A Nonlinear Semiactive Rear Differential Control in Rear Wheel Drive Vehicles

Many vehicle control systems are based on the yaw rate error to help the driver during oversteer and understeer conditions. The control systems usually operate on brake pressures distributions such as ESP and/or on active steering control (front and rear steering control). The main contribution of this paper is to show that vehicle dynamic performance can be improved by an electronically controlled semiactive rear differential which is based on yaw rate and rear wheel speed measurements. A nonlinear first order reference model for the yaw rate and for the rear wheels speed difference dynamics driven by the driver steering wheel input is employed. The controlled system shows new stable cornering manoeuvres and enlarged stability regions; moreover safety is increased in emergency conditions in which the driver does not react to a sudden external disturbance, since the regions of attraction are enlarged by feedback. The activation of the control law is based on Lyapunov techniques. Several simulations are carried out on a standard small SUV CarSim^reg car model to confirm the analysis and to explore the robustness with respect to unmodelled dynamics such pitch, roll and nonlinear combined lateral and longitudinal tire forces according to combined slip theory.