Adaptive feedforward thickness control in hot strip rolling with oil lubrication

The application of oil lubrication in hot strip rolling has many advantages for the process, i.e., reduced roll force, reduced energy consumption, better strip surface quality, and reduced roll wear. On the other hand, oil lubrication may also induce severe disturbances for several control loops in a tandem hot rolling mill, which can reduce the final strip quality or might even jeopardize a stable operation of the plant. For example, the widely used automatic gauge control (AGC) has a limited bandwidth to ensure stability in the whole operating range and thus cannot (dynamically) reject the thickness deviations caused by switching on and off the oil lubrication. For this reason, the effect of lubrication is analyzed in detail and mathematically modeled in this work. Based on this dynamic lubrication model together with a hydrodynamic roll gap model and a mill stand model, two feedforward control approaches for the strip thickness are presented. These controllers counteract the negative effects of varying lubrication. Moreover, the feedforward controllers are extended by a moving horizon estimator to track changes of uncertain parameters of the underlying models based on the measured roll force. The estimated parameters are instantly used by the feedforward controller. Simulation studies using an experimentally validated simulation model of a whole mill stand show that the proposed adaptive feedforward control approach provides a significant improvement of the strip exit thickness accuracy for strips with oil lubrication compared to conventional controllers.