Looper H-infinity control for hot-strip mills

H-infinity (H_∞) control theory has been applied to some engineering fields including the steel making process. The H_∞ control theory makes it possible to apply multivariable control with frequency domain design method and to get robust stable control systems. A looper H_∞ multivariable control system for hot-strip finishing mills is developed and achieves better results than conventional control methods. Looper control is one of the most important control items in hot-strip mills. A looper is placed between rolling stands and is used for controlling the interstand strip tension. It is very important to control stably both a looper and strip tension at their target values. Especially, we intended to reduce strip tension fluctuation more than looper angle fluctuation, because strip tension severely affects thickness and width gage. This paper presents the design method of the looper H_∞ control system. Also showing some actual control data, we compare the performance of conventional PI control, noninteractive control (NIC), and H_∞ control