## 4 System-Level Design
### Requirement Definition  
According to the MBSE and RFLP approach, the design of the interface started from the requirements definition. Building a complete and consistent set of requirements is a crucial step in system designing. In fact, a bad specification of them, can lead to significant costs, schedule overruns, and insufficient quality of the system. Therefore, the requirements definition for the proposed mechatronic solution was performed using the Black-Box analysis.  
The system was considered a black box, leaving out its internal structure. It was analysed from an external point of view, adopting a top-down approach and leveraging SysML diagrams to define progressively a consistent set of requirements [36].  
The \"global mission\" fixes the main purpose of the system and must be defined as one or more requirements. Usually, it is expressed as a textual definition. According to the final goal of the present paper, the main requirement has been issued as follow:  
_\"The system must allow the integration between cobots and mobile robots belonging to the identified systems set, by improving safety and minimizing cost, height and energy consumption\"_.  
Figure 7: BBA and WBA linked to the RFLP approach.  
Several sub-requirements were derived from the main requirement and hierarchically organized. A SysML requirements diagram (Figure 8) was reported to highlight the relationship between the main requirement and its sub-requirements.  
The definition of the life cycle led to the identification of aspects related to all the life phases of the system: designing, prototyping, manufacturing, assembly, maintenance, and dismissing. A set of corresponding requirements was captured.  
Modeling the system context and the external entities, which interact with the system, was a fundamental step. The following external actors were identified:  
* **Power source:** it gives power to the system;
* **Environment:** dust and dirt always present in every environment;
* **Cobot:** it is physically connected to the system and it exerts its weight on it;
* **Mobile Robot:** the system is mounted on it and it provides for the handling of the system;
* **External device:** it is the device used to allow communication with the system. It exchanges data about cobot and mobile robot required motion with the system.
* **Human operator:** he operates in the same workspace of the system or its sub-systems.  
Figure 9 shows the system context and represents the external actors and how they interact with the mechatronic interface.  
Then, the system was analyzed focusing on its outward behavior. The system usage during the operating phase was detailed and described using the State Machine Diagram in Figure 10.  
The modeling of the external behavior of the system was deepened by identifying the actors the system interacts with for each service.  
The services provided by the system were identified focusing on each user operating mode. The following services were identified:  
* **Data acquisition and elaboration:** the external device sends data while the cobot receives data.
* **Motion:** the power source powers the system to allow the motion of the cobot;
* **Safety stop:** if collision occurs, the system stops and an alert is sent to a human operator.  
Figure 8: Initial requirements related to the “Global mission” of the interface.  
Requirements were derived from the information coming from each step. The list of the identified requirements and sub-requirements has been used as baseline for the next phases (i.e., Functional, Logical, and Physical phases). The complete set of requirements is listed in Table 1:  
Figure 10: State machine diagram representing user operating modes.  
Figure 9: System context and external entities.