Enabling Bidirectional Thrust for Aggressive and Inverted Quadrotor Flight

Quadrotors have become the most common and highly popularized small aerial vehicles among robotics researchers, consumers and commercial users. Traditionally, these platforms have been designed with each of the four propellers to spin in the designated direction (two clockwise and two counter-clockwise) to produce unidirectional thrust-this allows the vehicle to oppose gravity when in its nominal hover orientation. In this paper, we present a quadrotor which is capable of bidirectional thrust actuation: it is generated by reversing the direction of the driving motors and hence propeller spins. This configuration is motivated by the desire to increase the vehicle's agility, as well as to imbue it with functionalities not available to the standard unidirectional thrust platforms. We present the dynamics model of a quadrotor with bidirectional thrust, the controller adapted to this configuration, and details of the implementation of the bidirectional capability in hardware and software, using the state-of-the-art Pixhawk micro-controller and PX4 flight stack. The transient thrust characteristics of a symmetric propeller are experimentally determined, highlighting the maximum rate of change and dead-zone specific to bidirectional thrust. Results starting from simulation, to hardware-in-the-loop testing, to experiments conducted with an outdoor platform are presented for a half flip maneuver, demonstrating the performance and upside down hovering of the vehicle.