Research On Low-speed Steering Resistance Torque Model Of Vehicles And Active Disturbance Rejection Control For Hybrid Power-EPS

Electric Power Steering(EPS)is not suitable for heavy-duty commercial vehicles due to large front axle load and power limitation of existing vehicle power supply system in heavy-duty commercial vehicles especially under the condition of pivot or low-speed steering.Aiming at the problem that the EPS systems under existing vehicle power systems cannot provide enough power for heavy-duty commercial vehicles under pivot or low-speed steering conditions,this thesis proposes a new EPS system which is based on the hybrid power system constituted by the vehicle power system and the supercapacitor(SC)in parallel,and thus be abbreviated as SC-EPS,to promote the application of EPS in heavy-duty commercial vehicles.In order to provide a theoretical basis for the intervention-withdrawal mechanisms of the super-capacitor in SC-EPS,the change rules of low-speed steering resistance torque with vehicle speed and steering wheel angle need to be explored,so as to determine the operation modes of the SC-EPS system based on this.When the system is in hybrid power supply mode,the Active Disturbance Rejection Control(ADRC)scheme is proposed to restrain the assist chattering caused by the voltage disturbance problem of super-capacitor.The main research contents of this thesis are as follows:Firstly,the models of pivot steering friction torque between tire/pavement and pivot steering resistance torque of vehicles are established based on calculus theory,which are verified by theoretical calculation,simulation and experiment.The result of model simulation is consistent with that of vehicle experiment,and the change rules of pivot steering resistance torque with front axle load,tire pressure and steering wheel angle are analyzed subsequently.Secondly,the values of low-speed steering friction force between tire/pavement and the exponential expression of the equivalent steering friction coefficient with the vehicle speed are obtained by means of finite element simulation and numerical fitting,and the model of low-speed steering friction torque between tire/pavement is established subsequently.Then the model of low-speed steering resistance torque is established based on the two-degree-of-freedom vehicle model and the "Magic Formula" tire model,which is verified through simulations and experiments to explore the change rules of low-speed steering resistance torque with vehicle speed and steering wheel angle.The consistency of simulation results and experimental results indicates that the established model of lowspeed steering resistance torque has a good accuracy.The surface diagram of low-speed steering resistance torque with vehicle speed and steering wheel angle for heavy-duty commercial vehicles is obtained based on the model simulation results.Thirdly,the analysis of algorithm principle and design of structural model for the second order ADRC controller of SC-EPS system under the hybrid power supply model are carried out.Then some input signal simulations are well performed,which verified the rationality of the selected parameters and the validity of the established model.Fourthly,the assist characteristic curves of SC-EPS system are designed,and the relationships between the target assist current with steering wheel input torque and vehicle speed are obtained.The mathematical models and simulation models of mechanical drive parts and assist motor of SC-EPS system are established,and the PID control scheme and ADRC control scheme are selected respectively to carry out the simulation and analysis for the SC-EPS system under the hybrid power supply mode.The simulation results show that the effect of ADRC control is better than PID control in terms of suppressing disturbance signals.Finally,the power steering characteristic curve of the design is preliminarily verified by the test bed of the electric circulating ball steering gear.To sum up,the model of low-speed steering resistance torque established in this thesis has a good accuracy in reflecting the laws of its change with the vehicle speed and steering wheel angle,and the ADRC control scheme proposed in this thesis has a good effect on restraining the assist chattering caused by the voltage disturbance of supercapacitor in the hybrid power supply mode.