Influence of Tire Dynamics on Slip Ratio Estimation of Independent Driving Wheel System

The independent driving wheel system, which is composed of in-wheel permanent magnet synchronous motor（I-PMSM） and tire, is more convenient to estimate the slip ratio because the rotary speed of the rotor can be accurately measured. However, the ring speed of the tire ring doesn’t equal to the rotor speed considering the tire deformation. For this reason, a deformable tire and a detailed I-PMSM are modeled by using Matlab/Simulink. Moreover, the tire/road contact interface（a slippery road） is accurately described by the non-linear relaxation length-based model and the Magic Formula pragmatic model. Based on the relatively accurate model, the error of slip ratio estimated by the rotor rotary speed is analyzed in both time and frequency domains when a quarter car is started by the I-PMSM with a definite target torque input curve. In addition, the natural frequencies（NFs） of the driving wheel system with variable parameters are illustrated to present the relationship between the slip ratio estimation error and the NF. According to this relationship, a low-pass filter, whose cut-off frequency corresponds to the NF, is proposed to eliminate the error in the estimated slip ratio. The analysis, concerning the effect of the driving wheel parameters and road conditions on slip ratio estimation, shows that the peak estimation error can be reduced up to 75% when the LPF is adopted. The robustness and effectiveness of the LPF are therefore validated. This paper builds up the deformable tire model and the detailed I-PMSM models, and analyzes the effect of the driving wheel parameters and road conditions on slip ratio estimation.

PHENOMENON OF CARVED DRIVING WHEELS

A newly found phenomenon of carved driving wheels of a rear-wheel-drive tractor used in an airport is discussed. The circum of every driving wheel is damaged at three regions, which distribute regularly and uniformly. Everyday, the tractor tows a trailer which are times heavier than the tractor, and moves on the same road in the airport. The phenomenon is explained by the torsional self-excited vibration system of the powertrain. The simplified torsional vibration system is discribed by a 2-order ordinary differential equation, which has a limit circle. Experiments and numerical simulations show the followings： Because of the heavy trailer, the slip ratio of the tractor＇s driving wheels is very large. Therefore, there is severe torsional self-excited vibration in the tractor＇s drivetrain, and the self-excited vibration results in severe and regular fluctuations of the rear wheel＇s velocity. The severe fluctuations in velocity fastens the damage of the driving wheels. At the same time, the time interval in which an arbitrary point in the circum of the driving wheel contacts with the road twice is two times more than the period of the torsional self-excited vibration, and this times explained the existence of three damaged regions. At last, it points out that the phenomenon can be avoided when the torsional damping is large enough.

Four Wheel Independent Drive Electric Vehicle Lateral Stability Control Strategy

In this paper,a kind of lateral stability control strategy is put forward about the four wheel independent drive electric vehicle.The design of control system adopts hierarchical structure.Unlike the previous control strategy,this paper introduces a method which is the combination of sliding mode control and optimal allocation algorithm.According to the driver’s operation commands(steering angle and speed),the steady state responses of the sideslip angle and yaw rate are obtained.Based on this,the reference model is built.Upper controller adopts the sliding mode control principle to obtain the desired yawing moment demand.Lower controller is designed to satisfy the desired yawing moment demand by optimal allocation of the tire longitudinal forces.Firstly,the optimization goal is built to minimize the actuator cost.Secondly,the weighted least-square method is used to design the tire longitudinal forces optimization distribution strategy under the constraint conditions of actuator and the friction oval.Beyond that,when the optimal allocation algorithm is not applied,a method of axial load ratio distribution is adopted.Finally,Car Sim associated with Simulink simulation experiments are designed under the conditions of different velocities and different pavements.The simulation results show that the control strategy designed in this paper has a good following effect comparing with the reference model and the sideslip angle is controlled within a small rang at the same time.Beyond that,based on the optimal distribution mode,the electromagnetic torque phase of each wheel can follow the trend of the vertical force of the tire,which shows the effectiveness of the optimal distribution algorithm.