Scale Dependence of Force Patterns During the Scanning of a Surface by a Bare Finger

This study investigated how the geometry of a touched surface influences forces felt during the scanning of an object with the finger. Although prior research, and basic mechanical considerations, have shed light on force production during haptic interaction with a touched object, surprisingly little is known about how to relate surface detail at different scales to the specific patterns of force that are observed. To address this, we designed an apparatus that could accurately measure normal and tangential forces between a finger and a surface. We fabricated sinusoidal surfaces with precisely controlled geometry, and measured spatial variations in resultant forces generated while subjects repeatedly scanned the surfaces at specified speed and pressure. Subsequent analysis revealed that the resulting force patterns varied in an organized way with spatial scale, and that fluctuations, in the form of aperiodic signal components that proved difficult to model, played an increasingly important role as the spatial scale of the surface geometry decreased. The results may help to explain differences in how surface detail is recovered by the haptic perceptual system at different length scales.