  Intuitive and efficient physical human-robot collaboration relies on the
mutual observability of the human and the robot, i.e. the two entities being
able to interpret each other's intentions and actions. This is remedied by a
myriad of methods involving human sensing or intention decoding, as well as
human-robot turn-taking and sequential task planning. However, the physical
interaction establishes a rich channel of communication through forces, torques
and haptics in general, which is often overlooked in industrial implementations
of human-robot interaction. In this work, we investigate the role of haptics in
human collaborative physical tasks, to identify how to integrate physical
communication in human-robot teams. We present a task to balance a ball at a
target position on a board either bimanually by one participant, or dyadically
by two participants, with and without haptic information. The task requires
that the two sides coordinate with each other, in real-time, to balance the
ball at the target. We found that with training the completion time and number
of velocity peaks of the ball decreased, and that participants gradually became
consistent in their braking strategy. Moreover we found that the presence of
haptic information improved the performance (decreased completion time) and led
to an increase in overall cooperative movements. Overall, our results show that
humans can better coordinate with one another when haptic feedback is
available. These results also highlight the likely importance of haptic
communication in human-robot physical interaction, both as a tool to infer
human intentions and to make the robot behaviour interpretable to humans.
