Cost Effectiveness of Coverage in Reliable Factory Automation Systems

Abstract

Fault-Tolerant (FT) architectures are often used in the context of Industrial Automation, especially in safety-critical systems. Coverage is a very important parameter; it measures the ability of these systems to detect and recover from expected failures. This paper investigates the effect of changes in the value of the Coverage parameter on the steady state availability of several commonly used FT architectures, namely Triple Modular Redundancy (TMR), Sift-Out and Reconfigurable Duplication. When comparing TMR and Sift-Out, it is found that, at relatively low values of coverage, adding redundancy will reduce availability and increase downtime; this is a counter intuitive result. For Reconfigurable Duplication, it is found that, at low Mean Time To Repair (MTTR), the effect of small changes in coverage has little effect on architecture availability. Hence, it is preferable to invest in the quality of the system's redundant modules instead of the quality of the error detection and recovery mechanisms. In contrast, at higher MTTRs, a small increase in the coverage affects availability; hence, investing in the quality of error detection will lower downtime and decrease profit loss.