  Robot touch can benefit from how humans perceive tactile textural
information, from the stimulation mode to which tactile channels respond, then
the tactile cues and encoding. Using a soft biomimetic tactile sensor (the
TacTip) based on the physiology of the dermal-epidermal boundary, we construct
two biomimetic tactile channels based on slowly-adapting SA-I and
rapidly-adapting RA-I afferents, and introduce an additional sub-modality for
vibrotactile information with an embedded microphone interpreted as an
artificial RA-II channel. These artificial tactile channels are stimulated
dynamically with a set of 13 artificial rigid textures comprising raised-bump
patterns on a rotating drum that vary systematically in roughness. Methods
employing spatial, spatio-temporal and temporal codes are assessed for texture
classification insensitive to stimulation speed. We find: (i) spatially-encoded
frictional cues provide a salient representation of texture; (ii) a simple
transformation of spatial tactile features to model natural afferent responses
improves the temporal coding; and (iii) the harmonic structure of induced
vibrations provides a pertinent code for speed-invariant texture
classification. Just as human touch relies on an interplay between
slowly-adapting (SA-I), rapidly-adapting (RA-I) and vibrotactile (RA-II)
channels, this tripartite structure may be needed for future robot applications
with human-like dexterity, from prosthetics to materials testing, handling and
manipulation.
