Operator of the paper machine watches traditionally the pulp layer which is formed, as the dilute wood fibre suspension flows from the headbox onto a moving, endless cloth belt called the wire. The main aim of his visual inspection is to trace the dry line or, in more physical terms, the line at which the free water disappears from the surface of the pulp. However, by naked eye he is neither able to observe more than a part or parts of the dry line nor to evaluate it quantitatively, because it changes continuously form and location, and his observations are at the same time disturbed by bright reflections and shadows. Straightforward substitution of the human vision by camera does not as such make the wire view any better, and replacement of the human brain by computer does not lead to a better extraction of the dry line from the deficient image obtained.

Fig1. Oblique view of the wire shows the power of strucured light in exposing the dry line.

The problem of dry line determination has now been solved by an inventive MV approach, while due attention has been paid to the laws of optics in structuring the illumination and in locating and orienting the video camera. The measurement systems developed provide full, correct and undisturbed information of the dry line for real time processing in computer. They exclude the blinding spots and present the total dry line range of the wire as a composition of two homogeneous, light and dark, fields which are separated from each other by the dry line. This range is repeatedly imaged, and the dry line extracted by an edge detection algorithm reproducing it as a digital vector. The dry line data thus obtained is displayed to the machine operator both graphically and by characteristic numbers in real time. The operators have accepted favourably this information which is exactly what they have always tried to trace by eye, since the prototype of the modern paper machine was built 200 years ago.

Fig. 2 Relatively regular, digitally extracted dry line on teh unprocessed wire image backgraound. Left: Distance from deadbox in meters. Uppermost: Monumentaru mean position of dry line. Middle: CD coordinate (slice actuator number). Curve at left relates to CD average of light received.

Use of the digital dry line signal has recently been extended to automatic control of the dry line in machine direction (MD). A feedback control loop has been implemented, in which the momentary average location of the dry line is compared with a given setpoint value and the headbox slice adjusted in accordance with their difference. The control system has been tested thoroughly and held in routine use for one and half years in a large paperboard machine of StoraEnso Co in Imatra, Finland. During this time, it has been proven that the machine vision based control keeps the average of the dry line at its setpoint eliminating fast the effects of disturbances which enter the wire from the headbox. It removes the need of human monitoring of the dry line, also during grade changes, and effects a fast transfer to a new setpoint. - Because the CD form of each dry line measured is already available in the form of 512 readings, the use of this fast information, together with the CD data obtained from the scanning instruments at the end of the machine, is presently being developed towards a better control of both the dry line and the final product profiles.

if the paper machine is provided with an overall automation system, 'ike in the ease described, it is practical to interface the dry line analyzer with it, in implementation of the control loop. Such interfaces have this far been constructed for transfer of dry line data to ABB/Accuray 1180 and Honeywell/Measurex MX Open systems. The methods developed for measurement and control of the dry line have been patented in several countries and can be made available by licence.

During grade change, all inputs of the paper machine chage over wide ranges resulting, after an initial dealy, in decrease of the dry basis weight of product from 200 to 178 grams per square meters (upper record). The control described reacts faster and keeps the CD average of dry line within +18/-12 mm of the setpoint (lower record), i.e. well within the permitted range of +/-400 mm.

For more detailed information please contact:

Prof Antti Niemi
Helsinki University of Technology
P.O.box 5400 
FIN-02015 HUT
Finland

Tel. +358 9 451 5202
fax  +358 9 451 5208
mobile +358 50 551 0712

Antti.J.Niemi@hut.fi