Model-based control of the transverse strip shape and position in a hot-dip galvanizing line

Hot-dip galvanizing is the state-of-the-art technology to produce zinc coats on steel strips. The primary objective of this process is to establish a zinc layer with a defined uniform thickness. In this way, the zinc consumption and thus the operating costs of the plant can be significantly reduced without violating the coating thickness requirement. One condition to achieve a homogeneous zinc layer is a uniform air gap between the strip and the gas wiping dies, where excess liquid zinc is blown off. Hence, a flat transverse strip profile at the gas wiping dies is required. In modern plants, electromagnetic actuators are used for contactless vibration and shape control of the strip. In the considered application, the control input (force of electromagnetic actuators), the sensor output (measured transverse strip displacements near the electromagnets), and the system output to be controlled (transverse strip displacement at the gas wiping dies) are usually located at different positions along the strip, which makes the overall control task quite challenging. In this work, model-based methods are developed to improve the homogeneity and accuracy of the zinc coating thickness compared to state-of-the-art methods. To this end, an estimator of the flatness defects is designed and validated for different test strips and settings of the plant. Using the validated mathematical model, a simulation study is conducted to compare the state-of-the-art control method with the developed feedforward controller. In order to suppress persistently exciting disturbances at the position of the system output, a control concept is developed and successfully validated by means of an experimental test rig that mimics the essential properties of the industrial hot-dip galvanizing line.