Study On Electric-hydraulic All-wheel Steering System For Four-axle Heavy Truck

Maneuverability and stability of multi-axle vehicles can be greatly improved byall-wheel steering technology. In this paper, a heavy four-axle truck is studied. The first andsecond axle of the vehicle is equipped with hydraulic power steering system and the thirdand fourth axle is equipped with electric-hydraulic steering system, which enables the truckto be steered with all wheels. This paper focues on the electric-hydraulic steering systemapplied on the third and fourth axle and the control algorithm of the all-wheel steering.Furthermore, with the use electric-hydraulic steering tester and dSPACE controller, asemi-physical simulation was carried out. Main research contents are given as follows:(1) A nonlinear four-axle truck model with22degrees of freedom (DOF) wasdeveloped. Simulation results between the22DOF model and TruckSim model wasshowed the accuracy of the model. In addition, based on vehicle dynamic equations, a newmethod to solve the tire load for four-axle truck was proposed. Different from the steadystate coordinate equation, the suspension damp, roll speed and roll acceleration were takeninto consideration to solve the tire load in more accurate way in transient state.(2) Center-position cylinder was adopted in the electric-hydraulic steering system forall-wheel steering. Then a transfer function mode for electro-hydraulic servo system wasdeveloped, in which symmetry valves control single rod cylinder, load inertia, stiffness,damp and friction were taken into consideration. With the help of the transfer functionmode, the frequency respond analysis was carried out to determine the proportional gain inthe PID control strategy and the bandwidth of the system and the steady-state tracking error.At last, the electro-hydraulic steering system model was developed in AMESim, andparameterized simulation of PID controller parameters, oil pressure, the equivalent loadparameters, servo proportional valve characteristic parameters and the delay time of thecontroller were conducted, which can draw a conclusion that the electric-hydraulic systemwas reliable.(3) Aiming at the studied four-axle vehicle, four steering modes were proposed: lowspeed coordinated steering mode, crab mode, Auto mode and traditional steering mode.Aiming at low speed coordinated steering mode, a control algorithm based on Ackermansteering was proposed. Simulation results show that the proposed algorithm can effectivelyreduce the turning radius and reduce tire wear. Aiming at crab mode, a control algorithm with zero yaw rate condition was proposed. Simulation results verify that this algorithm canmake the vehicle steer almost in the way of lateral moving parallel. Aiming at Auto mode,using velocity, steer angle of first axle and yaw rate as inputs, a feedforward plus feedbackcontrol algorithm with lead element and lag element was proposed. Based on this algorithm,steady state analysis, transient analysis, frequency domain analysis consideringimplementing agencies and robustness analysis were carried out, and the effect of all-wheelsteering on the full vehicle handling and stability was obtained essentially. Due to thedifferent load for heavy truck and different road friction coefficient have severe effect ontyre lateral stiffness, a new control algrithm considered vehicle load and road frictioncoefficient is given. The simulation results showed it has more robust and improvesvehicle’s stability. For Auto model control algrithm which outputs large oppsite-steer in lowspeed which made wheels unaccoordinate in steering and aggravate tire wear, a new controlalgorithm with steer angle constraint is proposed. The simulation results show it goodperformance. At last, only auto mode and crab mode are persisted for four-axle truck bycomparing the working range of four modes.(4) The steering tester is designed to research the performance of the electric-hydraulicsteering system. It involves in the following: imitate steering resistance and self-aligningtorque, design steering signal generator, hydraulic system, and electrical control system.Based on tester and dSPACE, semi-physical simulation of all-wheel steering was carriedout. The test results indicate that when steering input is low frequency and larger steeringanger, the system based on proportional valves has good performance. And the wheel steerangle, vehicle yaw rate and lateral acceleration are good agreement with the theoreticalvalues. This verified that the proposed control algorithm of all-wheel steering. If thefrequency of steering input increases, the test results deviated with the theoretical results, sothis requires the fast response actuator in the all-wheel steering system.