The ThyssenKrupp production facility in Calvert, Ala., isn’t a typical manufacturing plant. As the largest steel-mill facility in the country, it is the size of a small town and boasts a production capacity of up to 5.3metric tons of steel each year, including mild steels, high-strength and advanced high-strength steels, as well as high-carbon and structural steels. Miles of hot-rolled bands and pickled coils, as well as full-finished cold-rolled, galvanized, galvannealed, aluminized, and galvalume products are shipped from this facility on a daily basis.
To meet end-user demand, equipment runs continuously, moving, shaping, and cooling tons of metal every hour. It falls to a team of operators and technicians to ensure that each machine continues to operate, and that there is no deviation in quality and specifications whatsoever. Banks of computers monitor every machine, alerting the team to deviations as small as a few microns.
“Even a tiny discrepancy in the thickness of a slab on the line can quickly affect thousands of pounds of steel, considering the speed at which our processing lines move,” explains Automation Technician Rodney Willard. “Consistency and quality control are critical to ensuring that our customers get the exact product they specified.”
ThyssenKrupp employs advanced Hydraulic Gap Control (HGC) systems to keep steel traveling on the hot-roll lines at the proper thickness. The key to these systems resides in four position-indication sensors bolted onto the sides of the HGC cylinders. The sensors monitor deviations with resolution down to 1 μm, and communicate that data to the control room. Any discrepancy is immediately evident, allowing workers to compensate in the most-efficient method possible.
SMS Siemag in Germany, a world leader in rolling mills, supplies the complete plant, including its electric and automation systems. As part of the original plant design, incremental linear encoders were used to monitor the HGC cylinders, due to their precise measurement capabilities. The downside is that these encoders are expensive, rely on delicate electronics to measure and communicate signals, are susceptible to pressure and vibration, and require occasional replacement.
“These sensors operate in hot, hostile environments where pressure, vibration, and exposure to water are essentially constant,” explains Willard. “Maintaining and replacing failing linear encoders was a major source of downtime on the hotroller lines.”
Two years ago, Electrical Maintenance Manager Alec Glenn proposed switching to magnetostrictive sensors from MTS Sensors, Cary, N.C., already installed on other equipment in the plant. As an initial trial executed with support from SMS Siemag, Glenn swapped out half of the encoder-style sensors on a roughing mill stand.
“Since installing the MTS sensors in 2010, we haven’t replaced any of them,” says Glenn. “In that same period, we’ve had numerous issues with the linear encoders. At first, one of our major concerns was whether the MTS sensors could match the accuracy of the digital linear encoders. It turns out that the difference between the two technologies is negligible. Combined with improved reliability and lower price, the R-Series is a much better fit for our operations — so we’re switching all equipment to these.”
R-Series sensors report with linearity as low as ±20μm with a fast update rate and work with different output protocols such as SSI, DeviceNet, Profibus, DP, EtherCAT, and EtherNet/IP. Recent advances in testing protocols, due to the MTS test case, have provided the data necessary to specify these sensors for even the most demanding rolling mill applications. In addition to improved reliability in rougher environments, Willard also notes ease of installation as an unanticipated benefit.