  A shared grasp is a grasp formed by contacts between the manipulated object
and both the robot hand and the environment. By trading off hand contacts for
environmental contacts, a shared grasp requires fewer contacts with the hand,
and enables manipulation even when a full grasp is not possible. Previous
research has used shared grasps for non-prehensile manipulation such as
pivoting and tumbling. This paper treats the problem more generally, with
methods to select the best shared grasp and robot actions for a desired object
motion. The central issue is to evaluate the feasible contact modes: for each
contact, whether that contact will remain active, and whether slip will occur.
Robustness is important. When a contact mode fails, e.g., when a contact is
lost, or when unintentional slip occurs, the operation will fail, and in some
cases damage may occur. In this work, we enumerate all feasible contact modes,
calculate corresponding controls, and select the most robust candidate. We can
also optimize the contact geometry for robustness. This paper employs
quasi-static analysis of planar rigid bodies with Coulomb friction to derive
the algorithms and controls. Finally, we demonstrate the robustness of shared
grasping and the use of our methods in representative experiments and examples.
The video can be found at https://youtu.be/tyNhJvRYZNk
