Compared with centralized drive electric vehicles, the mechanical link between the driver’s action and actuator of distributed drive electric vehicles (DDEV) has been removed and replaced with X-by-wire system (e.g., drive-by-wire and brake-by-wire), and distributed drive electric vehicles utilize in-wheel motors to drive the wheels such that the torque of each wheel can be controlled independently and fast, which significantly simplify the traditional vehicle structures aand improve transmission efficiency, and further it provides unique advantage with more flexibilities in vehicle dynamics control (e.g.,generate the external yaw moment). The the direct yaw control system (DYC) provided by steer-by-wire technique, together with active front steering system (AFS), possesses potentiality to improve vehicle handling performance and lateral stability of distributed drive electric vehicles. However, performance and stability of vehicle motion control systems heavily depend on accurate knowledge of important vehicle parameters,meanwhile, advanced control technology in vehicle lateral dynamics control systems is also very crucial for enhancing vehicle stability and handling performance of distributed drive electric vehicles.With regard to distributed drive electric vehicles, this dissertation studied parameter estimation and lateral stability with robust control strategy for the lateral dynamic system, the main contents are as follows:
(1) Research the problem of "nonlinear function × gaussian density" in bayesian filtering solved with third-order spherical-radial cubature criterion, the new cubature kalman filter theory under gaussian bayesian nonlinear filtering framework is introduced into the vehicle dynamics state estimation system, the CKF estimation algorithm is designed to estimate longitudinal velocity, lateral velocity and vehicle sideslip angle utilizing real-time measurements of in-vehicle sensors in distributed drive electric vehicles. And then the high-fidelity co-simulation platform between Carsim and Matlab/Simulink for the DDEV is established, simulations evaluate the performance of the designed CKF estimation.
(2)To real-time estimate vehicle roll angle,lateral tire-road forces and tire-road friction coefficient, the nonlinear Dugoff dynamic tire model is applied to describe transient characteristics,dual CKF is designed to jointly estimate vehicle state and parameter for DDEV, the first CKF estimate vehicle velocity, vehicle roll angle,lateral tire-road forces while the second CKF estimate tire-road friction coefficient based on estimated vehicle states from the first CKF.In order to effectively improve the real-time, accuracy and stability of the joint estimation in dual CKF, the square root cubature kalman filter is also applied.Simulation is implemented to evaluate the performance of the proposed dual CKF estimation.
(3)The phase plane approach is employed to analyze the nonlinear motion stability in vehicle yaw dynamics system,and the the phase space about vehicle yaw motion is plotted as vehicle sideslip angle and yaw rate, and then the lateral stability region where critical stable boundary depends on the unsteady equilibrium point-saddle point is analyzed in term of tire-road friction coefficient,steering wheel angle and longitudinal velocity.Uncertain factors such as tire cornering stiffness and vehicle mass in vehicle lateral dynamics are represented via the norm uncertainty. To address the importance of time-varying longitudinal velocity for vehicle lateral stability control, uncertainty about longitudinal velocity is also considered.Combined AFS and DYC control-oriented vehicle lateral dynamics with multi-uncertainties is developed.The uncertain vehicle lateral dynamics system is augmented with dynamic error, and then the resulting robust feedback controller is finally designed, which ensures the corresponding closed-loop system is asymptotically stable with a quadratic H? performance.Simulation results show that the proposed controller can effectively preserve vehicle yaw stability even when the vehicle undergoes extreme double-lane change maneuvers under high longitudinal velocity.
(4)In order to improve vehicle roll stability, the nominal value of vehicle yaw rate is corrected according to the defined roll stability index that is comprised of vehicle load parameter, vehicle roll angle and critical lateral acceleration. Roll stability control-oriented vehicle roll dynamics with uncertainties is developed.The control saturation is also considered due to the physical limitations of actuators, active roll robust controller considering control saturation is designed with Lyapunov theory under unified framework of anti-saturation H? control, the feasibility conditions are derived from Finsler recursive theorem so that the closed-loop system is asymptotically stable and possesses H? level. Simulations show active roll robust controller has lateral stability margin and can avoid vehicle instability caused by the vehicle large roll posture.
(5)To demonstrate the effectiveness of proposed estimation algorithm and robust control strategy, road experiments are carried out on distributed drive electric vehicles experimental platform.All road experiments were conducted under low speed due to safety concerns.J-turning and serpentine turning are tested to assess estimation algorithm,and half circle and two long-time continuous intense steering tests are implemented to evaluate robust control strategy.Experiments results verify the effectiveness and the feasibility of developed estimation algorithm and robust control strategy.