Control oriented modeling for enhanced yaw stability and vehicle steerability

Emerging drivetrains open new possibilities in vehicle stability control. In this paper, we consider a vehicle powered by four independently driven wheel motors, one for each wheel, with steer-by-wire. We explore the possibilities of using steering and individual wheel acceleration on top of braking to enhance stability and steerability. A 7-DOF model is developed to capture the vehicle longitudinal, lateral and yaw motion and the other four degrees of motion representing the wheel dynamics. However, this 7-DOF model is rather complicated for controller design due to the coupling of the inputs, and because the tire forces are highly nonlinear and coupled when the tires are at their limits. A control oriented model is developed that decouples the inputs and facilitates ease in control algorithm development. In addition, the longitudinal and lateral forces are also decoupled based on detailed analysis of the tire behavior at their limits. Simulation results of both the 7-DOF model and the control oriented model are compared to show that the control oriented model is capable of capturing the vehicle behavior at safety-critical cases.