Improved Color and pH Control Using Adaptive Deadtime Compensation, 1998 Process Control, Electrical & Info. Conference Proceedings

Product uniformity and paper machine stability were improved on two paper machines through the use of an advanced control method that reduces the effects of changes in process inputs and system response characteristics. The new adaptive method has been implemented to control color on two paper machines, and has reduced color variability by more than 30%. The same method was implemented to improve the control of we end pH on both machines and has reduced wet end pH variability by more than 60%. The improved wet end pH control has resulted in a 10% reduction in sizing chemicals as well as a significant improvement in sizing uniformity. The new adaptive strategy also reduced color and sizing rejects.

Transport and measurement delays, also known as deadtime, exist in many critical control loops in a pulp and paper mill and often result in less than optimal control. Deadtime compensation algorithms have been available since the 1950’s; however, their performance depends on the accuracy of the process model. In cases where the dynamics the process vary, such as in response to different grades, these algorithms can cause large oscillations and instabilities in the control system. In this paper, an easy-to-use method for adapting the model to changing process conditions is presented. Additional features of this adaptive compensation method are that it is fairly easy to understand and tune, and that it can be implemented on most digital controllers.