Research On Vehicle Stability Control Considering Input Constraints

Vehicle has brought great convenience to people's lives,and its stability has always been the focus of attention.In this process,the Advanced Driver Assistant Systems(ADAS)has been developing rapidly,and the automatic driving technology is the mainstream direction of the automotive industry in the future.Especially when the friction coefficient of road is relative low,the reachable driving state of vehicle is limited.In this case,the stability and cruise capability of the vehicle should be taken seriously.In view of the existing research concerning vehicle stability,the longitudinal force and lateral force of the tire are saturated under extreme conditions,resulting in no solution to the problem of tire force distribution.A tire force distribution scheme based on dynamic target adjustment is proposed.When the friction coefficient of the road is low to a certain threshold,the executable drive space of vehicle is limited.The given speed and front wheel angle are restricted within a certain range,which is essentially the input constrained problem of the nonlinear control system.The research work of this paper is supported by the key special projects of the National Key Research and Development program for new energy vehicles,“Electric Vehicle Intelligent Assisance Driving Technology R&D and Industrialization”(2016YFB0101102).The proposed solution enables the vehicle to realize the optimal redistribution of the tire forces on the low-adhesion roads.In this paper,a seven-degree-of-freedom vehicle model considering lateral wind disturbance is established,and the problem of trajectory tracking and queue keeping are involved.In the high-level controller,the longitudinal velocity,lateral velocity and yaw rate of the vehicle are used as virtual inputs to control the longitudinal distance between the vehicle and the preceding one,the lateral displacement deviation and the tangential angle deviation relative to the reference trajectory,respectively.Then,a reasonable Lyapunov augmentation function is set up to control the virtual input error.By using the Back-stepping method,a robust controller for vehicle stability is designed.At this point,the resultant forces and resultant moments of maintaining the stability of the vehicle are obtained,and the next should be consider how to allocate them reasonably to each wheel.In the lower controller,through the coordinate transformation of the vehicle and the tire to manifest the instantaneous angle between the longitudinal and lateral force.Taking the tire friction circle as the limiting condition,the effective range of generalized resultant force and resultant torque under different road friction coefficients is deduced,and the output of the upper controller is compared with the derived feasible region.If the output of the stability controller exceeds the planned feasible region,the reconstruction coefficient is introduced to reconstruct high-level controller with the structure integrity.At the same time,the input attenuation coefficient is introduced to reduce the front wheel angle of the vehicle and reduce the longitudinal driving speed.According to the listed vehicle dynamic balance equation and the principle of coefficient adjustment deduced from the two-degree-of-freedom reference model,the different driver operation characteristics is combined to make the vehicle realize the optimal redistribution of tire forces after adjusting the control target.Based on the proposed scheme,there is a feasible solution to the problem of tire force distribution under the road with low Adhesion coefficient.Under the constraint of equality of force and moment and the inequality of tire forces,the distribution results of braking force can be obtained through the quadratic programming algorithm.The nonlinear vehicle model is a typical redundant control system.There are multiple methods to combine tire force as the control torque,but this process is often unattainable on the low-friction road surfaces.The scheme proposed in this paper enables the vehicle to maintain a stable operation while redistributing the tire force,so that the vehicle does not deviate from the original track and maintains an ideal tracking distance with the preceding vehicle.Finally,the trajectory tracking model V2 R and the queue maintenance model V2 V of the vehicle are established in Matlab/Simulink.The desired control target is set up together with the classical seven-degree-of-freedom vehicle model,and the effect of Back-stepping controller under different working conditions is verified.It shows the effectiveness of the designed controller and the feasibility of the proposed scheme for the input constrained system,which indicates that this paper has a certain theoretical basis and engineering significance for the development of automatic driving technology.