Abstract:
In order to be able to adjust the tension and position of a material web, for example, a paper web, that should be printed by a printer, an electrostatically-acting braking device is described that provides a charger, for example, a corotron, and a braking plate, between which the material web runs. The charger charges the material web, which is thereby drawn to the braking plate, lies on the braking plate, and is braked via friction. It is advantageous that the charge voltage of the charger and be controlled via sensors for the web tension and web position and can very quickly react to the signals of the sensors.

Description:
BACKGROUND OF THE INVENTION 
   The adjustment of the tension of a material web is always of importance when the material web must be processed exactly. A typical example is an electrographic printing device in which a recording medium is printed as a material web in a printing group. Such electrographic print devices are presumed in the following discussion. 
   In such an electrographic print device, a clean/faultless print image is only created when the material web (e.g., a paper web) is supplied to the printing group of the print device in a taut, pre-stressed state. For this, it is known from German Patent Document No. DE 197 49 603 C2 to provide a loop-puller device via which the material web is guided. The loop-puller device comprises a loop-puller roller, under spring tension and movable in a predetermined region, via which the material web is stressed. 
   In normal operation, the deflection of the loop-puller roller is sufficient to keep the material web stressed. However, there are situations in which additional measures are necessary in order to keep the material web in a stressed state. For these cases, the position of the loop puller roller is scanned and, dependent on its position, a control signal is generated with which a braking device is triggered that brakes the material web such that the tension of the material web is maintained. As a braking device, a negative pressure brake is used that pulls on the material web dependent on the control signal and thus brakes it. This is described in German Patent Document No. DE 197 49 603 C2. 
   In operation, the transport of the material web should be designed such that, for example, the material web optimally, rapidly reaches its desired position upon starting. The braking device should be designed such that various widths of the material web, material webs with marginal perforations, etc. can be used. However, there are problems with the use of a negative pressure brake as a braking device when, for example, the material web is corrugated or comprises holes (material webs with marginal perforations). However, a braking device should function with material webs of various widths, material webs with marginal perforations and such with cut-outs. The braking force should thereby be able to be adjusted quickly. 
   For this, in addition to the web tension, the position of the material web must also be maintained. This is necessary in order to achieve consistent processing results. Due to inhomogeneities of the material web or non-continuous interferences via the processing process, a regulation of the web tension and the web position is mostly necessary. What is particularly problematic is mutual influencing of the parameters, i.e., upon change of the web position, a change of the web tension or the web tension distribution over the width of the material web results. 
   To adjust the web position, a rotating frame can be used with which the material web can be deviated multiple times. The material web can thereby be displaced depending on the relative position of the deflections. A device to deflect a material web is, for example, known from German Patent Document No. DE 198 27 254 A1. 
   SUMMARY OF THE INVENTION 
   The problem to be solved by the invention is to provide an apparatus that, in all operating types of a device processing the material web and for all types of material webs, ensures that the tension of the material web on the transport path is always maintained by the apparatus, for example, through the printing group of a print device, and in addition, the position of the material web is kept stable. 
   This problem is solved by utilizing an apparatus to adjust tension of a material web that is moved through a processing station processing the material web, comprising: a braking device acting electrostatically on the material web that is arranged in a transport path. 
   The problem is further solved by an appertaining method for using the apparatus in an electrographic print device, the method comprising adjusting at least one of tension and position of a recording medium that is transported through a print group to generate print images. Various developments are described below. 
   The advantage of the electrostatically-acting braking device is that a lesser mechanical effort is required, no negative pressure feed is necessary, a fast regulation capability is given, and no adaptation to the width of the material web is necessary. 
   According to various embodiments of the invention discussed below, the electrostatically-acting braking device is realized in a simple manner as a braking plate arranged on the one side of the material web and charger arranged on the other side of the material web. The braking plate exhibits a different potential relative to the potential of the charger, for example, it may be grounded. When a charge voltage is applied to the charger, the material web over the charger is electrostatically charged and thereby drawn to the braking plate and there braked via friction. It is advantageous that the braking force can be easily adjusted, in that the charge voltage applied to the charger is correspondingly changed. 
   A corona device, for example, a corotron of known design, can be used as a charger. The corona device appropriately extends at least over the width of the material web. It can be arranged parallel to the material web or at an angle to the material web and above/below the material web based on height. In the latter case, the charging of the material web changes corresponding to the relative position over the width of the material web. The result is that the material web is braked differently over its width due to different charge and the position of the material web can thereby be altered. A different influencing of the material web can also be achieved in that the corona device is comprised of a plurality of individually controllable corotrons or sections of a corotron (single corotrons). 
   In order to electrostatically neutralize the material web again after the influencing by the braking device, it is appropriate to arrange a discharging device, for example again, a corotron, after the braking device. This results in the material web exhibiting a defined electrostatic charge state. 
   When a sensor to measure the web tension (web tension signal), a sensor to measure the web width (web width signal), or a sensor to measure the web position (web position signal) is arranged along the path of the material web, the charge voltage of the braking device can be regulated dependent on their sensor signals. Since the braking device reacts very quickly to a change of the charge voltage, changes in the web tension, the web position or web width can be compensated for very quickly. Changes in the web width alone (without change of the position of the material web) thereby require no change of the charge voltage. 
   Only when the position of the material web is to be corrected and the corona device is comprised of individually controllable single corotrons is a web width signal necessary in order to specifically control the individual corotrons with a corresponding charge voltage, via which the material web can be significantly differently charged over its width in order to then be able to brake the material web differently over its width. 
   The invention is very advantageously suited for the adjustment of the voltage and position of a recording medium in an electrographic print device. The braking device can then be arranged before the print group in order to achieve that the printing on the recording medium is clean. It is appropriate to arrange the web voltage sensor after the braking device and the web position sensor immediately before the print group. 

   
     DESCRIPTION OF THE DRAWINGS 
     The invention is further explained using an exemplary embodiment that is shown in Figures. 
       FIG. 1  is a block diagram illustrating the design of an electrostatically-acting braking device; 
       FIG. 2  is a block diagram illustrating the course of the material web given use of an electrostatically-acting braking device with braking force changing over the width; 
       FIGS. 3 and 4  are pictorial schematics illustrating an example of a corona device to generate a change that can change over the width of the material web; and 
       FIG. 5  is a block diagram as shown in  FIG. 2  illustrating the use of an angled charger. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  illustrates an embodiment of a design of an electrostatic braking device  11  that can be used to brake a material web  10  (e.g., a paper web) that is, for example, processed by a processing station. The processing station can, for example, be the print group of an electrographic print device. 
   The braking device  11  comprises a charger  14 , for example, a charging corotron, and a grounded braking plate  13 . The material web  10  is guided through between the charger  14  and the braking plate  13 . When a charge voltage AV is applied to the charger  14 , this generates an electrostatic field via which the material web  10  is charged. The material web  10  is thereby drawn to the braking plate  13  and there braked via friction. Via adjustment of the height of the charge voltage AV for the charger  14 , the braking force of the braking device  11  can be adjusted, since the force with which the material web  10  is pressed on the braking plate  13  is also correspondingly adjusted. 
   In order to neutralize the charge on the material web  10 , after the braking device  11  a discharging device  15  can be arranged that, for example, can be comprised of a discharging corotron. The advantage of the discharging device  15  is visible in that the material web  10  is subsequently at a defined charge state. 
   In order to control the braking device  11 , a web tension sensor  19  can be arranged after the braking device  11  whose web tension signal BS proportional to the web tension is supplied to a control circuit  16 . According to the web tension signal BS, this controls a voltage supply  17  that correspondingly adjusts the charge voltage AV. The charge level of the charger  14 , and thus the braking force of the braking device  11 , is thereby influenced, such that the desired web tension is achieved. 
     FIG. 2  shows the course of the material web  10  through a processing station  12 , for example, a print group of an electrographic print device. A plurality of sensors are arranged along the web course. In addition to the web tension sensor  19 , a web position sensor  20  and a web width sensor  21  are provided. The material web  10  is moved with the aid of drive rollers  22  that, for example, can lie before and after the processing station  12 . 
   When only the web tension should be adjusted, the charger  14  can be placed parallel to the material web  10 . However, it is also possible to arrange the charger  14  in a different manner relative to the material web  10 . This is particularly necessary for the adjustment of the web position. In order to correct the web position of the material web  10 , a web position signal BL from the web position sensor  20  can be used. This is in turn supplied to the control circuit  16 , which correspondingly influences the charge voltage AV of the voltage supply  17 . From the mutual evaluation of the web position signal BL and of the web tension signal BS, both the web tension and the web position can be corrected. 
   In order to influence the web position, the charger  14  can be realized as a corona device that is arranged based on height above/below the material web  10  at an angle to the material web  10 , or which is comprised of separately controllable single corotrons. With such a corona device, a varying charge, and therewith a varying braking force, can be generated over the width of the material web  10 . The web tension distribution is thereby changed over the width of the material web  10  and correspondingly changes over a non-homogenous deformation of the material web  10  in the course of the web position. 
   Resulting from  FIGS. 3 and 4  is a corona device  23  that is fashioned such that the material web  10  can be differently, highly charged along its width. In  FIG. 3 , the corona device  23  is comprised of corotron sections  24  (effectively individual corotrons  24 ) to which the charge voltage AV can be separately applied via switch  27 .  FIG. 4  shows a corona device  23  that is realized as glass-cladding corotron in which the shield  26  shielding the corotron wire  25  is divided into sections  28  (separately controllable individual corotrons thereby again effectively result) to which the charge voltage AV can respectively be applied via switch  27 . In both exemplary embodiments, a different charge of the material web  10  over its width is possible. 
   With the aid of the web tension sensor  19 , a regulation of the web tension is thus possible; with the aid of the web position sensor  20 , a correction of the web position is thus possible. When a web width sensor  21  and its web width signal BR are additionally used, the charge region of the braking device  11  can be adjusted; this is reasonable when a corona device  23  according to  FIG. 3  or  4  is used as a charger  14 . The individual corotrons  24 ,  28  with which the material web  10  can be charged over its width can then be specifically controlled, such that such a braking force distribution is achieved that it is not necessary to change the position of the material web  10 . 
   The sensors  19 ,  20 ,  21  can be realized corresponding to those disclosed in German Patent Document No. DE 692 12 111 T2. 
   The inventive braking device  11  can be particularly advantageously used to adjust the tension or position of a paper web in an electrographic print device. This has the advantage that the effort is low and the braking effect can be rapidly adapted to the respective web circumstances. 
   For the purposes of promoting an understanding of the principles of the invention, reference has been made to the preferred embodiments illustrated in the drawings, and specific language has been used to describe these embodiments. However, no limitation of the scope of the invention is intended by this specific language, and the invention should be construed to encompass all embodiments that would normally occur to one of ordinary skill in the art. 
   The present invention may be described in terms of functional block components and various processing steps. Such functional blocks may be realized by any number of hardware and/or software components configured to perform the specified functions. For example, the present invention may employ various integrated circuit components, e.g., memory elements, processing elements, logic elements, look-up tables, and the like, which may carry out a variety of functions under the control of one or more microprocessors or other control devices. Similarly, where the elements of the present invention are implemented using software programming or software elements the invention may be implemented with any programming or scripting language such as C, C++, Java, assembler, or the like, with the various algorithms being implemented with any combination of data structures, objects, processes, routines or other programming elements. Furthermore, the present invention could employ any number of conventional techniques for electronics configuration, signal processing and/or control, data processing and the like. 
   The particular implementations shown and described herein are illustrative examples of the invention and are not intended to otherwise limit the scope of the invention in any way. For the sake of brevity, conventional electronics, control systems, software development and other functional aspects of the systems (and components of the individual operating components of the systems) may not be described in detail. Furthermore, the connecting lines, or connectors shown in the various figures presented are intended to represent exemplary functional relationships and/or physical or logical couplings between the various elements. It should be noted that many alternative or additional functional relationships, physical connections or logical connections may be present in a practical device. Moreover, no item or component is essential to the practice of the invention unless the element is specifically described as “essential” or “critical”. Numerous modifications and adaptations will be readily apparent to those skilled in this art without departing from the spirit and scope of the present invention. 
   REFERENCE LIST 
   
       
         10  material web 
         11  braking device 
         12  processing station 
         13  braking plate 
         14  charger 
         15  discharging device 
         16  control circuit 
         17  voltage supply 
         19  web tension sensor 
         20  web position sensor 
         21  web width sensor 
         22  drive roller for the material web 
         23  corona device 
         24  corotron section=individual corotrons 
         25  corotron wire 
         26  corotron shield 
         27  switch 
         28  shield sections (individual corotrons) 
       BS web tension signal 
       BL web position signal 
       BR web width signal