Hot strip mill multivariable mass flow control

Hot strip mills are processes with a very high throughput of products. They are highly developed capital intensive and competitive businesses where control systems are essential to maintain product quality and production efficiency. Even small improvements in the accuracy of the product and mill availability yield significant savings for the steel producer. To this end the development and trial implementation of a multivariable controller to control the gauge and mass flow for one stand and interstand in a tandem hot strip mill is described. A feature of the design is that no new or additional equipment to that installed for the conventional control system is required for the multivariable design. Gauge control is essential to product quality while regulating mass flow is important for stable operation of the complete mill. Gauge and mass flow are tightly coupled. The control design, which uses state feedback and estimation, is described along with the issues necessary for a practical implementation. Inherent in the multivariable design is the explicit ability to trade-off gauge and mass flow performance against each other. The performance of the controller from a trial implementation on a seven-stand hot strip mill is analysed and compared with the existing mill controllers.