Note: Descriptions are shown in the official language in which they were submitted.
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. ,
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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.
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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
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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.
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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
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- 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).
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Optical gratings constitute a conventional
solution. If it is wished to avoid a mechanically moved
grating, inter alia because they cannot
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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.
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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
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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.