At present, most of the multi-variable flatness control systems of cold rolling mills are developed based on global optimisation methods. However, the global optimisation methods always occupy a large amount of the controller resource, leading to a reduced control efficiency and real-time performance. In order to improve the efficiency of multi-variable flatness control, a new control method, based on the relay optimisation, has been proposed. First of all, the multi-variable optimisation model of flatness control is converted into a series of univariable optimisation models in the relay mode. Then the optimal adjustments of various flatness actuators will be obtained in cyclic optimisation processes. Moreover, the structure of flatness control system and the time delay compensation strategy have been designed in accordance with the proposed method. It is found that the proposed control method is efficient and effective for the multi-variable flatness control process by means of numerical experiments and applications.
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