predetermined tolerance. Similarly, by acquiring measures of straightness or curvature for multiple segments  10 A along the length of a piece of linear stock material  10 , an overall measure of curvature or straightness of the linear stock material  10  can be identified and compared with predetermined tolerances. Those of ordinary skill in the art will recognize that the segments  10 A over which the curvature or straightness is measured need not be discrete, and in fact, can overlap to provide a more accurate measure of the characteristics of the linear stock material  10 . By storing the measures of curvature or straightness for linear stock material  10  exiting a mill  12  over time, trends or changes in the characteristics of the linear stock material  10  output can be noted and flagged for operator inspection or correction. 
   Turning to  FIG. 7 , an alternate embodiment of the present invention is shown in which the imaging sensors  104 X and  104 Y in each imaging module  102  are replaced with a pair of distance measurement sensors  200 A and  200 B mounted in a predetermined and known configuration. Each distance measurements sensor  200 A and  200 B is disposed within a common cross-sectional plane of the linear stock material  10 , and aligned along an axis which is perpendicular to the exterior surface of the linear stock material  10 . However, unlike the imaging sensors  104 X and  104 Y, the distance measurements sensors  200 A and  200 B are not typically disposed in an orthogonal relationship to each other, but rather, are separated by an acute angular orientation α, preferably having an arc of about 28 degrees about a common centerpoint. The acute angular orientation between each distance measurement sensor  200 A and  200 B enables the measurement system  100  to accommodate linear stock material  10  having different outer diameters without requiring reconfiguration. 
   Each distance measurement sensor  200 A and  200 B is configured to acquire a distance measurement between the distance measurement sensor  200 A and  200 B, and a respective point  202 A,  202 B on the surface of the linear stock material  10  co-linear with each detector axis  204 A,  204 B. In one configuration, each distance measurement sensor  200 A,  200 B is configured with a laser projection system to project a point of laser light onto the points  202 A,  202 B on the surface of the linear stock material  10 . An optical sensor disposed within each distance measurement sensor  200 A,  200 B obtains an image of the projected point, and calculates a distance thereto. 
   For linear stock material having an outer diameter known to within a predetermined tolerance, the central processing system  106  is configured with a software algorithm to utilize the distance measurements obtained by each distance measurement sensor  202 A, and  202 B together with the predetermined and known configuration of the distance measurements sensors  202 A and  202 B, to identify the coordinates of a centroid C of the linear stock material in the cross-sectional plane using conventional geometric relationships. Curvature of the linear stock material  10  results in variations in the measured distances to the points  202 A and  202 B along the length of a segment  10 A, as each point  202 A and  202 B shifts about the exterior surface of the segment  10 A, and accordingly changes the centroid coordinates within the respective cross-sectional planes. Once a set of centroids C are identified for a segment  10 A of the linear stock material  10 , the straightness of curvature of the segment  10 A is determined as previously described in relation to a determined virtual axis VA for each segment  10 A. 
   Those of ordinary skill in the art will recognize that a set of centroids C for a segment  10 A of linear stock material  10  may be determined using a variety of techniques and components without departing from the scope of the present invention.