  Receiving a hug is one of the best ways to feel socially supported, and the
lack of social touch can have severe negative effects on an individual's
well-being. Based on previous research both within and outside of HRI, we
propose six tenets ("commandments") of natural and enjoyable robotic hugging: a
hugging robot should be soft, be warm, be human sized, visually perceive its
user, adjust its embrace to the user's size and position, and reliably release
when the user wants to end the hug. Prior work validated the first two tenets,
and the final four are new. We followed all six tenets to create a new robotic
platform, HuggieBot 2.0, that has a soft, warm, inflated body (HuggieChest) and
uses visual and haptic sensing to deliver closed-loop hugging. We first
verified the outward appeal of this platform in comparison to the previous
PR2-based HuggieBot 1.0 via an online video-watching study involving 117 users.
We then conducted an in-person experiment in which 32 users each exchanged
eight hugs with HuggieBot 2.0, experiencing all combinations of visual hug
initiation, haptic sizing, and haptic releasing. The results show that adding
haptic reactivity definitively improves user perception a hugging robot,
largely verifying our four new tenets and illuminating several interesting
opportunities for further improvement.
