Experimental investigation of frictional properties of the human fingerpad

Abstract

A 2-DOF robot was modified to serve as a high precision tactile stimulator, controlled to indent and stroke the fingerpad of human subjects. It was configured to deliver various dynamic stimuli, such as different indentation depths, stroke velocities, and stroke directions. Three kinds of transparent indentors were used in the experiments: glass, polycarbonate, and acrylic planar surfaces. During stroking, the normal and shear forces were recorded by a 2-axis force sensor. A videomicroscopy system was set up to capture the image sequences of the contact region between the fingerpad and the indentor surface while stroking. The stimulator and the videomicroscopy system were synchronized to match the images with corresponding force data. Five subjects participated in this experiment. The data show distinct frictional behaviors for different indentors. For the glass surface, the curves of normal as well as shear forces increased smoothly to steady state values. When the indentation depth was higher, the normal and shear forces were larger, but the friction coefficient was smaller. When the stroke velocity increased, the normal force was about the same for a given indentation depth, while the shear force and the friction coefficient increased. The stroke direction did not significantly influence the results. The image sequence shows that the relative motion, or slip, between the fingerpad and the glass plate indentor began at the periphery of the contact region and propagated towards the center. Under all the stroke conditions, the glass plate slipped smoothly across the fingerpad. In contrast, both polycarbonate and acrylic surfaces exhibited stick-slip phenomenon consistently. An analysis of the stick-slip frequency and the stick-slip shear force was conducted with respect to various indentation depths and various stroke velocities. A hypothesis about junction forming rate and junction breaking rate was proposed based on adhesion theory and the observation of images as well as force measurements. This was used to explain the different results from the glass plate indentor and the polycarbonate plate indentor. The frictional properties of the fingerpad is shown to be similar to those exhibited by rubber-like materials.