The advanced set-up and control system for Dofasco´s tandem cold mill

The new coupled pickle line and cold mill (CPCM) has been processing steel since December 1991 at Dofasco Inc. in Hamilton, Ontario, Canada. This mill rolls both sheet and tin plate. The work rolls for the last stand are often textured by grit blasting to roll “matte” finished strip. A new computer-targeted control system was developed for the roll speeds and gaps, specifically for matte rolling. The new set-up system uses Stone´s model, which is more appropriate for the limited reductions evident with the rougher rolls. Conventional automatic gauge control (AGC) systems using thickness measurements after the last stand are limited in performance by the effect of long transport delays. These limitations are particularly critical during mill acceleration and deceleration. The use of a thickness gauge before the stand with matte rolls allows the development of AGC systems using shorter control loops for final gauge control. The AGC system uses a Smith predictor for transport lag, (phase shift), and compensation. The combination of the shorter AGC loop and the Smith predictor allows higher gains to be used in the control system. This set-up and control system has contributed to Dofasco´s position as a market leader in thickness performance