Research On Full-hydraulic Power Steering System With Dual-mode Operation

With the combination of industrial intelligence and construction machinery industrialization,there are higher requirements for working comfort and efficiency.Improving the steering system performance of off-road vehicle has become the focus of research.In the process of frequent steering operation,the drivers directly control the steering wheel,whose control performance directly affects the working efficiency and steering safety.Reasonable design and improvement of the existing steering system is an effective technical means to improve the steering performance of articulated vehicle.This thesis is funded by the National Key Research and Development Program.Based on the traditional steering system with load sensing and flow amplification,a dual-mode operation full-hydraulic power steering system with steering wheel and joystick is designed.Through simulation and experimental analysis,compared with the traditional steering wheel control system,the system can better realize the steering control of articulated off-raod vehicles,and has more stable handling performance.The main contents of this thesis are as follows:(1)A full dual-mode operation full-hydraulic power steering system is proposed.According to the structural characteristics of traditional steering system with load sensing and flow amplification,the dual-mode operation full-hydraulic power steering system is proposed.The nonlinear mathematical models of power module,steering wheel control pilot module,joystick control pilot module and steering execution module are established.The transfer function between proportional amplifier,electromagnetic proportional directional valve,flow amplifying valve and steering cylinder is derived,which lay a theoretical foundation for the control strategy and performance simulation of the steering system.The working principle and mode switching control strategy of the proposed system are studied.The functions of the dual-mode operation full-hydraulic power steering system are as follows:(1)Integrated control of steering and shift by joystick;(2)Steering wheel priority control steering system;(3)Priority steering with hydraulic control;(4)Variable angle ratio control related to speed.(2)The matching strategy of the equivalent hydraulic damping for the pilot circuit is proposed and multi-objective optimization design is carried out.The simulation model under equivalent load of the dual-mode operation steering system is established based on AMESim.The performance of the key components and the steering sensitivity of the system is verified and analyzed.Based on the simulation results and fluid resistance theory,the matching strategy of the equivalent hydraulic damping for the joystick control pilot circuit is proposed.Multi-objective optimization design of the equivalent damping hole under maximum steering load is carried out based on genetic algorithm.After optimization,the effective pilot valve spool displacement increases from5.23 to 8.46 mm,with an increment rate of 61.75%.The joystick can control the steering system in a more extensive and effective swing range.According to the hydraulic and control principle of the proposed steering system,the design of the test bench with equivalent load is carried out.The performance of the steering wheel control system,joystick control undamped and damped system,and switching of the two control systems is tested.The feasibility of the proposed steering scheme and the effectiveness of the control strategies are proved preliminarily.According to the analysis of the experimental results,the joystick control system increases the damping structure,the effective swing angle range of the joystick increases from 6°?12° to 9°?20°,with an increment rate of 83%,and effectively alleviate the oversensitive phenomenon of the joystick control steering system.(3)A speed-dependent variable-sensitivity steering system control strategy is proposed.The kinematic equation of the articulated loader during steering is established.The coupling mechanism between the systems is clarified.Based on minimum energy principle,Newton's second law and D 'Alembert principle,the dynamics loader model is established.The coupling characteristics of tire forces is analyzed.The mechanical-electro-hydraulic simulation model of the vehicle is established to verify the kinematics and dynamics mechanism of the steering system,and the mechanical properties of the tires during steering are obtained.The state-space model of joystick controlled steering system is established.According to the physical model of the electro-hydraulic position feedback system and the position control requirements of the steering cylinder,an adaptive feedback linearization sliding mode control strategy is proposed.The yaw rate gain of the front and rear vehicle bodies is used as the basis of the control strategy design,and a speed-dependent super twisting sliding mode control strategy is proposed.The proposed control strategies are simulated and analyzed.Compared with the conventional sliding mode control algorithm,the tracking accuracy and running stability are greatly improved.(4)The self-developed 5-ton articulated loader is used as the carrier to verify the performance of the dual-mode operation full-hydraulic power steering system.The articulated loader with dual-mode operation full-hydraulic power steering system is independently developed.The control performance and ergonomics of the proposed steering system are tested.The test results show that the joystick control steering system has good stability,which verifies the effectiveness of the proposed control strategy.The yaw angular velocity change trend of the front and rear frame is linearly related to the joystick swing angle,and the angular velocity change gain increases with the increase of vehicle speed.The proposed steering control strategy has good handling characteristics.The stable switching of the dual-mode can be completed within 1.3s,which verifies the effectiveness of the proposed control strategy,and the dual-mode operation steering system improves the driving safety of the whole vehicle.According to the analysis and calculation of the test data based on the RULA evaluation method,the joystick control steering system can significantly reduce the risk of upper limb muscle injury and improve the sitting comfort.The dual-mode operation full-hydraulic power steering system and control strategies proposed in this thesis are helpful to improve the steering convenience and working efficiency of the loader,which is of great significance for the research of articulated vehicle steering system and the application and popularization of the joystick control steering technology.