Simcenter Madymo Simcenter Madymo - Belt retractor modelling

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 and an advanced seat belt retractor.

Fig. 1 - Demonstrator with MB retractor model

Three sled pulse profiles are made available to the user which can be selected through a DEFINE variable called LOADCASE in the main GROUP_DEFINE (global scope):

• frontal-braking-crash: 2-stage longitudinal braking pulse (0.4g and 0.8g) followed by an impact at a maximal deceleration of 25g
• frontal-braking: Longitudinal braking pulse at 0.8g
• lateral-braking: Left-right lateral pulse with combined 2-stage longitudinal braking pulse (0.4g and 0.8g)

The MSB's activation and deactivation times as well as the webbing on spool value, the trigger time for the pyrotechnical pretensioner and the time set to switch from the load limiter's first to second stage have been added as load case parameters (global scope) and referred to in the GROUP_DEFINE inside the retractor's SYSTEM.MODEL (local scope).

Note: in the model, the webbing on spool parameter (WOS) only affects the film spool effect in the retractor (represented in the characteristic FilmSpool_char). The load limitation is however not affected by this parameter.; in the reality, a torsion bar is often used as a load limiting device (torque-rotation driven) and thus affected by the amount of webbing left on the spool. Even though this behavior has been characterized by a force-stroke function, the effect of the webbing on spool has not been implemented in this conceptual model.

Fig.2 - Load case definitions

Note: the user can extend the number of scenarios by defining his/her own scenarios.

The model is structured with distinctive SYSTEM.MODEL's so that the entire retractor model including the seat belt can be easily copied/pasted to any other application model the end user wants to use it in:

• Sled_sys: contains the sled model with the rigid bench and footrest
• Hybrid_III_50th: contains the occupant (ATD) model
• Contact_sys: contains all contact interactions defined between the different system models
• Belt_sys: contains retractor model and the 3-point seat belt model

Note that the TIMESTEP variable shown on the figure above will need to be copied as well since this variable is also used in the seat belt's FE models as MIN_STEP in CONTROL_FE_TIME_STEP.

Fig. 3 - Overview of the model structure

Under the GROUP_DEFINE inside the SYSTEM.MODEL Belt_sys (local scope), a set of system variables has been defined along with a description. The belt design parameters are divided into two categories:

• for the retractor: here the user can adapt the locking sensitivity of the retractor, the pull in force of the recoil spring, the load limitation force and maximal strokes, the pyrotechnical pretensioner force and stroke, the characteristics of the reversible pretensioner, etc.
• for the webbing: properties, such as density, width, thickness for the automated alignments between MB segment and FE belt can be edited. The tensile behavior of the belt is characterized in separate CHARACTERISTIC.LOAD/MATERIAL's and FUNCTION.XY's in the SYSTEM.MODEL

Fig. 4 - Retractor model parameters

The retractor is modelled in a conceptual way, consisting of an assembly of rigid bodies and translational joints put in series together with restraints, each representing a dedicated function of the retractor. The seat belt webbing consists of a mix of 2D segments using FE membrane elements (where interaction with the occupant is likely to occur) and 1D segments defined through BELT elements. The lower belt attachment points (buckle and anchor) as well as the upper one (D-ring) are rigidly fixed to the sled using BODY.RIGID's, JOINT.BRAC's and POINT.OBJECT_MB's.

Note: The belt attachment points could also be attached to bodies of a more complex multi-body chain, by simply changing the parent body and position in the dedicated CRDSYS_OBJECT.MB. This could for instance apply when the user has already created a deformable buckle MB model, either or not including a pyrotechnical pretensioner.

Fig. 5 - Belt attachment points

The kinematic multi-body chain of the retractor model was designed as described below (the order of the joints in the kinematic chain reads from top to bottom):

• Retractor kinematic chain (RetractorChain_jnt): this is a bracket joint that is connected to the body used to position the retractor in the vehicle/sled coordinate system. The RetractorChain_bod body is the first body of the entire multi-body chain.
• Recoil (or motor) spring (WebbingAttachmentOnSpool_jnt): the user defines the force of the recoil spring. The retractor joint will lock when the webbing payout velocity exceeds the threshold (user-defined, occurrence MANY) OR when the vehicle deceleration exceeds the threshold (user-defined, occurrence MANY) OR when the pyrotechnical pretensioner is triggered (user-defined, occurrence ONCE) OR when the tension at the retractor outlet exceeds 1.5x the force of the recoil spring (occurrence MANY) OR when the MSB is activated (time window)
• Reversible pretensioner (MSB_jnt): the user defines the time window during which the MSB is active (i.e. engaged to the retractor spool to pull in belt webbing), the pulling force and the maximal pulling velocity as well as the releasing force and maximal releasing velocity. When the MSB is deactivated, the MSB will release the same amount of webbing reeled in during its entire activation time window, after which it permanently locks. During activation, the MSB may temporarily lock in position in case the tension on the belt exceeds the pulling force of the MSB (occurrence MANY) OR in case the MSB drives backwards (occurrence MANY) AND the MSB is activated (time window). The MSB will permanently lock as soon as the pyrotechnical pretensioner is triggered.
• Pyrotechnical pretensioner (PyrotechnicalPretensioner_jnt): the user defines the activation time, the maximal force and maximal belt pull-in stroke. The joint will permanently lock in case the maximal stroke is reached (occurrence ONCE) OR in case the joint starts sliding backwards (when the tension on the belt becomes higher than the pulling capacity of the pretensioner) (occurrence ONCE)
• Load limitation (LoadLimiter1_jnt & LoadLimiter2_jnt): the user defines the load limitation level (defined as constant) and the maximal belt payout for each stage as well as the time when the load limiter is set to switch from stage 1 to stage 2. The load limitation stage 1 is activated when the retractor is in LOCK status (occurrence MANY). As soon as the maximal stroke is reached (occurrence ONCE) OR the time is reached when the load limitation is set to switch from level 1 to 2 (occurrence ONCE), LoadLimiter1_jnt will lock and LoadLimiter2_jnt will be set to FREE.
  Note: the existing load limiting functions could be adjusted to represent a more complex or realistic load limitation profile such as progressive or degressive.
• Film spool effect (FilmSpool_jnt): this joint always remains active. The FilmSpool_bod body is the end body of the kinematic chain of the retractor and it connects to the first 1D belt segment.

Outputs have been defined for the retractor'sjoints so that the user can assess the contribution of the different parts of the retractor on the webbing payout/pull-in.

Fig. 6 - Retractor joint outputs

The joint outputs is Retractor_jpsout (.jps file) and writes out the displacement for the following translational joints:

WebbingAttachmentOnSpool_jnt: amount of webbing grabbed in or released by the spool when not locked

MSB_jnt: amount of webbing pulled in and released by the MSB

PyrotechnicalPretensioner_jnt: amount of webbing pulled in by the pyrotechnical pretensioner

LoadLimiter1_jnt: amount of webbing released by the load limiter stage 1

LoadLimiter2_jnt: amount of webbing released by the load limiter stage 2

FilmSpool_jnt: amount of webbing released by the film spool effect

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