Flatness Profile Computation
Three-dimensional surface reconstruction of a rolled product using slit scanners involves two separate stages: intraframe and interframe processing. Once a frame with the projection of the laser stripe onto the surface of the product is acquired, it must be processed in order to determine the surface profile. To this purpose, the intraframe processing is split up into three major tasks. The first task consists of extracting the laser stripe from the image and computing the strip profile expressed in the image coordinate system. The second task translates each point of the detected stripe to obtain the profile of the surface expressed into world coordinates. The third task involves software tracking to compensate for the effects of small lateral displacements while the strip is being processed. Figure 13 shows the steps carried out by the intraframe processing stage in order to compute the position of the fibers of the strip for each frame.
F13 :
Flatness profile computation: (a) Intraframe processing stage and (b) interframe processing stage.
Image not available.
The intraframe processing stage in the proposed flatness inspection system is based on both the laser stripe extraction method proposed in this paper and the camera-calibration procedure described above. The translation between coordinate systems is carried out by means of a lookup table computed during camera calibration. This stage is carried out in real time.
The interframe processing stage must use an indirect method to obtain the 3-D strip surface map because there is no information about the length of the fibers in the surface profile. The strip surface map is then used to compute the length of the fibers in each integration interval, making it possible to comptute flatness profiles.
Once the strip has moved forward by the length corresponding to an integration interval, the interframe processing stage must reconstruct the 3-D surface of the strip in this interval in order to compute a new flatness profile. Figure 13 shows the steps carried out by the interframe processing stage for each integration interval of the strip. This stage is also carried out in real time.
The information obtained from the laser beam projection onto the strip surface in the intraframe processing stage, together with the speed of the strip movement, makes the computation of the 3-D surface reconstruction of the strip possible.
The acquisition rate of the camera of the flatness inspection system is automatically adjusted, depending on the strip movement speed. The speed of the strip is measured by means of a laser surface velocimeter, an optical device for noncontact measurement of 1-D surface velocities.
Measuring the length of the fibers at fixed longitudinal sections of the 3-D surface map is the main task required for computing the flatness of the strip. The length of each fiber is computed in the interframe processing stage, once the intraframe stage has provided the position of the fibers in the scene for each image processed in the integration interval. The length is computed using an integration technique, based on both the height variations of the fiber between consecutive images and the speed of the strip while moving along the production line, defined by
where L j is the computed approximation of the length of the fiber f j ; h j i is the height of the fiber f j measured in the frame i ; t i is the time stamp when the frame i was taken; u i is the average speed of the strip between the frames i and i−1 ; and n is the total amount of height frames in the integration interval. Figure 14 shows a diagram representing the approximation carried out in this integration technique.
F14 :
Approximation of the length of a rolled product fiber.
Once the length of each fiber of the strip in the whole integration interval is computed, the system is able to compute a flatness profile of the strip expressed in I-units using Eq. 1. Computing a flatness profile for each integration interval of the strip makes the computation of the flatness map of the whole strip possible (see Fig. 6).