Just Feeling the Force: Just Noticeable Difference for Asymmetric Vibrations

Previous research showed that haptic feedback, in the form of asymmetric vibrations, can be used to provide directional cues to the operator in a laboratory setting. Nevertheless, it is unclear how these vibrations should be designed for pilots controlling their aircraft using a side-stick. This paper aims to determine the magnitude and shape for which vibrations can still be perceived as directional cues, for one fixed frequency based on literature. The threshold magnitude of two forcing function shapes (triangular and saw tooth) was determined for both pulling and pushing cues in a just-noticeable-difference experiment. Participants were asked to report the direction at varying input magnitudes while exerting different offset force levels on the stick at different positions. Results confirmed all hypotheses: they indicated a lower perception threshold for the asymmetric saw tooth shaped vibration compared to a triangular shaped; higher offset force decreased the threshold in the opposite direction; and stick position had no effect on the obtained thresholds. Based on the experiment we advise to use saw tooth vibrations with an amplitude higher than 0.094 Nm.