Research On The Emergency Collision Avoidance Control Strategy Of Intelligent Vehicle Braking/steering

In recent years,vehicle active collision avoidance technology has become one of the research and development hotspots in the field of active safety by avoiding or mitigating collision hazards by changing the movement state of the vehicle.Among them,the currently more mature automatic emergency braking system assists the driver in emergency braking to avoid rear-end collision with the vehicle in front at a critical moment,and the effect is remarkable.However,considering the longitudinal distance required for collision avoidance,the vehicle can avoid more collisions under high relative speed,low overlap rate and low attachment conditions.Therefore,from the perspective of further improving the adaptability of the vehicle's emergency collision avoidance conditions,this paper focuses on the collision avoidance method decision strategy,emergency steering trajectory planning method,trajectory tracking control algorithm and emergency braking steering joint control.The main works have been done are as follows:(1)The emergency braking process of the vehicle was analyzed,and the latest emergency braking point and the latest first-level braking point in different dangerous situations ahead were given.Proposing a polynomial trajectory planning method to limit lateral acceleration.This method derives the expression of the inward acceleration of the planned trajectory period when determining the motion state parameters at the beginning and end of the planning.In order to improve the stability of the vehicle for steering and collision avoidance,a limit value for the lateral acceleration peak was set,and the collision avoidance trajectory at various vehicle speeds was determined according to the acceleration peak equation under this limit.Combining the determined trajectory and lateral avoidance distance,the latest steering point was obtained by numerical calculation method.Finally,the actual distance between the vehicle and the obstacle in front was compared with the latest first-level braking point,the latest emergency braking point,and the latest steering point to decide the most appropriate emergency collision avoidance mode.(2)Researching emergency steering tracking algorithm based on model predictive control.Aiming at the critical requirements of the real-time control system under emergency conditions,the linear region in the tire cornering characteristics was taken and a simplified vehicle model was derived under the assumption of a small angle.Combined with the Taylor-approximated linearization method,the simplified vehicle model was further linearized,and a linear time-varying prediction equation in a finite time domain was established.Aiming at the requirements of trajectory tracking accuracy and stability,an objective function that comprehensively considers the minimum error of the desired trajectory,the increment and amplitude of the control quantity reflecting the characteristics of the actuator was designed,and the constraints of the center of mass slip angle and tire slip angle were added ensuring stable tracking of vehicles under emergency conditions.Finally,after derivation,the objective function and constraints were transformed into a quadratic programming problem that the computer can quickly solve.(3)Aiming at the problem that the vehicle's high speed and large lateral acceleration are prone to rollover,an emergency braking and steering joint control scheme was proposed to ensure that the vehicle passes the planned obstacle avoidance trajectory safely.First,a reasonable rollover stability evaluation index was obtained based on the vehicle roll simplified mechanical model.The evaluation index was verified by the vehicle dynamics model established in the software Carsim,and the rollover dynamic characteristics of the vehicle at high speed steering were summarized.Then in the joint control,the MPC control algorithm was used laterally to calculate the front wheel turning angle of the tracking trajectory.The upper vertical controller calculated and outputed the longitudinal deceleration based on the difference between the expected rollover stability index and the actual state of the vehicle,and the lower layer built an inverse longitudinal dynamics model based on Simulink.(4)Constructing a Carsim/Simulink joint simulation platform to verify the effectiveness of emergency collision avoidance trajectory planning,steering trajectory tracking algorithm and brake steering joint control method on high-adhesion and low-adhesion roads at 60 km/h,80 km/h,and 100 km/h.The results showed that thetrajectory planning method designed in this paper has obvious real-time and adaptability under various typical working conditions.The trajectory tracking controller has good tracking performance.When driving at medium and high speeds on a good road,the tracking error is limited and the stability performance indicators are within the constraints.When driving on slippery roads,the controller's control effect on medium and high speeds is still good,and the dynamic performance indicators are basically within the stable range.The combined braking and steering control method can effectively reduce the lateral acceleration of the vehicle on high and low attached roads,improve the stability of rollover,and also improve the trajectory tracking level to a certain extent.The maximum braking force that the low adhesion coefficient pavement can provide to the vehicle is limited.When the vehicle has excessive lateral acceleration,the braking speed does not decrease significantly,and the risk of rollover still exists.This is not the case on the high adhesion coefficient pavement.