Increasing efficiency of M-out-of-N redundancy

Abstract

In industrial automation, unexpected failures often generate high direct and indirect costs. To achieve high availability, the critical parts of the system are typically redundant. M-out-of-N (MooN) redundancy is a widespread redundancy pattern because it offers low failover times. This pattern has however two major disadvantages. First, the voter, an essential entity in MooN patterns, is itself a single point of failure. Second, it introduces delay to the control process and significant network load when deployed with multiple voters. This article proposes new MooN redundancy patterns for distributed control systems. These eliminate the voter as a single point of failure and reduce the network load and delay of the MooN redundancy. For each of these patterns we introduce an algorithm, which computes a near-minimum instance for the MooN problem. We validate our solution against minimum values and present a case study giving the delays introduced through our redundancy patterns and the induced network load.