Fail-Safe Controller Architectures for Quadcopter with Motor Failures

TL;DR

This paper presents fail-safe quadcopter architectures that maintain stability during motor failures, demonstrating that a two-propeller setup is more efficient and robust than a three-propeller configuration.

Contribution

It introduces and compares two fail-safe architectures for quadcopters, highlighting the superior robustness and efficiency of the two-propeller design under failure conditions.

Findings

Two-propeller architecture is more stable and robust.

Higher yaw rate improves stability in fail-safe mode.

Both architectures maintain altitude with PID control.

Abstract

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.