Decentralized Kinematic Control of Payload Transport by a System of Mobile Manipulators

In this paper, we examine creation of a decentralized kinematic control scheme for a composite system of two (or more) wheeled mobile manipulators that can team up to cooperatively transport a common payload. Each mobile manipulator module consists of a differentially-driven wheeled mobile robot (WMR) with a mounted planar two-degree-offreedom (d.o.f) manipulator. A composite multi-degree-offreedom system is formed when a payload is placed at the endeffectors of multiple such modules with significant advantages. However, the nonholonomic/holonomic constraints and active/ passive components within the composite vehicle need careful treatment for realizing the payload transport task. Hence, we first verify that arbitrary desired end-effector motions can be accommodated, within the feasible motion distributions of the articulations and the wheeled base. Then, we develop motionplans by which this desired end-effector motion can be actively realized, using only the limited active motion-distribution of the differentially-driven wheels. Finally, we deploy this in the form of a two-level hierarchical control framework, with an upper-level planning of the steerable active vector-fields and a lower-level posture stabilization control of the individual WMRs. Preliminary experimental results from the decentralized-control implementation for a two-module composite vehicle are also presented. Keywords—Mobile Manipulator, Nonholonomic Constraints, Twist Analysis, Decentralized Control.