A New Strategy for Control of Tandem Cold Metal Rolling

The control of tandem cold rolling of metal strip constitutes a significant engineering challenge to control engineers. This is due mostly to the complexity of the interactions between the process variables, nonlinearities which change with the operating conditions, and long speed- dependent time delays. The present technology is based on control strategies that decompose the problem into several independent control sub-problems and as a result yield limited capability for improvement in performance. This paper describes a new control strategy wherein a state-dependent algebraic Riccati equation is solved pointwise to establish a control law for a multi-input-multi-output (MIMO) controller with appropriate trimming functions. The new controller coupled to a nonlinear model of the process was tested by simulations. The resulting tolerance in mill exit thickness was held to about .2% for typical rolling operations, which compares favorably to the tolerances of .5% to .8% using existing controllers. In addition to improved performance, the simplicity of the new control method reduces the efforts required for design, commissioning, and maintenance.