The invention relates to an operating method for an electric window lifter for moving a rear pane ( 3 ) in a motor vehicle, in which the rear pane ( 3 ) is moved in a lowering direction (−z) along an approximately s-shaped path of movement. In the case of trapping, the movement of the rear pane ( 3 ) in the lowering direction (−z) is stopped and the rear pane ( 3 ) is moved in the lifting direction (+z).

BACKGROUND 
     1. Field of the Invention 
     The invention relates to an operating method for an electric window lifter for moving a rear pane. The invention furthermore relates to a control device for carrying out the method. 
     2. Description of the Related Art 
     Electric window lifters are installed in vehicles and, in response to a control command, move the vehicle panes, in particular also a “rear pane”, into a desired position. The term rear pane is understood to mean a side pane in a vehicle, arranged behind what is referred to as the B pillar or adjoining a front pane associated with the driver's or passenger's door on the same side. When closed, a front edge of the rear pane adjoins the B pillar. The term vehicle “B pillar” is generally used to refer to the connecting pillar between the vehicle floor and the vehicle roof, approximately in the middle of a passenger compartment. Many coupés or cabriolets do not have a B pillar. In the case of such vehicles without a B pillar, the front edge of the rear pane adjoins the rear edge of a front pane, with a sealing lip generally being fitted on the front edge. 
     Especially in the case of cabrios and coupés, path-guided window lifters are therefore used. This is because the restriction on installation space due to the rear wheel housing makes it necessary to pivot the rear pane about the horizontal transverse direction of the vehicle (y direction) as said pane is lowered. Thus, the rear pane is not moved in a straight line but substantially on an s-shaped path. In order to implement such a pivoting operation of the window pane, a first and a second guide rail are provided, each running along a curved path, at least in sections. Also provided is a driver, which can be moved in a direction of movement which is substantially vertical (z direction) overall by means of the first and the second guide rail. The driver serves to connect the window pane to be moved to the window lifter. A window lifter of this kind is disclosed in German patent application DE 10 2004 048 017 A1, for example. 
     On the s-shaped path, the front edge of the rear pane approaches the B pillar or the front pane. As the front edge approaches the B pillar or the front pane, parts of the body or objects may be trapped. 
     SUMMARY 
     It is the underlying object of the invention to provide an operating method for an electric window lifter in which, in the case of trapping, release from trapping is made possible. 
     With respect to the method, this object is achieved, according to the invention, by means of an operating method for an electric window lifter for moving a rear pane in a motor vehicle, in which the rear pane is moved in a lowering direction along an approximately s-shaped path of movement. In this method, in the case of trapping the movement of the rear pane in the lowering direction is stopped and the rear pane is reversed, i.e. moved in the lifting direction. 
     The operating method according to the invention is independent of the presence of an anti-trap protection system, which is generally activated during the movement of the vehicle pane into the closed position, and of the manner in which a case of trapping is detected. Thus, for example, a case of trapping can be recognized or detected on the basis of a change in the drive or motor current, in particular a rise in the current, or a change in the rotation speed, in particular a drop in the rotation speed, or by the evaluation of a sensor signal from a trap sensor. 
     According to an advantageous development, in the case of trapping the front side pane (front pane) on the same side as the rear pane, i.e. that on the same side of the vehicle (driver or passenger side), is held in position at the same time as the rear pane is reversed, and operation of a window lifter for the front pane is blocked. Holding and blocking the front pane prevents damage/injury occurring due to the rear pane being moved along a trapped object, especially part of the body. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An illustrative embodiment of the invention will be explained in greater detail below with reference to a drawing, in which: 
         FIG. 1  shows one portion of a side part of a motor vehicle with a front pane and a rear pane and, in schematic representation, a control device for a rear window lifter motor and a front window lifter motor, 
         FIG. 2  shows the side part of the motor vehicle with the front pane and the rear pane in a closed position, 
         FIGS. 3-6  show the side part of the motor vehicle in accordance with  FIG. 2  with the rear pane in various positions, 
         FIG. 7  shows the side part of the motor vehicle in accordance with  FIG. 2  with the rear pane in an open position, 
         FIG. 8  shows a case of trapping with the rear pane in one position, and 
         FIG. 9  shows a variation diagram with a rotation speed characteristic of the rear window lifter motor and a position characteristic for the rear pane. 
     
    
    
     In all the figures, corresponding parts and variables are provided with the same reference signs in all cases. 
     DETAILED DESCRIPTION 
       FIG. 1  shows a side part  1  of a motor vehicle with a movable front pane  2  and a movable rear pane  3  and, in schematic representation, a control device  4  for operating a front window lifter motor  5  and a rear window lifter motor  6 . In this case, a front window lift button  7  and a rear window lift button  8  are connected to the control device  4 . The front and rear window lifter motors  5 ,  6  are furthermore connected to the control device  4 . A rotation speed monitor  9  is connected to the rear window lifter motor  6  and to the control device  4 . 
     When the front or rear window lift button  7 ,  8  is actuated, the front or rear window lifter motor  5 ,  6  is operated by the control device  4 . In this case, the front pane  2  or rear pane  3  is moved in accordance with a button command. During the movement of the rear pane  3 , the rotation speed monitor  9  records the rotation speed n of the rear window lifter motor  6 , and the control device  4  analyzes the rotation speed n in order to detect a case of trapping. 
       FIG. 2  shows the side part  1  of the motor vehicle with the front pane  2  in the closed state. Like the front pane  2 , the rear pane  3  is in a closed position P 0  in the closed state. In the closed position P 0 , a front edge  10  of the rear pane  3  adjoins the rear edge  12  of the front pane  2  by means of a fitted sealing lip  11 . In this case, the front edge  10  and the rear edge  12  are oriented horizontally with respect to the z axis, which is based on the typical vehicle coordinate system. The adjoining position of the edges  10 ,  12  prevents relative wind and rain from penetrating into the passenger compartment. 
       FIGS. 3 to 6  show the rear pane  3  in various positions as it is moved between the closed position P 0  and an open position P 5 . As the rear pane  3  is moved in the xz plane, a rear pane corner  13  formed at the transition between the rear pane front edge  10  and the rear pane top edge  14  moves along an s-shaped trajectory  15 . 
     In  FIG. 3 , the rear pane  3  is shown in a first position P 1 . In this position, the rear pane  3  is tilted slightly in the (−)x direction about a y axis and offset in the (−)z lowering direction relative to the closed position P 0 . In position P 1 , a gap  16  which increases in the (+)z lifting direction is thus formed between the front edge  10  and the rear edge  12 . 
       FIG. 4  shows the rear pane  3  in a second position P 2 , in which the rear pane  3  is at a level approximately half way between the closed position P 0  and the open position P 5 . In position P 2 , the rear pane  3  is tilted slightly in the (+)x direction in the xz plane, and the front edge  10  and the rear edge  12  form a gap  16  which tapers slightly in the (+)z lifting direction but is open. 
       FIG. 5  shows the rear window pane  3  with the rear pane corner  13  in a third position P 3 , in which it has moved along trajectory  15 , with the front edge  10 , which is tilted in the (+)x direction, forming with the rear edge  12  a virtually closed gap  16  which tapers in the (+)z lifting direction and, in the (−)z lowering direction, adjoins a door body top edge or a trim strip  17  located there. 
       FIG. 6  shows the rear pane  3  in a fourth position P 4 , in which the rear pane corner  13 , a shoulder of the front edge  10  and the front pane top edge  14  project beyond the trim strip  17 . In this case, the rear pane corner  13  and the front edge  10  are at a distance from the rear edge  12 . 
       FIG. 7  shows schematically the rear pane  3  in the open position P 5 , in which the rear pane  3  has been moved in the (−)z lowering direction and retracted into the side part  1 . 
       FIG. 8  shows a case of trapping in position P 3  (in accordance with  FIG. 5 ), in which an object  18  is trapped in the gap  16 . The s-shaped movement of the rear pane  3  means that the gap  16  tapers in the (+)z lifting direction in such a way that it is not possible to free the object  18  by movement in a direction in the xz plane. 
       FIG. 9  shows, in a variation diagram, a rotation speed characteristic 19 for the rear window lifter motor  6  in positions P 1  to P 5  as the rear pane  3  is moved in the xz plane. Here, a position characteristic shows the position of the rear window corner  13  along the z axis. At an initial time t 0 , the rear pane  3  is in the open position P 5 . In a first interval I 1 , the rear pane  3  is moved in the (+)z lifting direction from position P 5 , via positions P 4 , P 3 , P 2  and P 1 , into the closed position P 0 . Owing to the system slack, an abrupt rise in the rotation speed n of the rear window lifter motor  6  at the beginning of the first interval I 1 , with a subsequent rapid drop merging into a normal rotation speed range, is observed. Toward the end of the first interval I 1 , the approach to the closed position P 0  is associated with a drop in the rotation speed n. The first interval I 1  ends with the rear window  3  in the closed position P 0 , and the rear window lifter motor  6  stops. 
     In a second interval I 2 , the rear pane  3  remains in the closed position P 0 . In this case, the rotation speed n=0. In a third interval I 3 , the rear pane  3  is moved in the (−)z lowering direction. Owing to the system slack, the magnitude of the rotation speed n initially rises abruptly but then decreases as the window lifter mechanism between the rear window lifter motor  6  and the rear pane  3  comes into nonpositive engagement. As the rear pane  3  is moved in the (−)z lowering direction, the rear window lifter motor  6  moves in the opposite direction, this being illustrated by a negative rotation speed n. 
     At time t 1 , the case of trapping illustrated in  FIG. 8  occurs. In this case, the rotation speed (−)n falls rapidly owing to the trapped object  18 . The control device  4  recognizes the case of trapping from the drop in rotation speed detected by means of the rotation speed monitor  9  and stops the rear window lifter motor  6 . At the same time, the front window lifter motor  5  is switched to a holding state, in which the front pane  2  stops and can no longer be moved. In the case of trapping, the control device  4  switches over the rear window lifter motor  6  immediately in such a way that the rear pane  3  is moved into position P 1  in accordance with  FIG. 3 , as illustrated in a fourth interval I 4 . 
     In a fifth interval I 5 , the rear pane  3  remains in position P 1  to enable the trapped object  18  to be freed. Position P 1  is advantageous since the gap  16  between the front edge  10  and the rear edge  12  is widened in the (+)z lifting direction. This simplifies the freeing of the trapped object  18 . Once the object  18  has been freed, the rear pane  3  is moved into the open position P 5  in a sixth interval I 6 .