Integrated Control Of Active Steering And Hybrid Braking In Distributed Electric Drive Vehicles

In recent years, most of the research and development(R&D) have focused on new energy vehicles, namely, vehicle electrification, due to the oil crisis and the more and more strict standards on vehicle emissions as well as safety. However, this poses considerable challenges and needs revolutionary alternatives to the well-established traditional vehicle technologies associated with powertrains, chassis design and layout, and vehicle dynamics and stability. Inevitably, significant efforts have been made in the control and energy management of powertrain systems, however, the impact of vehicle electrification on vehicle dynamics and control systems has received a little attention. For instance, with current technology, the battery packs are still extremely heavy, which tends to increase the sprung mass and roll moment of inertia and even the height of vehicle center of gravity(c.g.), and then deteriorates the roll stability and poses a higher challenge in vehicle roll controls. Even if guaranteeing the stability of the vehicle, there still have the challenges on saving energy of motors. So, the integrated control of vehicle stability and energy efficiency for new energy vehicles is a new research topic. The electric motor is the key component connecting electrified vehicle performance with vehicle dynamic. However, due to the limitation of the maximum regenerative power and the battery state of charge(So C), the maximum braking torque of individual motor is unable to meet the needs of the emergency braking case, so a hybr id braking based on the electric motor and the friction brakes has emerged. The wheel slip control based on the hybrid braking will be in a more complex coupling mechanism, which results in some difficulties, such as how to determine the optimal split betw een electric and friction brake torque as well as how to deal with the coupling between the braking energy recovery and tire nonlinear characteristics.Because of these, this article focuses on the integrated control of active front independent steering(AIFS, proposed in recent years) and hybrid braking based on the distributed electric drive vehicle. It pays attention to the difference between electric vehicles and internal combustion engine vehicles in terms of vehicle dynamics and control, considering t he impact of the wheel slip control with new hybrid braking on ABS and ESP, and plays to the strengths of active front independent steering and four-wheel independent hybrid braking in term of vehicle stability control and breaking energy recovery. Finally, an integrated control strategy of AIFS and hybrid braking based on hierarchical structure and advanced control allocation is proposed to ensure the vehicle stability and energy efficiency. The main work is as follows:(1) Vehicle dynamics modeling and verification. Firstly, the actuator model of AIFS system is established including the multi-body dynamics model and the mathematical model for analyzing suspension characteristics and full vehicle performances and vehicle stability control, respectively. Se condly, the actuator model of hybrid braking system is also built, containing a pade approximation for the time delay and the processing of the discrete state equation model. Then, vehicle dynamics model and tire models are built for verifying the effect o f the controller. Finally, a verification of full vehicle model is carried out to guarantee the precision of the model using two business vehicle dynamics softwares of ADAMS/car and Car Sim.(2) Vehicle state estimation. Vehicle dynamics control is based on accurately determining the vehicle motion states, driving environment, road and tire adhesion. Therefore, the estimation of vehicle states is performed using Kalman Filter(KF) algorithm, such as vehicle sideslip angle, roll angle, roll rate, etc. A real vehicle road test is implemented to verify the estimation of vehicle states based on the attitude and position integrated navigation system. In addition, a sliding-mode observer is built for estimating the tire longitudinal force and it has been validated by the simulation method.(3) Firstly, this dissertation creatively analyzes the impact of AIFS system on the suspension characteristics and full vehicle performances from the perspective of multi-body dynamics, which provides the effective guidance on veh icle stability control. Then, a sliding mode controller with good robustness to parameter perturbation as well as rule-based allocation method is designed for vehicle stability control. Besides, a new wheel angle distribution algorithm of the AIFS system i s presented based on control allocation method, in terms of the fact that the rule-based allocation algorithm may cause lower adaptivity and obtain an optimal result difficultly. Finally, the control effect is verified by numerical calculation.(4) The current studies on hybrid braking torque blending are focused on the open-loop planning method, which will result in the deviation between the desired torque value and the actual hybrid braking torque response. Therefore, a predictive dynamics control allocation(PDCA) method is employed to determine the optimal split between electric and friction brake torque, which can explicitly consider the actuator dynamics by formulating them as equality constraints. In addition, two control allocation methods of static control allocation and dynamic control allocation are adopted to compare the distribution effects with PDCA. On the other hand, a new control algorithm of wheel slip control(WSC) for hybrid braking is proposed based on the sliding mode extremum-seeking algorithm(SMESA), in terms of the fact that the road adhesion coefficient is complex and variable and the optimal slip ratio is difficult to be estimated accurately, and deducing the SMESA’s sliding mode condition. A straight-line emergency braking situation is performed to verify the above-mentioned control method firstly. When braking in a bend, the active front steering(AFS) compensation method and the method of amending control law are both carried out to improve vehicle lateral stability considering th e coupling dynamics between the wheel slip and tire longitudinal and lateral force. Moreover, numerical calculation is performed to verify the control effect.(5) In order to guarantee vehicle stability and energy efficiency, an integrated control strategy of AIFS and hybrid braking is proposed. Firstly, the overall structure of the integrated control system is programmed, and the coordinated control rule is designed through the phase plane analysis of vehicle roll and yaw dynamics; Secondly, the solver generated for PDCA and its compiling process based on an embedded convex optimization code generator of CVXGEN is introduced. Then, the integrated control strategy designed to be a hierarchical structure is studied specially, and the coordination layer is designed using the coordinated control rule to switch different control modes. In the first layer, the sliding mode controller is employed to calculate the generalized forces for compensating the vehicle stability based on the identified driver’s intention and the estimated vehicle states. In the second layer, an optimization-based control allocation strategy is used to map the upper level and wheel slip control inputs to actual actuator commands of AIFS and four-wheel independent hybrid braking, taking into account the actuator constraints. In the third layer, the PDCA method is employed to determine the optimal split between electric and friction brake torque based on the braking actuator commands, which ensures vehicle stability and energy efficiency. Nume rical simulation studies have been conducted to evaluate the proposed control algorithm using the double lane change maneuver and the fishhook maneuver.