Kinematic Discrepancy Minimization for AWD Terrain Vehicle Dynamics Control

Abstract

Stability of motion, turnability, mobility and fuel consumption of all-wheel drive terrain vehicles strongly depends on engine power distribution among the front and rear driving axles and then between the left and right wheels of each axle. This paper considers kinematic discrepancy, which characterizes the difference of the theoretical velocities of the front and rear wheels, as the main factor that influences power distribution among the driving axles/wheels of vehicles with positively locked front and rear axles. The paper presents a new algorithm which enables minimization of the kinematic discrepancy factor for the improvement of AWD terrain vehicle dynamics while keeping up with minimal power losses for tire slip. Three control modes associated with gear ratio control of the front and rear driving axles are derived to provide the required change in kinematic discrepancy. Computer simulation results are presented for different scenarios of terrain and road conditions. The effectiveness of the proposed control algorithm was analytically proved by modeling the same vehicle with no kinematic discrepancy control. INTRODUCTION The phenomenon of kinematic discrepancy is an inherent property of all-wheel drive (AWD) vehicles with positively engaged drive axles. By this effect is meant the difference of theoretical velocities (no slip occurs) of the front and rear wheels. Kinematic discrepancy influences significantly power distribution between the driving axles and wheels and as consequence fuel consumption, mobility, and stability. Kinematic discrepancy was originally introduced in [1, 2] and then research was continued by [3-7]. A generalized approach to kinematic discrepancy of multi-wheel drive vehicle is presented in [8]. An AWD vehicle can feature kinematic discrepancy in the case of being equipped with locked up power dividing units, or positively engaged PDUs. Figure 1 presents an example of the positively engaged PDU, i.e. a free-running clutch. It makes possible automatic engagement and disengagement of a driving axle. When power is transmitted only to the rear wheels, ring 1 that is set into rotation by the front wheels that are still not connected to the drive, but receive their rotation from the road, rotates at higher angular velocity than ring 2 that is set into rotation by the vehicle’s engine. This takes place due to kinematic discrepancy between the front and rear wheels.