Stability analysis of grasped object by soft-fingers with 3-dimensional deformation based on moment stability

This paper analyze stability of an object grasped by soft-fingers in 3-dimensional space based on moment stability. We firstly define the moment stability as a criterion for stability of a grasped object when the object is perturbed for the orientation. In detail, the moment stability means that the grasping force can be the restoring force when the object is perturbed. Based on the moment stability, the stability condition of an object grasped by hard-fingers is derived. We indicate that contact points to satisfy the condition are restricted to upper locations of the center of mass and the class of the object shape is only a kind of hollow objects. Next, the condition of an object grasped by semispherical soft-fingers thirdly is considered. Two-fingered grasp by soft-fingers can satisfy the force-closure which is traditional major criterion for static grasp. The consideration shows a novel result that two-fingered grasp can satisfy the moment stability with only two fingers while the hard-fingers need at least three numbers for the force-closure and can not satisfy the moment stability. On the other hand, it is indicated that the torsion moments do not effect on the moment stability well and it is necessary to add the third finger in a condition of the contact points. Numerical examples finally are shown.