Grasping of deformable 3D objects under gravity

Abstract

The research area of robotic grasping of soft objects is difficult for multiple reasons: high cost computation of deformation, and the changes of the wrench space, contact areas, as well as point wise contact modes inside the areas. This thesis describes modeling of grasped soft objects and recovering their gravity-free shapes. Chapter 1 will introduce the background and related work. Chapter 2 will focus on grasping and picking up soft objects. In the beginning, some results will be shortly described on resisting a third force applied on a 2D rigid body . The results will show that one of the forces must lie on an edge of the friction cone in order to achieve the optimized total normal force. Studying grasping of soft 2D objects will pave the way for picking up 3D objects. Even though there are some similar methods used in both situations, such as the four events of contact establishment, contact break, stick-to-slip and slip-to-stick, which can happen in the finger’s squeezing process, there is still some significant difference between the 2D and 3D cases. This difference goes beyond just adding one more dimension, because the gravity effect has to be considered in 3D cases. In Chapter 3, the focus will be on recovering the shape of a 3D object. Since we use its shape under gravity to construct the stiffness matrix and compute deformations, errors are observed in the experiment. The reason behind this, is that the stiffness matrix in conventional FEM practice already encodes the effect of gravity, which is considered again in the constitutive equation used for computing deformation. Then a numerical iteration method will be introduced to recover the gravity free shape.