Year 1995
Juha Piipponen

Basis Weight Control of Plies of Multiply Board in Machine Direction and Cross-Direction

The aim of this study was to develop methods to improve control of individual layer in both machine and cross direction. Basis weight control of individual layer on machine direction is based on feedforward control with thick stock flow and consistency. These is no way to measure the dry basis weight of a single layer in cross-direction.

New methods for measuring basis weight of layer were tested. Measurements and control schemes based on them were tested on a full-scale board machine. Consistency measurements of thick stock and short circulation were evaluated for machine direction control. In cross-direction control gamma backscatter -based weight measurement and observation of dryline were examined.

Basis weight control in machine direction can be improved by improving control of thick stock consistency. New microwave-based consistency measurement method was found to better than the older methods based on viscosity of fiber network. The estimation and control of bases weight can also be don by using consistency measurements of headbox and white water. By this the measurement point can be moved closer to sheet formation. Basis weight measurement at wire section depends strongly on water removal and thus not on dry basis weight, which makes the method unsuitable in this application.

Basis weight measurement gives fairly good of cross-direction profile of dry weight. Most important disturbances are shrinkage profile and cross-direction profile of drainage. The control can be done by least-squares optimisation based on static response model. Observation of dryline by CCD-camera and image analysis was found very difficult to implement on this boardmachine so that the results would be good enough for control. The method does not seem very promising for this application.

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