Coordinated Control Method Of Motion Control System For In-wheel Motor Electric Vehicle

In-wheel motor electric vehicle changes the structure and driving mode of chassis system,and it is a widely concerned future electric vehicle model.The four wheel torques can be adjusted independently and continuously,which provides greater flexibility for vehicle motion control and leads to a lot of challenging control problems.In addition,the development of x-by-wire technology in chassis-control provides an opportunity to explore vehicle coordinated motion control from the perspective of longitudinal-lateral-vertical dimensions.Researching on the in-wheel motor electric vehicle,this paper explores the method for improving the longitudinal-lateral-vertical motion performance by coordinated control based on the current structure of motion control system.The objective of longitudinal-lateral-vertical coordinated control varies under different driving conditions.Thus,this paper divides the longitudinal-lateral-vertical motion control into longitudinal-lateral motion control and lateral-vertical motion control.The controller design is simpled and the coordinated objective can also be satisfied on different driving conditions.Firstly,wheel slip control is studied to solve the anti-skid problem of the in-wheel motor electric vehicle.A piecewise linear modeling method of longitudinal tire force is proposed,which has a unified expression of longitudinal tire force under different road friction coefficient.Furthermore,taking the wheel speed and vehicle speed as varying parameters and taking the coupling from other wheels into account,a model for wheel slip control is established.In order to solve the torque oscillation problem in wheel slip control,a feedforward controller is designed to reduce the adjustment range of feedback controller,and an H_∞feedback controller design method based on gain scheduling is proposed.Test results of the experimental vehicle on low adhesion road show that the proposed method can effectively control the wheel slip and improve the driving performance of the vehicle.Aiming at the cornering condition with longitudinal-lateral motion control perfor-mance requirements and taking the advantage of the over-actuated characteristics of in-wheel motor electric vehicle and the coupling characteristics of the longitudinal-lateral dynamics,a nonlinear model predictive control method for tracking the vehicle speed and ensuring the yaw stability by adjusting the wheel slip is proposed.This method extends target from the single lateral motion control to the longitudinal-lateral motion coordinated control,and can meet different control modes and requirements by adjusting the weight-ing matrix,so as to effectively improve the longitudinal-lateral motion performance of vehicles under different driving conditions.The hardware-in-the-loop simulation results under multiple driving conditions verify the feasibility and effectiveness of the proposed longitudinal-lateral motion coordinated control method.The existing research results of longitudinal-lateral motion control are obtained on the condition that all the in-wheel motors work normally.On the contrary,the control performance will not be guaranteed and even the vehicle may be unstable in case of motor failure.Therefore,this paper further explores the lateral motion control method when the in-wheel motor fails by using Barrier Lyapunov Function.In this method,when the in-wheel-motor fault causes the vehicle state to move to the instability region,it is constrained in the elliptical region of an equilibrium point around the stability boundary.The proposed method can not only prevent the vehicle from instability,but also ensure the lateral motion performance of the vehicle.Under the condition of high-speed sharp turn,the requirement of considering both the lateral and vertical control performance is brought by the coupling between the lateral and vertical motion.Taking the in-wheel motor electric vehicle equipped with magnetorheological semi-active suspension as the object and considering inconsistent control objectives for suspension damping force of restraining roll dynamics and reducing wheel dynamic load,the damping force control method is researched firstly.This paper presents a piecewise linear modeling method of damping force based on the experimental data and analyzes the variation characteristics of damping force and its action law on lateral dynamics.A cost function characterizing lateral and roll dynamics is proposed.Then,a optimization design method of suspension damping force is put forward.This work provides a basis and idea for the design of semi-active suspension controller considering roll and lateral motion performance.The sliding-mode controller design method for yaw stability under the coordination of damping force controller is further explored.The relationship between the desired yaw rate,the desired lateral velocity and the steady-state value is analyzed.The parameter optimization design method of sliding mode controller synthesizing overshoot and regulation time performance is given.The simulation results show that the proposed method can effectively reduce the yaw moment demand when the yaw performance is close to that of the non-coordinated yaw stability control.The above longitudinal-lateral and vertical-lateral coordinated controller design methods are applicable and effective for the condition that vehicle states stay in the stable region,such as conventional driving conditions and the extreme driving conditions where the wheel lateral force is in the saturation zone.However,for the condition to change the understeer behavior,such as high-speed U-turn,drift movement can more effectively improve the vehicle’s passing capacity by coordinating the lateral and longitudinal motion to drag the vehicle states into the unstable region.Thus,the drift control by coordinating longitudinal and lateral motion is studied.The reported researches mainly design the con-troller based on the linearization model of the unstable equilibrium point(saddle point).In contrast,this paper explores the motion characteristics and laws of vehicle state during drift motion,as well as the design and control methods of desired vehicle states.The dynamic model is established with the rear tire slip angle and vehicle yaw rate as state variables.A nonlinear function is used to characterize the influence of rear wheel lateral force on drift motion.The steady-state yaw rate and front lateral tire force are calculated by rear lateral tire force,and the calculation problem of balance point is reduced to the design problem of rear lateral tire force.Based on the analysis of the action mechanism and law of the rear tire slip angle on the equilibrium point,the conditions for stabilizing the equilibrium point and the optimal calculation method of the desired rear tire slip angle are given.Different from the controller design method based on the linearization model of equilibrium point in the existing literatures,the proposed design method of nonlinear static feedback controller can guarantee the closed-loop stability in a large region near the equilibrium point,and has better control performance when the vehicle states are far away from the equilibrium point.The simulation results under different driving conditions show that the proposed drift control method has better U-turn passing performance.