The ongoing trend in automotive electronics to incorporate active safety systems has led automobile manufacturers to incorporate antirollover capabilities into vehicle chassis control systems, such as those for antilock brakes and traction control. This trend is driven by the National Highway Traffic Safety Administration (NHTSA), which mandates that all 2011 model year vehicles and beyond have antirollover controllers.
This requirement is based on NHTSA analysis of accident data from rollover crashes. For example, in 2001, according to NHTSA's National Center for Statistics and Analysis, 10,138 people died in rollover crashes, representing 32% of occupant fatalities for the year. Implementing active safety mechanisms to lower the risk of vehicle rollover could potentially reduce fatalities.
One method of lowering the risk of rollover is to implement an Electronic Stability Control (ESC) that applies differential braking based on measured and estimated vehicle states. This article highlights the use of model-based design to develop and automatically optimize an ESC for sport-utility vehicles (SUVs), those with the greatest rollover risk.
Vehicle and controller model
A central concept in model-based design is the executable specification, or model, that describes the dynamic behavior of the system. A validated model of the automobile, in this case a high-fidelity model of an SUV, can be leveraged to achieve a significant savings in the controller development cost and time. Numerical simulation of the model can be used to study the vehicle response to various steering maneuvers and these tests can be repeated easily while varying parameters like road surfaces, tire models, and vehicle properties. Additionally, the models can be used in the development and verification of the embedded control system.
The vehicle used in this article is representative of a midsize SUV. The vehicle model is available in CarSim®, a commercial off-the-shelf vehicle dynamics simulation tool. The vehicle model performance has been verified against test data and is suitable for simulating vehicle response under significant roll motions. The model has dual independent front suspension and a solid rear axle that supports the sprung mass. It is a nonlinear mathematical model with degrees of freedom for the sprung mass, each axle, each wheel, steering, and braking system, and can be customized using different vehicle parameters as well as road and environmental conditions.
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The CarSim user interface is used to set up vehicle parameters.
The figure above shows the CarSim user interface and some of the physical vehicle parameters used to build up the vehicle model. These parameters can be modified separately from the controller parameters to test the behavior of the controller under different vehicle conditions, such as single occupant, multi-occupant, and high center of gravity.
The vehicle model used for this paper applies steering inputs concordant with the NHTSA fishhook maneuver, a standard maneuver used to assess dynamic vehicle stability. This test is designed to mimic the actions a driver might perform to avoid a sudden obstacle in his or her path. For the numerical simulation, we set up this steering input for the SUV model and verified that in the absence of an ESC, the vehicle exhibits rollover.