IEEE TRANSACTIONS ON CONTROL SYSTEMS TECHNOLOGY, VOL. 5, NO. 6, NOVEMBER 1997 571 On-Line Sensing of Paper Machine Wet-End Properties: Dry-Line Detector Allan P. Kjaer, Peter E. Wellstead, William P. Heath Abstract— This paper is concerned with the task of sensing in web-forming process control loops. Conventional and newly proposed measurement techniques are discussed and a new type of measurement is suggested. The new measurement method adresses the problem of paper machine “wet-end” sensing and is based on a CCD camera system which can detect the dry-line. Both the theoretical and practical aspects of the invention are investigated and the sensing system is developed into a portable measurement system which is applied to a paper machine. The paper machine measurements are analyzed and presented using nonparametric techniques based on analysis of variance. Com- pared to traditional scanning gauge systems the dry-line detector allows for a large reduction in the transport delay, higher data density and sampling rate, and better machine directional (MD), and cross-directional (CD) signal decomposition. Index Terms— Analysis of variance, cross-direction, dry-line, image transforms, machine-direction, nonparametric techniques, paper industry, plastic film industry, three-dimensional systems, two-dimensional systems. I. INTRODUCTION AND REVIEW OF EXISTING SENSING TECHNIQUES A. Introduction S ENSING is one of the weak points in paper machine profile control loops because none of the existing measure- ment techniques [1]–[3] can supply the amount of information required by the machine directional (MD) and cross-directional (CD) loops (see, e.g., [4]). This implies that most manufac- turers of CD equipment have a problem in demonstrating that their sensor system can meet the specifications. One of the typical problems for the manufacturers is the separation of the MD part of the signal from the CD [5], [6]. Of the other problems it is appropriate to mention the very low data density and the large transport delays caused by the position of the profile measurement at the drying section output [7], [8]. Consequently, there is a demand for improved CD sensing in order that better control can be obtained. The most frequently used sensing method is a point mea- surement device in scanning mode (see Fig. 1). This creates Manuscript received December 4, 1995; revised December 28, 1996. Recommended by Associate Editor, N. Yoshitani. This work was sponsored by SERC Grant 9R/M21692 under the LINK scheme and NATO Research Grant 910 435. A. P. Kjaer is with the The Danish Steelwork, 3300 Frederiksvaerk, Denmark. P. E. Wellstead is with the Control System Center, UMIST, M60 1QD Manchester, U.K. W. P. Heath is with Lucas Strategic Planning, AVSD, Solihull, B90 4JJ, U.K. Publisher Item Identifier S 1063-6536(97)07777-4. Fig. 1. Zig-zag measurement pattern. A point measurement device (scanner) is moved back and forth over the web. The speed of the device, , is much smaller than the speed of the web, . This can in the worst case produce an angle, , less than one degree. The measurement of the web properties is therefore extremely sparse. a zig-zag measurement pattern where the speed of the web is much greater than the speed of the scanner. As a result, the measurement process provides only sparse information concerning the web variations. Typically, a paper profile scanning device will actually measure only about 0.001% of the surface of the sheet. Thus the control process is fundamentally limited by the restricted area of the web which is scanned. A further problem with conventional sensing is aliasing. The sampling process of the scanner is associated with a burst of energy (beta rays, near infrared, visual light, X-rays, or other types of spectral energy) and an averaging process applied to the transmitted information over the sample period. Because the measurement is obtained directly as a discrete value it is not possible to limit the measurement bandwidth before the sampling and hence aliasing can occur. The sampling rate in the CD is relatively high and only very narrow web streaks with significant power at high frequencies might cause aliasing. In the MD, however, the values are obtained at such 1063–6536/97$10.00  1997 IEEE