Abstract:
Provided is a vehicle occupant restraint system which can reduce the load on the chest and head of the vehicle occupant in an impact situation while reliably preventing the submarine phenomenon. By restraining the pelvis and the adjacent parts of the vehicle occupant before the load is applied to the chest and head, the load is more favorably distributed, and the critical parts are favorably protected. This can be accomplished by activating a restraining member for raising an appropriate part of the vehicle seat immediate upon detecting an impact.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     Copending U.S. patent applications Ser. No. 09/334,116 filed Jun. 15, 1999, and Ser. Nos. 09/525,994, 09/526,405 and 09/526,650 all filed Mar. 15, 2000 are directed to similar subject matters, and the contents of these applications are incorporated herein by reference. This application is a continuation-in-part application of Ser. No. 09/525,980 filed Mar. 15, 2000 which is now abandoned. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a vehicle occupant restraint system which is adapted to prevent a vehicle occupant from slipping forward under the seat belt in case of an impact situation such as a vehicle crash. 
     BACKGROUND OF THE INVENTION 
     A seat belt is widely used for restraining a vehicle occupant to prevent the vehicle occupant from being thrown forward in case of an impact situation such as a vehicle crash, and hitting the instrumental panel or the steering wheel. An inflatable air bag is being used in an increasing number of vehicles to even further enhance the effectiveness of the seat belt. However, depending on the way the vehicle occupant is seated, the vehicle occupant could slip forward under the seat belt if the seat belt fails to restrain the pelvis of the vehicle occupant in an effective way. This is called “submarining”, and substantially impairs the effectiveness of the seat belt. 
     It was therefore proposed to provide a moveable member inside the seat bottom so as to be deployed and restrain the thighs of the vehicle occupant in case of a vehicle crash. Such a thigh restraining member may be actuated typically by using a pyrotechnic actuator (see Japanese patent laid open publication No. 01-122504). 
     In such a vehicle occupant restraining device, typically, the front end of the seat bottom is raised so as to raise the knees of the vehicle occupant. This prevents the knees of the vehicle occupant from hitting the dashboard or other members opposing the vehicle occupant at the time of a vehicle crash, but the hip of the vehicle occupant inevitably slips forward to a certain extent and the reduction in the effectiveness of the vehicle seat belt due to the submarine phenomenon was not entirely avoided. 
     In view of minimizing the maximum value of the deceleration acting on the vehicle occupant at the time of a vehicle crash, it is desirable to transmit the deceleration to the vehicle occupant as soon as possible immediately following the occurrence of a vehicle crash. Because the integral value of the deceleration of the vehicle occupant from the occurrence of a vehicle crash to a complete stop is equal to the speed of the vehicle immediately preceding the vehicle crash, and is therefore fixed, any delay in the transmission of deceleration to the vehicle occupant needs to be made up for by a high level of deceleration in a later stage of the vehicle crash. In an actual vehicle crash, such a delay in the transmission of deceleration to the vehicle occupant is substantial because of the slack that may be present in the seat belt and the property of the seat belt to elongate when subjected to a tension. 
     On the other hand, if the restraining capability of the seat belt is simply increased by using a pretensioner device or other means for removing the slack and/or prohibiting the elongation of the seat belt, the upper part of the vehicle occupant such as the head and chest of the vehicle occupant are subjected to an excessive level of deceleration, and this is not desirable in view of minimizing the injury to the vehicle occupant. 
     BRIEF SUMMARY OF THE INVENTION 
     In view of such problems of the prior art, a primary object of the present invention is to provide an anti-submarine vehicle occupant restraint system which can reduce the load on the chest and head of the vehicle occupant in an impact situation. 
     A second object of the present invention is to provide a vehicle occupant restraint system which can reliably prevent the submarining of the vehicle occupant at the time of a vehicle crash. 
     A third object of the present invention is to provide such a vehicle occupant restraint system which is simple in structure. 
     According to the present invention, such objects can be accomplished by providing a vehicle occupant restraint system for restraining a lower part of a vehicle occupant in a vehicle crash situation, comprising: a vehicle seat incorporated with a seat belt; a sensor for detecting an impact; a restraining member supported by a vehicle seat so as to be moveable from a retracted position concealed inside the seat to a deployed position projecting upward from the retracted position to restrain a forward movement of a hip area of the vehicle occupant; and a power actuator provided in association with the vehicle seat for actuating the restraining member to the deployed position according to a detection signal from the sensor; the power actuator being adapted to deploy the restraining member substantially before the seat belt effectively restrains the vehicle occupant. 
     Upon conducting extensive research, the inventors have discovered that the injury to the vehicle occupant is often caused by a high level of acceleration that is applied to the vulnerable part of the vehicle occupant such as the chest and head, and that the mechanical strength of the waist of a vehicle occupant is high enough to withstand the forward force at the time of an impact with a certain margin. It was therefore concluded that the injury to the vehicle occupant can be minimized by appropriately selecting the force and timing of restraining the pelvis and the adjacent parts (such as thighs and hip) relative to the timing of the seat belt restraining the chest area of the vehicle occupant. 
     Because the load acting on the vehicle occupant is thereby more distributed, the maximum load on the critical parts of the human body such as the chest and head can be reduced. In particular, the pelvis or waist and the parts adjacent thereto have a relatively high mechanical strength as compared to other parts of the human bone structure, and are relatively free from load at the time of an impact if the vehicle occupant is seated so that an early increase in the load on this part would not create any significant problem. 
     Preferably, a vertical height of the restraining member at the deployed position is equal to or higher than an ischium of the vehicle occupant seated in the seat. Thereby, the forward movement of the hip of the vehicle occupant can be effectively restrained, and the desired timing for restraining the hip of the vehicle occupant can be achieved. 
     According to a preferred embodiment of the present invention, a vertical height of the restraining member at the deployed position is at least 20 mm higher than an ischium of the vehicle occupant seated in the seat, and projects 20 mm or less into the part of the thighs of the vehicle occupant immediately above the restraining member at the deployed position. 
     It was also found to be desirable to restrain a part of the thighs of the vehicle occupant which is relatively close to the pelvis or more particularly the ischium of the vehicle occupant. According to a preferred embodiment based upon such a recognition, the restraining member at the deployed position is higher than an ischium of the vehicle occupant seated in the seat, and is 80 mm to 220 mm ahead of the ischium of the vehicle occupant. 
     The power actuator can be made highly compact when it consists of a pyrotechnic power actuator comprising a cylinder, a piston received therein, and a propellant received in the cylinder for rapidly increasing an inner pressure of the cylinder so as to produce a thrust that will rapidly push the piston out of the cylinder. To evenly apply a restraining force to the thighs of the vehicle occupant, and ensure a necessary mechanical strength without requiring any massive components, the restraining member may comprise a laterally extending main member supported by free ends of a pair of arms having base ends pivotally supported by a seat frame. 
     Typically, the pivotally supported base end of each arm is located behind the main part of the restraining member, and the power actuator is located behind of the main part of the restraining member. 
     Because the restraining member is desired to continue to apply a restraining force to the vehicle occupant for a certain period of time following a vehicle crash, it is desirable to provide a one-way lock mechanism in the power actuator or other parts of the restraint system for substantially preventing a reversing movement of the restraining member once it is deployed. 
     The orientation of the arm can be reversed, and by so doing, it is possible to retain the restraining member at its deployed position once it is deployed without requiring any special arrangement because the inertia acting on the vehicle occupant urges the restraining member toward its deployed position as long as it exists. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Now the present invention is described in the following with reference to the appended drawings, in which: 
     FIG. 1 is perspective see-through view of a vehicle seat embodying the present invention; 
     FIG. 2 is a schematic side view of the vehicle seat; 
     FIGS. 3 a  and  3   b  are views similar to FIG. 2 showing two different modes of positioning the occupant restraining member; 
     FIG. 4 is an exploded perspective view of the vehicle occupant restraint system; 
     FIG. 5 a  is a side view of the active side of the arrangement for actuating the occupant restraining member; 
     FIG. 5 b  is a sectional view taken along line IIIb—IIIb of FIG. 5 a;    
     FIG. 6 is a side view showing the passive side of the arrangement for actuating the occupant restraining member; 
     FIG. 7 is a sectional view of the pyrotechnic actuator for actuating the occupant restraining member; 
     FIGS. 8 a,    8   b  and  8   c  are graphs showing the levels of deceleration acting on the head, chest and hip of the vehicle occupant; and 
     FIG. 9 is a view similar to FIG. 2 showing an alternate embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1 and 2 show a vehicle seat  1  which includes a seat bottom  2 , and is fitted with a seat belt  40 . The seat bottom  2  comprises a rectangular seat frame  3  made of steel which is padded with foamed urethane and other suitable materials. The seat bottom  2  is incorporated with a vehicle occupant restraint system  4  embodying the present invention which includes an active subassembly  4   a  and a passive subassembly  4   b,  and a restraining member  13  extending between the working ends of the active and passive subassemblies  4   a  and  4   b.  The active and passive subassemblies  4   a  and  4   b  are fixedly secured to either side of the seat frame  3 . The occupant restraint system  4  is adapted to raise the restraining member  13  at the time of a vehicle crash to restrain the thighs of the vehicle occupant, and prevent the forward movement of the hip and waist of the vehicle occupant. Preferably, the restraining member  13  is provided with a suitable deformability to absorb the impact of the vehicle crash in an optimal fashion. 
     The seat frame  3  is formed with a projection  14  provided immediately in front of the restraining member  13  and defining an upper surface substantially flush with the upper surface of the restraining member  13  so that the restraining member  13 , in its retracted position, would not apply any undesirably localized pressure to the thighs of the vehicle occupant seated in this seat. 
     As best shown in FIG. 2, the restraint system  4  is disposed in such a manner that the restraining member  13 , in its deployed position, engages a part of the vehicle occupant which is somewhat offset from a mid point of the thigh bones toward the hip joints. It was found experimentally by using dummies (HYBRID-III 50th percentile male dummies) that the vehicle occupant can be effectively restrained by selecting the deployed position of the restraining member  13  to be somewhat offset from a mid point of the thigh bones toward the hip joints, and higher than the ischium O or the lowermost part of the pelvis P of the vehicle occupant. 
     If the deployed position of the restraining member  13  is more forward than the mid point of the thighs of the vehicle occupant, the restraining member  13  is unable to restrain the hip of the vehicle occupant at an early enough timing to avoid an excessive deceleration to be applied to the head and chest of the vehicle occupant by the restraining force of the seat belt. The desired range of the deployed position of the restraining member  13  extends from 80 mm to 220 mm as measured from the ischium O of the vehicle occupant seated in the seat. Because the ischium O of the vehicle occupant is typically in the range of 100 mm to 130 mm ahead of the rear end of the seat bottom  2 , the desirable region corresponds to the range of 180 mm to 350 mm as measured from the rear end of the seat bottom. 
     The fore-and-aft length of a vehicle seat bottom is typically in the range of 530 to 580 mm, the range of 180 mm to 350 mm corresponds to about 34% to 66% of the fore-and-aft length of the seat bottom as measured from the rear end of the seat bottom when the fore-and-aft length of the seat bottom is 530 mm, and to about 31% to 60% of the fore-and-aft length of the seat bottom as measured from the rear end of the seat bottom when the fore-and-aft length of the seat bottom is 580 mm. 
     The vertical height of the restraining member at its deployed position should be at least 20 mm higher than the level L of the ischium O of the vehicle occupant seated in the seat. If it is less than 20 mm, the restraining member is unable to effectively restrain the forward movement of the hip or pelvis P of the vehicle occupant. On the other hand, it is also desirable to control the vertical lift of the restraining member so as to avoid any inadvertent injury to the thighs of the vehicle occupant, and should be 20 mm or less above the level M of the lower surface of the part of the thighs of the vehicle occupant immediately above the restraining member. 
     The desired area for the deployed position of the restraining member based on the foregoing considerations is indicated by the hatched area A in FIG.  2 . 
     Based on the considerable that a child seat may be placed on the seat bottom or a child whose feet do not reach the floor may be seated on the seat bottom, the projection of the restraining member  13  should be 10 mm or less as measured from the part of the upper surface N of the seat bottom immediately above the restraining member. 
     FIG. 3 a  shows an example in which the deployed position of the restraining member  13  is at the rear extreme of the desirable region indicated by the hatched area A, and FIG. 3 b  shows an example in which the deployed position of the restraining member  13  is at the front extreme of the desirable region A. The shape and size of the projection  14  should be selected according to the particular position and size of the restraining member. 
     Referring to FIGS. 4 and 5 a,  the active subassembly  4   a,  which is attached to the right side of the seat frame  3  in this embodiment, comprises an elongated casing  5  having a rectangular cross section, an arm  6  pivotally attached to a front end of the casing  5  via a pivot collar  7   c  passed through holes  5   g  and  7   b  formed in the casing  5  and arm  6  so as to be rotated vertically, and a power actuator  8  received in the casing  5  and adapted to actuate the arm  6  when required. One end of the restraining member  13  is welded to a free end  6   a  of the arm  6 . 
     The power actuator  8  is provided with a one-way lock mechanism  9  which retains the working end of the power actuator (or a piston rod  21   b  of a piston main body  21   a  received in the power actuator  8  as described hereinafter) at its deployed position once the power actuator  8  is activated. The free end of the piston rod  21   b  is connected to the arm  6  via a pin  11  passed through a slot  6   b  formed in the arm  6  at a position offset from the pivot collar  7 . The pin  11  for actuating the arm  6  is also passed through a pair of horizontal slots  15   c  formed on either side of the casing  5  so that the piston rod  21   b  may be guided linearly along a central axial line of the cylinder  23  of the power actuator  8 . A rubber ring  31  is interposed between a flanged head of the pivot collar  7   c  and the opposing surface of the casing  5  to accommodate a slight axial displacement of the pivot collar  7   c.  Such an axial displacement of the pivot collar  7   c  creates a dimensional tolerance for the lateral dimension of the seat frame  3 . 
     The casing  5  comprises a main body  5   a  having an open right side, and a lid member  5   b  closing the open right side of the main body  5   a.  In the assembling process, the power actuator  8  and the one-way lock mechanism  9  are introduced into the main body  5   a  of the casing  5  from sideways, and the lid member  5   b  is then placed over the open right side of the main body  5   a.  The lid member  5   b  is secured to the main body  5   a  by crimping tabs  5   f  formed in the casing main body  5   a  and passed into corresponding slots formed in the lid member  5   b.    
     As best shown in FIG. 5 b,  the power actuator  8  includes a cylinder  23  having a circular cross section. The corresponding parts of the casing main body  5   a  and lid member  5   b  are suitably curved in cross section so as to conform to the outer profile of the cylinder  23  of the power actuator  8 . Furthermore, the casing main body  5   a  and lid member  5   b  are provided with rectangular openings  5   d  and  5   e,  respectively, so as to receive and engage a flange  23   b  of the cylinder  23 . Because the casing main body  5   a  and lid member  5   b  are curved in cross section, the edges of the openings  5   d  and  5   e  engaging the flange  23   b  are also curved so that the flange  23   b  is supported by the edges of the opening  5   d  and  5   e  evenly, symmetrically and over a large area. Therefore, the cylinder  23  can be supported in a secure manner, particularly when the actuator  8  is actuated and the rear surface of the flange  23  is pushed against the corresponding edge of the openings by the reaction caused by the activation of the power actuator  8 . The casing  5  is attached to the seat frame  3  by a pair of threaded bolts  7   a  and  10   a,  one  7   a  passed through the pivot collar  7   c  pivotally supporting the arm  6 , and the other  10   a  passed through an opening  10   a  provided in the rear end of the lid member  5   b.    
     Referring to FIG. 6, the passive subassembly  4   b  comprises an elongated casing  15  having a rectangular cross section, an arm  16  pivotally attached to a front end of the casing  15  via a pivot collar  17  so as to be rotated vertically, a rod  18  similar to the piston rod  21   b  having a front end pivotally supported by a pin  12  received in a horizontal slot  15   c  formed in the arm  16 , and a one-way lock mechanism  19 , which is also similar to the one-way lock mechanism  9  of the active subassembly  4   a,  is attached to the casing  15  via rectangular openings  15   d  and  15   e  formed in the casing  15  similar to the openings  5   d  and  5   e  of the active subassembly  4   a.  The rod  18  is provided with circumferential grooves or a threaded portion so as to favorably cooperate with the one-way lock mechanism  19 . The other end of the restraining member  13  is welded to a free end  16   a  of the arm  16 . 
     The casing  15  of the passive subassembly  4   b  also comprises a main body  15   a  having an open left side, and a lid member  15   b  closing the open left side of the main body  15   a.  In the assembling process, the one-way lock mechanism  19  is introduced into the main body  15   a  of the casing  15  from sideways, and the lid member  15   b  is then placed over the open left side of the main body  15   a.  The lid member  15   b  is secured to the casing main body  15   a  by crimping tabs  15   f  formed in the casing main body  15   a  and passed into corresponding slots formed in the lid member  15   b.  The casing  15  is attached to the seat frame  3  by a pair of threaded bolts  17   a  and  20   a,  one  17   a  passed through the pivot collar  17   c  pivotally supporting the arm  16 , and the other  20   a  passed through an opening  20   b  provided in the rear end of the lid member  15   b.  A rubber ring  32  is interposed between a flanged head of the pivot collar  17   c  and the opposing surface of the casing  15  to accommodate a slight axial displacement of the pivot collar  17   c.  Such an axial displacement of the pivot collar  17   c  creates a dimensional tolerance for the lateral dimension of the seat frame  3 . In short, the passive subassembly is similar to the active subassembly except for the absence of the power actuator. 
     Referring to FIG. 7, the power actuator  8  comprises a cylinder  23  which is received and fixed in the casing  5  of the active subassembly  4   a,  a piston main body  21   a  received in an inner bore  23   a  of the cylinder  23 , and a gas generator  24  received in the part of the cylinder  23  more toward the base end thereof than the piston main body  21   a.  A compression coil spring  26  is interposed between the piston main body  21   a  and the gas generator  24  via a resilient seal member  25  to normally urge the piston main body  21   a  in the direction of activation (projecting direction). The seal member  25  may consist of any suitable kind such as an O-ring which has a resiliency in the axial direction, and is effective in preventing the leakage of generated gas. 
     The front end of the power actuator  8  is provided with the one-way lock mechanism  9 . The one-way lock mechanism  9  comprises a housing  28  surrounding the piston rod  21   b  and fixedly attached to the casing  5 , and the housing  28  accommodates therein an engagement piece  19 , and a compression coil spring  30  urging the engagement piece  19  toward the base end of the piston rod  21   b  or the cylinder  23 . The engagement piece  19  is gradually reduced in outer diameter from the free end of the piston rod  21   b  to the base end thereof. The inner bore of the housing  28  includes a large diameter portion  28   a  and a tapered portion  28   b  which is gradually reduced in inner diameter away from the large diameter portion  28   a.  Therefore, in the state illustrated in FIG. 7, the engagement piece  29  is pressed onto the tapered portion  28   b  of the housing  28  and engages the outer circumferential surface of the piston rod  21   b  under the biasing force of the spring  30 . As the piston rod  21   b  moves in the projecting direction, the engagement piece  29  is dragged in the projecting direction of the piston rod  21   b  against the spring force of the spring  30 . As it moves toward the large diameter portion  28   a,  the engagement piece  29  moves away from the piston rod  21   b  so that the piston rod  21   b  is allowed to move freely. 
     When the piston rod  21   b  is pushed back into the cylinder  23 , the engagement piece  29  moves toward the tapered portion  28   b  under the spring force of the spring  30  and engages the outer circumferential surface of the piston rod  21   b  so that the piston rod  21   b  is securely fixed relative to the cylinder  23 . The inner circumferential surface of the engagement piece  29  is provided with annular grooves or thread grooves while the outer circumferential surface of the piston rod  21   b  is provided with corresponding annular grooves or thread grooves  21   c.  Therefore, as the piston rod  21   b  is pushed into the cylinder  23 , the inner circumferential surface of the engagement piece  29  engages the outer circumferential surface of the piston rod  21   b  so that these two parts are firmly retained to each other, and are kept at a fixed position. 
     In the embodiment described above, there was only one engagement piece, but it is also possible to provide two or more similar engagement pieces around the piston rod  21   b.    
     The gas generator  24  is electrically connected to a control unit  42  (FIG. 2) which activates the gas generator  24  by receiving a signal from an impact sensor  41  upon detection of a deceleration indicative of a vehicle crash. Deceleration sensors for such a purpose are well known in the art, and reference should be made to available prior art references for more details. 
     Now the mode of operation of this embodiment is described in the following. First of all, under a normal condition, because of the engagement between the inner circumferential surface of the engagement piece  29  and the thread grooves  21   c  on the outer circumferential surface of the piston rod  21   b , the piston main body  21   a , the arm  6  engaged thereby, and the restraining member  13  are kept stationary and prevented from any inadvertent movement. However, when a vehicle crash is detected by the impact sensor  41 , gas is generated from the gas generator  24 , and the resulting rapid increase in the inner pressure of the cylinder  23  instantaneously pushes out the free end of the piston main body  21   a  along with the piston rod  21   b  from the cylinder  23 . As a result, the arm  6  attached to the free end of the piston rod  21   b  turns in counter clockwise direction as indicated by the imaginary lines in FIG. 5 a , and the restraining member  13  is raised so that the submarining of the vehicle occupant can be prevented. The deployment of the restraining member  13  is typically accompanied by the corresponding deformation or destruction of the cushion members of the seat bottom  2 . Even after the generation of gas from the gas generator  24  has ceased, and the drive force of the power actuator  8  has been lost, the raised restraining member  13  would not come down, and maintains the action to prevent submarining by virtue of the one-way lock mechanisms  9  and  19 . 
     The deployment of the restraining member directly prevents the forward movement of the hip and waist of the vehicle occupant at the time of a vehicle crash, and this causes an early buildup of deceleration in the hip and waist of the vehicle occupant. This indirectly causes the corresponding build up of deceleration in the chest and head of the vehicle occupant. By thus causing the vehicle occupant, in particular to the head and chest of the vehicle occupant which are known to be more vulnerable than the hip and waist of the vehicle occupant, to experience the vehicle deceleration from an early phase of a vehicle crash, it is possible to minimize the maximum deceleration that is applied to the head and chest of the vehicle occupant. 
     As the vehicle occupant starts to be thrown forward, the waist or the pelvis of the vehicle occupant is relatively immediately restrained and the deformation of the restraining member absorbs a certain amount of the impact energy of the vehicle occupant (about 30 to 40 ms after the impact). The subsequent forward movement of the vehicle occupant causes the tension of the seat belt to reach its maximum level, or the pelvis to be fully restrained, and the pelvis experiences the maximum acceleration level (40 to 50 ms after the impact). The maximum acceleration level on the head occurs thereafter (50 to 60 ms after the impact). In this regard, it is important to deploy the restraining member substantially before the belt effectively restrains the vehicle occupant. This can be accomplished by restraining an appropriate part of the thighs of the vehicle occupant at an appropriate timing as discussed above. 
     In the experiments conducted by the inventors, a HYBRID-III 50th percentile male dummy was seated in a conventional seat of a compact car, and the vehicle was allowed to make a frontal crash substantially without any slack in the seat belt. Similar experiments were also conducted by using a vehicle seat incorporated with an occupant restraint system according to the present invention. In the graphs shown in FIGS. 8 a,    8   b  and  8   c,  the broken lines indicate the time histories of deceleration measured at various parts of the vehicle occupant at the time of a vehicle crash when only a seat belt is used and no arrangement is made to restrain the thighs of the vehicle occupant, and the solid lines indicate the time histories of deceleration measured at various parts of the vehicle occupant at the time of a vehicle crash when the vehicle occupant restraint system according to the present invention described above was used in combination with a seat belt. 
     The double-chain-dot lines indicate the time histories of deceleration measured at various parts of the vehicle occupant at the time of a vehicle crash when the vehicle occupant restraint system which is similar to the present invention but is adapted to raise a front end of the vehicle seat bottom was used in combination with a seat belt. According to the experiments conducted by the inventors, it was found that the restraining the front end of the vehicle seat or a lower part of the thigh bones (femurs) of the vehicle occupant is not as effective as one would like to expect. It is believed that restraining a part of the thigh bones near the hip joints is more effective in restraining the vehicle occupant than restraining a part of the thigh bones near the knee joints. It is also important to determine the vertical height of the deployed position of the restraining member  13  in relation to the ischium of the vehicle occupant so as to prevent the hip of the vehicle occupant from slipping forward in an early phase of a vehicle crash. Such a slipping causes the seat belt to restrain the chest of the vehicle occupant before the hip of the vehicle occupant is restrained by the restraining member, and prevents the desired minimization of the maximum value of the deceleration that is applied to the vulnerable part of the human body. 
     In the embodiment described above, the vehicle occupant restraint system comprised a passive subassembly and an active subassembly. If desired, it is possible to use two active subassemblies. In this case, it is possible to reduce the size of the power actuator for each active subassembly for a given power required in adequately actuating the restraint system 
     FIG. 9 shows a second embodiment of the present invention in which the subassemblies are oriented differently from those of the previous embodiment. The arms  6  and  16  as well as the restraining member  13  are provided in the rear ends of the subassemblies  4   a,  and the power actuator  8  is provided in the front end of the active subassembly  4   a.  In this case, the inertia of the vehicle occupant at the time of a frontal crash tends to raise the restraining member  13  as opposed to the foregoing embodiment. Therefore, according to this embodiment, the power that is required to deploy the restraining member  13  may be reduced. 
     Also, even when the position for deploying the restraining member  13  is in a relatively rear part of the seat bottom, according to this embodiment, the subassembly would not interfere with the frame or other structure of the seat bottom. By proper selection of the geometry of the arms  6  and  16 , it is possible to eliminate the need for the one-way lock mechanism because the restraining member  13  may be held in the deployed position as long as the vehicle occupant is subjected to deceleration and continues to apply a pressure to restraining member  13  towards its deployed position. 
     Although the present invention has been described in terms of preferred embodiments thereof, it is obvious to a person skilled in the art that various alterations and modifications are possible without departing from the scope of the present invention which is set forth in the appended claims.