Enveloping Grasp with a New Detaching Strategy

For enveloping an object placed on a table, human often utilizes the wedge—effect where a simple pushing motion of the bottom part of object makes the object detach from the table. Through grasp experiments by human, we newly found an interesting behavior where human changes his (or her) finger posture from upright to crooked ones after all fingers make contact with the object. This motion is termed as the detaching assist motion (DAM) and greatly contributes to achieving a detaching motion even for an object which is small enough to ensure that such a wedge-effect is not available. We first analyze this motion by utilizing a specially designed force sensing system, and then analytically examined the condition leading to the DAM by using a simple finger-object model. We also implement it into the grasping procedure of a three-fingered robot hand, especially for handling a small size of object.