Phase Difference Between Normal and Shear Forces During Tactile Exploration Represents Textural Features

Contact forces and skin deformation induced during tactile exploration have been investigated in the frequency domain to understand finger-material interaction. Their power spectra are one of the representative feature quantities that have been associated with the surface properties of materials. However, thus far, the phase information of these quantities has not been studied. Furthermore, most previous studies focused on uni-dimensional signals such as forces in either the normal or tangential directions. We investigated the phase differences between normal and shear forces induced during tactile exploration. The results showed that the phase differences between these two axial forces differ among materials and that they exhibit features different from their power spectra. These results indicate that the phase difference between two axial forces should be taken into account to understand the finger-material interactions during tactile exploration.