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Adaptive control of sheet caliper on paper machines

Caliper control is carried out in the calendering process in order to obtain even thickness of the sheet both in machine-direction and cross-direction. To control caliper, the local temperature of the calender rolls is manipulated. The relationship between temperature and caliper is nonlinear as heating and cooling must be represented by different linear processes. Both models feature timevarying parameters. The correlations among actuators across the roll are described in the interaction matrix of the C D model developed from mill data analysis. Generalized Predictive Control (GPC) is applied under system constraints in machine-direction (MD) and cross-machine direction (CD) control. This control algorithm can handle the timevarying parameters, couplings, and variable delays that are present in the caliper control system. The constrained optimal control signals of GPC are obtained by using the Lagrangian multiplier method. The on-line estimator is based upon Recursive Least Squares with Exponential Resetting and Forgetting factors. Simulations show satisfactory performance of constrained GPC in both M D and CD caliper control. The outputs track set-point changes closely and still have small variations even with parameter changes. Load disturbances added in the C D control system are rejected very well.