By the term power grasp in the physiology of human manipulation, a particular type of hold is indicated, that uses not only the fingertips but also the inner phalanges of the hand for constraining the object. In robotics, this concept can be extended to robotic systems composed of multiple actuated limbs (such as arms, fingers, or legs) cooperating in the manipulation of an object. Power grasp (also indicated by 'enveloping' or 'whole-limb') operations that exploit any part of the limbs to contact the object are considered in this paper. In particular, the problem of decomposing the system of contact forces exerted between the robot limbs and the object, in order to apply a desired resultant force on the object (and/or to resist external disturbances) is studied. The peculiarity of whole-limb systems is that contacts occurring on links with limited mobility, such as the inner links of a robot arm or hand, and even on fixed links (a robot chest or palm), are possible. Although the potential usefulness of whole-limb manipulation is demonstrated by biomorphic examples as well as by practically implemented robotic devices, present methods for grasp analysis cannot directly deal with these types of grasping mechanisms. We propose a modification of known force decomposition analysis that generalizes to enveloping grasping. The results of the proposed technique provide a basis for the realization of real-time optimal control of whole-limb manipulation.
