Cable-suspended load lifting by a quadrotor UAV: hybrid model, trajectory generation, and control

In the last years, autonomous aerial vehicles have become promising mobile robotic platforms capable of manipulating external objects. In particular, quadrotors, rotorcrafts with four propellers, have been used for aerial transportation of cable-suspended loads. A critical step before transporting a payload is the lift maneuver. However, the analysis and planning of this maneuver have received a little attention in the literature so far. In this work, we decompose the cable-suspended load lifting into three simpler discrete states or modes: Setup, Pull, and Raise. Each of these states represents the dynamics of the quadrotor-load system at particular regimes during the maneuver. Furthermore, we define a hybrid system based on these states and show that it is a differentially-flat hybrid system. Exploiting this property, we generate a trajectory by using a series of waypoints associated with each mode. We design a nonlinear hybrid controller to track this trajectory and therefore execute the lift maneuver. We verify the proposed approach by carrying out experiments on an actual quadrotor with a cable-suspended load.