We propose a grasp stability index which takes into account the elastic deformation of fingertips. In this index, an evaluation value of grasp stability is derived as the minimum energy which causes slippage of a fingertip on a grasped object surface. Elastic potential energy of the fingertips and gravitational potential energy of the object are considered in this index. It is ensured that any fingertip does not slip on a grasped object surface if the external energy added to the object is less than the evaluation value. This index has good features as follows. (i) Contact forces corresponding to the grasping state are taken into account. (ii) It is possible to derive a condition of stable grasp including the kinetic energy of a grasped object. The effectiveness of this index is verified through several numerical examples.

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We aim to develop a multi-fingered hand which grasps various objects. We develop a finger equipped with soft fingertip and two layers nails. The first layer of the nails is thin and long. It is possible to insert it into the bottom of the object. The second layer of the nails is thick and short. It supports the elastic force of soft fingertip. We develop a planner for selecting a grasp style according to the height of the object. The planner can grasp a low height object by using a multi-fingered hand. We demonstrate the feasibility of the proposed method with simulation. In addition, it is shown that the developed hand can grasp various objects through experimental results.