Modeling and analysis of multiple cooperating robot manipulators

Abstract

The situation of two or more manipulators simultaneously grasping and transferring a common payload is investigated. In particular, for a given payload motion trajectory, the problem of distributing the load between the various arms is considered. There are in general an infinite number of choices for this distribution, corresponding to an infinite number of ways in which the individual end effectors may impart forces and moments to the payload to achieve the desired motion. In this work, the load distribution is related to and interpreted at both the payload object and the manipulator joint levels. Initially, previous approaches to load distribution using criteria related to the object are investigated, considering in particular the effects of the resulting solutions on the object. In particular, the effects of internal loading, that is, forces and moments experienced within the object that result in no motion, are analyzed. It is shown that an assumption commonly made regarding the generation of such internal loads by end effector force/moment solution sets is invalid in general. This may result in problems due to unexpected effects of unmodeled internal forces. A new representation of end effector force/moment solutions is introduced which eliminates this problem, and additionally yields new insight into the effects of load distribution schemes at both object and jointspace levels. Using this representation, and introducing a transformation of the load distribution problem to the manipulator joint level, algorithms for sharing the motion between the arms based on jointspace criteria are developed. These algorithms allow application of any criteria satisfying certain functional forms, where the selection of the criteria will be application dependent. The relationship between object-based and joint-based criteria is investigated. In the above, the focus is on the effects of load distribution at different parts in the system, and on increasing the understanding of the structure of the multiple arm situation. To this end, a structure for the control of the payload object motion and the internal loading of the payload is developed. This structure allows the application of a number of differing control philosophies. All developments are presented within a unifying framework in which the determination and effects of sharing the load between the arms may be seen clearly.