Nonlinear control in rolling mills: a new perspective

This paper is concerned with the application of modern nonlinear control techniques to the thickness control in rolling mills. It turns out that the performance 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 nominal operating point, because they are not limited to a more or less small neighborhood of the operating point, but are valid all over the operating range. Practically, these nonlinear control concepts are a possible answer to the trend of modern rolling mills toward tighter thickness tolerances, thinner final strip thicknesses, faster production rates and shorter off-gauge lengths. However, a straightforward application of the well established nonlinear control theory does not always lead to a control concept that is practically feasible. It is rather unavoidable to consider all the special features and properties of the plant. By means of the gap control of a mill stand with a hydraulic positioning system, the authors show in detail the advantages of a nonlinear control approach. Finally, simulation and measurement results demonstrate the feasibility and the excellent performance of the proposed design.