Reliability and Availability Basics
E. Topuz
Dogu~ University, Electronics and Communications Eng. Dept. Zeamet Sokak 21, Acibadern - Kadikoy, 34722 Istanbul, Turkey E-mail: etopuz@.tr
A3 = 0.9979 A4 = 0.9993
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BB4 6 months 3 hours
IEEE Antennas and Propagation Magazine, Vol. 51, No.5, October 2009
parameters . On the other hand, if a subsystem containing two identical modules is considered fully operational when either one of its modules is operational, then one of them is clearly redundant. Such subsystems may be modeled as a parallel connection of two (or more) BBs. A topological model for reliability analysis of David's smart home is depicted in Figure I . The basic system consists of a cascade of BB2, BB3, BB4, and the ten sensors, S\ through SIO' which make up BB I . Redundant BBs, which may be added to the basic system in order to achieve the required overall availability performance, are indicated with a bar on top, and connected in parallel with their twins. The objective is the calculation of the availability performance of the entire system, based on the life distributions of the BBs (and their redundant twins) from which it is composed.
Table 1. The availability performance of the four building blocks. MTBF MTTR Cost Unit Availability Al = 0.986 1.0 BB1 0.1 years 12 hours Az = 0.9986 2.0 1 day BB2 2 years BB3 4 years 3 days 1.5 0.5
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The first step in reliability analysis is to break up the system at hand into simpler modules and subsystems. The appropriate fragmentation level is determined either by the availability of vendor-provided reliability data at this level, or by the time or cost advantages of performing reliability tests on chosen subsystems, rather then on the entire system. In the present case, we are given that reliability calculations for David's smart house project involve four subsystems or "building blocks" (BB). The second step is to prepare a reliability model of the system. At this stage, we decide the parallel and serial connectivity of the system. We note that when a system consisting of a number of building blocks is considered as not operational in case of the failure of anyone BB, then it may be modeled as a series chain of BBs, each characterized by its own reliability performance
Hale Waihona Puke 3.ABB2 houses the man-machine interface and computational intelligence. The RF subsystem, including transponder and antenna, make up BB3, and BB4 is responsible for providing power to all other BBs. The smart house system will be considered to be unavailable during periods of time wherein anyone of the building blocks does not function as designed. The method to be used for decreasing downtime is to use redundancy. In case a replica of any BB needs to be purchased for this purpose, this redundant BB will be integrated into the system as a hot-standby unit.
1. Introduction
n item or system is specified, designed, and procured to meet a functional requirement, and it is important that it satisfies this requirement. It is also desirable that the item or system should be predictably available. This depends upon its reliability and availability. Weare tempted to take for granted that everything around us will function as designed on a continuous basis. Except for rare events to the contrary, this is precisely what we experience in our daily lives. Rare though it may be, the occasional malfunctioning of a device may cause us varying degrees of inconvenience, say, while brewing coffee, driving to a meeting, or needing to sniff some oxygen from a scuba tank in an underwater cave. We thus need to work out efficient ways of minimizing the risk of possible damage that may result from device or system failures in a given application. This can be achieved via reliability engineering, which provides a rigorous framework for determining appropriate system design, production, test and evaluation procedures, and operation and maintenance policies. In what follows, we will present a grossly simplified and condensed review of the basics of reliability, availability, and maintainability calculations. The reader is referred to the numerous excellent texts on reliability engineering for indepth analysis of the subject [1-3]. Before discussing the rules of the game, let us recall the quiz problem: David plans to convert his house into a "smart" house. He would like us to calculate the yearly downtime of the built-in intelligence, and the price he would have to pay for reducing this period. David provided the following inputs: 1. For reliability calculations, the smart house can be broken down into four subsystems or building blocks (BB). For each BB, we are given its relative cost and the reliability parameters mean time between failure (MTBF) and mean time to repair (MTTR) (see Table 1). BB1 consists of ten sensors with a dedicated communications infrastructure. All sensors and related communications in BB1 have identical failure and repair rates, and BB1 will be considered unavailable in the case of a malfunction of a single sensor.