Performance And Benefits Of Fiber Orientation CD Control with Bias Target Optimization, TAPPICon25

This paper presents the performance and benefits of a multivariable model predictive controller for cross directional (CD) fiber orientation (FO) processes with bias target optimization. This control can effectively eliminate problems with sheet flatness uniformity (warp, twist, etc) from board machines by providing simultaneous optimization of top and bottom layer fiber orientation profiles together with a derived proxy for twist.

Most paper mills that perform manual control of flatness depend on laboratory analysis of fiber orientation for both top and bottom layers. This analysis demands separation of layers before testing. This is a challenging task that requires skilled personnel. Process adjustments may be necessary prior to sampling to make layer separation possible. Like any process variable, sheet flatness uniformity (warp, twist, etc) changes and drifts over time, often going outside specifications. Since the lab samples are typically collected only at the end of jumbo reel, and time is required to process them in the lab, there is a large amount of product made that is not checked, resulting in customer claims.

Manual intervention based on on-line measurement of fiber orientation is possible, but requires continuous monitoring and frequent actions. Due to intrinsic multivariable nature of flatness, effective manual intervention needs a level of skill that is not available in most mills, and certainly not present around the clock. By contrast, automatic fiber orientation CD control with bias target optimization was able to correct flatness condition very quickly and continuously.

Twist controls such as those discussed here have been in use in liquid packaging and board mills in Europe for many years. Field application data collected from a European mill is presented in this paper to illustrate the performance and benefits of this type of control.

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