An omnidirectional mobile manipulator (OMM) provides extended capabilities to a robotic system since it enlarges its workspace at the expense of an omnidirectional mobile base, which is nowadays a reality. When various OMM transport an object, the resultant constrained OMM dynamics is quite complex that demands effective control schemes without neglecting dominant nonlinear effects nor interaction forces. In this paper, it is proposed a decentralized cooperative control for a set of k OMMs (k-OMM) subject to homogeneous holonomic constraints imposed by the object being manipulated. A chatterless integral sliding mode force-position control guarantees exponential tracking whose implementation does not require any knowledge of dynamics. This controller is decentralized in a sense that neither dynamics nor interaction forces of the i-OMM are required for the j-controller, and it is cooperative by correcting collectively the velocity deviation due to the pushing of the other j-OMM. Each omnidirectional base provides object mobility along passive smooth velocity fields to avoid obstacles on the floor, which is synthesized as the secondary task when resolving redundancy, and the force control stabilizes internal forces to ensure object grasp, which are declared together with end-effector position, as the primary task. Simulations are discussed for two 8-DoF Kuka youBot OMM system navigating in a cluttered environment.

A passivity-based model-free control scheme for an underwater fully actuated vehicle-manipulator system (UVMS) in contact tasks is proposed. Orthogonalized motion and force second-order sliding modes are enforced for all time for the redundant noninertial robotic UVMS, including when it is subject to a class of fluid disturbances. To this end, we first determine the constrained dynamics using the quasi-Lagrangian formulation to explicitly characterize hydrodynamic fluid perturbations. Our scheme aims at exploiting the structural properties of Lagrangian systems, then we derive a mapping between the quasi-Lagrangian UVMS to its equivalent Lagrangian form, and study the conditions for open-loop passivity preservation during the interaction of postures and contact constraints between the end effector of the UVMS and the rigid contact surface of bulky objects in cluttered submarine environments. Internal motions are simultaneously computed by solving, in the tangent subspace of the contact manifold, a hierarchy of secondary tasks to satisfy the posture constraints. More importantly, the solution shapes the extended errors that are used to preserve passivity and to enforce dissipativity so as to guarantee local exponential stability without any knowledge of the complex UVMS dynamics, and the energetic performance of the UVMS in closed-loop. Illustrative simulations are discussed to show the feasibility of the proposed scheme.

In this paper, a new hybrid active mechanical technique is proposed, which nullifies the torque coming from gravitational acceleration by means of controlled displacements of the center of mass of each link. Modeling, design and control issues are presented as well as some preliminary experimental results illustrate this principle.

The paper is devoted to the problem of nonlinear robust control design for underwater vehicle/manipulator (UVM) systems composed of a freefloating platform equipped with a robot manipulator. The different bandwidth characteristics of the composite vehicle/arm dynamics are used as a basis for the control design via singular perturbation theory. The main contribution of this paper is that a robust nonlinear control is only required in the slow-subsystem (vehicle dynamics). Thus, this control is only used to compensate the coupling effects from the arm to the base. The paper also presents a comparative study between simple PD, a partial linearized control and the nonlinear robust feedback.