Nonlinear Optimal and Multi-Loop Flatness-Based Control of the 6-DOF Coaxial Rotor UAV

Coaxial rotor 6-DOF UAVs can find use in several defence and civilian tasks. In this article, two different control methods are proposed for the control of this type of drones: (i) nonlinear optimal control and (ii) multi-loop flatness-based control. The dynamic model of the coaxial rotor drone is formulated and differential flatness properties are proven about it. To apply the nonlinear optimal control method, the dynamic model of the coaxial rotor drone undergoes approximate linearization with first-order Taylor-series expansion and through the computation of the associated Jacobian matrices. To apply multi-loop flatness-based control, the dynamic model of the UAV is decomposed in two subsystems connected in chained form. This means that the state vector of the second subsystem becomes virtual control input to the first subsystem, while the virtual control input of the first subsystem becomes a setpoints vector for the second subsystem. The two proposed control schemes ensure stabilization and precise flight-path tracking for the coaxial rotor UAV. Both control methods avoid changes of state variables and complicated state-space model transformations.