Abstract:
An end of a seat belt is connected to an actuator that can selectively remove a slack from the seat belt. The actuator includes a main actuator unit, a flywheel which provides a maximum inertia effect for a given space, and a cushioning member for decelerating a movement of the moveable end of the seat belt following a certain initial travel of the moveable end. The main actuator unit produces an early rise in the vehicle occupant deceleration, and the cushioning member smoothly connects the time history of the vehicle occupant deceleration to a ride-down condition whereby the deceleration acting on the vehicle occupant is favorably spread over time, and the maximum level of the deceleration can be minimized. The flywheel contributes to a favorable shaping of the time history of the vehicle occupant deceleration.

Description:
CROSS REFERENCE TO RELATED PATENTS (PATENT APPLICATIONS)  
       [0001]    Reference should be made to the following commonly assigned prior patents (copending patent applications), and the contents of these patents (patent applications) are hereby incorporated in this application by reference.  
                                       Our Ref:   Patent (Application) Number   Issue (Application) Date                   F650   6,193,296   Feb. 27, 2001       F651   6,1865,74   Feb. 13, 2001       F655   6,2030,98   Mar. 20, 2001       F685   6,2541,64   Jul. 3, 2001       F686   09/608,669   Jun. 30, 2000       F687   09/648,190   Aug. 23, 2000       F716   09/729,973   Dec. 6, 2000       F781   10/005,739   Nov. 6, 2001       F817   unknown       F818   unknown       F819   unknown                  
 
     
    
     
       TECHNICAL FIELD  
         [0002]    The present invention relates to a vehicle occupant protection system for improving the crash safety of the vehicle.  
         BACKGROUND OF THE INVENTION  
         [0003]    In recent years, motor vehicles have been often fitted with a pretensioner device which positively increases the tension of the seat belt for restraining the vehicle occupant at the time of a crash and improves the protection of the vehicle occupant. The deceleration acting on the vehicle occupant who is restrained to the seat by a restraint device such as a seat belt starts rising only when the forward inertia force acting on the vehicle occupant at the time of the crash has started to be supported by the seat belt. As it is not possible to eliminate a certain amount of resiliency and slack in the seat belt, the deceleration of the vehicle occupant reaches a peak level only when the vehicle occupant has moved forward a certain distance under the inertia force and the elongation of the seat belt has reached its maximum extent. The peak value of the deceleration of the vehicle occupant gets greater as the forward displacement of the vehicle occupant under the inertia force increases, and is known to be substantially larger than the average deceleration of the passenger compartment of the vehicle body.  
           [0004]    When the relationship between the vehicle body deceleration and the vehicle occupant deceleration is compared to the relationship between the input and output of a system consisting of a spring (vehicle occupant restraint device) and a mass (mass of the vehicle occupant), it can be readily understood that the maximum elongation and time history of the spring are dictated by the waveform (time history) of the vehicle body deceleration. Therefore, it can be concluded that the waveform of the vehicle body deceleration should be controlled in such a manner that not only the average deceleration acting on the vehicle body is reduced but also the overshoot of the vehicle occupant deceleration due to the elongation of the spring (vehicle occupant restraint device) is minimized.  
           [0005]    In the conventional vehicle body structure, the impact energy is absorbed by a crushable zone, consisting of an impact reaction generating member such as side beams and gaps defined between various components, provided in a front part of the vehicle body, and the waveform of the vehicle body deceleration is adjusted by changing the resulting reaction properties by means of the selection of the dimensions and deformation properties of such parts. The deformation mode of the vehicle body other than the passenger compartment at the time of a crash may also be appropriately selected so that the deceleration of the passenger compartment of the vehicle body may be reduced, and the deformation may be prevented from reaching the passenger compartment. Such vehicle body structures are proposed in Japanese patent laid open publication (kokai) No. 07-101354.  
           [0006]    It is important to note that the injury to the vehicle occupant at the time of a vehicle crash can be minimized by reducing the maximum value of the acceleration (deceleration) acting on the vehicle occupant which is dictated by the waveform (time history) of the vehicle body deceleration. It is also important to note that the total amount of deceleration (time integration of deceleration) which the vehicle occupant experiences during a vehicle crash is fixed for the given intensity of crash (or vehicle speed immediately before the crash). Therefore, as shown in FIG. 6 for instance, the ideal waveform (time history) of the vehicle body (seat) deceleration (G 2 ) for the minimization of the vehicle occupant deceleration (G 1 ) should consist of an initial interval (a) for producing a large deceleration upon detection of a crash, an intermediate interval (b) for producing an opposite deceleration, and a final interval (c) for producing an average deceleration.  
           [0007]    The initial interval allows the vehicle occupant to experience the deceleration from an early stage so that the deceleration may be spread over an extended period of time, and the peak value of the deceleration to be reduced. According to a normal vehicle body structure, owing to the presence of a crushable zone in a front part of the vehicle and a slack and elongation of the restraint system such as a seat belt, it takes a certain amount of time for the impact of a crash to reach the vehicle occupant. The delay in the transmission of deceleration to the vehicle occupant must be made up for by a subsequent sharp rise in deceleration according to the conventional arrangement. The final interval corresponds to a state called a ride-down state in which the vehicle occupant moves with the vehicle body as a single body. The intermediate interval is a transitional interval for smoothly connecting the initial interval and final interval without involving any substantial peak or dip in the deceleration. Computer simulations have verified that such a waveform for the vehicle body deceleration results in a smaller vehicle occupant deceleration than the case of a constant deceleration (rectangular waveform) for a given amount of deformation of the vehicle body (dynamic stroke).  
           [0008]    According to the conventional vehicle body structure, the vehicle body components of the crushable zone start deforming from a part having a relatively small mechanical strength immediately after the crash, and a part thereof having a relatively high mechanical strength starts deforming thereafter. As a result, the waveform of the crash reaction or the vehicle body deceleration is small in an early phase, and then gets greater in a later phase so that the vehicle occupant deceleration cannot be adequately reduced. To eliminate such a problem, it has been proposed to obtain a prescribed amount of reaction force by making use of the collapsing of the side beams and to maintain a stable reaction by providing a plurality of partition walls in the side beams (Japanese patent laid-open publication (kokai) No. 07-101354). However, such previous proposals can only maintain the vehicle body deceleration at an approximately constant level at most, and are unable to provide a more effective deceleration waveform.  
           [0009]    To minimize the adverse effect of the resiliency of the seat belt, it is known to provide a pretensioner device in association with the seat belt to positively tension the seat belt at the time of a vehicle crash. According to another previously proposed structure, at least one of the anchor points of the seat belt is attached to a member which undergoes a movement relative to the remaining part of the vehicle which tends to increase the tension of the seat belt in an early phase of a vehicle crash. Such devices are beneficial in reducing the maximum level of deceleration acting on the vehicle occupant at the time of a vehicle crash, but a device capable of more precise control of the vehicle occupant deceleration is desired.  
           [0010]    Referring to FIG. 7, the vehicle occupant deceleration G 1  and vehicle body deceleration G 2  correspond to the input and output of a transfer function representing a two-mass spring-mass system consisting of the mass Mm of a vehicle occupant  2 , a spring (such as a seat belt), and a vehicle body mass Mv. More specifically, the vehicle body deceleration G 2  can be given as a second-order differentiation of the coordinate of the vehicle body mass Mv with respect to time.  
           [0011]    However, in an actual automotive crash, if a three-point seat belt is used, the shoulder belt portion of the seat belt which can be considered as a spring engages the chest of the vehicle occupant corresponding to the center of the vehicle occupant mass Mm so that the shoulder belt portion can be considered as consisting of two springs, one extending between the chest and shoulder anchor, the other extending between the chest and the buckle anchor.  
           [0012]    If the seat belt is entirely incorporated to the seat, the shoulder anchor and buckle anchor move as a single body, and the two parts experience an identical deceleration. In such a case, it can be assumed that the seat belt can be given as a composite of two springs, and the deceleration acting on the shoulder anchor and buckle anchor is identical to the input to the two-mass spring-mass system or the vehicle body deceleration.  
           [0013]    Now, suppose if the buckle anchor point is fixedly attached to the vehicle body while the shoulder anchor is capable of movement relative to the vehicle body as an example in which the two anchor points undergo different movements relative to the vehicle body. In such a case, because the shoulder anchor and buckle anchor experience different decelerations, the springs cannot be simply combined or the decelerations acting on the shoulder anchor and buckle anchor cannot be simply equated to the vehicle body deceleration.  
           [0014]    Meanwhile, the external force acting on the chest wholly consists of the force received from the seat belt. Therefore, if the time history of the load acting on the seat belt in the direction of deceleration agrees with the time history of the spring load in the two-mass spring-mass system, the chest receives the same deceleration waveform as the response of the vehicle occupant mass of the two-mass spring-mass system to the optimum waveform of vehicle body deceleration. This enables the vehicle occupant to reach the ride-down state in which the vehicle occupant is restrained by the seat belt substantially without any delay and the relative speed between the vehicle body and vehicle occupant is zero (no difference between the vehicle occupant deceleration G 1  and vehicle body deceleration G 2 ).  
           [0015]    To achieve a time history of the seat belt that produces such a state, it suffices if the time history of the average deceleration of the shoulder anchor and buckle anchor (or vehicle body) is equal to the optimum waveform of the vehicle body deceleration. Introducing the concept of the waveform of average vehicle body deceleration allows an identical result in reducing the vehicle occupant deceleration as controlling the vehicle body deceleration so as to achieve the optimum waveform to be achieved.  
           [0016]    The early rise in the tension of the seat belt to apply the deceleration to the vehicle occupant from an early stage can be most conveniently provided by a pyrotechnical actuator typically using a propellant. Pyrotechnical actuators are widely known in such applications as vehicle air bags and pretensioners. However, it was found due to the nature of its structure which relies on the generation of high pressure gas that such an actuator alone may not be able to produce a desired time history of the deceleration of the vehicle occupant. It was found that the provision of inertia mass prevents an oscillatory movement to the moveable end or vehicle occupant during the activation of the actuator. The inventors have discovered that such a problem can be overcome by adding a suitable amount of mass to the actuator end of the seat belt in combination with a cushioning member.  
         BRIEF SUMMARY OF THE INVENTION  
         [0017]    In view of such problems of the prior art, a primary object of the present invention is to provide a vehicle occupant protection system which can improve the protection of the vehicle occupant at the time of a vehicle crash for a given dynamic stroke or a deformation stroke of a front part of the vehicle body.  
           [0018]    A second object of the present invention is to provide a vehicle occupant protection system which can maximize the protection of the vehicle occupant with a minimum modification to the existing vehicle body structure.  
           [0019]    A third object of the present invention is to provide a vehicle occupant protection system which can maximize the protection of the vehicle occupant without increasing the weight of the vehicle body or taking up any significant amount of space in the passenger compartment.  
           [0020]    According to the present invention, such objects can be accomplished by providing an automotive vehicle occupant protection system, comprising: a seat supported on a floor of a vehicle body; a seat belt provided in association with the seat and including a moveable end; an actuator connecting the moveable end of the seat belt to a part of the vehicle body to selectively remove a slack from the seat belt; and a control unit including a deceleration sensor for detecting a frontal vehicle crash meeting a prescribed condition; the actuator including a mass member attached to the moveable end of the seat belt, an arrangement for amplifying an inertia effect of the mass on a movement of the moveable end of the seat belt, and a main actuator unit adapted to move the mass member in a direction to remove a slack from the seat belt immediately upon detection of a frontal vehicle crash.  
           [0021]    Thus, upon detection of a crash, the main actuator which typically consists of a pyrotechnic actuator increases the restraint of the seat belt by moving the moveable end of the seat belt so that a deceleration greater than the average deceleration (vehicle deceleration) is produced in the vehicle occupant. Because the seat belt and vehicle occupant behave as a spring mass system, an oscillatory motion of the vehicle occupant tends to be induced. Such an oscillatory motion is obviously undesirable to the end of minimizing the peak value of the deceleration acting on the vehicle occupant. Therefore, according to the present invention, an increased amount of mass is added to the spring mass system to prevent any such undesirable oscillatory motion of the moveable end of the seat belt or the vehicle occupant from occurring. The provision of the mass member also facilitates the control of the time history of the output of the actuator to best achieve the desired acceleration control for the vehicle occupant.  
           [0022]    The system preferably includes a cushioning member for decelerating a movement of the moveable end following a certain initial travel of the moveable end. Thus, after the moveable part has moved by a prescribed distance, the movement is prevented by the cushioning member and an opposite deceleration is produced in the vehicle occupant so that the vehicle occupant and vehicle body move as a single body in a final phase of the crash, and decelerate at the average deceleration. This achieves a waveform of vehicle body deceleration suitable for the minimization of the deceleration of the vehicle occupant.  
           [0023]    The anchor points may be provided in appropriate parts of the vehicle body, but all or some of them may be provided on parts of the seat. According to a preferred embodiment of the present invention, to obtain a highly predictable result, the seat belt may comprise three anchor points including a shoulder anchor, a seat bottom side anchor provided near a seat bottom on a same side as the shoulder anchor, and a buckle anchor provided near the seat bottom on an opposite side of the shoulder anchor.  
           [0024]    Preferably, the mass member comprises a flywheel which provides a large amount of inertia without taking up any significant amount of space. The actuator may comprise wire including a first end connected to a working end of the main actuator unit, an intermediate part wound around a rotary shaft having the flywheel integrally or otherwise functionally connected thereto, and a second end attached to the moveable end of the seat belt. In particular, by choosing the rotary shaft to have an appropriate diameter, it is possible to magnify the inertia effect of the flywheel at will. The inertia effect gets greater as the diameter of the rotary shaft is reduced.  
           [0025]    According to a particularly preferred embodiment of the present invention, the main actuator unit includes a cylinder integrally attached to a side of a seat bottom of the seat, a piston slidably received in the cylinder, and a pyrotechnic gas generator provided on one end of the cylinder, the wire being attached to the piston. However, it is also possible to use other sources of energy for the actuator including pre-loaded springs. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]    Now the present invention is described in the following with reference to the appended drawings, in which:  
         [0027]    [0027]FIG. 1 is a schematic side view of the outline of the vehicle body structure fitted with a vehicle occupant protection system embodying the present invention;  
         [0028]    [0028]FIG. 2 is an overall perspective view of the seat fitted with the vehicle occupant protection system according to the present invention;  
         [0029]    [0029]FIG. 3 is an enlarged sectional view of the vehicle occupant protection system;  
         [0030]    [0030]FIG. 4 a  is a schematic view of the vehicle body in an initial phase of the crash;  
         [0031]    [0031]FIG. 4 b  is a schematic view showing an intermediate phase of the crash;  
         [0032]    [0032]FIG. 4 c  is a schematic view showing a final phase of the crash;  
         [0033]    [0033]FIG. 5 a  is a sectional view showing the state of the actuator in an initial phase of the crash;  
         [0034]    [0034]FIG. 5 b  is a sectional view showing the state of the actuator in an intermediate phase of the crash;  
         [0035]    [0035]FIG. 5 c  is a sectional view showing the state of the actuator in a final phase of the crash;  
         [0036]    [0036]FIG. 6 is a diagram showing the waveforms of the vehicle occupant deceleration and vehicle body deceleration; and  
         [0037]    [0037]FIG. 7 is a conceptual diagram showing the relationship between the vehicle occupant, vehicle body and seat belt at the time of a vehicle crash. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0038]    [0038]FIG. 1 schematically illustrates the overall structure of a vehicle incorporated with a vehicle occupant protection system embodying the present invention. The vehicle occupant protection system includes a seat belt  3  for restraining the vehicle occupant  2  to the seat  1 . As shown in FIG. 2 also, the seat belt  3  has three anchor points, and an end of the seat belt is connected to a retractor  4  integrally provided in a shoulder part of the seat  1 , another end fixedly attached to a side of the seat bottom on the same side as the shoulder anchor, and an intermediate part having a tongue plate that is latched to a buckle  6  attached to the side of the seat bottom on the other side of the shoulder anchor. Therefore, the vehicle occupant  2  who is seated in the seat  1  is integrally restrained to the seat  1  by the seat belt  3 . The seat  1  is attached to a floor  5  via seat rails la so as to be adjustable in the fore-and-aft direction.  
         [0039]    An actuator  8  is integrally attached to a side of the seat bottom of the seat  1 . As also shown in FIG. 3, the actuator  8  comprises a cylinder  8   a  extending in the fore-and-aft direction, a piston  8   b  coaxially received in the cylinder  8   a , a cylinder cap  8   c  integrally attached to an end (rear end with respect to the vehicle body) of the cylinder  8   a , and a gas generator  8   d  connected to the cylinder cap  8   c  so as to communicate with the interior of the cylinder cap  8   c . The front end of the cylinder  8   b  in this case opens out to the atmosphere. The actuator  8  is integrally provided with a bracket  8   j  by which the actuator  8  is bolted down to the frame of the seat bottom.  
         [0040]    To the piston  8   b  is connected an end of wire  9  which extends out of an opening  8   e  provided in the rear end of the cylinder cap  8   c . The outer end of the wire  9  is wound around a rotary shaft  10   a  which is rotatably supported behind the actuator  8  and is integrally and coaxially provided with a flywheel  10 , and then further extends to an extreme end which is connected to a base end of the buckle  6 . Therefore, as the piston  8   b  is driven forward by the gas generator  8   d  and the wire  9  is pulled forward as a result, the wire  9  pulls the buckle  6  in the direction to increase the tension of the seat belt  3  or the restraint on the vehicle occupant  2  while the intermediate part of the wire  9  wound around the rotary shaft  10   a  causes the flywheel  10  to rotate. The intermediate part of the wire  9  and rotary shaft  10   a  are received in a casing  8   f  which is attached to the rear end of the cylinder cap  8   c.    
         [0041]    The outer circumferential surface of the piston  8   b  is provided with a reversing preventing ring  8   g  which allows the forward movement of the piston  8   b  by inclining itself to one side but prevents the rearward movement of the piston  8   b  by wedging into the inner circumferential surface of the cylinder  8   a.    
         [0042]    A tubular cushioning member  8   i  is provided on an outer end of an opening  8   h  of the casing  8   f  facing the buckle  6  in a coaxial arrangement, and the free end of the tubular cushioning member  8   i  opposes the base end of the buckle  6  defining a gap of a prescribed dimension d therebetween. A bellows cover  11  covers the wire  9  connected to the base end of the buckle  6  and the tubular cushioning member  8   i , and extends between the base end of the buckle  6  and the opposing end of the casing  8   f . The bellows cover  11  has an adequate rigidity to support the buckle  6  in a substantially fixed manner, but demonstrates a flexible that allows the bellows cover  11  to axially compress so as to accommodate the movement of the buckle  6  in the direction to increase the tension of the seat belt  3 .  
         [0043]    The actuator  8  described above thus comprises the cylinder  8   a , piston  8   b , cylinder cap  8   c  and gas generator  8   d , and is designed to provide a primary acceleration that increases the restraint on the vehicle occupant  2 . The tubular cushioning member  8   i  provides a secondary acceleration which controls the acceleration provided by the actuator  8  as will be described hereinafter.  
         [0044]    To the gas generator  8   d  is connected a signal line from a control unit  12  mounted to an appropriate part (such as the floor  5 ) of the vehicle body and incorporated with a crash sensor which, for instance, may consist of a G sensor. The crash sensor provides a crash detecting signal to the gas generator  8   d  when a crash meeting a prescribed condition is detected. In response to a crash detecting signal, the gas generator  8   d  instantaneously produces expanding gas which is then introduced into the cylinder cap  8   c.    
         [0045]    Referring to FIGS. 4 a  to  4   c  and  5   a  to  5   c,  the mode of operation of the embodiment of the present invention is described in the following by taking an example of a frontal crash onto a fixed structure.  
         [0046]    [0046]FIG. 4 a  shows a state of an initial phase (interval a of FIG. 6) immediately following the occurrence of a crash. The front end of the vehicle body collapses, and the front ends of side beams  13  integral with the floor  5  undergo a compressive deformation as shown in the drawing. The crash sensor incorporated in the control unit  12  detects the vehicle body deceleration resulting from the vehicle crash exceeding a prescribed intensity, and the control unit  12  judges the condition that is produced. If the control unit  12  judges that the condition meets the prescribed criterion, the gas generator  8   d  is activated.  
         [0047]    The expanding gas produced from the gas generator  8   d  is introduced into the cylinder cap  8   c  as indicated by the arrows in FIG. 5 a , and the pressure of the expanding gas pushes the piston  8   b  in the forward direction with respect to the vehicle body. As a result, the buckle  6  which is connected to the piston  8   b  via the wire  9  starts moving in the direction to increase the restraint of the seat belt  3  on the vehicle occupant  2  while the rotary shaft  10   a  and flywheel  10  start rotating. The movement of the buckle  6  causes the bellows cover  11  to collapse, and accelerates as the pressure of the generated gas increases.  
         [0048]    The early rise in the tension or load acting on the seat belt  3  corresponds to an increase in the restraint on the vehicle occupant  2  and the deceleration of the vehicle occupant from an early phase of the crash. The resulting rise in the seat belt load is earlier than that provided by a conventional seat belt which is simply secured at three anchor points in restraining the vehicle occupant from being thrown forward under the inertia force. Therefore, the deceleration of the vehicle occupant is made to rise from a very early part of the crash as indicated by G 1  in FIG. 6.  
         [0049]    [0049]FIG. 4 b  shows a state in an intermediate phase of the crash (interval b of FIG. 6). As the collapsing of the front part of the vehicle body progresses, the piston  8   b  of the actuator  8  moves further forward with respect to the vehicle body as indicated in FIG. 5 b . As the piston  8   b  moves further forward, the base end of the buckle  6  eventually collides with the cushioning member  8   i , and this decelerates the movement of the buckle  6 , thereby producing an opposite (forward with respect to the vehicle body) acceleration to the vehicle occupant. This produces an effect equivalent to that produced by an acceleration directed in the opposite direction to the deceleration resulting directly from the crash acting on the passenger compartment. To better achieve such an effect, the effective mass of the flywheel  10  and diameter of the rotary shaft  10   a  as well as the acceleration of the flywheel  10  at the time of colliding with the cushioning member  8   i  are appropriately adjusted. It is preferable to design the properties (such as elongation and spring properties) of the seat belt  3  and the properties (such as impact absorbing property) of the cushioning member  8   i  so that the speed and deceleration of the vehicle occupant  2  coincide with those of the vehicle body (seat  1 ) upon completion of the acceleration in the opposite direction acting on the buckle  6  during this intermediate phase.  
         [0050]    [0050]FIG. 4 c  shows a state of a final phase (interval c of FIG. 6) of the crash. During the final phase, the movement of the buckle  6  is further decelerated by the cushioning member  8   i , and the piston  8   b  eventually comes to a complete stop. As a result, the buckle  6  also stops moving any further, and is retained at this position until the end of the vehicle crash by virtue of a reversing preventing ring  8   g.    
         [0051]    During this final phase, once the speed and deceleration of the vehicle occupant agree with those of the vehicle body (seat  1 ), there is no relative movement between the vehicle occupant  2  and vehicle body (seat  1 ), and the vehicle occupant  2  continues to decelerate as a single body with the vehicle body (seat  1 ). In other words, the maximum value of the vehicle occupant deceleration G 1  can be reduced by achieving a ride down state in which the relative speed between the vehicle occupant  2  and vehicle body (seat  1 ) is minimized and the difference between the vehicle occupant deceleration G 1  and vehicle body deceleration G 2  is minimized.  
         [0052]    Thus, the process described above can substantially reduce the vehicle occupant deceleration by controlling the deceleration produced in the buckle  6  so as to follow the optimum deceleration waveform or by designing the actuator  8  so as to produce the optimum deceleration waveform.  
         [0053]    Thus, according to the foregoing embodiment, upon detection of a crash, the main part of the actuator consisting of a pyrotechnical actuator increases the restraint of the seat belt by moving the moveable part provided on the seat serving as a part of the vehicle body so that the vehicle occupant is allowed to experience an early rise in deceleration. Then, after the moveable part has moved by a prescribed distance, the movement is prevented by the cushioning member and an opposite deceleration is produced in the moveable part so that the vehicle occupant and vehicle body move as a single body in a final phase of the crash, and decelerate at the average deceleration. This achieves a waveform of vehicle body deceleration suitable for the minimization of the deceleration of the vehicle occupant. As a result, not only a substantial reduction in the vehicle occupant deceleration can be achieved with a smaller vehicle body deformation (dynamic stroke) but also the displacement of the vehicle occupant in the passenger compartment relative to the vehicle body can be reduced even more than possible by providing a load limiter in the restraining device to reduce the vehicle body deceleration. The smaller displacement of the vehicle occupant reduces the possibility of a secondary collision.  
         [0054]    When an end of the seat belt is attached to the seat, and an intermediate part of the seat belt is attached to the moveable part via a buckle, the vehicle occupant and seat can be joined integrally to each other by using a conventional three-point seat belt incorporated to a seat so that the cost of the system can be minimized without requiring any substantial change to the existing system.  
         [0055]    Although the present invention has been described in terms of a preferred embodiment 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. For instance, all or some of the anchor points may be provided on parts of the vehicle body other than those on the seat. The actuator also may consist of actuators other than pyrotechnic actuators such as spring loaded actuators without departing from the spirit of the present invention.