Towards modular cooperation between multiple nonholonomic mobile manipulators
Bhatt, Rajankumar M
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In recent times, there has been considerable interest in creating and deploying modular cooperating collectives of robots. Interest in such cooperative systems typically arises when certain tasks are either too complex to be performed by a single robot or when there are distinct benefits that accrue by cooperation of many simple robotic modules. In this dissertation, we examine these aspects in the context of cooperative payload manipulation and transport by wheeled mobile module collectives with tightly-coupled dynamics arising from the physical interactions between various modules. Our basic module is a wheeled mobile manipulator (WMM) formed by mounting a multi-link manipulator on top of a disk wheeled mobile base. By merging mobility with manipulation, mobile manipulators derive significant novel capabilities for enhanced interactions with the world. However, a careful resolution of the redundancy and active control of the reconfigurability created by the surplus articulated degrees-of-freedom and actuation is key to unlocking this potential. Addition of nonholonomic constraints creates additional challenges when disk wheeled mobile base is used and needs to be carefully addressed. The primary control challenges arise due to the dynamic-level coupling of the nonholonomy of the wheeled mobile bases with the inherent kinematic and actuation redundancy within the articulated-chain. The solution approach leverages the kinetic-energy metric of the underlying articulated mechanical systems to create a dynamically-consistent and decoupled partitioning into external (task) space and internal (null) space dynamics. The independent controllers developed within each decoupled space facilitate active internal reconfiguration in addition to resolving redundancy at the dynamic level. Specifically, two variants of null-space controllers are implemented to improve disturbance-rejection and active reconfiguration during performance of end-effector tasks by a primary end-effector impedance-mode controller. These algorithms are evaluated within an implementation framework that emphasizes both virtual prototyping (VP) and hardware-in-the-loop (HIL) testing. Simulation and experimental results are used to highlight aspects of implementation in a real-time sensor-based control framework to fully exploit the novel capabilities of such nonholonomic wheeled mobile manipulators. We then envisage the use of such WMM modules capable of autonomous control in a decentralized cooperative payload transport application. The various actuation schemes used on individual WMMs, their interactions with each other through the payload and presence of nonholonomic constraints significantly affect the overall system performance. We leverage the rich theoretical background of analysis of constrained mechanical systems to provide a systematic framework for formulation and evaluation of system-level performance on the basis of the individual-module characteristics. The composite multi-degree-of-freedom wheeled vehicle, formed by supporting a common payload on the end-effectors of multiple individual WMMs, is treated as an in-parallel system with articulated serial-chain arms. The system-level model, constructed from the twist- and wrench-based models of the attached serial-chains, is then systematically analyzed for performance in terms of mobility and disturbance rejection. Finally, a 2-module composite system example is used to highlight various aspects of methodical system model formulation and the effects of selection of active, passive or locked articulations on the mobility and disturbance rejection of the system.