## 2 State of Art
### Collaborative Robots  
Collaborative robots are robots able to work side by side with human operators [9]. According to the ISO 10218-2 [10] a cobot is a \"robot designed for direct interaction with a human within a defined collaborative workspace\". Cobot applications have proved that is important to define the boundary of the collaborative workspace [11] which is the appointed space for Human-Robot Interaction (HRI).  
About safety, industrial robots and cobots are quite different. Cobots use several and concurrent solutions to ensure a safe interaction with operators and the surroundings. They are built with lightweight materials and designed with rounded shapes. The cobots are equipped with a set of sensors to detect the surrounding space and the human presence. Finally, to reduce the potential damage to humans, they have torque and force sensors to detect unwanted contacts. Therefore, the collaborative robots are largely used where the collaboration with humans can increase the performance of the production or overcome the technical limitations without physical fences [12].  
To classify the available commercial cobots and to identify the differences among them, three cobot market maps was developed by sampling a representative group of cobots based on the following criteria of interest for the systems integration: (i) the payload, (ii) the reach, (iii) the footprint and (iv) the weight. The weight and the footprint are selected as the main binding criteria. Indeed, the cobot should be mounted on a mobile robot that must be strong and large enough to support it. The footprint defines the minimum dimension that the mobile robot, and consequently the interface, should have. In the second stage, the payload and the reach have to be considered to evaluate the possible working conditions where the cobot should be adopted. The maximum payload combined with the weight of the cobot provides the minimum necessary capacity of both the mobile robot and interface. Moreover, the maximum reach of the cobot involves two important aspects: (i) safety and (ii) feasibility. Indeed, the reach determines which part of the human body can be achieved by the cobot and the effect on the stability of the mobile manipulator.  
All the examined cobots are organized by three maps where the weight is compared respectively versus the payload (Figure 1), the reach (Figure 2) and the footprint (Figure 3). Each selected brand is represented using a defined color according to the figures' legend.  
Figure 1: Collaborative Robots Market: weight vs. payload.  
The results highlight that cobots are characterized by: (i) a weight between 11 and 61 kg; (ii) a diameter of footprint between 120 and 310 mm; (iii) a reach between 500 and 1800 mm; (iv) a payload between 2 and 18 kg. The worst case, characterized by the sum of the weight and the maximum payload, is 71 kg. Furthermore, the robot with the maximum footprint has a diameter of 310 mm, a reasonable value that falls into the range of most mobile robots. Indeed, that is an important limitation for the dimension of the mobile robot and the interface, and it needs to be considered. Figures 1-3 show that 80% of cobots have a weight lower than 40 kg, and only 25% of cobots have a footprint higher than 230 mm. Therefore, a representative group of the most common cobots can be characterized by the following features:  
1. weight up to 40 kg,
2. payload between 2 and 18 kg,
3. reach from 500 to 1800 mm and
4. footprint up to 230 mm.  
Figure 3: Collaborative Robots Market: weight vs. footprint.  
Figure 2: Collaborative Robots Market: weight vs. reach.