Patent ID: 8162095

Claim:
A by-wire vehicle steering system providing a steering movement of at least one steerable wheel of a vehicle, the steering movement being input by a driver via a steering control element, the vehicle steering system comprising: a steering gearbox; an electronic control unit which receives an input signal from the steering control element; an actuator operatively arranged directly or indirectly upstream of the steering gearbox, the actuator being actuated by the electronic control unit; a hydraulic default system, the hydraulic default system having a hydraulic feed unit coupled to the steering control element and a hydraulic actuating unit coupled to the steering gearbox or forming the steering gearbox; wherein the hydraulic feed unit and the hydraulic actuating unit are coupleable by way of two hydraulic lines such that when the actuator is non-functional, a steering control element input is transformed into a corresponding wheel steering angle via the hydraulic feed unit and the hydraulic actuating unit, said two hydraulic lines between the hydraulic feed unit and the hydraulic actuating unit being connectable together; a proportional servo steering valve by which the two hydraulic lines are connectable together, a flow cross-section of the proportional steering valve being continuously adjustable between completely closed and completely opened positions; wherein even when the actuator is functional, the steering valve is not in the completely opened position, but rather in a closed position or in a partially opened position, when the system intends to cancel an essentially fixed correlation between the steering control element input and the wheel steering angle when the steering valve is closed; and wherein the actuator is actuated at least when the steering valve is completely closed such that a desired hand torque, which can be sensed by the driver and is a function of boundary conditions, adjusts for a respective steering process at the steering control element.