Analysis of End-Stop Oscillations in an Asymmetric Hydraulic Steering Unit

Danfoss Power Solutions ApS (DPS) has a product line of hydrostatic steering units for heavy-duty machines. The focus of this paper is on oscillations occurring in an asymmetrical hydraulic steering unit called sSteering. This concept, which is hydraulic asymmetric, increases the steering responsiveness between the steering wheel input and the output. One of the challenges with this asymmetry refers to the end-stop situation. This challenge is related to the left end-stroke of the cylinder, where the concept suffers from a relatively low end-stop torque, that indicates to the operator that the maximum turning angle is reached. Different solutions to increase this torque have been investigated, but each with the drawback of introducing underdamped oscillations of the steering wheel. The objective of the current paper is thus to investigate the concept and identify the root cause for the underdamped response.
The end-stop situation is analyzed through a lumped-parameter model, which is able to imitate the dynamics causing the oscillations. The oscillations seen in the model, are found to be present when the torque input is in a limited range. Through analysis of the model responses, the root cause is found to be a chain of events, which is initiated by the opening of a hydraulic bleed row in the end-stop situation. This bleed row has an impact on the torque balance of the steering unit’s internal components, which initiates the underdamped oscillations of the steering wheel. Finally, a sensitivity analysis
is conducted, to clarify which model parameters that have a crucial impact on the oscillations, and to understand how these oscillations may be dampened.