  Sound is a rich information medium that transmits through air; people
communicate through speech and can even discern material through tapping and
listening. To capture frequencies in the human hearing range, commercial
microphones typically have a sampling rate of over 40kHz. These accessible
acoustic technologies are not yet widely adopted for the explicit purpose of
giving robots a sense of touch. Some researchers have used sound to sense
tactile information, both monitoring ambient soundscape and with embedded
speakers and microphones to measure sounds within structures. However, these
options commonly do not provide a direct measure of steady state force, or
require electronics integrated somewhere near the contact location. In this
work, we present AcousTac, an acoustic tactile sensor for electronics-free
force sensitive soft skin. Compliant silicone caps and plastic tubes compose
the resonant chambers that emit pneumatic-driven sound measurable with a
conventional off-board microphone. The resulting frequency changes depend on
the external loads on the compliant end caps. We can tune each AcousTac taxel
to specific force and frequency ranges, based on geometric parameters,
including tube length and end-cap geometry and thus uniquely sense each taxel
simultaneously in an array. We demonstrate AcousTac's functionality on two
robotic systems: a 4-taxel array and a 3-taxel astrictive gripper. AcousTac is
a promising concept for force sensing on soft robotic surfaces, especially in
situations where electronics near the contact are not suitable. Equipping
robots with tactile sensing and soft skin provides them with a sense of touch
and the ability to safely interact with their surroundings.
