Analysis Of Steering Characteristics And Multi-objective Angle Control For Multi-axle Steering Vehicle

With the development of the world economy and the need of military,multi-axle steering vehicles have been used more and more in various fields.But its large size and heavy load also cause some problems,such as maneuverability,stability and tire wear.While,the reasonable and effective angle control technology can play a key role in improving the above problems.In this paper,the steering characteristics of a six-axle steering vehicle is studied.Then,four Ackerman equivalent steering models are proposed to analyze the steering characteristics of the multi-axle steering vehicle and the steering control strategy of each axle.Finally,a layered steering control strategy is proposed to control each axle of the vehicle effectively,and the results of the multi-objective steering control strategy is shown.The main contributions of this dissertation are listed as follows.Firstly,a nonlinear 3DOF vehicle dynamic model is established with Lagrange dynamics equation theory.Three Submodels of hydro-pneumatic suspension,tire model and steering model of highway driving mode are included.The hydro-pneumatic suspension model is modeled by the method of the combination of test and curve fitting.The damping characteristics of the hydro-pneumatic suspension is simulated by high frequency reciprocating vibration,and the stiffness characteristics of the hydro-pneumatic is simulated by low frequency reciprocating vibration.This experimental modeling method is very practical and conform to the real system.The Uni Tire model proposed by academician Guo Konghui is used to built the tire model in pure side-slip condition,and it is embed into the vehicle model.Then,the model is verified by the actual vehicle test.Secondly,a linear 2DOF vehicle model is established to study the steering characteristics of the multi-axle steering vehicle.Subsequently,the characteristics of different steering modes are described in detail.The calculation method of solving the minimum turning radius of the vehicle is also elaborated.The optimization objective of this method is to minimize the steering space,the steering angle of each axle is the optimization variables,and the constraints are that the deviations of the steering angle of each axle are no more than 2°.According to this method,the final minimum turning radius is less than 6m,which greatly improves its steering flexibility.The steering characteristics of steady-state and transient-state are also discussed in this section.Equivalent steering center distance and equivalent side slip angle coefficent are proposed to express the steady-state side slip angle gain.Equivalent wheelbase and equivalent steady coefficient are also deduced to express the steady-state yaw rate gain.Then,the influence of steering center distance and tire side-slip stiffness on some important steady parameters are analyzed systematically.And the transient-state characteristics is also discussed from three aspects which are time domain,frequency domain and complex domain.Furthermore,in order to simplify the study of the characteristics of multi-axle steering vehicle and the control of the steering angle of each axle in the condition of non Ackerman angle relationships,four Ackerman equivalent steering model are proposed.The derivation process of the four equivalent models and their respective characteristics are elaborated in detail.Meanwhile,the validity of these four Ackerman equivalent steering model is also verified from three aspects which are time domain,frequency domain and complex domain.At last,based on the third Ackerman equivalent steering model,a steering angle controller which is divided into three layers is designed.The upper controller is designed according to the ideal vehicle model and the objective vehicle model.According to the requirement of side-slip angle of each axle for uniform tire wear and the design of the upper controller,the steering angle of each axle of the lower layer can be allocated.While,the last control layer is a fine tune controller,which is designed to micro regulate the steering angle of each axle to maintain the task of tracking the ideal vehicle model and the performance of even tire wear.The results show that the proposed control strategy not only improves the steering flexibility at low speed and vehicle stability at high speed,but also has a good inhibition effect on reducing tire wear.Major Innovations of the Dissertation:(1)A nonlinear 3DOF vehicle dynamic model is established for the research of steering angle control of each axle of multi-axle steering vehicles to enhance the maneuverability,stability and reduce tire wear;(2)Equivalent steering center distance and equivalent side slip angle coefficent are proposed to express the steady-state steering characteristics and the transient-state steering characteristics is also discussed from the aspects of time domain,frequency domain and complex domain;(3)Four Ackerman equivalent steering model are proposed to simplify the study of the characteristics of multi-axle steering vehicle and the control of the steering angle of each axle in the condition of non Ackerman angle relationships;(4)A novel control strategy which is constructed with three layers is proposed for steering angle control of each axle to improve the vehicle maneuverability and stability and reduce tire wear rate.