Transformation of foldable robotic hand to scissor-like shape for pinching based on human hand movement

Increasing the number of degrees of freedom for multi-finger robotic hands is necessary to achieve high performance. However, this increases structural complexity and the obtained improvement may be small. Humans change the shape of their hands by extending or bending the fingers to apply force to an object through contact with a wide surface or two or more fingers. In some cases, continuous finger movements are not necessary or some fingers do not make contact with the object. A robotic hand with a small number of degrees of freedom could effectively use its fingers to perform many tasks by properly arranging the fingers, increasing the movable range of joints, and utilizing the back and sides of the fingers. This paper proposes a hand system and conducts a theoretical analysis of the transformation of the hand shape into a scissor-like motion to handle a cylindrical object. It is found that the scissor-like motion is unsuitable for cylindrical objects that exceed a certain size. Experiments show the effectiveness of the proposed hand system. The correlation between the contact position of a finger with an object and the success ratio of pinching is demonstrated. Furthermore, a control system that can switch from pinching to grasping when the robot judges that pinching is difficult is developed and experimentally validated.