US20080121362A1

US20080121362A1 - 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

Info

Publication number: US20080121362A1
Application number: US11/938,929
Authority: US
Inventors: Armin Bauer; Robert Kling
Original assignee: Voith Patent GmbH
Current assignee: Voith Patent GmbH
Priority date: 2006-11-27
Filing date: 2007-11-13
Publication date: 2008-05-29
Also published as: EP1925725A1; DE102006055823A1

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).

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention
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.
2. Description of the Related Art
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.
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.
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.
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.
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.
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.
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.
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

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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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

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:

FIG. 1 is shows an exemplary signal flow of an optical web tear detection device upon occurrence of a web tear;

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;

FIG. 3 shows an exemplary signal flow of an inventive solution according to the present invention;

FIG. 4 shows the detection of a fibrous web tear by a Fourier spectrum;

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

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.

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

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.
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.
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.
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.
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.
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.
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).
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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

1 Machine for producing a fibrous web
2 Drying section
3 Fibrous web
4 Drying cylinder
5 Dryer fabric
6 Vacuum device
7 Guide roller
8 Web tear detection device
9 Cut-off apparatus
10 Characteristic color pattern
11 Thread
12 Spiral thread
13 Colored stripe
14 Weft thread
15 Warp thread
16 Spiral
Bmax Maximum width
Bmin Minimum width
DF Thread thickness
DS Spiral thickness
F Color intensity
f Frequency
I Intensity
L Running direction (arrow)
S Trigger threshold value
t Time
v Speed (arrow)

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.