Study On The Vehicle Dynamics Control Principle And Strategy Of Distributed Drive Electric Vehicle

No matter considering from the environment problem or energy trouble, modernelectric vehicles are the main direction of future automotive technology research. Compareto other forms of power system, because of the outstanding advantages: the better dynamicscontrollability, the higher efficiency of drive system and better system reliability, thedistributed electric vehicles have been a hotspot of research in and abroad.In this paper, based on the distributed electric vehicles with four in-wheel motors,vehicle dynamics control principle and control strategies are research priorities. The mainadvantages in the dynamics control of distributed electric vehicles are expressed as follows:more controllable degrees of freedom because of four independent continuous controllablewheel torques, the motor torque control with faster response and higher control accuracy.This paper is studied on how to make full use of these advantages and exploit potentialitiesin improving the vehicle stability, handiness and energy economy of distributed electricvehicles by use of dynamics control. The impact of torque distribution on vehicleperformance and the motor torque characteristics for the handling and stability controlperformance are analyzed. And detail the stability control strategy under emergencyconditions and the vehicle dynamics control strategy taking into account the economy andsafety.In this paper specific studies are as follows:(1) Research on the wheel torque distribution methodThe drive system with distributed independent controllable wheel drive torques is oneessential difference between distributed electric vehicles and conventional vehicles. In thepaper, the impact of torque distribution between front and rear wheels on vehicleperformance is analyzed respectively from two aspects: steering characteristic and energy economy by use of dynamics principle analysis and simulation. Then different distributionmethods are evaluated comprehensively and the results are shown as follows: the energysaving and safety are two control objectives and it is difficult to be met at the same time,sometimes they are even mutually contradictory. Because of this, this paper presents a novelmethod for wheel torque distribution: according to driving conditions and stability state ofthe vehicle, timely change the control target of torque distribution. In the safe drivingconditions, when there is no demand yaw moment control, the driving torque are distributedbetween the front and rear wheels for energy saving. When there is yaw moment controlrequirement, the driving torque is regulated between the left and right wheels for safety. Bythis way, with the premise of meet the yaw moment requirement, the energy efficiency of thedrive system can be maximized.(2) Research on the stability control with drive motor and hydraulic braking system asactuatorsWhether in the conventional vehicle or in the distributed electric vehicle, the vehiclestability control is very important. For the characteristics of the distributed electric vehicle,making full use of the motor torque and braking pressure, the estimation algorithm of theside slip angle and friction coefficient in order to simplify the algorithm complexity andimprove the estimation accuracy while is studied. In terms of stability control strategy,making full use of the advantages of the motor torque control, a yaw torque distributedmethod between the in-wheel-motor system and hydraulic braking system is proposed.Firstly, differential drive and differential brake are realized using the in-wheel-motors byoptimal torque allocation. If the motor torque is insufficient, the hydraulic brakecompensating the remaining yaw moment demand. At last the state estimation algorithm andstability control strategy are all verified by simulation. The results also show that thestability control performance with drive motor and hydraulic braking system as actuators isbetter that with only the hydraulic braking system as actuator.(3) Research on the vehicle dynamics control strategy taking into account the drivingsafety and energy saving The distributed electric vehicle in conventional driving conditions also has the greatdevelopment potential in addition to the similar traditional stability control. Through thecoordination control of the motor and hydraulic braking system, the handling stabilityintegrated control strategy is proposed in which the steady state is used as the judgmentcondition. At last combine the pedal resolution strategy and wheel torque distributionmethod, to complete the integrated dynamics control system. The integrated dynamicscontrol system can significantly enhance the handling stability under various operatingconditions and increase the drive system efficiency to some extent, which is verified bysimulation.(4) Experimental research of the integrated dynamics control system for the distributeddrive electric vehicleBased on the theoretical analysis and simulation study, the process and results ofexperiment are introduced, which include the EHB HIL test and the actual vehicle test onroad. The real-time performance for the control algorithm is verified through the HIL test. Atthe same time, the influence of the real characteristics of the hydraulic system for the controlsystem is tested. At last a distributed electric vehicle platform with four in-wheel motors isdeveloped and the vehicle road test is carried on to validate the effectiveness of the dynamicscontrol system. The effectiveness of wheel torque distribution algorithm in generalconditions is demonstrated. And the results show that the dynamics control system cansignificantly improve the vehicle stability in the limit conditions.This paper has some innovations in the following two aspects:(1) A novel method for wheel torque distribution is proposed: Because the energysaving and safety are two control objectives and it is difficult to be met at the same time, andsometimes they are even mutually contradictory. It is necessary to timely change the controltarget of torque distribution according to driving conditions and stability state of the vehicle.In the safe driving conditions, when there is no demand for yaw moment control, the drivingtorque are distributed between the front and rear wheels for energy saving. When there isyaw moment control requirement, the driving torque is regulated between the left and right wheels for safety. By this way, with the premise of meet the yaw moment requirement, theenergy efficiency of the drive system can be maximized.(2)Vehicle dynamics control strategy taking into account the driving safety and energyis proposed: Through the coordination control of the motor and hydraulic braking system,the handling stability integrated control strategy is proposed. Combined with the wheeltorque distribution method, the integrated dynamics control system can significantlyenhance the handling stability under various operating conditions and increase the drivesystem efficiency to some extent.