## 3 The Cooperative Cargo Transportation Strategy  
Among some applications listed, this manuscript focuses its efforts on implementing a control strategy for transport with a heterogeneous formation during lifting, transposing, and delivering loads. To stress the multirobot collaboration, we assume the environment has a physical barrier that makes it impossible for ground robots to cross from one side to another, for instance, a UGV trying to reach the opposite side of a broken bridge. Figure 4a illustrates the ground robots P1 and P2, the aerial vehicle B1, the cargo C1, and also a trailer T1 used to transport B1. Because there is an obstacle positioned between the UGVs, we assume that it will be necessary to use three robots, two UGVs (P1 and P2, respectively) and one UAV (B1) for practical validation of a real cargo transport strategy. Once the robot P1 cannot overcome the imposed barrier, [PERSON] reaches the other side of the barrier at a predefined coordinate, and then receives the load, which is lifted, transported, and delivered by a UAV. Our delivery strategy is composed by four steps, as follows. 1. [leftmargin=*]
2. Cargo (C1) needs to be taken from city A to city B, but we previously know of the existence of a recently collapsed bridge between the two cities. A single vehicle (P1) cannot reach city B, and requires a second vehicle (P2) on the opposite side to accomplish the delivery task. 3. Once P1 arrives at the bridge, [PERSON] is called to pick up the load on the other side of the bridge. 4. After [PERSON] arrives to serve [PERSON], the load must be transferred between them by using B1. With P1 and P2 waiting at rest, [PERSON] begins the aerial transposition above the fallen bridge by picking up the load from P1 and placing it over [PERSON]. Once this is done, [PERSON] goes back to meet [PERSON] and lands on it. 5. After overcoming the obstacle, the cargo over [PERSON] goes to the desired destination, whereas [PERSON] and [PERSON] returns to city A to receive a new delivery order. Figure 4 presents in detail the events of the delivery task described above, and how it will be implemented in practice. Initially, Figure 4a shows the agents that compose the entire formation and how they are arranged in space. Figure 3: Example of delivery operation. Figure 4: Sequence of events of formation during the experiment. (**a**) Agents of heterogeneous formation. (**b**) [PERSON] starts moving towards the desired point. (**c**) [PERSON] starts its movement to give assistance to [PERSON]. (**d**) [PERSON] takes off to pick up [PERSON]. (**e**) [PERSON] passes with [PERSON] under the obstacle. (**f**) [PERSON] delivers [PERSON] on P2 and returns to T1 to land. (**g**) All formation returns to their respective destination. (**h**) Task is completed successfully. Figure 4b, [PERSON] goes to the desired target close to the obstacle. At the same time, [PERSON] is carried on the trailer T1; note also that [PERSON] is carried on P1. Figure 4c shows [PERSON] starting this movement; it goes to the opposite side of the obstacle to receive [PERSON]. Then, [PERSON] takes off to position itself over P1. After that, [PERSON] captures [PERSON] by using the electromagnet E1 (see Figure 4d). Now, [PERSON] transports C1 from P1 to P2, as depicted in Figure 4e. Being over P2, [PERSON] decouples C1 and returns to land on T1 after stabilizing the pendulum motion of E1 (see Figure 4f). In the sequel, P1 and P2 go to their respective pickup/delivery points (Figure 4g). Finally, the transportation task is successfully completed as shown in Figure 4h. It is important to stress that P1 cannot move freely during the positioning task, because of T1 attached to it. In other words, there are some maneuvers that are impossible to do, such as sharp bends, where T1 can collide with P1. To overcome this movement constraint, we define a set of way-point that results in a smooth and approaching maneuver.