Safety restraint

Abstract: A vehicle safety restraint seat belt incorporating a load limiting effect to allow a limited and controlled forward motion of the occupant after the retractor has locked to decrease the forces exerted by the belt on the occupant's torso. The webbing is attached to a structural part of the vehicle via a load limiting device, which comprises: a container defining a flow path for a fluid, an actuator arranged within the container and operable to act on the fluid, and a restriction in the flow path of the fluid, wherein the actuator is movable within the container under the action of load transmitted to the webbing from the vehicle occupant under extreme acceleration or deceleration conditions, to move the fluid through the restriction in the flow path thereby to dissipate a predetermined portion of the load on the webbing. A piston cylinder arrangement may be used or a rotary paddle array, or forms of vane pumps or peristaltic pumps.

Description: The present invention relates to a safety restraint for use in a vehicle and particularly to a seat belt. Modern seat belts comprise a length of webbing arranged to pass diagonally across the torso of a vehicle occupant and generally horizontally across the hip region of the vehicle occupant (so-called lap portion of the belt). This is known as a three point belt system. One end of the belt webbing is firmly attached to a structural part of the vehicle such as the floor, and the other end is attached to the spool of a retractor which itself is firmly attached to a structural part of the vehicle, usually the side, B-pillar. The retractor automatically keeps any slack in the belt wound onto the spool and thus keeps a tension in the belt. Between the retractor and the other fixed point, a fastening element such as a metal tongue is fixed to the belt with which it can be fastened into a buckle which itself is attached to fixed part of the vehicle on the other side of the occupant seat. A clock spring in the retractor allows pay out of webbing under the influence of relatively gentle forwardly directed inertia of the vehicle occupant, for example to allow for normal movement of the occupant such as reaching forward to activate in car controls, glove compartments or door pockets. In the event of a crash the sudden high forward momentum of the occupant activates a crash sensor which locks the spool against rotation and prevents forward motion of the occupant to prevent him colliding with the internal fixtures of the vehicle such as the steering wheel, dashboard or windscreen. However it has been found in high velocity crashes that the sudden locking of the seat belt can itself cause injury to the occupant due to the sudden impact of the torso with the belt webbing. In recent years it has been proposed to introduce a load limiting effect into the seat belt system so as to allow a limited and controlled forward motion of the occupant after the retractor has locked. This decreases the forces exerted by the belt on the occupant's torso. Load limiting proposals are described in EP 0 297 537 wherein a plastically deformable member is used in the retractor, and particularly between the spool and inner most winding of the belt webbing. Alternative load limiting proposals are known in which crushable bushings or nuts or deformable torsion bars are placed in the retractor in the force path between the spool locking mechanism and belt webbing. These proposals are complex and expensive and require the retractor to be specially designed and constructed to incorporate them. FR 2 306 376 describes a vehicle safety restraint according to the preamble of claim 1 and comprising a load limiting arrangement with a piston and concentric tubes which are picked up with increasing belt pay-out to increase the restraining effect as a crash pulse progresses. The present invention proposes an improved, simpler and more cost effective load limiting arrangement for seat belts. According to the present invention there is provided a vehicle safety restraint comprising: seat belt webbing for restraining a vehicle occupant, the webbing being attached to a structural part of the vehicle via a load limiting device, the load limiting device comprising: a container defining a flow path for a fluid, an actuator arranged within the container and operable to act on the fluid, and a restriction in the flow path of the fluid, wherein the actuator is movable within the container under the action of load transmitted to the webbing from the vehicle occupant under extreme acceleration or deceleration conditions, to move the fluid through the restriction in the flow path thereby to dissipate a predetermined portion of the load on the webbing, the actuator comprising a piston and the container comprises a cylinder, and one of the piston and the cylinder is attached to an end of the belt webbing and the other one of the piston and the cylinder is attached to the structural part of the vehicle, and being arranged so that the cross sectional area which it presents to the fluid is variable wherein the piston is formed of a core piston and a plurality of concentric tubes which are dropped by the core piston as it travels down the cylinder so as to decrease the restraining effect of the restraint as the crash pulse progresses. The restriction may comprise one or more holes in the piston itself so that under pressure fluid is forced from one part of the cylinder, on the high pressure side of the piston, to the other part of the cylinder, on the low pressure side. Alternatively there may be a feedback pipe external to the cylinder to feed fluid under pressure from the high pressure side of the piston to the low pressure side. This feedback pipe may have a further restriction or a valve. The valve may be adjustable to offer a predetermined resistance to fluid flow in dependence upon the force damping characteristics required for the particular seat belt. The valve may be actively controllable, for example in response to a gauge constantly monitoring the pressure of the fluid in the cylinder. In this way the load perceived by the vehicle occupant can be actively tailored, not only to the vehicle concerned, but also to the circumstances which generated the load: the severity of the crash pulse, the weight of the occupant and the activation of a secondary restraint such as an airbag, or the occurance of a second crash. This results in a decrease in the restraining effect as the crash pulse progresses because the predetermined load at which load limiting becomes operational decreases as the belt payout increases. This is particularly useful, for example, if an airbag is fitted to the vehicle, because that will take over the restraining effect and stopping the seat belt completely at an early stage of the crash pulse is not important and the piston loses sections of its effective cross section as it travels down the cylinder. The exact arrangement can be easily adapted to the effect which is required. For a better understanding of the present invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings in which: Figure 1 illustrates a vehicle safety restraint which in not part of the present invention. Figure 2 is a graph illustrating force against belt extension for the example of figure 1. Figure 3 is a perspective view of a first embodiment of the present invention. Figures 4a, 4b and 4c are cross-sectional views of a part of the embodiment of figure 3. Figure 5 is a cross-sectional view of a development of a second embodiment. Figure 6 is a transverse cross-sectional view of the embodiment of figures 3, 4 and 5. Figures 7, 8 and 9 illustrate further examples which are not part of the present invention. The figures show a cylinder 1 with a fixing hole 2 at one end for connection to a structural part of the vehicle, either at the buckle end or at the sill end of the seat belt. Inside the cylinder 1 is a piston 3 connected by a cable 5 to seat belt webbing 6 to provide a load limiting arrangement between a structural part of the vehicle and the seat belt webbing of a three point system, either at the buckle end or the sill end. Inside the cylinder 1 is a fluid 7 which may be a gas or a liquid or some other extrudible material such as a gel or a foam composition. The exact nature of this extrudible and/or compressible material is chosen to suit the load limiting characteristics required of the load limiting arrangement. A crush tube 8 may optionally be arranged coaxially with the cylinder to tune the load limiting effect to increase the energy absorption over a particular range of belt extension, specifically near to the end of travel of the piston. The force is controlled by the size of a restriction in the flow path. The restriction may be a hole in the piston as shown in figure 1 where the piston is shown as a simple piece not in accordance with the invention. A spring 11 is also included to bias the piston in the direction of least extension, ie in the rear position. The spring also increases the limiting load. The effect can be seen figure 2 where the upper curve A indicates a high velocity crash force extension curve relationship for the piston-cylinder arrangement of figure 1 and the line B indicates a low velocity crash force belt extension curve relationship. C indicates the crash pulse without any force limiting. When a crash is indicated, a crash sensor signal causes the retractor to lock up against further webbing extension. The force felt by the occupant rises relatively steeply in the first part of the curves A and B, mirroring the crash pulse C itself. There will be some forward movement of the occupant due to belt slack being taken up (film spool effect). This allows a limited belt extension of the distance x after the spool has locked. However when a predetermined level of load is applied to the piston and the film spool effect is over, the inherent inertia of the system is overcome and the piston begins to move along the cylinder 1 applying a force to the fluid or gel 7 in the cylinder 1 and forcing it through a restriction. This absorbs some of the load which would otherwise be felt by the occupant and allows a further webbing extension y, at a more constant load, as indicated by the relative plateaus in the graphs A and B. In the embodiment of graph A, the fluid compression characteristics exhibit a gradient and thus the plateau between x and y is not so flat as for graph B. Towards the end of the cylinder the piston encounters the crush tube 8 which slows its movement. The force felt by the occupant rises again relatively sharply and then exhibits another plateau as indicated between y and z in the graphs A and B. When the crush tube 8 is almost completely crushed the force on the occupant will rise again steeply at z. However at this point the force of the crash is dissipating as can be seen by the crash pulse indicated by line C. Of course the system could be made such that the tube crush force is lower than the fluid compression or extrusion force. In this case the graphs A and/or B will be effectively the same shape but the plateau between x and y will be attributable to the crush tube rather than the fluid and that between y and z to the fluid rather than the crush tube. Figure 3 illustrates an embodiment in which a feed back loop pipe 10 carries the fluid 7 from one part to the other part of the cylinder as the piston 3 is pulled along the cylinder under tone from the seat belt. A control valve or restriction 9 is optionally introduced into the feedback pipe 10 so as to effect further control and tuning of the system. The fluid may be gas or liquid and the viscosity of the fluid together with the size of the return tube, controls the force required. In figure 4 a variable size piston surface is used to tune the system further. The arrangement is such that the piston surface area decreases with increasing belt extension so that the limiting load decreases and more payout is allowed for less load, for example as the crash pulse dissipates, or as a secondary restraint such as an airbag comes into operation. This produces a more constant force on the vehicle occupant. The piston is formed with a small central circular part 3a attached to cable 5, with a series of concentric cylinders 3b, 3c and 3d fitting snugly inside each other. At the beginning of the crash pulse the central piston part 3a moves together with all the concentric cylinders, in the direction of arrow E and thus presents a maximum piston surface area to the extrudible/compressible fluid 7. This is shown in figure 4a and gives the maximum resistance to movement of the piston and thus the minimum load limiting effect. However as the belt extension increases, the outer concentric cylinder 3d reaches the end of cylinder 1 and is stopped by it. The adjacent cylinder 3c slides past the outer one as the pull on the belt and thus on cable 5 is still sufficient to overcome the friction between the concentric cylinders. However the effective piston surface area, as shown in figure 4b is much reduced and thus the resistance to piston movement through the fluid is decreased, allowing a greater webbing payout for a fixed load, ie the load limiting effect is decreased. As the belt extension increases further each of the outer concentric cylinders 3b to 3d come to a stop at the end of cylinder 1 and only the central piston part 3a continues to move. However at this stage as shown in figure 4c an even smaller piston surface is presented and thus the load limiting effect is minimal. The level of load limiting usually rises relatively rapidly as the velocity of the crash increases, and can be a function of the square of the velocity of the crash. This is a characteristic of fluids and is a well known phenomenon in standard damping technology. Thus in a high speed crash the damping effect of the fluid filled load limiter is very much higher than the effect in a low speed crash. Obviously this is advantageous. In general, the force absorbed is given by FOC V n where n is between 1 and 2 depending upon the properties of the fluid employed and the particular valve design. The embodiment of figures 5 and 6 is a variation on the embodiment of figures 3 and 4 in which the concentric tubes 3a, 3b, 3c and 3d are held together by a spring 11. These concentric cylinder tubes each have three lugs so that they are held in the position shown in figure 5 under the action of spring 11 under normal, non-load limiting, conditions. However when the webbing 6 is put under load and the piston is pulled in the direction E, the inner piston 3a picks up each of the tubes 3b, 3c and 3d in turn so that the rate of extension of the webbing is varied during the full stroke of the piston. However, this arrangement of the piston picking up cylinders is not within the scope of the invention. In this arrangement the effective surface area of the piston increases with increasing extension of the webbing as the crash pulse develops and thus the piston moves more fluid and requires more force. A load limiter of this type could be used to come into effect earlier in the crash pulse than the load limiter shown in figure 4c where the surface area of the piston effectively decreases. A valve 9 is incorporated in the feedback return pipe 10 to restrict the flow of fluid to increase or decrease the force. The valve could be present in the factory or controlled according to crash conditions or occupant size or position. A pressure sensor 16 may be used to collect data to control the valve. In the example of figure 7 the inside of the cylinder 1 is profiled to give a predetermined rate of webbing extension for a given load. In parts, the cylinder widens allowing fluid to leak past the piston: the size of the leakage area controls the force. For example the cross-sectional area of the cylinder increases with increasing belt extension at the beginning of the stroke but decreases towards the end of the stroke. The particular profile will be chosen to suit the load limiting characteristics, the expected crash pulse and the reaction of the particular safety restraint involved. The piston 3 also has an internal tapering profile to give additional load limiting near the end of the stroke as the last bit of fluid 7 is trapped and compressed. The internal conical profile of piston 3 fits over the externally tapering portion 12 at the other end of the cylinder 1. The piston is biased by spring 11 to its start position as shown in figure 7. After the initial extension of the webbing, because of the expanding cross-sectional profile of the cylinder 1, there is an annulus around the periphery of the piston 3 and the fluid 7 is forced through this annulus thus absorbing some of the energy allowing webbing extension and limiting the load on the vehicle occupant. Figure 8 shows an example in which the cable 5 is profiled to provide the appropriate load limiting characteristics. In the rest position no exhaust port exists between the cable and the end of the cylinder 1. As the piston moves in the direction E the profiled cable 5 (which could also be a solid rod or shaft) first decreases in cross-section and therefore effectively opens up an exhaust port 13. This allows outflow of fluid 7 which absorbs some of the energy of the crash and limits the load felt by the occupant. The actual shape of the cable or rod 5 is chosen to provide the appropriate load limiting characteristics. A spring (not shown) may be used in the cylinder 1 to bias the piston 3 to the webbing retracted position. A fluid return pipe may be used to channel fluid from one side of the piston to the other to provide load limiting without leakage of fluid out of the cylinder. In this case a variable diameter seal will be provided at the external wall of the cylinder to compensate for the profiling on the cable. Figure 9 illustrates another example wherein a ball valve 14 biased by a spring is used to control exhaust vents 13. The piston 3 is biased in its rest position by spring 11 inside cylinder 1 and the fluid 7 is on the spring side of the piston. A vent 15 is provided on the other side of the piston 3 to avoid a vacuum being created. A sensor 16 is incorporated to sense the pressure of the fluid 7 and control the ball valve 14 accordingly. The piston 3 as shown in figures 7 to 9 is not in itself within the scope of the invention.

Claims: A vehicle safety restraint comprising: seat belt webbing (6) flor restraining a vehicle occupant, the webbing (6) being attached to a structural part of the vehicle via a load limiting device, the load limiting device comprising: a container (1) defining a flow path for a fluid, an actuator (3) arranged within the container (1) and operable to act on the fluid, and a restriction in the flow path of the fluid, wherein the actuator (3) is movable within the container (1) under the action of load transmitted to the webbing (6) from the vehicle occupant under extreme acceleration or deceleration conditions, to move the fluid through the restriction in the flow path thereby to dissipate a predetermined portion of the load on the webbing (6), the actuator comprising a piston (3) and the container comprises a cylinder (1), and one of the piston (3) and the cylinder (1) is attached to an end of the belt webbing (6) and the other one of the piston and the cylinder is attached to the structural part of the vehicle, the piston (3) being arranged so that the cross sectional area which it presents to the fluid is variable, and wherein the piston (3) is formed of a core piston (3a) and a plurality of concentric tubes (3b, 3c, 3d); characterised in that said concentric tubes (3b, 3c, 3d) are dropped by the core piston (3a) as it travels down the cylinder (1) so as to decrease the restraining effect of the restraint as the crash pulse progresses. A vehicle safety restraint according to claim 1 wherein the restriction comprises one or more holes in the actuator so that under pressure fluid is forced from one part of the container, on the high pressure side of the piston, to the other part of the container, on the low pressure side of the piston. A vehicle safety restraint according to claim 1 wherein the restriction comprises a feedback pipe (10) external to the container to feed fluid under pressure from the high pressure side of the piston (3) to the low pressure side of the piston. A vehicle safety restraint according to claim 3 wherein the feedback pipe (10) comprises a restriction (9). A vehicle safety restraint according to claim 4 wherein the feedback pipe restriction comprises a valve (9). A vehicle safety restraint according to claim 5 wherein the valve (9) is adjustable to offer a predetermined resistance to fluid flow in dependence upon the force damping characteristics required for the seat belt. A vehicle safety restraint according to claim 5 or claim 6 wherein the valve (9) is actively controllable. A vehicle safety restraint according to claim 7 comprising a gauge arranged to monitor the pressure of the fluid in the cylinder and connected to the valve to control the valve in dependence upon the monitored pressure. A vehicle safety restraint according to claim 7 or claim 8 wherein the valve (9) is connected to be controlled in dependence upon any one or any combination of features taken from the list of: the type of vehicle; the severity of the crash pulse; the weight of the occupant; the activation of a secondary restraint; and the occurance of a second crash.

Inventors: BLACKADDER DAVID, GORDON IAN ALEXANDER, HARTE JOSEPH PATRICK, PARK ANDREW, REES ELIZABETH, SMITHSON ALAN GEORGE, TAYLOR JOHN, BLACKADDER, DAVID, GORDON, IAN ALEXANDER, HARTE, JOSEPH PATRICK, PARK, ANDREW, REES, ELIZABETH, SMITHSON, ALAN GEORGE, TAYLOR, JOHN
Applicants: BREED AUTOMOTIVE TECH, BREED AUTOMOTIVE TECHNOLOGY, INC.
Application year: 1998
Main classification: B60R22
IPC codes: B60R22