An Optimal Control Method for Improvement in Tandem Cold Metal Rolling

Controlling the tandem cold rolling of metal strip is a significant engineering challenge mostly because of the complexity of the interactions between the process variables, the nonlinearities which change with the operating conditions, and the long time delays which change significantly with the mill speed. The present technology is based on methods that divide the control problem into several independent control sub-problems, which results in a limited capability for improvement in performance. This paper describes a new control strategy wherein the control of the mill is treated as a single interconnected problem that is addressed by solving a state-dependent algebraic Riccati equation pointwise to establish a control law for a multi-input-multi-output (MIMO) controller which is enhanced by appropriate trimming functions. Simulation testing of the new controller coupled to a nonlinear model of the process showed that the tolerance in mill exit thickness could be held to about .2% for typical rolling operations, which compares favorably to the tolerances of .5% to .8% using existing techniques. In addition to this improvement in performance, the simplicity of the new control method also offers the potential to reduce the efforts required for design, commissioning, and maintenance.