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Multivariable Control of Cold Tandem Mill

The main function of the control system of a tandem cold-rolling mill is to produce the strip with a prescribed thickness accuracy and to maintain the interstand tensions within a a reasonable range in the rolling operation, against disturbances such as the mill input thickness, the stand set-up error, the drift of ingoing strip hardness, the coefficient of friction and so on. In a conventional control scheme, the output thickness is controlled by adjusting the screw position, while the interstand tension is controlled by updating the roll speed. Since the output thickness is strongly coupled with interstand tension, it is natural to consider that the integration of thickness control and tension control will result in a better control performance. In this paper we demonstrate an integrated control scheme for output thickness and interstand tension using the modern state-space approach. A state-space model for 5-stand coldrolling mill is derived and an optimal feedback control law is calculated for this model, incorporated with an integral action to eliminate the tension off-set. The output thickness of the preceding stand is employed as a feedforward signal to update the screw position of each stand. Simulation results are exhibited in comparison with that of a conventional control scheme. It is concluded that our system demonstrates a better control performance over the conventional one. Moreover it suggests an approach for shape control by adjusting weighting matrices of the performance index.