Unmanned Aerial Vehicles (UAVs) are known to be advantageous for many applications in security, monitoring of natural risks of environment, management of ground installations, and agriculture. In recent years, the share of autonomous payload transportation by rotary-wing UAVs is growing rapidly as well. Such vehicles allow operation in limited and confined environments, urban areas or even indoor sceneries. Yet, the implementation of such systems is limited due to the limited lifting capabilities of most of the existing hovering vehicles.
We address this issue by utilizing a slung-load approach in which the payload is hauled by multiple quad-rotors. The idea is to explore the possibility to use such “of-the-shelf” vehicles that were originally designed to operate independently, and connect them by cables to a payload in order to perform a cooperative transportation mission. In such a system, each vehicle is independently assigned with a trajectory while treating cable forces as additional external disturbance loads. This setting naturally raises questions regarding its feasibility and characteristics. However, if this system is indeed feasible, it constitute a valuable and extremely modular and effective arrangement for a variety of payloads by non-dedicated vehicles while the number of UAVs that are assigned for each mission is changed according to the payload weight, while no further adjustments are required. In the current study, a multiple quadrotor-single payload (MQSL) transportation system is analyzed using numerical simulative approach.
As opposed to the common “constant rotor force coefficients approach”, the quadrotor dynamics is modeled using detailed rotor aerodynamics, which is founded on an extended Blade-Element-Theory and enables simulation of unique and important flight regimes such as slow descent and gusty conditions. Additional feature of the present modeling is a refined cable simulation. This model consists of breaking the cable into finite segments which are elastically connected along with a local aerodynamic drag model for each segment. These enable simulation of the cable bending modes and a capability to simulate loose cables, which are typically not taken into account in full-scale helicopter slung-load models. Stability analysis and typical normal modes of the system will be presented. The suggested methodology includes an extension of feedback linearization control algorithm for quadrotor vehicles, in addition to a trajectory scheduling algorithm for individual quadrotor vehicle.
Within the validity checks of the proposed setting, the MQSL dynamics and control simulation was also successfully integrated with a Rapidly-exploring random tree (RRT) path planning algorithm. This algorithm provides collision free, reference path in a tight and confined urban and indoor environments.
Several examples of transportation scenarios, of pre-defined trajectories such as circular motion, slalom and “bug trap escaping path” will be demonstrated. The results of the present simulative study of MQSL systems give rise to a cautious optimism about the possibility of implementing such systems for modular payload transportation.
