Characteristics of human fingertips in the shearing direction

The shearing strain of the human fingertip plays an important role in the d etermination of the optimal grasping force and in the perception of texture . Most research concerned with the mechanical impedance of the human finger tips has treated the orthogonal direction to the tip surface: and little at tention has been paid to the tangential direction. This paper describes imp edance characteristics of the human fingertips in the tangential directions to the tip surface. In the experiment, step and ramp shearing forces were individually applied to the tips of the thumb, middle finger, and little fi nger. Dynamics of the fingertips were represented by the Kelvin model. Expe rimental results show that each fingertip had different properties with res pect to the shearing strain versus the applied force, and that the thumb ha d the strongest shearing stiffness among these three digits. Moreover, the shearing stiffness depended on the direction of the applied force? and the stiffness in the pointing direction was stronger than that in the perpendic ular direction. As the contact force in the orthogonal direction to the fin gertip surface was increased, the shearing stiffness and viscosity increase d without regard to the load speed of the shearing force. Furthermore, it i s shown that the average strain rate of the fingertip in the tangential dir ection to the fingertip surface became slower and converged to a constant v alue with higher contact forces.