Hot-dip galvanizing is a standard technology to produce coated steel
strips. The primary objective of the galvanizing process is to establish a homogeneous zinc layer with a defined thickness. One condition to achieve this objective is a uniform transverse distance between the
strip and the gas wiping dies, which blow off excessive liquid zinc. Therefore, a flat
strip profile at the gas wiping dies is required. However,
strips processed in such plants often exhibit residual curvatures which entail unknown flatness defects of the
strip. Such flatness defects cause non-uniform air gaps and hence an inhomogeneous zinc coating thickness. Modern hot-dip galvanizing lines often use electromagnets to control the transverse
strip profile near the gas wiping dies. Typically, the control algorithms ensure a flat
strip profile at the electromagnets because the sensors for the transverse
strip displacement are also located at this position and it is unfeasible to mount displacement sensors directly at the gas wiping dies. This brings along that in general a flatness defect remains at the gas wiping dies, which in turn entails a suboptimal coating.In this paper, a model-based method for a feedforward control of the
strip profile at the position of the gas wiping dies is developed. This method is based on a plate model of the axially moving
strip that takes into account the flatness defects in the
strip. First, an estimator of the flatness defects is developed and validated for various test
strips and settings of the plant. Using the validated mathematical model, a simulation study is performed to compare the state-of-the-art control approach (flat
strip profile at the electromagnets) with the optimization-based feedforward controller (flat
strip profile at the gas wiping dies) proposed in this paper. Moreover, the influence of the distance between the gas wiping dies and the electromagnets is investigated in detail.
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