Active modulation of torque distribution for dual-motor front- and rear-axle drive type electric vehicle based on slip ratio

Abstract: Research and development of electric vehicles (EVs) are being rapidly performed to cope with the latest environmental and energy issues. Studies of techniques for the efficient use of electrical energy are very important for the development of EVs. To this end, it is necessary to develop EVs that can deliver efficient running performance on various road surfaces. As we all know, the vehicle performances such as the drivability, the stability and the economics, are strongly influenced by its driveline architectures. New driveline architectures with 2 motors respectively installed at the input of the front and rear axles have yielded a new generation of four-wheel-drive vehicle, which is known as the front- and rear-wheel-independent drive type electric vehicle (FRID EV). Benefit from the mechanical layout, the longitudinal forces on the front and rear wheels can be adjusted flexibly according to the road and the driving conditions. This allows to design active control systems that are capable of altering the behavior of the vehicle and make it possible to achieve excellent driving performances via the reasonable control method that uses the structural features of the FRID EV. Therefore, in order to improve the traction performance and avoid the excessive slip of the driving wheels, which is also beneficial to energy saving during running, an active torque distribution method in consideration of the optimal slip ratio control for the front- and rear-wheel-independent drive type electric vehicle, which is based on the friction coefficient / slip ratio curve, was put forward in this paper. The pattern of the control method was designed as 2 levels, the top gives the total driving torque assignable in consideration of the driver's expectations and various road surfaces, and the underlying is responsible to reasonably distribute the torque given by the top level between the front and rear axles. Taking the longitudinal, lateral, the yawing modes of motion and the revolving of each wheel into consideration, the seven-degree-of-freedom dynamic model of an FRID EV was set up in this paper. In view of the advantages of sliding mode control (SMC) that can overcome the uncertainty of the system and is robust to external noise disturbances, the parameter perturbation and imprecise-model dynamics, especially for nonlinear systems, the SMC controller for the front and rear drive motors was designed to optimize the slip ratio of each wheel on the basis of the dynamic model. Using the exponential approach law, the approach motion (non-sliding mode) reached the switch surface in finite time, and buffeted less in the process of approaching by introducing a continuous function instead of the sign function. The correctness of this design was verified by the acceleration simulation tests on a split-μ and a joint road surface based on the MATLAB/Simulink software (SIL) platform and the hard-ware-in-the-loop (HIL) simulation system, respectively. The simulation results on the split-μ road surface show that the slip ratio of each wheel can be kept stably within the level of 0.12 around by actively modulating the distribution of the torque between the front and rear motors, which therefore inhibits effectively the excessive slip that may be caused by the wheels on the road with low friction coefficient, although the terminal velocity after 8 seconds is close to the case of equal torque distribution. But when it comes to the joint road surface, the acceleration performance is improved remarkably with the terminal velocity increased by 9.9%. Besides, during the process of turning on a uniform low-μ road surface, the slip ratio of inner wheels is limited within 0.2, which improves the steering capability of the vehicle. Besides, the simulation results of SIL and HIL are essentially in good agreement.