  A safe controller for multicopter is proposed using control barrier function.
Multicopter dynamics are reformulated to deal with mixed-relative-degree and
non-strict-feedback-form dynamics, and a time-varying safe backstepping
controller is designed. Despite the time-varying variation, it is proven that
the control input can be obtained by solving quadratic programming with affine
inequality constraints. The proposed controller does not utilize a cascade
control system design, unlike existing studies on the safe control of
multicopter. Various safety constraints on angular velocity, total thrust
direction, velocity, and position can be considered. Numerical simulation
results support that the proposed safe controller does not violate all safety
constraints including low- and high-level dynamics.
