DEPENDABILITY ANALYSIS OF HYBRID REDUNDANCY SYSTEMS USING FUZZY APPROACH
With the increasing demand for high reliability in mission critical systems such as space shuttle, digital flight and real time control to mention a few, reliability analysis of fault tolerant systems continues to be the focus of most researchers. The reliability analysis of triple modular redundancy (TMR) and hybrid redundancy (TMR with spares) systems is in general carried out with the assumption of failure rate being precise. However, in practice failure rate is imprecise due to the uncertainties of system operation. In this paper, the dependability analysis of hybrid redundancy systems (HRS) comprising of N modular redundancy (NMR) and standby redundancy is presented assuming failure rates and repair rates as fuzzy numbers. Each module of the NMR is assumed to have access to a number of cold spares and a repair facility. A Markov model for the HRS is developed. As the Markov model parameters may not be precisely known due to various reasons, vertex method and a cut method is applied. These methods allow uncertainty based parameters that are represented as fuzzy numbers. The dependability measures such as availability and reliability are obtained. A comparative study of the fuzzy results and the conventional results using probability concepts is presented.