Contact Mechanics and Nonlinear Contacts Stiffness for Hemielliptical Soft Fingertip in Grasping and Manipulation

Soft finger is commonly used as fingertip in robot and prosthesis hand applications, to provide the stability in grasping and manipulation. The study of contact mechanics for the soft finger in previous researches has been carried on the hemispherical and hemi-cylindrical structures, whereas this study concentrates on the use of a hemi-elliptical structure, which represents more realistic soft fingertips. In this study, a nonlinear contact mechanics model has been established to relate the vertical depression of a hemi-elliptical soft fingertip to the apply load as power-law equation. Then, the nonlinear contact stiffness of the hemi-elliptical soft fingertips under applying a normal force was derived. The influence of hemi-elliptical geometry on the vertical depression equation and stiffness was analyzed by introducing a new dimensionless parameter factor (𝜅). Stiffness relationship of Hertzian contact for linear elastic materials is shown to be a special case of the general model presented in this paper. Experimental results are presented to confirm the theoretical analysis using three different silicone rubbers. The results clearly indicated that the nonlinear contact stiffness increases with the increase of curvature ratio (Rx/Ry).