This article proposes a technique to calculate the coefficient of friction for the fingertip– object interface. Twelve subjects (6 males and 6 females) participated in two experiments. During the first experiment (the imposed displacement method), a 3-D force sensor was moved horizontally while the subjects applied a specified normal force (4 N, 8 N, 12 N) on the surface of a sensor covered with different materials (sandpaper, cotton, rayon, polyester, and silk).Thenormal forceand thetangential force(i.e., the force due to the sensor motion) were recorded. Thecoefficient of friction(µd) was calculated as the ratio between the tangential force and the normal force. In the second experiment (the beginning slip method), a small instrumented object was gripped between the index finger and the thumb, held stationary in the air, and then allowed to drop. The weight (200 g, 500 g, and 1,000 g) and the surface (sandpaper, cotton, rayon, polyester, and silk) in contact with the digits varied across trials. The same sensor as in the first experiment was used to record thenormal force(in a horizontal direction) and thetangential force(in the vertical direction). Theslip force(i.e., the minimal normal force or grip force necessary to prevent slipping) was estimated as the force at the moment when the object just began to slip. The coefficient of friction was calculated as the ratio between the tangential force and the slip force. The results show that (1) the imposed displacement method is reliable; (2) except sandpaper, for all other materials the coefficient of friction did not depend on the normal force; (3) theskin–sandpapercoefficient of friction was the highest µd= 0.96 ± 0.09 (for 4-N normal force) and theskin–rayonrayon coefficient of friction was the smallest µd= 0.36 ± 0.10; (4) no significant difference between the coefficients of friction determined with the imposed displacement method and the beginning slip method was observed. We view the imposed displacement technique as having an advantage as compared with the beginning slip method, which is more cumbersome (e.g., dropped object should be protected from impacts) and prone to subjective errors owing to the uncertainty in determining the instance of the slip initiation (i.e., impeding sliding).
A Technique to Determine Friction at the Fingertips