Integrated control of active rear differential and front wheel steering in rear wheel drive vehicles

Many vehicle control systems were designed and implemented in the last years to enhance performances and stability acting on front and/or rear steering actuators [1,2,3] and/or individual wheel braking [8]; the development of electroactuated differentials allows to transfer torque between wheels from the faster to the slower one in semiactive differentials [11,12,13,14] and in both directions in active differential [15]. Current research trends are focused on the integration of several actuators to improve vehicle dynamics performance while reducing tire forces [4,5,6]. In this paper the integration of the active differential with the active front steering is investigated in order to: (i) improve traction by avoiding the drawbacks due to the mechanical self-locking differential action which may cause undesired understeering and oversteering behaviours; (ii) improve vehicle dynamics by suppressing resonances and enlarging the bandwidth for the yaw rate tracking dynamics. The main contribution is to show that an integrated proportional integral active front steering control and proportional yaw moment control from the yaw rate tracking error can assign all the eigenvalues of the linearised single track steering dynamics in any operating condition (without assuming the availability of lateral speed measurements) when the vehicle is not neutral so that resonances are suppressed and the yaw rate dynamics is improved. When the vehicle is neutral only two eigenvalues may be assigned while the third one is negative real. Since the wheel speed dynamics is faster then the lateral vehicle dynamics, the required yaw moment, which is proportional to the yaw rate error, is given as a reference signal for the rear wheel speed difference. The torque to be transferred by the active differential between the inner and the outer wheels is then computed as a proportional integral control based on the error between the measured rear wheel speed difference and the corresponding reference signal. The control strategy for the active differential is not only aimed at keeping the rear wheel speed difference at a designed value but is designed to produce a yaw moment which contributes to improve the vehicle dynamics. Several simulations are carried out on a standard CarSim® small SUV 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. Responses to increasing driver step inputs show no overshoots confirming that resonances are suppressed. A µ split braking test and …