Advantage of variable stiffness of human fingers for key insertion task

This study verifies an effect of variable stiffness of a fingertip to increase stability of a pinching object and to widen controllable range of object compliance. Human intelligently controls compliance of the grasping object for desirable contact state by adjusting his/her body joint's stiffness. The compliance contributes to tolerance of a positioning error and to absorption of an impulse force from an external environment. For example, a holding force of a key adjusts according to the task progress. We pinch a key softly in order to fit its orientation to a keyhole at the beginning of the key insertion. However, we must increase its grasping force in order not to slip off the key, if the resistance to insert the key or displacement of key angle is large. The stiffness of fingers needs to be adjusted according to the resistance and displacement to execute the task smoothly without increase of grasping force during the key insertion task. Numerical simulations in this paper reveal that an effect of the variable stiffness of the fingers is more dominant than that of displacement of contact point for grasping with rolling contact of fingertip from the viewpoint of stability of the pinching object and controllable range of object compliance. We found an advantage of the variable stiffness for dexterity to insert the key smoothly.