Abstract:
A seat belt retractor has a retractor frame, a belt spool rotatably mounted in the frame, an electric drive motor, a reduction belt gear with a toothed belt connecting the electric motor permanently with the belt spool, and a winding spring functionally arranged between the frame and the belt spool, permanently biasing the belt spool with a winding moment. The electric motor is controlled to either counteract or assist the winding spring.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a seat belt retractor comprising a retractor frame, a belt spool rotatably mounted in the frame, an electric drive motor and a reduction gear coupling the electric motor to the belt spool.  
         BACKGROUND OF THE INVENTION  
         [0002]    Conventional seat belt retractors have a winding spring permanently engaged between the frame and the belt spool to bias the belt spool in the winding direction. The winding spring must be dimensioned to overcome frictional resistance from various sources such as deflection rings and passengers cloths over which the belt webbing slides, thus ensuring the belt being substantially free of slack. On the other hand, belt tension is detrimental to comfort and, in fact, is one of the reasons for not fastening a seat belt. Sophisticated mechanisms have been developed to reduce belt tension over a limited range of belt length withdrawn from the spool to enhance comfort.  
           [0003]    Another approach is to replace the winding spring with an electric drive motor. The electric drive motor can be easily controlled to develop appropriate belt tension in all circumstances, including pre-crash tensioning of the seat belt. In the event of an electric power failure, however, no winding function is available, and the seat belts cannot be used, nor can they be stowed away by winding on their belt spools.  
         SUMMARY OF THE INVENTION  
         [0004]    The present invention provides a seat belt retractor with an electric drive motor wherein a winding function is available even in case of an electric power failure. According to the invention, the belt retractor comprises a retractor frame, a belt spool rotatably mounted in the frame, an electric drive motor and a reduction belt gear with a toothed belt connecting the electric motor permanently with the belt spool. A winding spring is functionally arranged between the frame and the belt spool, permanently biasing the belt spool with a winding moment. The electric motor is controlled to either counteract or assist the winding spring. In case of an electric power failure, the winding spring must drive the belt spool and simultaneously entrain the electric motor through the reduction gear that now acts as a step-up gear. As a first requirement, the reduction gear must be reversible, i.e. it must transmit rotation in both directions. A second requirement is that the reduction gear should have a high efficiency thereby limiting the necessary spring force. A belt gear inherently satisfies both requirements.  
           [0005]    In the preferred embodiment of the invention, a length of belt webbing withdrawn from the belt spool is detected. The length of belt webbing withdrawn is compared to predetermined threshold values to discriminate between a belt wearing condition and a belt non-wearing condition. The electric motor is driven in the wearing condition with current of a first polarity and adjusted to counteract the winding spring for appropriate belt wearing comfort, and is driven in a non-wearing condition with current of a second, opposite polarity adjusted for full retraction of belt webbing on the belt spool. Thus, belt tension can be adjusted for an optimum comfort after an initial tensioning to remove excessive belt slack, and increased in the non-wearing condition to safely retract, in combination with the winding spring, the belt webbing on the belt spool. The winding spring is dimensioned to overcome frictional resistance and mass inertia in the entire belt system, including the reduction gear and the electric motor. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]    Further advantages and features will become apparent from the following description of a preferred embodiment with reference to the drawings. In the drawings:  
         [0007]    [0007]FIG. 1 is a sectional view of a seat belt retractor;  
         [0008]    [0008]FIG. 2 is a block diagram of an electronic control circuit; and  
         [0009]    [0009]FIG. 3 is a diagram of belt forces vs. withdrawn webbing length. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0010]    The seat belt retractor in FIG. 1 has a frame  10  with two opposed walls  10   a,    10   b  wherebetween a belt spool  12  is rotatably mounted. Wall  10   b  of frame  10  has an extension  10   c  whereon an electric drive motor  14  is mounted. The electric motor  14  has a stator  16  carried by a bearing sleeve  18  that has an axial end fixed in an opening of wall extension  10   c.  The electric motor  14  has an external rotor  20  connected to a shaft  22  rotatably mounted in and axially extending through bearing sleeve  18 . Shaft  22  has an output end projecting from the bearing sleeve  18  with a pinion  24  attached thereto for joint rotation. An externally toothed wheel  26  is attached to one axial end of belt spool  12  for joint rotation. A toothed belt  28  is trained about pinion  24  and wheel  26 . Wheel  26  has an outer diameter much greater than that of pinion  24 , thereby forming a reduction belt gear coupling the electric motor  14  to belt spool  12 .  
         [0011]    Wheel  26  has an annular cavity to form a spring cage for accommodation of a helical winding spring  30 . Winding spring  30  has an outer end connected to wheel  26  and an inner end connected to a cylindrical bushing  32  fixed in an opening of wall  10   b  of frame  10 . Bushing  32  includes a bearing for rotatably mounting spool  12  on frame  10 .  
         [0012]    On its axial side opposite wheel  26 , belt spool  12  has an axial end rigidly connected to a locking wheel  34  for co-operation with a locking pawl  36  controlled by a solenoid  38 .  
         [0013]    An electric control unit is mounted on a printed circuit board  40  and includes a number of electronic components such as a capacitor  42 , power FET transistors  44  and an integrated circuit incorporating a microprocessor. The printed circuit board  40  extends parallel to wall extension  10   c  and is slightly spaced therefrom.  
         [0014]    An elongate cover  50  is fitted over wheel  26 , belt  28 , pinion  24  and partially over the components of the electronic control circuit and is attached to frame  10 . A hood  52  is fitted over the electric motor  14  and also attached to frame  10  so as to form a continuous enclosure with cover  50  to accommodate the electric motor, the electronic control unit and the reduction belt gear. Another cover  54  is fitted over the locking mechanism formed by locking wheel  34 , pawl  36  and solenoid  38 .  
         [0015]    The electronic control unit shown in FIG. 2 includes a microprocessor  60  with a number of inputs and outputs. Outputs of microprocessor  60  are connected to a driver circuit  62  the outputs of which are in turn connected to control gates of power FETs  44   a,    44   b,    44   c  and  44   d.  Each power FET  44   a - 44   d  drives one winding  46   a,    46   b,    46   c  and  46   d  of stator  16 . Current sense resistors R 1 , R 2  are connected in series with the windings of stator  16 . Each of the current sense resistors R 1 , R 2  provides a voltage drop indicative of current flowing through the windings of electric motor  14  and applied to a pair of inputs of microprocessor  60  through an input driver  64 . A pair of HALL detectors H 1 , H 2  are connected to corresponding inputs of microprocessor  60 . HALL detectors H 1 , H 2  are associated with rotor  20  of electric motor  14  to detect rotational positions of rotor  20 . By detecting rotational positions of rotor  20 , microprocessor  60 , on the one hand, controls commutation of electric motor  14  and, on the other hand, counts incremental steps of rotation so as to keep track of the absolute angle of rotation of belt spool  12  and, therefore, of webbing length withdrawn from belt spool  12 . Another input of microprocessor  60  is connected to a pre-crash sensor  66  mounted in the vehicle where the seat belt retractor is installed.  
         [0016]    Operation of the seat belt retractor will now be explained with reference to FIG. 3. In FIG. 3, line S shows the force permanently developed by winding spring  30  and appearing as a belt tension force. The belt tension force is a function of belt length withdrawn from belt spool  12 . This force rises from an initial value of 5 N (fully retracted belt) to a value of above 10 N (more than 1,000 mm of belt length withdrawn). Any force developed by electric motor  14  is either added to or subtracted from the force developed by winding spring  30 , depending on the sign of torque transmitted from motor  14  to belt spool  12  via the reduction belt gear. The resulting belt tension force is that experienced by the occupant wearing the seat belt, and is also that responsible for winding belt webbing on belt spool  12 .  
         [0017]    In the preferred embodiment, a first, relatively high level of belt tension is substantially constant over the length of belt webbing withdrawn from belt spool  12 , as indicated by line ( 1 ) in FIG. 3. The first level ( 1 ) is used for retracting the belt webbing on spool  12 . It results from the combined forces of winding spring  30  and motor  14 , the force developed by motor  14  being indicated by line M( 1 ) in FIG. 3. Force M( 1 ) is initially on the order of 2N (fully retracted belt webbing), decreases to 0 after withdrawl of a small length of belt webbing (about 300 mm in FIG. 3) and changes sign to reach negative values compensating for the increasing forces developed by winding spring  30 . A second, relatively low level of belt tension is also substantially constant over the length of belt webbing withdrawn from belt spool  12 , as indicated by line ( 2 ) in FIG. 3. The second level ( 2 ) is used after the occupant has buckled the seat belt and is dimensioned for comfort. It also results from the combined forces of winding spring  30  and motor  14 , the force developed by motor  14  being indicated by line M( 2 ) in FIG. 3. Force M( 2 ) is initially negative on the order of −1N (fully retracted belt webbing) and steadily increases to higher negative values to compensate for the increasing forces developed by winding spring  30 . Switching between the first and second levels ( 1 ) and ( 2 ) is controlled by microprocessor  60  as a function of the length of belt webbing withdrawn from belt spool  12 , as determined by the count of incremental rotation steps of motor  14 , and comparing the current length of belt webbing with predetermined thresholds.  
         [0018]    A third, much higher level of belt tension forces (not shown in FIG. 3) is used in a pre-crash situation as signalled by pre-crash sensor  66 .  
         [0019]    In the event of an electric power failure, the winding spring  30  still provides a winding force according to line S in FIG. 3, only somewhat reduced by frictional losses in the reduction belt gear. Winding spring  30  is dimensioned to provide a sufficient winding force under all circumstances to safely retract the belt webbing on belt spool  12 .