Abstract
Flatness sensors are required for quality control of metal sheets obtained from steel coils by roller leveling and cutting systems. This article presents an innovative system for real-time robust surface estimation of flattened metal sheets composed of two line lasers and a conventional 2D camera. Laser plane triangulation is used for surface height retrieval along virtual surface fibers. The dual laser allows instantaneous robust and quick estimation of the fiber height derivatives. Hermite cubic interpolation along the fibers allows real-time surface estimation and high frequency noise removal. Noise sources are the vibrations induced in the sheet by its movements during the process and some mechanical events, such as cutting into separate pieces. The system is validated on synthetic surfaces that simulate the most critical noise sources and on real data obtained from the installation of the sensor in an actual steel mill. In the comparison with conventional filtering methods, we achieve at least a 41% of improvement in the accuracy of the surface reconstruction.
Highlights
Laser based optical sensors have been widely used in industrial environments for various applications like online programming, part measurement and quality control, and part identification and localization.The requirements for the surface quality of sheet metal products are continuously increasing.Flatness defects are a major problem in many industrial areas such as architectural panel works [1,2]and the automotive industry [3], just to name a few
Using a combination of both pieces of information, we developed a method capable of obtaining a precise surface reconstruction that is robust against all kinds of noise sources [32,33]
An alternative sensor calibration approach consists of finding the mapping function between the image plane and laser plane, treating the sensor as a black box carrying out a plane mapping function
Summary
Laser based optical sensors have been widely used in industrial environments for various applications like online programming, part measurement and quality control, and part identification and localization. It is possible to compute the surface height from the laser line segmented on the image, where each column of the imaging sensor corresponds to a virtual fiber of the metal sheet These measurements are sensitive to the noise produced by some other mechanical processes, e.g., degreasing, cleaning, polishing, shearing, and transporting roll systems, as well as the illumination effects from the ambient light and the speckles generated by the laser coherent light. The contributions of this paper are the following: (1) the design and implementation in a real industrial operation of a surface flatness sensor that uses two parallel laser lines and an industrial high speed area scan camera providing height maps of the metal sheets moving below the sensor; and (2) a noise removal system based on Hermite polynomial interpolation that effectively removes high frequency noisy wave patterns induced in the sheet by mechanical manipulation.
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