Research On Longitudinal And Lateral Coupling Motion Control Of Intelligent Vehicle Based On Integrated Brake-by-Wire System

The intelligent motion control strategy relies on the driving,braking,steering and other actuators of the chassis system to regulate the kinematic and dynamic state of the vehicle in response to the various types of target trajectory signals input by the intelligent driving system.In the development of traditional intelligent motion control strategies,in order to reduce the design complexity,independent longitudinal and lateral motion control methods are mostly used,and acceptable control performance is achieved in steady-state driving conditions with small longitudinal and lateral accelerations.However,the longitudinal and lateral motions of the vehicle are not completely independent from each other,but there is a nonlinear coupling relationship.When the longitudinal and lateral accelerations exceed the dynamic domain boundary of the steady-state driving condition,the vehicle enters a strong transient driving condition.In this case,not only the tire lateral cornering stiffness and other system parameters will change with the intense vehicle motion,but also the vehicle longitudinal and lateral dynamics will show a significant nonlinear coupling relationship due to the change of vehicle centripetal force,tire vertical load distribution and other factors.As the longitudinal and lateral accelerations move closer to the limits of road adhesion,the vehicle will enter a critical instability condition.In order to ensure the control margin of the vehicle,the intelligent driving system usually limits the size of lateral acceleration during trajectory planning,so the vehicle often performs larger longitudinal acceleration under critical instability conditions,causing a larger change in longitudinal speed,resulting in more significant longitudinal and lateral coupled nonlinear dynamics of the vehicle.Due to the longitudinal and lateral independent design of intelligent motion control strategy is difficult to cope with these problems,its control performance in strong transients,critical instability driving conditions will have a large decline,and even cause vehicle instability.In addition,intelligent motion control strategies also rely on braking systems with high precision,fast response,and strong consistent pressure control performance,thus promoting the development of electronic stability control systems to integrated brake-by-wire systems.However,this "machine-electric-hydraulic" coupled brake-by-wire system often faces nonlinear problems such as hydraulic time-varying characteristic,mechanism uncertainty friction and motor electrical characteristic coupling when braking pressure building,which affects the pressure control performance.This paper,which is relies on the 13th Five-Year National Key R&D Program Project "Research on the Construction of Hardware-in-the-loop Test Environment and Simulation Test Technology for Autonomous Electric Vehicles"(2018YFB0105103)and the National Natural Science Foundation of China Project "Research on Human-Machine Coupling Mechanism and Human-like Autonomous Driving Decision Planning Strategy in Mixed Traffic Environment"(52172386),researches and establishes longitudinal and lateral coupling motion control of intelligent vehicle based on integrated brake-by-wire system.The research focuses on the vehicle longitudinal lateral coupling trajectory tracking control strategy considering the uncertainty of system parameters under strong transient conditions,designs the stability control strategy considering the nonlinear dynamics characteristics of vehicle longitudinal lateral coupling under critical instability conditions,and develops the brake pressure control method to solve multiple nonlinear problems.The following is a brief description of the above work:(1)A longitudinal-lateral coupled motion control strategy framework and target trajectory generation method for intelligent vehicles are designed.Firstly,a comprehensive analysis of the intelligent vehicle motion control strategy is carried out,and the design objectives of "high applicability","simple system architecture" and "excellent comprehensive performance" are proposed.The characteristics of vehicle motion in different dynamic regions of steady state,strong transient and critical instability driving conditions are compared,the necessity of each motion control function is explained,and the specific requirements from vehicle motion control to integrated brake-by-wire control system are proposed;Then,a longitudinal-lateral coupled motion control strategy framework for intelligent vehicles covering the vehicle motion control layer and the pressure control layer of the integrated brake-by-wire control system is designed.The main functions of each layer are described,the interaction information between the layers is clarified,and the interaction logic within the layers is designed;In addition,a target trajectory generation method for longitudinal-lateral coupled motion control of intelligent vehicles is proposed for integration testing of the intelligent motion control strategy.A multi-objective behavior decision strategy for intelligent vehicles is designed based on hybrid collision risk assessment model,critical braking safety distance model and lateral lane change safety constraints.A longitudinal-lateral coupled lane change trajectory planning strategy is designed by fusing deep reinforcement learning,genetic algorithm and imitation learning,and the trajectory planning training results are analyzed.(2)A vehicle longitudinal-lateral coupled trajectory tracking control strategy considering the uncertainty of system parameters is proposed.First,the kinematic and dynamical performance of the intelligent vehicle in following the target trajectory is analyzed,and a longitudinal-lateral coupled nonlinear trajectory tracking model of the vehicle considering the uncertainty of the system parameters is established;Subsequently,a trajectory tracking controller with low algorithmic complexity and followable parameter calibration is proposed based on the theory of nonlinear triple-step method for solving the longitudinal-lateral coupled nonlinear dynamics problem faced by vehicles in strong transient driving conditions;Then,a system parameter adaptive law is introduced in the trajectory tracking controller based on Lyapunov stability theory,which effectively compensates the influence of the uncertainty of system parameters such as tire cornering stiffness caused by the intense motion of the vehicle on the trajectory tracking control performance;Then,the robustness of the closed-loop trajectory tracking control against system parameter uncertainties and unmodeled disturbances is proved using the input-to-state stability theorem.Finally,simulation comparison tests are designed to demonstrate the good control performance of the designed trajectory tracking strategy.(3)The longitudinal-lateral coupling stability control strategy of the vehicle under critical instability conditions is proposed.First,a vehicle motion control coordination strategy is constructed.The stability boundary parameters are obtained by the phase plane method to identify the vehicle driving stability state,and the motion control mechanism is relied on to dynamically adjust the working state of the trajectory tracking and stability controller,which dynamically resolves the conflict when the two work together and guides the vehicle motion control allocation strategy to select the appropriate output;Subsequently,an adaptive linear regression algorithm and a dimensionless performance evaluation weighting function are designed to effectively weigh the contradiction between the cyclomatic complexity and performance of the control strategy system and reasonably guide the Takagi-Sugeno fuzzy model to segmentally fit the nonlinear dynamics characteristics of vehicles in different speed ranges;Then the vehicle stability controller is successfully built by combining dynamic output feedback robust control theory and parallel distributed compensation method to cope with the longitudinal-lateral coupling nonlinear dynamics of the vehicle under critical instability conditions;Finally,the designed simulation tests prove that the stability control strategy can work well with the trajectory tracking control to control the intelligent vehicle smoothly and accurately to complete the trajectory tracking task under the critical instability working condition.(4)An integrated brake-by-wire pressure control strategy considering multiple nonlinear characteristics is designed.First,the working principle of the brake-by-wire system is systematically introduced and the key braking system model including the time-varying characteristics of the hydraulic system and uncertain friction is comprehensively constructed;The brake pressure control framework is built to successfully sort out and coordinate the brake pressure building relationship between servo brake unit,switching and functional valve unit,which is used to match the pressure control requirements of power cylinder and wheel cylinder under different braking modes;Next,based on the robust sliding mode control and radial basis neural network,a multi-closed-loop power cylinder pressure control strategy is designed to effectively solve the multiple nonlinear problems of hydraulic time-varying characteristic disturbance,mechanism uncertainty friction hindrance and motor electrical characteristic coupling in the braking process of the "machine-electric-hydraulic" coupled brake-by-wire system.Then,through the analysis of the working principle of the inlet/outlet solenoid valve involved in wheel cylinder pressure control and experimental testing,a simple and intuitive MAP of wheel cylinder pressure increase/decrease characteristics was extracted,and the design of wheel cylinder pressure control strategy is realized.Finally,through bench test,it is proved that the designed pressure control strategy can effectively ensure the pressure control performance of the braking system with high precision,fast response and strong consistency under single wheel,multi-wheel and power cylinder braking modes.(5)Finally,based on the real brake system,the self-designed high-power drive board and d SPACE equipment,the hardware-in-the-loop integration test platform is built and the integrated brake-by-wire system is successfully installed on a electric vehicle platform through the cooperation between the university and the enterprise.In the hardware-in-the-loop test,all the control algorithms of this paper are tested and analyzed comprehensively by designing working conditions including longitudinal emergency braking,constant speed lane change,brake-steer cooperative lane change,brake-steer cooperative lane change triggered stability control,etc.,which verified that the designed algorithms can help the intelligent vehicle to complete the driving tasks safely and precisely under different working conditions.The performance of this paper’s intelligent motion control strategy is verified by the real vehicle platform test.