WO2012126608A1

WO2012126608A1 - Knee airbag module with a two-stage inflating unit

Info

Publication number: WO2012126608A1
Application number: PCT/EP2012/001218
Authority: WO
Inventors: Thomas Reiter; Nico Wallat
Original assignee: Autoliv Development AB
Current assignee: Autoliv Development AB
Priority date: 2011-03-22
Filing date: 2012-03-20
Publication date: 2012-09-27
Anticipated expiration: 2013-09-22
Also published as: DE102011014650A1

Abstract

A knee airbag module comprising an inflating unit (14,16)and a knee airbag (12) is described. Said knee airbag module can be inflated by the inflating unit (14,16) between a lower area of a dashboard (40) and the knee-shin area of a vehicle occupant. In order to achieve good protection both for large, heavy occupants and for small and light occupants, the inflating unit (14,16) is formed as a two-stage system, wherein the second stage is ignitable with a time offset in respect to the first stage. The first stage is formed by a quick cold gas generator (14) that is dimensioned such that it can deploy the knee airbag completely. The second stage is a hot gas generator (16), usually a pyrotechnic gas generator.

Description

Knee Airbag Module with a Two-Stage Inflating Unit

Description

Technical field of the invention

The invention is concerned with a knee airbag module according to the preamble of claim 1 .

Knee airbag modules are becoming increasingly widespread in the automobile construction, and are used in particular in passenger cars with a very hard crash pulse. Such knee airbag modules always have a housing, a knee airbag folded into this housing and an inflating unit for this knee airbag. The housing is located in the dashboard or in an area of the ceiling of the foot well adjoining the dashboard and the knee airbag is designed so that, after being inflated by the inflating unit, it is located at least sectionally between a lower area of the dashboard and the knee-shin area of the vehicle occupant.

The inflated knee airbag of the knee airbag module has two tasks: firstly, it prevents the occupant's knee-shin area from hitting the hard dashboard, and secondly it antagonises a forward displacement of the pelvis and thus contributes to the overall restraining of the occupant. Because the occupant is coupled to the knee airbag at a very early stage, in particular much earlier than the impact on a passenger airbag, the knee airbag plays a very special role in occupant safety, because it has a great effect on the occupant's kinematics. The knee airbag is particularly important if the occupant is unbelted, but it can also make an important contribution to restraining a belted occupant gently and safely. If there is no knee airbag module and if the occupant is wearing a seatbelt, the belt is the sole means of restraint for the occupant in an initial stage of the restraining process and the seat belt has to restrain the occupant until the front airbag, for example the passenger front airbag, is fully deployed. To achieve this goal it is frequently necessary that the belt restrains the occupant in particular in the shoulder area with very great force, which can lead to injuries in the shoulder and chest area. A knee airbag that is inflated early and provides sufficient restraining force can re- duce the forces that act between the belt and the occupant at this early point in time, because forces from the upper body can be transmitted into the motor vehicle through the essentially horizontally stretched thighs and the knee airbag. It is therefore seen that the knee airbag can play an important role for both: for belted and for unbelted occupants.

Prior art

A problem is that if the restraining forces of the knee airbag were sufficient for a heavy man, for example a 95% man, the permissible thigh forces for a 5% woman would be exceeded, so that, at present, for use with a heavy man, in particular if he is unbelted, knee airbags have to remain behind what they could actually achieve for this constellation.

From generic DE 198 42 672 A1 a knee airbag module with two gas generators is known. The reasons for providing two gas generators are mainly of mechanical / geometrical nature. Further it is proposed that the gas generators are ignitable at the same time or with an offset. The ignition with a time offset, or the ignition of only one gas generator which does not fill the airbag completely is said to be of an advantage in case of an out of position event.

Summary of the invention

Starting from this, the present invention sets itself the problem of providing a knee airbag module that provides good protection for both: tall and heavy occupants and small and light occupants.

This problem is solved by a knee airbag module with the characteristics of claim 1. According to the invention, the inflating unit that is provided to inflate the knee air- bag comprises two stages, wherein the second stage is ignitable with a time offset with regard to the first stage. The two stages are of different types, namely such that the gas exhausted by the first stage is colder than the gas exhausted by the second stage. In praxis that usually means that the first stage is a so called cold gas generator or a hybrid inflator and the second stage is a pyrotechnic gas generator. It is of no relevance if the two gas generators share a common housing or are spaced apart from another. The cold gas generator or hybrid inflator fills the knee airbag very quickly with a specific pressure, which is chosen such that even in the case of a 5% woman it does not lead to an exceedance of the permissible thigh forces. The first stage is dimensioned such that its gas brings the knee air- bag to complete deployment, that means that it reaches - at least if it deploys unhindered without hitting an obstacle - its maximum depth and volume The depth is here defined as the distance between the impact wall and the support wall of the knee airbag. The pyrotechnic gas generator, which is ignited with a time offset and exhausts much slower hot gas, then increases the pressure through additional gas and by the gas in the knee airbag being warmed up. However, this is done preferably at a time at which a coupling between the occupant's knee-shin area and the impact surface of the knee airbag has already taken place, so that the load from the airbag itself is reduced. In spite of the great restraining force that the knee airbag can make available after being filled by both stages, the collision impulse of the impact surface is relatively low. Through the re-heating of the gas by means of the pyrotechnic gas generator a relatively long standing time is achieved for the knee airbag with a relatively constant pressure level, which is advantageous in many accident scenarios. In this way it can be achieved that the coupling of the knee-shin area to the knee airbag takes place at a very early stage - but not with too much force - and the maximum restraining force is built up with a time offset. In this way, the high force impulse that impacts the knee-shin area when the expanding airbag hits it is reduced and, in spite of this, an on the whole very high restraining force is made available that has the above-mentioned positive effects with a belted occupant and with an unbelted occupant. A further advantage of using the combination of a cold gas generator and a pyrotechnic gas generator is that in a temperature range between -35°C and 80°C the previously occurring performance fluctuations can be clearly reduced. The ignition offset between the first and the second stage is preferably maximum 15 ms, and further preferably between 5 ms and 15 ms.

In some load cases, the second stage is not required at all; this is the case in particular with a light occupant (5% woman), in particular if the latter is wearing a seatbelt. In this case, the ignition of the second stage does not bring any additional protection in many cases, but can mean an additional load without any benefit. In accordance with claim 6 it is therefore to be preferred that a controlling unit for triggering the inflating unit is provided, wherein the controlling unit ignites the second stage only if at least one additional condition is fulfilled in addition to the detection of a frontal or a laterally offset frontal collision. This additional condition may be in particular the detection of an unbelted occupant and/or the detection of an occupant whose weight exceeds a predetermined value. A controlling unit of this type can also preferably calculate the matching ignition offset between the two stages (insofar as they are both ignited) from the given parameters.

The invention will now be explained in detail by means of an embodiment with regard to the Figures. Wherein:

Brief description of the drawings

Figure 1 : shows a schematised longitudinal section through the passenger area of a passenger car,

Figure 2: shows what is shown in Figure 1 after the ignition of the first stage of the inflating unit, and

Figure 3: shows what is shown in Figure 2 and a passenger sitting on the passenger seat. Detailed description of preferred embodiments

The invention will now be described by means of the best and most elaborate embodiment. Following the description of this embodiment, simpler variants are referred to that work with fewer sensors and that can also supply satisfactory results for many cases of application.

Figure 1 shows schematically a vehicle seat 50, namely a passenger seat, and the dashboard 40 located in front of the vehicle seat 50. A housing 10 of a knee airbag module is arranged in a lower area of the dashboard 40 respectively in a transition area between dashboard 40 and the ceiling of the foot well. This is therefore a so-called "low mount" knee airbag module. However, it must be stated at this point that the present invention can also be used with knee airbag modules with other geometries. A knee airbag 12, which in case of an accident can be in- flated between a lower area of the dashboard 40 and the knee-shin area of an occupant seated on vehicle seat 50, is folded into the housing 10 .

As can be seen from Figure 2, the knee airbag 12 comprises an impact wall 12a facing towards the knee-shin area of the occupant, and a support wall 12b facing the lower area of the dashboard. The distance between the impact wall 12a and the support wall 12b is referred to as depth d.

To inflate the knee airbag there is an inflating unit that is controlled by a controlling unit 20 and that has two stages. In the preferred embodiment shown the two stage nature of the inflating unit is achieved by having two gas generators, namely a first gas generator 14 and a second gas generator 16. In the embodiment shown, the two gas generators 14, 16 are each formed as elongated cylinders, which are located within the housing 10. However, other geometries are possible as well. The first gas generator 14 is a cold gas generator and the second gas generator 16 is a pyrotechnic gas generator. The controlling unit 20 has two outputs, namely a first output 26 and a second output 27, wherein the first output 26 is assigned to the first gas generator 14 and the second output 27 to the second gas generator 16. In addition, the controlling unit 20 has a first input 21 , a second input 22 and a third input 23. The first input 21 is connected to a crash sensor 56, the second input 22 to a belt fastening sensor in a belt buckle 54 and the third input 23 to a weight sensor 52 in the seating surface of vehicle seat 50. The controlling unit 20 is shown here as a separate component; considered physically, it can also be a part of a vehicle ECU. Insofar, the term "controlling unit 20" is to be understood as a functional term.

The knee airbag module works as follows: if the controlling unit 20 receives a signal via the first input 21 that there has been a frontal collision or a laterally offset frontal collision, the controlling unit 20 ignites the first gas generator 14, that is the cold gas generator, via its first output 26 at the earliest possible time. Cold gas generators are very fast, so that there is a very early coupling between the occupant's knee-shin area and the knee airbag 12. The cold gas generator exhausts an amount of gas that is sufficient in order to deploy the knee airbag completely, such that it reaches its maximum volume and depth d (Figure 2 and 3). Depending on what the controlling unit 20 continues to receive through its inputs 21 , 22, 23 as information, it triggers the second gas generator 16 with a specific time offset, which, for example, can amount to 10 ms. An example of a decision-making matrix is given in the following table:

This decision-making matrix is to be understood only as an example, the values depend on the concrete circumstances of the vehicle. In addition, the decision- making matrix can always be designed to be much more differentiated and in par- ticular it is also possible that there is no fixed time offset, but that an ideal time offset is calculated from the given parameters, which can also contain the severity of the accident. In cases in which the occupant is wearing a seatbelt and the second gas generator 16 is not ignited, the restraint function of the seatbelt with which the occupant is restrained is only slightly supported and it is prevented that the occupant's knee-shin area hits the dashboard 40. Very large forces are not transmitted into the occupant's thigh area in this case. In cases in which the second gas generator 16 is ignited, the knee airbag takes over a significant part of the restraining work, so that correspondingly large forces pass into the thighs of the occupant who is to be protected.

As already mentioned, the force peaks are already avoided solely on the basis of the time-offset ignition of the two gas generators and because of their different characteristics, so that in some application cases it can be possible to do without weight sensor 52 and without the belt sensor in belt buckle 54, wherein the second gas generator 16 is then always ignited. Mixed forms between the preferred embodiment described above and what has just been said are also possible, for example in such a way that the second gas generator 16 is always ignited in case of an unbelted occupant or always if a defined weight of the occupant is ex- ceeded.

Reference list

10 Housing

12 Knee airbag

12a impact surface

12b support surface

14 First gas generator (cold gas generator)

16 Second gas generator (pyrotechnic gas generator)

20 Controlling unit

21 First input

22 Second input

23 Third input

26 First output

27 Second output

40 Dashboard

50 Vehicle seat

52 Weight sensor

54 Belt buckle

56 Crash sensor

d depth

Claims

1. Knee airbag module comprising an inflating unit and a knee airbag (12) being inflatable between a lower area of the dash board (40) and a knee- shin-area of a car occupant by the inflating unit, said knee airbag having an impact wall and a support wall, the distance between the impact wall and the support wall defining the depth of the knee airbag, wherein the inflating unit has two stages with the second stage being ignitable with a time offset in respect to the first stage,

characterized in that the gas exhausted by the first stage is colder than the gas exhausted by the second stage and in that the knee airbag (12) reaches its maximum depth (d) and volume, if only the first stage of the inflating unit is ignited.

2. Knee airbag module according to claim 1 , characterized in that the inflating unit is a two stage gas generator.

3. Knee airbag module according to claim 1 , characterized in that the inflating unit is comprised of two gas generators, such that the first stage is formed by a first gas generator (14), namely a cold gas generator or a hybrid infla- tor, and the second stage is formed by a second gas generator (16), namely a pyrotechnic gas generator.

4. Knee airbag module according to one of the preceding claims, character- ized in that the time offset amounts up to 15 ms.

5. Knee airbag module according to claim 4, characterized in that the time offset is between 5 and 15 ms.

6. Knee airbag module according to one of the preceding claims, characterized in that a controlling unit (20) for controlling the inflating unit is provided, wherein the controlling unit (20) ignites the second stage only, if addition- ally to the detection of a frontal or a laterally offset frontal crash at least one additional condition is given.

Knee airbag unit according to claim 6, characterized in that the additional condition is the detection of an unbelted occupant.

Knee airbag according to claim 6 or claim 7, characterized in that the additional condition is the detection of an occupant whose size and/or weight exceeds a predetermined value.