Looper H-infinity control for hot strip mills

The H-infinity control theory has been applied to some engineering fields including the steel making process. The H-infinity control theory makes it possible to apply multivariable control with frequency domain design method and to get robust stable control systems. The author developed a looper H-infinity multivariable control system for hot strip finishing mills, and have obtained better results than conventional control methods. The looper control is one of the most important control methods in hot strip mills. The loopers are placed between each rolling stand, and control the interstand strip tension. It is very important to control stably both looper and strip tension at each of their target values. Especially, the authors intended to reduce strip tension fluctuation than looper angle fluctuation, because strip tension severely affects thickness and width gage. This paper presents the design method of the looper H-infinity control system. Also showing some actual control data, they compare the performance of conventional PI control, noninteractive control, and H-infinity control.