Fail-Safe Controller Architectures for Quadcopter with Motor Failures
This work addresses safety and reliability for drone operations in scenarios like delivery or inspection, though it is incremental as it builds on existing control methods like PID and LQR.
The researchers tackled the problem of enabling quadcopters to fly safely after motor failures by developing and testing two fail-safe controller architectures for three- and two-propeller operation, finding that the two-propeller architecture is more efficient, robust, and stable, with higher yaw rates improving stability in fail-safe mode.
A fail-safe algorithm in case of motor failure was developed, simulated, and tested. For practical fail-safe flight, the quadcopter may fly with only three or two opposing propellers. Altitude for two-propeller architecture was maintained by a PID controller that is independent from the inner and outer controllers. A PID controller on propeller force deviations from equilibrium was augmented to the inner controller of the three-propeller architecture. Both architectures used LQR for the inner attitude controller and a damped second order outer controller that zeroes the error along the horizontal coordinates. The restrictiveness, stability, robustness, and symmetry of these architectures were investigated with respect to their output limits, initial conditions, and controller frequencies. Although the three-propeller architecture allows for distribution of propeller forces, the two-propeller architecture is more efficient, robust, and stable. The two-propeller architecture is also robust to model uncertainties. It was shown that higher yaw rate leads to greater stability when operating in fail-safe mode.