Nonlinear finite element analysis is employed to study the soft-finger contact mechanics. Two fingertips of the same material but with different sizes are analyzed. The results are compared with experiments to support the power-law theory proposed by Xydas and Kao (1999). The material properties used in the FEM analysis are based upon uniaxial compression experiments conducted using the actual fingertip material. The coefficient of friction is also experimentally determined and applied in the simulation. Comparisons of numerical as well as experimental radii of contact are offered in conjunction with the power-law theory for soft fingers. The pressure distribution profile at the contact zone, obtained from the finite elements analysis, is plotted and the order of the generalized pressure distribution profile is calculated. Finally, the influence of friction over the contact area is investigated using FEM analysis.

We investigate the influence of size and material properties of fingertips on the power-law equation for soft-finger contacts. Fingertips made of different materials of various sizes were tested, and the parametric relationship between normal forces and radii of contact was found. The experimental results validate that the growth of contact radius, a, is a function of the normal force, N, according to the power law, that is, a=cN/sup /spl gamma//, for nonlinear elastic soft fingers with large deformation. The experimental results suggest that /spl gamma/ is a material-dependent constant, decreasing with the hardness of the soft-finger materials and ranges from 0 to 1/3. In addition to material properties, the constant c is dependent upon the size and geometry. Overall, the constant c increases as the soft finger becomes softer and larger, and decreases when the soft finger becomes harder and smaller. An INSTRON Model 1011 equipment was employed to conduct the experiments with different fingertip materials, loading rates, and ranges of normal force.

A new theory in contact mechanics for modeling of soft fingers is proposed to define the relationship between normal force and area of contact for soft fingers by considering the soft finger materials as nonlinearly elastic. The results show that the radius of contact is proportional to the normal force raised to the power of /spl gamma/ which ranges from 0 to 1/3. This new theory subsumes the Hertzian contact model for linear elastic materials where /spl gamma/=/3. Experiments are conducted to validate the theory using artificial soft fingers made of various materials such as rubber and silicone. This theory provides basis for constructing friction limit surface numerically. Combining the results of the contact mechanics model with the contact friction model the normalized friction limit surface is derived for anthropomorphic soft fingers.