Optimising the roll-sway motion cues available from a short stroke hexapod motion platform

This paper presents findings from research conducted at the University of Liverpool aimed at optimising the motion cues available from a short-stroke hexapod motion platform. Piloted simulations were conducted for a typical helicopter low-speed sidestep manoeuvre. To correctly simulate the sidestep manoeuvre the motion platform must translate laterally at the same time as it rolls. If the motion in these two axes is not properly harmonised then the pilot can experience significant false motion cues. This is a particular concern for short-stroke hexapod platforms, where displacement limits can severely constrain the available lateral travel particularly during motion in multiple axes (e.g. roll and sway). During the experiment the motion filter gains in the roll and sway axes and the roll-axis motion filter break-frequency were varied. Objective and subjective measures of pilot performance and motion fidelity were gathered for each motion filter configuration, the latter using a new motion fidelity rating scale. The key findings show that acceptable motion cues could only be achieved by careful harmonisation of the motion filter gains in the roll and sway axes. A high gain in the roll axis coupled with a low gain in the sway axis resulted in motion which was abrupt and uncomfortable. On the other hand, too large a gain in the sway axis resulted in extreme lateral displacements of the motion platform leading to undesirable side-effects. The phase distortion between the visual and platform motion cues, introduced by the roll-axis motion filter, also had a significant impact on the pilot’s perception of motion fidelity. These results are presented in the form of proposed motion fidelity criteria for short-stroke hexapod platforms and compared with results from previous research conducted on a range of large motion systems.