Measuring arrangement in a shortened dry end of a tissue machine

FIELD OF THE INVENTION

The invention relates to an apparatus for a papermaking machine, the apparatus comprising: an active airfoil having a wall defining a web-supporting surface adjacent to which a paper web travels in the papermaking machine, the wall defining openings through which air is discharged to form an air layer along the web-supporting surface for supporting and stabilizing the paper web; and a measuring device integrated into the active airfoil and directed from the web-supporting surface onto the paper web for measuring at least one property of the paper web, the measuring device being operable to measure the web property from only one side of the web. The invention also relates to a dry end of a papermaking machine comprising: a dryer for drying a paper web, and a rotatably driven reel spool for winding the paper web thereon located downstream of the dryer.

BACKGROUND OF THE INVENTION

US 2002 060 036 A1 describes an apparatus as defined in the preamble of claim 1, wherein the measuring device has a sensor which is traversable in the cross-machine direction.

DE 19912500 A1 describes an apparatus for determining properties of a travelling paper web, comprising a radiation source for illuminating a plurality of measuring points on the paper web, a detector with a detecting surface, and an optical device for imaging of the measuring points on the detector. The optical device may comprise a plurality of optical fibers.

DE 19709963 A1 describes an apparatus for monitoring the production of a paper web with a spectrometer operating in the near-infrared range. A cross bar is positioned at a distance from the web. A plurality of optic fibers are arranged in the cross bar, a first end area of said fibers being directed towards the web so that the end area captures reflected light along a track assigned to each end area. The optic fibers transmit the detected light to a commutator.

In the production of paper, it is common practice to monitor the condition of the paper web at certain points along the web's passage through the papermaking machine, and to adjust the papermaking process as necessary depending on the condition of the web. For instance, in the dry end of a paper machine, the web exiting from the final drying section of the machine is typically monitored to measure properties such as the basis weight and moisture. In particular, the profiles of such properties in the machine direction (MD) and cross-machine direction (CD) are deduced from the measurements. An ideal web would have flat MD and CD profiles of all parameters of interest. In the real world, such profiles are never flat at all times. Through monitoring of the profiles and appropriate adjustment of the papermaking process in a closed-loop control, however, the profiles can be maintained close to the desired flat state.

Various types of sensor devices have been used for monitoring the properties of a moving paper web. The sensor devices typically comprise optical sensors employing light transmission through the web and/or reflection from the web, and detection of the transmitted or reflected light, from which paper properties of interest are deduced. With most types of sensors, it has been the conventional practice to mount the sensors on a measuring frame whose only or primary purpose is to support the sensors. For example, in a conventional tissue machine employing a final Yankee dryer, a measuring frame located just downstream of the Yankee dryer and upstream of the reel-up supports sensors for measuring basis weight and moisture content of the finally dried web.

In some paper machines, the sensors are traversed in the cross-machine direction. The direction of traverse is normally substantially perpendicular to the direction of movement of the web. The sensors therefore measure properties of diagonal samples of the web, rather than the whole web. Measurements are made at substantially the same plurality of locations across the machine during each traverse, and they may be made while traversing the sensors in one or both directions across the web. Measured variations in web properties of interest are commonly separated by means of numerical algorithms into estimates of the MD and CD variations. The usual separation methods attempt to identify MD variations and to separate them from the scan data, and the remaining variations are considered to be CD and random variations. MD variations of high frequency cannot be separated and are commonly deemed to be random variations. Variations designated as random are often removed by filtering. MD variations of low frequency may be substantially identified and separated with any of several numerical algorithms. Such algorithms include averaging, exponential filtering, or Kalman filtering applied to each cell.

A drawback of the conventional arrangement is that the measuring frame takes up space in the machine direction and, consequently, the draw between the Yankee dryer or other final dryer and the reel-up becomes somewhat long. As a further consequence of this long draw, the paper web must be supported between the final dryer and reel-up, or else the web will not be stable and will not be capable of supporting its own weight without risk of breaking. Thus, sophisticated supporting equipment is required.

Traversing sensors also tend to collect fibers and debris and hence must be regularly cleaned to maintain their proper operation and to prevent dust accumulation that can present a fire hazard.

Still another disadvantage of typical measuring arrangements is that basis weight is measured by placing a source of radioactive isotopes on one side of the web and a detector on the other side of the web. The detector receives the radioactive rays after their passage through the web and deduces basis weight based on the degree to which the web absorbs the radiation. Not only are radioactive emissions potentially hazardous to personnel, but the through-web transmission technique requires that the web traverse an open draw in the region of the measuring arrangement. In tissue machines, this is disadvantageous because the tissue web is weak and hence can tend to break in open draws.

SUMMARY OF THE INVENTION

The present invention seeks to address the above-noted needs, by providing measuring arrangements that facilitate shortening of the dry end of a tissue machine and that allow paper properties to be measured without the requirement of an open draw. The present invention is characterized by the features defined in claim 1, namely in that said measuring device is a fiber optic measuring device having a plurality of non-traversing sensors in form of optical fibers each having a sensing end, the sensing ends of the optical fibers facing the paper web that travels over the web-supporting surface and being spaced apart in a cross-machine direction, said optical fibers being arranged to measure said at least one property at a plurality of locations spaced across the paper web, and that the optical fibers are routed internally within the active airfoil, whereby the sensing ends of the optical fibers are arranged in one or more apertures formed through said wall that defines the web-supporting surface of the active airfoil. In accordance with the invention, the paper web is measured for basis weight and other parameters while supported on the airfoil. To this end, a reflectance measurement technique is used in which measuring beams (e.g., electromagnetic waves, acoustic waves such as ultrasonic energy, light waves in the visible or invisible spectrum, or the like) are emitted onto the web on the web support and are reflected from the web back to a sensor. Thus, no open draw is required because the sensors that emit and receive the beams are located on only one side of the web.

The active airfoil generally comprises a panel defining a web-supporting surface and a plurality of other walls joined to the panel so as to form an internal chamber that is supplied with air under pressure. The airfoil defines one or more air outlets that discharge air from the chamber and along the web-supporting surface to form an air layer that supports a moving paper web traveling along the web-supporting surface. The fiber optic measuring device comprises a plurality of optical fibers having sensing ends. The optical fibers are arranged in the airfoil such that the sensing ends of the fibers face the moving paper web through one or more apertures in the web-supporting panel of the airfoil. The sensing ends of the fibers are spaced apart in the cross-machine direction so that paper properties can be sensed at a plurality of widthwise locations along the web. The optical sensors preferably employ a reflectance measurement technique in which light waves are emitted from ends of some of the optical fibers and reflected waves are received by ends of others of the optical fibers.

In accordance with a preferred embodiment of the invention, the opposite ends of the optical fibers are connected to a sampling device located remote from the airfoil. The sampling device sequentially samples the optical output signals from the optical fibers, and provides samples of the signals to a further device such as a computer, which can determine the web MD and CD profiles therefrom. The sampling device is capable of sampling all of the optical fibers across the entire width of the web much faster than a traversing sensor can be moved across the width, thereby enabling high- and low-frequency MD variations to be detected. The sampling device can be a mechanical device such as a rotating device that is rotated to be coupled sequentially with the ends of the optical fibers arranged about a circular path; alternatively, the sampling device can accomplish the sampling electronically.

In one embodiment of the invention, an active airfoil preferably supports the web from the final drying device up to the reel-up in a tissue machine so that there is no open draw, or at most a very short open draw between the airfoil and the reel-up. When the active airfoil is coupled with reflectance sensors mounted within the airfoil, a particularly compact dry end is provided.

More particularly, an apparatus for a dry end of a tissue machine includes a rotatable reel spool onto which the paper web is wound to form a paper roll, and an active airfoil extending from the dryer to the paper roll. The active airfoil in some embodiments has a downstream edge that forms a nip with the paper roll through which the paper web is guided onto the paper roll. In other embodiments, the airfoil does not form a nip with the paper roll, in which case there can be a very short free draw between the downstream edge of the airfoil and the paper roll. Where the active airfoil forms a nip with the paper roll, the active airfoil can be movable relative to the reel spool for controlling the nip load in the nip. Advantageously, the active airfoil can be rotatable about a pivot axis for controlling the nip load. Alternatively or additionally, the active airfoil can include a downstream edge portion that is flexible and bears against the paper roll to form the nip.

The invention enables a number of advantages to be achieved over conventional paper machines. The airfoil with integrated fiber optic measuring device requires no traversing measuring head, and hence complicated traversing mechanisms and the vibrations and cleaning problems that are associated with such measuring heads are eliminated. Integration of the measuring device into the active airfoil rather than on a separate measuring frame or the like also saves space and reduces the footprint of the machine. The optical fibers are routed internally within the active airfoil, and thus will not cause a dust accumulation that could be a fire hazard. Additionally, MD variations are easily detectable by monitoring each sensor at a given widthwise position along the web with a high frequency or even continuously.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features, and advantages of the invention will become more apparent from the following description of certain preferred embodiments thereof, when taken in conjunction with the accompanying drawings in which:

• FIG. 1 is a diagrammatic depiction of a dry end of a paper machine in accordance with one embodiment of the invention having a driven reel spool closely coupled to the creping doctor with a pivotable active airfoil therebetween, with the reel spool in a lower position relative to the airfoil;
• FIG. 2 is a view similar to FIG. 1, but with the reel spool in an upper position relative to the airfoil;
• FIG. 2A is a fragmentary plan view of the active airfoil as viewed along line 2A-2A in FIG. 2;
• FIG. 3 is a schematic depiction of a sampling device connected to the optical fibers of the airfoil.

DETAILED DESCRIPTION OF THE DRAWINGS

The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout.

FIG. 1 depicts the dry end of a papermaking machine in accordance with a first embodiment of the invention, suitable for making tissue. The paper web, as is conventional, is dried on a Yankee dryer having a heated dryer roll 20 rotating in the direction of arrow 22. The web is removed from the roll 20 and preferably creped by a creping doctor 24 having a doctor blade 26. A cleaning doctor 28 arranged after the creping doctor cleans the surface of the roll. Alternatively, the doctor 28 can be used for removing and creping the paper web from the roll 20 when the doctor 26 is out of service for replacement or maintenance. The web creped from the dryer roll 20 proceeds over a short draw to a driven reel spool 30 rotating in the same direction as that of the dryer roll 20. In the draw between the creping doctor 24 and the reel spool 30, the web is stabilized by an active airfoil 32 having its upstream edge adjacent the creping doctor blade 26 and its downstream edge proximate the paper roll 34 building on the reel spool 30. The airfoil 32 advantageously extends across the full width of the paper web in the cross-machine direction. The airfoil 32 is mounted so as to be rotatable about a pivot axis 36 located near the upstream edge of the airfoil and extending parallel to the cross-machine direction. Thus, the airfoil can be pivoted to keep the downstream edge of the airfoil in a desired position relative to the growing paper roll 34. An actuator 40 provides the actuation force pivoting the airfoil 32 as the paper roll grows. The airfoil 32 acts to suppress flutter of the web, which can occur particularly with webs of low basis weight traveling at high speeds.

The airfoil comprises an active airfoil that uses pressurized air to create a directed air flow for supporting and assisting the web's movement. Such an active airfoil is described, for example, in U.S. Patent No. 5,738,760, the disclosure of which is incorporated herein by reference. Briefly, the airfoil has a panel 42 defining a web-supporting surface 44 along which the paper web travels, and a plurality of other walls 46, 48 that, together with the panel 42, define an internal chamber into which pressurized air is supplied. Openings 49 in the panel 42 discharge air from the chamber in a direction generally parallel to the web-supporting surface 44 of the panel, thus creating a layer of moving air that supports the paper web.

FIG. 2 shows an alternative embodiment of the invention, which is similar to that of FIG. 1 except that the reel spool 30 is arranged in an upper position relative to the active airfoil 32 so that the paper web is wound onto a lower side of the building paper roll 34, which rotates in an opposite direction to that of the Yankee dryer 20.

The airfoil 32 also includes an integrated fiber optic measuring device 60. The fiber optic measuring device 60 comprises a plurality of optical fibers 162, shown schematically in FIG. 3, that are routed along the airfoil 32, internally in the airfoil. Each optical fiber 162 has a sensing end for receiving light and transmitting the light along the fiber to an opposite end thereof where the transmitted light is detected and properties of the paper web are deduced therefrom. The sensing ends of the fibers 162 face the paper web traveling along the web-supporting surface 44 of the airfoil. To this end, the panel 42 of the airfoil includes a plurality of apertures 146 therethrough, and the ends of the optical fibers 162 are received in these apertures so that the sensing ends of the fibers are substantially flush with the web-supporting surface. It is also possible to employ a slot-shaped aperture (or more than one such aperture) in the airfoil panel 42 and to arrange a plurality of optical fibers in a single aperture, as opposed to having a separate aperture for each fiber.

Regardless of how the fibers 162 and aperture(s) 146 are arranged, the sensing ends of the fibers are spaced apart along the cross-machine direction, as shown in FIG. 2A. The fiber ends are spaced across substantially the entire width, or over only a portion of the width, of the paper web at predetermined intervals, e.g., about 50-150 mm, and preferably about 100 mm. Although the fiber ends are shown in FIG. 2A as being arranged in a straight row and with uniform spacing, the invention is not limited to any particular configuration and placement of the fiber ends; alternative placements can be used depending on which portions of the paper web are to be monitored.

FIG. 3 shows a sampling device 70 that receives the opposite ends of the optical fibers 162 and detects the light transmitted by each fiber. While the light transmitted by each fiber may be continuous, the sampling device may only periodically detect the light transmitted by any particular fiber. Thus, for example, the sampling device may be a mechanical device such as the rotary device shown in FIG. 3, in which a member 72 carrying a detector 74 revolves such that the detector 74 is brought into alignment with the end of each optical fiber 162 in turn. Each optical fiber thus is sampled once per revolution of the member 72. The signal sampled from each fiber is then communicated to a processor 80 such as a programmed computer, which is operable to deduce properties of the paper web from the signals. The sampling device 70 can send signals to the processor 80 as optical signals over a fiber optic cable, in which case the processor 80 is operable to convert the optical signals to electrical signals that are then quantified and used in calculating properties of the paper web. Alternatively, the sampling device can convert the optical signals from the fibers into electrical signals and can send the converted electrical signals to the processor.

Although a rotary sampling device is shown, alternatively a linearly moving sampling device could be used. It is also possible to employ a sampling device that samples the fiber optic signals by electronic sampling rather than mechanical sampling. It will also be recognized that the sampling device and processor could be integrated into the same device, if desired. Furthermore, each optical fiber could have its own dedicated device continuously converting the optical signal into electrical signals so that all optical signals of all fibers are simultaneously converted into electrical signals that are either continuously or periodically monitored.

As shown in FIG. 2, in addition to the fiber optic measuring device 60, the dry end of the paper machine can also include further sensors 61-64 in various locations for measuring web properties. Advantageously, the sensor 61 can comprise an infrared temperature sensor placed upstream of the creping doctor 24 for measuring web temperature prior to the web being creped from the dryer roll 20. It has been found that there is a good correlation between web moisture content and the web temperature measured by an infrared temperature sensor. Accordingly, the web temperature measured by the sensor 61 can be used for determining web moisture content going into the dry end. Sensors 63 and 64 can be used for measuring the speed of the web.

The sensors 60 measure paper properties preferably using a reflectance technique. Measuring beams such as electromagnetic waves, ultrasonic energy, light waves in the visible or invisible spectrum, or the like, are emitted by the sensors onto the web passing along the airfoil, and reflected measuring beams from the web are received by the sensors and processed to deduce web properties. In particular, the moisture content and/or basis weight of the paper preferably are determined. Moisture content of the web can be measured using infrared sensors and techniques; such techniques are known.

Prior to the invention, the measurement of basis weight has been carried out by placing a source of radioactive isotopes on one side of the web and a detector on the other side. The detector receives the radioactive rays that pass through the web and deduces basis weight based on the amount of absorption of the radioactivity by the web. In accordance with the invention, however, measuring beams such as infrared waves are reflected from the web and the reflected measuring beams are received and analyzed using spectral analysis methods. Basis weight is correlated with changes in the spectral content of the reflected measuring beams, such that based on the spectral analysis the basis weight of the paper is deduced.

It will be appreciated by persons skilled in the art that the principles of the invention are not limited to being applied in paper machines employing a Yankee dryer as the final dryer device, but can also be applied in other types of machines such as those employing one or more through-air dryer (TAD) units as the final dryer(s).

The invention enables a number of advantages to be achieved over conventional paper machines. The reflectance measurement of paper basis weight and other parameters enables open draws to be eliminated and close-coupling of the reel-up to the drying section, thereby lessening the likelihood of web breaks as well as reducing overall machine length and providing a compact arrangement. With respect to the embodiments employing the active airfoil with fiber optic measuring device, the device requires no traversing measuring head, and hence complicated traversing mechanisms and the vibrations and cleaning problems that are associated with such measuring heads are eliminated. Integration of the measuring device into the active airfoil rather than on a separate measuring frame or the like also saves space and reduces the footprint of the machine. The optical fibers can be routed internally within the active airfoil, and thus will not cause a dust accumulation that could be a fire hazard.

Furthermore, the invention makes it relatively easy to detect both high- and low-frequency MD variations in web properties, because the optical fiber signals can be sampled at a rate faster than the shortest expected period of MD variations, or can even be continuously monitored if desired.

Many modifications and other embodiments of the invention will come to mind to one skilled in the art to which this invention pertains having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. For example, the embodiments illustrated and described herein as having a Yankee dryer could instead have other types of drying devices such as through-air dryers. Therefore, it is to be understood that the invention is not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims.