Periodic Disturbance Suppression in a Steel Plant with Unstable Internal Feedback and Delay

During continuous casting, especially of modern steel grades or at higher casting velocities, undesired oscillating disturbances may occur, which strongly impair the quality of the final product, or even lead to costly plant downtime. In this paper, both a novel model based explanation of these level oscillations in terms of unstable internal feedback and delay time, as well as a corresponding control to reject the oscillations are presented. The first part of the paper shows how these oscillations arise from couplings inside the strand and how they can be explained in terms of approximated periodic oscillations. To this end, both a new approach to model this oscillatory behavior as well as a frequency analysis is shown. This plant structure poses difficulties for control, as there is a substantial delay time in the plant and the disturbing oscillations depend on the quantitatively unknown internal feedback. To cope with these problems, a new control structure is proposed. Further a prediction based approach is presented, which allows to eliminate the periodic patterns in the oscillations and thus to lead those to rest.