Controller for reducing excursions in tensions, thicknesses and looper positions during threading of a hot metal strip rolling process

The highly complex and nonlinear nature of the tandem hot metal strip rolling process presents a difficult control challenge. The control of the threading phase of this process is especially difficult as, in addition to the hostile rolling environment which precludes the location of certain sensors important for control, the model of the process changes rapidly as the head end of the strip is sequenced from stand to stand during threading. To improve the quality of the final product during threading it is necessary to reduce excursions in the strip tension, looper position, as well as strip thickness. This paper extends our previous work to develop an improved controller for this process to include the threading phase. The controller resulting from our previous work in this area is based on the use of an augmented state-dependent Riccati equation technique which has been shown by simulation to be highly successful for control of the threaded tandem mill. In this paper we present a comprehensive model of the threading process plus the results of our initial work to develop a suitable controller which handles the rapid changes in the model during threading. Simulations demonstrate the successful results of this work.