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Optical contour measurement and 3D surface inspection of rebars

LAP GmbH Laser Applikationen has been developing, producing and marketing laser systems for industrial
and medical applications since 1984. The LAP portfolio includes laser components and specific system solutions for projection and non-contact measurement in industry, and for repeatable positioning of patients undergoing radiotherapy.

In many fields, LAP has introduced new optical procedures which have made possible a number of innovative production processes. These include efficient production of sheet steel and wire, and also new quality assurance procedures for line pipes. Accurate and extremely robust systems were needed in rolling mills in particular. Such systems must be protected against steam, scale, dust, vibrations, elevated temperatures and collision with structural steel elements.

Since their market launch LAP’s CONTOUR CHECK measuring systems have proved highly successful in a large number of long product rolling mills, where they have become an indispensable asset of the rolling process. The product family comprises the following parts

▲CONTOUR CHECK WIRE – diameter measurement ▲CONTOUR CHECK BAR & TUBE – measurement of medium diameter wire rod, bars and tubes at temperatures up to 200°C ▲CONTOUR CHECK ROUND – the standard option for measuring round sections ▲CONTOUR CHECK EDGE – scanning of square, flat and hexagonal sections ▲CONTOUR CHECK SHAPE – control of all dimensions and detection of (dimensional) rolling process of long products ▲CONTOUR CHECK PROFILE – modular solution for sections of complex shapes.

LAP has now added a new function to its CONTOUR CHECK SHAPE system in order to meet the growing demand for precise measurements of the contours Authors: Dr Uwe Knaak and Marianna SchroeterLAP GmbH Laser Applikationen of ribbed steel (rebars). Traditional systems based on the shadowing method have been unable to provide this kind of measurement because the ribbed surface of the bars obstructs the view of part of the bar body. To overcome this problem LAP have developed its ‘light-sectioning’ method which guarantees full coverage of the complete cross-section of rebar by high-precision measurements. This system depicts the true cross-section of the bar without omissions and with no shadow effects. It scans up to 2,000 complete sections per second and recognises geometrical deviations caused by rolling defects, including deviations of the characteristic values of rebar, such as rib height or the contour of the bar body.

The system supplies, in real time, reliable geometric data  that do not require interpretation, which form the basis for an optimal adjustment of the rolling stands, and which generates warnings and alarms if the contour data deviate from specification. The benefits are that less time is needed for fine-tuning the rolling parameters after product changes, and the fine-tuning process itself is more accurate resulting in a higher percentage within specification after a size change, leading, in turn to higher mill yields. The system also determines, along the length of the stock, the off-size length, which is necessary for optimising head and tail end cropping. It also makes a significant contribution to cutting mill operating costs insofar as it is no longer necessary to cut off-line test samples from every coil in the finishing shop. At many mills this effect alone enables a CONTOUR CHECK system to recover its costs in only a few months.

On steel bar and wire, CONTOUR CHECK SHAPE detects
shape defects such as: ▲Overfill and underfill on one or both sides ▲Incorrect roll-gap setting ▲Ovality ▲Roll misalignment ▲Roll wear

Unequivocal automatic classification of rolling defects means that their causes can be systematically eliminated and rolling parameter settings are no longer dependent on subjective estimates or on the skills of the operator. Additionally, the system detects defects of the surface, both isolated ones, such as rolling inclusions, scale, scabs or scoring, and cyclical ones, such as roll spalling or roll eccentricity.

A special algorithm ensures that the orientation of the rolling stock is known at all times. This is important because it is not always possible to install the measuring system close to the rolling stand and the section may have rotated between the roll gap and the location of the measuring system.

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