Coupling Bifurcation Of Vehicle Driving Torque And Steering Angle And Solution Of Driving Stability Region

The bifurcation characteristics is not only the main content of the nonlinear vehicle dynamics mechanism research,but also is the most intuitive,the most effective method from the perspectives of driving the car stability.But,at present the dynamic nature and the integrated dynamics control criterion under the instable driving condition combined with steering condition still lacks the complete theory research.Recent years,the development of tire dynamics provides the foundations,for which is able to reveal the driving and steering coupling bifurcation characteristics effects on vehicle stability under the driving and steering combined condition.But due to the lack of deep study of coupling bifurcation characteristics,the bifurcation mechanism,which only takes the steering angle as a parameter,can not fully uncover the coupling bifurcation characteristics,and also cannot explain the drive torque and the drive mode influence on vehicle handling stability.Therefore,based on the analyzing the driving influence on vehicle dynamics,study of the driving and steering coupling bifurcation will deepen the mechanism understanding of vehicle coupling bifurcation,and will provide a scientific basis for engineering practice.The common automotive integrated control now is the association of the independent control systems to obtain the best control effect through some control method,such as coordination control.These independent control systems are usually on the basis of steering bifurcation mechanism.But under the driving and steering combined condition,the the coupling features between tire longitudinal forces and lateral forces must make to bifurcation characteristics change.Moreover,the corresponding stable driving area will also be reassessed,and the corresponding control method also be adjusted.At present,however,many stability region studies based on steering bifurcation do not consider the driving influence,the conclusions and results still can't be used in steering and driving integrated control directly.So the stability region research based on the driving and steering coupling bifurcation is not only the advantage that automotive research achievements of nonlinear dynamics was applied to engineering practice,but also can provide the unified dynamics criterion of the vehicle handling stability integrated control and safety assistant driving system design.Based on the inner relations among planar motion,wheel rotation and tyre dynamics characteristics,the related study of driving and steering coupling bifurcation characteristics,as well as the driving stability region was achieved in this dissertation.The concrete research content is as follows:(1)Five degrees of freedom(5DOF)vehicle dynamics model involved the driving factor is established.The uniform slip equation was proposed.Based on the wheel rotation dynamics analysis,the classical tire longitudinal slip rate equation is verified under instable driving situation.A uniform slip equation is proposed by deduced on the extension of classical definition of slip rate equation.The 5DOF planar motion nonlinear dynamics vehicle model is established.Based on the conventional 3DOF vehicle system model and the tyre combined slip feature,a 5DOF planar motion nonlinear dynamics vehicle model is established under the consideration of wheel rotation equation.The simulations are carried out to validate the slip equation and the 5DOF vehicle model.The tire dynamics simulation test illustrated the effectiveness of the uniform slip equation in tyre bombined slip and the correctness of the tyre dynamics analysis.The typical driving condition of the vehicle simulation,including straight line driving and the global phase space characteristics with driving torque is equal to the initial state wind resistance,verified that the 5DOF model is more superior than 3DOF.The 5DOF model can be used to express the driving impacts on the vehicle dynamic characteristics.These research provide the foundational model for whole dissertation.(2)Vehicle dynamics equilibriums solution and bifurcation confirmation.Vehicle dynamics equilibriums solution method based on hybridization of Genetic Algorithm and Quasi-Newton Method is introduced.A hybrid method is proposed for vehicle dynamics equilibrium search,which combines real-coded Genetic Algorithm with Quasi-Newton gradient method.The hybrid method was verified by equilibriums solution search of Van der pol equation and Duffing equation.Vehicle dynamics equilibriums were searched.5DOF vehicle dynamics equilibriums with variable driving condition and front steering angles are searched using proposed algorithm.And the hybrid method is verified by comparing with conventional GA solutions.Equilibrium bifurcation was confirmed.According to the definition of static bifurcation of nonlinear systems,the equilibrium bifurcation of 5DOF vehicle system was confirmed.By the characteristics analysis of the global phase space,front wheel steering angle and driving torque effect on equilibrium bifurcation was done.(3)Analysis of driving effect on vehicle dynamics with no front wheel angle input.The driving torque effect on vehicle dynamic characteristics is analysed.Take rear-wheel-drive mode as an example,analysis were achieved under the condition is as follows:There is no front wheel angle input.Constant driving torque was kept to accelerate with different initial velocity.The simulation results include multiple initial values global phase space characteristics and tyre mechanical characteristics time sequence with single initial value,etc.The driving mode effect on vehicle dynamic characteristics is done.Take the constant driving force as an example,analysis were achieved under the condition is as follows:there is no front wheel angle input.Different driving mode involves front-wheel-drive mode,rear-wheel-drive mode and four-wheel-drive mode.And initial velocity is identical.The simulations include multiple initial values global phase space characteristics and tyre mechanical characteristics time sequence with single initial value,too.Analysis of driving torque effect on bifurcation is performed.Based on further analysis of constant driving torque tests and the bifurcation diagram of the system state variables,it is obvious that the 5DOF vehicle dynamics system bifurcation phenomenon will occur as the drivding torque values increased with no front wheel steering angle input.These results illustrate that the driving effect on the vehicle dynamics is obvious,and it is unreasonable to ignore the driving effect when analyzing the bifurcation characteristics.(4)Analysis of the driving and steering coupling bifurcation characteristics.Bifurcation analysis with periodical front wheel steering angle input was finished.The 5DOF vehicle system model was transformed into 7DOF autonomous equation with the front wheel steering angle as intermediate variable.From the two aspects of constant driving torque and constant angle amplitude,the bifurcation diagrams were calculated.The evolution process of dynamic bifurcation was analysed.Numerical results include the phase space trajectory,state variables time sequence,poincare section,Lyapunov index and and power spectral density.The driving and steering coupling bifurcation characteristics is analyzed.Base on the former simulations,it is illustrate that the bifurcation parameter values will influence each other with the driving and steering combination.And the driving and steering mechanism of the coupling bifurcation is revealed.These achievements show that the change of driving torque and the front wheel angle will lead to dynamic bifurcation of the automotive systems.And the driving and steering coupling bifurcation mechanism is revealed.(5)Solution of driving stability region based on the driving torque and steering angle bifurcation.The driving stability region is calculated.In combination with the physical significance of vehicle system dynamics equilibrium,the driving stability region definition based on equilibrium bifurcation characteristics was proposed.By the hybrid algorithm,stability region was solved.The driving simulation has been carried out to verify the correctness of the stability region.Some typical initial points were selected to conform the correctness of driving stability region.Simulation results include phase space analysis,state variables time series and body posture analysis and so on.These study is a contribution to vehicle handling stability integrated control and safety assistant driving system design,which provides a new method and theory basis.To sum up,the driving(including driving torque and driving mode)and steering coupling effect on handling stability was discussed comprehensively.The driving and steering coupling dynamics bifurcation mechanism of the vehicle handling stability was revealed.The driving stability region based on driving force and steering angle was calculated.These study provides fundamental for vehicle handling stability integrated control and safety assistant driving system design.