The demand for power steering systems primarily stemmed from the need for providing assistance to operators in achieving their heading directions, especially as transportation vehicles kept growing in size and mass. While the main requirements for the primitive systems were adequate assistance levels and acceptable controllability, today’s requirements are much different. Given the increased awareness and attention to energy efficiency, productivity, and safety, the field for researching alternative technologies is virtually open. Present-day power steering architectures include hydraulic, electric, and electro-hydraulic structures, which vary based on their energy source, energy transmission, and energy management schemes. Hydraulic power steering is plagued with poor energy efficiency mainly due to throttling losses associated with hydraulic control valves. Electric power steering systems offer better energy efficiency with on-demand power supply and result in improved packaging constraints, but suffer from power limitations at larger scales. State-of-the-art electrohydraulic steering systems take advantage of the high power density and efficiency of fluid power, but use electronically controlled valves that still suffer from energy inefficiency.
This paper introduces a novel electro-hydraulic power steering system that utilizes a proven energy-saving technology, pump displacement control (DC), which eliminates throttling losses associated with hydraulic control valves by controlling the displacement of a variable displacement pump. DC has been applied to the working hydraulics of mobile machines, active vibration damping, and machine power management strategies. However, for the first time this technology is being investigated and applied to steering systems, which presents a unique opportunity to solve the pressing challenges and highlight the benefits of such systems. A DC steering system lends itself to high energy efficiency (lower fuel consumption/emissions), greater machine productivity and reduced operator fatigue, and active safety for counteracting instabilities. The paper presents the new concept; reviews the dynamic models of the hydraulics subsystem and the dynamics subsystem; summarizes the system synthesis with focus on stability; and finally describes the primary controller design based on modern control techniques via state space formulation and full state feedback methods. To allow for designing a controller at the system level, vehicle dynamics equations of motion and pressure-flow equations are merged together to form a single-input single-output (SISO) system with the desired input and output in mind. Simulation results are provided to validate the control algorithm relative to the specified performance requirements.