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Steven D. Beard
Presenter
NASA Ames Research Center

The Vertical Motion Simulator at NASA Ames Research Center utilizes custom nonhydrostatic hydraulic actuators to drive its roll, pitch, yaw and longitudinal axis. These
actuators have operated flawlessly with one exception; they exhibit a phenomenon that is
detrimental to the motion fidelity called “Turnaround Bump” (TAB). TAB occurs in all
hydraulic actuators with seals at the onset of motion (extending or retracting) when the
friction on the seals changes from the higher static (no motion) to the lower dynamic
(motion) friction. The sudden change in the friction force causes an acceleration spike that
can be felt by the pilot thus giving a false motion cue. In order to reduce TAB without going
to costly hydrostatic actuators, a cost effective mechanism was designed that constantly
rotates the piston rod end of the actuator so that the piston rod never stops moving relative
to the seals even when the actuator stops moving in the linear direction.

Download the full paper in PDF (0.9 Mb).

A hardcopy of this document is available by request from Kathleen Starmer.