Calculating Static Rollover Threshold Using Automotive Tilt Table Event and MotionSolve
Altair EmployeePreface:
The automotive Static Rollover Threshold (SRT) refers to the maximum tilt angle at which a vehicle is likely to tip/roll over during a static situation. The SRT can be used to predict the vehicle lateral acceleration in which rollover may occur. Vehicles with a low Static Rollover Threshold (SRT) are at a greater risk of rolling over compared to those with a higher SRT.
The SRT is commonly determined by completing a tilt-table event, in which the vehicle is slowly rotated on a platform until wheel liftoff is observed. The final tilt-table angle and vehicle body angle are recorded and used to determine the SRT.
This article has been written to detail how to import an existing tilt table Analysis, attach the Analysis to an existing vehicle model, and predict the vehicle’s Static Rollover Threshold.
Introduction:
The following list summarizes the specific event details that we need to emulate in the simulation:
- The vehicle shall begin at rest on a stationary table.
- The steering wheel shall be locked, and the parking brake shall be applied.
- The table shall slowly rotate at an approximate speed of 0.0016 rad/s.
- This low speed is chosen to reduce any transient input to the system.
- Wheel lift off can be measured using a contact sensor and lift-off is often determined when a single wheel or all upper wheels have separated from the table surface.
- The table angle and the vehicle body angle are measured throughout the event to determine the induced body roll.
- Depending on requirements, there may also be a small curb or trip rail in contact with the sidewall of the lower tires. This is to ensure sliding does not occur prematurely before liftoff can be measured. This can also be achieved by using high friction surfaces for the table in practice and in simulation.
Understanding The Model Definition in Motionview:
Base Model Preparation:
The Pickup Truck example was chosen for this demonstration, but any model can be used with the tilt table analysis. To begin, ensure the Vehicle Tools extension is loaded by navigating to File >> Extensions >> Vehicle Tools, then toggling on the extension:
Next, first load the Pickup_Truck.MDL model from the Entity Browser >> Entities >> Vehicle Tools >> Examples. If the Entity Editor is not shown, then navigate to View >> Entity Browser (F6).
Example models from the Entity Editor feature many entities that allow for the simple introduction of driven events. In this case, we need to remove most of those features since this event is stationary. Delete (or deactivate) the optional body clamp joints, differential torque, engine torque output, braking system, and Altair Driver Model (as shown):
Event Import and Preparation:
For the event entities, you can import the MDL Definition (File >> Import >> MDL Definition) called “tilt_table_event_2024_1_v00.MDL”. Make sure you import the Analysis at the top level of the assembly (called Model).
This will create a new Analysis and event specific entities, including the table body, table graphic, revolute joint, revolute motion, wheel lock motions, wheel lift sensors, and event outputs:
The only critical modification to make after importing the event is to reassign the AutoTire Road Markers from the default markers to the new “table ref” marker in the tilt table analysis:
You can now run the model to see the tilt table event in action!
Tilt Table Event Specifics:
In this analysis, there exists a table Body that will rotate with a set velocity via the “table motion” Motion entity. The AutoTires react to a road that is attached to the table Body as rotation is induced. The standard runtime for this event is 500 seconds, which will induce approximately a 45-degree angle on the table at a rotational velocity of 0.09 degrees/second.
5 Sensors exist in the Analysis that will halt the simulation if the Sensor conditions are met. The first 4 Sensors read the vehicle vertical load (measured from individual wheel hubs) and will halt the simulation if the vertical load reaches nearly 0 N. This signals single wheel lift-off, and thus the simulation is ended. The final sensor will measure the lateral displacement of the vehicle and halt if it exceeds 1000 mm as it is likely sliding off the tilt table.
To simply the import of the Tilt Table Analysis, it has been designed with several Attachments to limit the amount of modification needed for non-default vehicle models. If the Analysis is selected, the following Attachments will be visible in the Entity Browser:
These vehicle model entities are used to position the Bodies, Points, Markers, and Motions found in the Analysis. When first importing the Analysis, please ensure that all Attachments reflect your current vehicle model. If you are ever confused as to what attachments to use, please refer to this example with the Pickup_Truck.MDL and the entities listed above.
Event Outputs:
To simulate the event, simply open the Run Motion Analysis window and run the model Offline:
After simulating the event, we can review the results in HyperView and HyperGraph. Simply select the Results button to launch a new session of HyperView by default with the animation loaded:
Within HyperView, you can review the animation to ensure the setup of your model was correct. You can also add more windows to your current page by changing the single window icon to 2, 3, or 4 windows (shown):
The new windows can be modified to be HyperGraph (Line Chart) instances by selecting the Change Type Icon in the top right of the window or by changing the displayed application in the ribbon:
In the HyperGraph instance, you can plot the generic outputs for the model, or the two custom outputs featured in this event. Output Sensor Loads and Output Angles are two expression-based outputs that display the vertical loads on the wheel hubs (used for the sensor wheel lift Halt), the lateral displacement of the vehicle relative to the table (used for the sensor sliding Halt), the table angle, and the induced roll angle of the vehicle relative to the table angle.
In the top right plot, you can see that the L2 wheel end experiences lift-off and is the reason for the simulation being halted prior to the 500 second runtime. The final table angle and vehicle roll angle can be extracted for determining the relationship between the current vehicle parameters and the rollover threshold.
Save Tilt Table Analysis As MDL Definition:
If you modified the Analysis to be compatible with a different vehicle model and want to be able to import the modified event in the future, you can export the Analysis as a MDL Definition by right clicking on the Analysis (or System) and selecting Export:
Lastly, make sure the System you have selected is the Analysis (not the entire Model) and give the file a name.
After exporting, this MDL Definition can be imported as many times as needed into as many models as needed.
Conclusions:
This article has provided an overview for the use of the supplied Tilt Table event. The specific entities, use case, and review methods have been detailed to be integrated seamlessly into your next MotionView project. By using this MDL Definition, one can easily predict the Static Rollover Threshold for their vehicle models and better understand the handling limits of their vehicle.
Hopefully this process can be of use to you in your MotionView journey!
Completed Analysis:
Tilt Table MDL Definition:
Helpful Links:
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