Influence of Measurement Errors on CD Profile Quality, 1997 Process Control, Electrical & Info. Conference Proceedings

Cross direction (CD) profile control systems on modern paper machines or coaters produce a 2 s value of the controlled profile, which is very close to the measurement accuracy of today’s scanners. For a proper judgment of a given CD profile it is therefore necessary to know exactly how much measurement error contributes to the 2 s value displayed by the quality control system and how far paper quality is affected if an automatic profile control system is used. Recent research on this topic on a paper machine equipped with a headbox - having narrow-spaced dilution valves for CD profile control - showed that the measurement error leads to an indicated basis weight profile error by the quality control system with a 2 s value of 0.21% based on 138 data values (which was the number of actuators), assuming the paper has an absolutely uniform quality. The quality of the scanner used was middle range, whereas some scanners installed in paper machines produce an accuracy lower or higher by about a factor of 2. The effect of erroneous measurements on an automatic CD profile control system is discussed in detail. Finally an easy-to-use test procedure is described for evaluating the stable profile measurement error and the measurement noise of a CD profile scanner.