  This study seeks to understand conditions under which virtual gratings
produced via vibrotaction and friction modulation are perceived as similar and
to find physical origins in the results. To accomplish this, we developed two
single-axis devices, one based on electroadhesion and one based on out-of-plane
vibration. The two devices had identical touch surfaces, and the vibrotactile
device used a novel closed-loop controller to achieve precise control of
out-of-plane plate displacement under varying load conditions across a wide
range of frequencies. A first study measured the perceptual intensity
equivalence curve of gratings generated under electroadhesion and vibrotaction
across the 20-400Hz frequency range. A second study assessed the perceptual
similarity between two forms of skin excitation given the same driving
frequency and same perceived intensity. Our results indicate that it is largely
the out-of-plane velocity that predicts vibrotactile intensity relative to
shear forces generated by friction modulation. A high degree of perceptual
similarity between gratings generated through friction modulation and through
vibrotaction is apparent and tends to scale with actuation frequency suggesting
perceptual indifference to the manner of fingerpad actuation in the upper
frequency range.
