The Grasp Acquisition Strategy of the Velvet II

Fig. 1. A power grasp of a wooden object with the Velvet-II dexterous gripper. In this work we exploit the combined effects of the under-actuation and the active surfaces of the Velvet II dexterous gripper. The aim is to achieve a firm enveloping grasp, starting from an initial pinch grasp. The pull-in grasping strategy described here turns out to be very useful in untidy environments where the scene is populated by many objects in a cluttered disposition. In this context, a nimble fingertip grasp is more likely to be feasible than a robust enveloping grasp [1], [2]. The Velvet-II is composed of two fingers, each of which can be regarded as a RR planar manipulator with a distal and a proximal phalanx each. One motor in the palm drives the opening and closing of the fingers through an under-actuated mechanism. During the closure, the fingers are extended and form a V-like shape while moving without obstruction. When the proximal phalanges are blocked by an object however, the closure behavior changes and the distal phalanges continue to rotate achieving a stable enveloping grasp. Additionally, the gripper comprises four independent active surfaces, one for each phalanx. These are implemented by controlled conveyo