Coverage influence on the dependability of PLC architectures

The paper illustrates a way of using analytical methods in order to analyze the dependability of several replicated PLC architectures characterized by imperfect coverage. Firstly, we establish the general modeling frame of the dependability and we state the assumptions taken into account for modeling the reliability and safety of PLC systems. Then, the considered architectures are described by the means of block diagrams, functioning mode and associated Markov models. For this purpose three fault coverage factors are considered. The comparative analysis of PLCs architectures dependability is done considering systems safety, reliability and MTTHE, and takes into account several scenarios for the three fault coverage factors. The analysis shows that higher coverage bring consistent improvements in the safety and MTTHE of replicated architectures and constitutes a valuable tool for designers, due to the fact that it shows where and how to interfere in order to improve the overall system dependability.