Soft grasping mechanism of human fingers for tomato-picking bionic robots

Soft grasping is a great challenge for picking robots and its bionic inspiration originates from human fingers. In this study, the hand was scanned to obtain the internal structure of fingers by a computerized tomography (CT) scanner, and the soft contact mechanical index a was defined for characterizing the degree of softness of a finger region during gentle grasping. The effects of mechanics and structure of finger tissues on the soft contact me-chanical index were investigated by finite element analysis and multiple linear regression. The finite element models of the 14 finger regions were split into 6 different groups by a hierarchical cluster analysis. In each group, a mathematical model was established to link the soft contact mechanical index with the mechanics as well as the structure of finger tissues. In most finger regions, their soft contact mechanical index significantly depended on the elastic moduli of the skin and subcutaneous tissue (E-skin, E-tissue), the Poisson's ratio upsilon(tissue) and the thickness T-tissue of the subcutaneous tissue (p < 0.05). The E-tissue showed the most contribution on the soft contact me-chanical index of a finger region, followed by upsilon(tissue), T-tissue, and E-skin. This study demonstrated how the me-chanics and structure of the human finger quantitatively affect its soft contact mechanical behavior during gentle grasping and further provided a bionic basis for developing robotic fingers with varying degrees of softness, particularly for fruit picking.