Optimum Separation of MD and CD Product Variations - The Scanning Measurement Challenge, 1991 Process Control, Electrical & Info. Conference Proceedings

Increasing the number of discrete measurements made during each scan across the sheet improves process visibility, by uncovering a wider range of machine-direction and cross-direction product variations. Higher resolution measurement also introduces more machine-direction variability into unprocessed "now" measurement profiles. Unless the incremental variance is removed by alternative methods, more trending will be required to determine accurate profiles for cross-direction control purposes. Unfortunately, increased exponential trending will reduce profile measurement response.

To avoid the need for increased trending, slower scanning has been suggested as a means to reduce the amount of high-frequency MD variability imported into untrended "now" measurement profiles. To avoid compromising speed of response by reducing scan rates, this paper proposes alternative techniques that reduce the influence of both high and low-frequency MD variability on the final control profile. Easter scanning, parallel measurement data processing, and adaptive profile trending permit higher frequency MD and CD measurement, without compromising the speed or accuracy of control profile determination.