| Electrification not only represents the development direction of future vehicles,but also is a key technical way to deal with many challenges such as energy conservation and environmental protection in the development of the automotive industry.Distributed drive electric vehicle(DDEV)with four in-wheel independent motor has become a research hotspot in recent years due to its high transmission efficiency and high actuation flexibility.However,it is a research direction of chassis control on how to coordinate reasonably under multi-degree of freedom,distribute active front wheel steering and four-wheel motor torque,make full use of the ability that each actuator can adjust independently,improve the handling performance,and realize the goal of safety.Furthermore,the research of safety and energy-efficient also puts forward higher practical demand for chassis control of four in-wheel drive electric vehicle,and has become a research focus in this field.In order to solve the above issues,four wheel drive electric vehicle with active front wheel steering is taken as the research object,and the integrated chassis control strategy and torque distribution method are studied in this paper.In previous studies,vehicle stability control and energy-efficient were often independent of each other,and the effects of interaction and coupling were ignored.In contrast,the control strategy of this paper is under an overall objective function,and the operation of each actuator is optimized synchronously.The control strategy revolves around the idea of "first local mechanism analysis,off-line optimization,and then overall online global optimization".Local optimization provides vehicle handling,stability and energy-efficient tracking target for global optimization,and global optimization achieves dynamic balance of tracking target.This paper first establishes the vehicle simulation model and motor model suitable for functional development requirements according to the actual needs of the vehicle.Based on the established model,the vehicle handling and stability control area is systematically analyzed and given.Furthermore,starting from the vehicle stability,the state observation and design of the key state needed for the integrated chassis control are carried out,which provides accurate information feedback for the control strategy.On this basis,the integrated chassis control strategy based on stability and the integrated chassis control strategy based on stability and energy-efficient are systematically studied.Finally,through the real vehicle test,it is proved that the integrated chassis control strategy has good stability performance and energy-efficient effect.The specific work of this paper is as follows:First,this paper studies the vehicle dynamics.a fourteen degrees of freedom dynamic model which can accurately reflect the motion of vehicle body,suspension and wheel is established.The accuracy of the model is verified by simulation,which lays a foundation for vehicle handling and stability.In order to achieve the goal of energy-efficient in this paper,the map characteristics of the hub motor are tested,which lays a foundation for the following research of energy-efficient control strategy.In order to solve the problem of vehicle stability control,the phase plane analysis method is used to reveal the variation of phase plane stability region with longitudinal speed,front wheel angle and friction coefficient.The control area is further divided according to the identification results,and the vehicle stability judgment conditions are given,which lays a foundation for the selection of reference targets in the later integrated chassis control.Secondly,the state observer is designed for the key variables needed for integrated chassis control.Based on the principle of information fusion estimation and the observation characteristics of kinematics and dynamics,the observers of longitudinal speed and vehicle sideslip angle are designed respectively.In the speed observation,according to the kinematic information and dynamic information feedback of the in-wheel motors,the nonlinear extended Kalman filter is used to estimate longitudinal speed,and then the two kinds of estimation information are processed by information fusion,and the observation results are analyzed statistically.In the vehicle sideslip angle observation,considering that the kinematics estimation method is affected by sensor noise.In this paper,based on the two degrees of freedom model,a steady-state model suitable for the observation of vehicle sideslip angle is derived.Combined with the simplified UniTire model,a closed-loop estimation method is designed to dynamically adjust the parameters of the steady-state model.A simple structure and good suppression of the sensor noise are constructed.Furthermore,the observation performance of the existing observation methods is simulated and analyzed,and the characteristics are summarized.On this basis,combined with the advantages of the existing observation methods,through feature extraction,the fusion rules are worked out to improve the estimated bandwidth of the steady-state observation structure.The results of simulation and statistical analysis show that the improved estimation method has better estimation bandwidth and can achieve accurate estimation of vehicle sideslip angle.Thirdly,the integrated handling and stability chassis control strategy is studied in detail.Based on the UniTire Model proposed by Professor Guo Konghui,the simplified expression of equivalent tire cornering stiffness of the UniTire model under the combined condition is obtained,and it is revealed that the cornering stiffness under the combined condition is affected by the tire slip ratio and the synthetic force.On this basis,an eight degrees of freedom(order)prediction model considering vehicle dynamics and combined condition of tire characteristics is constructed,which can comprehensively reflect the motion of vehicle body and tire slip,as well as the nonlinear of tire characteristics under critical maneuver.Furthermore,the model predictive control method is used to design the mixed objective function,and the integrated chassis control strategy for vehicle stability and tire slip is proposed.The mixed objective functions include: Taking the vehicle stability and tire slip ratio are controlled by sideslip angle,yaw rate and four-wheel longitudinal motion as the first objective.Through off-line optimization analysis,the desire objectives of active front wheel steering and four-wheel torque are found out as the second objective,so that the optimal solution set of the objective function is always around the desire center.The stability of the optimal solution set is the third objective to prevent chatter in control actions.In the solution of model predictive control,the solution of model predictive control is accelerated by the multiple centrality corrections interior point algorithm considering constraints,and the real-time optimization problem of integrated chassis control is solved.The simulation results show that the strategy can effectively control the vehicle stability and tire slip ratio,and achieve the goal of handling and stability.Then,the integrated chassis control strategy of stability and energy-efficient is studied in detail.On the one hand,based on the map characteristic of the tested hub motor,the efficiency characteristic of the motor is expressed as a function of the torque.By establishing the minimizing objective function of input power,the off-line optimization analysis is carried out,and the map table of desired four-wheel torque distribution is determined,which provides a desired target for integrated chassis control strategy to realize drive energy-efficient.On the other hand,the four in-wheel independent drive combined with the front-wheel active steering system enables dynamic adjustment of the tire slip,which provides sufficient freedom to eliminate unnecessary tire slip energy.Thus,tire slip energy can be further reduced compared to conventional vehicles when achieving the same drive and steering requirements.Based on the tire theory,this paper proposes a semi-empirical UniTire slip energy model for tire slip energy estimation in control strategy,and systematically analyzes tire slip force and slip power under combined conditions.Furthermore,combined with the previous stability control strategy,an integrated chassis control strategy with integrated stability and energy-efficient is proposed,and tire slip energy is introduced to track the target in the formulation of the control strategy.The realization of energy-efficient target and slip energy target is to unify the energy optimization of motor and tire into the integrated control strategy from the point of view of energy,which is beneficial and complementary to reduce the overall energy consumption of the vehicle.Then,the proposed integrated chassis control strategy is analyzed by simulation,and the integrated control of the vehicle chassis,which takes into account the energy of the motor and the tire slip energy,is very advantageous for simultaneously reducing the output power and the tire slip energy.Finally,according to the integrated chassis control strategy proposed in this paper,the real test of four in-wheel drive electric vehicle is carried out.The real car is equipped with four hub motors studied in this paper to form a drive system.The vehicle verification is in the professional cold zone test site.The real vehicle test mainly includes the verification of the handling stability and tire slip energy on the low friction coefficient,in which two representative tests,slalom and double lane chance,are selected to verify the integrated chassis control strategy proposed in this paper.It is proved that the integrated chassis control proposed in this paper is effective at the maximum passing speed.Simultaneously,it is verified that the integrated chassis control strategy can also overcome tire slip and reduce tire slip energy on ice straight road.The main innovations of this paper are as follows:(1)A sideslip angle observer with a kinematic and steady state model is established.The observer is developed based on the steady-state model.The simplified UniTire model is embedded to form a dynamic closed-loop.At the same time,the high-frequency transient estimation ability of the kinematics method is taken into account,and the dynamic feature extraction fusion estimation result is realized.An accurate sideslip angle observer is achieved.(2)An eight degrees of freedom prediction model considering vehicle dynamics and combined condition of tire characteristics is established,which can comprehensively reflect the motion of vehicle body and tire slip,as well as the nonlinear variation of tire characteristics.On this basis,using the model predictive control,an integrated chassis control strategy for comprehensive control of vehicle stability and tire slip is proposed.(3)A semi-empirical UniTire slip energy model is proposed to accurately describe the mechanical properties of tire slip.By considering the efficiency of the motor and the energy consumption of tire slip,an integrated chassis control strategy for handling stability and energy-efficient is proposed. |
PDF Full Text Request