CA2295543A1

CA2295543A1 - Method and device for determining the thickness of paper or cardboard by measuring on a continuous material web

Info

Publication number: CA2295543A1
Application number: CA002295543A
Authority: CA
Inventors: Hermann Hartenstein; Uwe Lampe; Christoph Roth
Original assignee: Individual
Current assignee: Siemens AG
Priority date: 1997-07-07
Filing date: 1998-07-07
Publication date: 1999-01-21
Also published as: ES2185212T3; DE19830323A1; PT995076E; DE59805827D1; EP0995076A1; WO1999002941A1; EP0995076B1; ATE225497T1

Abstract

The invention relates to a method for determining the thickness of a web of continuous material, in which contactless optical methods are used, and measuring takes place with optical spectra, preferably in the infrared (IR) range. In accordance with the invention, continuous spectra are evaluated by chemometric methods and/or neuronal networks, from which the thickness of the material web is advantageously determined by using training sets. An appropriate device for carrying out the method is characterized by a spectrometer (1) having at least one light source, means (10, 10', ...) for transferring the light to the material web (100) and devices (30, 31, 31') for measuring the light intensity after transmission through the material web (100) and for subsequent evaluation in order to determine the material thickness.

Description

r ' ' CA 02295543 2000-O1-OS
GR 97 P 3452 P FILE, P+N-~~ T'~I$ ~
Description T~AI~SLATI(~~
Method and apparatus for determining the thickness of paper or cardboard by measurement on a continuous material web The invention relates to a method for determining the thickness of paper or cardboard by measurement on a continuous material web, non-contacting optical methods being used. In addition, the invention also relates to associated apparatus for implementing the method.
In practice in the paper industry, hitherto the paper thickness on a continuous paper web has been measured with contact by pressing on the paper and while traversing over the entire width of the paper web. Non-traversing measurements over the entire material web width, which can be up to 10 m in paper mills, have hitherto not been used.
Non-contacting measurements which use optical principles, such as laser measurements or spectroscopy, have certainly also been proposed for measuring the thickness of paper, but cannot be used successfully in practice on high-speed paper webs.
EP 0 250 365 A2 discloses methods and associated apparatus with which properties of two-dimensional objects, that is to say including the thickness among other things, are intended to be measured optically and without contact. In this case, operations are carried out in each case with at least three discrete wavelengths, the transmission or the reflection capacity of the object is registered, and the required characteristic value is determined by means of correlation with known characteristic values of known objects. , ' CA 02295543 2000-O1-OS

The object of the invention is, in contrast, to specify a different optical method for measuring the thickness of paper or cardboard on a high-speed material web, and to provide the associated apparatus.
With respect to the method, according to the invention the object is achieved by the sequence of method steps according to patent claim 1. Advantageous developments are specified in the subclaims. Associated apparatus is identified in the substantive claims.
In the invention, the electromagnetic radiation used is preferably the infrared range, of this preferably the range of the near infrared (NIR:
0.8 - 2.5 Vim) and the medium infrared (MIR: 2.5 - 10 Vim). It is essential that continuous spectra are registered and evaluated in these spectral ranges.
It is therefore possible for a measurement to determine the paper thickness on a paper web running at high speed in the papermaking machine to be carried out in particular without contact and online, specifically with the aid of infrared spectroscopy. It has been found that a precise determination of the thickness is possible from the absorption of the spectra by the cellulose constituents of paper or cardboard. The measurement of the absorption can preferably be carried out by means of a linear array of a number of sensors as an array transverse to the paper web. Traversing over the paper web is then not necessary.
In accordance with the Lambert-Beer law, the absorption of electromagnetic radiation depends on the layer thickness of the absorbing substance. It is therefore possible to measure the absorption of electromagnetic radiation in transmission through the paper. The paper thickness is then determined from the measured values of the absorption.

' GR 97 P 3452 P

Since the composition of paper or cardboard varies from grade to grade, irrespective of the paper thickness, it has an influence on the absorption of infrared light, which can change as a result at constant paper thickness. In order to register such effects, it is necessary to determine the influence of the paper composition on the absorption. For this purpose, measurements are likewise made with continuous spectra in the specified spectral range.
The evaluation of the continuous spectrum specifically to determine the thickness of the continuous material web is carried out in particular with the aid of chemometric methods, for example PCA
(Principal Component Analysis) and/or PLS (Partial Least Square). Alternatively, evaluation via neural networks is also possible. In both cases it is advantageous to employ training sets of papers with known characteristics for the measurement and evaluation system. It is possible to set up suitable models in this way.
As already mentioned, the absorption in paper or cardboard depends significantly on the composition of the paper or of the cardboard, for which purpose, depending on the known raw material composition, such as pulps, waste paper, fillers, different training sets are stored, in which the known relationships between absorption and composition and material thickness are taken into account. Since no change to the raw material composition takes place during paper or cardboard production during the production of one grade, the change from one training set to another is then necessary only in the event of a grade change if a change to the raw material composition is associated with it.
In addition to the evaluation of the continuous spectra, in order to calculate the paper thickness, other measured data relating to the paper ,r CA 02295543 2000-O1-OS

or cardboard or else from the papermaking machine can be included. In particular, the porosity or the formation of the paper, which can be measured directly by optical methods, can be taken into account. It is also possible to calculate material variables indirectly during production via a combination of relevant machine parameters, such as vacuum, speeds, press pressure, steam consumption.
Traversing advantageously becomes superfluous as a result of a linear array of infrared sensors being fitted to a measuring frame transversely with respect to the paper web, so that simultaneous measurement of the transverse profile of the paper is made possible.
The,number of measurement points required is predefined by the number of actuators on the papermaking machine, for'example on the headbox. The number of measurement points can be increased by means of synchronized deflection of the measuring frame with the sensor array?.
Further details and advantages of the invention emerge from the following figure description of exemplary embodiments with reference to the drawing, in which paper manufacture is specifically discussed, this applying in a corresponding way to cardboard. In the figure, in each case in a rough schematic illustration, Figure 1 shows an online spectrometer on a paper web for a sequential measurement in the transverse direction of the paper web, Figure 2 shows a modification of Fig. 1 with a line of light sources, Figure 3 shows an online spectrometer for a simultaneous measurement in the transverse direction of the paper web, and Figure 4 shows an online spectrometer which measures against a reflective roll, using diffuse reflection.

,, CA 02295543 2000-O1-OS

The method according to the invention emerges from the common description of the individual figures:
In the figures, in each case identical parts have identical reference symbols.
The basic construction of an online spectrometer for the measurement in the transverse direction on a paper web running at high speed comprises:
- one or more light sources - individual glass fibers from the light source to the paper web, with which parallel illumination of the paper web is achieved - measuring devices for the absorption/
transmission as response signals following the interaction of the light with the continuous paper web - a computer (PC) with suitable software for controlling the measuring instrument, for picking up the signals and evaluating the signals.
In Figure 1, a light source 1 is fitted at a suitable position above a paper web 100. The light is led toward the paper web, for example by means of glass fibers 10, 10', ..., so that the result is uniform illumination in the lateral direction of the continuous paper web. In this case, the number of glass fibers 10, 10', ... to be connected depends on the number of measurement points to be implemented transversely with respect to the paper web. Measurements should advantageously be carried out in the optical range of the NIR (near infrared: 0.8 - 2.5 Vim), since inexpensive glass fibers of adequate length and quality are available for this spectral range, and a measurement in transmission is possible, even through relatively thick paper.
The light passes through the paper web 100. The measurement of the remaining light intensity and therefore the absorption can be carried out in two alternative ways:
- sequential measurement across the paper web, using an optical multiplexer - simultaneous measurement across the paper web using an array of spectrometers using microsystem technology.
In the case of a sequential measurement across the paper web 100, the measurement points are measured one after another transversely with respect to the paper web 100. In this case, the lateral local resolution depends only on the number of measurement points implemented, the resolution in the longitudinal direction depends on the measurement speed of the spectrometer and the number of measurement points.
According to Figure 1, such a spectrometer comprises a sufficient number of glass fibers 20, 20', ...
corresponding to the measurement points to be implemented, a multiplexer 25, a monochromator (not illustrated in detail) and a detector 30. As a result of the use of the glass fibers, the latter components can be operated physically separated from the paper web 100, so that they are in particular not exposed to the high temperature of the paper web running at high speed in the production process.
As an alternative to optical multiplexing, Figure 2 illustrates the construction of a line of light sources corresponding to the number of measurement points. The light sources 1, 1', ... are switched on and off one after another in accordance with the desired measurement frequency. In each case, only the measurement point from which the spectrum is to be picked up is then illuminated. In this way, complicated optical multiplexing is dispensed with, since the switching of the light sources can be performed purely electronically.
One monochromator is needed for the evaluation.
For this purpose, in principle two implementations are possible, specifically - optical gratings - so-called AOTFs (Acoustical-Optical Tunable Filters).

. CA 02295543 2000-O1-OS
' GR 97 P 3452 P
- 6a -Optical gratings constitute a conventional solution. If it is wished to avoid a mechanically moved grating, inter alia because they cannot . CA 02295543 2000-O1-OS

_ 7 _ be used to implement short measurement times in the range of a few ms, a detector array is needed for this type of monochromator. On the other hand, the AOTF acts directly as a monochromator, so that only the light S with the desired wavelength passes to the crystal.
The AOTF is a material whose refractive index for visible light or infrared light can be adjusted by applying an acoustic vibration, so that only light of the desired wavelength can pass through the monochromator to the detector.
Since drifting of the individual components of the spectrometer, in particular the light source and the detectors, cannot be ruled out, the possibility of a reference measurement must be provided. In the simplest case, a glass fiber is led directly from the light source to the detector for this purpose. An additional measure is a suitable mathematical pretreatment of the continuous spectra.
The detector has to be cooled in order to improve the signal stability, for which purpose a Pettier element is sufficient. In order to drive the spectrometer and to evaluate the measurement results, an industrial PC is needed. The spectrometer, including the PC, can be accommodated at the point of use, for example in a temperature-controlled container.
In the case of a simultaneous measurement across the paper web, an array of miniature spectrometers using microsystem technology (MST) is built up transversely with respect to the paper web.
The significant advantage of such a system resides in the fact that the measurement on the paper web can be carried out virtually in real time. This means that the maximum local resolution can be achieved, laterally as in the longitudinal direction.

Using an arrangement according to Figure 3, the measurement can be improved considerably, above all on high-speed papermaking machines. In principle, the microspectrometer contains the same components as Figure 1, but the multiplexer 20 is omitted.
In Figure 4, a measuring arrangement corresponding to Figure 3, in which optical fibers 15, 15~, ... operate as transmitting/receiving optical fibers, measures against a reflecting roll 16 behind the paper web 100, using diffuse reflection. In principle, the spectrometer contains the same components as in Figure 1, with an optical multiplexer 25 and detector 30. A construction corresponding to Figure 2 or Figure 3 is likewise possible.
In order to determine the continuous spectra, the infrared spectrum of the paper is measured in the NIR range, in particular or, if necessary, also in the MIR range. The measured spectra are then preprocessed, for which purpose suitable software is available. This includes, inter alias - smoothing the spectra - zero-line correction - normalization - averaging - identifying outlyers, that is to say disturbances which are produced, for example, by measurements on extended dirt spots - forming derivatives or integrals - identifying peaks with regard to their intensity, peak width and area integral.
The actual evaluation follows: using chemometric methods, known per se, for the evaluation of spectra, such as in particular so-called Principal Component Analysis (PCA) or the methods of the least _ g _ squares (PLS - Partial Least Squares), it is possible to process continuous spectra with the aid of a computer. At the same time, in order to determine suitable characteristic variables, the procedure corresponding to the earlier application PCT/DE 97/02987 is followed, in that document continuous spectra being evaluated for process management and process optimization during paper manufacture and, in particular, state and/or process models being derived. Now, by means of suitable modeling and the setting up of training sets, the determination of the thickness of paper or cardboard is specifically made possible, to be precise under practical conditions in a paper mill on a material web running at high speed.
In addition to the chemometric methods specified, use may also be made of neural networks. In both cases, suitable training sets are needed to form the yodel relating to the paper properties, so that the necessary data are supplied to the computer. The paper thickness is calculated from the data by suitable software.

Claims

claims

1. A method for determining the thickness of paper or cardboard by measurement on a continuous material web, non-contacting optical methods being used for the measurement, having the following features:
- the measurement is carried out with continuous optical spectra, preferably in the infrared (IR) range, - for this purpose, light with a predefined spectral range is radiated uniformly in the lateral direction toward the continuous material web, - after the light has interacted with the continuous material web, continuous spectra are evaluated as response signals by chemometric methods and/or neural networks, and the thickness of the material web is determined from these.

2. The method as claimed in claim 1, characterized in that training sets are used for the evaluation by the chemometric methods and/or the neural networks.

3. The method as claimed in claim 1, characterized in that the training sets take into account different raw material compositions of the material, in particular the type of pulps, waste paper, fillers in paper webs.

4. The method as claimed in claim 3, characterized in that the empirical relationships between absorption and composition of the material and the thickness of the paper web are stored in the training sets.

5. The method as claimed in claim 1, the continuous material web being the paper web in a papermaking machine, characterized in that, in order to determine the thickness of the paper web, measurement data registered on the papermaking machine are included, for example properties such as the porosity, the formation and other properties which can be measured optically.

6. The method as claimed in claim 5, characterized in that parameters of the papermaking machine itself, for example vacuum, speed, press pressure, steam consumption or the like, are also taken into account.

7. The method as claimed in one of the preceding claims, characterized in that measurements are carried out in the direction transverse to the direction of the continuous material web, using a linear array of infrared sensors.

8. The method as claimed in claim 7, characterized in that, for the measurement on continuous paper webs, the number of measurement points is determined by the number of actuators on the papermaking machine, in particular on the headbox.

9. The method as claimed in claim 7, characterized in that, during the measurement, the array is shifted in the lateral direction in order to increase the number of measurement points per sensor.

10. An apparatus for implementing the method as claimed in claim 1 or one of claims 2 to 9, characterized by a spectrometer (1) having at least one light source, by means (10, 10', ...) for transmitting the light toward the material web (100), in order to ensure parallel illumination, and by devices (30, 31, 31', ...) for measuring the light intensity after transmission through the material web (100) and for evaluating the continuous spectra.

11. The apparatus as claimed in claim 10, characterized in that the means for transmitting the light (10, 10', ..., 20, 20', ...) are glass fibers.

12. The apparatus as claimed in claim 10, characterized in that the device for measuring the light intensity contains an optical multiplexer (25) for a sequential measurement across the material web (100).

13. The apparatus as claimed in claim 10, characterized in that the device for measuring the light intensity contains a detector array (31, 32, ...) for a simultaneous measurement across the paper web (100), said array being mounted on a measuring frame.

14. The apparatus as claimed in claim 13, characterized in that the detector array (31, 32) comprises individual spectrometers using microsystem technology (MST).

15. The apparatus as claimed in claim 10, characterized by suitable software for controlling the measurement and recording signals and for signal evaluation by means of a computer.

16. The apparatus as claimed in claim 15, characterized in that the computer for signal evaluation can carry out the following measures, individually or in combination:
- smoothing the spectra - zero-line correction - normalization - averaging - identifying outliers, that is to say disturbances which are produced, for example, by measurement on extended dirt spots or the like - forming derivatives or integrals - identifying peaks with regard to their intensity, peak width and area integral - calculating the principal components, such as PCA or PLS.

17. The apparatus as claimed in claim 15, characterized in that the evaluation in the computer is carried out by neural networks.