Abstract
In the past many efforts to improve the suspension performance have been made regarding the optimization of kinematics, elastokinematics and steering geometry. Nowadays, innovative mechatronic suspension systems enable to overcome characteristic restrictions of conventional passive systems. Thus they allow obtaining the whole potential of tires in critical driving situations and improving vehicle dynamics. Another challenge in design of passenger cars is packaging. Left and right wheels are connected via anti roll bar and steering rack, i.e. those suspension components cross the space where usually the power train is placed. Single wheel steering systems avoid these restrictions, opening new design possibilities and reducing assembly efforts regarding the whole vehicle.
In this paper an innovative electric single wheel steering system, based on a wheel guiding spring strut, is presented. An electric motor which supplies the steering torque and a gearing are integrated into the spring strut. Thus, besides guiding the wheel vertically, the suspension strut also generates the steering motion directly. Moreover, removing the steering column and the steering rack from the design simplifies vehicle topology and increases passenger safety in case of a crash. The presented actuator can also be applied for additional rear wheel steering systems. The presented electric single wheel steering system is analysed regarding power consumption, fail-safe behaviour and influence on vehicle dynamics. For this purpose, a non-linear full vehicle model is developed in Matlab/Simulink®. The model is validated with measurement data. The presented suspension strut is modeled as part of a modified McPherson suspension in the multiphysical simulation system Dymola® and embedded in the full vehicle model mentioned above, building a co-simulation environment.
With these models, all important design parameters of the electric system are derivated (e.g. necessary power, maximum motor torque, manipulating speed, etc.). Energy consumption is investigated at common operating conditions as well as in critical driving situations. Furthermore parameter studies (e.g. rotational stiffness and damping) are performed and results are analyzed regarding their influence on vehicle dynamics. Conclusions about the rotational stability of the wheel are drawn. Fail-safe behaviour of the vehicle in case of power loss of the steering system is presented and possible improvements are shown.
KEYWORDS: electric steering, active suspension, steer-by-wire, vehicle dynamics, fail safe