Modeling, Simulation, and Experimental Investigation of an Electrohydraulic Closed-Center Power Steering System

In steering-related active safety systems, active steering is a key component. Active steering refers to the possibility to control the road wheel angle or the required torque to turn the wheels by means of an electronic signal. Due to the high axle loads in heavy vehicles, hydraulic power is needed to assist the driver in turning the wheels. One solution to realize active steering is, then, to use electronically controlled valves that are of closed-center type. This means that the assistance pressure, or force, can be set to any feasible value and still benefit from the high power density of fluid power systems. A closed-center solution also implies that a significant reduction in fuel consumption is possible. This paper investigates such an electrohydraulic power steering system, and a comparison with the original system is also made. The findings have shown that while a high response of the pressure control loop is desired for a good steering feel, instability might occur at higher steering wheel torque levels. This has effectively been shown and explained by simulation and hardware-in-the-loop simulation, together with linear analysis. For any desired boost curve, the response of the pressure control loop must be designed to preserve stability over the entire working range.