The ability to manufacture steel with a high degree of flatness has always been a prime objective of the steel industry. Not only is planity a critical property that has a direct bearing on the users’ ability to utilize material efficiently, it can also decisively enhance a manufacturer’s competitiveness. The advent of the latest generation of non-contact, high precision flatness measurement technology from Sweden has already consigned all conventional methods to the history books and is fast establishing unsurpassed benchmarks.
In their endeavour to attain improved levels of productivity and increasingly high standards of quality, a growing number of steel and metal manufacturers have sought new ways and opportunities to meet increasing market demands for ever flatter products. Although non-contact measurement of manifested flatness for steel and plate is by no means a new phenomenon to the steel industry, the methodology used to accomplish this has witnessed significant development since its inception, with major technological advances enabling manufacturers to realize their highly ambitious goals.
Widely recognized as one of the foremost authorities in this field, the Linköping-headquartered enterprise Shapeline AB in Sweden has taken its competitors and rivals to task with its development of a non-contact, precision flatness measurement technology that has confounded the method’s sceptics and detractors. A claim vindicated by the success the company has experienced in attracting the attention of the industry’s blue-chip heavyweights across the globe, translating the interest into a growing list of international advocates and references.
“Unlike our competitors, Shapeline already opted more than ten years ago to research and develop a range of planity measurement and control solutions not only based on computation of the I-unit profile, but also the laser-line triangulation principle in combination with retrieval of the material’s topographical data”, explains company founder and Managing Director, Dr. Pär Kierkegaard. “Laser-line triangulation is based on a red diode laser with a line producing optics that spreads the laser light in one dimension to create a fan of light. Shapeline uses refractive optics that make the intensity even, assuring even measurement accuracy across the width.
“The line is focused on the material and clearly visible. If the surface is uneven, it may be observed with the naked eye as a slight bending of the laser line,” says Dr. Kierkegaard. “To obtain the shape automatically and more precisely, the continuous laser line is observed with a 2D matrix camera. The line is subsequently projected in the camera image across the picture and the software can detect the line for every column in the picture. The usual way is to detect the theoretical centre of gravity of the line in every point with sub-pixel accuracy. This ensures the measurement is not only precise, but also independent of the actual intensity and surface reflectivity. The intensity itself is not important, only the position of the line.
“Since the camera may have several hundred columns, the number of measurement points is high. Shapeline solutions can provide over 4,000 points across the width by using several synchronized cameras. Since all measurement points are acquired simultaneously, we can ‘freeze’ the shape in terms of flatness and transverse bend measurement – more commonly known as crossbow – to produce high-precision reports.”
The double laser line approach also differentiates itself from other methods in that it permits local surface slope measurement, both in width and length, in addition to 3D-positions that can be used for material vibration compensation and, to some extent, elastic deformation compensation. “By projecting two laser lines one after another in the transport direction, we are able to measure the local surface gradient”, explains Dr. Kierkegaard. “A shape defect always produces a change in the local gradient, whereas a vibration does not. Even if there is a vibration, the gradient is the same. In fact, an elastic deformation gives no or very limited gradient change unless the material reshapes, i.e. the buckles move.” The resultant compensation has the advantage of enabling measurement of manifested flatness with high resolution even of material that is moving on roller tables or experiencing severe vibrations in the lines.
According to Dr. Kierkegaard, the method was first conducted in controlled laboratory conditions and following fine tuning and refinement was tested by several customers before ultimately being fully commissioned in both strip and plate lines, initially on conveyor belt lines and, following further research and development, roller tables.
“Non-contact flatness measurement of thin steel plate on roller tables has always represented the most difficult application manufacturers have had to battle with. However, we have been able to adapt and refine the double laser-line triangulation principle adopted by Shapeline – in combination with the retrieval of topographical 3D-data and I-unit computation – to create a reliable, high-precision solution for non-contact measurements on roller tables.”
Qualifying as the world’s foremost manufacturer of quenched and tempered heavy plate, SSAB Oxelösund began within the commissioning of a new facility for the production lines of thin steel plates in 1998. As requirements for optimum productivity and quality were very high combined with a rising user demand for high flatness, SSAB saw the long-term opportunities of working with Shapeline and entered a co-operation. Today, Shapeline systems are in operation on no less than seven of the production lines at the SSAB steelworks in Oxelösund.
As customers have discovered, the advantages to be gained by using Shapeline technology are manifold. Higher standards of quality result in higher levels of satisfaction among clients who can further strengthen their market positions. Improvements in production efficiency made possible by measurement on-line yield significant savings in terms of time and costs. As Ola Hägglund, Head of Plate Division, SSAB Oxelösund, explains, thanks to Shapeline, SSAB is now in a position whereby it can conduct precise, objective and systematic measurement and product development warranting benefits in both the short and long-term.
“When SSAB entered the market some years back with thin plate products, we embarked on a co-operation with Shapeline that allowed us to solve a number of levelling issues”, recalls Ola Hägglund. “Since then we have, together with the help of Shapeline, improved our ability to resolve levelling process problems to a point where we are now able to provide our customers with a product that is defined.
“Without Shapeline technology we wouldn’t be able to produce and deliver closely defined products in accordance with the most stringent customer specifications as we do today,” continues Hägglund. “It also sharpens our competitive edge in terms of the capacity of our production lines. In a nutshell, our production costs would be higher without Shapeline.
“Having begun with on-line measurement systems for conveyor belt lines, we have now found a solution for non-contact measurement on roller tables. This is an example where use of Shapeline technology can be extended and an illustration of how co-operation has improved our flexibility in the measurement of plate.
“In terms of benefits, it would be true to say that reductions in overheads and lead times combined with a higher quality product have not only enabled us to gain a better price for our products but also to establish ourselves as preferred supplier with a large number of customers,” he adds. “This is especially important when the market is in decline.”
The improvements in efficiency generated by the ability to feed steel plate directly into the manufacturing line process at the normal production rate also render considerable space savings. “Bearing in mind the effects on material and limitations with regard to material handling make manual measurements on-line impossible, Shapeline’s contact-free planity measurement has meant a great deal to SSAB Oxelösund,” says Anders Carlestam, Project Leader SSAB Oxelösund. “The technology enables us to deliver material with tolerances finer than our competitors can guarantee.
“Implementation of on-line measurement technology not only allows us to detect the very smallest defects and improve the already high levels of performance, but has also allowed us to resolve some technical issues we weren’t even aware of earlier. This has subsequently enabled us to help our customers optimise their own production procedures,” adds Carlestam.
“In terms of amortization, the Shapeline system already paid for itself during the start-up phase of the quenching line. But we can see on-going major economic benefits are resulting from reduced scrap metal volumes and improved productivity.”
Shapeline’s co-operation with SSAB Oxelösund looks set to continue in the future. “Based on our positive experiences to date, we can already say that on-line flatness measurement technology will be a feature of any new plate lines we design in the future,” states Jan Steninger, Strategic Investments Manager SSAB Oxelösund. “It enables us here at SSAB to achieve savings in terms of workshop floor space, manpower and heavy equipment.
“There is no doubt that if you cannot measure accurately, you cannot improve the product. Shapeline technology has enabled us to deliver products over and above the accepted norms and specifications, and attain the highest standards in terms of quality, uniformity and flatness that are best suited to market demands,” concludes Steninger.
By providing accurate, high-precision measurement results, the double laser-line triangulation principle is well on the way to establishing itself as the prime on-line measurement principle of manifested flatness for flat metal products. To this end, Shapeline has developed a highly functional and flexible programme of systems geared to suit a wide range of metal production applications thus making reliable on-line planity measurement a reality in a fast growing number of strip and plate production facilities across the globe.
Paul R. Compton is a British freelance journalist resident in Sweden since 1992 and a regular contributor to European business titles and publications since 1986.