We discuss grasp stability under gravity where each finger makes soft-finger contact with an object. By clustering polygon models of a finger and an object, the contact area between a finger and an object is obtained as the common area between an object cluster and a finger cluster. Then, by assuming the Winkler elastic foundation, the pressure distribution within the contact area is obtained. By using this pressure distribution, we show that we can judge grasp stability under soft-finger contact. We further consider defining a quality measure of a soft-finger grasp by assuming that although the gravitational force is applied to an object, the direction of gravity is unknown. To demonstrate the effectiveness of the proposed approach, we show several numerical examples.

This paper presents a grasp planner which allows a robot to grasp the constricted parts of objects in our daily life. Even though constricted parts can be grasped more firmly than convex parts, previous planners have not sufficiently focused on grasping this part. We develop techniques for quadric surface approximation, grasp posture generation, and stability evaluation for grasping constricted parts. By modeling an object into multiple quadric surfaces, the planner generates a grasping posture by selecting one-sheet hyperbolic surfaces or two adjacent ellipsoids as constricted parts. When a grasping posture being generated, the grasp stability is evaluated based on the distribution of the stress applied to an object by the fingers. We perform several simulations and experiments to verify the effectiveness of our proposed method.