WO1990009908A1

WO1990009908A1 - Inflatable air bag with pressure control valve

Info

Publication number: WO1990009908A1
Application number: PCT/US1990/000891
Authority: WO
Inventors: David S. Breed; Denny Meredith; Anthony S. Pruszenski, Jr.
Original assignee: Automotive Technologies International, Inc.
Priority date: 1989-02-23
Filing date: 1990-02-21
Publication date: 1990-09-07

Abstract

Designs to air bags (2, 45) are disclosed which allows high pressure air to vent from the air bag (2, 45) but maintains a sufficient pressure in the bag to protest an occupant in a secondary collision. A variable orifice design (6, 30, 71, 91) and viscous flow-restricting devices are disclosed to provide for air bag venting. A multi-compartment air bag (45) of which the periphery (41) is stiffer than the center (43) to reduce the tendency of occupants to slide off the air bag (45) is also disclosed.

Description

Inflatable Air Bag with Pressure Control Valve

BACKGROUND OF THE INVENTION Inflatable restraints commonly known as air bags are being adopted in increasing numbers by the world's automobile manufacturers. Industry experts expect that sometime during the 1990s all newly manufactured cars and most trucks will be equipped with inflatable restraints. Although many lives have already been saved and numerous injuries mitigated the inflatable restraint has changed little since approximately 10,000 vehicles were manufactured by General Motors in the early 1970s.

Although significant improvements have been made to inflators or gas generators and crash sensors there has been little or no improvement to the gas bag itself. The driver's side bag, for example remains a relatively crude device created by joining two pieces of heavy neoprene coated nylon together.

Gas bags contain orifices or holes for exhausting the gas. Thus, typically within one second after the bag is inflated the gas has been completely exhausted from the bag through the vent holes. This imposes several limitations on the restraint system. Take for example the case where an occupant is wearing a safety belt and has a marginal accident such as hitting a small tree which is sufficient to deploy the air bag but it is not really needed since the driver is being restrained by his safety belt. If the driver has lost control of the car and is traveling at 30 MPH, for example, and has a secondary impact one second or about 50 feet later, this time with a large tree, the air bag will have become completely deflated and would not be available to protect the occupant in this secondary life threatening impact.

In other situations, the occupant might be involved in an accident which exceeds the design capability of the restraint system. These systems are typically designed to protect an average size male occupant in a 30 MPH barrier impact. At higher velocities the maximum chest deceleration experienced by the occupant can exceed 60 G's and become life threatening. This is particularly a problem in smaller vehicles where air bag systems typically only marginally meet the 60 G maximum requirement or with larger or more frail occupants.

There are many cases particularly in marginal crashes where existing crash sensors will cause the air bag to deploy late in the crash. This can result in an out-of-position occupant. Other cases of out-of-position occupants are standing children or the forward motion of occupants during panic breaking prior to impact especially when they are not wearing safety belts. The deploying air bag in this situation can cause injury to the out-of-position occupant.

The air bag must be available to protect an occupant for at least the first 100 - 200 milliseconds of the crash. Since the air bag contains large vents the inflators must continue to supply gas to the air bag to replace the gas flowing out of the vents. As a result, inflators are usually designed to produce about twice as much gas than is needed to fill the air bag. This, of course, increases the cost of the air bag system as well as its size, weight and total amount of contaminants, which are exhausted into the automobile environment.

Current air bags contain a single chamber, which is filled with the gas produced by the gas generator. Thus, the entire air bag has approximately the same stiffness. This can result in an occupant sliding off the airbag in an oblique impact. It is desirable for the outer perimeter of the airbag to have a higher stiffness than the interior to deflect the occupant towards the center softer portion of the airbag.

It is the purpose of the invention described herein to substantially solve the limitations listed above.

SUMMARY OF THE INVENTION it is a principal object of this invention to provide an air bag design which will be available in the event of multiple impacts where the air bag is not fully utilized during the initial impact.

It is another object of this invention to provide an air bag which will automatically adjust to limit the maximum force transmitted to the occupant. It is a further object of this invention to retain the gas in the air bag for a substantial period of time until it is impacted by an occupant.

It is another object of this invention to provide an air bag design which minimizes the total gas required from an inflator.

It is another object of this invention to provide an air bag system design which minimizes the total size and weight of the inflator.

It is an additional object of this invention to minimize the total amount of gas and contaminants produced by all of the inflators in the vehicle.

An additional object of this invention is to reduce the injury potential to an out-of-position occupant from the deploying air bag.

It is a further object of this invention to provide an airbag which is stiffer at its periphery than at its center so as to reduce the tendency of occupants to slid off the airbag.

Further objects and advantages of this invention will become obvious with reference to the detailed discussion of the preferred embodiments described below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of an inflated driver's side air bag shown in conjunction with the steering wheel and containing a variable orifice according to the teachings of this invention.

FIG. 2 is an expanded view of a variable orifice from FIG. 1.

FIG. 3 is a cross sectional view of the variable orifice of FIG. 1.

FIG. 4 is a cross sectional view of another design of a variable orifice. FIG. 5 shows a plot of the chest acceleration of an occupant and the occupant motion using a conventional air bag.

FIG. 6 shows the chest acceleration of an occupant and the resulting occupant motion when the variable orifice of this invention is utilized.

FIG. 7 is a variable orifice controlled by the fabric tension of air bag material.

FIG. 8 shows a composite flap valve construction.

FIG. 9 is a composite flap valve in its open position.

FIG. 10 shows a driver's side compartmentalized air bag.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 illustrates generally a driver side air bag system shown in the inflated condition containing a variable orifice of this invention. An air bag 2 is shown supported by steering wheel rim 3 and steering wheel hub 4. Variable orifice 6 is attached to the air bag material 8 as described below.

FIG. 2 is an enlarged view of the variable orifice 6 of FIG. 1 showing the attachment to the air bag material with most of the material shown cut away.

FIG. 3 is a cut away view of the variable orifice and bag material shown in FIG. 2 looking along line AA. An upper plastic valve seat 21 is riveted to a lower plastic member 22 by means of a plurality of rivets 23. A valve member 25 is designed to seal on valve seat 26 which is part of rim 21. Valve 25 is held against valve seat 26 by means of spring 27 and spring retainer post 28.

When the bag is deployed pressure builds in the bag as a result of the burning of a propeilant in the gas generator. When the bag is completely filled with gas, the pressure continues to rise until the inflator propeilant is exhausted or the pressure within the bag reaches a value such that the force from the gas on valve 25 is sufficient to overcome the spring force from spring 27 and open the valve 25. This permits gas to escape from the air bag and maintains a constant pressure in the air bag.

FIG. 4 shows a cross-sectional view of another pressure controlled valve. Flap valve 30 is hinged to lower plate 33. Torque spring 31 exerts a biasing force on valve 30 and prevents valve 30 from opening before the pressure inside an air bag rises to a designed value. The mechanism is covered by upper cap 32, which is attached to lower plate 33 by a plurality of rivets 34 with air bag material 35 clamped on the periphery of lower plate 33. If the pressure inside an air bag is high enough, flap valve 30 opens and gas is released from the air bag into atmosphere through exit ports located around the rim of upper cap 32. The arrows in FIG. 4 show the direction of gas flow.

FIG. 5 shows a typical chest G pulse experienced by an occupant and the resulting occupant motion when impacting an air bag during a 35 MPH frontal impact in a small vehicle. When the variable orifice air bag is used in place of the conventional air bag the chest acceleration curve is limited and takes the shape similar to a simulation result shown in FIG. 6. Since it is believed to be the magnitude of the chest acceleration that injures the occupant, the injury potential of the air bag in FIG. 6 is substantially less than that of FIG. 5.

Since the variable orifice remains closed as long as the pressure in the air bag remains below the set value, the inflator need only produce sufficient gas to fill the air bag once. This is approximately half of a gas which is currently produced by standard inflators. Thus the use of a variable orifice significantly reduces the total gas requirement and therefore the size, cost and weight of the inflator. Similarly, since the total amount of gas produced by all inflators in the vehicle is cut approximately in half, the total amount of contaminants and irritants is similarly reduced or alternately each inflator used with the variable orifice air bag is now permitted to be somewhat dirtier than current inflators without exceeding the total quantity of contaminants in the environment. This in turn, permits the inflator to be operated with less filtering, thus further reducing the size and cost of the inflator.

The characteristics of inflators varies significantly with temperature. Thus, the mass flow rate of gas into the air bag is a significant function of the temperature of the inflator. inflator. In conventional fixed orifice air bags, the gas begins flowing out of the air bag as soon as positive pressure is achieved. Thus, the average pressure in the air bag similarly varies significantly with temperature. The use of a variable orifice system as taught by this invention however, permits the bags to be inflated to the same pressure regardless of the temperature of the inflator. Thus, the air bag system will perform essentially the same whether operated at cold or hot temperature, removing one of the most significant variables in air bag performance. The air bag of this invention provides a system which will function essentially the same at cold and hot temperatures.

The variable orifice air bag also solves the dual impact problem where the first impact is sufficient to trigger the crash sensors in a marginal crash where the occupant is wearing a safety belt and does not interact with the air bag. Later in a subsequent more serious accident, the air bag will still be available to protect the occupant. In conventional air bags using a fixed orifice the gas generator may have stopped producing gas and the air bag may have become deflated.

Since the total area available for exhausting gas from the air bag can be substantially larger in the variable orifice air bag, a certain amount of protection for the out of position occupant is achieved. If the occupant is close to the air bag when it deploys, the pressure will begin to build rapidly in the air bag. Since there is insufficient time for the gas to be exhausted through the fixed orifices, this high pressure results in high accelerations on the occupants chest and can cause injury. In the variable orifice case, however, the pressure will reach a certain maximum in the air bag and then the valve would open to exhaust the gas as fast as the gas generator is pumping gas into the air bag. Thus maintaining a constant and lower pressure than in the former case. Naturally, the bag must be sufficiently deployed for the valve to be uncovered so that it can operate.

Many geometries can be used to achieve a variable orifice in an air bag. These include very crude systems such as slits placed in the bag in place of round exhaust vents, rubber patches containing one or more holes which are sewn into the bag such that the hole diameter gets larger as the rubber stretches in response to pressure in the bag, plus a whole variety of flapper valves similar to the two disclosed in this invention. Slit systems so far have not worked well in experiments and rubber patches are affected by temperature and thus are suitable only for very crude systems. Similarly, the bag itself could be made from a knitted material, which has the property that its porosity is a function of the pressure in the bag. Thus, once again, the total amount of gas flowing out of the bag becomes a function of the pressure in the bag.

Although we have illustrated the case where the pressure is essentially maintained constant in the bag through the opening of a valve, it is possible that for some applications a different function of the pressure in the bag may be desirable. Thus a combination of a fixed orifice and flapper valve might be desirable.

Besides the above embodiments of controlling pressure in the air bag, the opening area of an orifice can also be controlled by the fabric tension in the air bag material. FIG. 7 illustrates an elastic clip mounted on an air bag. When the pressure inside the air bag rises, the tension of the air bag fabric 72 will increase. This tension force will open the clip 71. The higher the pressure, the wider the clip will open. Therefore, the opening area of the orifice is regulated by the pressure inside the air bag.

FIG. 8 shows the construction of a composite layer, made of a center layer 81 , one or more layers of fibers 82, and outer layers 83. These fibers are aligned with the deflection axis, and they are separated by a thermoplastic or other compliant matrix. A suitable polymer would be flexible over extreme temperature regimes and bondable to the fibers and fabric. Urethane and silicon are both able to meet these requirements. It is possible to use a fairly stiff matrix material between upper and lower fibers and a softer material as the outer most layer. This hybrid construction will synergisticly give a better seal with lower flexural stresses.

FIG. 9 shows another pressure regulating valve made of the composite layer in FIG. 8. The valve is constructed by laminating carbon or other stable fibers, indicated by lines 91 in FIG. 9, into a compliant matrix with good extensional limits. The matrix is formed into small bags 92 and 93 with exit orifices 94 which are open as the bag valve is inflated. The valve assembly 90 is attached to the air bag material over two orifices. The opening of bags 92 and 93 are constrained by side limiters 95. In the closed position, the two sides are designed to fold into the exit orifices. Note that the fibers 91 are in cross directions, which stiffens the symmetrical cantilever attachment and distributes the load into the substrate.

The regulating bags 92 and 93 are initially closed by the spring formed by the fibers. When the pressure exceeds the design lower limit, then the orifices are opened. These orifices increase in size and/or numbers as the pressure increases and allow more gas to escape. At maximum design flow the orifices are designed to be of sufficient area to drop the upper pressure limit across the valve.

The composite shown in FIG. 8 is composed of 0.004 inch diameter graphite fibers 82 with an elastic modiolus of 40 million psi. The center layer 81 is 0.003 inch polyester or acrylic sheet. The center layer could be a Urethane. The outer layers are heat bondable Urethane sheet or a suitable thermoplastic such as acrylic or CAB. This system has a calculated unit E1 product of 1.7 pound*inch² The beam is calculated to deflect about 2.4 inches at 2 psi differential. This is the main reason to have the side limiters. It is possible to design a suffer system without side limiters by increasing the number of layers in the flap construction. Also, the system will stiffen by the fourth power of the beam length. In other words, if the beam length is reduced from 2 inches to 1 inch, it will give a 16 fold reduction in deflection. This calculates a deflection of approximately 0.15 inch at 2 psi.

The purpose of adjusting the opening area of an air bag vent hole is to control the gas flow rate out of the vent hole according to the pressure inside the gas bag. If the pressure is higher, then the area of the vent hole becomes larger and allows more gas to flow out. By regulating the pressure inside an air bag, the force applied on an occupant is minimized.

The flow through the vent hole in an air bag is proportional to the square root of the pressure difference between the inside and outside of the air bag, because the gas flow is of inertial type. The same object of adjusting the flow rate by a variable orifice area can be largely achieved by making the flow rate more responsive to the pressure difference. For example, if the flow through an orifice is in the viscous domain, then the flow rate is proportional to the pressure difference. Thus the flow rate of a viscous flow is a stronger function of the pressure difference than an inertial flow through an orifice. Practically, this can be accomplished by installing a viscous flow-restricting device.

Since viscosity is a function of temperature, the flow rate of a viscous flow will depend on the temperature of gas. However, gas exiting from an inflator into an air bag is generally in the vicinity of 750 F. Due to this high temperature, the gas temperature is not greatly affected by the ambient temperature in the passenger compartment. Even if the temperature does change, the pressure also changes and since the viscosity of gases decreases with decreasing temperature and since the pressure in the bag will also decrease with temperature, the two effects partially cancel each other. Based on this observation, the viscous flow device can be designed to operate without significant variation caused by the temperature changes.

The flow-restricting device can be constructed by a slice cut from a cluster of very fine tubes, a porous restrictor, or a piece of porous material or filter material. Such devices allow gas to flow through many fine openings instead of a single vent hole. If a considerable thickness is used relative to the diameter of the tubes, the flow rate through these devices is controlled by the viscosity of the gas. A critical parameter in these devices to achieve the viscous effects is the equivalent flow length/diameter ratio. This parameter is a function of many factors including the gas viscosity, the gas temperature, the equivalent opening diameter, and the pressure difference, and thus will vary for different designs.

It is not trivial to select an appropriate diameter for the purpose of this invention. Because the diameter will determine the Reynolds number of the flow, which will decide the characterizes of the flow and the needed length/diameter ratio. Moreover, the vent area to achieve the required flow rate for evacuating an air bag is a function of the diameter and the length. An analysis of the gas flow, using typical parameters of an air bag, is as follows:

In an air bag design, the following parameters will be calculated or estimated:

Po = initial gas pressure in bag

Pa = atmospheric pressure

R = gas constant

To = initial temperature in bag

V = bag volume

v = gas velocity

γ = specific heat ratio

μ = gas viscosity

t = bag natural evacuation time For flow through a circular passage, including entrance and exit effects

(Huπsaker & Rightmire,"Engineering Applications of Fluid Mechanics," McGraw-Hill, 1947, pp. 132.)

Thus, the ratio of the v² to v pressure drops is

where Re = pvd/μ is the Reynolds number. If this ratio is to be held to 10%, that is, the inertial effects and small compared to the viscous effects, then the desired length to diameter ratio of the flow passage is

Then

For estimation purposes, consider an air bag deflating without impact; the bag volume V is approximately constant. During deflation,

where m is the mass of gas in the bag. Now

so

where A is the exit flow area, and

This can be integrated to give

If in time t the difference P - Pa is reduced to 10% of its initial value, then

Also, then

To insure laminar flow, the Reynolds number must be less than about 2,000.

The maximum Re occurs initially:

When this is combined with Eq. (1 ),

and

Then for each hole diameter d, the minimum L/d is determined from (4), and Re max is checked. Some L/d greater than this is chosen, and the total exhaust area A is found from (2).

Example:

Po = 20 psi Pa = 15 psi

R = (497)2 m² /S² Rº (air) To = 1,200 R° V = 4,000 in³

γ = 1.4 μ = 5 x 10⁹ psi sec (air at 1200 R)

t = 0.05 sec

( L/d)min = 12,250 d Re max <= 34,400 d

d

(a) d = 0.01", L min = 1.23"

(b) d = 0.0001", L min = 0.000123", A = 125,000 L

(c) d = 10^-6 in, L min = 1.23 x10^-8 in, A = 1.25 x 10⁹ L

(d) d = 0.001", L min = 0.0123", A = 1 ,250 L

The above analysis indicates that the equivalent diameter can be in the range of 0.0001 to 0.001 inch, the thickness (or length) 0.0001 to 0.01 inch, and the total vent area is 15 to 250 square inches. (These parameters are interrelated.)

FIG. 10 is a sketch of a multi-compartment air bag 45. There are a multiplicity of orifices 42 between a central chamber 43 and a neighboring outside chamber 41. The orifices can be just openings on the material separating the chambers or they can be constructed of a different material that allows gas to flow through. The gas generated by inflator 40 enters the outside chamber 41 first and then flow through orifices 42 into central chamber 43 to a vent hole 44. Due to the direction of the gas flow shown by arrows in the figure, the gas pressure in the peripheral chamber 41 remains higher than that in the central chamber 43. Consequently, the peripheral portion of the bag is stiffer than the central part to reduce the possibility of driver sliding off the airbag.

Although some preferred embodiment have been disclosed and described in detail herein, it should be understood that this invention is in no sense limited thereby and its scope is to be determined by that of the appended claims.

Claims

CLAIMS 1 claim:

1. Inflatable occupant restraint apparatus for protecting an occupant in a vehicle crash comprising:

(a) an inflator;

(b) a gas bag in communication with said inflator, adapted to be filled with the gas from said inflator;

(c) an exhaust vent, in said gas bag;

(d) means to vary the effective cross section area of said vent in response to pressure from within said gas bag.

2. The invention in accordance with claim 1 , wherein said responsive means comprises a spring loaded valve.

3. The invention in accordance with claim 2, wherein said valve remains closed until a predetermined pressure is exceeded.

4. The invention in accordance with claim 1 , wherein said responsive means comprises an elastic clip, which controls said vent area accordingly with the pressure in said gas bag.

5. The invention in accordance with claim 1 wherein said responsive means comprises composite material flaps, in which fibers deflect and adjust said vent area accordingly with the pressure in said gas bag.

6. The invention in accordance with claim 1 wherein said composite material is formed into a bag valve, which remains closed until a designed pressure is exceeded.

7. An inflatable occupant restraint apparatus comprising:

(a) an inflator;

(b) a gas bag adapted to be mounted to said inflator;

(c) a vent in said gas bag;

(d) means for preventing said vent from opening until a predetermined pressure is achieved in said gas bag; whereby said gas bag compensates for the variability in inflator performance as a function of temperature.

8. An inflatable occupant restraint apparatus comprising:

(a) an inflator;

(b) a gas bag with a multiplicity of compartments, adapted to be mounted to said inflator;

(c) means for controlling gas flow from said inflator to said compartments such that the peripheral portion of said gas bag is stiffer than the central portion; whereby said means reduces the occupant's tendency to slide off said gas bag.

9. Inflatable restraint apparatus for protecting an occupant in a vehicle crash comprising:

(a) an inflator;

(b) a gas bag in communication with said inflator, adapted to be filled with the gas from said inflator:

(c) at least one exhaust vent in said gas bag;

(d) means of providing viscous effects on the gas flow through said exhaust vent.

10. The invention in accordance with claim 9, wherein said means of providing said viscous effects comprises a porous restrictor.

11. The invention in Accordance with claim 9, wherein said means of providing said viscous effects comprises a piece of porous fabric covering or comprising said exhaust vent.

12. The invention in accordance with claim 9, wherein said means of providing said viscous effects comprises a plurality of fine tubes.