Dynamics and variance control of hot mill loopers

Poor control of hot strip mill loopers degrades strip width and gauge, and may even lead to mill breakdowns due to instability. In this study, dynamics of the looper–strip system and the control challenges it poses are discussed, and covariance control theory is applied to variance control design for loopers. Since looper disturbances have a deterministic nature, and their accurate modelling is a challenge that is not easily addressed, the problem represents a case in which a variance controller has to be designed without an explicit disturbance model. Control performance is assessed on a full-stand, nonlinear high fidelity finishing mill simulator, and comparisons to a conventional control system are provided.