New Possibilities using Modern Optics in Paper Production

Sammanfattning: During the last decades the need to control the paper production process has grown, due to the technological development and the continuing increases in quality demands. Recent development of sensors and fast, high-capacity computers open up new possibilities for the development of high capacity monitoring systems for the paper industry. Optical methods have advantages for performing fast and high-geometrical resolution measurements without any contact with the paper. However, this will require an increased measurement capacity, signal processing as well as a deepened understanding of "what to measure". One important quality parameter of the paper is its homogeneity, i.e. having as low variations as possible over the paper sheet in terms of different paper properties. Therefore, the need to monitor the distribution of different substances in the paper on-line have become important. In newsprint the main constituent is wood fibers, carrying cellulose, hemicellulose and lignin. Measuring the distribution of these constituents in the paper could, for example, be used as an indicator of the homogeneity of the final paper product. However, the print quality is governed by how well the smallest print entity, the individual halftone dot is reproduced on the paper. Therefore, the properties of the newsprint at precisely that spot on the paper where the dot is to be printed govern the result. Consequently, a high geometrical-resolution of the sensors of the monitoring system is required for measuring such variations in the paper. A robust sensor with high geometrical-resolution has been developed and tested for on-line measurements in paper production. The sensor is designed for measuring the fluorescence response from lignin in paper. A fast system for monitoring and recording the signal from the new sensor has also been designed and built. With this system a whole tambour, about 60 km of web length, can be sampled along a line with a physical distance between adjacent samples of 25e-6 m at a web speed of 30m/s. With this system the behavior of the paper machine can be studied at different length scales, from sub-millimeter variations to behaviors taking place at the scale of many kilometers of produced paper. The sensor system has been evaluated and tested on-line at newsprint mills. From these measurements it could be concluded that the system can be adapted to the industrial environment and give valuable results related to the variations in the process. The high geometrical resolution of the fluorescence sensor has opened up the possibility of measuring small variations in the paper structure. Another example that has been demonstrated is the possibility to measure imprints in the paper made by the forming fabric. This property has been used to develop a fast and robust method for measuring the paper shrinkage. The work presented in this thesis has contributed to the creation of a platform for the development of future sensors and data acquisition systems for on-line quality monitoring of paper production. The experiments made at the paper mills have demonstrated that the use of high resolution optical sensors and high-capacity data acquisition systems can provide new input for our understanding of the dynamics of the manufacturing process.