Stiffness and contact mechanics for soft fingers in grasping and manipulation

In this paper, nonlinear stiffness of contact for soft fingers, commonly used in robotic grasping and manipulation, under a normal load is studied. Building upon previous research results of soft-finger contact expressed in the power-law equation, the equation for the nonlinear stiffness of soft contact was derived. This new theory relates the approach displacement (or the vertical depression) of soft fingertips with respect to the normal force applied. The nonlinear contact stiffness is found to be the product of an exponent and the ratio of the normal force versus approach displacement. Stiffness relationship of Hertzian contact for linear elastic materials is shown to be a special case of the general theory presented in this paper. Experimental results are used to validate the theoretical analysis. In addition, potential applications to fixturing are discussed.