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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 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.

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The standard second order single track linearized model for four-wheel-steering vehicles is considered: yaw rate and lateral speed are the outputs to be controlled while front and rear steering angles are the control inputs (only additive steering angles with respect to the pilot commands are considered for the front wheels).It is shown that the lateral speed dynamics and the yaw rate dynamics can be decoupled by feeding back longitudinal speed, yaw rate and lateral acceleration measurements: lateral speed measurements are not required. The yaw rate tracking error dynamics follow a second order reference model with arbitrary poles, while the lateral speed dynamics tend exponentially to zero with a vehicle-dependent time constant and lateral acceleration tends to be proportional to the yaw rate.Simulations on a nonlinear third order single track model show significant improvements in the closed loop behaviour: larger stability regions, larger bandwidth, resonances suppression, improved manoeuvrability. A key feature of the inputoutput decoupling control is the improved comfort since both the lateral speed and the phase lag between lateral acceleration and yaw rate are greatly reduced.