Patent Publication Number: US-2007107867-A1

Title: Method for determining paper strength

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      This invention relates to a method for determining at least one strength property of a fibrous web during its production, the fibrous web being in particular a paper web, paperboard web or tissue web.  
      2. Description of the Related Art  
      Using the methods known from the prior art it is possible to determine strength properties of paper such as bursting strength, elongation at break or tensile strength only offline, meaning in the laboratory after taking a sample.  
      This has the disadvantage of not being able to react directly during production to fluctuations of the web strength beyond the tolerance range, which can lead to considerable waste production because of the time delay thus caused.  
      Furthermore, the laboratory measurements are time-consuming and therefore cost-intensive.  
      What is needed in the art is a method of the type initially referred to, with which the described disadvantages no longer arise.  
     SUMMARY OF THE INVENTION  
      According to an embodiment of the present invention, a method for determining at least one strength property of a fibrous web during its production has the following steps: determining the web draw and the tension of the fibrous web resulting therefrom; determining the actual state of at least one controlled variable influencing the strength property of the fibrous web; and determining the actual state of the at least one strength property of the fibrous web by way of a model which creates a dependency of the strength property on the web draw, the web tension and the at least one controlled variable.  
      The method of the present invention is based on the idea of determining at least one strength property indirectly by means of a suitable model which takes account of directly measurable properties of the fibrous web and of controlled variables that can be influenced during the production process. Hence it is no longer necessary, as known from the prior art, to measure a strength value directly, which up to now is possible only in the laboratory.  
      Rather, in order to determine a strength property indirectly, the method of the present invention proposes to determine the web draw and the web tension resulting therefrom. This is particularly advantageous because today already the web draw and the web tension resulting therefrom are determined on paper machines in order to control the suitable coil hardness and error-free winding when winding up the paper web for example. Therefore, for the method of the present invention it is possible to use sensors which are already installed on the paper machine.  
      The at least one strength property can be controlled during production of the fibrous web on the basis of its determined actual value. Thus, an embodiment of the present invention provides for the following additional process steps: comparing the actual state of the at least one strength property with the desired state of the at least one strength property; and changing the actual state of the at least one controlled variable in order to establish the desired state of the at least one strength property.  
      The present invention thus creates an automatic machine control system which is capable of optimizing, during production, the controlled variables or process parameters with regard to the strength of the fibrous web.  
      In concrete terms the at least one strength property of the fibrous web can include its bursting strength, its longitudinal and/or transverse elongation at break, its tensile strength and its breaking length.  
      The web draw and the web tension resulting therefrom are determined in the winding up area. The sensors provided on the paper machine for controlling the winding up process can thus be used.  
      To ensure a continuous control of the at least one strength property during the production process, the web draw and the web tension, as well as the actual state of the at least one controlled variable, are determined online. For the online determination it is possible for the web draw and the web tension and/or the at least one controlled variable to be determined continuously or in fixed time intervals for example.  
      When drawing up the model, account is taken of measured values of the at least one strength property in order to create a comparison between the values of the at least one strength property which are determined indirectly using the web draw and the web tension and the values of the at least one strength property which are determined directly from the fibrous web, thus assuring that the model used meets the requirements. The model can be in particular a self-learning model which in the event of deviations between indirectly and directly determined values of the at least one strength property automatically carries out a corresponding adjustment of the parameters.  
      The at least one strength property is measured on a sample of the fibrous web, which was taken from the finish-produced fibrous web or from the finish-wound reel. The directly measured values of the at least one strength property are determined in this case in the laboratory.  
      Furthermore, measured values of the at least one strength property can also be used for the continuous or regular calibration of the sensors measuring the web draw and the web tension.  
      The model used in the method of the present invention takes account of not only the web draw, the web tension and the actual state of the at least one controlled variable but also of the type of fibrous web and/or specific characteristics of the production process used and/or the design of the machine producing the fibrous web, which means that the model is refined further.  
      As the result of the further refinement, the actual state of the at least one strength property indirectly determined with the model corresponds very closely to the real value of the actual state of the at least one strength property, as can be determined with the direct measurement in the laboratory.  
      The strength of the fibrous web is co-defined by the composition of the pulp suspension for example, meaning by the chemical additives, the fibrous material or fibrous materials used, the ratio of fibrous materials to each other when more than one fibrous material is used, or by the filler material fraction for example. An embodiment of the present invention provides accordingly for the at least one controlled variable to include the composition of the pulp suspension used for the production of the fibrous web.  
      An additional or alternative embodiment of the present invention provides for the at least one controlled variable to include the ratio of web draws at various points of the fibrous web production process. For example, the bursting strength of the fibrous web is co-defined essentially by the ratio of the web draw upstream from the last press nip of the press section to the web draw downstream from the last press nip of the press section.  
      It is also possible with the method of the present invention for the model to take account of several controlled variables.  
      Another embodiment of the present invention provides for a transverse profile of the at least one strength property to be determined by the fibrous web being divided into several sections extending respectively over a subwidth of the fibrous web and by the web draw and the web tension resulting therefrom then being determined for each section.  
      The transverse profile of the at least one strength property can be determined on a web cutting machine for example. In this case the machine-wide reel is divided into several narrow paper reels. Through measurement of the web draw and web tension at each individual reel combined with the evaluation using the model of the invention, it is thus possible to determine and control the at least one strength property during production without any direct laboratory measurement. 
    
    
     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 drawing, wherein:  
       FIG. 1  is a schematic representation of a machine suitable for performing the method of the invention. 
    
    
      Corresponding reference characters indicate corresponding parts throughout the several views. The exemplification set out herein illustrates one embodiment of the invention, 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 drawing, there is shown a paper machine  1  with a pulp preparation section  2 , a forming section  3 , a press section  4 , a drying section  5  and a winding apparatus  6  for winding up a finish-produced paper web  7 .  
      The paper machine  1  also has a control unit  8  for controlling the paper production process. The control unit  8  includes a subunit  9  with which the at least one strength property, in this concrete case the bursting strength of the paper web  7 , is controlled during the production processes.  
      Provided in the area of the winding apparatus  6  is a sensor  10  with which the web draw and the web tension of the paper web  7  resulting therefrom can be determined.  
      According to the method of the present invention, the subunit  9  for controlling the bursting strength of the paper web  7  performs the following steps. Using the sensor  10 , the web draw and the tension of the paper web  7  resulting therefrom are determined. Furthermore, the actual state of the fibrous material composition in the pulp preparation section is determined using sensors  11 , and the actual state of the ratio of the web draw upstream from the last press nip of the press section  4  to the web draw downstream from the last press nip of the press section  4  is determined using sensors  12  and  13 .  
      The data thus obtained is sent to the subunit  9  which, using a model, determines the bursting strength of the paper web  7  on the basis of this data and with account taken of other process data supplied by a process data monitoring unit  14  and, for example, with account taken of the type of paper web  7  and/or specific characteristics of the production process used and/or the design of the paper machine  1 . Using the subunit  9  it is also possible to calibrate the sensors  11  to  13  on the basis of bursting strength values which were determined in the laboratory on the finish-produced paper web  7 .  
      Using the subunit  9 , the actual state of the bursting strength is then compared with the desired state of the bursting strength and, on this basis, the actuators  15 ,  16  and  17  are actuated by the subunit  9  in order to adjust the actual state of the bursting strength to the desired state by the actual state of the controlled variables—fibrous material composition in the pulp preparation section  2  and ratio of the web draw upstream from the last press nip of the press section  4  to the web draw downstream from the last press nip of the press section  4 —being changed accordingly.  
      On the embodiment presented, the web draw and the web tension during winding up, as well as the controlled variables, are defined online and continuously.  
      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.