Grasping spherical deformable objects using parallel gripper with planar or concave jaws

The goal is to demonstrate that spherical concave jaws of a parallel gripper induce smaller indentation displacements and smaller contact pressures to spherical deformable objects, compared to planar concave jaws. A Festo electric parallel gripper type HGPLE-14-60 is used here to grasp spherical deformable objects with gripping forces from 0 to 50 N. It is equipped with two types of parallel almost rigid jaws: planar jaws, versus spherical concave jaws with spherical concavities of radii 75 mm and 50 mm. The advantage of using spherical concave jaws rather than planar jaws is proved analytically for a purely Hertzian theoretical spherical object, and experimentally for two spherical objects with nonlinear viscoelasticity: a tennis ball and a small deformable water ball. So, the obtained results show clearly that indentation displacements and contact pressures are smaller when using the spherical concave jaws and quantify this reduction. Further publication will concern the experimental grasping of apples and oranges using spherical concave jaws versus planar jaws.