# Advancing Digital Twin-Based Collision Avoidance:
## 4 Functional Safety for Wireless Systems
### Background
#### 4.1.1 Functional Safety  
Functional safety is defined as \"the part of the overall safety of the machine and the machine control system, which depends on the correct functioning of the safety control system and other risk-reducing measures\" by the IEC 62061 [17]. The IEC 62061 stands as an international standard developed by the International Electrotechnical Commission (IEC) to guide the design, implementation, and maintenance of safety-related control systems in machinery. Highlighting a holistic approach to the functional safety lifecycle, the standard mandates a thorough risk assessment to identify and evaluate potential hazards associated with machinery. This process facilitates the determination of necessary risk reduction measures. Introducing the concept of performance levels (PLs), expressed as PL \(a\) to \(e\), it quantifies the reliability and performance of safety functions. Aligned with IEC 61508, IEC 62061 employs Safety Integrity Levels (SILs) as a complementary measure. The standard mandates thorough documentation to demonstrate compliance and adherence to functional safety requirements. Given its international nature, IEC 62061 provides a globally recognized framework for addressing functional safety in machinery, making it an essential reference for academic inquiry into safety-related electrical, electronic, and programmable electronic control systems. Transitioning to the realm of wireless systems, the discussion extends to the emerging challenges and considerations in ensuring functional safety in wireless communication within industrial and critical applications. ISO 13849-1 [18] is a standard that provides guidelines for the design and integration of safety-related control systems in machinery. It introduces the concept of performance levels as a measure of the reliability and performance of safety functions. The performance level is a quantitative measure of the ability of a safety-related control system to perform its safety function under specified conditions. It uses the performance-level classification from PL a (low reliability) to PL e (high reliability). The selection of the appropriate performance level depends on the risk assessment for the specific machinery and its associated safety functions. To assess the risk, three criteria are used [18]. The first criteria is the degree of danger; S1: light reversible injury, S2: serious irreversible injury. The second criteria is the exposition to danger; F1: rare or short exposition to danger, F2: frequent or long exposition to danger. The third criteria is the possibility of danger avoidance; P1: possible under certain circumstances, P1: hardly possible. Table 3 shows which outage frequency is required to reach which PL and SIL. Often, it is wrongly assumed that a single value, such as the \(PFH_{D}\) (Probability of Dangerous Failure per Hour) of a system, is enough to define the performance level of a system. However, this is a misconception. First, performance level is defined for a safety function and not for a system. Second, we must consider the complete chain of systems involved that perform the safety function together. For example, the hardware or the software of the system might also occasionally fail. For a system with \(N\) safety-related \begin{table}
\begin{tabular}{c c c} \hline \hline PL & **Mean Probability [1/h]** & **SIL** \\ \hline a & \(\geq\)10\({}^{-5}\) to \(<\)10\({}^{-4}\) & - \\ b & \(\geq\)3 \(\times\) 10\({}^{-6}\) to \(<\)10\({}^{-5}\) & 1 \\ c & \(\geq\)10\({}^{-6}\) to \(<\)3 \(\times\) 10\({}^{-6}\) & 1 \\ d & \(\geq\)10\({}^{-7}\) to \(<\)10\({}^{-6}\) & 2 \\ e & \(\geq\)10\({}^{-8}\) to \(<\)10\({}^{-7}\) & 3 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Relationship between Performance Level (PL), mean probability, and Safety Integrity Level (SIL).