Abstract:
A device for controlling substrate web tension includes first and second driving units with respective driving rollers around which the substrate web is partially guided and tensioned. The first and second driving units have different rotational moments of inertia. A measuring unit measures the tension of the substrate web. A control unit has first and a second filtering units with different filtering characteristics for filtering the measured value. The control unit includes a first control device for controlling the first driving unit based on the measured value filtered by the first filtering device, and a second control device for controlling the second driving unit based on the measured value filtered by the second filtering device.

Description:
TECHNICAL FIELD OF THE INVENTION 
       [0001]    The present invention relates to a device and a method for controlling the tension of a substrate web, said substrate web being conveyed along a transport path. 
       BACKGROUND OF THE INVENTION 
       [0002]    A plurality of machines for processing substrate webs, for example, paper webs or fabric webs is known. These processing machines can perform a multitude of process steps on the substrate web such as, for example, printing, embossing and/or cutting. Most process steps require that the substrate web display a tension that is as uniform as possible. If the substrate web is not tensioned uniformly, process errors may occur in the substrate web such as, for example, shifts of printed images, as well as tears and/or creases. In order to maintain a uniform tension of the substrate web such processing machines comprise tensile stress control arrangements. 
         [0003]    In particular in multi-color printing machines, it is necessary that the substrate web tension of a substrate web be controlled as accurately as possible in order to enable printing in a register-perfect manner. The description hereinafter thus refers to the example of a printing machine in greater detail; however, it should be noted that both the device, as well as the method for controlling the substrate web tension, can be used with other processing machines. 
         [0004]    A known design of a tensile stress control device comprises a force-measuring box that measures the tension of the substrate web and emits a corresponding measuring signal. The measuring signal is used for the control of an associate driving unit. 
         [0005]    Depending on the size of the driving unit that is used for controlling the tensile stress, there are limits regarding the disturbances of the substrate web tension that can be corrected by the driving system. However, the remaining errors and fluctuations are not acceptable, in particular in the case of printing machines because said errors and fluctuations can seriously compromise the printed image. 
         [0006]    For example, in reel fed printing machines there are existing strict requirements regarding the achievable tensile stress and the allowed tensile stress errors. It is impossible to meet these requirements with the aforementioned conventional method, wherein a tensile stress control is based on a not specifically processed measured value from force-measuring boxes. 
         [0007]    With regard to the dimensioning of drive and control of a printing machine, in most cases the problem occurs that an electric motor with strong torque is necessary for the required mean substrate web tension. However, the rotors of these motors display large moments of inertia and long coil delay times that make it difficult to correct high-dynamic disturbances of the substrate web tension. Here the coil delay time is understood as the ratio of coil inductance to coil resistance. Conversely, it can be said that small electric motors displaying a small moment of inertia of the rotor and low coil delay times provide the dynamics necessary for correcting the highly dynamic disturbances but cannot provide the required torque. 
       SUMMARY OF THE INVENTION 
       [0008]    It is, therefore, the object of the invention to create a control device and a control method that make it possible to avoid the aforementioned problems. 
         [0009]    This object is achieved with a device in accordance with Claim  1 , a printing machine in accordance with Claim  5 , as well as a with a method in accordance with Claim  10 . Additional embodiments are disclosed by the subclaims. 
         [0010]    In particular, the device for controlling the tension of a substrate web, said web being conveyed along a transport path, comprises a first driving unit with a first driving roller around which the substrate web is partially guided and tensioned, and comprises a second driving unit with a second driving roller around which the substrate web is partially guided and tensioned. The first and the second driving units feature different rotational moments of inertia. Further provided are a measuring unit for providing a measured value of the tension of the substrate web and a control unit in connection with said measuring unit and said first and second driving units, said control unit comprising a first and a second filtering unit displaying different filtering characteristics for filtering the measured value. The control unit comprises a first control device for controlling the first driving unit based on the measured value filtered by the first filtering unit, as well as a second control device for controlling the second driving unit based on the measured value filtered by the second filtering unit. Thus, the driving units can be activated in a manner that has been advantageously adapted to the dynamic behavior that is affected by the rotational moments of inertia of said driving units. 
         [0011]    Preferably, the filtering units comprise, respectively, a low-pass filter and a high-pass filter. Consequently, a driving unit with a small rotational moment of inertia can compensate for rapid tension changes measured with the use of a measured value filtered with the high-pass filter, and a driving unit with a large rotational moment of inertia can compensate for slow tension changes measured with the use of a measured value filtered with the low-pass filter. 
         [0012]    In one exemplary embodiment of the device, at least one of the filtering units displays a filtering characteristic that depends on the conveying speed of the substrate web along the transport path. Thus, different dynamic behaviors of the driving units in the different speed ranges can be taken into consideration when said driving units are being activated. 
         [0013]    In one exemplary embodiment of the device, the first driving unit displays a higher rotational inertia than the second driving unit. This is the case if, on the one hand, a heavy driving unit displaying high torque capacity is used and, on the other hand, a light-weight driving unit displaying low torque capacity is used. In this manner, it is possible to correct fast and slow disturbance values of the substrate web tension. 
         [0014]    Furthermore, a printing machine is disclosed that, in addition, comprises a main drive for conveying a substrate web along a transport path, at least one printing unit and a device for controlling the tension of the substrate web in accordance with one of the aforementioned embodiments. The main drive prespecifies the process speed or printing speed. 
         [0015]    Preferably, the control unit of the device for controlling the substrate web tension communicates with the main drive of the printing machine. Thus, a printing speed or process speed generated by the main drive can be taken into consideration during the control operation. 
         [0016]    In the printing machine, the measuring unit of the device for controlling the tension of the substrate web is advantageously arranged between the second driving unit and the main drive—viewed in the direction of the transport path. 
         [0017]    Preferably, the printing machine comprises a control unit for controlling the main drive, said control unit controlling a printing speed generated by the main drive. Thus, the printing speed can be generated by a powerful main drive, and disturbances of the substrate web tension can be corrected by the dynamic driving units. 
         [0018]    Furthermore, the object of the invention is achieved by a method for controlling the tension of a substrate web, wherein the substrate web is at least partially guided and conveyed around a first and a second driving roller that feature different rotational moments of inertia. 
         [0019]    This method comprises the following steps: measuring the tension of the substrate web and generating a measured value as a function of said tension; parallel filtering of the measured value in a first and in a second filtering unit displaying different filtering characteristics; controlling the first driving unit depending on the measured value filtered by the first filtering unit; and controlling the second driving unit depending on the measured value filtered by the second filtering unit. Thus, the measured value is simultaneously used as the control value for two different driving units and is, for this purpose, processed with respect to the dynamic characteristics of the driving units. 
         [0020]    Preferably, the step of filtering the measured value comprises low-pass filtering and high-pass filtering so that fast and slow disturbances of the substrate web tension can be corrected by a fast-responding driving unit and by a slow-responding driving unit, respectively. 
         [0021]    Advantageously, the method comprises conveying the substrate web by means of a main drive along the transport path. This means that the printing speed is generated by a main drive using a simple control, and disturbances of the substrate web tension can be corrected by the first and second driving units. 
         [0022]    In one exemplary embodiment of the method, the step of filtering the measured value occurs depending on a conveying speed of the substrate web along the transport path, so that any different dynamic behavior of the driving units in different speed ranges can be taken into account. 
         [0023]    With the method, preferably a nominal tension of the substrate web is taken into consideration regarding the control of the first driving unit. Furthermore, an offset preferably is taken into consideration regarding the control of the second driving unit. 
         [0024]    The invention, as well as additional details thereof, will be explained in detail hereinafter with the use of exemplary embodiments and with reference to the figures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]      FIG. 1  a schematic side view of a printing machine, said printing machine comprising a device for controlling the tension of a substrate web; 
           [0026]      FIG. 2  a schematic view of the control concept of the device for controlling the tension of a substrate web; and 
           [0027]      FIG. 3  a flow chart showing the process of a method for controlling the tension of a substrate web. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0028]    It should be noted that the terms top, bottom, right and left, as well as similar directions used in the description hereinafter, relate to alignments or arrangements shown in the figures and are only intended for describing the exemplary embodiments. However, these expressions must not be understood to have a restrictive meaning. 
         [0029]      FIG. 1  shows a schematic side view of a printing machine  1  as an example of a processing machine. The printing machine  1  comprises a feeder  2  with a first substrate web roll  3  and an output means  4  with a second substrate web roll  5 . A substrate web  7  extends along a transport path from the first substrate web roll  3  to the second substrate web roll  5 . Between the feeder  2  and the output means  4  and along the transport path of the substrate web  7 , there is a printing region  8  with several printing stations  9  for different colors being arranged in said printing region. The substrate web  7  is also guided in the printing machine  1  over at least one deflecting roller  11  in order to define the transport path and the wrap angle, these enabling a reliable transmission of the driving torques of the rollers to the substrate web  7 . Furthermore, the printing machine  1  comprises a main driving unit  13  with a main driving roller  14 , the latter being intended for conveying the substrate web  7  from the first substrate web roll  3  in the direction toward the second substrate web roll  5 . 
         [0030]    Furthermore, the printing machine  1  comprises a tension control device  16  that is arranged between the first substrate web roll  3  in the feeder  2  and the printing region  8  comprising the print heads  9 . The tension control device  16  comprises a control unit  17 , a first driving unit  18  with a first driving roller  19 , and a second driving unit  20  with a second driving roller  21 , as well as a measuring unit  22  with a measuring roller  23 . It should be noted that the measuring unit  22  may also be designed without measuring roller, said unit being able to measure the tension of the substrate web  7 . The first driving roller  19 , the second driving roller  21  and the measuring roller  23  are arranged in such a manner that the substrate web extends in the form of an S-shaped loop around the rollers  19 ,  21  and  21 ,  23 . 
         [0031]    The control unit  17  of the tension control device  16  comprises a first control device  24  for controlling the first driving unit  18  as well as a second control device  25  for controlling the second driving unit  20 . For easier explanation, the first and the second control devices  24  and  25  are shown as separate control devices in  FIGS. 1 and 2 ; however, in practical applications, they may also be components of a control unit, said unit controlling a multitude of processes of the printing machine  1 . 
         [0032]    A signal line  26  is disposed to connect the measuring unit  22  with the first control device  24  which, in turn, is connected with the associate driving unit  18  via an additional signal line  27 . In the same manner, the measuring unit  22  is connected—via a signal line  28 —with the second control device  25  which, in turn, is connected with the associate second driving unit  20  via a signal line  29 . The main driving unit  13  is connected with the first control device  24  via a signal line  30  and is further connected with the second control device  25  via a signal line  31 . 
         [0033]      FIG. 2  shows, in detail, the design of the first and second control devices  24 ,  25 . The first control device  24  for the first driving unit  18  comprises a low-pass filter block  34 , a PI controller block  35 , a position control block  36 , a speed control block  37 , a torque control block  38 , as well as a function or mapping block  39 . An input of the low-pass filter block  34  is connected with an output of the measuring unit via the signal line  26 , in order to receive a measured value that is in relation to the measured tension of the substrate web  7 . Between the low-pass filter block  34  and the PI controller block  35  there is arranged a node  40 , wherein a nominal tension, as well as the negative output signal of the low-pass filter block, are combined and transmitted to the PI controller block  35 . Between the output of the PI controller block  35  and the input of the position control block  36 , there are sequentially arranged two nodes  41 ,  42 . At node  41 , the output signal of the PI controller block  35 , said signal having been processed by means of the function (1+X), as well as a speed signal from the main driving unit  13  are combined and transmitted further to the subsequent node  42 . At node  42 , the output signal of the node  41 , as well as a negative output signal of the mapping block  39  are combined and transmitted to the input of the position control block  36 . Another node  43  is arranged between the position control block  36  and the speed control block  37 . At the node  43 , an output value of the position control block  36 , as well as an actual rotational speed signal of the motor, said signal having a negative sign, are combined by the first driving unit  18  and are transmitted to the input of the speed control block  37 . The output of the speed control block  37  is connected with the input of the torque control block  38 . The output of the torque control block  38  is connected with the first driving unit  18 . 
         [0034]    The second control device  25  comprises a high-pass filter block  46 , a P controller block  47 , as well as a torque control block  48 . The output of the measuring unit  22  to which the measured value based on the substrate web tension is applied, is connected with the input of the high-pass filter block  46  via the signal line  28 . A node  49  is arranged between the high-pass filter block  46  and the P controller block  47 . At the node  49 , a value of 0, as well as a negative output signal of the high-pass filter block  46  are combined and transmitted to the input of the P controller block  47 . Another node  50  is arranged between the output of the P controller block  47  and the input of the torque control block  48 . At node  50 , the output value of the P controller block  47  and an offset are combined and transmitted to the input of the torque control block  48 . A rotational speed limit tapped from the second driving unit  20  is transmitted to the input of the torque control block  48  in order to act there on the input value into the torque control block  48 . The output of the torque control block  48  is connected with the second driving unit  20 . 
         [0035]    The main driving unit  13  is connected with the low-pass filter block  34  via signal line  30  and is connected with the high-pass filter block  46  via signal line  31 . Thus, depending on the control pattern, the filtering characteristics of the low-pass filter block  34  and/or the high-pass filter block  46  can be adjusted depending on the process speed or printing speed of the main driving unit  13 . It should be noted that the connection of the main driving unit  13  with the high-pass and low-pass filter blocks  34 ,  46  of the first and second control devices  24 ,  25  is not absolutely necessary. 
         [0036]    With reference to  FIG. 3 , the basic design of the control method is described hereinafter in greater detail with the tension control device  16  in operative position. In this method, the tension of the substrate web  7  is basically measured with the aid of a measuring unit  22 , and its measured value is processed further in different filtering devices and used for controlling the first and second driving units. 
         [0037]    To put it more precisely, the tension of the substrate web  7  is measured with the aid of the measuring unit  22  in step  60 . The output signal resulting therefrom or the measured value of the measuring unit  22  is then transmitted parallel to the low-pass filter block  34  being the first filtering unit (step  61 ) and also to the high-pass filter block  46  being the second filtering unit (step  62 ). A first filtered measured value results from the processing operation in the low-pass filter unit  34 , said first filtered measured value being used for controlling the first driving unit  18  (step  62 ). A second filtered measured value results from the processing operation in the high-pass filter block  46 , said second filtered measured value being used for controlling the second driving unit  20  (step  64 ). 
         [0038]    As is obvious from  FIG. 2 , the instantaneous printing speed or process speed is additionally transmitted from the main driving unit  13 —via the signal lines  30 ,  31 —to the low-pass filter block  34  as well as also to the high-pass filter block  46 . The filtering characteristics of the low-pass filter block  34  and/or the high-pass filter block  46  are then adjusted based on the process speed, for example, as a function of a mapping field, a function, or as a function of a prespecified setting. As it were, it may, depending on the process speed, be desirable to change the dynamic behavior of the first and the second driving units. However, it should be noted that the signal lines  30 ,  31  and the adjustment of the filtering characteristics that depends on the process speed are not necessary for each exemplary embodiment. 
         [0039]    Processing of the first filtered measured value as the output value of the low-pass filter block  34  will now again be described in more detail with reference to  FIG. 2 . The first filtered measured value is given a negative sign and transmitted to the node  40  of the first control device  24 . At the node  40 , the first filtered measured value is added to a prespecified nominal tension, and the result is transmitted as the input value into the PI controller block  35  and processed there. The output value of the PI controller block  35  is processed using the function (1+x) and transmitted to the node  41  of the first control device  24 . The node  41  also receives a speed signal from the main drive  13 . The output value of the node  41  is then transmitted further to the node  42 . The actual rotational speed of the motor is continually tapped from the first driving unit  18  and transmitted to the function or mapping block  39 . In the mapping block  39 , the actual rotational speed of the motor is changed in accordance with a mapping field, a function or a look-up table, and is also transmitted—as the changed actual rotational speed of the motor with a negative sign—to the node  42 . In node  42 , the changed negative actual rotational speed of the motor is combined with the output value of the node  41  and transmitted as the input value into the position control block  36  and processed there. The output value of the position control block  36  is transmitted to the node  43 , with the actual rotational speed of the motor of the first driving unit having a negative sign also being transmitted to said node. The negative actual rotational speed of the motor and the output value from the position control block  36  are combined in node  43  and entered as the input value in the speed control block  37  and processed there. The output value resulting from the speed control block is transmitted directly as the input value into the torque control block  38 . The torque control block  38  processes this input value, ultimately transmitting said input value to the first driving unit  18 . 
         [0040]    Now, processing of the measured value of the measuring unit  22  in the second control device  25  will also be described in greater detail with reference to  FIG. 2 . The output value or measured value of the measuring unit  22  is transmitted to the input of the high-pass filter block  46  and processed therein. The output value of the high-pass filter block  46  is given a negative sign and transmitted to the node  49 . The node  49  also picks up a control value “0” and transmits the value resulting from the two input values as the input value to the P controller block  47 , where the input value is processed. The output value exiting from the P controller block is combined with a prespecified offset at node  50  and made available as the input value to the torque control block  48 . However, before the combined offset and the output value of the P controller block  47  are used as the input value for the torque control block  48 , there is a check to determine whether this input value for the torque control block  48  exceeds the rotational speed limit that is being tapped from the second driving unit  20 . If the rotational speed limit is exceeded, the input value for the torque control block  48  is limited. If the rotational speed limit is not exceeded, the input value is made available to the torque control block  48  without any further change. The torque control block  48  processes this input value and outputs its output value to the second driving unit  20 . 
         [0041]    In summary, it can be said that the measured signal of a measuring unit is first split and then filtered separately, and then subjected to separate signal processing in order to find its use in the control of different driving units. Thus, the measured signal is used, at the same time, as a as a control value for two different driving units and is processed, for this purpose, in view of the dynamic characteristics of the driving units. 
         [0042]    In the practical application in a reel fed printing machine displaying a printing speed of 0.13 m/sec to 2.5 m/sec, a substrate web tension on the order of 120 to 550 N with a tolerance of 1% is generated with the aid of the presented device and the method for controlling the substrate web tension shown here. 
         [0043]    The invention has been described with reference to a preferred exemplary embodiment, wherein individual features of the described exemplary embodiment may be left out, unless they are absolutely necessary. For the person skilled in the art, there are numerous possible and obvious modifications and embodiments, without departing from the inventive idea. 
       PART LIST 
     FIG. 1: 
       [0000]    
       
           1  Printing machine 
           2  Feeder 
           3  Web roll 
           4  Output means 
           5  Web roll 
           7  Substrate web 
           8  Printing region 
           9  Printing station 
           11  Deflecting roller 
           13  Main driving unit 
           14  Driving roller 
           16  Tension control device 
           17  Control unit 
           18  First driving unit 
           19  First driving roller 
           20  Second driving unit 
           21  Second driving roller 
           22  Measuring unit 
           23  Measuring roller 
           24  First control device 
           25  Second control device 
           26  Signal line 
           27  Signal line 
           28  Signal line 
           30  Signal line 
       
     
       FIG. 2: 
       [0000]    
       
           19  Actual rotational speed limit 
           21  Rotational speed limit 
           48  Torque control 
           38  Torque control 
           50  Offset 
           47  P Controller 
           37  Speed control 
           46  High-pass filter 
           36  Position control 
           35  PI Controller 
           40  Nominal tension 
           34  Low-pass filter 
       
     
       FIG. 3: 
       [0000]    
       
           60  Measuring of the tension of the substrate web Measured value 
           61  Processing in the first filtering device (low-pass filter) 
           62  Processing in the second filtering device (high-pass filter) First filtered measured value 
           63  Activating the first driving unit Second filtered measured value 
           64  Activating the second driving unit