Exchanging objects with humans through handovers is a key feature for any robot operating side by side with humans. The studies on the topic tackle various problems, such as the hand dexterity, the communication cues, the arms motions, the forces, the head motions, but most of them consider a handover as an independent action, decorrelated from the plan it is part of. In this paper, we consider situations where it might be necessary (or preferable) to achieve several handovers in order to transfer an object from one agent to another one. This problem complexity grows accordingly with the number of agents that might be involved in the task, making a classical approach under efficient. We propose a graph-based approach enabling a fast computation of a solution taking into account different parameters linked to the humans comfort and preferences. An abstract model of the task is also used as a heuristic to guide the search in the graph, a search which is performed with a Lazy Weighted A*. The method computes which agents (human or robot) to use and where handovers should be performed. It also computes motion plans for each robot, ensures that humans can reach handover places and preserves comfort of human partners by reducing, for instance, their efforts. The method has been implemented and is demonstrated on various simulated multi-agents environments and on our two PR2 robots interacting with two humans.

We claim that the activity consisting in providing route directions can be best dealt with as a joint task involving the contribution not only of the robot as a direction provider but also of the human as listener. Moreover, we claim that in some cases, both the robot and the human should move to reach a different perspective of the environment which allows the explanations to be more efficient. As a first step toward implementing such a system, we propose the SVP (Shared Visual Perspective) planner which searches for the right placements both for the robot and the human to enable the visual perspective sharing needed for providing route direction and which makes the choice of the best landmark when several are available. The shared perspective is chosen taking into account not only the visibility of the landmarks, but the whole guiding task.

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