  Realistic display of tactile textures on touch screens is a big step forward
for haptic technology to reach a wide range of consumers utilizing electronic
devices on a daily basis. Since the texture topography cannot be rendered
explicitly by electrovibration on touch screens, it is important to understand
how we perceive the virtual textures displayed by friction modulation via
electrovibration. We investigated the roughness perception of real gratings
made of plexiglass and virtual gratings displayed by electrovibration through a
touch screen for comparison. In particular, we conducted two psychophysical
experiments with 10 participants to investigate the effect of spatial period
and the normal force applied by finger on roughness perception of real and
virtual gratings in macro size. We also recorded the contact forces acting on
the participants' finger during the experiments. The results showed that the
roughness perception of real and virtual gratings are different. We argue that
this difference can be explained by the amount of fingerpad penetration into
the gratings. For real gratings, penetration increased tangential forces acting
on the finger, whereas for virtual ones where skin penetration is absent,
tangential forces decreased with spatial period. Supporting our claim, we also
found that increasing normal force increases the perceived roughness of real
gratings while it causes an opposite effect for the virtual gratings. These
results are consistent with the tangential force profiles recorded for both
real and virtual gratings. In particular, the rate of change in tangential
force ($dF_t/dt$) as a function of spatial period and normal force followed
trends similar to those obtained for the roughness estimates of real and
virtual gratings, suggesting that it is a better indicator of the perceived
roughness than the tangential force magnitude.
