QMT Features: June 2015
Hot measurement for rolling optimisation
Nextsense has developed a process to measure hot rolled steel between passes without having to cool it down – thus speeding roll setup*

Before producing a new hot rolled profile the roll stand set-up has to be optimised. The ideal parameters are established by the repeated separation, cooling and measuring of the profile shape between passes – a very time and resource consuming process.

Even in modern rolling mills with a high level of automation you often meet very simple and sometimes time-consuming methods for verifying shape and dimension of rolled product especially in the intermediate state between roll passes. In many cases, cutting samples and comparing the shape on a profile projector is still state of the art. For checking the pass filling of the rolls some mills still use the techniques of watching the burn-off of wooden boards held onto the hot product.

On the other hand there are big profile measurement systems which are accurate and quick, but expensive and inflexible in terms of quickly changing to different position in the rolling mill
Working with hot-rolled special steel profile manufacturer Mannstaedt GmbH, the Austrian metrology company NextSense GmbH has developed a device for measurement of hot rolled profiles between single passes. Using laser light section technology this enables the measurement of a complete profile (360°) of variously shaped objects within a few seconds, regardless of temperatures above 1,000°C, dust, steam and vibrations.

The Nextsense Calipri Hot system significantly shortens the set-up time per rolling and these time savings directly translate into a higher utilization of the rolling mill. In addition to the reduced set-up time, the measurements provide more information due to the hot measuring method. Previously, hot state dimensions could not be measured. Calipri Hot now makes it possible to measure them directly, allowing for a more precise adjustment of the rolls.

Thus, it is no longer necessary to wait until the sample has cooled down to calculate its dimensions. This gives substantial benefits, especially for the development of new roll pass designs.  Mannstaedt produces special steel sections for diverse industrial applications. Compared to rolling mills for commodity steel production, these specialised applications call for very small production lots, leading to frequent profile changes on both of its rolling mills. Consequently, the mills very often have to be completely reset. For example, in 2014, a total of 479 campaigns were rolled on the smaller, the light-section mill.

Whenever a new campaign is started, samples had to be cut from the rolled billet after each single pass to check the product quality. These samples must be cooled, sawed, deburred and inspected by means of a manual inspection device and/or a profile projector. This is a very time-consuming procedure.

Using Calipri Hot eliminates these processes because the instrument is placed around the section directly in the rolling mill at the position to be measured. The expectation was that by placing the instrument around the rolling stock, it should be possible to capture the complete contour with a laser-based system and measure a closed contour of the section at any intermediate stage. The measured results could be directly compared with the nominal values and used to correct the roll gap, if necessary.

Thanks to its mobile design, the Calipri Hot system can be used anywhere in the rolling mill. Hence, measurements can take place at any intermediate rolling stage. As it is possible to make a virtual cut through the rolling stock to be measured and display the quality-relevant data in a matter of seconds, approx. five minutes of time could be saved in each campaign. With almost 500 campaigns rolled on the light-section mill every year and based on the calculated five minutes of saved time per campaign, the annual time saving would amount to approx. 50 hours. Multiplying these hours by the costs per hour of rolling mill operation would provide appreciable savings.

The fact that the assessment of the dimensional accuracy is available in much less time means the operators can react faster and initiate any corrective measures more quickly. This not only applies to measurements after a changeover to a new size, but also to general quality monitoring along the production chain. Using Calipri Hot, for example, at the cooling bed for checking the final contour would make it possible to recognize out-of-spec contours at an earlier stage and reduce the amount of unusable material.

In addition to reducing the test times and the amount of set-up material, Calipri Hot provides previously unavailable information about the current rolling process. This new information helps optimize the rolling process. For example, the available data could be used to investigate all roughing passes of the pass designs. In this case, the instrument would serve not only as a measuring but also as a development tool.

NextSense is a young company founded in 2007, which offers laser-based profile and surface measurement system in the rapidly growing market for optical sensors. One of its most important products is a hand-held laser based profile measurement system for measuring wear of railway wheels for the periodic maintenance of railway vehicles called Calipri. The new Calipri Hot system is a further development of the Calipri System.
Calipri is based on the well-known laser light sectioning technique whereby a laser projects a line onto the profile to be measured. This laser line is observed by a camera under a triangulation angle a. The shape of the section to be measured can be reconstructed from the image of the laser line. Using this method in a small hand-held device raises two fundamental problems. The problem of positioning errors, particularly if the laser plane is not exactly perpendicular to the section to be measured, is overcome by using multiple laser lines – if the orientation of the measurement head changes, then this causes a shift in the lines relative to each other. The second problem is that it is not possible to to cover the whole section to be measured with one laser and one camera.

Using two cameras and two lasers in a fixed relationship would end up in a device which is not operable anymore as a mobile device.  This is solved by scanning the object to be measured dynamically and thus producing a multitude of measurements with unknown geometric relationship, each showing just a part of the whole section. From these measurements, an automatic selection of those sections needed for a good reconstruction is done. These sections are aligned to each other in the next step using a best fit approach.
The aligned measurements are smoothed and sampled equidistantly, thus providing a closed contour of the cross section to be measured. Based on this contour arbitrary dimensions like length, angles or radii can be measured either interactively by mouse or by pre-defining measurement rules for a certain cross section and applying automatically to each measurement.

Since it isn’t convenient to perform measurements of sections above 1000°C using a hand-held sensor, Nextsense and Mannstaedt developed a simple mechanical system to move the sensor around the rolled profile automatically.

In contrast to previous profile measurement systems there is no special demand for precision or thermal stability, so the mechanical system can be very simple and light construction. This is because measurements are not aligned to each other by using calibrated sensor positions, but by aligning the data by best fit methods to adjacent datasets.

The picture shows the first prototype in operation. The C-Frame allows to move the device in and out while the bar is running through the line. The measurement is started by pushing a button on the outside of the housing. This forces the sensor to move around the bar forwards and backwards again, which takes approximately 10 seconds. During this movement approximately 70 single images are taken by the camera from different directions and preprocessed on the sensor. The sensor itself (in the figure in top right position) is protected with a heat shield and is moved along a 270° degree circle track by means of a gear wheel. The whole time needed for one measurement is less than 30 seconds.
The Calipri technology can be applied to nearly any arbitrary cross section. The only precondition is the need for some distinct features on the contour, which allow the software to match each new measurement unambiguously to the previously recorded ones. Profiles such as rails, beams and so on meet this requirement. For rectangular profiles, the edge radius suffices. The only critical sections are round bars, since the profile shape looks exactly the same from all directions. To overcome this limitation, static reference objects can be mounted in the field of view of the sensor which allow to align subsequent measurements by means of these pass points.

The Calipri Hot system is currently under development and the first prototypes are being tested in different mill environments. Successful tests on hot rectangular billets with 140 x 100 mm have been performed in a rolling mill in Austria. The first system used permanently under production conditions will be installed at the rolling mill of Mannstaedt GmbH in Troisdorf probably in the fourth Quarter of 2015. It is planned to market the system in different sizes for different measurement ranges starting with 2016.

* Based on a paper by A. Niel, NextSense GmbH, Austria; Ch. Trappmann, Mannstaedt GmbH, Germany.
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