Delivering better productivity
Machine vision technology has been used by the metal industry to an increasing extent over the last 15 years to solve challenging dimension and position measurement problems - and has produced good results in areas such as higher productivity and better and more consistent quality.
The VISI Edge Guiding and Centring System developed by Vision Systems has been at the front of the field in pioneering and expanding this technology.
Photocell-based optical systems have been the conventional solution for edge guidance and centring needs when handling strip material on many processing lines in the metal industry. The technology used has several drawbacks, however. In particular, it involves a lot of moving parts and specialised electronics, and has a high maintenance requirement. The advantages of CCD line scan camera technology for strip edge tracking and position control, as well as for strip centring, in comparison, are obvious.
The VISI system, based on CCD line scan camera technology, is an ideal solution for today's mills operating with a minimum of maintenance personnel and with an automation system for controlling their measurement and monitoring equipment. The VISI system is able to work as an independent measurement and control device, feeding all the required information into the automation system.
The VISI system is based on the latest measurement and control technology, and is extremely accurate. Strip edge tracking is performed by a VISI line scan camera with an appropriate lighting unit. Either reflected light or backlight can be used, depending on the requirements of the processing line.
VISI line scan cameras are highly reliable with no moving parts, and provide high-resolution, non-contact measurement. Their user-friendly Windows interface enables parameters for a wide range of measurements to be selected very easily.
All the information required can be sent from the cameras to the PLC or other automation system via a serial link. For an edge guiding application on an on-coiler, one or two cameras are installed on one or both edges of the strip. For a strip centring application, on a decoiler or guiding roll, two cameras are required on both edges of the strip.
Successful track record
Rautaruukki Steel's hot strip mill in Raahe is a particularly good reference for VISI systems, as they are used on all the processing lines at the company's Sheared Strip Products Department today, with great success.
Initially, VISI systems were installed on the push pickling and cut-to-length lines, and in the skin passing mill. These lines were previously equipped with photocell-type sensors, which used electric motors to traverse the strip to measure variations in width. This made strip edge position measurement slow and inaccurate, however. Additionally, the traversing system was prone to wear or breakdown, and the mirrors used with the photocells could easily become contaminated by dirt or oil. This often resulted in the failure of the edge guiding system. In worst-case incidents, the result was a defective strip and possibly damage to the rolls of the skin passing mill. Installing VISI systems solved these problems.
The mill then acquired a VISI system for both of its slitting lines. This time, the systems were interfaced to a PLC so that real-time camera signals could be shown on the monitoring screen in the control room. Soon after, the highly advanced VISI 5000 camera-based system (with PLC interface) was installed in the skin passing facility.
During the last 10 years, VISI systems have also been installed on a number of lines at the Avesta Polarit Mill in Tornio. Lines equipped with VISI systems today include two slitting lines, two cut-to-length lines, a tension levelling line, and four annealing/pickling lines.
As at Rautaruukki, VISI systems were initially used to replace photocell-type systems. More recently, however, they have been acquired as the original edge guiding or centring systems for new processing lines installed at the mill.
The VISI systems installed at Raahe and Tornio, as well as at Rautaruukki Steel's Hämeenlinna Works, have provided a significant improvement in the accuracy of edge position control and strip centring, as well as a reduction in downtime. In particular, the accuracy of strip edge position control provided by the VISI system permits narrow tolerances in the shearing of raw strip edges, resulting in a significantly higher production yield. Extremely accurate strip width measurement is also possible using VISI systems.
Maintenance for a VISI system typically involves no more than an annual check and possible readjustment of camera alignments, which can be done during a planned shutdown. In the case of VISI systems using back light illumination; even cleaning the lighting unit to remove dirt and oil is a minor job today.