Looper and Tension Control in the Hot Strip Finishing Mill Based on Decentralization and Coordination

This paper presents a new control method for interstand tension and looper height in the hot strip finishing mill. In the finishing mill, several passes of rolling are executed by tandem rolling in the presense of interstand tension. Tension can be adjusted either by manipulating roll speed or by using the looper, which is implemented between stands. Since tension and looper height control is the key to successful mill operations, several multivariable control schemes have been applied to cope with interactions between tension and the looper. However, they lead to complicated controllers which are hard to be tuned on site. The proposed method is based on decentralization and coordination. First, the iteractions are evaluated using the structured singular value to ensure the applicability of decentralized control. Then, a decentralized controller based on the two-degree-of-freedom IMC (Internal Model Control) sturucture is applied. Moreover, impedance control of the looper is introduced to retrieve the coordinated action of the looper and to improve transitional responses of tension. The control scheme enables stepwise commissioning of control systems, which is very helpful for on-line tuning. The control scheme has been successfully applied to the tension and looper control system in a hot strip mill.