Variable Speed Scanning • Improving Measurement of Paper Quality, 2012 PaperCon Conference

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It has long been known that the CD profiles from conventional scanning sensors contain a mixture of MD and CD variations. This is often referred to as MD aliasing. The reasons for these phenomena are reviewed. Simulation studies have shown that scanning a moving web with a more random velocity pattern can reject much of this MD aliasing effect. In particular, fast MD variability can be kept from affecting the CD profile where it would result in erroneous control actions. Recently, one vendor has introduced a scanner that uses variable speeds for scanning. CD velocity is changed depending on direction and the operational situation. The results of a real mill experiment scanning forward and back and at the edges at different rates are presented. Variable speed scanning provides more realistic MD measurements and CD profiles than conventional single speed scanning. As part of this development, a number of tools have been built into the quality control system (QCS) to aid mill personnel in detecting and troubleshooting process variability problems. Real mill cases are presented where these tools found process issues.