  Cooperative transportation by multiple aerial robots has the potential to
support various payloads and to reduce the chance of them being dropped.
Furthermore, autonomously controlled robots make the system scalable with
respect to the payload. In this study, a cooperative transportation system was
developed using rigidly attached aerial robots, and a decentralized controller
was proposed to guarantee asymptotic stability of the tracking error for
unknown strictly positive real systems. A feedback controller was used to
transform unstable systems into strictly positive real ones using the shared
attachment positions. First, the cooperative transportation of unknown payloads
with different shapes larger than the carrier robots was investigated through
numerical simulations. Second, cooperative transportation of an unknown payload
(with a weight of about 2.7 kg and maximum length of 1.6 m) was demonstrated
using eight robots, even under robot failure. Finally, it was shown that the
proposed system carried an unknown payload, even if the attachment positions
were not shared, that is, even if the asymptotic stability was not strictly
guaranteed.
