Study On Vehicle Nonlinear Dynamics And Control Under Extreme Drifting Maneuvers

With the development of autonomous driving technology at home and abroad and the increase of people’s requirements for the active safety of vehicles,the research on the nonlinear dynamics and control of vehicles is particularly important under extreme conditions.Vehicle control under extreme conditions has always been the most challenging domain to meet the functional safety requirement of semi/fully autonomous vehicles.Drifting is a unique dynamic phenomenon of a vehicle under extreme conditions which is caused by the demand for high-speed cornering in motorsport.The driver deliberately puts the vehicle in an oversteering state by making the rear wheels or all tires in a full-slip state,and accurately controls the throttle and steering to make the vehicle achieve fast steady-state cornering.Due to the saturation of the rear tires,the vehicle in drifting is very unstable,and an experienced professional driver is required to keep the vehicle from getting out of control.This obviously does not meet the requirements of the current automotive safety system,but the research on drifting helps to fully understand the dynamic mechanism and the possibility of control of the vehicle under extreme conditions,thereby improving the autonomous vehicle’s controllability and emergency hedging capabilitiesDrifting involves complex nonlinear dynamics and control.During the drifting,the vehicle is under the combined extreme conditions of driving,braking,and steering.The tire’s lateral slip rate and longitudinal slip rate are both very large,far exceeding the linear region of the tire mechanical characteristics.Within such scope of slipping,tire force and moment would have speed-dependent friction feature.Drifting control is the control of the vehicle’s steady cornering with rear tire saturation.Because the rear tire is always saturated during the drifting,the vehicle is extremely unstable and the control must have strong robustnessIn this paper,the following three aspects were studied:nonlinear tire modeling and vehicle modeling,drifting dynamics mechanism analysis,drifting controller design and simulation.The main research contents and conclusions are as follows:Firstly,combining the accuracy of the model and the practicability of the control design,the tire model under the conditions of pure lateral slip/combined lateral and longitudinal slip and the two/three-degree-of-freedom vehicle models were established respectively.The pure lateral UniTire model with speed-dependent tire-road friction was established considering the strong nonlinearity and dynamic friction characteristics of the tire under drifting maneuvers,and the parameters of the tire model were identified based on the test data.Then,a tire model under the condition of combined lateral and longitudinal slip was established based on the pure lateral UniTire model,and two/three-degree-of-freedom vehicle models were established based on the vehicle parameters of the D-class SUV in CarSim.Secondly,the mechanism of drifting dynamics was analyzed.First,the equilibrium state of the two-degree-of-freedom vehicle model was analyzed.Based on this,the equilibrium state of the three-degree-of-freedom vehicle model was analyzed.The analysis of the equilibrium state was mainly carried out from three aspects:changing the steer angle with constant vehicle speed,changing the vehicle speed with the constant steer angle,and using the phase portrait to analyze the properties of the equilibrium point and describe the state changes of the system near the equilibrium point.The analysis results show several key characteristics of drifting.The first is that drifting corresponds to the vehicle’s open-loop unstable equilibrium point because drifting is usually accompanied by countersteer,large sideslip angle,and obvious rear wheel slip.The second is that the introduction of the dynamic friction coefficient in the tire model has a significant impact on the drift equilibrium state with the lateral force of the front and rear tires and the yaw rate at the drift equilibrium increasing or decreasing correspondingly with the change in the friction coefficient.The third is that the change of sideslip angle near the drift equilibrium has a non-minimum phase characteristic and the change of the yaw rate has a significant impact on the sideslip angle.The last is that the rear drive force could control the lateral dynamics around the drift equilibrium as a result of the coupling between the rear tire lateral and longitudinal forces that arises from rear tire saturation.Thirdly,combining with the change mechanism of the drift equilibrium,the controller for automated drifting along the circular trajectory was designed and simulated based on the principle of sliding mode variable structure control.The lookahead error and the sideslip angle were chosen as the control states,and first-order feedback control was used to control the lookahead error,and sliding mode variable structure control was used to control sideslip angle The vehicle simulation demonstrated the effectiveness of the control algorithm based on Simulink/CarSim platform.The simulation results showed that the control algorithm could accurately track the circular trajectory while drifting.