Simcenter Madymo - Scalable passenger airbag model

The passenger airbag model consists of a rectangular container made of MB surfaces, a fabric envelop made of 2D membrane elements, internal straps made of 2D membrane elements for the connections to the fabric envelop and 1D strap elements in between. The airbag cushion is inflated by a user-defined gas generator. The whole airbag model is attached to a rigid body called AirbagMount_bod, itself mounted on a translation joint to allow deployment above its theoretical mounting location to avoid initial intersection with the dashboard for example at T0 (see section Initial settings below). The position of the MB surfaces modelling the module are automatically adjusted depending from the design parameters chosen and do not require any user action; a default gap of 2mm (not adjustable) guarantees a constant gap between the FE mesh and the MB surfaces of the container. The entire model is defined in a single include file called PAB_inc.xml and its position as well as its design are controlled by DEFINE variables.

 

Fig. 1 - Passenger airbag model

 

Positioning parameters

The airbag can be attached to any existing rigid body in the user's input deck and positioned using the positioning parameters listed below. The airbag mount positioning parameters position the center of the rear upper edge of the module part of the airbag at the junction with the deformable part of the airbag mesh (see fig. 3) in the local coordinate system of the attachment body. The fabric element size is about 15mm with the airbag sized to its default value (500mm width, 640mm high and 600mm length).

 

Fig. 2 - Positioning parameters

 

Design parameters

Many design parameters are made available to the user so that the inflated shape of the airbag can be adjusted to a specific interior by means of scaling parameters in term of length (x-direction), width (y-direction) and height (z-direction). In the vertical direction, two different scaling parameters are available to independently adjust the height of the airbag above and below the XY reference plane - defined as the plane passing through the top corners of the airbag module part (see below). Seen from the top, the airbag has a usual trapezoidal shape. When scaled to its desired inflated shape or scaled to its initial shape, the module including both venting holes are not affected by both scaling types; only the depth of the module (Z-direction only) will be scaled depending from the value defined by the user for the so-called dead volume (see below the list of design parameters). The figure below shows the scaling directions as well as the XY reference plane and the origin of the airbag mesh.

Fig. 3 - Parametrizable driver airbag

 

All the available design parameters are listed below:

• AB_width; it defines the overall width of the airbag measured between the most outwards points in the Y-direction of the mesh's local coordinate system. The width at its rearmost point (edge shared with the module part) is fixed to 190mm.
• AB_length; it defines the overall length of the airbag measured between the foremost and the rearmost point in the X-direction of the mesh's local coordinate system.
• AB_heighttop; this parameter defines the height of the airbag above the XY reference plane.
• AB_heightbottom; this parameter defines the height of the airbag below the XY reference plane.

 

Fig. 4 - Examples of designs shown in reference state

 

Note: The four design parameters listed above define the overall size of the airbag and thus its volume. In its default size (AB_width=500, AB_length=600, AB_heighttop=300 and AB_heightbottom_340), the volume of the airbag chamber with untensioned fabric is about to 92 litres. Since the volume of the airbag cannot be easily calculated, the user can perform a null run with the parameter AB_iniscale=1.0 and check the volume of the airbag chamber at T0 in the .fhs file.

 

• AB_upperstraplength; this defines the length of the internal upper strap - measured at Y0 of the airbag's local coordinate system, connecting the rear edge of the module to the upper part of the airbag cushion. This parameter is used in the LENGTH attribute of the 1D elements and is automatically calculated taking into account the length of both (scaled) 2D strap elements.
• AB_lowerstraplength; this defines the length of the internal lower strap, measured at Y0 of the airbag's local coordinate system, connecting the front edge of the module to the lower part of the airbag cushion. This parameter is assigned to the LENGTH attribute of the 1D elements and is automatically calculated taking into account the length of both (scaled) 2D strap elements.

 

Fig. 5 - 1D/2D strap elements inside the airbag chamber

 

• AB_modulevol; it represents the dead volume of the airbag chamber which includes for instance the space around the gas inflator but it does not include the space occupied by the scaled part of the airbag; a quick check can be performed by performing a null run to get the volume of the airbag chamber at T0. This parameter will basically affect the depth of the module (Z-direction) but not in the X- and Y- directions, meaning that the size of the vent holes modelled at the bottom of the module will remain unaffected by the scaling*. In its reference state, the volume of the module is 931cc (or 7x7x19cm).

 

Note here that this is a single chamber airbag model, using the uniform pressure method for thermodynamics calculations. This means that the actual position of the vent hole(s) is not critical in itself.

 

Fig. 6 - Module and vent holes

 

• AB_ventdia; this defines the diameter of each circular vent holes (2), normally cut in the fabric to allow venting of the inflated gas. The vent hole elements being however defined on the module part of the airbag (not scaled), the vent hole has a constant size over time. To prevent immediate venting after trigger of the inflator, the user can tune the two next parameters, which are linked to the CDT function of the MATERIAL.HOLE definition (outflow scaling vs. time function).
• AB_exhauststartdelay; this defines the duration after trigger as of when the outflow through the vent hole is enabled.
• AB_fullexhaustduration; this defines the duration of the ramp up from start of exhaust to full exhaust (linear behavior assumed between AB_exhauststartdelay and AB_fullexhaustduration).

Alternatively, the outflow through the vent holes can be controlled by over-pressure using the two next parameters AB_ventdpex and AB_ventdtex, for instance to model a membrane tearing at a certain over-pressure. In that case, use very small values for AB_exhauststartdelay and AB_fullexhaustduration (the values must be strictly greater than zero), so that the function equals to 1 as soon as the airbag is triggered. The outflow will nevertheless start as soon as the below mentioned conditions are satisfied.

 

• AB_ventdpex; over-pressure condition to start outflow through the vent holes. The default value is 0.
• AB_ventdtex; over-pressure duration condition to start outflow through the vent holes after the over-pressure condition above has been satisfied. The default value is 0.
• AB_leakagedia; this defines the diameter of a fictive circular vent hole through which outflow of gas leakage through sews, inflator etc. is possible. Contrary to the regular vent holes cut in the fabric, the outflow is enabled as soon as the relative pressure in the airbag chamber exceeds AB_leakagedpex.
• AB_leakagedpex; over-pressure condition to start outflow for leakage. Set value to 0 to activate outflow all the time.
• AB_jetori; this defines the orientation around the Y-axis of the jet. The origin of the jet is located at the middle of the volume formed by the module (after scaling). The jet rectangular base has a width of 150mm and length of 30mm.
• AB_iniscale; this parameter is automatically used to scale down the airbag to its initial state since this airbag model uses the IMM2 method i.e. used for scaled and not folded airbags.
• AB_inflatedgasmass; this represents the total gas mass inflated into the airbag chamber. This parameter effectively applies a scaling factor in Y on the default mass flow rate vs. time function.
• AB_perm; permeability constant factor of airbag fabric used in permeability model 1.

 

Fig. 7 - Examples of scaling for initial state 

 

Note: excessive downscaling can lead to instabilities as soon as the airbag is triggered - i.e. when it switches from the rigid to the deformable state, if the self-contact is switched ON (see below).

 

Initial settings

The initial setting parameters available are:

• AB_ttf; this defines when the airbag inflator is triggered.
• AB_selfcnt; by default, the value is set to ON and it is recommended to keep it, despite the additional computation efforts required by this contact. The user is however given the option to switch it OFF to speed up the computation time.
• AB_initialzoffset; even if the size of the MB airbag container is automatically adjusted, it may be necessary to have the airbag container located above its theoretical mounting position in the dashboard to prevent possible initial intersections of the airbag fabric mesh and the dashboard surface. This parameter allows to start the inflation process above the position of the theoretical mounting location position of the airbag. The container and the supported part of the airbag are then moved to its final position using a linear profile for which the user can specify the times at which the motion starts and the time it requires to get to its final position.
• AB_motionstart; it defines how long after trigger when the container start to move towards its final position.
• AB_motionduration; this parameter defines the time needed to move from the initial position to its final position.

 

To check the initial and end positions of the airbag and its container, the user can play the animation within the XMADgic viewer. If no initial offset is needed, set the parameter AB_initialzoffset to 0. The example below shows a user-case when the scaled airbag interferes with the dashboard at T0. Defining a vertical offset allows to position the airbag higher and remove any initial intersections at T0. Later during the deployment, the module is progressively moved to its final position inside the dashboard.

 

Fig. 8 - Example of interferences with the dashboard without vertical offset at T0

 

Please contact your local Madymo Support organization for further questions/details on this topic.