Ultrasonic non-destructive on-line estimation of the tensile stiffness of a running paper web

Abstract

A new method for measuring the elastic properties of paper on-line in a paper machine by using ultrasound has been developed. The unique feature of the method is that a continuous stochastic ultrasonic signal is used for measurements, which is generated by means of a dry friction between a running paper web and a rigidly fixed friction head. The signals from the web are picked up by an array of air-coupled ultrasonic receivers, which have no mechanical contact with the paper web. The ultrasound velocity in the web is obtained by means of the correlation processing. By using advanced digital signal processing, the tensile stiffness index (TSI) of the running web is determined. From the sensor we get the TSI values both in the machine direction and the cross-machine direction. The on-line ultrasonic sensors have been installed on two different paper machines for more than two years. This paper describes the background of the method, as well as some experiences and applications from two mill installations.