Next Generation Mold Level Control: Development of LevCon 2.0

Siemens VAI develops the next generation of the successful mold level controller LevCon. The new controller uses the most modern control principles from optimal control theory while being robust enough to operate in the difficult conditions of a steel work. One prerequisite for the application of modern control theory is a good mathematical model of the process. In the past, the models used for mold level control design were quite simple, neglecting some of the important limiting factors such as surface resonance waves of the mold level. For the derivation of a resonance surface wave model, extensive water model trials have been carried out. With the help of the plant model, modern control theory is applied and a mathematically optimal controller feedback is designed. It offers an increased disturbance rejection compared to earlier algorithms, which is significantly reducing the mold level deviation due to disturbances as, e.g., unclogging events. However, there are still disturbances that cannot be eliminated completely, like mold level hunting caused by instationary bulging. In the past, mold level hunting was reported to be a very slow disturbance with a wavelength of approximately one roller pitch and a frequency below 0.1 Hz. During the development work, it turned out that also higher harmonics of this base frequency exist with wavelengths of 1/2, 1/3, up to 1/7 of the roller pitch. These high frequency fluctuations cannot be eliminated by feedback control only. However, an additional feed-forward loop which utilizes the withdrawal drives power for direct compensation of the mold level hunting steel flow is developed. With this approach, it is possible to reduce even fast mold level hunting.