Dual adaptive control of paper coating

The paper is concerned with the cross directional (CD) control of coat weight on industrial bent blade coaters. The coater is a coupled multivariable process whose gain drifts over time and often switches sign. Current industrial practice is to switch off automatic control when the loop becomes unstable due to gain sign reversal. Because of this, the standard industrial controller is rarely on for more than half of the blade life. The control strategy developed in the paper is based primarily on viewing the coater as a plant with uncertain linear characteristics. An active suboptimal dual controller minimizes a nonlinear performance index designed specifically to reflect the peculiarities of the process. Thus, no heuristic logic is needed. The controller, which takes into consideration the loading and bending limitations imposed by the actuators and the blade, is coupled with an adaptive Kalman filter for parameter estimation. An approximate analytical control law was derived to allow for easy implementation. The proposed strategy was successfully applied to an off-machine bent blade coater. The results of a series of trials show the advantages of including probing in the control signal, and that the adaptive Kalman filter was capable of tracking gain variations. The dual controller yielded substantial quality improvement and was able to control the process throughout the entire blade life. The developed strategy was well accepted by the company.