Study On Integrated Control Of Dual-mode Interconnected Suspension And Active Front Steering

Pursuing more superior safety and comfort are the two important directions for the development of automotive technology,and the improvments of vehicle lateral and roll stabilities and the ride comfort are the significant manifestations for the two major directions.As known to all,steering system and suspension system,which are the two key sub-systems of the vehicle chassis system,play a critical role in enhancing vehicle stability and ride comfort,respectively.However,it should be noted that the two sub-systems couple with each other through tire force when they work together on the chassis dynamics system,thus a collaborative problem emerges when the two coupled sub-systems are controlled simultaneously.Since the traditional method of independent optimal control can not produce the optimal comprehensive performance,to excavate the maximum functional potential of the steering and suspension sub-systems,an integrated control system is necessary to develop to coordinate the overlapping and conflicting functions of the two coupled systems.As a result,this dissertation has carried out the following studies for the integrated control between steering system and suspension system to improve the vehicle stability and ride comfort.(1)A vertical and lateral coupled full-car dynamics model of 9-DOF was established,and the dynamics characteristics of the bottom execution systems were especially emphasized.Therefore,a "Mechanical-Liquid-Gas" coupled nonlinear mathematic model of the hydraulic actuator of suspension sub-system,and a dynamics model for the active front steering(AFS)and electric power steering(EPS)structure-integrated sub-system were established,aiming to check the effectiveness of the control system developed later within the capabilities of the executive systems.The proven vehicle dynamics commercial software including Carsim and AMESim,and bench test were performed to ensure the accuracy of the established models.(2)To make some preparations for the follow-up proposed passive/active switched hydraulically interconnected suspension(HIS)aiming to improve vehicle anti-roll performance,the dynamics characteristic analysis and parameter optimization for the HIS system were carried out.Specially,based on the stiffness and damping mathematical models of the bounce and the roll modes of the hydraulic interconnection system,the stiffness and damping characteristics,and frequency response of the hydraulic interconnection system under the two modes were analyzed respectively in the time domain and the frequency domain.Subsequently,parameter sensitivity under the two modes of the passive HIS system was analyzed using Isight software,and then according to the results of the sensitivity analysis,a parameter optimization scheme was proposed and the corresponding optimization results were obtained.(3)Integrating the principle of state estimation and optimization and combining the Unscented Kalman Filtering(UKF)and the Particle Swarm Optimization methods,the parameter identification for the tire model was completed.The work in details is as follows,taking vehicle lateral acceleration and yaw rate as the measurement outputs,the preliminary estimation result was obtained by the UKF method,which provides initial values and reference optimization ranges for the PSO algorithm,so that to realize the offline identification of the pure lateral model of the nonlinear tire and provide accurate tire model for the model-based state estimation and control system.Subsequently,a state estimation system was established by employing the UKF method,and typical states were selected as the accuracy verification representative of the observing system,then a real test was performed by using a GPS/INS integrated navigation.(4)The active interconnected anti-roll control mode(AHIS mode)and non-interconnected ride comfort control mode(IASS mod)were defined.As for the AHIS mode,an objective function for vehicle body roll angle was constructed to ensure a smooth body posture control process,and an active anti-roll control system was designed by using the backstepping algorithm.Considering that the passive HIS has higher roll stiffness than traditional suspension but with little influence on vertical mode,we proposed a passive/active switched HIS control method,and for the time delay problem,it was tackled by combining a nonlinear backstepping algorithm and an improved Smith Predictive compensation algorithm.As for the IASS mode,a nonlinear filtering method was developed to make trade-off between the vertical acceleration and the suspension deflection.Meanwhile,a multi-objective control system was developed with consideration of the nonlinear characteristic of the hydraulic actuator.Based on the above researches,we proposed a demand-oriented dual-mode switched active suspension control method to take advantages of the two modes.(5)An intervention criterion was developed for the AFS system which uses as an emergency control technology,namely,the intervention time of the AFS system for stability control was suggested to be determined by the tire linear/nonlinear working state,which can avoid being too early intervention to conflict with the driver's intentions and too late intervention to lose the ability of recovering stability.According to the intervention criterion,a rapid judging method was proposed to ascertain the nonlinear region for lateral tire force,then a controller was designed by using the sliding mode control algorithm.Finally,as for the AFS and EPS structure-integrated system,a collaborative control method was developed to solve the discomfort problem caused by abrupt change of steering torque produced by the AFS's intervention.(6)Considering the constraint of road adhesion coefficient,a phase-plane method of "sideslip angle-the rate of sideslip angle" was improved to determine the stability regions of sideslip angle and yaw rate,then a lateral stability region of vehicle was enveloped by the upper,the lower,the left and the right bounds,and the road adhesion constraint.On this basis,the switching criteria of ride comfort control and handling stability control were designed for the IASS mode,and the switching criteria of yaw rate control,sideslip angle control,and their proportional weights control was designed for the AFS system.Meanwhile,a phase-plane method of "body roll angle-body roll rate"(?-?')was improved to determine the roll stability region,and then a switching criteria for the AHIS mode and the IASS mode was developed based on the ?-?' phase-plane.According to the above criterias,the collaborative strategy for the vertical and lateral integrated system was developed,and the integrated control system imcluding organization layer,coordination layer and executive layer was completed using hierarchical control method.