GB2388426A

GB2388426A - Optical detection of the tension profile of a web

Info

Publication number: GB2388426A
Application number: GB0210498A
Authority: GB
Inventors: Timothy Clarke; Peter Graham Long
Original assignee: University of York
Current assignee: University of York
Priority date: 2002-05-08
Filing date: 2002-05-08
Publication date: 2003-11-12
Anticipated expiration: 2022-05-08
Also published as: GB2388426B; GB0210498D0

Abstract

An apparatus and method for measuring a lateral tension profile of a web (2) whereby at least one fluid jet (3) provides a substantially uniform fluid cushion across the width of the web (2) at a predetermined point along the length of the web (2) such that the fluid cushion displaces the web (2) when the tension across the width of the web (2) varies; a light source (4), arranged on the opposite side of the web (2) to the at least one fluid jet (3), provides a beam of light across the width of the web (2) at the same predetermined point along the length of the web (2) such that the light beam is adjacent the fluid cushion; at least one detector (5) detects an image of the beam of light from across the width of the web (2); at least one analyser (6) analyses the image of the beam of light and determines the lateral tension profile at the said predetermined point along the length of the web (2) by detecting variations in the displacement of the web (2). The apparatus (1) may be operable to measure the lateral tension profile along a predetermined length of the web (2). The image of the beam of light may be a real or virtual image.

Description

APPARATUS AND METHOD TO MEASURE A TENSION PROFILE

OF WEB

The present invention relates to an apparatus and method to measure a tension profile 5 of a web of material. The present invention relates in particular, though not exclusively, to an apparatus and method to measure a lateral tension profile of a web of material.

Materials, such as aluminium foil, plastic film and paper, are often produced in 10 continuous lengths and rolled onto spools. Such continuous lengths of material are commonly known as webs. Manufacturers have discovered that it is very difficult to maintain a constant tension in the web as it moves through machinery or is rolled onto or off a spool. As a result the tension across the web varies. It has been found that the tension can vary along the length and/or width of the web of material.

15 Variation in tension leads to a lack of planarity in the final product due to stretching, creasing or wrinkling due to slackness, fluting (rippling along the length of the web at sections under tension, see Figure 7), flutter (instability at edges of the web, see Figure 6), printing misregistration, cutter misalignment and breaks in the web. These problems lead to tremendous stock waste, low quality final products and exacerbate 20 plant downtimes.

Prior attempts at taking continuous non-intrusive measurements of the tension profile of a web have not been successful. Load cells, attached to the various segments of a roller, have been used to measure the load of the roller acting on the web material 25 and can determine the tension profile. However, the loads cells are expensive and the cost of fitting the cells to the roller is substantial. Furthermore, the load cells are prone to drifting and affected by vibration, mechanical stress and changes in temperature. Hand-held devices, placed adjacent the web to detect either the counter-

pressure of compressed air blown toward,th,e web or the reaction force of the web 30 have been developed but found limiting. These devices may only be used for occasional observations and are not suitable for continuous monitoring. The most

popular tension profile measuring system so far uses a laser beam to take a zigzag! raster prod le of the web. However, the resolution of the detected profile across both the width and length of the web is very poor and so subsequent analysis is difficult.

Another prior art system maintains a thin layer of air between a sensor and web. As

S the web moves over the sensor air pressure detectors measure the pressure of the air film, which is thought to correlate with the tension profile of the web.

According to a first aspect of the present invention, there is provided an apparatus for measuring a lateral tension profile of a web, the apparatus comprising at least one 10 fluid jet to provide a substantially uniform fluid cushion across a width of the web at a predetermined point along a length of the web such that the fluid cushion displaces the web when a tension across the web varies, a light source, arranged on an opposite side of the web to the at least one fluid jet, to provide a beam of light across the width of the web at the same predetermined point along the length of the web such I 15 that the light beam is adjacent to the fluid cushion, at least one detector to detect an image of the beam of light from across the width of the web, at least one analyser to analyse the image of the beam of light and determine a lateral tension profile at said predetermined point along the length of the web by detecting variations in the image due to the displacement of the web.

Preferably, the apparatus is operable to measure the lateral tension profile along a predetermined length of the web.

The fluid of the said fluid jet may be air. Alternatively, the fluid may water, oil 25 and/or a fluid stream of magnetic particles.

In one embodiment of the present invention the at least one detector detects a virtual image of the beam of light.

, 30 Tn another embodiment of the present invention the at least one detector detects a real image of the beam of light. Preferably, the real image is formed on a surface of the

web. Or the apparatus further comprises a screen arranged such that the real image of the beam of light is formed on the screen.

Preferably, the at least one detector comprises a camera. Even more preferably the 5 least one detector comprises six cameras.

The at least one analyser comprises a computer including software to process the detected image, analyse variations in the image and determine the lateral tension profile of the web. Preferably, the computer software includes an image capture 10 program and line tracking application to process the detected image in real time. I Furthermore, the computer software may include an algorithm to analyse variations in the image and determine the lateral tension profile of the web.

According to a second aspect of the present invention, there is provided a method of 15 measuring a lateral tension profile of a web, the method comprising the steps of creating a substantially uniform fluid cushion across a width of the web at a predetermined point along a length of the web such that the fluid cushion displaces the web when a tension across the width of the web varies; providing a beam of light across the width of web on an opposite side of the web and adjacent to the fluid 20 cushion; detecting an image of the beam of light from across the width of the web; analysing the image of the beam of light to detect variations in the image due to the displacement of the web and determining the lateral tension profile of the web at the predetermined point along the length of the web.

25 Preferably, the method may further comprise the step of moving the web length-

ways. The method may additionally include the step of measuring the lateral tension profile of a web along a predetermined length of web.

Advantageously, the method further cones the step.of analysing the detected 30 image in real-time. Subsequently, the method - erably comprises the steps of processing data relating to the detected image, providing co-ordinates to locate the..DTD:

image with reference to the predetermined point along the length of web; displaying the lateral tension profile of the web.

Embodiments of the present invention seek to provide an apparatus and method that 5 can continuously detect and determine the tension profile of a web in real-time.

Embodiments of the present invention also seek to provide accurate measurements that can be used by a tension control system to help overcome variations and achieve substantially uniform tension in the web. Embodiments of the present invention further seek to provide an apparatus and method that is suitable for the factory 10 environment, is able to determine a tension profile for many types of web material I : and can cope with mechanical stress, variations in temperature and variable web speeds. Embodiments of the present invention are operable to determine the lateral web tension profile at a predetermined point along the length of the web and the lateral web tension profile along a predetermined length of the web.

For a better understanding of the present invention and to show how it may be carried into effect, reference shall now be made by way of example to the accompanying drawings, in which.

FIGURE I depicts an overview of an embodiment of an apparatus to measure a lateral tension profile of a web according to the present invention; FIGURE 2a depicts a cross-sectional view of a mouth of a nozzle of an air jet; FIGURE 2b depicts a profile of an air pressure at a surface of a web produced by the air jet of Figure 2a; FIGURE 3 depicts a 3-D profile showing the effects of increasing the air pressure on 30 the displacement of a web; '

r FIGURE 4 depicts a further embodiment of the present invention where a virtual image of a beam of light is produced; FIGURE S depicts a 3-D profile showing a variation in lateral tension of a web along a length of web; FIGURE 6 depicts a 2-D profile showing flutter across a width of a web; FIGURE 7 depicts a 2-D profile showing fluting along a length of a web.

l An apparatus (1) to measure a lateral tension profile of a web (2) comprises at least one fluid jet (3), a light source (4), at least one detector (5) and at least one analyser (6). The fluid jet (3) is arranged to provide a substantially uniform fluid cushion I S across an entire width of the web (2) at a predetermined point along a length of web (2). The fluid cushion displaces the web (2) if the tension across the web (2) varies. A light source (4) is mounted on an opposite side of the web (2) to the air jet (3). The light source (4) provides an incident beam of light across the entire width of the web (2) at the same predetermined point along the length of the web (2) such that the 20 beam of light is adjacent the fluid cushion. An image of the beam of light from across the width of the web (2) is detected by at least one detector (5). The at least one analyser (6) then analyses the image to determine the lateral tension profile at said predetermined point along the web (2) by detecting the variations in the image due to the displacement of the web (2).

The fluid jet (3) forces fluids from a chamber (a) and through a nozzle (9) towards the web (2). The chamber (8) and nozzle (9) are designed to produce a cushion of fluid at the surface of the web (2) that has a substantially uniform velocity and pressure. The fluid jet (3) may use air, wader, oil, and/or a fluid stream of magnetic 30 particles or any other suitable fluid to produce a iFo-Mid cushion. The fluid jet (3) may include a single nozzle (9) with a mouth that preferably extends at least

across the width of the web (2). Alternatively, the fluid jet (3) may include multiple nozzles (9) or the present invention may include multiple fluid jets (3) each with a single nozzle (9) to provide a uniform fluid cushion across the entire width of the web (2). The embodiment depicted in Figure 1 shows an air jet (3) that blows air 5 from the chamber (8) and through the air nozzle (9) towards the web (2). In order to provide a uniform air cushion the feed velocity of the air within the chamber (8) is kept below a speed of 2m/s. Uniformity is further enhanced by dropping the pressure across the nozzle air entry points located at the base of the nozzle (9). Figure 2a depicts a cross- sectional view of the mouth of an air nozzle (9). The mouth of the 10 nozzle (9) is shaped to help produce the uniform cushion of air at the web (2). Figure 2b depicts a pressure profile at the surface of the web (2) produced by the nozzle (9) shown in Figure 2a. Figure I depicts an air jet (3) with a single nozzle with a mouth length of 1.6m. As a result a uniform air cushion is provided across the 1.5m wide web (2).

The fluid cushion displaces the web (2) if the tension across the web (2) varies. If the web is slack then a fluid cushion of air, of] and water etc. will deflect the web (2) pushing it away from the fluid jet (3). The amount of deflection is equivalent to the slackness of the web (2). Of course, if the web (2) is taut then the fluid cushion has 20 substantially no effect and the web (2) does not move. If the web material is magnetic then the fluid jet (3) may use a fluid stream of magnetic particles to produce a magnetic fluid cushion. If such a web (2) is slack then the magnetic fluid cushion will deflect the web (2). The web (2) may either be deflected towards or away from the magnetic fluid cushion depending on the polarity of the cushion. Effectively, if the 25 tension across the width of the web (2) varies then regions of the web (2) are deflected by the fluid cushion whilst other regions are unaffected. The amount of displacement of the web (2) also increases if the velocity of the fluid jet (3), and hence pressure, increases. Figure 3 depicts a three-dimensional profile showing the effect of increasing the air pressure of the air cushion on the displacement of the web 30 (2). The width of the web (2) is divided into four sections and nine different air pressures are tested. The profile shows that the displacement of the web (2) increases

as the air pressure increases. It can also be seen that the tension across the width of the web (2) varies. Furthermore, Figure 3 shows that the tension across each quartile varies since the deflection across a section changes.

5 The apparatus ( I) to measure a lateral tension profile of the web (2) may be included as part of the continuous production line. As a result, the newly produced web (2) moves through the apparatus prior to being rolled onto a spool. Alternatively, the apparatus (1) may be included as part of a further processing step e.g. heating, chemical-treatment, slitting, coating, drying or combining with other web materials.

10 Respooling mechanisms are often employed to unwind the web (2), convey the web (2) through the measuring apparatus (1) and further processing systems and then respool the web (2). Other external mechanisms may be used to move the web (2) length-ways through the apparatus. The fluid cushion also has the effect of supporting the web (2) as it moves lengthways through the apparatus (1). Fluid jets IS (3) that have this effect are commonly known as 'flotation nozzles'. Figure I shows that as the web (2) is conveyed through the measuring apparatus, it passes over roller (10). Figure S depicts a lateral tension profile of a length of web (2), where the web (2) is moving through the measuring apparatus at 60m/s and the profile represents 8 seconds worth of data i.e. a 480m length of web (2).

The light source (4) is arranged on the opposite side of the web (2) to the air jet (3) so that an incident beam of light is provided across the width of the web (2) at exactly the same point along the length of the web (2), directly adjacent the air cushion. The light source produces a line of light that stretches across the entire width of the web 25 (2). The light source may be a fluorescent tube lamp, laser device or any other appropriate light source. In the embodiment shown in Figure 1, a fluorescent tube measuring 1.8m in length is used to provide a strip of light across the l.Sm wide web (2). 30 An image of the beam of light is produced. The type of image produced is dependent upon the material of the web (2). A virtual image is produced when reflective, shiny

f and/ or non-coated materials are tested, e.g. un-coated polyester films and shiny metallic foils. A real image is produced when non-reflective, diffuse materials are tested, e.g. dry paper, textiles, metallic foils with a marl finish or satin finish. In order to improve the image production efficiency of a web surface with a satin finish the 5 light source is mounted such that the incident beam of light is at least >75 in a direction normal to the web. The resolution of the image is improved as a result.

Figure 4 depicts another embodiment of the present invention where a virtual image of the reflected beam of light is formed. The virtual image is detected by a plurality 10 of cameras (5) (only one shown) and the image is fed to a computer (6) to be i analysed (not shown). If the tension across the web (2) is perfectly uniform the virtual image of the line of light reflected from across the web (2) is straight. If the web (2) is displaced due to a variation in tension, then the reflection of the beam of light is not uniform. Consequently, the virtual image is displaced at the same point 15 where the web (2) is displaced. As a result, the virtual image of the reflected beam of light varies and appears to be wobbly. Ripples in the image are equivalent to slack portions in the web (2) displaced by the fluid cushion. Figure 4 shows that the virtual image magnifies the displacement of the web (2) by a factor of two. Accordingly, the resolution of the virtual image is clear and the variations are easily detectable by the 20 cameras (5).

Figure I depicts an embodiment of the present invention where a real image of the beam of light is formed. The real image may be formed on the surface of the web (2) or on a screen. In Figure I the image is formed on the surface of the web (2) and 25 detected by at least one detector (5) (not shown) positioned behind the protective screen (7). The protective screen (7) includes a viewing window (not shown) for the at least one detector (5). As with the embodiment discussed above, the real image of the line of light is straight if the tension across the web (2) is uniform. However, if the tension varies across the width of the web Em Ipd thy web is displaced by the air 30 cushion, the image of the beam of ligtititt" quifon4.e at least one detector (5) is

( mounted such that the entire image of the width of the web (2) is captured. The images captured are fed to a computer (6) to be analysed (not shown).

Embodiments of the present invention may employ multiple detectors (5) to detect 5 the image of the beam of light from across the entire width of the web (2). It has been found that a single camera (5) is impractical to use. In order to view the entire width of the web (2) a single camera (5) requires a very wide-angle lens and the light source (4) has to be twice the length of the width of the apparatus (1). Six detectors (5) have been found to be a preferred number, where each detector (5) is arranged to observe 10 the equivalent of slightly more than one sixth of the width of the web (2). As a result no part of the image of the beam of light from across the web (2) can be missed.

Suitable detectors such as cameras are used in the present invention. Cameras are small, have excellent resolution and require short optical path lengths. Thus, the space required is minimised and may be used for other pieces of equipment. The six 1 S cameras may be configured to capture 25 frames/sec.

The effective vertical resolution of the cameras (5) used to detect the image is increased by expanding the horizontal resolution by 3.8:1 using an anamorphic lens combination. As a result each camera (5) is able to scan a width of approximately 20 250mm and scan a vertical range of 90mm. She anamorphic lens arrangement is constructed using cylindrical lenses in a Galileo telescope configuration. 50mm focal length mount spherical lenses are used on the cameras (5) to keep the aperture of the anamorphic lenses as small as possible. l his provides an effective pixel image height of about 120 microns. The cameras (5) are able to resolve an image to approximately 25 a tenth of a millimetre. Consequently, the analyser is able to determine displacements approximately one tenth of a millimetre in the web (2).

The images detected by the coheres (5) are fed to a computer (6) (not shown) to be processed into useful data and analysed. The computer (6) runs a software program to 30 acquire continuous images as fast as possible so that a detailed lateral tension profile can be subsequently plotted for the web (2). The real-time software comprises an

image capture program, line-tracking application and an algorithm. The image capture program and line-tracking application process the detected images in real time. The line-tracking application controls the flow of data from the detectors (5) and provides co-ordinates to help locate the detected image in relation to the length 5 and width of the web (2). The processed data may be then stored on hard disk and subsequently on CD-ROM for batch processing at a later date. The algorithm uses the relationship between the deflection of the web (2) and the deflection of the image of the beam of light to determine the lateral tension in the web (2). The displacement of the image is directly related to the displacement of the web (2), which is turn is 10 directly related to the tension in the web (2). T bus, the algorithm allows the image displacement to be analysed and subsequently calculates the tension across the web (2) Having calculated the lateral tension values the software goes on to process this 15 information in order to display the results. Three-dimensional and/or two dimensional profiles of the web (2) are produced to show how the tension varies.

These profiles are displayed on a monitor (not shown). Multiple lateral tension measurements may be collated to form a profile of the tension along a predetermined length of web (2). Such a profile is depicted in Figure 5. In this case the web (2) was 20 travelling through the measuring apparatus (1) at 60m/s and Figure 5 represents 8 seconds of data. The three-dimensional profile clearly shows that the tension varies substantially from one side of the web (2) to the other and this variation in tension is maintained along the length of the web. Effectively, one edge of the web (2) is slacker than the other.

The algorithm is able to take advantage of historical location information acquired by previous scans in order to reduce analysis time; it is unaffected by line width variations or fluttering in the web (2). The software is able to produce a calibration file to help post- processing alignment. - distortions in the line location data 30 determined by the line-tracking application, due to lens aberration, is rectified during algorithmic analysis in order to avoid delays in the real-time analysis. Furthermore,

the calculated lateral tension results may be fed to a tension control system to help overcome the variations in tension and achieve a substantially uniform tension in the wed (2).

Claims

1. An apparatus for measuring a lateral tension profile of a web, the apparatus 5 comprising (a) at least one fluid jet to provide a substantially uniform fluid cushion across a width of the web at a predetermined point along a length of the web such that the fluid cushion displaces the web when a tension across the width of the web varies; 10 (b) a light source, arranged on an opposite side of the web to the at least one fluid jet, to provide a beam of light across the width of the web at the same predetermined point along the length of the web such that the light beam is adjacent to the fluid cushion, (c) at least one detector to detect an image of the beam of light from across the width 15 of the web, (d) at least one analyser to analyse the image of the beam of light and determine a lateral tension profile at said predetermined point along the length of the web by detecting variations in the image due to the displacement of the web.

20

2. An apparatus as claimed in claim 1, wherein the apparatus is operable to measure the lateral tension profile along a predetermined length of the web.

3. An apparatus as claimed in any one of claims I or 2' wherein the fluid of the said fluid jet is air.

4. An apparatus as claimed in any one of claims 1 or 2, wherein the fluid of the said fluid jet is water.

5. An apparatus as claimed in any one of claims 1 or 2, wherein the fluid of said 30 fluid jet is oil.

6. An apparatus as claimed in any one of claims 1 or 5, wherein the fluid of said fluid jet is a fluid stream of magnetic particles.

7. An apparatus as claimed in any preceding claim, wherein the at least one detector 5 detects a virtual image of the beam of light.

8. An apparatus as claimed in any one of claims I to 6, wherein the at least one detector detects a real image of the beam of light.

10

9. An apparatus as claimed in claim 8, wherein the real image is formed on a surface of the web.

10. An apparatus as claimed in claim 8, wherein the apparatus further comprises a screen arranged such that the real image of the beam of light is formed on the 1 5 screen

11. An apparatus as claimed in any preceding claim, wherein the at least one detector comprises a camera.

20

12. An apparatus as claimed in any preceding claim, wherein the least one detector comprises six cameras.

13. An apparatus as claimed in any preceding claim, wherein the at least one analyser comprises a computer including software to process the detected image, analyse 25 variations in the image and determine the lateral tension profile of the web.

14. An apparatus as claimed in claim 13, wherein the computer software includes an image capture program and line tracking application to process the detected image in real time..!> en.

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15. An apparatus as claimed in claims 13 or 14, wherein the computer software includes an algorithm to analyse variations in the image and determine the lateral tension profile of the web.

5

16. A method of measuring a lateral tension profile of a web, the method comprising the steps of: (a) creating a substantially uniform fluid cushion across a width of the web at a predetermined point along a length of the web such that the fluid cushion displaces the web when a tension across the width of the web varies; 10 (b) providing a beam of light across the width of web on an opposite side of the web and adjacent to the fluid cushion; (c) detecting an image of the beam of light from across the width of the web; (d) analysing the image of the beam of light to detect variations in the image due to the displacement of the web and determining the lateral tension profile of the web 15 at the predetermined point along the length of the web.

17. A method according to claim 16, further comprising the step of moving the web length-ways. 20

18. A method according to claim 16 or 17, further comprising the step of measuring the lateral tension prof le of a web along a predetermined length of web.

19. A method according to any one of claims 16 to 18, further comprising the step of analysing the detected image in real-time.

20. A method according to claim 19, further comprising the steps of (e) processing data relating to the detected image; (f) providing coordinates to locate the image with reference to the predetermined point along the length of web; 30 (g) displaying the lateral tension profile of the web.

21. An apparatus for measuring a lateral tension profile of a web substantially as hereinbefore described with reference to the accompanying drawings.

22. A method for measuring a lateral tension profile of a web substantially as 5 hereinbefore described with reference to the accompanying drawings.

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