  Being motivated by ceiling inspection applications via unmanned aerial
vehicles (UAVs) which require close proximity flight to surfaces, a systematic
control approach enabling safe and accurate close proximity flight is proposed
in this work. There are two main challenges for close proximity flights: (i)
the trust characteristics varies drastically for the different distance from
the ceiling which results in a complex nonlinear dynamics; (ii) the system
needs to consider physical and environmental constraints to safely fly in close
proximity. To address these challenges, a novel framework consisting of a
constrained optimization-based force estimation and an optimization-based
nonlinear controller is proposed. Experimental results illustrate that the
performance of the proposed control approach can stabilize UAV down to 1 cm
distance to the ceiling. Furthermore, we report that the UAV consumes up to
12.5% less power when it is operated 1 cm distance to ceiling, which is
promising potential for more battery-efficient inspection flights.
