Abstract:
A belt pretensioner in a vehicle, comprising a piston-cylinder device with a cylinder chamber containing an explosive charge. The piston has a piston rod, which projects out of the cylinder on the side of the piston opposite to the cylinder chamber with the explosive charge, and is joined to a cable, which drives, via a drum and a planetary gear set, a belt reel mechanism in the winding-up direction when the explosive charge is detonated. The cylinder chamber has an evacuation opening, which communicates with a spill valve, which is controlled by an electronic control unit in response to a sensor, which senses the weight of the occupant.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
         [0001]    This application claims priority to Swedish Application No. 0003681-4, filed Oct. 12, 2000.  
         BACKGROUND OF INVENTION  
         [0002]    Technical Field. The present invention relates to safety devices. More specifically, the invention relates to a device for pretensioning a safety belt web joined to a reel mechanism in a vehicle.  
           [0003]    Background Information. Belt pretensioners are typically used to take up slack in motor vehicle safety belts, e.g., in a collision, between the web windings on the spool of the reel mechanism. At the same time, pretensioners tension up the web against the passenger&#39;s body in order to prevent, as much as possible, sliding under the belt or jerking back and forth, resulting in whiplash. It is known that the heavier the person, the higher the pretensioning force required for eliminating slack in the belt. Further, pretensioning which is too forceful can injure a small, light person.  
           [0004]    Various devices and methods for adapting the pretensioning force to the weight of the passenger are known. For example, DE-OS 38 17 942 teaches two different solutions, both based on controlling pressure in a cylinder chamber at expansion by the drive gas formed at detonation, whereby the pressure for a lighter person is lower than that for a heavier person. In both solutions, a mechanical device found in the seat is utilized. This device reacts to pressing down the seat cushion. In one case, the volume of the cylinder chamber is controlled by the mechanical device displacing a movable wall in the chamber. If the person is heavy, the wall is set so that the volume will be small. If the person is light, the volume is increased so that the force on the person will be less. In the latter case, a valve slide is controlled relative to a number of evacuation openings from the cylinder chamber so that gas in the chamber is evacuated earlier the lighter the person is. In the first case, the weight adaptation of the pretensioning is infinitely variable, while in the second case it is effected in six steps.  
           [0005]    DE 296 12 781 U1 teaches a device for weight adaptation of the belt pretensioning in only two steps. Here, a piston-cylinder device is used with two propulsion charges, both of which are set off if the person on the seat is heavy. If the person is light, only one of the propulsion charges is set off.  
           [0006]    In the above known solutions, the piston-cylinder devices are pulling, i.e., the starting position is with the piston rod out of the cylinder. Consequently, the expansion volume for the propulsion charges will be situated on the piston rod side of the piston. Therefore, during expansion the piston rod is drawn into the cylinder.  
           [0007]    With the above solutions, complete sealing between the piston rod and the opening in the cylinder wall is difficult to achieve. Therefore, leakage of propellant gas is practically unavoidable. Accordingly, there is a need for a pretensioner that avoids the problem of propellant gas leakage.  
         SUMMARY OF INVENTION  
         [0008]    The pretensioner of the present invention includes a cylinder and a piston displaceable in the cylinder and having a piston rod. The piston rod is joined to one side of the piston and extends through an opening in one end wall of the cylinder. The pretensioner further includes a motion-transmitting element acting between the piston rod and a reel mechanism. Upon displacement of the piston in one direction, the element achieves rotation of the reel mechanism in the winding-up direction of the belt web. The pretensioner further includes both a pyrotechnic charge that, when detonated, causes a pressure increase in a cylinder chamber between the piston and a cylinder end wall for displacement of the piston in one direction, and a controller for controlling the pressure in the cylinder chamber depending on the weight of the person held in by the belt.  
           [0009]    One purpose of the present invention is to achieve a device of the type described above that makes possible secure precision adaptation of the pretensioning to the weight of the passenger. An additional purpose of the invention is to achieve a device which eliminates the risk of propellant gas leaking out between the piston rod and the cylinder opening through which the piston rod extends.  
           [0010]    This is achieved according to the invention by virtue of the fact that the cylinder chamber is delimited between the side of the piston opposite to the piston rod and the second end wall of the cylinder. Further, the motion-transmitting element is arranged to rotate the reel mechanism in the winding-up direction of the belt web when the piston is displaced in the projection direction of the piston rod. The cylinder chamber has an outlet that communicates with a spill valve, which can be set between various degrees of opening and a completely closed position.  
           [0011]    By rotating the reel mechanism in the winding-up direction of the belt, thereby using a pushing instead of a pulling movement of the piston rod, the expansion chamber is placed on that side of the piston which does not have any through-hole for the piston rod. This eliminates one source of leakage for the propellant gas. By combining this arrangement with a spill valve that can be set at a completely closed position, the piston-cylinder device can be blocked in the pretensioned position.  
           [0012]    In a preferred further development of the device according to the invention, this possibility is utilized in combination with a positional sensor coupled to the control unit, or “roll-over sensor”. This sensor senses if the vehicle is upside-down, thereby providing a signal to the control unit to close the spill valve, thus blocking the device in the pretensioned position so that the passenger is held securely against the seat and will not risk hitting his head against the vehicle ceiling.  
           [0013]    In a further development of the device according to the invention, the motion-transmitting element is joined to and partially wound up on a drum. This drum is solidly joined to a ring gear of a planetary gear set, so disposed and coupled to the reel mechanism that the gear ratio between the ring gear and the reel mechanism is 1:1 when the ring gear rotates in the winding-up direction of the belt web, and is geared up upon rotation in the opposite direction so that winding up of a certain length of the motion-transmitting element on the drum of the ring gear corresponds to a several times longer feeding-out of the web from the reel mechanism.  
           [0014]    In previously known designs, the force-limiting function is achieved by plastic deformation of a torsion element in the reel mechanism. This function occurs subsequent to pretensioning upon reeling out of the belt due to the load caused by vehicle retardation from the passenger&#39;s body against the belt. In practical embodiments, the feed-out length of the belt is, for design reasons, limited to approximately 300 mm. By utilizing a planetary gear set between the motion-transmitting element and the reel mechanism, the feed-out length can be increased by at least fifty percent without any problem. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0015]    The invention will now be described in more detail with reference to examples shown in the accompanying drawings, where  
         [0016]    [0016]FIG. 1 a  shows a schematic longitudinal section through one embodiment of a piston-cylinder device with a valve and a motion-transmitting element according to the invention, and  
         [0017]    [0017]FIG. 1 b  shows an enlargement of the valve in FIG. 1 a  with associated means for controlling the valve,  
         [0018]    [0018]FIG. 2 shows a schematic longitudinal section through a reel mechanism with a first embodiment of a planetary gear set,  
         [0019]    [0019]FIG. 3 a  shows a partial enlargement of the planetary gear set in FIG. 2 with associated blocking means in the pretensioning position,  
         [0020]    [0020]FIG. 3 b  shows a schematic side view of the gear set in FIG. 3 a,    
         [0021]    [0021]FIG. 4 a  shows a partial enlargement of the planetary gear set in FIG. 2 with associated blocking means in the force-limiting position,  
         [0022]    [0022]FIG. 4 b  shows a schematic side view of the gear set in FIG. 4 a,    
         [0023]    [0023]FIG. 5 shows a schematic longitudinal section through a second embodiment of a planetary gear set, and  
         [0024]    [0024]FIG. 6 shows a diagram of the belt displacement as a function of time during the pretensioning and force-limiting stages. 
     
    
     DETAILED DESCRIPTION  
       [0025]    In FIG. 1 a , reference  1  designates a piston-cylinder device with a cylinder  2  and a piston  3  displaceable in the cylinder. The piston  3  is joined to a piston rod  4 , which extends through an opening  5  in one end wall  6  of the cylinder  2 . The end wall also has a vent or ventilation opening  7 . The opposite end wall  8  of the cylinder  2  and the piston  3  together define an expansion chamber  9  in which a pyrotechnic charge  10  with a detonator (not shown) is placed. The charge  10  is preferably exploded by retardation-sensitive means, which are known per se and therefore not shown in more detail here.  
         [0026]    The cylinder wall in the area of the expansion chamber  9  is made with an opening  11  leading to a valve device  12 . Through this opening  11 , propellant gas in the expansion chamber can escape to the environment.  
         [0027]    The valve device  12  (illustrated more fully in FIG. 1 b ) has a valve slide  14  displaceable in a housing  13 , joined to a rotatable set screw  15  in a threaded bore  16  in the housing. The set screw  15  is rotatable with the aid of a servomotor  17 . The servomotor  17  is controlled by an electronic control unit  18  based upon one or more signals. These signals may include, for example, a signal from a weight-sensitive sensor  19 , which in one embodiment can be built in to the weight-bearing portion of the vehicle seat (not shown) for registering the weight of the person sitting in the seat. The signals may further include a signal from a roll-over sensor  20  that senses if the vehicle is about to end up upside-down.  
         [0028]    At the end of the piston rod  4  there is a pulley or roller  21 . On either side of the cylinder  2  there are at least two additional rollers  22  and  23  over which run a closed cable loop  24 . Two portions or parts  24   a  and  24   b  of the loop converge into a single portion or cable  25 , which is joined to a drum  26  ( see, FIGS.  2 - 5 ).  
         [0029]    The drum  26 , via a planetary gear set  27 , is arranged to drive a reel mechanism, which is known per se and generally designated  28 , as will be described below. By this arrangement of a pushing piston rod instead of a pulling piston rod, one opening in the end wall  8  on the expansion chamber side of the piston  3  is eliminated. Arranging the cable loop  24  around the piston  3  assures that there will be a symmetrical load on the piston rod  4 .  
         [0030]    Once all passengers are seated and the driver turns the ignition key, the sensors  19  and  20  cooperating with the control unit  18  are activated. The control unit  18 , via the servomotor  17 , sets the valve slide  14  to a position that is calculated to provide the optimal catch sequence for the occupant. The relationship between the occupant&#39;s weight and the size of the valve opening  11  can be either pre-determined by tests, and the values stored in the control unit, or can be calculated based upon the sensed weight with the opening  11  adjusted based upon the calculated value. Generally, the higher the weight, the smaller the valve opening. If the roll-over sensor indicates that the vehicle is about to end up upside-down, the control unit  18  closes the valve  12 , fixing the reel mechanism  28  in its pretensioning position by blocking the piston  3 . The lack of a piston rod opening in the expansion chamber  9  further assures that gas cannot leak out and change the position of the piston.  
         [0031]    [0031]FIG. 2 illustrates the planetary gear set  27  and the reel mechanism  28 . The latter is of a type known per se and has a belt cylinder  30  with a belt web  31  wound up on the cylinder  30 . The cylinder  30  has a shaft  32  which, via a web jerk and compartment retardation sensitive retractor mechanism  33  (indicated only schematically here), is joined joined to a shaft  34  of the planetary gear set  27 , shown in greater detail in FIGS. 3 a ,  3   b ,  4   a  and  4   b.  The shaft  34  is rotatably mounted in a housing  35 , which can be either solidly joined or made in one piece with the housing in which the belt cylinder shaft  32  is mounted. In this manner, the housing  35  is solidly mounted in the vehicle.  
         [0032]    Referring to FIGS. 3 a ,  3   b ,  4   a  and  4   b,  the planetary gear set shaft  34  carrying a sun gear  36  is illustrated. The sun gear  36  is able to interact with one or more first planet gears  38  carried by a planet carrier  27 . The planet gears  38  are able to interact with one or more second planet gears  39  carried by a second planet carrier. The second planet gears  39  are able to interact with a toothed rim  40  on the interior of the drum  26 , forming the ring gear of the planetary gear set  27 . A first blocker or blocking means in the form of a spring  41  loaded pin  42  in the end wall of the drum is provided. The pin  42  extends into a cavity  43  in the sun gear  36 , and thus the ring gear  40  and consequently also the drum can be locked to the sun gear  36 . In another embodiment, a further blocking means is provided in the form of a second pin  45  loaded by a snap-spring  44  and disposed in a bore in the planet gear carrier  37 . By projecting the pin  45  into a cavity  46  in the housing  35 , the planet gear carrier  37  can be locked to the housing.  
         [0033]    The starting position is illustrated in FIGS. 3 a  and  3   b.  In this position the pin  42  locks the sun gear  36  to the ring gear  40 , providing a gear ratio of about 1:1 between the shaft  34  and the drum  26 . The pin  42  is kept in position by a cylindrical body  47 , which is held in an arcuate groove  48  in the end wall of the drum  26 .  
         [0034]    In a collision, the retractor mechanism  33  locks together the shaft  32  of the cylinder  30  and the planetary gear set  34  when the charge  10  in the cylinder expansion chamber  9  is detonated. The cable  25 , which is joined to and is somewhat wound up on the drum, then turns the drum an angle dependent on the length of stroke of the piston  3  and the diameter of the drum  26 . In this manner, the belt web  31  is pretensioned so that the slack of the web windings on the cylinder  30  is taken up and the belt tightened against the occupant, with the tightening sequence determined by the setting of the valve  12 .  
         [0035]    When the piston  3  has reached its end position in the cylinder  2 , the pretensioning is completed. The forces on the belt web then change directions as the occupant loads the belt during the retardation. This also reverses the torque direction in the planetary gear set, thereby initiating the force-limiting stage illustrated in FIGS. 4 a  and  4   b.  The shaft  34  with the sun gear  36  begins to rotate in a direction opposite to the original pretensioning direction. The cylindrical body  47 , held by friction against the housing  35  under the influence of the spring  41 , is displaced in the groove  48  to the position shown in FIG. 4 b.  The spring  41  can now move the pin  42  out of the cavity  43 , as is shown in FIG. 4 a.  The sun gear  36  can now rotate relative to the drum  26 . Torque is transmitted from the sun gear  36  to the planet gears  38 ,  39  and planet gear carrier  37 , prompting an initial relative movement between the first planet gear  38  and the planet gear carrier  37 . The pin  45 , which during the pretensioning stage was kept with its end inserted into a flanked depression  49  in the first planet gear  38  under the force of the snap-spring  44 , is now pressed towards the planet gear carrier  37 . The carrier  37  is provided along a circle directly opposite the pin with a plurality of uniformly spaced depressions  46 . The snap-spring  44  will snap the opposite end of the pin  45  into one of the depressions  46 , locking the planet gear carrier  37  to the housing  35 . Thereby, there will be a gear ratio between the shaft  34  of the planetary gear set and its ring gear  40  corresponding to the relationship between the number of teeth on the sun gear  36  and the ring gear  40 . A similar ratio is obtained between the return stroke of the piston and the feed-out length of the belt. The force by which this process occurs is determined by the setting of the valve  12  and, thus, adapted to the occupant&#39;s weight.  
         [0036]    According to one embodiment of the invention, the belt displacement as a function of time is illustrated in FIG. 6. As can be seen from the diagram, during the pretensioning phase the belt is pulled in up to about 5 cm in approximately 15 ms. Thereafter, the belt is fed out during the force-limiting phase about 45 cm in approximately 75 ms. This implies that the entire belt displacement process takes approximately 90 ms, and that the gear ratio in the planetary gear set is approximately 9:1, if the diameter of the drum and the medium diameter of the fed out web windings from the cylinder  30  are approximately equal.  
         [0037]    In order to obtain a large gear ratio in the planetary gear set without incurring a drum diameter so large that space problems are created, it is preferable to arrange the gearing in two steps. One example of such a planetary gear set is shown in FIG. 5. The gear set has a housing  50  in which a first shaft  51  is rotatably mounted. The shaft  51  carries a planet gear carrier  52  on which first and second planet gears  53  and  54 , solidly joined to each other, are mounted. A first ring gear  55 , disposed on the inside of a drum  56  joined to the cable  25 , is joined to a second shaft  57  mounted in the housing. The first ring gear  55  is able to engage the first planet gears  53 . A second ring gear  58  is able to engage the second planet gears  54  and is joined to a sleeve  59  mounted concentrically in the housing with the first shaft  51 . The shaft  51  is connectable to the belt cylinder shaft  32  with the aid of a belt jerk and passenger compartment retardation-sensitive retractor mechanism (not shown in more detail here).  
         [0038]    With the aid of a blocker or blocking means (not shown in more detail here), for example, one-way clutches, saw tooth blocking mechanisms or snap-locks of a type known per se, the first shaft  51  can be locked during the pretensioning phase relative to the second shaft  57  to establish a gear ratio of about 1:1. During the force-limiting phase, the second ring gear  58  can be locked to the housing  50  while the shafts  51  and  57  are released relative to each other, thus providing a high gear ratio between the shafts  51  and  57 .  
         [0039]    The present invention makes possible a substantially accurate adaptation of the catching phase to the weight of the occupant, both during the belt pretensioning phase and during the force-limiting phase during the subsequent belt feed-out. By having a long belt feed-out, it is possible to optimize the force-limiting phase. Finally, the be closed after the pretensioning phase so that the occupant is held securely in his seat.  
         [0040]    While there has been disclosed effective and efficient embodiments of the invention using specific terms, it should be well understood that the invention is not limited to such embodiments as there might be changes made in the arrangement, disposition, and form of the parts without departing from the principle of the present invention as comprehended within the scope of the accompanying claims.