Fingertip haptics: Preliminary experiments on the perception of slip in haptic feedback

As a first step towards displaying fingertip sensations to the user, this thesis investigates the perception of relative motion, or slip, between the finger and a surface with three psychophysical experiments. The first experiment investigates how well humans discriminate the velocity of a surface beneath the fingertip. Sensitivity to slip velocity is highly dependent on surface geometry, with perception of a dotted surface (Weber fraction 0.10) about twice as good as the perception of a textured (rough) surface (Weber fraction 0.24) at low speeds. The experiment also identifies the slip direction discrimination, averaging 6.5 degrees.; The second experiment attempts to discover a relationship, if any, between tactile speed perception and kinesthesia. Results from thirty-six subjects show that kinesthesia dominates the perception of velocity, but that tactile sensations of slip do affect perception. The results also show evidence of a mechanism that affects kinesthetic perception of velocity that may involve generalization of end point velocity.; The third experiment confirms that tangential displacements of fingertip skin during moving contact contributes to the perception of features. This final experiment tasks subjects to follow a path given three different factors: the first factor is the absence of tangential displacements (1) within the contact area and (2) on the whole finger pad. Thirty-three subjects completed the testing, showing that the absence of tangential forces severely impairs tracking performance, but also revealing surprising performance without skin stretch present. The results suggest that subjects do not use shape to track features, but some combination of shape and tangential forces (presumably due to friction) together.; Finally, this research makes use of the well-known "waterbed" model of fingertip deformation to begin to understand the sensitivity to tangential skin displacement. The analysis reveals that although tangential skin displacements are relatively small in magnitude compared to normal displacements, the percent change in tangential displacement due to variations in membrane properties is much greater than for normal displacements. The results of the model and all of the experiments lead to a recommendation for the use of slip display in virtual or tele-operated environments, and a proposal for display device design.