# Improved Safety Analysis Integration in a Systems Engineering Approach
## 3 Improved SafeSysE Methodology  
In this section, an overview of the project is given. It aims at integrating safety in the systems engineering approach in order to ensure consistency between models and to improve the system architecture and behavior. As shown in Figure 1, the process is composed of four main steps.  
The first step is the automatic generation of a Preliminary Failure Mode and Effects Analysis (PFMEA) from SysML diagrams by using a Safety Profile [16]. Then, a safety expert completes the PFMEA with different information in order to obtain an FMEA with Severity (S), Occurrence (O), and Detectability (D) parameters. For more detail about the FMEA generation please refer to Section 4.1. In the second step, the generated FMEA is performed and used to make some architecture modifications in order to improve its reliability. In this work, redundancy is used as a solution to reduce the risk of critical components (Section 4.2). When the system architecture is modified, it is necessary to specify its behavior. Therefore, a redundancy profile is created in order to generate a DFT from SysML diagrams (Section 5.1). In the fourth step, the generated DFT is analyzed and the minimal cut sequences are used to create a state machine diagram to describe the system behavior (Section 5.2).  
In the Improved SafeSysE, the generation of FMEA from SysML diagrams is the same as in SafeSysE [16]. Then, by using the Risk Priority Number (RPN) of FMEA, some architecture modifications are made as shown in [8]. The improvements in Improved SafeSysE is that we use the criticality instead of RPN because RPN can not always identify the true critical components. Furthermore, Improved SafeSysE extends [15] and describes the generation of a DFT from SysML diagrams, which considers redundancy between components. Once the DFT is generated, Improved SafeSysE allows the generation of a STate Machine (STM) using information from qualitative analysis of DFT.  
The overall methodology is described by an activity diagram which represents the detailed process as shown in Figure 2. This process is divided into three parts: the systems engineering part, the safety assessment part, and the decision making part. The starting point of the process is a set of initial requirements which describes the needs of the system. Each activity has a data store which allows the storage of different information.  
For the SE part, different activities like defining requirements, system architecture, system behavior, and functions are modeled by SysML diagrams. The first activity is the definition of the systems requirements which describes the functionality of the system. To identify these requirements, some SysML diagrams can be used such as the requirement diagrams, the block definition diagrams, and the use case diagrams. The second activity is the definition of the system functions. These functions are suggested by using the system requirements identified in the previous step. The system functions are modeled, in SysML, by a set of activity diagrams. Each activity diagram describes the decomposition of a given function into sub-functions and it represents the transformation of input flows into output flows. Once the system functions are defined, the system architecture is built by assigning each component to its corresponding functions. In SysML, the system architecture is modeled by a BDD and an IBD, which describe the components of the system and the interactions between each other. Finally, the system behavior in SysML is modeled by a set of state machines which aims at describing the behavior of all components or sub-systems of the system.  
The decision making part and the SA part of the detailed process will be discussed in the next section.  
Figure 1: Improved SafeSysE: flowchart of the general methodology.