# Strategies and Methods for the Fault-Tolerant Function Development of Multi-Domain Systems +
## 4 Emergence of Faults  
One may ask why faults still emerge in current products which are the result of state-of-the-art design processes with elaborate performance and quality management. An example of such faults might be the catalytic converter of a motor-bike. Insufficient petrol quality may cause wear of this component. The lambda-sensor placed after the catalytic converter would detect this wear and an adaptation of the motor control might accommodate this fault. Nonetheless, a fault can emerge in this scenario as a result of an unexpected environmental condition. This section seeks to explore such causes of faults. Up to now, no concise scientific analysis concerning the typical causes of faults in system development and operation has been performed and reported in the literature. Some insights can be gathered from the literature concerned with robust design (e.g., ref. [60]), with condition monitoring [61], with mechanical systems failure [62], and from semi-scientific publications [63]. Analysis of this kind of literature, the industrial experience of the authors, as well as consideration of numerous engineering bachelor's and master's degree theses led to the insight that many problems arising in product development today do not occur because of direct faults of single components but because of complex or unexpected situations and system constellations or because of the intransparent effects of vibrations or because of intransparent multi-domain effect chains and other unknown interdependencies. In current industrial product development, powerful processes, methods, and tools exist to detect and accommodate direct faults of sensors and actuators. However, in certain areas, faults occur leading to different consequences up to catastrophic failure. Figure 5 summarises complex or unexpected situations and system constellations, intransparent effects of vibrations, and intransparent multi-domain effect chains which may lead to the emergence of faults. \begin{table}
\begin{tabular}{c c} \hline \hline
**Approach** & **Essential Characteristics** \\ \hline \multicolumn{3}{c}{Global approaches for multi-domain systems development} \\ \hline Model-Based System Engineering (MBSE—[49; 50; 51]) & Product and process models in the centre of the planning and execution of the system development process \\ \hline Graph-Based Design Languages (GBDLs—[52]) & GBDLs allow unlimited accessibility of model contents, interconnection of model components, and model interoperability. \\ \hline \multicolumn{3}{c}{Global approaches for fault accommodation} \\ \hline Fault-Tolerant control (FTC—[59]) & Possibilities to accommodate faults by means of a robust control design or active fault-tolerant control \\ \hline Fault-Tolerant Design (FTD—[3]) & FTD summarises fault accommodation possibilities by means of enabling or easing FTC or by inherently fault-tolerant design characteristics \\ \hline \multicolumn{3}{c}{Approaches for functional development} \\ \hline Integrated Function Modelling (IFM—[5]) & A framework that allows modelling of the functions of technical systems with transparent interconnections between states, actors, operands, and processes \\ \hline Function Failure Identification and Propagation (FFIP) method [55] & FFIP allows identifying what functions are likely to fail or degrade and the overall effect of this failure or degradation \\ \hline Companion Flow State Logic (FSL) method [57] & FSL focusses on non-nominal states, enables reasoning on the state of energy, material, and signals, as well as assessment of failure propagation over potential flows \\ \hline Uncoupled Failure Flow State Reasoned (UFFSR) [58] & UFFSR focusses on non-nominal states, enables reasoning on the state of energy, material, and signals, as well as assessment of failure propagation over potential flows (UFFSR is an improvement of FSL) \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Strategic and methodical approaches.