| With the development of science and technology, the in-wheel motor electric vehicle will be a important development direction of the electric vehicle industry in the future relying on its own technical advantages. The in-wheel motor electric vehicle is a new kind of driving form, so it is necessary to analyze the dynamics control characteristics. Supported by International Technology Cooperation Project of National Ministry of Science and Technology ?Development of Electric Vehicle Technology Platform Driven by Advanced In-wheel motors‘, this paper designs the direct yaw moment control(DYC) system and the state observer based on the dynamics principle of in-wheel motor vehicle. A simulation platform, a real-time test platform and a test vehicle based on rapid prototyping technique are established to verify the effect of control strategy. The main contents are as follows:For the characteristics of in-wheel motor vehicle, this paper designs the estimation method of longitudinal velocity, tire-road friction and vehicle sideslip angle systematically. Firstly, using the method of kinematics, and integrating the GPS information and the on-board INS information longitudinal velocity can be estimated rapidly and accurately. Secondly, this paper distinguishes the μ-s curve into the linear region, the approximate linear region and the nonlinear region. The process mathematical model of K was built on the basis of the proportional relations in the linear region of μ-s curve. The recursive least squares method(RLS) is chosen to identify K. Different K values corresponds different pavement. Finally, vehicle sideslip angle observer is designed based on UKF. A 3-DOF nonlinear vehicle dynamics model considering longitudinal, lateral and yaw directions is established. Accuracy of tire force calculation is key to the quality of the observer. A modified Dugoff tire model, featuring the consideration of dynamic characteristics, is utilized to improve the accuracy of tire lateral force calculation. Off-line simulation and field test results show that the observer can provide the precision values of the vehicle state.Based on the hierarchical architecture control method, the DYC system is designed. Present the relationship between yaw rate/sideslip angle and vehicle dynamics. This paper designs the upper controller based on the stability boundary of sideslip angle-sideslip angle rate phase plane. Analyze the changing of phase plane when the external environment factors such as vehicle velocity, tire-road friction and front wheel angle change. Then,determine the calculation method for stability boundary of phase plane and quantify the degree of vehicle stability. A sliding mode controller is introduced to calculate the control efforts. Off-line simulation is made to verify the reasonability of stability boundary determined and the effect of upper controller based on the co-simulation platform.In the lower controller, sum of squares of utilization rate of the tires is selected as the optimized target of stability control, and the yaw moment is allocated to each motor optimally considering the tire-road adhesion and peak torque of motors. The rear weight coefficient of utilization is no smaller than the front axle. In addition, the influence of in-wheel motor‘s transient response characteristics to the DYC system is analyzed. Based on the DYC system of which actuators are in-wheel motors, a closed loop transfer function is established. The frequency domain analysis is made to calculate the demand range of torque response time. And simulation results show that the response time proposed can meet the demand of direct yaw moment control system.A real-time test platform is established on PXI to verify the DYC system, and the selection and debugging of vehicle model, sensors, controller and actuators is made for the real-time platform. In the establishment of platform,in view of the traditional motor modeling problem of low precision, the finite element analysis and modeling of permanent magnet synchronous motor is carried on to improve the simulation precision. FPGA module is used to make a high-speed operation of electric model to improve the real-time performance. Compare the test results and the simulation results,it can be seen that finite element model of motor can reflect the real working performance accurately, and is very suitable for the construction of the real-time platform. Based on the real-time test platform, several driving circles are made to verify the real-time performance of DYC algorithm and the coordination between the various modules.A test electric vehicle driven by in-wheel motors is made using the Motohawk rapid prototyping controller. The state observer and DYC algorithm are downloaded into the vehicle controller. According to the vehicle parameters and expected performance, match the parameters of the motors and power battery. In the process of vehicle manufacture, limited by technological level and cost, test vehicle is installed with two in-wheel motors in the rear axle. The multi-link rear suspension system is designed to cooperate with in-wheel motors well, which can guarantee the vehicle with good motility. In order to ensure the smooth progress of field test, INS and acquisition system are installed. Finally the field test results validate effect of state observer and the effect of the DYC system. |
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