Modeling and Control of Plate Mill Processes

Abstract: In this thesis two processes have been modeled and controlled, namely the plate thickness control and the slab temperature control. The classical solution for the thickness control problem can't handle asymmetric hardness variations and the estimation of the controlled output introduces stability problems. A nonlinear multivariable model for the thickness control system is derived. The parameters of the model for the rolling mill are identified using data from The Danish Steel Works Ltd. and the model is used for designing a nonlinear multivariable thickness control algorithm. The algorithm is evaluated using computer simulations and the new controller is able to handle the asymmetric case and gives a more accurate thickness control compared to the existing control system. The new control strategy is also found to be stable. The task of the slab temperature controller is to ensure a high furnace throughput, and a proper heating quality while minimizing the energy consumption. Models for the slab and furnace temperatures are identified using data collected during normal production at The Danish Steel Works Ltd. A new way of obtaining parameters for the existing slab temperature control system is described. Furthermore, a nonlinear controller is designed which it is shown to be is globally asymptotically stable. The performance of the linear and nonlinear controller are compared using computer simulations which show that the performance of the nonlinear controller is superior to the performance of the linear controller. The linear slab temperature controller design has been implemented in the slab temperature control system at The Danish Steel Works Ltd. The experimental results indicate that the new controller parameters have lead to a 10% increase in furnace capacity, with unchanged heating quality. Data from the reheat furnace no. 2 indicate that the energy consumption has not been increased by the adjustments.