U.S. patent application number 11/938929 was filed with the patent office on 2008-05-29 for method for detecting a fibrous web tear in a drying section of a machine for producing the fibrous web and a dryer fabric for performing the method.
Invention is credited to Armin Bauer, Robert Kling.
Application Number | 20080121362 11/938929 |
Document ID | / |
Family ID | 38740987 |
Filed Date | 2008-05-29 |
United States Patent
Application |
20080121362 |
Kind Code |
A1 |
Bauer; Armin ; et
al. |
May 29, 2008 |
METHOD FOR DETECTING A FIBROUS WEB TEAR IN A DRYING SECTION OF A
MACHINE FOR PRODUCING THE FIBROUS WEB AND A DRYER FABRIC FOR
PERFORMING THE METHOD
Abstract
This invention relates to a method for detecting a tear in a
fibrous web (3) in a drying section (2) of a machine (1) for
producing the fibrous web (3), whereby the fibrous web (3) is
passed through the drying section (2) by way of at least one dryer
fabric (5). The tear is detected by at least one web tear detection
device (8) and a cut-off apparatus (9) for the fibrous web (3) is
activated by the web tear detection device (8). Use is made of a
dryer fabric (5) with at least one characteristic color pattern
(10) which is reliably detected by the web tear detection device
(8) in the event of a tear in the fibrous web (3). In addition the
present invention relates to a dryer fabric (5) for a drying
section (2) of a machine (1) for producing a fibrous web (3).
Inventors: |
Bauer; Armin; (Osterreich,
DE) ; Kling; Robert; (Osterreich, DE) |
Correspondence
Address: |
TAYLOR & AUST, P.C.
P.O. Box 560, 142. S Main Street
Avilla
IN
46710
US
|
Family ID: |
38740987 |
Appl. No.: |
11/938929 |
Filed: |
November 13, 2007 |
Current U.S.
Class: |
162/198 ;
162/263 |
Current CPC
Class: |
D21F 7/04 20130101 |
Class at
Publication: |
162/198 ;
162/263 |
International
Class: |
D21F 7/12 20060101
D21F007/12 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 27, 2006 |
DE |
10 2006 055 823.5 |
Claims
1. A method for detecting a tear in a fibrous web in a drying
section of a machine for producing the fibrous web, comprising the
steps of: passing the fibrous web through the drying section by way
of at least one dryer fabric; detecting the tear in the fibrous web
by way of at least one web tear detection device; and activating a
cut-off apparatus dependant upon a signal from the web tear
detection device, said at least one dryer fabric having at least
one characteristic color pattern detectable by said web tear
detection device in the event of the tear in the fibrous web.
2. The method of claim 1, wherein said at least one characteristic
color pattern is formed by a plurality of colored stripes including
at least two colors, said colored stripes having a predetermined
alignment.
3. The method of claim 2, wherein said colored stripes are aligned
transverse to a running direction of said at least one dryer
fabric.
4. The method of claim 1, wherein said at least one characteristic
color pattern is formed by a plurality of differently colored
threads.
5. The method of claim 4, wherein said plurality of differently
colored threads include differently colored weft threads.
6. The method of claim 5, wherein said differently colored weft
threads include a white polytetrafluorethylene thread, said
plurality of differently colored threads including red polyester
warp threads.
7. The method of claim 6, wherein said weft threads have a
thickness that is used as a minimum width (Bmin) of said
characteristic color pattern.
8. The method of claim 4, wherein said at least one dryer fabric
includes a spiral mesh, said characteristic color pattern being
produced by differently colored spirals.
9. The method of claim 8, wherein one spiral thickness (DS) of one
of said spirals is used as a minimum width (Bmin) for said
characteristic color pattern.
10. The method of claim 9, wherein a maximum width (Bmax) of said
characteristic color pattern results from a maximum permissible web
length which does not cause any damage on the machine for producing
the fibrous web before said cut-off apparatus is activated by said
web tear detection device.
11. The method of claim 1, further comprising the step of coloring
said at least one dryer fabric to produce said characteristic color
pattern.
12. The method of claim 11, wherein said coloring step includes
using thermography to produce said characteristic color
pattern.
13. The method of claim 1, wherein said at least one dryer fabric
has a plurality of regions with only some of said plurality of
regions contain said characteristic color pattern.
14. The method of claim 1, wherein said detecting step includes a
step of making a spot measurement of one of the fibrous web and
said at least one dryer fabric with one of a laser beam and a light
beam.
15. The method of claim 1, wherein said detecting step includes a
step of making a line measurement of one of the fibrous web and
said at least one dryer fabric with a line camera.
16. The method of claim 1, wherein said detecting step includes a
step of making an area measurement of one of the fibrous web and
said at least one dryer fabric with a camera.
17. The method of claim 1, wherein said detecting step includes a
step of making use of a difference between a non-periodic
stochastic signal of the fibrous web and a periodic signal of said
characteristic color pattern.
18. The method of claim 1, wherein said detecting step includes the
steps of: evaluating a signal using one of a Fourier and a power
spectrum of said signal, said signal being generated by said tear
detection device and relating to a tear in the fibrous web; setting
at least one trigger threshold value at a frequency characteristic
of said at least one dryer fabric.
19. A dryer fabric for a drying section of a machine for producing
a fibrous web, the machine including a web tear detection device
and a cut-off apparatus, the dryer fabric comprising: one of a
woven, a non-woven and a spiral mesh structure; and at least one
characteristic color pattern associated with said structure, said
at least one characteristic color pattern being detectable by the
web tear detection device in the event of a tear of the fibrous
web.
20. The dryer fabric of claim 19, wherein said at least one
characteristic color pattern is formed by a plurality of colored
stripes including at least two colors, said colored stripes having
a predetermined alignment.
21. The dryer fabric of claim 20, wherein said colored stripes are
aligned transverse to a running direction of the dryer fabric.
22. The dryer fabric of claim 19, wherein said at least one
characteristic color pattern is formed by a plurality of
differently colored threads.
23. The dryer fabric of claim 22, wherein said plurality of
differently colored threads include differently colored weft
threads.
24. The dryer fabric of claim 23, wherein said differently colored
weft threads include white polytetrafluorethylene threads, said
plurality of differently colored threads including red polyester
warp threads.
25. The dryer fabric of claim 24, wherein said weft threads have a
thickness that is used as a minimum width (Bmin) of said
characteristic color pattern.
26. The dryer fabric of claim 22, wherein said at least one dryer
fabric is a spiral mesh, said characteristic color pattern being
produced by differently colored spirals.
27. The dryer fabric of claim 26, wherein one spiral thickness (DS)
of said spiral is used as a minimum width (Bmin) for said
characteristic color pattern.
28. The dryer fabric of claim 27, wherein a maximum width (Bmax) of
said characteristic color pattern results from a maximum
permissible web length which does not cause any damage on the
machine for producing the fibrous web before the cut-off apparatus
is activated by the web tear detection device.
29. The dryer fabric of claim 19, wherein said characteristic color
pattern is made by subsequent coloring of the dryer fabric.
30. The dryer fabric of claim 29, wherein said characteristic color
pattern is produced by using thermography.
31. The dryer fabric of claim 19, wherein the dryer fabric has a
plurality of regions with only some of said plurality of regions
containing said characteristic color pattern.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates to a method for detecting a fibrous
web tear in a drying section of a machine for producing the fibrous
web, whereby the fibrous web is passed through the drying section
by way of at least one dryer fabric, whereby the tear is detected
by at least one web tear detection device and whereby a cut-off
apparatus for the fibrous web is activated by the web tear
detection device. In addition the invention relates to a dryer
fabric for a drying section of a machine for producing a fibrous
web, in particular for performing the inventive method.
[0003] 2. Description of the Related Art
[0004] During the production of all paper grades, formed
essentially from fibrous suspensions, the fast and reliable
detection of tears in the fibrous web during its production is
extremely important in order to prevent damage to parts of the
machine used to produce the fibrous web.
[0005] At points in which the fibrous web is in free draw, web
tears can be detected very reliably, for example by light barriers.
However, light barriers are unsuitable in situations in which the
fibrous web rests on a skin acting as a web carrier, for example on
a forming mesh, a press felt or a dryer fabric. Here use is usually
made of optical systems such as laser or fiberoptic sensors, with a
detector and a light or radiation source of the web tear detection
device mounted on the same side of as the fibrous web.
[0006] Different web tear detection methods are known. For example,
with the known "color detection" method, the difference in color
between the fibrous web and the dryer fabric is used to detect the
web tear. By way of example, FIG. 1 shows the signal flow, plotted
in a color intensity/time diagram (F-t diagram), of an optical web
tear detection device upon occurrence of a web tear. Upon
overshooting or undershooting of a preselected trigger threshold
value S (dashed line) of the color intensity F, a signal is sent to
a cut-off apparatus. The trigger threshold value S for the signal
to the cut-off apparatus can lie at, for example 50% of the color
intensity F. This method works particularly well with a distinct
color difference such as that between a white fibrous web and a
green dryer fabric.
[0007] A drawback of the color detection system is that it becomes
more and more unreliable as the color difference between the
fibrous web and the dryer fabric decreases. This is the case, for
example, on machines for producing paperboard or packing paper,
where brownish paper or paperboard webs lie on red or amber dryer
fabrics. Here it is no longer possible to differentiate clearly
between the fibrous web and the dryer fabric. This results in
either web tear detection without a tear or in a tear without web
tear detection. The former leads to an unnecessary stoppage of
production and hence to financial loss for the plant operator, the
latter runs the risk of causing damage to the machine.
[0008] Furthermore, dryer fabrics can become severely soiled the
longer they are used, as the result of which the actual color of
the dryer fabric is covered by dirt deposits. In such cases also it
has been discovered that a color detection system no longer works
reliably.
[0009] Another known method is the "detection of changes to light
scatter (pseudo-structure detection)", whereby light is scattered
differently on the structured dryer fabric than on the fibrous web.
This difference is used to detect web tears.
[0010] It has been discovered that pseudo-structure detection
systems likewise do not work reliably. This is owed presumably to
soiling in the dryer fabric or to the transparency of the still
partly wet fibrous web.
[0011] What is needed in the art is a dryer fabric that results in
more reliable detection of web tears as compared to the prior art
in particular in single-row drying sections of a machine for
producing a fibrous web.
SUMMARY OF THE INVENTION
[0012] The present invention includes a method that uses a dryer
fabric with at least one characteristic color pattern which is
reliably detected by the web tear detection device in the event of
a fibrous web tear. The object of the invention is completely
accomplished in this manner.
[0013] By using at least one characteristic color pattern, which is
less vulnerable to soiling of the dryer fabric or discolorations of
the fibrous web and/or the dryer fabric, the reliability of the
fibrous web tear detection is notably improved, in particular with
regard to the conventional methods.
[0014] Furthermore, according to the present invention, more than
one characteristic color is now monitored than compared with the
conventional methods. The reliability of the tear detection is
improved further through the combination of the two color
measurements.
[0015] Yet another advantage of the present inventive method is
that the at least one color pattern to be detected is far larger
than the structures of the dryer fabric and therefore less
vulnerable to soiling in the dryer fabric.
[0016] The term "color" is used in the context of this disclosure
to mean light with a certain wavelength and/or light from a certain
wave length range such as the R channel on an RGB sensor. This
covers UV and IR radiation.
[0017] In one embodiment of the present invention, the
characteristic color pattern of the dryer fabric is formed by
several color stripes which are differently colored and aligned,
preferably transverse, to the running direction of the fabric. In
this case the colored stripes of the dryer fabric can have a
plurality of different colors. This multicoloredness contributes
likewise to a distinct improvement in the reliability of the tear
detection.
[0018] In practical conditions the characteristic color pattern of
the dryer fabric is formed by several differently colored threads.
During production of the dryer fabric it is possible therefore to
influence its later use selectively and at a relatively low
cost.
[0019] In one embodiment of the present invention the
characteristic color pattern of a woven dryer fabric is produced by
differently colored weft threads. For example, white
polytetrafluorethylene can be used for the weft threads and red
polyester for the warp threads. The basic color of the dryer fabric
can be red, for example, and every 30 cm there can be a white
colored stripe, with a black colored stripe arranged alongside
every fourth white colored stripe. Preferably one thread thickness
of the weft thread is used as the minimum width for the
characteristic color pattern.
[0020] In another embodiment of the present invention the
characteristic color pattern of a spiral mesh is produced by
differently colored spirals. Due to the method used to produce
spiral meshes, the meshes include differently colored spirals that
can be produced without any great additional effort and hence
without extra costs as compared to single-colored meshes. In this
case preferably one spiral thickness of the spiral is used as the
minimum width for the characteristic color pattern.
[0021] In addition, the maximum width of the characteristic color
pattern can result from the maximum permissible web length which
does not cause any damage on the machine for producing the fibrous
web before the cut-off apparatus is activated by the web tear
detection device. A typical value would be, for example, a 10 meter
web length, meaning the color pattern should be repeated on the
dryer fabric no later than every 10 meters. Colored stripes which
are much wider than 10 meters would thus make no sense.
[0022] The colored stripes of the characteristic color pattern can
vary of course in their width. For example, a dryer fabric can be
made of 90 cm wide red colored stripes and 10 cm wide white colored
stripes.
[0023] In addition, the characteristic color pattern can also be
produced by subsequent coloring of the dryer fabric, preferably by
way of thermography. This has the advantage of being able to use
identical dryer fabrics, produced if necessary in large quantities
at low unit costs, for various applications on account of different
colorings.
[0024] It is also possible to use a dryer fabric with a
characteristic color pattern arranged in only some regions. The
colored stripes can thus have only a certain stripe length, meaning
they must not necessarily extend over the entire width of the dryer
fabric. Under certain circumstances this can result in the
advantage of reduced production costs for the dryer fabric.
[0025] The detection of the fibrous web tear takes place, according
to the present invention, by using various measurement methods: for
example taking a spot measurement with a laser beam, a light beam
or the like, a line measurement with a line camera or the like,
and/or an area measurement with a CCD camera or the like. All
measurement methods are characterized by high reliability and good
usability even in difficult environments.
[0026] In addition, one embodiment of the present invention makes
use of the difference between a non-periodic, in particular
stochastic signal, of the fibrous web and a periodic signal of the
characteristic color pattern in order to detect the fibrous web
tear. In other words, the change of color of the colored stripes
or/and the periodic signal generated by the color change must be
detected.
[0027] The detected fibrous web tear is evaluated, preferably by
way of a Fourier or power spectrum of the signal, whereby at least
one trigger threshold value is set at a frequency characteristic
for the dryer fabric. The characteristic trigger frequency results
from the width of the colored stripe and the speed of the fibrous
web. In addition it is advantageous for the characteristic
frequency to be adjustable by selecting the widths of the colored
stripes.
[0028] This object of the invention is accomplished with a dryer
fabric of the type initially referred to in that said fabric has at
least one characteristic color pattern which is reliably detected
by a tear detector in the event of a fibrous web tear.
[0029] The object of the invention is completely accomplished in
this manner and the previously mentioned advantages of the
invention are thus obtained.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] The above-mentioned and other features and advantages of
this invention, and the manner of attaining them, will become more
apparent and the invention will be better understood by reference
to the following description of an embodiment of the invention
taken in conjunction with the accompanying drawings, wherein:
[0031] FIG. 1 is shows an exemplary signal flow of an optical web
tear detection device upon occurrence of a web tear;
[0032] FIG. 2 shows a schematic representation of a subsection of a
machine for producing a fibrous web according to an embodiment of
the present invention;
[0033] FIG. 3 shows an exemplary signal flow of an inventive
solution according to the present invention;
[0034] FIG. 4 shows the detection of a fibrous web tear by a
Fourier spectrum;
[0035] FIG. 5 shows details of a plan view of a dryer fabric formed
from threads, the fabric having an embodiment of a characteristic
color pattern; and
[0036] FIG. 6 shows details of a plan view of a dryer fabric formed
from spiral threads, the fabric having another embodiment of a
characteristic color pattern of the present invention.
[0037] Corresponding reference characters indicate corresponding
parts throughout the several views. The exemplification set out
herein illustrates one embodiment of the invention, in one form,
and such exemplification is not to be construed as limiting the
scope of the invention in any manner.
DETAILED DESCRIPTION OF THE INVENTION
[0038] Referring now to the drawings, and more particularly
presented, schematically in FIG. 2, is a side view of a detail of a
drying section 2 of a machine, generally designated by 1, for
producing a fibrous web 3. Within machine 1, drying section 2
performs in a known manner the function of removing moisture from a
produced and/or processed fibrous web 3, meaning the function of
drying it.
[0039] The drying is done in the example shown in FIG. 2 by way of
contact drying, whereby fibrous web 3, being fed from the left in
the drawing, is dried by direct contact with a plurality of drying
cylinders 4 on the one hand and an endless dryer fabric 5
circulating around drying cylinders 4 on the other hand.
[0040] Presented in FIG. 2 are two dryer fabrics 5 in their
complete path of rotation, whereby each of dryer fabrics 5
circulates respectively around one group of drying cylinders 4. To
be more precise, each dryer fabric 5 undulates in the region of
drying cylinders 4, whereby at the wave peak dryer fabric 5 is
deflected by a drying cylinder 4 and at the wave valley by a
suction roller of a respective vacuum device 6. After leaving the
last drying cylinder 4 of the respective group, dryer fabric 5 is
returned by way of several guide rollers 7 to the first drying
cylinder 4 of the group.
[0041] To perform the inventive method for detecting a tear in
fibrous web 3 in drying section 2 of machine 1 for producing
fibrous web 3, provision is made for each group of drying cylinders
4 to have at least one web tear detection device 8 (symbolically
represented), which is described later. Furthermore, provision is
made for each group of drying cylinders 4 to have at least one
cut-off apparatus 9 (only schematically represented) for fibrous
web 3, which is activated by web tear detection device 8.
[0042] Further, in each group of drying cylinders 4 use is made of
a dryer fabric 5 woven from threads and having at least one
characteristic color pattern 10, which is reliably detected by web
tear detection device 8 in the event of fibrous web 3 tearing.
[0043] For detecting the tear in fibrous web 3 use is made of
either a spot measurement with a laser beam, a light beam or the
like, a line measurement with a line camera or the like, and/or an
area measurement with a CCD camera or the like. As these preferred
measurement methods are known to those skilled in the art, they
will not be described in more detail at this point. To detect a
tear in fibrous web 3 use is made accordingly of the difference
between a non-periodic, in particular, stochastic signal of fibrous
web 3 and a periodic signal of a characteristic color pattern 10.
To detect a tear, the change of color of characteristic color
pattern 10 or the periodic signal generated by the color change of
characteristic color pattern 10 must be detected.
[0044] The detected tear in fibrous web 3 is evaluated by way of a
Fourier or power spectrum of the signal, whereby at least one
trigger threshold value S is set at a frequency f characteristic
for dryer fabric 5 (see for example FIG. 4).
[0045] In this case the characteristic trigger frequency results
from the width of characteristic color pattern 10 and the speed v
(arrow) of fibrous web 3.
[0046] Now additionally referring to FIG. 3, there is shown an
exemplary signal flow of an inventive solution in a color
intensity/time diagram (F-t diagram). Upon overshooting of a
preselected trigger threshold value S (dashed line) of the color
intensity F, a signal is sent to a cut-off apparatus 9 of the
respective group of drying cylinders 4. The trigger threshold value
S for the signal to cut-off apparatus 9 can thus lie, for example,
at 50% of color intensity F. After tearing of the fibrous web, the
various colors, in this case, the two colors 1 and 2, of the
characteristic color pattern are detected.
[0047] Now, additionally referring to FIG. 4 there is shown the
detection of a tear in a fibrous web by way of a Fourier spectrum,
once in the situation existing before the tear (representation on
the left) and once in the situation existing after the tear
(representation on the right). The respective intensity/frequency
diagram (I-f diagram) presents the characteristic frequency f of
the respective color pattern. In the situation existing before the
tear, the characteristic frequency f does not overshoot the trigger
threshold value S at any time. By contrast, in the situation after
the tear, the characteristic frequency f overshoots the trigger
threshold value S at least once, whereby a signal to the cut-off
apparatus 9, shown schematically in FIG. 2, of the respective group
of drying cylinders 4 is generated.
[0048] Now, additionally referring to FIG. 5 there is shown details
of a plan view of a dryer fabric 5 formed from threads 11, fabric 5
having a first embodiment of a characteristic color pattern 10. And
FIG. 6 shows details of a plan view of a dryer fabric 5 formed from
spiral threads 12, the fabric having a second embodiment of a
characteristic color pattern 10.
[0049] The two presented dryer fabrics 5 are both excellently
suited for performing the inventive method. Each dryer fabric 5 has
at least one characteristic color pattern 10 which is reliably
detected by a tear detector of web tear detection device 8 in the
event of a tear in the fibrous web 3.
[0050] The characteristic color pattern 10 of each dryer fabric 5
shown in FIG. 5 is made of several colored stripes 13, which are
differently colored, arranged preferably transverse to the fabric's
running direction L (arrow) and made of several differently colored
threads 14. Shown, by way of example, is only one colored strip 13,
which is made of seven differently colored threads 14. In this case
it is possible of course for different threads 14 to have the same
color.
[0051] The characteristic color pattern 10 on woven dryer fabric 5
in FIG. 5 includes seven preferably differently colored weft
threads 14. Weft threads 14 are made from white
polytetrafluorethylene and the warp threads from red polyester.
Alternatively, but not explicitly shown, it is possible for dryer
fabric 5 to have the basic color red and for there to be a white
colored stripe every 30 cm, with a black colored stripe lying
alongside every fourth white colored stripe.
[0052] In addition, characteristic color pattern 10 on a woven
dryer fabric 5 has a minimum width Bmin in the range of one thread
thickness DF of weft thread 14.
[0053] By contrast, characteristic color pattern 10 on dryer fabric
5 constructed as a spiral mesh in FIG. 6 includes differently
colored spirals 16. In this case the characteristic color pattern
10 has a minimum width Bmin in the range of one spiral thickness DS
of spiral 16.
[0054] On both embodiments characteristic color pattern 10 has a
maximum width Bmax resulting from the maximum permissible web
length which does not cause any damage on machine 1 for producing
fibrous web 3 before cut-off apparatus 9 is activated by web tear
detection device 8.
[0055] Furthermore, colored stripes 13 can vary in their width. For
example, dryer fabric 5 can be made of 90 cm wide red colored
stripes and 10 cm wide white colored stripes 13. Alternatively, the
characteristic color pattern 10 can also include a subsequent
coloration of dryer fabric 5, preferably by way of thermography,
which is known to those skilled in the art, and it can also exist
on dryer fabric 5 in certain regions only. In other words: colored
stripes 13 can have a certain length, meaning they must not
necessarily extend over the entire transverse width of dryer fabric
5.
[0056] In summary the present invention also develops a dryer
fabric of the type initially referred to such that a more reliably
detection of web tears as compared to the prior art is made
possible, in particular in single-row drying sections of a machine
for producing a fibrous web.
[0057] While this invention has been described with respect to at
least one embodiment, the present invention can be further modified
within the spirit and scope of this disclosure. This application is
therefore intended to cover any variations, uses, or adaptations of
the invention using its general principles. Further, this
application is intended to cover such departures from the present
disclosure as come within known or customary practice in the art to
which this invention pertains and which fall within the limits of
the appended claims.
LIST OF REFERENCE NUMERALS
[0058] 1 Machine for producing a fibrous web [0059] 2 Drying
section [0060] 3 Fibrous web [0061] 4 Drying cylinder [0062] 5
Dryer fabric [0063] 6 Vacuum device [0064] 7 Guide roller [0065] 8
Web tear detection device [0066] 9 Cut-off apparatus [0067] 10
Characteristic color pattern [0068] 11 Thread [0069] 12 Spiral
thread [0070] 13 Colored stripe [0071] 14 Weft thread [0072] 15
Warp thread [0073] 16 Spiral [0074] Bmax Maximum width [0075] Bmin
Minimum width [0076] DF Thread thickness [0077] DS Spiral thickness
[0078] F Color intensity [0079] f Frequency [0080] I Intensity
[0081] L Running direction (arrow) [0082] S Trigger threshold value
[0083] t Time [0084] v Speed (arrow)
* * * * *