A Technique to Determine Friction at the Fingertips

Abstract

This paper proposes a technique to calculate the coefficient of friction for fingertip-object interface. Twelve subjects (6 males and 6 females) participated in two experiments. During the first experiment (the "imposed displacement" method) a 3D force sensor was moved horizontally while the subjects applied a specified normal force (4N, 8N, 12N) on the surface of a sensor covered with different materials (sandpaper, cotton, rayon, polyester, and silk).The normal force and the tangential force (i.e. the force due to the sensor motion) were recorded. The coefficient of friction (µ d ) 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 (200g, 500g and 1000g) 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 the normal force (in a horizontal direction) and the tangential force (in the vertical direction). The slip 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 of the normal force; (3) the skin-sandpaper coefficient of friction was the highest µ d = 0.96±0.09 (for 4N normal force) and the skin-rayon rayon coefficient of friction was the smallest µ d = 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 to the beginning slip method, which is more cumbersome (e.g. dropped object should be protected from impacts) and prone to subjective errors due to uncertainty in determining the instance of the slip initiation (i.e. impeding sliding).

Grasp modelling with a biomechanical model of the hand

“…Tables 5 and 6 compare the experimental finger forces registered during the static hold of the cylinders to the contact forces and torques estimated by the model and three different modifications. The original model ('Estimated 1') considered the minimisation of the sum of the squared muscle stresses (maximisation of endurance) to solve the indeterminate problem, and a friction coefficient of 0.8, obtained from Savescu et al (2008). The disagreement between the experimentally measured normal forces and the estimated ones is evident.…”

“…Taking into account the data reported by Savescu et al (2008), the effect of changing this coefficient to the very low value of 0.3 has been checked (Tables 5 and 6). Although the model estimates greater normal forces, the total grasping force estimated by the model for grasping the lighter cylinder is still far from the registered one.…”

“…The values of  and K have been obtained from Savescu et al 2008) and Hajian and Howe (1997), respectively.…”

“…The authors are already working on developing a neural network able to automatically obtain both postures and avoid the use of registered data. The data reported by Savescu et al (2008) were used to select the appropriate friction coefficient (0.8) between the hand and the cylinder material being grasped. The results of the simulation of grasping both cylinders were the grasping postures, the contact points, the contact normal directions, the contact finger forces and torques, and the muscle force distribution.…”

Evaluation of Human Prehension Using Grasp Quality Measures

“…Friction constraints are derived based on general expressions for the non-planar contacts of elastic bodies, taking into account the local geometry and structure of the objects in contact. The values for the human hand skin friction coefficient and the stiffness modulus have been obtained from [29] and [30] respectively. …”