Nonlinear control in rolling mills: A new perspective

This paper is concerned with the application of modern nonlinear control te chniques to the thickness control in rolling mills. It turns out that the p erformance of the closed loop can be significantly improved by taking into account the essential nonlinearities of the plant to be controlled already in the controller design. These nonlinear control strategies totally differ from the common approach of linearizing the nonlinear system around a nomi nal operating point because, they are not limited to a more or less small n eighborhood of the operating point, but are valid all over the operating ra nge. Practically, these nonlinear control concepts are a possible answer to the trend of modern rolling mills toward tighter thickness tolerances, thi nner final strip thicknesses, faster production rates and shorter off-gauge lengths. However, a straightforward application of the well established no nlinear control theory does not always lead to a control concept that is pr actically feasible. It is rather unavoidable to consider all the special fe atures and properties of the plant. By means of the gap control of a mill s tand with a hydraulic positioning system we will show in detail the advanta ges of a nonlinear control approach. Finally, simulation and measurement re sults will demonstrate the feasibility and the excellent performance of the proposed design.