Picking up Soft 3D Objects by “Feeling ” When Without Tactile or Force Sensing

This paper describes a strategy for a robotic hand to pick up deformable 3D objects from a table. Inspired by the human hand behavior, the robotic hand employs two rigid fingers to first squeeze a soft object until it “feels ” the object to be liftable. Such “feeling ” is provided by a (virtual) liftability test that is repeatedly conducted during the squeeze. Passing of the test then triggers a lifting action. Throughout the manipulation the object’s deformation and its state of contact with the fingers and the table are being tracked based on contact events. Deformable modeling uses the finite element method (FEM) while slip computation employs the homotopy continuation method, based on the contact displacements induced by finger movements. Experiment was conducted over daily items ranging from vegetables to a toy. A simulation-based comparison between deformable grasping and rigid body grasping reveals why soft objects are easier to pick up than hard ones, and demonstrates how a grasping strategy for rigid objects could fail on soft objects in certain situations. KEY WORDS — Deformable grasping, gravity, liftability test, contact modes, finite element method.