Robot Grasp Control

Abstract

Grasp control refers to the art of controlling the motion of an object by constraining its dynamics through contacts with a hand. The process of controlling the grasp is not limited to robotic hands only but also applies to human hands (Johansson and Edin 1991) and to all other mechanisms using contact constraints to control the motion of the manipulated object (Brost and Goldberg 1996). A crucial role in the control of grasping is played by contact constraints. All the interactions between the robotic hand and the grasped object occur at the contacts whose understanding is paramount (Salisbury and Roth 1983). The unilateral nature of contact interaction in grasping makes the control problems much more challenging than cooperative manipulation where multiple arms hold the object rigidly allowing bilateral force transmission at each contact point (Chiacchio et al. 1991). The importance of unilateral contact constraints in grasping led a large part of the literature to focus on the closure properties of the grasp (Bicchi 1995). Those properties refer to the ability of a grasp to prevent motions of the grasped object relying only on unilateral frictionless constraints in case of form closure (Reuleaux 1876) and on contact constraints with friction in case of force closure (Nguyen 1988). While form closure is a purely geometric property of the grasp and depends on where the unilateral contact points are on the object, force closure depends on the ability that the robotic hand has to resist and apply forces to the object through the contacts while satisfying the friction constraints. In other terms force closure directly involves the control of the robotic hand kinematics and not only the geometry of the contacts (Bicchi 1995). This entry focuses on force-closed grasps. The optimal choice of the contact points on the object surface is a critical issue known as grasp planning. Among the many optimal criteria that have been proposed in the literature to choose the contact points, I want to recall the one proposed in Ferrari and Canny (1992) where the grasping configuration is evaluated according to the magnitude of the largest worst-case disturbance wrench that can be resisted by the grasp. Many approaches have been studied in the literature on grasp planning in the presence of uncertainties. The uncertainty can be either due to the shape of the object which is partially known or partially sensed as in Goldfeder et al. (2009) or due to the errors in positioning the fingers on the object during the grasping (Roa and Suarez 2009). In what follows all the parameters of the grasp including those related to the hand, the object, and the contact points are assumed to be known with no uncertainties. The main objective of grasp control is that of tracking a desired trajectory with the grasped object by applying a set of contact forces satisfying the friction constraints (Bicchi and Kumar 2000). Complex in-hand object motions can be obtained by rolling and sliding the contact points on the object surface as proposed in Montana (1988) or by using finger gaiting to get large-scale motions (Han and Trinkle 1998). This entry deals with non-rolling and non-sliding contact points