Grasp-state plane analysis of two-phalanx underactuated fingers

Abstract

This paper presents a new technique to analyze the grasp stability of two-phalanx underactuated fingers of general architecture using a grasp-state plane approach. Similarly to the state plane analysis for dynamical systems, a grasp-state plane technique is very elegant and efficient to study the grasp stability of two-phalanx underactuated fingers. The concept of underactuation in robotic fingers—with fewer actuators than degrees of freedom—allows the hand to adjust itself to an irregularly shaped object without complex control strategy and numerous sensors. However, in some configurations, the force distribution of an underactuated finger can degenerate, i.e. the finger can no longer apply forces on the object. This situation leads in some cases to the ejection of the object from the hand. This paper focuses on two-phalanx fingers and studies their ability to seize objects with a secure grasp, considering practical issues on the grasp, namely the local geometry of the contact, the influence of design parameters and friction. A grasp-state representation which allows to accurately visualize the contact state trajectory as well as equilibrium and unstable situations is presented.