Characteristics of human fingertips in the shearing direction.

The shearing strain of the human fingertip plays an important role in the determination of the optimal grasping force and in the perception of texture. Most research concerned with the mechanical impedance of the human fingertips has treated the orthogonal direction to the tip surface, and little attention has been paid to the tangential direction. This paper describes impedance 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 finger. Dynamics of the fingertips were represented by the Kelvin model. Experimental results show that each fingertip had different properties with respect to the shearing strain versus the applied force, and that the thumb had 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 perpendicular direction. As the contact force in the orthogonal direction to the fingertip surface was increased, the shearing stiffness and viscosity increased without regard to the load speed of the shearing force. Furthermore, it is shown that the average strain rate of the fingertip in the tangential direction to the fingertip surface became slower and converged to a constant value with higher contact forces.