AC Vector Drives, PLCs Help Roll Out Steel Carpet
If a company plans on churning out 500 feet of cold-rolled steel per minute and add various protective coatings to each mammoth roll, it's going to need a durable and highly accurate automation system.L-S II Electrogalvanizing Company (Columbus, O.), a joint venture of LTV Steel and Sumitomo Metal Industries, faced this dilemma when the company began operations in 1991.
If a company plans on churning out 500 feet of cold-rolled steel per minute and add various protective coatings to each mammoth roll, it's going to need a durable and highly accurate automation system.
L-S II Electrogalvanizing Company (Columbus, O.), a joint venture of LTV Steel and Sumitomo Metal Industries, faced this dilemma when the company began operations in 1991. L-S II turned to Yaskawa Electric America Corp. (Northbrook, Ill.) to provide programmable logic controllers (PLCs) and more than 150 variable speed ac vector drives required for the production line.
Confronted with L-S II's demanding control system requirements, Yaskawa's first task was to control the steel plating current based on the varying production line speeds needed to accomplish different plating options. These requirements allow L-S II's customers to specify different plating weights, zinc versus nickel-zinc plating, or choose between single- or double-sided plating.
Maintaining line speed
System precision was further essential because the steel plating process at L-S II is continuous. The end of one coil with one set of plating specs is welded to the beginning of a new coil with another set of plating specs. Likewise, the steel in a preplating electrolyte bath must keep moving at a minimum speed to prevent sulfuric acid from eroding it. At the same time, the plating current control must vary with the operator-selected line speed determined by the type of plating the customer has requested.
Yaskawa's engineers solved these problems by developing an elaborate weld tracking system with Yaskawa PLCs, using inputs from strategically placed weld detectors along with customer order information from the plant's computer system. As weld points pass through each plating cell, the PLC switches from the previous coil's plating data to the next.
With new data sets for each coil in place, Yaskawa was also able to resolve L-S II's varying line speed requirements. Slack from the looping towers (see photo) helps keep the line's center section running above a minimum speed for the plating process. This slack also allows either end of the line to be stopped and started independently—to weld steel onto the line, or shear the finished coils coming off it. Yaskawa's VS-676 ac vector drives control the line's speed. The drives are given digital references over communication links from the PLCs.
The drives also provide accurate speed feedback to the PLCs over the same communication links, while the speed control loop is located in the drive itself.
Project installation and commissioning were completed on schedule, and L-S II has operated successfully for the past six years. Mike Murdock, an electrical engineer with L-S II who helped construct the facility and currently oversees the electrical line maintenance, calls the control system a complete success. "The drives, motors, and controls installed by Yaskawa have performed reliably with very little maintenance," he says. Mr. Murdock adds that because of the high level of automation achieved with the drives and PLCs, L-S II has been able to maintain product quality with less staff than similar steel-producing facilities.
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