Research On Hydraulic Power Steering With Active Steering System Control Of Buses

High-speed buses can be prone to rollover accidents with huge casualties under some extreme conditions, such as sideslipping and drifting because that large passengers, huge volume and mass inertia, high and constantly changing center of gravity, time lag of the working mechanism as well as comparatively large length-width ratio of underpan make the correction effect of control system of longitudinal force of tire on bus body decrease greatly. Therefore, handling stability active technology, control strategy and robustness remain one of the important research topics to be solved in the domain of bus security.Active Front Steering technology is one of the latest developing directions in the vehicle steering system which can improve the handling stability and security of the vehicle effectively by controlling the angular displacement and torque transfer characteristics of the steering system directly and easily by optimizing the force of tire and distribution moment, resisting stiffness perturbation and external disturbance and providing an additional yawing moment for the vehicle. Although electrically driven superimposed AFS system has been applied to some high-end passenger vehicles, it is difficult to control in the buses because of the large volume of the power motor and high cost of equipments.Therefore,it is not with AFS on buses.Hydraulic servo system is the reasonable solution to the steering system of various large wheeled vehicles with the advantages of powerfulness, fast response, high control accuracy and good reliability. In terms of this, the research group developed a new-type hydraulic-powered and active steering system with independent intellectual property and applied it to the buses to equip it with electric hydraulic power steering function as well as the active front wheel steering function like small cars, which provided an important active security technology for improving the handling stability by preventing sideslipping and drifting. The paper conducted an in-depth research on the system design, performance analysis and control methods of the new hydraulic-powered active steering system and anti-drift AFS stability control of buses. The main work was as follows:A dynamic model, steering model and a tire model of bus were established to improve the adaptability and fitting accuracy of the tire model under different road conditions by discussing the improved methods of Pacejka tire model. Bus 9-DOF dynamics model based on Simulink software and ADMAS/Car were set up based on with which as the simulation platform for research on the anti-drift AFS stability control of the bus. Snake-shaped orbit around pile testing and double lane change overtaking experiment were carried out to testify the accuracy of Simulink model of bus.An electric hydraulic power servo steering and electro-hydraulic servo AFS control system was put forward based on the thought of triple-redundant reliability to match the performance parameter of design system by analyzing three basic working modes of the new hydraulic steering system, namely, comprehensive steering "road feel", response characteristics and control accuracy. A new mathematical model of the hydraulic steering system was set up to deduce the transfer function of the displacement system of the AFS hydraulic servo system. A coupling stiffness model of AFS subsystems’ double- stretching telescopic hydraulic cylinder steering system was established to analyze the system stability, response characteristics, errors and their influencing factors and corresponding solutions were proposed.A control strategy of a new neural network adaptive sliding mode for the pump speed of PMSM servo system in the new hydraulic steering system was proposed to select the sliding surface in form of integration and utilize RBF network to make some online estimations of changes of control law as well as compensate for changes by combining with the adaptive algorithm in order to weaken the chattering of the conventional sliding mode and improve the performance of variable hydraulic pressure source control system. AFS new hydraulic servo system has the characteristics of a complex working condition, constant changing parameters and high nonlinearity. Third-order nonlinear control model was set up by adopting inversion algorithm to design the first-order and second-order subsystems in order to maintain the robustness of system uncertainty caused by sliding mode technology. Terminal sliding algorithm exponential order was introduced in the third-order subsystems to eliminate buffeting, thus to improve the overall quality of the system.A joint simulation was carried out for the new hydraulic-powered active steering system. A new hydraulic and mechanical system model of the hydraulic-powered active steering system was established based on AMESim with the PMSM speed servo controller model and electro-hydraulic servo AFS displacement system controller model being set up by using Simulink. A joint simulation model was set up with Simulink as the main control software to conduct a joint simulation for those proposed control strategies.Test shelf for the data acquisition and motion control of the new hydraulic-powered active steering system was set up to introduce the hardware composition and software function. Contrast tests of the high speed conditions of bus were carried out to show a consistence of the theoretical analysis, simulation analysis and experimental results and the rationality and effectiveness of the new hydraulic steering system.The dynamic characteristics of the vehicle steering on the basis of stability factors is analyzed and study the law of axial lateral force variation when a bus was experiencing the drifting. Sensing detection and estimation system of bus dynamics and dynamic parameters was designed based on a predicting mechanism of the axle lateral force saturation to propose sliding mode control method of anti-drift AFS. A simple decoupling algorithm of sliding mode to obtain a comprehensive feedback from yaw angular velocity and core side slipping angle and to simplify the designing complexity of this kind of sub-drive control system was realized by introducing a virtual intermediate variable. Anti-drifting AFS decoupling sliding Simulink controller was set up to conduct a simulation of typical sliding and drifting working conditions by taking buses equipped with the new hydraulic-powered active steering system as the control object. The results show that the anti-drift AFS sliding decoupling control strategy has a relatively strong robustness for uncertain nonlinear dynamic systems of buses which increases its tracking performance of the desired trajectory.