Research On Estimation Of Vehicle States And Parameters And Integrated Control Method For Vehicle Stability

The vehicle is a complex dynamics system with high nonlinearity, model uncertainties, parameter perturbation and external disturbances. When the vehicle is made a big steering at high speed, tire lateral force is easy to achieve adhesion limit. When the side force of vehicle goes into saturation limit, vehicle is difficult to operate for non-professional drivers, and may go into lateral instability. With the development of auto technology, the speed of vehicle is higher and higher; and how to guarantee the lateral stability of vehicle when turning at high speed is an important research direction of the vehicle stability control at present. Among various kinds of vehicle active safety control system, active steering system and active braking system are the main methods to improve the vehicle active safety and stability. There is a trend that the actuator of vehicle stability control system is being standardized, and developing an advanced control method can ehance the performance of vehicle stability control system. At the same time, estimating the state and parameter information of the vehicle is the key problem of the research of vehicle stability control, and is the premise and necessary condition to realize the closedloop feedback control.The integrated control strategy that can be against the model uncertainty and improve the stability of the vehicle is the research focus in the paper. This paper also studies the adhesion coefficient and mass side-slip angle prediction technology. The main research contents are as follows:(1) The 2-DOF vehicle model and Fiala tire model are used for the estimation of adhesion ground coefficient; the 8-DOF vehicle model of four wheels considering the roll situation and the Magic Formula tire model are used in the vehicle simulation; the 2-DOF vehicle model and the Magic Formula tire model are used for the vehicle sideslip angle estimation; single-point preview driver model which is based on the “preview-following” theory is established; and double lane-change input path is fitted by Cubic spline curves. Central steering tests of real vehicle are done by steering robot in order to verify the validity of the vehicle simulation model, and the availability of the established simulaiton model is verified by comparing the lateral acceleration, the yaw rate curves of the simulation model and the test vehicle.(2) In order to improve the estimation accuracy of the vehicle sideslip angle, the extended Kalman filter is used to estimate the sideslip angle. This paper develops a data acquisition system of sideslip angle based on differential GPS, in which the sidelip angle between vehicle longitudinal axis and the velocity vector is obtained by Gauss projection coordinates transformation. Then the feasibility and validity of the extended Kalman filter are verified by the vehicle sideslip angle acquisition equipment based on GPS.(3) The method of estimating the total vehicle steering resistance torque based on the current of SBW steering motor is given in the paper. Total steering resistance torque caused by tire six-component forces is analyzed by using the spatial coordinate transformation, and then the tire self aligning moment can be extracted from the total steering resistance torque. A nonlinear observer based on Fiala model and two degrees of freedom vehicle model is used to estimate road friction coefficient. In the paper, the total kingpin steering resistance torque is obtained from the force sensor attached to the left and right of the steering rod, and the tire self aligning moment can be extracted from the total kingpin steering resistance torque. Then the self aligning moment is used to estimate road friction coefficient, the results demonstrate the effectiveness of the method of nonlinear observer. This method can identify road friction coefficient early when the tire lateral force has not reached the saturation region.(4) The current researches of active front steering system mainly focus on advanced control method design. Because of the tire nonlinearity and vehicle’s parameters’ uncertainty, robust control methods have been widely used in active front steering control, however, the robust control is at the cost of performance so that the robust controller is a little conservative and has low performance for SBW control. Therefore looking for a new control method that can overcome the contradiction between the performance and stability of controller is the key issue. In this paper, a generalized internal model robust control(GIMC) that can overcome the contradiction between performance and stability is used in the SBW control. In GIMC, the Youla parameterization is used in an improved way. Simulations of double lane change(DLC) maneuver and that of braking on split-μ road are conducted to compare the performance and stability of the GIMC control, the nominal performance PID controller and the 2-DOF H∞ controller. Simulation results show that the high nominal performance PID controller will be unstable under some extreme situations because of large vehicle’s parameters variations, 2-DOF H∞ controller can not consider the limitaion of the road adhesion, and only the GIMC controller overcomes the contradiction between performance and robustness, which can both ensure the stability of the SBW controller and guarantee the high performance of the SBW controller.(5) The stability of vehicle is analyzed by ?? ?? phase plane of vehicle in the paper, and the linear stability boundary is determined by linearizing the stability boundary of phase plane. Active steering control characteristics is analyzed, and the coordination distribution logic for active steering and active brake control is proposed based on the stability region of phase plane. The yaw rate and sideslip angle are used as two control variables in the integration control. According to the upper road friction coefficient, the nominal yaw rate and the nominal sideslip angle are corrected. The GIMC obtained by modifying internal model control(IMC) control structure is proposed for the integrated control of SBW and DYC to surmount the conflict between performance and robustness. Double lane change(DLC) simulation is developed to compare the performance and the stability of the GIMC strategy, the PID controller based on the reference vehicle model and the 2-DOF H∞ controller. Simulation results show that the PID controller may oscillate and go into instability in severe driving conditions because of large variations of tire parameters, the 2-DOF H∞ controller can not enhance the stability of vehicle because 2-DOF H∞ controller can not consider the limitaion of the road adhesion, only the GIMC controller can both ensure the robustness and the high performance of the integrated control of SBW and DYC.