Modeling of a hydraulic steering system and evaluation of its performance theoretically

In this study a linear and two different non-linear models for the hydraulicaüy assisted power steering system were developed. The power piston, which has different areas on its either side, and the non-linear control valve characteristics were assumed as the common properties in the non-linear models. A mass load was assumed in one of the non-linear models. In the other model a load that includes the wheel tire elasticity and the dry friction between the wheel and road was assumed. The control input to the system is the steering angle applied by the driver to the steering wheel, whereas the disturbance input is the force applied by the road to the wheels. The main output of the system is the joint position of the lever that orients the wheel. However, the position of the control valve spool, the valve opening and the moment felt by the driver on the steering wheel were also investigated in the study. The parameter values of a middle class passenger vehicle were used in the study. Step, ramp and sinusoidal control inputs and step disturbance input were used to evaluate the performance in the linear system model. Step and ramp control inputs and step disturbance input were used to obtain the time responses of non-linear system models. The effects of the system parameters on system behavior were also investigated. The results show that 5-95% rise time of the system is about 0.01 sec. The system is sufficiently stiff against external disturbance forces and the movement of the wheel-rotating joint is only a few hundredth's of a mm, if an external force of 500 N is applied. The system output can follow the system input without any amplitude loss in the 10-150 rad/sec frequency range. Key Words : Hydraulic power steering, vehicle steering systems, hydraulic follow-up mechanism.