A non-contacting online caliper measurement has been papermakers' dream for over two decades. Currently, paper thickness is measured using buttons contacting the paper web on both sides. In such a configuration, paper thickness is assumed to be the distance between the contacting surfaces and determined by a magnetic measurement principle. However, this arrangement of contacting measurement has several disadvantages including sheet marking, hole creation, dirt build-up on the contacting buttons, wearing of the contacting surfaces, and even sheet breaks. Moreover, the current trends in paper manufacturing, especially the increasing use of recycled raw materials are necessitating the development of a more reliable thickness measurement solution that is not affected by dirt and other material on paper or board sheet surfaces. So far, a non-destructive, on-line thickness measurement has not been successfully applied in paper production environment. Recently, Metso Automation has successfully piloted in several mills a caliper sensor that does not contact the sheet on both sides and is able to measure paper thickness with sub-micron accuracy. The new sensor is based on single sided laser triangulation. This paper presents the measurement set-up and discusses the challenges encountered. Measurement results obtained in mill trials with various paper grades are reviewed and compared to those made simultaneously with contacting, on-line sensors and off-line laboratory results of the same sheet. Factors affecting the measurement with conventional and optical thickness sensors are also discussed.

Date Published: 13 June 2005
PDF: 9 pages
Proc. SPIE 5856, Optical Measurement Systems for Industrial Inspection IV, (13 June 2005); doi: 10.1117/12.611890

Published in SPIE Proceedings Vol. 5856:
Optical Measurement Systems for Industrial Inspection IV
Wolfgang Osten; Christophe Gorecki; Erik L. Novak, Editor(s)