###### Abstract  
This paper presents a practical validation of a heterogeneous formation of mobile robots in performing a load lifting, transportation, and delivery task. Assuming that an unmanned ground vehicle (UGV) is unable to perform a mission by itself due to the presence of an obstacle in the navigation route, an unmanned aerial vehicle (UAV) is then assigned to lift the cargo over this UGV, transport the obstacle, and deliver over another UGV. The UAV uses an electromagnetic actuator supported by a cable to pick up the load, the mass of which is 32% of that of the UAV. Experimental results demonstrate that the developed system is capable of performing cargo transport missions and can be scalable for applications such as package delivery in urban or remote areas and supply delivery in conflict or disaster zones.  
multirobot systems; aerial-ground cooperation; load transportation +
Footnote †: journal: _Sci._  
0000-000-000-000-0000-0000-0000-0000-0000-0000-0000-0000-0000-0000-0000-0000-0000-0000-0000-0000-0000-00000-00000-00000-00000-0000  
Recent publications proposed control solutions for the manipulation and transportation of suspended payloads by using quadrotors that work cooperatively. In [12], it is stated that the control problem for the cable-suspended cargo approach is addressed in a way similar to that of a pendulum-stabilization problem. However, the cargo adds passive degrees of freedom to the UAV, directly affecting its dynamic characteristics and generating swings during flight. To do so, the controller must deal with such weight fluctuations and their consequences. The most traditional way is to stabilize and minimize cargo oscillations, neutralizing these unwanted movements. In [16], a case in which the UAV cannot lift the cargo is addressed. In this case, the strategy is to drag it laterally over the surface to the destination, whenever there is a possible route.  
In contrast to most approaches, some researchers treat cable conditions as different subsystems in a hybrid dynamic model, as can be seen in [17; 18], in which each subsystem has its specific and specialized controller, and a supervisory system is responsible for switching between these simpler controllers.  
One way to transport a payload is by attaching it to the UAV's rigid body via mechanical or electromagnetic devices, robotic grippers, or robotic hands [19; 20; 21; 22; 23; 24]. Although this option provides a simpler way by which to attach or release a payload, it reduces the agility of the rotorcraft according to [12]. Another way to perform the transport is to use suspended cables also with actuators composed of robotic arms, magnetic grippers, or mechanical devices [25; 26; 27]. This paper focuses on the latter approach, because our goal is to preserve the manoeuvrability of the UAV during execution. Furthermore, we aim to contribute to the literature with a practical demonstration of a task of grasping, lifting, carrying, and delivering a load by suspended cables. In this context, this present work aims to present an experimental implementation of a cooperative load transportation by using a heterogeneous formation. In summary, a control scheme guides a group of unmanned ground vehicles (UGVs) and a UAV and determines the best way to move a load between the UGVs. Finally, to perform the transport task, an electromagnetic system is attached to the suspended cable connected to the UAV.  
To address the topics involved, this paper is divided into several sections, starting with Section 2, in which some possible real-life applications involving cooperation between aerial and ground-based multirobots are given to exemplify the importance of this work. Section 3 explains how the delivery strategy was developed based on the positioning steps and the robots. Section 5 presents some experiments performed and discusses their results, and also provides additional information about the cargo-actuator system. Finally, Section 6 highlights our concluding remarks.