Control-oriented modeling of sheet and film processes

Sheet and film processes pose a challenging identification and control problem due to high complexity, poor conditioning, and limited input–output data. The interaction between model accuracy and closed‐loop performance is explored for sheet and film processes using a model decomposition in terms of two static orthogonal matrices in series with a diagonal transfer‐function matrix. It is shown that the accuracy of the diagonal elements of the transfer‐function matrix directly specifies the closed‐loop performance achievable by a model‐based controller. This motivates the development of a combined identification and control procedure in which the controller is designed to be robust to model inaccuracies quantified during identification. The resulting controller is compared to an industrially accepted controller design method for two examples, including a simulated blown‐film process. Based on the theoretical results and simulations, it was concluded that the poor performance often reported for existing industrial sheet and film process‐control systems is likely due to signs of the model gains being incorrectly identified.