Simcenter Madymo Simcenter Madymo - Steering column and steering wheel modelling

The Multi-Body steering column and steering wheel models consists of an assembly of kinematic joints, rigid bodies and MB surfaces. Both models are defined in a separate SYSTEM.MODEL so that the user can exchange them easily.

 

Fig. 1 - Steering column and steering wheel Multi-Body models

 

Steering column model

The steering column model only consists of the steering column itself, i.e. including the height and depth adjustment mechanism (driver comfort settings) and the built-in collapsing device (in-crash safety feature). The entire MB model is defined as an include file and a set of DEFINE parameters were made available to the user to easily position, define the geometry, and set the steering column as needed. The three categories of DEFINE's (positioning, design and initial setting parameters) are listed in the local GROUP_DEFINE of the steering column's SYSTEM.MODEL.

 

Fig. 2 - DEFINE parameters

 

The positioning parameters allow the user to position and orientate the steering column in the environment by mean of a reference body parameter (SC_attachmentbody), position (SC_mountposX/Y/Z) and orientation parameters (SC_mountrotX/Y/Z). The origin is defined as the centre of rotation of the steering column's adjustment mechanism.

Among the design parameters, the user defines the total length of the steering column via the design parameter SC_length (defined as the distance from the centre of rotation to the supposed attachment point of the steering wheel hub). The initiation position of the steering column can be adjusted in height with the variable SC_initangle (expressed in °) and in depth with the variable SC_initdepth (expressed in m); keeping these two variables to zero means mid-height/mid-depth. Additionally, the static/dynamic friction values in the RESTRAINT.JOINT's of the adjustment mechanism for both rotation and translation can be adjusted;

• SC_adjustanglefric; value of dynamic/static Coulomb friction loads (resistive torque expressed in Nm) in the revolutional joint (SteeringColumn_AdjustmentHeight_jnt)
• SC_adjustdepthfric; value of dynamic/static Coulomb friction loads (resistive force expressed in N) in the translational joint (SteeringColumn_AdjustmentDepth_jnt)

End-stop in the adjustment mechanism are modelled for each joint (SteeringColumn_AdjustmentHeight_jnt and SteeringColumn_AdjustmentDepth_jnt) by means of characteristics & functions with bottoming-out after the maximal rotation/displacement range is achieved.

 

Fig. 3 - Bottoming-out functions for adjustment range

 

For the collapsing device, a few basic design parameters are made available to modify the behavior of the collapsing device. These are:

• SC_collapse_forceinit; it represents the force threshold to initiate the deformation of the collapsing device - such as the force required to break a fuse pin for example
• SC_collapse_forcebegin; the resistive force in the sliding mechanism just after the deformation initiated
• SC_collapse_forceend; the resistive force in the sliding mechanism just before reaching the maximal stroke of the steering column (linear behavior assumed)
• SC_collapse_stroke; the maximal stroke of the collapsing device

 

The figure below shows the characteristic of the collapsing device (force vs. displacement) implemented in the model. The user is obviously free to replaced the default profile of the force-displacement function with a more representative behavior, coming from their own component tests for example.

Fig. 4 - Deformation characteristic of the collapsing device

 

Finally, the initial settings parameters allow the user to set the steering column in the desired position (within the adjustment range) in height (SC_initialangle) and depth (SC_initialdepth). If the user does not want to model a collapse-able steering column, the deformation joint (SteeringColumn_CollapsingPart_jnt) can be locked by setting the parameter SC_collapse_status to LOCK.

Note: it is not intended that the steering column adjustment mechanism releases during a crash. But if the load transferred by the occupant to the steering column exceeds the strength of the mechanism locking the steering column in position, a release is possible. This can be simulated by allowing the joints of the adjustment mechanism to move by setting SC_adjust_status to FREE. 

Additionally, the rotation of the steering wheel (in pre-crash or in-crash simulations) can be simulated with ease by defining a rotation profile over time in the function SteeringColumn_Rotation_fun (defined outside the include file). If a steering wheel is attached to the end body of the kinematic chain of the steering column (SteeringColumn_End_bod), the steering rim will automatically rotate along.

 

Fig. 5 - Function to simulate steering wheel rotation

 

Steering wheel model

The steering wheel model includes the steering rim with the spokes and the centre part of the steering wheel (hub). Similarly to the steering column, the entire MB model is defined as an include file and a set of DEFINE parameters were made available to the user to easily position and define the geometry of the steering wheel. The three categories of DEFINE's (positioning, design and initial setting parameters) are listed in the local GROUP_DEFINE of the steering wheel's SYSTEM.MODEL.

 

 

Fig. 6 - DEFINE parameters

 

The positioning parameters for the steering wheel are similar to the steering column which allows the user to attach the steering wheel of any existing rigid body in the model.

Regarding the geometry of the steering wheel, the hub and the rim are independently defined from each other which allows more flexibility in the final design.

For the hub, three parameters available are:

• SW_huboffset; the offset w.r.t the position of the attachment body
• SW_hubheight; the height of the hub
• SW_hubouterdia; the outer diameter from the hub

And for the rim, the design parameters are:

• SW_rimsectiondia; diameter of the rim section
• SW_rimdia; diameter of the rim
• SW_rimoffset; offset of the rim w.r.t the attachment body
• SW_spokeoffset; offset of the spoke attachments w.r.t the attachment body
• SW_spokethickness; thickness of the spokes
• SW_lowerspokeangpos; angular position of the lower spokes w.r.t the uppermost point on the rim
• SW_lowerspokewidth; width of the lower spokes
• SW_upperspoekangpos; angular position of the upper spokes w.r.t the uppermost point on the rim
• SW_upperspokewidth; width of the upper spokes

 

The figure below shows examples of possible steering rim designs by varying the above mentioned parameters. Note that only circular steering rim designs are possible.

 

Fig. 7 - Examples of possible steering rim design

 

In order to represent the deformation of the steering rim (bending), deformation (revolutional) joints were added for the bottom and top portions of the rims, associated to RESTRAINT.JOINT's and CHARACTERISTIC.LOAD's (the user is free to change the default stiffness characteristics of each rim portions). The positions of the deformation joints are automatically calculated based on the orientation of the spokes as depicted below with an example of deformed shape. On the figure below, the coloring of the ellipsoids indicates which ellipsoids are attached to which body in the model (default):

• pink to the upper rim (deformable)
• orange to the hub (rigid) and
• yellow to the lower rim (deformable)

 

Fig. 8 - Deformations of the rim

 

If the deformation at the connection between the steering wheel hub and the steering column needs to be modelled, a deformation (universal) joint and deformation characteristics have been already implemented in the model. A universal joint was chosen since only bending was assumed and no torsion. The joint is located at the same position of the steering wheel's attachment body (end of the steering column).

 

 

Fig. 9 - Deformations at the connection between steering column and hub

 

If no deformation of either the steering rim or the hub is needed, the deformation joints can be kept as rigid by setting the parameters SW_hub_status and SW_rim_status to LOCK.

 

Important remark concerning the modelling of the rim deformation:

The default set-up is suitable for spokes positioned as depicted on figure 8. However, if the rim spokes are positioned differently, for instance for a steering rim connected to the hub with two spokes only, the deformation joints will be correctly positioned but the bodies assigned to the ellipsoids are not judicious if kept as default. In that case, the user will have to:

• re-assign to the correct body in the BODY attribute and
• change the factor 1.0 to 0.0 of the Z-offset in the POS attribute so that the deformed shape will look more realistic.

 

 

Fig. 10 - Updates required in case of different spoke positions

 

How to connect a driver airbag to the steering wheel?

It is common practice in Madymo modelling when creating an airbag model, which may include both the airbag module (MB/Facet/FE) and the airbag cushion (FE), to use a reference body on which the entire airbag is attached to. This obviously makes easier the position of the entire airbag in the environment. To attach the driver airbag to the hub of the steering column, the user must add a rigid connection (with a JOINT.BRAC for instance) between the airbag's reference body and the SteeringWheel_Hub_bod and possibly align the coordinate system of the airbag so that its orientation is correct.

 

Fig. 11 - Body of the steering wheel used to attach driver airbag

 

In the example shown below, the airbag's reference body is called AirbagMount_bod and a bracket joint Airbag_jnt with parametrized positioning of the parent body (using CRDSYS_OBJECT.MB in combination with DEFINE variables for BODY, POSITION and ORIENTATION referencing) is used to rigidly connect the entire airbag model to the steering wheel's hub in the right position and orientation w.r.t to the SteeringWheel_Hub_bod's coordinate system and position.

 

Fig. 12 - Airbag connection to steering wheel's hub

 

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