  Distributed tactile sensing for multi-force detection is crucial for various
aerial robot interaction tasks. However, current contact sensing solutions on
drones only exploit single end-effector sensors and cannot provide distributed
multi-contact sensing. Designed to be easily mounted at the bottom of a drone,
we propose an optical tactile sensor that features a large and curved soft
sensing surface, a hollow structure and a new illumination system. Even when
spaced only 2 cm apart, multiple contacts can be detected simultaneously using
our software pipeline, which provides real-world quantities of 3D contact
locations (mm) and 3D force vectors (N), with an accuracy of 1.5 mm and 0.17 N
respectively. We demonstrate the sensor's applicability and reliability onboard
and in real-time with two demos related to i) the estimation of the compliance
of different perches and subsequent re-alignment and landing on the stiffer
one, and ii) the mapping of sparse obstacles. The implementation of our
distributed tactile sensor represents a significant step towards attaining the
full potential of drones as versatile robots capable of interacting with and
navigating within complex environments.
