In-Flight Cable Length Control for Improved Quadrotor-Based Suspended Load Transportation

Load transportation (LT) using quadrotor unmanned aerial vehicles (UAVs) has drawn increasing interest from the robotics community in recent years. Two main alternative load carrying modalities have been investigated. Namely, the load is either rigidly attached to the body of the quadrotor (LT-RA), or it hangs from the quadrotor via a cable, resulting in a constant-length cable-suspended load transportation system (SLT-CL). However, cable-suspended load transportation systems featuring an in-flight actively controlled cable length (SLT-AL) have rarely been studied. The SLT-AL system introduced in this letter allows for active in-flight control of the cable length to enable safe and efficient load transportation in cluttered environments. To this end, we propose a novel, versatile optimal trajectory planning method based on the direct collocation approach, which can be applied to all three types of load carrying systems (LT-RA, SLT-CL, SLT-AL). The method includes the coupled dynamics of the UAV and load and accounts for input constraints and obstacle avoidance. Results from numerical simulations and experimental tests indicate the advantages of SLT-AL over SLT-CL in terms of energy saving, tracking performance, and load swing suppression.