Hydraulic Power Steering System Design and Optimization Simulation

Hydraulic Power Steering System Design and Optimization Simulation

ABSTRACT

Hydraulic rack and pinion power steering system is a high bandwidth servo with stringent performance requirements on accuracy, reliability, and cost. Design of such a system can be best achieved by using a validated and user friendly computer simulation program. Hydraulic integrated power steering ( HIPS ) program has been developed using basic concepts from science and engineering. HIPS provides a design and test environment for the integrated steering and suspension system subjected to disturbance forces, which may be induced by pump flow oscillations and tire loads. Two real-world automotive hydraulic steering systems are simulated with HIPS. The simulation results agree closely with the dynamometer test results. The application of HIPS for design optimization is also demonstrated.

INTRODUCTION

Figure 1 shows a hydraulic rack and pinion power steering system, which is a high bandwidth nonlinear servo capable of generating a rack force of 4,000 N for cars and 6,000 N for small trucks. About 80% of the rack force comes from hydraulic power assist and the remaining 20% comes from driver’s effort. At on-center position of the steering wheel, a vane pump, which is driven by the engine, circulates the fluid in a closed -loop hydraulic circuit which includes a reservoir, vane pump with flow control and pressure relief, supply and return lines, and the rotary spool valve ( RSV ). The flow control valve which connects the discharge and inlet manifolds of the vane pump, regulates the pump flow rate into the supply line at 2.1 gpm for cars and 3.5 gpm for small trucks, for engine speeds above 900 rpm. When the steering wheel is turned by the driver, RSV diverts the supply line fluid flow to either side of the power piston for a right or left turn of the vehicle. At the same time, RSV passes an equal amount of fluid flow from the other side of the power piston through the return line to the reservoir.

This increases the differential pressure acting on the power piston and yields the desirable hydraulic power assist force. An optional speed sensitive steering controller modulates the fluid flow rate drawn by the RSV from the supply line, using signals from wheel speed sensors and wheel torque sensor. The driver’s steering effort is increased by lowering the hydraulic power assist during highway driving, hence improving driver’s feel of the road during city and highway driving. The steering torque is obtained when the rack force is applied to an off-center joint on the knuckle, which turns about the kingpin at the suspension strut, including a coil spring and damper assembly.

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