Straightness of Pipes Gauged Online
Michael D. Wheeler
A machine vision system that uses a pair of line-structured lasers and two CCD cameras could improve the speed and accuracy of inspecting the straightness of seamless steel pipes.
One of the last steps in pipe production involves taping the ends, and the quality of the finished product depends on precisely determining the straightness of each pipe. The most common methods for measuring large-scale straightness -- profile matching and laser alignment -- are ineffective, however, for automatic online measurement.
Researchers at Tianjin University in Tianjin, China, have devised a method that combines three-dimentional-vision measurement technology with ellipse fitting. They theorized that by determining the 3-D coordinates of a series of points along the central axis of the pipe, the straightness could be computed using mathematical models.
The scientists positioned a pair of line-structured laser sensors at various cross sections of the pipe. The laser lines cast on the pipe intersected at various points to create two distinct elliptical arcs. Two CCD cameras took images of the arcs, which were sent to a computer via acquisition hardware. Because the scientists used two arcs on different sections of the pipe to fit the ellipse, they obtained a more accurate central coordinate.
In tests conducted on 1.8-m-long steel pipes, the researchers discovered that the method was not only effective at yielding precise measurements, but was also quite fast -- less than 30 seconds on average to measure each pipe at various cross sections. And, because it is noncontact and suitable for an online processing environment, the technique could be used for many applications.
Lead researcher Changku Sun said that Tianjin Steel Pipe Corp. in northern China intends to install the system on its production line by the end of this year. The company produces more than 500,000 tons of seamless steel pipes each year.
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