Abstract:
A belt retractor for a vehicle safety belt comprises a frame, a belt spool which is rotatably arranged in the frame, and a drive motor which can cooperate with a drive wheel which is coupled with the belt spool. The retractor further comprises a locking catch and locking teeth into which the locking catch can be guided in order to lock the belt spool. The drive wheel is rotatable relative to the belt spool to a limited extent and the locking catch is controlled as a function of this relative rotation.

Description:
TECHNICAL FIELD 
   The invention relates to a belt retractor for a vehicle safety belt. 
   BACKGROUND OF THE INVENTION 
   A conventional belt retractor comprises a frame, a belt spool which is rotatably arranged in the frame, a drive motor which can cooperate with a drive wheel which is coupled with the belt spool, a locking catch and locking teeth into which the locking catch can be guided in order to lock the belt spool. 
   Such a belt retractor is known from the DE 100 18 972. The locking catch is biased by a spring in the sense of an engagement into the locking teeth. An electromagnet is provided, which in the ON-state holds the locking catch out of engagement with the locking teeth. 
   The object of the invention consists in improving the known belt retractor to the effect that a lower structural expenditure results. 
   BRIEF SUMMARY OF THE INVENTION 
   According to the invention, a belt retractor for a vehicle safety belt comprises a frame, a belt spool which is rotatably arranged in the frame, and a drive motor which can cooperate with a drive wheel which is coupled with the belt spool. The retractor further comprises a locking catch and locking teeth into which the locking catch can be guided in order to lock the belt spool. The drive wheel is rotatable relative to the belt spool to a limited extent and the locking catch is controlled as a function of this relative rotation. The belt retractor proposed is based on the fundamental idea of using the drive motor for the belt spool, which is already present in any case, also to control the locking catch. This takes place in that the drive motor is controlled such that the relative rotation between the drive wheel and the belt spool, necessary to control the locking catch, is produced. This makes it possible to dispense with additional components which were necessary hitherto to actuate the locking catch. 
   Preferably, provision is made that the locking teeth are constructed on a locking wheel which is securely connected with the belt spool, and that between the drive wheel and the locking wheel at least one restoring spring is provided, which biases the drive wheel relative to the locking wheel into an initial position. In so doing, preferably the locking catch is biased by a locking spring permanently in the sense of an engagement into the locking teeth, the drive wheel being provided with control teeth which are formed by depressions in the otherwise smooth outer peripheral surface of the drive wheel, and the locking teeth and the control teeth being constructed such that in the initial position between the drive wheel and the locking wheel, the outer peripheral surface of the drive wheel prevents the locking catch from being guided into the locking teeth. The restoring spring can be dimensioned here such that the drag moment of the current-free drive motor is greater than the moment of rotation which is necessary for rotating the drive wheel relative to the locking wheel. In this way, an emergency locking is achieved, which locks the belt spool in the belt band withdrawal direction when the drive motor is at a standstill. The restoring spring can also be dimensioned so as to be stronger, so that with a slow withdrawal of belt band from the belt spool, a relative rotation does not occur between the drive wheel and the locking wheel, but rather only on exceeding of a predetermined angular acceleration of the belt spool, because in this case the rotational moment of inertia of the drive motor additionally acts as a brake. In this way, a locking function can be realized, which corresponds to a conventional mechanical, belt band-sensitive locking. 
   According to a preferred embodiment of the invention, provision is made that the side of the locking teeth cooperating with the locking catch is not undercut. This makes it possible for the control teeth of the drive wheel to guide the locking catch out from the locking teeth when the belt spool is locked, without the belt spool having to be turned in the belt band winding direction for this purpose; such a rotation would be felt by the vehicle occupant as an unpleasant, brief increase to the belt band force. 
   Preferably, a winding spring is provided which acts upon the belt spool in the belt band winding direction with a moment of rotation which is greater than the drag moment of the drive motor when the latter is current-free. This also ensures an emergency function to the effect that in the case of an interruption in the electrical supply of the drive motor, or in the case of failure of its control, in each case it is ensured that the belt band is wound up correctly when the vehicle occupant removes the safety belt. 
   Preferably, an electronic control unit is provided, which controls the drive motor such that on withdrawing or winding up of the belt band, a predetermined belt band force is always in effect. The control unit recognizes the rotation of the belt spool, for example by means of a belt spool sensor, and controls the drive motor and hence the rotation of the belt spool such that the belt band is available in the desired manner. When the vehicle occupant puts on the safety belt, for example, the drive motor provides a moment of rotation which is opposed to the moment of rotation of the winding spring. In this way, a comparatively low belt band withdrawal force is produced. The moment of rotation provided by the drive motor is preferably varied here such that for the vehicle occupant a constant belt band withdrawal force is perceptible, although the moment of rotation exerted by the winding spring rises with increasing belt band withdrawal. The drive motor can also provide a moment of rotation on winding up of the belt band, which counteracts the moment of rotation of the winding spring. In this way, the comparatively high belt band winding force of the winding spring is reduced to a more comfortable amount. 
   The control unit may have a signal input for the vehicle acceleration, the control unit controlling the drive motor on exceeding a predetermined vehicle acceleration such that the drive wheel is brought into a locking position relative to the locking wheel, in which the locking catch engages into the locking teeth. The signal for the vehicle acceleration may originate from systems already present, for example from an electronic stability program or an anti-locking system, and supplied to the control unit via a vehicle bus system, for example a CAN-bus. In this way, the vehicle-sensitive locking of the belt spool can be realized with minimum expenditure. 
   Advantageous developments of the invention will be apparent from the subclaims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows in a diagrammatic exploded view a belt retractor according to the invention; 
       FIG. 2  shows in a perspective, diagrammatic view the belt retractor of  FIG. 1 , some components not being illustrated, for greater clarity; 
       FIG. 3  shows in a perspective, enlarged view the locking mechanism of the belt retractor of  FIG. 2  in the non-locked state; 
       FIG. 4  shows the locking mechanism of  FIG. 3  in the locked state; and 
       FIG. 5  shows a diagram of the curve of the belt band withdrawal force, the spring characteristic of the winding spring and also of the moment of rotation of the drive motor in various operating states. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   In  FIGS. 1 and 2  a belt retractor can be seen, which has a frame  10  and a belt spool  12  rotatably mounted in the frame. A vehicle safety belt can be received on the belt spool. Inside the belt spool  12 , a torsion rod  14  is arranged, which at its left-hand end with respect to  FIG. 1  is connected with the belt spool  12  for joint rotation therewith. At this end, a winding spring  16  is also connected, which biases the belt spool  12  in the belt band winding direction. A locking wheel  18  is connected for joint rotation with the right-hand end of the torsion rod  14  with respect to FIG.  1 . The locking wheel  18  has a smooth outer peripheral surface which is interrupted at regular intervals by depressions  20 . The depressions  20  have a side aligned in radial direction and also a side running obliquely from the deepest point of the depression to the outer peripheral surface, the radially aligned flank being that which with a rotation of the belt spool  12  and hence of the locking wheel  18  lags behind in the belt band unwinding direction. All the depressions  20  together form locking teeth on the locking wheel  18 . 
   Concentrically on the locking wheel  18 , a drive wheel  22  is arranged which is provided with two recesses  24 . In each recess  24  a tongue  26  engages, which is constructed on the locking wheel  18 . Between each tongue  26  and a wall of the recess  24 , a restoring spring  28  see  FIG. 3  is arranged, which each act upon the drive wheel  22  relative to the locking wheel  18  into an initial position shown in FIG.  3 . The drive wheel  22  is provided on its outer peripheral surface, in the same manner as the locking wheel  18 , with several depressions which are given the reference number  30  here. In contrast to the depressions  20  of the locking wheel  18 , the depressions  30  of the drive wheel  22  are constructed with two sides, arranged approximately symmetrically and running obliquely, so that control teeth are formed. 
   A drive motor  35  is coupled to the drive wheel  22  by a toothed belt  32  and a motor wheel  34 . The drive motor is preferably a synchronous external rotor motor. The drive motor is controlled by a diagrammatically illustrated control unit  38 , which receives signals from a diagrammatically illustrated vehicle acceleration sensor  40  and a diagrammatically illustrated belt spool sensor  44 , the latter detecting the rotation of the belt spool. 
   On the frame  10  of the belt retractor, two locking catches  46  are arranged, which lie diametrically opposite each other with respect to the axis of rotation of the locking wheel  18  and of the drive wheel  22 . The two locking catches  46  can be guided into the locking teeth of the locking wheel  18 . For this purpose, they are each acted upon by a locking spring  48  indicated in  FIG. 3  in the sense of an engagement into the locking teeth of the locking wheel  18 . 
   With the aid of  FIGS. 3  to  5 , the mode of operation of the belt retractor is described below. The starting point here is a state of equilibrium, in which the belt band forces, the moment of rotation of the winding spring and the moment of rotation of the drive motor are in equilibrium, so that the belt spool does not rotate. As the winding spring  16  is dimensioned such that with a current-free drive motor  36  it can rotate the belt spool in the winding direction, it is necessary for this state of equilibrium that the drive motor  36  largely compensates for the moment of rotation of the winding spring. The drive motor therefore attempts to turn the drive wheel with respect to  FIG. 3  in an anticlockwise direction. Only hereby is the drive wheel  22  situated relative to the locking wheel  18  in the initial position shown in  FIG. 3 , which is distinguished in that the depressions  20  of the locking wheel  18  are arranged staggered to the depressions  30  of the drive wheel  22 . In other words, the depressions of one wheel are always in an angular section in which the other wheel has a smooth outer peripheral surface. 
   If, starting from the state of equilibrium of  FIG. 3 , belt band is to be withdrawn from the belt spool, i.e. the belt spool rotates anticlockwise with respect to  FIG. 3 , the drive motor supports this, in order to reduce the force which is felt on the belt band to a more comfortable level, which lies below the level which would be necessary to overcome the winding spring. On unwinding of the belt band, the drive wheel  22  remains in the initial position relative to the locking wheel  18 , so that the locking catches  46  rest alternately on the outer surface of the drive wheel  22  and the outer surface of the locking wheel  18 ; in this way, they are prevented from engaging into the locking teeth of the locking wheel  18 . 
   If by a sudden, intensive traction on the belt band the belt spool is now acted upon very quickly in the belt band unwinding direction, it “overtakes” the drive wheel, the restoring springs  28  being compressed. The depressions  20  of the locking wheel  18  then come to overlap the depressions  30  of the drive wheel  22  see  FIG. 3 , and the locking catches  46  can engage into the nearest depression  20  of the locking wheel  18 ; the belt spool is then locked. This state is shown in FIG.  4 . 
   If, starting from the locked state, the locking is to be released again, the drive motor is guided such that the drive wheel  22  is turned anticlockwise with respect to FIG.  4 . In so doing, the corresponding sides of the depressions  30  of the drive wheel  22  ride up the tips of the locking catches  46  and press the latter radially outwards until they are guided out from the locking teeth of the locking wheel  18 . Here, it is not necessary to turn the belt spool slightly in the belt band winding direction, because the sides of the locking teeth, against which the locking catches  46  lie, are aligned radially. The locking is terminated again as soon as the locking catches  46  lie on the smooth outer peripheral surface of the drive wheel  22  and have left the corresponding depression  20  of the locking wheel  18 . 
   When the belt band is to be wound up on the belt spool, the holding torque of the drive motor  36  is reduced, so that the belt spool  12  turns in the belt band winding direction under the action of the winding spring  16 . 
   When the belt band is to be wound up on the belt spool  12 , it is merely necessary to reduce the holding torque of the drive motor  36 , so that the now predominant force of the winding spring provides for the belt spool turning clockwise with respect to  FIGS. 3 and 4 . 
   If a defect occurs on the motor or on the motor control, and the motor does not provide any driving torque at all, the winding spring  16  ensures that the belt band is wound reliably on the belt spool  12 . An emergency locking of the belt spool is also ensured. When belt band is withdrawn from the belt retractor, the drag moment of the drive motor  36  acts as resistance moment, plus an inertia-related torque, when the belt spool undergoes an acceleration of rotation. This resistance moment is sufficiently high in order to overcome the bias of the restoring springs  28 , whereby the drive wheel  22  arrives into the locking position (shown in  FIG. 4 ) relative to the locking wheel  18 . Then the locking catches  46  will engage into the nearest depression  20  of the locking wheel  18  and lock the belt spool. 
   By means of the control unit  38 , a plurality of comfort functions can be realized in the normal operation of the belt retractor. For example, it can be ensured that the belt band winding force on putting on the belt is lower than on taking it off, or also changes during the journey, depending of the manner of driving. In addition, it can be ensured that the belt band winding force does not change independently of the length of the belt band which is already wound. 
   Deviating from the embodiment which is shown, it is also possible to arrange the two locking catches so as to be not diametrically opposite each other, but rather to arrange them so as to be staggered with respect to the diametrical arrangement about an angle which corresponds to half the angle distance between two depressions. In this way, the rotation of the belt retractor required to reach the locked state can be reduced. It is basically also possible to use merely one single locking catch.