Simcenter Madymo Simcenter Madymo - Buckle pretensioner modelling using joint restraint

The demonstrator consists of a sled set-up with a rigid bench and footrest, an occupant (Hybrid-III 50th percentile male dummy) restrained by a standard 3-point seat belt, a simplified seat belt retractor (with load limiter only) and the buckle pretensioner. The pretensioner is modelled as a preloaded non-linear spring acting on a translational joint which is released at the desired time during the simulation. The non-linear spring is characterized with a force-stroke function that is parametrized for the user to easily adjust maximal belt pretensioning force and maximal pull-in stroke.

Fig. 1 - Demonstrator with MB buckle pretensioner model

In the Sled_sys SYSTEM.MODEL, a load case including a pre-crash longitudinal braking phase at 0.8g immediately followed by a crash pulse (25g) at T0 is defined. The buckle assembly and the pyrotechnical pretensioner are defined in two separate SYSTEM.MODEL's, so that that pretensioner system can easily be exchanged. The positioning of the buckle assembly and the pretensioner was facilitated using referencing to a coordinate system (CRDSYS_OBJECT.MB) for which all the positioning parameters are listed as a set of DEFINE parameters in their respective local GROUP_DEFINE's.

Fig. 2 - Positioning parameters

The design of the buckle assembly and the pretensioner is parametrized as well throughout a set of design parameters such as the pretensioner's maximal pull-in force and stroke, the pretensioning cable material and basic geometrical properties, the orientation of the cable outlet, the friction at the cable guide towards outlet, etc.

Fig. 3 - Design parameters

Notes: 

1) the bending, flexion and torsion of the cable is lumped in one single spherical joint for which the angular stiffnesses can be adjusted (linear behavior).

2) One end of the cable is connected to the piston of the pyrotechnical pretensioner assembly and the other end to the base of the seat belt buckle head. To enable the cable pull-in, there is a translational joint connected to the piston end of the kinematic chain. The translational joint remains locked until the pretensioner was triggered and locks again as soon as the pretensioner reaches maximal pull-in stroke or is overloaded by the belt pull-out applied by the moving occupant. The sleeve has been modelled with facet elements for visual purposes only. Two slender ellipsoids have been added to model the cable portion right below the buckle head to allow for contact definitions with the surrounding parts are needed. To do so, refer to the predefined GROUP_MB's BuckleHead_gmb and/or CablePortion_gmb. The use and definition of a stiff contact characteristic will be required.

Once the pretensioner design parameters are set, the user can define the initial settings:

• the activation time: PP_TTF
• the activation duration after trigger*: PP_activationduration
• the cable initial flexion, bending and torsion angles: BP_cableflexion(/bending/torsion)initial

*the pretensioner is modelled using a preloaded spring approach (force-stroke based characteristic) and the piston joint will lock when the pretensioner has reached its maximal stroke OR when the piston drives reverse. In case the tension on the cable remains lower than the pretensioner's pull-in force for an extended duration (for instance if the pretensioner would be triggered when the dummy does not sustain high deceleration yet), the piston may keep on trying to pull in belt for an unrealistic duration after the trigger. Therefore, this parameter is used to force the pretensioner joint to lock after a certain duration of activation.

Fig. 4 - Initial settings

Note: this buckle pretensioner on cable can also be used as a simple articulated buckle on rigid bracket (so without pretensioner and flexible cable). To do so, simply:

• Set BP_cablestate to LOCK --> it locks the spherical joint modelling the cable flexion and torsion
• Seat BP_pivotstate to FREE --> it unlocks the pivot of the buckle, allowing rotation around the Y-axis
• Define PP_TTF greater than TIME_END --> this will never activate the pretensioner and therefore keep the translational joint always locked
• Finally define the lower and upper boundaries of the rotation range of the buckle (BP_pivotminang & BP_pivotmaxang) and the friction load in the pivot (BP_pivotfricload). Also make sure that the initial buckle angle (BP_cableang) is between the lower and upper boundaries.

A few output definitions related to the buckle pretensioner are predefined such as the force generated by the pretensioner, the pull-in force generated at the buckle (through a fictive load cell) and the force in the cable portions around the routing point.

Fig. 5 - Predefined outputs

A session file with the above-mentioned signals is provided in the attachment as an example and is shown below.

Fig. 6 - Example of signal analysis

Notes: in the belt forces plots, the forces in the two cable segments (red and green) are shown to illustrate the force distribution around the cable guide. These forces remain constant after the pretensioner has locked since the joint CablePortion_Collapse_jnt and the piston joint PyrotechnicalPretensioner_Piston_jnt share the same state switch logic. The buckle head force signal (purple) in the plot actually represents the total force applied by the occupant to the buckle system.

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