Abstract:
A belt pretensioner and force limiter in a vehicle having a piston-cylinder device with a cylinder chamber containing an explosive charge in which the piston has a piston rod that protrudes from the cylinder and is joined to a cable. The cable, via a drum and a planetary gear set, drives a reel mechanism for the belt in a winding-up direction when the explosive charge is detonated. The planetary gear set is engaged so that the gear ratio is about 1:1 between the drum and the reel mechanism when the drum rotates in the winding-up direction. The planetary gear set is engaged in the opposite direction so that the return movement of the piston a certain distance in the cylinder corresponds to a several times longer feed-out of the web from the reel mechanism.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority to Swedish Application No. 0003682-2, filed Oct. 12, 2000. 
     BACKGROUND OF INVENTION 
     1. Technical Field 
     The present invention relates to safety belt devices. More specifically, the invention relates to a device for pretensioning and force-limiting a safety belt web joined to a reel mechanism in a vehicle. 
     2. Background Information 
     Belt pretensioners with force limiters are presently used for safety belts in motor vehicles in order to take up slack between the belt windings on the spool of the reel mechanism in the event of a collision. At the same time, the pretensioners tighten up the belt against the body of the occupant so as to avoid as much as possible sliding under the belt or jerking, along with a possible accompanying whiplash. This force-limiting function is achieved by controlling the feeding-out of a predetermined length of the belt after pretensioning. This reduces the force between the occupant and the belt web up to the point when the occupant must be caught completely by the belt to prevent his head from hitting interior components such as the steering wheel of the vehicle. 
     As indicated above, force-limiting after pretensioning occurs when the belt is fed out due to occupant load on the belt caused by vehicle retardation. In a previously known design, force-limiting after pretensioning is achieved by plastic deformation of a torsion element in the reel mechanism. In practical embodiments at desired force levels, the maximum possible feed-out length of the belt is limited by design to approximately 300 mm. However, MADYMO simulations with 95 percentile dummy and 35 miles per hour crash velocity have demonstrated that belt feed-out length should be on the order of approximately 450 mm in order to achieve optimum effect. Accordingly, there is a need for a pretensioner that enables a greater belt feed-out length. 
     SUMMARY OF INVENTION 
     The present invention provides a device for pretensioning and force-limiting a safety belt web joined to a reel mechanism in a vehicle. The device includes a cylinder and a piston which is displaceable in the cylinder and has 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 device further includes a motion-transmitting element acting between the piston rod and the reel mechanism. The element is able to achieve rotation of the reel mechanism in the winding-up direction of the belt web upon displacement of the piston in one direction. The device also includes a pyrotechnic charge that, when detonated, causes a pressure increase in a cylinder chamber between the piston and a cylinder end wall, thereby displacing the piston in one direction. 
     A purpose of the present invention is to achieve a device of the type described above that makes possible controlled feed-out of the desired belt length of approximately 450 mm during the force-limiting phase. This is achieved according to the invention by joining the motion-transmitting element to a ring gear of a planetary gear set so that displacement of the piston in the cylinder results in rotation of the ring gear. The planetary gear set is so disposed and coupled to the reel mechanism that the gear ratio between the ring gear and the reel mechanism is about 1:1 upon rotation of the ring gear in the winding-up direction of the web. The gear set is further disposed and geared up upon rotation in the opposite direction so that movement of the piston over a certain distance in the cylinder corresponds to a several times longer feed-out of the web from the reel mechanism. By utilizing a planetary gear set, the desired belt feed-out length can be simply achieved by adapting the gear up of the planetary gear set to the stroke length of the piston and the desired belt feed-out length. 
     In a further embodiment of the device according to the invention, the cylinder chamber has an outlet that communicates with a spill valve. The spill valve can be set between various degrees of opening and is controlled by a control unit in response to signals from a sensor for sensing the weight of the occupant. The degree of valve opening determines the resistance against the return stroke of the piston during the force-limiting phase. This implies that the larger the valve opening is, the less will be the resisting force of the belt against the occupant. By regulating the valve opening in relation to the weight of the occupant, the force limitation is regulated relative to the weight of the occupant. 
     In yet a further embodiment of the device according to the invention, the spill valve can be set to a closed position. The control unit is in this case also coordinated with a positional, so-called roll-over sensor, which senses the position of the vehicle. If the sensor indicates that the vehicle is about to end up upside-down after the pretensioning phase, the control unit will close the valve, which means that the occupant will be held securely against the seat and no belt feed-out can occur. To assure that the piston remains in its position when the valve has been closed, in a preferred embodiment said cylinder chamber is delimited between the side of the piston opposite to the piston rod and the second end wall of the cylinder, the motion-transmitting element being arranged to rotate the reel mechanism in the winding-up direction of the belt when the piston is displaced in the protrusion direction of the piston rod. This provides an expansion space without a through-hole for the piston rod, which would otherwise be a potential path for leakage. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     The invention will now be described in more detail below with reference to examples shown in the accompanying drawings, where 
     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, 
     FIG. 1 b  shows an enlargement of the valve in FIG. 1 a  with associated means for controlling the valve, 
     FIG. 2 shows a schematic longitudinal section through a reel mechanism with a first embodiment of a planetary gear set, 
     FIG. 3 a  shows a partial enlargement of the planetary gear set in FIG. 2 with associated blocking means in the pretensioning position, 
     FIG. 3 b  shows a schematic side view of the gear set in FIG. 3 a,    
     FIG. 4 a  shows a partial enlargement of the planetary gear set in FIG. 2 with associated blocking means in the force-limiting position, 
     FIG. 4 b  shows a schematic side view of the gear set in FIG. 4 a,    
     FIG. 5 shows a schematic longitudinal section through a second embodiment of a planetary gear set, 
     FIG. 6 shows a diagram of the web displacement as a function of time during the pretensioning and force-limiting stages. 
    
    
     DETAILED DESCRIPTION 
     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. 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. 
     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 into 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. 
     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 ,  23  over which runs a closed cable loop  24 . Two portions or parts  24   a,    24   b  of the loop converge in a single portion or cable  25 , which is joined to a drum  26  (see, FIGS.  2 - 5 ). 
     The drum  26 , via a planetary gear set  27 , is arranged to drive a reel mechanism, which is known per se and is 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 . 
     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  18 , or can be calculated based upon the sensed weights with the opening  11  adjusted based upon the calculated value. Generally, the higher the weight, the less 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  assures that gas cannot leak out and change the position of the piston. 
     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  that, 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 either be solidly joined or made in one piece with the housing  35  in which the belt cylinder shaft  32  is mounted. In this manner, the housing  35  is solidly mounted in the vehicle. 
     Referring to FIGS. 3 a,    3   b,    4   a  and  4   b,  the planetary gear set shaft  34  carries a sun gear  36 . The sun gear  36  is able to engage with one or more first planet gears  38  carried by a planet carrier  37 . The planet gears  38  are able to engage with one or more second planet gears  39  carried by a planet carrier. The second planet gears  39  are able to engage 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 , enabling the ring gear  40  and also the drum to be locked to the sun gear  36 . In another embodiment, a further blocker or 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  35 . 
     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 . 
     In a collision, the retractor mechanism  33  locks together the shaft  32  of the bobbin and the planetary gear set  27  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 . 
     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 the 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 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 , will now be 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. 
     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. 
     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 ,  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). 
     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 , thereby establishing 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 . 
     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 arrangement makes possible a roll-over function by virtue of the fact that the valve can be closed after the pretensioning phase so that the occupant is held securely in his seat. 
     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.