Rear Wheels Electro-Hydraulic Steering Control System with Reduced Performance Level Required

Performance requests and machine automation, in conjunction with new regulations for commercial and heavy duty vehicles, represent a difficult challenge for machine design. Machine control systems complexity and functional safety regulation, are complex requirements to deal with in new machines design. The paper describes a steering system design, of a 6 wheeled agricultural front articulated self-propelled machine, that must comply with new regulations in terms of functional safety. The vehicle steering is driven by an electro-hydraulic system, totally controlled using electronics; the rear wheels are independently steered and controlled. This architecture requires a highly functional safety performance level. The safety analysis of the system lead to a required performance level whose compliance is a challenging task, due to the software quality required, and to the cost of a fully redundant hardware, on both electronics and hydraulics side. In order to reduce the overall risk of the steering function, a thorough analysis on steering function and system characteristics was performed, considering that a pure hydraulic solution is not possible, due to the leakages in the steering systems that can change the parallelism of wheels. In the initial design, the most critical feature was found in the entirely electronic control system of the rear wheels. A complete revision of the functionality and a new approach to the steering function allowed a new design to be conceived. It fits the function requirement but requires a lower Performance Level. The paper explains the solution, the hazard analysis results and the approach to "risk reduction by electro-hydraulic design" that allowed a better solution, offering the same functionality.