Dynamic Control of Flatness and Elongation of the Strip in a Skin Pass Mill

Abstract

This paper proposes a dynamic control method of flatness and elongation of the strip in a skin pass mill. In the conventional feedback control, the target values of the flatness and the elongation are fixed. However, the elongation control to the fixed target value is often insufficient to achieve the strip flatness when the work roll deflection is caused by the rolling force manipulation of the elongation control. To improve the flatness control performance, the elongation control in consideration of the flatness is proposed. Periodically, the proposed method solves the optimization problem. The objective function including the control error of the flatness is minimized subject to the constraints such as the range of the elongation, strip thickness and control outputs. In addition, the feedforward control which suppresses the elongation deviations during mill speed change is also proposed. It is difficult to predict accurate rolling force online using a physical model because the computation load is very heavy. The proposed method utilizes the relationship between strain rate and deformation resistance of the strip which is measured offline and the designated rolling force change from the lowest to top rolling speed. This method doesn’t require a physical model and heavy computation load. The proposed method is evaluated by simulation and experiments and the results show that this control method has improved the flatness and the elongation control performance.