Effect of Layer Compliance on Frictional Behavior of Soft Robotic Fingers

Abstract

In this paper, the modeling and design for frictional loads applied by robotic fingertips through soft contact interface are investigated. The dependence of the sustainable friction forces and moments due to the normal load applied on the contact area is studied. A model that considers the dependence of both the contact area and the coefficient of friction on the application of normal load is proposed. Specimens of simplified fingertips, made with different materials and shaped with different thickness of the covering soft layer, are tested in order to validate the proposed model via designed experiments. The results obtained have very important relevance on the design of robotic hands and fingers equipped with soft pads or skins, and can help on the choice of suitable features for the different layers of contact pads or skins.

Experimental investigation of effect of fingertip stiffness on friction while grasping an object

“…However, the stiffness of the materials was not considered. In his dissertation [17], the similar numerical and experimental results to Tiezzi et al [8] were presented: if the fingertip stiffness was low, a large frictional force could be applied. As mentioned above, this is not always true.…”

“…The Amontons-Coulomb model is also considered valid for this contact. Unfortunately, it is not true for a fingertip with low stiffness [8]. Furthermore, considering that high fingertip stiffness affords relatively high manuability and low fingertip stiffness produces large frictional forces [9], robotic hands with changeable fingertip stiffness have been developed [6], [10].…”

“…Analyses of soft fingers were conducted by Kao et al [8], [11], [12] and Hirai et al [13], [14]. Xydas and Kao [11] presented the relationship between the normal force and the radius of the contact area when a semispherical fingertip contacts a flat surface, and used it to develop a model of the frictional condition.…”

“…Inoue and Hirai presented a model that considered both the normal and the tangential deformations. Tiezzi et al [8] presented a model of the frictional force and examined the effect of the fingertip compliance. Based on their theoretical and experimental results, they concluded that, if the fingertip stiffness was low, a large frictional force could be applied.…”

“…Specifically, under a particular condition, the frictional/resistible force increases with increasing fingertip stiffness. This phenomenon cannot be explained by conventional theories [8], [17] or a model based on the Hertz contact theory (for an example, see [19]). The resistible force consists of the frictional and (tangential) elastic forces, with the shearing frictional force constituting the major F. Fujihira is with the Graduated school of Natural science and Technology, Kanazawa University, Kakuma-machi, Kanazawa, 9201192, Japan (e-mail: fuji3134@stu.kanazawa-u.ac.jp).…”