Efficient coordination of a multirobot team is the key challenge in robotic application domains such as manufacturing, construction, and service robotics. In cooperative manipulation tasks, the system dynamics result from the complex interaction of several manipulators handling a common object. A comprehensive model is indispensable for a sophisticated model-based control design. An open problem is the modeling and analysis of the overall system dynamics including the manipulators' interaction wrenches. Based on the apparent end-effector dynamics in task space, in this paper we focus on the characterization of the interaction effects when manipulating a common object. We note the central role of the imposed kinematic constraints for the emerging system dynamics, their significance for the manipulator coordination in terms of control design, and the analysis of internal wrenches applied to the object. We derive fundamental properties of the cooperative manipulator system relevant to the manipulation task such as the apparent impedance with respect to external disturbances. An experimental study is conducted with two cooperating anthropomorphic manipulators supporting the relevance of our findings.

In cooperative manipulation tasks, load allocation is a crucial step in order to solve the intrinsic redundancy of the system. The desired wrench needs to be suitably distributed between the end-effectors to implement the desired motion of the manipulated object. In this framework, both the grasp kinematics and the individual capacities of each manipulator provide relevant constraints. On one hand, the end effector wrenches act on the object via the grasp geometry. On the other hand, the individual admissible payload further depends on the current configuration of the robots. In this paper we focus on a heterogeneous cooperative manipulation setting and we design a proper allocation strategy to distribute the desired object wrench, considering both constant and time-varying constraints for the load distribution. The relevance of our findings is illustrated by means of an experimental study involving two anthropomorphic robots manipulating a common object.

Knowledge of the human grasp pose is crucial in common control schemes for human-robot object manipulation tasks. Biased estimates of the grasp pose cause undesired interaction wrenches on the human partner, which disturbs the interaction and the recognition of motion intention. A use of wearable motion sensors for tracking the human motion facilitates the grasp pose estimation without a global sensing system. This paper presents an approach for estimating an unknown grasp pose of the human using wearable motion sensors while minimizing undesired interaction wrenches applied to the human. A condition necessary for convergence of the estimator together with appropriate robot motion strategies are provided. Estimation of relative orientation and displacement is performed online and based on minimizing the error in the least-square sense. The estimation process does not rely on a global sensing system and it considers only the measurements of the velocity and acceleration of the cooperating partners in their respective local frames. The approach is experimentally evaluated in a physical human-robot interaction scenario.

The load distribution strategy in cooperative manipulation tasks allocates suitable force and torque setpoints to an ensemble of manipulators in order to implement a desired action on the manipulated object. Due to the manipulator redundancy, the load distribution computed by means of a generalized inverse of the grasp matrix is not uniquely determined. Controversial results on the nonsqueezing property of specific load distributions exist in the literature. In this paper, we propose a new paradigm for the analysis of internal wrenches based on the kinematic constraints imposed to the manipulator ensemble. We unify previous results by showing that there exists no unique nonsqueezing load distribution and illustrate the consequences of our findings by means of several examples. In particular, the presented results provide a new perspective on the decomposition of interaction forces into internal and external components as required for cooperative multimanipulator control schemes.

Multi-robot cooperative manipulation of a common object requires precise kinematic coordination of the attached end effectors in order to avoid excessive forces on the object and the manipulators. A manipulation task is considered successful if the desired object motion and forces are tracked accurately. In this paper we present a systematic analysis on the effect of uncertain kinematic parameters on the tracking behavior in a planar manipulation task. An adaptive control scheme is proposed, which achieves the desired control goal asymptotically. The presented scheme employs the current force/motion data of the attached end effectors without relying on a common reference frame. The algorithm is applicable to common manipulator types with wrist-mounted force/torque sensors and implementable in real-time. The performance of the proposed control scheme is evaluated experimentally with two 7DoF manipulators who cooperatively manipulate an object of uncertain length.

Cooperative manipulation in robotic teams likely results in an increased manipulation performance due to complementary sensing and actuation capabilities or increased redundancy. However, a precise coordination of the involved manipulators is required in order to avoid undesired stress on the manipulated object. Extending the workspace of the robots by means of mobile platforms greatly enlarges the potential task spectrum but simultaneously poses new challenges for example in terms of increased kinematic errors. In this paper we show how kinematic errors in the closed kinematic chain originating from uncertainties in the geometry of object and manipulators limit the cooperative task performance. We extend an impedance-based coordination control scheme towards mobile multi-robot manipulation to limit undesired internal forces in the presence of kinematic uncertainties. Furthermore, we employ a task-space decoupling approach to reduce the impact of disturbances at the mobile platforms on the end effectors. The presented control scheme for cooperative, mobile dualarm manipulation is applicable in real-time and suitable for a team of heterogeneous manipulators. We evaluate the presented architecture by means of a large-scale experiment with four 7DoF manipulators on two mobile platforms.