H-infinity control system for a four-high stand rolling mill

In steelworks the demand for a higher product quality level has improved th e rolling mill process by means of a proper work roll speed control system. Firstly this paper proposes a lumped-parameter hot-rolling mill model. The rolling mill examined is reversible and equipped with an independent speed control system for each work roll (two single-input single-output systems) . Therefore, the same revolution speed, essential for a high-quality proces s, can only be ensured by equal set point values. Nevertheless, during the process, different disturbance load torques cause speed differences and, ev en if they are not noticeable, rolled section defects cannot be avoided. Th e second part of the paper shows that it is possible to design a proper H-i nfinity controller, to change the two independent control systems into a mo re complex multiple-input multiple-output system that will obtain decidedly better performances. Simulation examples have shown improvements.