3 1/2 DOF Polynomial Solution of the Inferential H2/H∞ Control Problem With Application to Metal Rolling

The solution of a new inferential 3 1/2-degree-of-freedom stochastic optimal control problem is discussed which may be either H2 or H∞ optimal. The system includes a 2 1/2-degree-of-freedom inferential tracking controller and a feedforward control action. The feedback controller must be robust and is therefore normally designed to minimize an H∞, criterion. However, the solution obtained is sufficiently general that the feedback, tracking or feedforward control functions can be chosen to minimize either an H2 or a H∞ criterion. The inferential control problem considered was motivated by a problem in the control of metal strip rolling mills. The design study presented considers the control of strip thickness given stand force measurements, and the use of feedforward signal from an upstream stand.