The ongoing trend in automotive electronics to incorporate active safety mechanisms has led automobile manufacturers to incorporate antirollover capabilities into traditional vehicle chassis control systems, such as those for antilock brakes and traction control systems that are now being enhanced to incorporate antirollover capabilities. This trend is driven by the US National Highway Traffic Safety Administration (NHTSA), which mandates that all 2011 model year vehicles and beyond have antirollover controllers. The 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 percent 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 a sport-utility vehicle (SUV).
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 development cost and time associated with controller design. 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's performance has been verified against test data and is suitable for simulating vehicle response under significant roll motions.
The vehicle model has dual independent front suspensions and a solid rear axle that supports the sprung mass. The nonlinear mathematical model has degrees of freedom for the sprung mass, each axle, each wheel, steering system, and braking system. The vehicle model can be customized using different vehicle parameters as well as road and environmental conditions.
Figure 1: Setting up the vehicle parameters using the CarSim user interface.
Figure 1 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.