Patent Publication Number: US-2006006269-A1

Title: Safety belt system for a motor vehicle and a method for controlling an electric motor in a safety belt system

Description:
TECHNICAL FIELD  
      The invention relates to a safety belt system for a motor vehicle, in particular to a safety belt system comprising a belt retractor which includes a belt spool and an electric motor, the belt spool being able to be turned by the electric motor at least in a belt webbing winding direction. The invention further relates to a method for controlling an electric motor in a safety belt system.  
     BACKGROUND OF THE INVENTION  
      In conventional safety belt systems, belt retractors with retraction springs are used which are able to completely wind up again the belt webbing withdrawn by an occupant, after he has put on the safety belt. The strong pre-stressing force of the retraction spring in the belt webbing winding direction leads, however, to restrictions in comfort in the use of the safety belt, because the safety belt must be withdrawn contrary to the resistance of the retraction spring and is felt in an unpleasant manner after the belt is put on by the occupant.  
      Safety belt systems with powerful electric motors and variable electronic controls are also known, which in a so-called “pre-crash” situation offer a high tensioning power for a pre-stressing of the belt webbing which is possibly placed only loosely against the occupant&#39;s body. The motors of such systems are, however, large, heavy and expensive, owing to the high requirements.  
      There is a need for a safety belt system with improved comfort.  
     BRIEF SUMMARY OF THE INVENTION  
      According to a first aspect of the invention, a safety belt system for a motor vehicle comprises a safety belt and a belt retractor, the belt retractor including a belt spool and an electric motor, the belt spool being able to be rotated by the electric motor at least in a belt webbing winding direction, wherein the electric motor being coupled to the belt spool without a clutch, the belt retractor further including a retraction spring which pre-stresses the belt spool in the belt webbing winding direction, the pre-tensioning force of the retraction spring being restricted to functionality in terms of comfort.  
      According to a second aspect of the invention, a safety belt system comprises a safety belt and a belt retractor, the belt retractor including a belt spool and an electric motor, the belt spool being able to be rotated by the electric motor at least in a belt webbing winding direction, wherein the electric motor being controlled by a control unit coupled to a braking system of the vehicle. The safety belt system according to the invention operates successfully with an electric motor of weaker design, and nevertheless provides as optimum preconditions as possible for a belt tensioning following a pre-tensioning. The invention is based on the finding that an accident is almost always preceded by an actuation of the brake of the vehicle. The connection to the braking system provides for a pre-tensioning on every actuation of the brake and hence provides for an optimum position of the safety belt (and indirectly also of the occupant) for a belt tensioning. As the electric motor is able to be activated by the occupant through an actuation of the brake, a retraction spring can either by dispensed with entirely, or it can be designed to be so weak that the comfort restrictions are reduced to a minimum. As the belt webbing winding is brought about by the electric motor, a belt webbing having more pleasant haptic characteristics can be used, which with the use of a conventional retraction spring would lead to distinct impairments in belt webbing winding. When using a control which can operate the electric motor in a currentless state with the brake not actuated, or in other situations in which a belt webbing winding is not suitable, a coupling for a selective drive of the belt spool is not necessary. Rather, the belt retractor is able at any time to tension the belt webbing in a braking process without prior coupling of the motor.  
      For the coupling of the electric motor to the belt spool, preferably a gear is provided which comprises a pinion driven by the electric motor and a toothed wheel coupled to the belt spool. With a transmission ratio of the gear in the range of approximately 30:1 to approximately 60:1, a relatively weak and favorably priced motor is sufficient in order to tension the belt webbing so taut that any belt slack is reduced to a sufficient extent.  
      A preferred embodiment of the invention provides for a retraction spring, arranged in the belt retractor, which pre-stresses the belt spool in the belt webbing winding direction. As already mentioned, for reasons of comfort, the retraction spring can be designed to less than two or even to less than one revolution up to reaching its maximum tension.  
      A particularly narrow structural form of the belt retractor can be realized in that the retraction spring is accommodated in the toothed wheel. The belt retractor is therefore also suitable for a safety belt system which is integrated in a vehicle seat.  
      The invention also provides a method for controlling the electric motor in a safety belt system according to the invention, in accordance with which the electric motor is activated in response to a braking process. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  shows a side view of a belt retractor for a safety belt system according to the invention in accordance with a first embodiment;  
       FIG. 2  shows an end face view of the belt retractor of  FIG. 1 ; and  
       FIG. 3  shows a side view of a belt retractor for a safety belt system according to the invention in accordance with a second embodiment.  
    
    
     DETAILED DESCRIPTION OF THE PREFFERED EMBODIMENTS  
      In  FIG. 1 a  belt retractor  10  is illustrated with a belt spool  14  mounted in a housing  12  for rotation about an axis A. The belt spool  14  has a winding section  14   a  which is delimited by two margin sections  14   b  and  14   c . A toothed wheel  16 , likewise rotatable about the axis A, is coupled to the belt spool  14  by means of a retraction spring explained in further detail below.  
      The belt retractor  10  further comprises a simple electric motor  18 , which is controlled by means of a control unit  20  (only illustrated symbolically), and can be operated in two directions of rotation. The control unit  20  is, in turn, coupled to the braking system of the vehicle and can evaluate signals which, for example, are generated by a brake assistant, a brake booster or simply by an actuation of the brake pedal of the vehicle, for a control of the electric motor  18 .  
      The electric motor  18  is fastened between two side parts of the housing  12  and drives a pinion  22  which meshes with the toothed wheel  16  so that the belt spool  14  can be turned by the electric motor  18  both in the belt webbing winding direction and also in the belt webbing withdrawal direction. The pinion  22  and the toothed wheel  16  form a gear which has a transmission ratio of 30:1 or 60:1, for example. No clutch is provided between the electric motor  18  and the belt spool  14 , i.e. the electric motor  18  can drive the belt spool  14  directly by means of the gear with no time delay.  
      In  FIG. 2  it can be seen that the retraction spring  24 , constructed as a spiral spring, is accommodated in the toothed wheel  16 . The inner end  24   a  of the retraction spring  24  is coupled to the belt spool  14 , the outer end  24   b  is constructed as a tappet and engages into a recess  16   a  on the toothed wheel  16 . Upon a rotation of the belt spool  14  in the belt webbing withdrawal direction, the retraction spring  24  provides for a pre-stressing of the belt spool  14  against the toothed wheel  16  in the belt webbing winding direction.  
      The retraction spring  24  is designed so as to be relatively weak, i.e. the spring force at maximum tension of the retraction spring  24  is very low compared with conventional retraction springs. After a rotation of the inner end  24   a  of the retraction spring  24 , which is coupled to the belt spool  14 , by a maximum of two revolutions in the belt webbing withdrawal direction, preferably after approximately 700 degrees, the outer end  24   b  of the retraction spring  24  entrains the toothed wheel  16  if the electric motor  18  is off (i.e. currentless). According to another variant of the belt retractor, the entrainment already takes place after a maximum of one revolution, preferably after approximately 350 degrees. In particular, the retraction spring  24  is designed such that it can not completely wind up the safety belt with the insert tongue in the non-buckled state; the insert tongue on the safety belt may be lifted by a maximum of 500 mm from the floor of the vehicle interior.  
      The mode of operation of the safety belt system according to the invention is described below, in particular with regard to how the electric motor  18  is controlled according to the invention.  
      Before a vehicle occupant puts on the belt, the electric motor  18  is currentless, so that it does not offer a significant rotational resistance either to a rotation in the belt webbing withdrawal direction or in the belt webbing winding direction. When the occupant withdraws a length of belt webbing, the belt spool  14  and thereby also the inner end  24   a  of the retraction spring  24  rotate in the belt webbing withdrawal direction. After just under two revolutions, the tensioned retraction spring  24  entrains with its outer end  24   b  the toothed wheel  16 , whereby the currentless electric motor  18  is co-rotated on further belt webbing withdrawal. Owing to the low resistance of the retraction spring  24  and the electric motor  18 , which is in a currentless state, the belt webbing withdrawal is possible in a very comfortable manner.  
      After the vehicle occupant has put on the belt, the retraction spring  24  relaxes and in so doing turns the belt spool  14  by just under two revolutions back in the belt webbing winding direction. Thereby, the belt webbing is placed loosely against the body of the occupant. If the occupant bends forward, the belt webbing follows this movement, i.e. belt webbing is withdrawn. For this, the occupant must merely overcome the low pre-stressing force of the retraction spring  24 , which represents a distinct advantage in terms of comfort compared with safety belt systems with conventional retraction springs. This applies in particular to the case of a safety belt with an inflatable section, which basically offers less wearing comfort than a conventional safety belt and, owing to the higher weight, would require an even stronger retraction spring in a conventional safety belt system without an electric motor.  
      In a braking process, the control unit  20  activates the electric motor  18 , so that the belt spool  14  is turned in the belt webbing winding direction and thereby the belt webbing is tensioned. As the belt webbing is already pre-tensioned by the retraction spring  24  in the belt webbing winding direction, on a rotation of the electric motor  18  the belt webbing is immediately wound. The control unit  20  sets the output of the electric motor  18  depending on the braking force. For example, via the pedal travel of the brake pedal, a potentiometer can be used to determine the current consumption of the electric motor  18 . The control unit  20  makes provision that the electric motor  18  is operated with maximum output at a full braking, in order to achieve a maximum belt tension. Thereby, optimum preconditions for a subsequent belt tensioning are provided, when an impact of the vehicle actually occurs.  
      After a braking process, the electric motor  18  assumes the currentless state again. By bending forward, the occupant can withdraw belt webbing again, which is then placed loosely against the occupant&#39;s body again by the pre-stressing force of the retraction spring  24 .  
      The controlling of the electric motor  18  makes possible a tensioning and relaxing of the belt webbing which can be repeated as often as desired, through actuation and releasing of the brake pedal and subsequent bending forward of the occupant, respectively.  
      In the case of a longer lasting actuation of the brake, e.g. at traffic lights or on an inclined road, the output of the electric motor  18  is reduced, or the electric motor  18  is switched off. The exceeding of a predetermined time duration serves as criterion.  
      The electric motor  18  is also switched off in situations in which the resistance against a rotation of the electric motor  18  exceeds a predetermined value or if the electric motor  18  is operated longer than a predetermined time duration.  
      When the occupant takes off the safety belt, owing to the weak design of the retraction spring  24  only a portion of the belt webbing is wound onto the belt spool  14 . Through an actuation of the brake pedal, the belt webbing can be fully wound.  
      An enhanced control provides for an automatic winding of the belt webbing after the safety belt has been taken off. In this case, the control unit  20  activates the electric motor  18  in response to a signal which is triggered by an opening of the belt buckle of the safety belt system or by the opening of the associated door of the vehicle. Such an activation can also take place with a time delay. After a predetermined time duration, e.g. 5 seconds, the control  20  switches the electric motor  18  off again.  
      An alternative embodiment of the safety belt system according to the invention operates successfully with a belt retractor without a retraction spring. In this embodiment, the toothed wheel  16  can be constructed for example in one piece with the belt spool  14 , or, as shown in  FIG. 3 , instead of a separate toothed wheel, outer teeth  26  can be provided, formed directly integrally with one of the margin sections  14   a  and  14   b  of the belt spool  14 . The gear formed between the electric motor  18  and the belt spool  14  can also have one or more additional pinions or toothed wheels  28  between the pinion  22  and the toothed margin section  14   b . In this embodiment, the supporting force in the belt webbing winding direction is provided solely by the electric motor  18 .