  Modular soft robots combine the strengths of two traditionally separate areas
of robotics. As modular robots, they can show robustness to individual failure
and reconfigurability; as soft robots, they can deform and undergo large shape
changes in order to adapt to their environment, and have inherent human safety.
However, for sensing and communication these robots also combine the challenges
of both: they require solutions that are scalable (low cost and complexity) and
efficient (low power) to enable collectives of large numbers of robots, and
these solutions must also be able to interface with the high extension ratio
elastic bodies of soft robots. In this work, we seek to address these
challenges using acoustic signals produced by piezoelectric surface transducers
that are cheap, simple, and low power, and that not only integrate with but
also leverage the elastic robot skins for signal transmission. Importantly, to
further increase scalability, the transducers exhibit multi-functionality made
possible by a relatively flat frequency response across the audible and
ultrasonic ranges. With minimal hardware, they enable directional contact-based
communication, audible-range communication at a distance, and exteroceptive
sensing. We demonstrate a subset of the decentralized collective behaviors
these functions make possible with multi-robot hardware implementations. The
use of acoustic waves in this domain is shown to provide distinct advantages
over existing solutions.
