Advanced Control Techniques for Rolling Mills

This paper addresses the problem of the product quality improvement in hot rolling mills. The simplest scheme for modeling the rolling process considers the reduction of a strip thickness by passing the hot strip through a preset roll gap between two work rolls. Each rolling stand is driven by a DC motor, which is equipped with a speed controller. Usually the two DC motors are separately controlled; i.e. each servo drive acts only on its motor, by imposing equal set points. An important condition to guarantee for a high product quality is the equalization of the motor torque acting on the two work rolls. Experimental data show that small differences between the angular speeds of the upper and of the lower work rolls lead to sensible differences between the two motor torques, deteriorating the product quality. A combined-control of the upper and lower DC servo-drives reduces the speed fluctuations; different control techniques, - including variable structure PI controllers - are proposed, evaluated and compared by extensive simulation tests.