Alias Impact on Control Profiles, 19PaperCon
In the paper industry it is well known that cyclical Machine Direction (MD) disturbances will cause false Cross Direction (CD) patterns that appear in profiles. When acted on by CD controls, these false patterns can excite CD actuators to move and cause the actual profile to degrade by building the inverse image of the alias and continually chasing this morphing pattern. This paper will review MD aliasing and show how one-dimensional (1D) MD/CD aliasing can produce false control profiles which degrade control performance. More complex two-dimensional (2D) MD/CD aliasing will be introduced along with the problems and additional difficulty that it poses. As CD controls typically use a filtered profile, the impact of aliases on the control profile are studied. In most cases a filter reduces the severity of aliasing to the control profile to an acceptable level that permits reasonable control actions that correct true CD variations without causing excessive errors due to aliasing. However, this may not always be true. Three types of 1D MD/CD aliases are identified: stable, moving and repeating. Stable and repeating aliases where the scan time and MD wavelength are harmonics1 cause special challenges. Futhermore, moving aliases are not benign and cause errors in control profiles and instability. Two myths for removing MD/CD aliases are debunked. Techniques for avoiding CD control problems caused by aliases are discussed.
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