Decentralized Kinematic Control of a Cooperating System of Mobile Manipulators

In this paper, we examine the development of a decentralized control framework for a modular system of wheeled mobile manipulators that can team up to cooperatively transport a large common object. Each individually autonomous mobile manipulator consists of a differentially-driven wheeled mobile robot (WMR) with a passive, two-degree-of-freedom, planar, revolute-jointed arm mounted in the plane parallel to the base of the WMR. The composite multi-degree-of-freedom vehicle, formed by placing a common object on the end-effector of two (or more) such mobile manipulator systems, possesses the ability to accommodate relative positioning errors of the mobile bases as well as change its relative configuration. Particular attention is paid for the development of kinematic control schemes for mobile manipulators, which take into account the non-holonomic constraints of the base and the presence of passive joints in the manipulator system. The control scheme developed for the individual mobile manipulators is then adapted for the decentralized kinematic control of two mobile manipulators carrying a common object along a desired trajectory. Experimental evaluation of the performance of the resulting approach and the ability of the overall collaborating system to accommodate, detect and correct for relative positioning errors between the mobile platforms is also presented. INTRODUCTION Our overall goal is the design, development and implementation of a flexible, scalable and modular framework for multiple individually autonomous mobile manipulators to team up to cooperatively transport large objects. Such frameworks for remotely-controlled or remotely-supervised cooperation of multiple autonomous mobile manipulators have many applications for material handling in diverse domains. Wheeled Mobile Robots (WMRs) have been extensively studied in the past by many researchers in the field of robotics. Designs for individual platforms, with multiple sets of disc wheels attached to a common chassis, are popular since this permits the load and the tractive forces to be distributed among the multiple wheels. However, the mobility and controllability of the resulting wheeled mobile platform, depends largely upon the type/nature and location of the attached wheels. Specifically, arbitrarily locating and actuating such sets of wheels often results in degradation of performance, due to the incompatibility of the wheel velocities. Hence the design and control of such vehicles needs to explicitly take into account maintenance of the compatibility conditions. In the plane, these conditions take the form of requirement for a common instantaneous center of rotation (ICR) for all the wheels. Most design approaches, therefore, consider the addition of active or passive articulations between the wheels and the chassis to satisfy this requirement. In our work, we consider the formation of larger composite wheeled vehicles by coupling together multiple individual differentially-driven mobile bases (each possessing a single rigid axle between two fixed disk wheels with the usual complement of nonholonomic constraints). As in the case of the individual platforms, two or more such wheeled mobile bases with rigid axles cannot be rigidly coupled to each other without experiencing degradation of performance. Hence, an intermediate compliant linkage (with at least three d.o.f.) needs to be introduced between the two axles to decouple the motions of the individual platforms [1]. Two such example vehicles, the CLAPPER and the OMNIMATE, featuring such a compliant linkage with two passive revolute joints and one passive prismatic joint, were built and tested. Such vehicles fall under the classification of multipledegree-of-freedom (MDOF) vehicles, since they possess more