An Improved Strip Thickness Controller for a Rolling Mill

Abstract

This paper describes an improved controller design for a conventional metal strip rolling mill with two work-rolls supported by larger diameter backup-rolls. The strategy is based on a model which represents the effects of backup-roll eccentricity, the plastic deformation process and elastic strains in the mill stand components. The strategy overcomes many of the fundamental difficulties inherent in earlier approaches. An important feature of the design is its ability to track an arbitrary backup-roll induced eccentricity disturbance whose waveform changes dynamically and whose period is not known accurately because speed and rotation measurements are only available for the work-rolls. The feedback controller design compensates for force, hysteresis, mill dependent parameters and for the inherently nonlinear response of the force adjusting mechanism. Simulation studies are presented showing the advantages of the new design and its insensitivity to plant parameter variations. The system has modest computation requirements and has been implemented on a modern, computer-controlled, five stand tandem cold mill at the Western Port Works of John Lysaght (Australia) Limited.