SUMMARYWheeled Mobile Manipulators (WMM) possess many advantages over fixed-base counterparts in terms of improved workspace, mobility and robustness. However, the combination of the nonholonomic constraints with the inherent redundancy limits effective exploitation of end-effector payload manipulation capabilities. The dynamic-level redundancy-resolution scheme presented in this paper decomposes the system dynamics into decoupled task-space (end-effector motions/forces) and a dynamically consistent null-space (internal motions/forces) component. This simplifies the subsequent development of a prioritized task-space control (of end-effector interactions) and a decoupled but secondary null-space control (of internal motions) in a hierarchical WMM controller. Various aspects of the ensuing novel capabilities are illustrated using a series of simulation results.

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Mobile manipulators derive significant novel capabilities for enhanced interactions with the world by merging mobility with manipulation. However, a careful resolution of the redundancy and active control of the reconfigurability, created by the surplus articulated DOFs and actuation, are the keys to unlocking this potential. Nonholonomic wheeled mobile manipulators, formed by mounting manipulator arms on disc-wheeled mobile bases, are a small but important subclass of mobile manipulators. 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 in this paper builds upon a dynamically consistent and decoupled partitioning of the articulated system dynamics between the external (task) space and internal (null) space. 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 and hardware-in-the-loop testing with representative case studies.Index Terms-Dynamic redundancy resolution, hardwarein-the-loop (HIL), nonholonomic wheeled mobile manipulator (NH-WMM), virtual prototyping (VP).

Our long-term goal is one of designing land-based vehicles to provide enhanced uneven-terrain locomotion capabilities. In this paper, we examine and evaluate candidate articulated leg-wheel subsystem designs for use in such vehicle systems. The leg-wheel subsystem designs under consideration consist of disk wheels attached to the chassis through an articulated linkage containing multiple lower-pair joints. Our emphasis is on creating a design that permits the greatest motion flexibility between the chassis and wheel while maintaining the smallest degree-of-freedom (DOF) within the articulated chain. We focus our attention on achieving two goals: (i) obtaining adequate ground clearance by designing the desired/feasible motions of the wheel axle, relative to the chassis, using methods from kinematic synthesis; and (ii) reducing overall actuation requirements by a judicious mix of structural equilibration design and spring assist. This process is examined in detail in the context of two candidate single-degree-of-freedom designs for the articulated-leg-wheel subsystems—a coupled-serial-chain configuration and a four-bar configuration. We considered the design synthesis of planar variants of the two candidate designs surmounting a representative obstacle profile while supporting a set of end-effector loads and highlight the key benefits in the presented results.

Our overall goal is to develop semi-autonomous and decentralized task performance capabilities during cooperative payload transport by a fleet of wheeled mobile manipulators (WMM). Each nonholonomic WMM consists of a planar two-link manipulator mounted on top of a differentially-driven wheeled mobile base. The nonholonomic base and the significant inherent redundancy create challenges for control of end-effector motion/force outputs. Nevertheless, realizing this capability is a critical precursor to decentralized payload manipulation operations. To this end, a dynamic redundancy resolution strategy is critical in order to control the dynamic interactions. The system dynamics are decomposed into a task space component (consisting of end-effector motions/forces) and a decoupled dynamically-consistent null-space part (of internal-motions/forces). A task-space controller is developed that allows each WMM module to be able to control its end-effector (motions/forces) interactions with respect to the payload. The surplus of actuation is then used to independently control internal-motions (of the mobile base) as long as they do not conflict with the primary goal. A variety of numerical simulations are then performed to test this capability of the end-effector and mobile base to independently track complex motion/force trajectories.