When designing a safe PLC, multiple factors must be taken into account, and specific design strategies are essential. A safety PLC focuses on internal diagnostics, combining both hardware and software to continuously monitor the device's operational status and detect any anomalies in real time. The software used in a safety PLC is developed using specialized techniques to ensure high reliability. Additionally, a safety PLC incorporates redundancy features, allowing the system to continue functioning even if part of it fails. It also includes an extra layer of safety mechanisms that prevent unauthorized access or modification of internal data through digital communication interfaces.
Unlike standard PLCs, a safety PLC must undergo certification by third-party organizations to meet strict international safety and reliability standards. This involves a systematic approach throughout the entire design and testing process. Experts from TUV in Germany and FM in the U.S. provide independent validation and verification of the design and testing procedures.
Safety PLCs use specialized electronic circuits, detailed diagnostic software, and comprehensive failure analysis to identify over 99% of potential internal component hazards. The Failure Mode, Effects, and Diagnostic Analysis (FMEDA) method guides the design process, helping to understand how each component might lead to system failure and how such failures can be detected. TUV engineers conduct failure testing as part of their certification process to ensure reliability.
The software in a safety PLC adheres to strict international standards, which require special techniques to avoid complexity. Detailed analysis and testing examine all interactions within the operating system, including real-time tasks like multitasking and interrupts. Additional diagnostic features, such as "program flow control" and "data validation," are implemented. Program flow checks ensure functions are executed in the correct sequence, while data validation ensures critical data is stored redundantly and validated before use.
During software development, safety PLCs undergo rigorous testing methods. One key test is "software failure injection," where the program is intentionally corrupted to verify that the PLC responds safely. All software is thoroughly documented so that third-party inspectors can understand its operation. Most software development does not follow this level of detail, which is why many programs contain undetected bugs. By following these stringent practices, safety PLCs achieve a higher level of dependability and security in critical applications.
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