Methodology And Optimal Control Of Energy Efficiency Improvement For Vehicle Steering-by-wire System

Steering by wire technology is one of the key technologies to realize the intellectualization,networking and electrification of automobiles.As essential components of the steering-by-wire system,the power of those motors are the bigger energy consuming components in the low-voltage electrical system of automobile.Energy consumed by the steering-by-wire system will result in load fluctuating of automotive power supply system and the problem of energy saving and emission reduction of automobile,which cannot be ignored as vehicles steering consequently during driving.In this paper,energy efficiency improvement methodology and optimal control of vehicle steering-by-wire system are discussed.Firstly,based on the knowledge of vehicle dynamics,the simulation platform of steering-by-wire system is built with Carsim and Matlab/Simulink software.Based on the simulation platform,the tracking characteristics and stability of steering by wire vehicle are tested in three steering ratio control modes,i.e.constant steering ratio,variable steering ratio and steering ratio calculated with PID algorithm.After that,the conclusion is drawn after analyzing the steering energy consumption of automobiles under common steering conditions that the energy consumption of steering by wire is greater than that of power steering,and an energy efficiency improvement method is proposed,which is based on structural improvement and optimization of original system structural parameters.Taking the electric drive steering unit as an example,the structural parameters optimization analysis of energy efficiency improvement is carried out with several parameter optimization methods after establishing the parametric model of the electric drive steering unit.Several results are acquired,which are the curve of mechanical system's efficiency varying with the pressure angle,and region distribution clouds of the system efficiency,reducer gear ratio and pressure angle of the rack.Based on this,a design scheme of two-gear deceleration ratio is proposed and preliminarily verified under the condition of double lane change.The results show that the comprehensive energy efficiency improvement is less than 5%.Then,based on the energy efficiency improvement method,structures of the electric drive steering unit and the steering wheel unit are improved and controlled with a low energy consumption controllable variable damping unit.The controllable variable damping unit are realized by an electric-driven variable damping rotary hydraulic damper designed according to the principle of variable volume,which also could be used as a torque commutation unit.The mathematical model of this damper is established,which include the guide curve equations of the damper blade,the method of calculating and adjusting the damping ratio,and so on.The simulation results show that the damper canadjust the damping ratio in a wide range.Based on the damper,the structure of the electric drive steering unit and the steering wheel system are improved.The structure of the improved electric drive steering unit is characterized by adding torque commutation unit between the motor and the pinion,so that the allign moment can be transmitted directly to the chassis through the torque commutation unit in some cases to reduce the energy consumption of steering.For this reason,algorithms of step control and tracking control are designed based on the principle of optimal energy consumption predictive control.By predicting the energy consumption of steering motor in the angle tracking mode and angle step increasing mode for a period of time,the control mode which consumes less energy is selected to achieve energy efficiency improvement.The simulation results of this control strategy based on Carsim-Simulink simulation indicate that the energy consumption can be saved by nearly 80% in the curve road condition but can not be realized in the double line-shifting condition due to the high angular speed.The structure of the improved steering wheel system is characterized by a new type of steering wheel device composed of a torsion spring,a motor and two dampers,which makes the artificial road feel torque basically provided by torsion spring and damper,so as to reduce the action time and output torque of the road feel motor.For this reason,a non-linear PID control algorithm is designed to control the damping ratio.The algorithm and strategy of the system are formulated,which include steering,holding,returning and steering wheel releasing freely.The algorithm designed to realize torque tracking,angle returning control and automatic switching control between different working conditions.Simulation results based on Carsim-Simulink show that the energy-saving effect of this control strategy is perfect,the energy-saving of curve road condition reaches 80%,and that of double-shift condition reaches 45%.Finally,the basic test bench platform of steering by wire system is designed.It is composed of three subsystems,the Senso-drive system,the electric drive steering unit,and the align moment simulation unit.The Senso-drive system is adopted to realize the simulation of road feel torque and the control of the steering wheel.The electric drive steering unit composed of electric drive steering gear device,MPC5604 P MCU and driving circuit is utilized to control the steering of the front wheel steering system.The align moment simulation unit is used to provide the align moment,which is composed of MPC5604 P microcomputer,AC servo motor,bracket,steering knuckle and king pin.This three units are communicated with CAN bus protocol to realize the validation of the steering by wire system.