Abstract:
A motorized seat belt retractor having a load limiter to control the tensile load on a seat belt webbing, especially when the webbing is withdrawn due to an emergency condition. The load limiter controls the tension by controlling the condition of the motor that drives the retractor.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority to and the benefit of U.S. Provisional Patent Application Serial No. 60/198,751 filed Apr. 21, 2000. The foregoing provisional application is incorporated by reference herein in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a seat belt retractor with a load limiter and a locking mechanism. More particularly, the present invention relates to a retractor having a motorized load limiter and locking mechansim. 
     Conventionally, retractors have include a mechanical load limiter such as a torsion bar or the like for limiting the load applied on a shoulder or chest of an occupant of a motor vehicle. In these conventional retractors the mechanical type of load limiter does not allow for the threshold setting for load on the webbing to be altered. Typically, the threshold value for the amount of tensile load to be applied to the webbing is determined according to the model type of automobile. This threshold value is fixed and cannot be easily altered. 
     Therefore, it is desired to develop a novel retractor in which the threshold value for tensile load on the seat belt webbing is adjustable. 
     SUMMARY OF THE INVENTION 
     To achieve the aforementioned object, the present invention provides a motorized seat belt retractor having a load limiter for controlling the tensile load on a webbing withdrawn in the event of emergency. The load limiter utilizes force generated by rotation of the motor shaft to control the tensile load. This approach to a load limiter is novel and may include a high-voltage motor having improved motor current rising characteristics during starting of the motor. 
     According to the structure of the present invention, the force generated by the rotation of the shaft of the motor can be utilized as the tensile load for the load limiter. This configuration allows for the easy adjustment of the threshold value of the tensile load on the webbing and permits the threshold value to be set over a wider range of possible values. 
     The load limiter of the motorized seat belt retractor includes a rotational resistance generating means which generates a rotational resistance force in response to the rotation of the shaft of the motor. According to this structure, during rotation of the shaft of the motor a rotational resistance force is generated due to the inertial moment of the shaft, this rotational resistance force can be positively utilized to control the tensile load on the webbing. 
     In the aforementioned motorized seat belt retractor which includes a load limiter for controlling the tensile load on a webbing withdrawn due to forward movement of an occupant in the event of emergency, the load limiter includes a rotational resistance generating means which provides for the generation of a rotational resistance force by the motor when the motor is short circuited. According to this structure, the rotational resistance force generated by the motor when short circuited can be positively utilized as the tensile load for the load limiter, thereby eliminating the necessity of a separate mechanical load limiter. Thus, the present invention provides for manufacturing motorized seat belt retractors of a smaller size and at a low cost. 
     According to the present invention, the rotational resistance generating means of the motorized seat belt retractor generates a rotational resistance force by switching the motor into either the short circuited state or the non-short circuited state according to predetermined sequence control. According to this structure, a desired tensile load on the webbing can be obtained by suitably setting the period in which the motor is in the short circuited state and the period in which the motor is in the non-short circuited state and sequentially switching the short circuited state and the non-short circuited state of the motor. 
     In the motorized seat belt retractor, the non-short circuited state can mean that the motor is energized. According to this structure, the tensile load on the webbing can be controlled by utilizing the rotational torque of the motor. The rotational resistance force of the motor in the non-short circuited state (i.e., energized state) is adjustable, thereby allowing a wider range for the setting value of the tensile load on the webbing. 
     In the motorized seat belt retractor, the non-short circuited state can mean that the motor is electrically connected via a resistor. According to this structure, the rotational resistance force of the motor in the non-short circuited state can be adjusted by changing the resistance value of the resistor, thereby allowing a wider range for the setting value of the tensile load on the webbing. 
     In the motorized seat belt retractor, the non-short circuited state can mean that the motor is electrically open. According to this structure, little rotational resistance force is generated by the motor in the non-short circuited state when the motor is electrically disconnected (open-circuit), thereby easily allowing a wider range for the setting value of the tensile load on the webbing at a low cost without any complex mechanism. 
     The load limiter of the motorized seat belt retractor includes a gear train which couples the shaft of the motor to a spool on which the webbing is wound. According to this structure, the tensile load on the webbing can be controlled not only by adjusting the rotational resistance force of the motor but also by changing the gear ratio of the gear train. 
     In the aforementioned rotational resistance generating means which generates rotational resistance force by the motor, the rotational resistance force further acts for preventing a rotational shaft of a motor from rotating in a direction of withdrawing the webbing. According to this structure, the rotating shaft of the motor is electrically locked, thus preventing rotation of the shaft at a suitable timing. 
     According to the present invention, described in general above, it is preferred that the rotational resistance force be generated at least by counter-electromotive force generated by the rotation of the rotational shaft of the motor when short circuited. According to this structure, the counter-electromotive force generated by the rotation of the rotational shaft of the motor when short circuited can be utilized as the locking mechanism, thereby eliminating specific control for driving the motor. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only, and are not restrictive of the invention as claimed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features, aspects and advantages of the present invention will become apparent from the following description, appended claims, and the accompanying exemplary embodiments shown in the drawings, which are briefly described below. 
     FIG. 1 is an exploded perspective view showing an embodiment of a retractor according to the present invention. 
     FIG. 2 is an explanatory view showing the mesh relation between gears of the retractor of the embodiment according to the present invention. 
     FIG. 3 a  is a view illustrating a state where the motor is rotated in the clockwise direction (CW direction) and 
     FIG. 3 b  is a view illustrating a state where the motor is rotated in the counterclockwise direction (CCW direction). 
     FIG. 4 is a graph schematically showing the relation between the rotational resistance force F [Nm] (Newton meter) of the short-circuited DC motor and time T [sec] (second) from a point where a vehicle collides with a wall ( 0  point in this figure) to a point where the vehicle completely crashes. Curves indicate cases which are different in the weight (Light, Middle, Heavy) of occupant in the vehicle, respectively. 
     FIG. 5 is a graph schematically showing the relation between the rotational resistance force F [Nm] (Newton meter) of the short-circuited DC motor and time T [sec] (second) from a point where a vehicle collides with a wall ( 0  point in this figure) to a point where the vehicle completely crashes. Curves indicate cases which are different in the collision speed (Low, Middle, High) of the vehicle, respectively. 
     FIG. 6 a  is a circuit diagram of the circuit including the retractor dc motor and a variable resistor. 
     FIG. 6 b  is a circuit diagram of the circuit including the retractor dc motor and a fuse. 
     FIG. 7 is a graph of the dc current/voltage applied to the retractor motor versus time. 
    
    
     DETAILED DESCRIPTION 
     Hereinafter, an embodiment of the present invention will now be described with reference to the drawings. It should be understood that the sizes, shapes, positional relation of respective components are schematically shown just for understanding the invention and that the numerical conditions stated in the following are just illustrative examples. 
     Hereinafter, an embodiment of a retractor according to the present invention will be described. FIG. 1 is an exploded perspective view showing the embodiment of the retractor according to the present invention. FIG. 2 is an explanatory view showing the mesh relation between gears of the retractor of this embodiment. It should be noted that the illustration of a pyrotechnic pretension mechanism is omitted in FIG.  1 . 
     The structure of the retractor of this embodiment will now be described with reference to FIGS. 1,  2 . The retractor  200  comprises the following components: a retainer  20 ; a DC motor  21  attached integrally to the retainer  20 ; a pinion  22  attached integrally to a motor shaft of the DC motor  21 ; a first gear  23  which is journalled by a projection formed on the retainer  20  and is in mesh or engaged with the pinion  22 . The first gear  23  is preferably an integral double gear comprising a large gear  23   a  and a small gear  23   b.  The pinion  22  is positioned to mesh with the large gear  23   a.    
     The retractor also includes a second gear  24  which is journalled by a projection formed on the retainer  20  and is in mesh or engaged with the first gear  23 . In particular, the retainer is engaged with the small gear  23   b.  The second gear  24  is preferably an integral double gear comprising a large gear  24   a  and a small gear  24   b.  The small gear  23   b  is in mesh or engaged with the large gear  24   a.    
     The retractor further includes a third gear  25  which is in mesh with the second gear  24 . In particular, the third gear is engaged with the small gear  24   b.  The third gear  25  is preferably an integral double gear comprising a large gear  25   a  and a small gear  25   b.  The small gear  24   b  is in mesh with the large gear  25   a.    
     The retractor also includes three planetary gears  26  which are in mesh with the third gear  25 . The planetary gears engage the small gear  25   b.  An internal gear  27  is also provided. The internal gear  27  has internal teeth  27   a  which engage with the three planetary gears  26 . The internal gear  27  includes external ratchet teeth  27   b  formed in the outer periphery of the internal gear  27 . 
     A pawl  30  is provided to engage with the external ratchet teeth  27   b,  and to thereby stop the rotation of the internal gear  27  in the clockwise direction. The pawl  30  is supported at a lever  31  comprising a spring at an end connected to the pawl  30 . The other end of the lever  31  includes a portion curled to form a ring member  32  that is formed in a curled portion of the other end of the lever  31 . The ring member  32  is wound on a projecting disk-like member  33 . The disk-like member  33  is integrally formed coaxially with the first gear  23 . A frictional piece  34  projects from the outer periphery of the disk-like member  33  and presses against the ring member  32  to apply friction. 
     The three planetary gears  26  are positioned on a carrier  35 . Three pins  36  are provided for rotatably supporting and securing the three planetary gears  26  to the carrier  35 . A speed-reduction plate  37  is interposed between the three pins  36  and the three planetary gears  26 . 
     A webbing W for restraining an occupant&#39;s body of which one end is fixed to a spool  38 . As shown in FIGS. 2 and 3 arrow A designates a direction of withdrawing the webbing W and arrow B designates a direction of retracting the webbing W. The spool  38  includes a tip portion  38   a  that passes through a rotational central hole of the carrier  35 . The tip portion  38   a  also passes through the rotational central hole of the third gear so as to be both slidable and rotatable relative to the third gear. On the other hand, the root of the tip portion  38   a  is fitted and fixed to the carrier  35 . 
     The retractor includes a cover  39  covering the entire of the force transfer mechanism or gear train. A plurality of screws  40  are provided for fixing the cover  39  to the retainer  20 . 
     A control circuit controls the connection of the DC motor  21  to be short-circuited or non-short-ciruited and also controls the rotation of the DC motor  21  in the clockwise (CW) direction or in the counterclockwise (CCW) direction. 
     As described herein, when the motor  21  is short circuited, no driving current is supplied to turn the motor shaft. In this condition, when the shaft of the motor attempts to rotate due to the rotational force transferred from the first gear and engaged pinion a counter electromotive force resists movement of the motor shaft. 
     As described herein, when the motor is non-short-circuited the motor may be located in an open-circuit or may be connected to a DC power source which supplies a driving current that generates a rotational force to drive the shaft in a chosen direction. 
     Hereinafter, description will now be made as regard to the operation of the retractor of the present invention with the aforementioned components. 
     FIGS.  3 (A) and  3 (B) illustrating the operation of this embodiment wherein FIG.  3 (A) is a view illustrating a state where the motor is rotated in the clockwise direction (CW direction) and FIG.  3 (B) is a view illustrating a state where the motor is rotated in the counterclockwise direction (CCW direction). 
     In the retractor  200 , as shown in FIG.  2  and FIG.  3 (B), the engaging pawl  30  is spaced apart from the external ratchet teeth  27   b  so that the internal gear  27  is not restricted in the normal state (i.e., not in an emergency such as emergency braking or a vehicle collision). In this normal state, because of the property of the planetary gear train, the rotational torque of the carrier  35  is not transmitted to the third gear. Therefore, the rotational torque of the spool  38  integrally fitted and fixed to the carrier  35  is not transmitted to the rotational shaft of the DC motor  21 , which is indirectly engaged with the third gear. 
     In the event of emergency, such as emergency braking and a vehicle collision, a pretensioning mechanism (for winding up the webbing W to increase the tension prior to the pyrotechnic pretension mechanism) is actuated according to output signals from an ABS (anti-skid or brake) mechanism (not shown) and/or a collision predictive device (not shown) in order to rotate the rotational shaft of the DC motor  21  in the CW direction as shown by the arrow in FIG.  3 (A). Then, the rotational torque of the pinion  22  in the clockwise direction is transmitted to the first gear  23  as a rotational torque in the counterclockwise direction (indicated by arrow). As a result, the pawl  30  engages with one of the external ratchet teeth  27   b  of the internal gear  27  to stop the rotation of the internal gear  27  in the clockwise direction (indicated by arrow). Therefore, the rotational torque of the third gear  25  can be transmitted to the carrier  35 , which is integrally fitted and fixed to the spool  38 . 
     In the case of the emergency condition, the rotational torque of the first gear  23  is transmitted to the second gear  24  as rotational torque in the clockwise direction (indicated by arrow). In addition, the torque is further transmitted to the third gear  25  as rotational torque in the counterclockwise direction (indicated by arrow). Due to the rotation of the third gear  25  in the counterclockwise direction, the small gear  25   b  of the third gear  25  is rotated in the counterclockwise direction so as to apply rotational torque in the clockwise direction (indicated by arrow) to the three planetary gears  26 . The three planetary gears  26  rotate in the counterclockwise direction (indicated by arrow) like planets around the small gear  25   b  and, during this rotation, engage with the internal teeth of the internal gear  27 . The internal gear  27  is stopped from rotating by the pawl  30 . Therefore, the carrier  35  rotates to journal the three planetary gears  26  in the counterclockwise direction (indicated by arrow). Because the spool  38  is fitted and fixed to the carrier  35 , which is rotating in the counterclockwise direction, the spool also rotates in the counter clockwise direction to wind up the webbing W (in the direction of arrow B). 
     Thereby, as described above, the rotational torque generated by the shaft of the DC motor  21  rotating in the clockwise direction is transmitted to the spool  38  as rotational torque for winding up the webbing W. 
     As impact is exerted on a vehicle body due to a vehicle collision, impact detecting signals are outputted from an acceleration sensor (not shown) and/or a crush sensor (not shown) whereby a pyrotechnic pretension mechanism (not shown) is actuated to retract the webbing W into the retractor  200 , as described above, thereby ensuring initial restraint of the occupant. 
     After the collision or impact occurs, the webbing W will be withdrawn (in a direction of arrow A in FIG.  3 (A)) by the inertial force of the occupant moving forwardly due to the collision. During this movement of the webbing W and the spool  38 , as shown in FIG.  3 (A), the torque applied to the spool  38  by withdrawing of the webbing W is transmitted to the DC motor  21  as rotational torque in the counterclockwise direction (in a direction opposite to the direction of arrow) because the engaging pawl  30  is engaged with the external ratchet teeth  27   b.  When the DC motor is short-circuited (i.e., the terminals are connected, but no external voltage is applied), the movement of the DC motor shaft created by the occupant&#39;s motion is opposed by a counter electromotive force (“counter emf”). As a result, the motor shaft provides a rotational resistance force that acts to prevent the rotation of the spool  38  and withdrawal of the webbing W. 
     The present invention provides for using the rotational resistance force created by the rotating DC motor shaft to provide a locking mechanism and/or the EA mechanism. It should be noted that the term “EA” is an abbreviation of “energy absorbing” meaning that impact (load) acting on an occupant&#39;s body is absorbed by a seat belt, and this term will be used generally hereinafter. 
     The characteristics of the rotational resistance force will now be described with reference to the drawings. FIG. 4 is a graph schematically showing the relation between the rotational resistance force F [Nm] (Newton meter) provided by the short-circuited DC motor and time T [sec] (second) from a point where a vehicle collides with a wall (0 point of this graph) to a point where the vehicle completely crashes. The three curves shown in FIG. 4, indicate situations where different weight occupants were located in the vehicle (i.e., Light, Middle and Heavy occupants). 
     FIG. 5 is a graph schematically showing the relation between the rotational resistance force F [Nm] (Newton meter) of the short-circuited DC motor and time T [sec] (second) from a point where a vehicle collides with a wall ( 0  point of this graph) to a point where the vehicle completely crashes or comes to rest. Curves indicate cases which are different in the collision speed (Low, Middle, High) of the vehicle, respectively. 
     As shown in FIG. 4, in the case of a light-weight occupant, the rising slope or inclination of the curve is relatively gentle (the solid line in the graph of FIG.  4 ). In the case of a heavy-weight occupant, the rising inclination of the curve is steep (the two-dot chain line shown in FIG.  4 ). In the case of a medium-weight occupant, the rising inclination of the curve is middle between the case of the light-weight occupant and the case of the heavy-weight occupant. Regardless of the occupant&#39;s weight, the descending slope or inclination of all of the cases are gentle. 
     Accordingly, by using the rotational resistance force as the EA mechanism, EA load is relatively gently increased against the light-weight occupant so that the total load on the light-weight occupant is relatively small. On the other hand, EA load is relatively steeply increased against the heavy-weight occupant so that the total load on the heavy-weight occupant is relatively large. The decrease in EA load is gentle regardless of the occupant&#39;s weight, achieving “soft landing” (it means “decrease in belt tension acting on the occupant is gentle with time elapsing”). 
     As shown in FIG. 5, the higher the speed of the vehicle when colliding with a wall, the higher the load limit of the rotational resistance force F (EA load limit) (the two-dot chain line in the graph of FIG.  5 ). The lower the speed of the vehicle when colliding with a wall, the lower the load limit of the rotational resistance force F (the solid line in the graph of FIG.  5 ). That is, the load limit is increased or decreased depending on the collision speed, exhibiting the ideal occupant restraint performance. 
     In case of conventional mechanical EA mechanism (e.g. a torsion bar) the rising inclination of EA load is constant so that the load limit is also constant regardless of the occupant&#39;s weight and the collision speed. The present invention improves on conventional methods and devices. 
     The load limit can be freely set in various manners as follows. For example, The gear ratio of the gears located between the shaft of the DC motor  21  and the web spool  38  may be changed. A change in gear ratio changes the load limit of the rotational resistance force transmitted from the motor  21  to the spool  38 . Also, a change in gear ratio changes the rising and descending slope of the force over time shown in FIG.  4 . 
     Further by way of example, the DC motor  21  may be attached to a circuit that includes a variable resistor  40 , as shown in FIG. 6 a.  The value of the resistor  40  may be changed in order to change the load limit of the force being transferred from the motor to the web spool  38 . Similarly, the value of resistance may be changed to adjust the rising inclination and the descending inclination of the curves shown in FIG.  4 . As the value of resistance is increased, the amount of force transferred from the motor  21  to the web spool  38  decreases. As a result, the load limit decreases, the rising inclination becomes gentler, and the descending inclination becomes steeper. In this case, a plurality of resistors having different values of resistance may be positioned in parallel and selectably connected to the circuit in such a manner as to automatically connect to a resistor having a value best suited to achieve ideal restraint performance. 
     Still further by way of example, a fuse  42  may be connected to the power supply for the motor  21 , as shown in FIG. 6 b.  The EA mechanism provided by the motor  21  can be released by opening the fuse and open-circuiting the motor to lower the EA load when current exceeds a predetermined value. 
     As described above, the DC motor  21  may be energized by a driving current to rotate in a direction for retracting the webbing W (the direction of arrow in FIG.  3 (A)). Rotation in this direction provides a rotational resistance force opposite to the force provided by the occupant. On the contrary, the rotational shaft of the DC motor  21  may be rotated in the direction of withdrawing the webbing W (the direction opposite to the direction of arrow in FIG.  3 (A)), to provide a force that subtracts from the conventional rotational resistance force. 
     The load limiter can also function as a locking mechanism, by providing sufficient rotational resistance force to cancel the rotational torque of the spool  38  acting in a direction of withdrawing the seat belt. 
     Alternatively, the motor  21  may be replaced with another one having different output. Thus, the load limit of the rotational resistance force F, the rising inclination, and the descending inclination can be adjusted by changing the motor rating. 
     As shown in FIG. 7, the time period t 1  of short circuit of the DC motor  21  and the time period t 2  of non-short circuit of the DC motor  21  may be freely changed to make a pulse-like rectangular wave in order to adjust the load limit of the rotational resistance force F, the rising inclination, and the descending inclination of the resistance force. For instance, as the time period t 1  is set longer than the time period t 2 , the load limit becomes higher, the rising inclination becomes steeper, and the descending inclination becomes gentler. On the contrary, as the time period t 2  is set longer than the time period t 1 , the load limit becomes lower, the rising inclination becomes gentler, and the descending inclination becomes steeper. 
     The timing for starting the EA mechanism can be controlled by an ECU (“Electronic Control Unit”) for commanding the ignition timing of an air-bag device or an ECU for a pretension mechanism. 
     It is preferable that the load limit of the rotational resistance force F, the rising inclination, and the descending inclination are suitably set according to the withdrawal characteristic of webbing W which is obtained from experiments using real cars with dummies. 
     A rotational shaft with a magnet in a copper tube may be used instead of the DC motor  21 , thereby removing the requirement to energize the motor and, thus, making EA mechanism at a low cost and with a simple structure. 
     Combinations of the EA mechanism and various pretension mechanisms such as a back pretensioner may provide more advantages. Further, a vehicle sensor may be incorporated in the retractor as an EA switch. 
     The method of using the rotational resistance force of the short-circuited motor as EA mechanism according to the present invention can be applied to a retractor of another type just like the aforementioned embodiment shown in FIG.  1 . 
     As discussed above, the present invention achieves suitable timing of locking. 
     Given the disclosure of the present invention, one versed in the art would appreciate that there may be other embodiments and modifications within the scope and spirit of the invention. Accordingly, all modifications attainable by one versed in the art from the present disclosure within the scope and spirit of the present invention are to be included as further embodiments of the present invention. The scope of the present invention is to be defined as set forth in the following claims.