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. Index Terms – Quasi periodic disturbance, delay system, disturbance suppression. I. THE CONTINUOUS CASTING PROCESS The main setup of a continuous casting plant, also described e.g. in [1], [2], [3], [4], and [5], is shown in Fig. 1. Liquid steel is transported through the ladle to the tundish, basically a liquid steel reservoir, where the level is kept constant. The level control therefore is not problematic and in this work not of our concern. From the tundish liquid steel flows through a nozzle and a pipe (usually a submerged entry nozzle) to the water cooled mold, where solidification starts. Supported through rolls and under constant cooling and thereby solidification the strand is transported downwards. At the horizontal end of the plant the strand is fully solidified and can be cut into slabs without interruption of the continuous process. Usually, i.e. for large slab formats, the casting speed (velocity of the strand) is given and only the inflow of the mold can be regulated through some hydraulic activated valve (e.g. stopper nozzle system or sliding gate system). The level of the liquid steel in the mold is needed as control input and is either measured with a radioactive or an electromagnetic method. The plant to be controlled has a simple structure as shown in Fig. 2. The servo system, which is fast compared to the rest of the plant, sets the flow through the pipe. The liquid steel flow through the pipe is modeled as simple time delay and represents a fall time.