## 2 State of Art
### Mobile Robots  
In manufacturing plants, mobile robots are widely adopted for logistic purposes. However, there is still confusion over the \"mobile robot\" term definition. It is mainly due to the existence of different kinds of mobile robots.  
The most common type of such mobile robot is the AGV, introduced in 1953 by Barret Electronics, an American company from Northbrook, Illinois [13]. An AGV is a simple and slow transporting system constrained to follow a wire on the floor instead of a rail. The following evolution of the technology, in particular the microprocessors, allowed to equip the AGVs with on-board computers able to store the programmed paths and initiate motion using directional systems to make the vehicle follow the desired path. Nevertheless, modern solutions of AGVs are laser-based capable of determining their position and direction from the reflection of the laser on specific reflective tapes on walls, poles, or fixed machines [14]. Laser emission allows detecting fixed or mobile obstacles to define different paths to reach the destination. In modern context, many factories usually adopt AGVs in the warehouses to carry materials over great distances autonomously providing efficient and flexible performances.  
On another hand, more recently, the AMRs [15] have been integrated into industrial applications. The main innovation compared to the AGVs is the capability to navigate autonomously from place to place and to execute specific tasks [6]. An AMR navigates using maps that can be loaded directly onto the robot, or that it builds autonomously on-site. If forklifts, pallets, people, or other obstacles are in its path, the AMR goes around safely, using the best alternative route.  
Similarly, as cobots, a mobile robots market map has been developed (see Figure 4) by sampling a representative group of mobile robots based on the following criteria of interest for the systems integration:  
1. payload: the mobile robot has to bear the load of the cobots and the workpiece;
2. battery life: for the autonomy of the mobile robot.
3. height: for the reachability of the human body part;  
The payload is the most important feature because the mobile robot has to carry not only the cobot but also the workpiece. Battery life could be a critical issue for the autonomy of the system and its operating performance. Height is the key feature for safety. Indeed, the human body parts reachable by the system are defined by the total height of the system obtained by combining the mobile robot and cobot heights [16]. The result of the market  
Figure 4: Mobile Robots Market: classification according to the payload, the height, and the battery charge (in hours) represented by the diameter size.  
analysis is presented in Figure 4. The color of the circle represents the brand while the diameter size represents the battery charge. Figure 4 shows four classes of mobile robots according to the payload: up to 300 kg, 500-600 kg, 1000 kg, and 1500 kg. Furthermore, the mobile robots within 300 and 400 mm height are characterized by the higher battery charge (bigger diameters). In conclusion, the 81% of the examined mobile robots can be characterized by the following features:  
1. Height up to 400 mm,
2. Payload between 90 and 1000 kg,
3. Battery charge between 8 and 15 h.