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This project deals with a semi-physical model for the dimensional stability properties (i.e. curl) of the carton board produced at AssiDomän Frövi paper mill in Sweden. AssiDomän Frövi AB is one of Sweden major cartonboard manufacturer, and produces some 300000 ton of board per year. Curl in paper is an important problem since the departure from the flat may seriously affect the processing of the paper. For this reason, customers impose quite restrictive limits on the allowed curvatures of the board. So, it is becoming more and more important to be able to produce a cartonboard with a curl within certain limits. Due to the economic significance of the curl problem, much research has been performed to find sheet design and processing strategies to eliminate or reduce curl. At AssiDomän Frövi , the problem of dimensional instability causes the loss of several tons of board every year, and it is one of the most troublesome quality variable to handle for the operators and process engineers. At present, curl is measured once every hour from samples taken at the end of each tambour. After conditioning for 5 minutes, the curl and twist measurements are taken by an optical device with a good accuracy. However, the curl may vary considerably across the web, and a previous investigation showed that the standard deviation of the measurements is very high if compared with their mean value. The measured curl is then presented to the operators in form of process flow diagram, profiles and historical tendencies. It is then up to the operator to do adequate changes of the settings on the board machine in order to get the curl within the wanted limitations. Due to the complexity of the process, the operators often control the quality variables by intuition and not by a verified procedure. In this scenario, it is of great interest to find a model which can be used for both controlling the process and to achieve an improved process knowledge. The modeling approach we used is based on grey-box modeling. The reasons for such an approach is that the physical process is very complex and nonlinear. The influence of some inputs is not entirely understood, and besides it depends on a number of unknown parameters and unmodeled/unmesurable disturbances. A similar approach was used in Petterson's bending stiffness predictor, which is nowadays used in the mill as a tool for the operators. |