Abstract
This paper presents a multibody dynamics mathematical model of a cornering vehicle which is fitted with hydraulic power-assisted steering system. The main aim is to develop a method and to assess the performance of a hydraulic power-assisted steering system of a selected passenger vehicle. The vehicle data were obtained from a manufacturer data sheet and were used in this study. The model has three DOFs namely lateral motion, yaw and roll, and an extra one DOF from the hydraulic power-assisted-steering assembly. The hydraulic power-assisted-steering model consists of the whole steering and the front wheels assembly. The control of the power assisted torque is derived from the `Hydraulic Boost Curve´ which is correlated with the steering wheel torque. The performance of the power-assisted steering systems is based on the steering gear stiffness and feel which are calculated using steering design parameters and the pressure boost rates. The derived equations of motions are solved using MATLAB/SIMULINK to obtain lateral accelerations, yaw velocities and steering gear stiffness and feel. The performances of a hydraulic power-assisted steering are assessed from the plots of the steering gear stiffness and feel versus the lateral accelerations and yaw velocities of a steady state cornering vehicle. The hydraulic power-assisted steering fitted on the vehicle analysed in this study was found to take the effort out of parking and slow speed manoeuvring; and provide higher steering gear stiffness at high lateral accelerations and angular velocities. The system also yields driver´s alertness and comfort during cornering by providing higher steering gear feel at low lateral accelerations and yaw velocities and lower steering gear feel at higher lateral accelerations and yaw velocities. However, the level of steering gear feel for both cases was found to be very low compared to the manual steering system and this may cause the driver to loose judgement on the forces acting at the road wheels.
Keywords:Mathematical modelling, Multibody Dynamics, Hydraulic Power-Assisted Steering, Steering Gear Stiffness and Feel, Steady State Cornering