Feedforward control of the transverse strip profile in hot-dip galvanizing lines. (August 2020)

• Record Type: Journal Article
• Authors: Saxinger, Martin; Marko, Lukas; Steinboeck, Andreas; Kugi, Andreas
• Is Part Of: Journal of process control. Volume 92(2020)
• Page Start: 35
• Page End: 49
• Publication Date: 2020-08
• Subjects: Distributed-parameter system -- Model-based control -- Feedforward control -- Steel industry -- Hot-dip galvanizing line -- Electromagnetic strip stabilizer -- Zinc coating; Process control -- Periodicals; Fabrication -- Contrôle -- Périodiques; Process control; Periodicals; Electronic journals; 660.281
• DOI: 10.1016/j.jprocont.2020.05.007
• Languages: English
• ISSNs: 0959-1524

Abstract

Abstract: 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. Graphical abstract: Highlights: A dynamic model for steel strips in industrial hot-dip galvanizing lines is derived. The model captures flatness defects of the strip and transverse loads. An estimator of the flatness defects is developed and validated in practice. A feedforward controller of the strip profile at an unmeasured position is developed. The feedforward controller is compared to existing control methods.