| Nowadays the Electric Vehicles (EV) are one of the effective ways to keep a sustainable development tendency under the pressure of the protection of natural environment and the shortage of oil resource in automobile industry. The four-wheels-independent-drive & steering(4WID/4WIS) vehicle has attracted more focus because of its advantages such as more flexible steering and driven styles. The controllability and stability can be improved obviously by controlling the steering and driving coordinated and integrated. Furthmore, the variable transmission ratio of steering can promote the safety of the vehicle very well, which shows that the vehicle has very wide prospection.Supported by Guangdong Provincial Natural Science Fund and Shenzhen Science and Technology Development Fund Project, the 4WID/4WIS vehicle is studied in this paper. After reviewed the current status of 4WID/4WIS technology, we focus on the steering stability control of the vehicle with the function of variable transmission ratio. The main work of this paper is as follows:Firstly, linear and non-linear models are established and driver model is introduced too. These above mentioned models are used to design the the reference model of linear and non-linear stability control systems and verify with the open-loop control and the closed-loop control algorithm. In the tyre modeling part of the complete-vehicle dynamical model, in order to facilitate the theoretical analysis, the Dugoff tyre model, which is capable of reflecting the non-linear features of tyre with clear physical meanings and simple form is used.Secondly, based on the optimum control principle, the active rear-wheel steering controller is designed to realize the function of variable transmission ratio in traditional mechanical steering, and guarantee drivers to acquire the ideal steering characteristics. In the design process of control strategy, the expected yaw velocity capable of symbolizing the ideal steering characteristics is designed based on the variable transmission ratio changing curve, and then the active rear-wheel controller is designed by adopting the LQR optimum control principle to realize the follow up for the expected yaw velocity, and improve the transient characteristics of driver steering. The simulation results show that the control strategy has good steering characteristics, and the active rear-wheel steering control strategy based on the variable transmission ratio meets the design requirements and could guarantee drivers to acquire ideal steering characteristics, and even guarantee vehicle to acquire manoeuvre stability under extreme working conditions.Thirdly, the slip angle of tyre is larger and falls into non-linear area of the lopsided characteristics in the process of slippage or high-speed steering. Meanwhile, considering the coupling and control redundancy of various systems, the integrated control on ARS controller and DYC controller is made. The ARS controller designed by adopting the linear sliding mode variable structure control could meet the demand on the stability control of vehicle in the linear region. For the stability control of vehicle in the non-linear region, the non-linear DYC is designed so as to improve the control performance of the controller in the non-linear region and under the extreme working conditions. When ARS controller can complete the tracking task independently, DYC controller will not work; when ARS controller can not complete the tracking task independently, the insufficient gain will be compensated by the DYC controller. The open-loop and the closed-loop simulation results show that the proposed integrated control system could realize the integrated control target better and provide margin for further utilizing the longitudinal control to stabilize vehicle control system.Finally, on the basis of 4WID/4WIS EV upon the four-wheel independent driving to be researched and developed independently, the architecture of hardware and software control system is introduced, and the theoretical research and experimental platform of steering stability control is established. The algorithm effectiveness of the the active rear-wheel steering control strategy based on the variable transmission ratio is verified through experiment.The ARS and DYC integrated control system upon the integrated variable transmission ratio raised by the paper can be used as the important theoretical basis and reference for the research of All-wheels-independent-drive & steering, and it has good universality. |
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