Abstract:
A method for the open-loop control of a wiper system ( 10 ), and a wiper system ( 10 ), in particular for a motor vehicle includes at least one drive device ( 16 ), at least one gear ( 14 ), which switches the motion of the drive device ( 16 ) to at least one wiper shaft ( 34 ) and has a gear ratio that depends on the position of the gear ( 14 ), and at least one open-loop control unit ( 24 ), which controls the torque that can be output by the drive device ( 16 ) by means of a driven shaft ( 26 ). The control unit ( 24 ) determines the torque that can be output by the drive device ( 16 ) at every instant, taking the gear ratio into account.

Description:
CROSS-REFERENCE 
   The invention described and claimed hereinbelow is also described in PCT/DE 02/02171, filed on Jun. 14, 2002 and DE 101 44985.2, filed Sep. 12, 2001. This German Patent Application provides the basis for a claim of priority of invention under 35 U.S.C. 119 (a)–(d). 
   BACKGROUND OF THE INVENTION 
   The invention relates to a wiper system and a method for open-loop control of a wiper system. 
   Numerous methods for open-loop control of wiper systems are known, for instance from German Patent Disclosure DE 197 00 457. In these methods, the wiper motor in each case is triggered such that the position of the wiper blade on the window always move along a desired trajectory. In simpler methods, the speed of the wiper motor and thus the speed of the wiper blade on the window is reduced in the region of the turning points. However, this involves commonly used closed-loop control mechanisms for positionally accurate control of the wiper blade. 
   SUMMARY OF THE INVENTION 
   The method of the invention for open-loop control of a wiper system has the advantage that the torque that can be output by the drive device is determined at every instant taking the gear ratios into account. In this way, an overload on the drive device, which ensues for instance whenever snow or ice on the windshield causes blockage of the wiper system, can be effectively prevented. Such blockage is especially problematic if blockage of the wiper system occurs just before the turning point of the wiper arm, is reached. Since the usual wiper systems drive the wiper arms via a crank gear, the greatest loads occur in that case, since the drive device comes to a stop precisely in the extended or the covering position. The maximum attracting torque of the drive device then acts on the wiper system with the least gear ratio. In theoretical terms, in that case with rigid components, infinitely high forces would occur because of the toggle lever effect, since the amount of the effective lever arm tends toward zero. 
   It is especially advantageous if the gear is embodied as a crank gear, and the torque that can be output is determined as a function of the position of at least one crank, since the position of the cranks is responsible for the torque transmission from the drive device to the wiper arm. 
   It is furthermore considered advantageous if the drive device outputs a rotary motion, the gear converts the rotary motion into a pendulum motion with two turning points, and the torque is reduced before the turning points are reached, since in these turning points, the crank of the drive device is located in the covered or extended position and as a result the transmitted torque is maximal. 
   It is also advantageous if the drive device consumes a current I, and the torque is effected by a limitation of the current I. In this way, the torque can be limited by means of a simple current limiter. 
   Advantageously, the limitation of the current I is effected by a characteristic curve that is stored in memory in the control unit, as a result of which a completely freely selectable current limitation as a function of the gear position is made possible. 
   It is furthermore advantageous if the drive device is controlled electronically and has a position and/or rotary motion detector, so that the current limitation can be adapted exactly to the current gear position. 
   It is especially advantageous if the drive device is embodied as a reversing motor, and after the turning point is reached, a higher torque is output by the drive device than before the one turning point is reached, in order to obtain an optimal blockage protection. 
   Furthermore, it is especially advantageous if the torque is reduced only whenever a rated rotational speed of a driven shaft of a motor of the drive device fails to be attained; at elevated rotational speeds of the driven shaft, the point of departure must be that no hindrance or even blockage of the driven shaft occurs, as a result of which a torque reduction becomes unnecessary. Only upon a hindrance or even blockage of the driven shaft is it necessary to reduce the torque that acts on the driven shaft. 
   The wiper system of the invention has the advantage that a marked weight reduction of the entire system is possible because of smaller dimensions of the mechanical components, without having to integrate additional open-loop control hardware into the system. The result is advantageously markedly lower costs for production and disposal of the entire system. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     One exemplary embodiment of the invention is shown in the drawings and explained in further detail in the ensuing description. Shown are: 
       FIG. 1 , a wiper system of the invention, shown schematically; 
       FIGS. 2   a – 2   f,  various gear positions of a wiper system of the invention; 
       FIG. 3 , a graph showing the gear ratio over the angular position of the motor crank of the drive device; and 
       FIG. 4 , a graph showing the torque demand and the torque limitation over the angular position of the motor crank. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   In  FIG. 1 , a wiper system  10  of the invention is shown in perspective. It essentially comprises a support tube  12 , a gear  14 , and a drive device  16 . The drive device  16  is embodied as an electric motor, which may be in the form of a reversing motor or a revolving motor. The drive device  16  has an armature shaft  18 , which drives a motor gear  20 . The motor gear  20  is disposed in a motor gearbox  22 , which is covered by a lid in which an open-loop control unit  24  is located. The motor gear  20  drives the gear  14  via a driven shaft  26 . To that end, the gear  14  has a motor crank  28 , which moves a thrust rod  30  that is rotatably joined to it. 
   If the drive device  16  is embodied as a reversing motor, then when current is supplied to the drive device, the motor crank  28  swings back and forth in pendulum fashion between two angular positions. In the case of a revolving motor, the motor crank  28  executes a revolving rotary motion. 
   Since the wiper system shown here is meant to drive two wiper arms, there are two thrust rods  30  disposed on the motor crank  28 , resulting in a symmetrical design. Below, only one side of the wiper system will be described, since the other side behaves symmetrically. 
   The thrust rod  30  is joined so as to move rotationally with a drive crank  32 , which upon a motion of the motor crank  28  sets a wiper shaft  34  into pendulum motion. The wiper shaft  34  is supported in a manner that is rotationally movable but axially fixed in a wiper bearing  36 . To that end, the wiper bearing  36  has a cylindrical bearing bush  38 . 
   If the driven shaft  26  now executes a circular motion, then the connection point between the motor crank  28  and the thrust rod  30  likewise moves along a circular path. The thrust rod  30  transmits this motion to the drive crank  32 , which thereupon sets the wiper shaft  34  into a pendulum motion. This pendulum motion takes place between two turning points WL 1  and WL 2 . 
   If the driven shaft  26  executes a pendulum motion, then the connection point between the motor crank  28  and the thrust rod  30  moves along a circular arc, and as a result the wiper shaft  34  is likewise set into a pendulum motion. This possibility that is realized by means of a reversing motor allows more freedom, in designing the wiper system  10 , in terms of the choice of the turning points WL 1 , WL 2  as well as other factors. In particular, in this way parking positions, which are assumed when the wiper system is not in operation, can be defined very simply. 
   The gear ratio of a crank gear of this kind varies continuously with the position of the gear, since the effective lever arm of the motor crank  28  and of the drive crank  32  depends on their deflection angle, since the moment applied to the wiper shaft depends on the vertical spacing of the longitudinal axis of the thrust rod  30  from the wiper shaft  34 , or the vertical spacing from the driven shaft  26  of the motor  16 . 
   In  FIGS. 2   a – 2   f,  various positions of the gear  14  are shown. 
   In  FIG. 2   a,  the gear  14  is in the insertion position. Here, the motor crank  28  and thrust rod  30  are located on a straight line, in such a way that the spacing between the driven shaft  26  and the connection point of the drive crank  32  and thrust rod  30  is maximal. A first turning point WL 1  of the wiper system is therefore also in this position. Naturally, the turning point WL 1  can also be located at some point reached earlier, if the drive device  14  is embodied as a reversing motor. In that case, the extended position is then not reached. 
   If the drive device  16  is a reversing motor, the motor crank  28  is not moved past the extended position, but instead moves clockwise, as represented by the arrow P. 
     FIG. 2   b  shows the gear  14  in a middle position, and  FIG. 2   c  shows it just before the covering position is reached. In the covering position, which is shown in  FIG. 2   d,  the motor crank  28  and thrust rod  30  are located on a straight line, in such a way that the spacing between the driven shaft  26  and the connection point of the drive crank  32  and the thrust rod  30  is minimal. A second turning point WL 2  of the wiper system is therefore also located in this position. Naturally, the turning point WL 2  can also be located at some point reached earlier, if the drive device  14  is embodied as a reversing motor, and the rotary motion of the motor crank  28  is reversed at an earlier point. 
     FIG. 2   e  shows the gear  14 , again just before the extended position is reached, when a reversing motor is used that has changed its rotary motion in the region of the covering position. The arrow P therefore also points counterclockwise here. 
     FIG. 2   f  shows the gear  14  just before the extended position is reached when a revolving motor is used, which always maintains the same rotary motion. The arrow P therefore points in the clockwise direction again here. 
   In  FIG. 3 , the gear ratio is shown over the deflection angle of the motor crank. The angle of 0° corresponds to the extended position, in which the longitudinal axis of the motor crank  28  and the longitudinal axis of the thrust rod  30  form parallels. At an angle of 180°, the gear is in the covering position. There, the two longitudinal axes of the motor crank  28  and of the thrust rod  30  are once again parallel, but the driven shaft  26  of the drive device  16  is concealed by the thrust rod  30 , so that the spacing between the driven shaft  26  and the connection point of the thrust rod  30  and the drive crank  32  is minimal. The angle of 360° naturally again corresponds to the extended position, that is, the angle of 0°. 
   The gear ratio Ü changes its sign at 180°, since the rotary motion of the motor crank  28 , in a revolving motor, remains the same but the direction of motion of the drive crank  32  and thus of the wiper shaft  34  reverses. The course of the gear ratio Ü is essentially sinusoidal; that is, the gear ratio varies continuously over the deflection angle of the motor crank  28 . As a result, the torque transmitted also differs at each angular position. 
   Typically, the components of the gear  14 , such as the motor crank  28 , thrust rod  30 , drive crank  32 , and the bearings, are dimensioned such that in the wiping operation, the maximum torque required does not damage the components in the event of blockage. According to the invention, it is now provided that the torque that can be output by the drive device  16  is limited as a function of the angular position of the motor crank  28  by means of the control unit  24 . 
   In  FIG. 4 , the torque demand and the torque that can be output are shown as examples over the deflection of the motor crank  28 . Since the force acting on the thrust rod  30  and thus on the drive crank  32  and the wiper shaft  34  is supposed to be limited, only a slight torque is needed at the turning points or at the turning point. In these regions, the vertical spacing between the thrust rod  30  and the driven shaft  26  is very slight, and as a result, despite the slight torque, a major force is produced. The force F should not be so high, however, that the thrust rod  30  or drive crank  32  or even the wiper arm will become warped in the event of blockage of the wiper blade or wiper arm. The range of rotation of the motor crank  28  is therefore divided up into a plurality of angular ranges WB 1  through WB 7 , and for each of these angular ranges WB 1  through WB 7 , its own maximum torque is defined. 
   In the first angular range WB 1 , the motor crank moves out of the extended position, up to an angle for instance of 10–20°. In this range, the torque that can be output is limited to 80% of the maximum torque that can be output. Next, in the second angular range WB 2 , the torque is limited to a set, maximum torque, which is meant to correspond to 100% and to the maximum torque required by the wiper system. This range therefore goes beyond the range of the maximum torque required. 
   In the third angular range WB 3 , the torque that can be output is again lowered to 80% of the maximum torque that can be output, and in a fourth angular range WB 4 , the torque is limited to approximately 50% of the maximum torque that can be output. This range is shortly before the turning point, for instance 10–15° before it, and ends at the turning point, in which the gear, that is, the motor crank  28  and the thrust rod  30 , are in the covering position. Upon reaching the turning point, in this example, the torque is again limited to the maximum torque that can be output, but here again, a reduced range, such as 80%, can be selected first. This range corresponds to the fifth angular range WB 5 . Approximately 20–30° before the turning point is reached, the torque is again limited in the sixth angular range WB 6  to 80%, and approximately 10° before the turning point is reached, in the seventh angular range, to approximately 50% of the maximum torque that can be output. 
   In  FIG. 4 , the drive device  16  is embodied as a reversing motor, which swings back and forth between two angular positions that do not correspond entirely to the extended and covering positions. For this reason, the dead center points at 0° and 180° of the position of the motor crank  28  are avoided. 
   Naturally, the exemplary embodiment shown in  FIG. 4  can be varied arbitrarily. For instance, a continuous course of the torque demand, or somewhat above the torque demand, can be set, as a result of which the function of the system is indeed improved, but increased open- and closed-loop control is needed.