Cooperative control of multiple robots to manipulate objects

Cooperative Control Of Multiple RobotsTo Manipulate ObjectsSusanna R. LeveroniJ. Kenneth SalisburyMassachusetts Institute of Technology, Department of Mechanical EngineeringArtificial Intelligence Laboratory, Cambridge, Massachusetts 02139ABSTRACTThis paper addresses the problem of using multiple robots together as a hand to reorient objects in theplane. A planner is described which generates a sequence of intermediate object motions and robot grasps,called a grasp gait, sufficient to achieve a final desired object orientation. These gaits are implementedusing 3 Phantom Haptic Interfaces, modified to act as robot fingers. The core control structure of theimplementation is presented. Finger force and position errors, as well as object position errors, arediscussed, and additional control algorithms are developed to correct for these errors. These gaits aredemonstrated to be robust to small disturbances using the additional control algorithms.Keywords: robot hands, manipulation, grasp gaits, grasp maintenance, object localization.1. INTRODUCTIONThe problem of using multiple robots as an articulated hand has long been of interest to the roboticscommunity. The early works of [Salisbury] and [Jacobsen] addressed hands able to grasp and manipulateobjects from a design point of view. The problem of choosing where to place fingers and how to allocategrasp forces in order to achieve stable grasps has been extensively studied, for example by[Lakshminarayanaj, [Nguyen], [Kerr and Roth], [Li and Sastry], and [Pollard]. Using hands to achievesmall object reorientations was studied by [Salisbury] and [Brock], among others. More recently it hasbeen recognized that to achieve a wide range of object motions by an articulated hand, grasp gaits must beused. Early examples of this approach include the work done by [Fearing], [Omata and Nagata], and [Chenand Burdick].This paper addresses a particular and instructive case of planning grasps for objects manipulated in theplane by a three-fingered hand. We focus on the problem of continuous reorientation of objects in the planevia sequences of stable, two-fingered grasps, finite reorientations, and regrasps. The problem is treated attwo levels: first, the planning of sequences of finger motions, and second, the construction of controlalgorithms and correction paradigms which enable the plans to be executed despite small errors in modelingand disturbance forces.2. GAIT PLANNINGIn this section we present tools developed to represent our gait planning problem in a tractable form, alongwith gait planning methods developed using this representation. This work is more thoroughly presented in[Leveroni and Salisbury] and is summarized here. In section 2. 1 we define the grasp map, similar to thecontact configuration space developed by [Chen and Burdick]. In section 2.2 we develop a workspace mapto represent workspace limits in a way that is compatible with the grasp map. In section 2.3 we combine thetwo maps in a way which allows us to represent grasp gaits. In section 2.4 we present two gait planningmethods using this representation and discuss the results of these two methods.