Structure optimization of multi-state system with time redundancy

In this article, a multi-state system with time redundancy where each system element has its own operation time is considered. In addition, the system total task must be performed during the restricted time. The reliability optimization problem is treated as finding the minimal cost system structure subject to the reliability constraint. A method for reliability optimization for systems with time redundancy is proposed. This method is based on the universal generating function technique for the reliability index computation and on genetic algorithm for the optimization. It provides a solution for the optimization problem for the complex series–parallel system and for the system with bridge topology. Two types of systems will illustrate the approach: systems with ordinary hot reserve and systems with work sharing between elements connected in parallel. Numerical examples are also given.     

Fault tolerance versus performance metrics for robot systems

The incorporation of fault tolerance techniques into robot systems improves the reliability, but also increases the hardware and computational requirements in the overall system. It is not always clear how to evaluate the merit, or ‘effectiveness’ of different fault tolerance approaches for a given application. In this paper, we present a new set of performance criteria designed to measure and compare the effectiveness of robot fault tolerance strategies. The measures, which are designed to evaluate fault tolerance/performance/cost tradeoffs, can also be used to evaluate pure performance or pure fault tolerance strategies. We show their usefulness using a variety of proposed fault tolerance approaches in the literature, focusing on multiprocessor control architectures.     

Analysis of fault tolerance and reliability in distributed real-time system architectures

Safety critical real-time systems are becoming ubiquitous in many areas of our everyday life. Failures of such systems potentially have catastrophic consequences on different scales, in the worst case even the loss of human life. Therefore, safety critical systems have to meet maximum fault tolerance and reliability requirements. As the design of such systems is far from being trivial, this article focuses on concepts to specifically support the early architectural design. In detail, a simulation based approach for the analysis of fault tolerance and reliability in distributed real-time system architectures is presented. With this approach, safety related features can be evaluated in the early development stages and thus prevent costly redesigns in later ones.     

Optimal number of redundant channels for a digital communication system

In many critical applications of digital systems, fault tolerance has been an essential architectural attribute for achieving high reliability. This paper considers the problem of determining the optimal number of redundant channels in a digital communication system. The expected system cost is obtained. We determine the number of redundant channels minimizing the expected system cost. A numerical example illustrates the techniques.     

Implementations of a four-level mechanical architecture for fault-tolerant robots

This paper describes a fault tolerant mechanical architecture with four levels devised and implemented in concert with NASA (Tesar, D. & Sreevijayan, D., Four-level fault tolerance in manipulator design for space operations. In First Int. Symp. Measurement and Control in Robotics (ISMCR '90), Houston, Texas, 20–22 June 1990.) Subsequent work has clarified and revised the architecture. The four levels proceed from fault tolerance at the actuator level, to fault tolerance via in-parallel chains, to fault tolerance using serial kinematic redundancy, and finally to the fault tolerance multiple arm systems provide. This is a subsumptive architecture because each successive layer can incorporate the fault tolerance provided by all layers beneath. For instance a serially-redundant robot can incorporate dual fault-tolerant actuators. Redundant systems provide the fault tolerance, but the guiding principle of this architecture is that functional redundancies actively increase the performance of the system. Redundancies do not simply remain dormant until needed. This paper includes specific examples of hardware and/or software implementation at all four levels.     

A high reliability two‐level fault tolerant mesh design and applications

Abstract

This paper presents a novel technique for the enhancement of the operational reliability of processor arrays by a multi‐level fault‐tolerant design approach. The proposed fault tolerant architecture uses a flexible reconfiguration of redundant nodes, thereby offering a better spare utilization than existing two‐level redundancy schemes. The spare nodes at each level can replace any of the failed primary nodes, not only at the same level but also those at the lower levels. The architecture can be adopted to increase the system reliability in Multi Chip Modules (MCMs). The main contributions of our work are the higher degree of fault tolerance, higher overall reliability, higher flexibility, and a better spare utilization.     

Markov chains and probabilistic computation-a general framework for multiplexed nanoelectronic systems

In emerging nanotechnologies, reliable computation will have to be carried out with unreliable components being integral parts of computing systems. One promising scheme for designing these systems is von Neumann's multiplexing technique. Using bifurcation theory and its associated geometrical representation, we have studied a NAND-multiplexing system recently proposed. The behavior of the system is characterized by the stationary distribution of a Markov chain, which is uni- or bi-modal, when the error probability of NAND gates is larger or smaller than the threshold value, respectively. The two modes and the median of the stationary distribution are the keys to the characterization of the system reliability. Examples of potential future nanochips are used to illustrate how the NAND-multiplexing technique can lead to high system reliability in spite of large gate error probability while keeping the cost of redundancy moderate. In nanoelectronic systems, while permanent defects can be taken care of by reconfiguration, probabilistic computation schemes can incorporate another level of redundancy so that high tolerance of transient errors may be achieved. The Markov chain model is shown to be a powerful tool for the analysis of multiplexed nanoelectronic systems.     

A method for evaluating fault coverage using simulated fault injection for digitalized systems in nuclear power plants

The fault coverage for digital system in nuclear power plants is evaluated using a simulated fault injection method. Digital systems have numerous advantages, such as hardware elements share and hardware replication of the needed number of independent channels. However, the application of digital systems to safety-critical systems in nuclear power plants has been limited due to reliability concerns. In the reliability issues, fault coverage is one of the most important factors. In this study, we propose an evaluation method of the fault coverage for safety-critical digital systems in nuclear power plants. The system under assessment is a local coincidence logic processor for a digital plant protection system at Ulchin nuclear power plant units 5 and 6. The assessed system is simplified and then a simulated fault injection method is applied to evaluate the fault coverage of two fault detection mechanisms. From the simulated fault injection experiment, the fault detection coverage of the watchdog timer is 44.2% and that of the read only memory (ROM) checksum is 50.5%. Our experiments show that the fault coverage of a safety-critical digital system is effectively quantified using the simulated fault injection method.     

Optimal reliability and maintainability allocation in manufacturing systems

In this article an attempt has been made to solve an important task in the reliability management of manufacturing systems: the definition of reliability and maintainability specifications of parts, the general objective being to increase productivity while maintaining costs low. An analytical approach has been considered to evaluate an average index of production capability in series-parallel systems. A heuristic optimization procedure has been developed to solve the following problems: (a) determine the optimal reliability and maintainability allocation of parts to achieve the maximum production index at a given cost; and (b) determine the optimal reliability and maintainability allocation of parts which minimizes the total cost of investments, subject to the constraint of meeting a system production requirement. Numerical examples prove the effectiveness of the proposed procedure and show that the application of optimization analyses can provide large gains in terms of either productivity or total cost.     

Strategy and tool development for nuclear power plant design synthesis with measurement of reliability and simplicity

In this work, the conceptual design supporting tools for nuclear power plants have been developed. These tools are made for system synthesis, complexity measure and reliability analysis.This design synthesis program combined with the reliability analysis program accomplishes the system synthesis. This design strategy can reduce mistakes, effort and time. This design tool, based on Prolog language, is applied to the auxiliary feedwater system. A logic based fault tree analysis program (LOFT) is also developed using Prolog language. As LOFT performs symbolic computation during the fault tree analysis, linking with knowledge-base systems is very easy and the partial usage of the program is possible. The importance measure of components obtained from the system reliability analysis and the complexity measure of the system give very important information to the system designer.     

Design optimization of a safety-instrumented system based on RAMS+C addressing IEC 61508 requirements and diverse redundancy

This paper presents the design optimization by a multi-objective genetic algorithm of a safety-instrumented system based on RAMS+C measures. This includes optimization of safety and reliability measures plus lifecycle cost. Diverse redundancy is implemented as an option for redundancy allocation, and special attention is paid to its effect on common cause failure and the overall system objectives. The requirements for safety integrity established by the standard IEC 61508 are addressed, as well as the modelling detail required for this purpose. The problem is about reliability and redundancy allocation with diversity for a series-parallel system. The objectives to optimize are the average probability of failure on demand, which represents the system safety integrity, Spurious Trip Rate and Lifecycle Cost. The overall method is illustrated with a practical example from the chemical industry: a safety function against high pressure and temperature for a chemical reactor. In order to implement diversity, each subsystem is given the option of three different technologies, each technology with different reliability and diagnostic coverage characteristics. Finally, the optimization with diversity is compared against optimization without diversity.     

Reliability assessment of reliquefaction systems on LNG carriers

The paper gives an introduction to reliability assessment of reliquefaction systems for boil-off gas (BOG) on LNG carriers with focus on redundancy optimization and maintenance strategies. The reliability modeling is based on a time-dependent Markov approach. Four different system options are studied, with varying degree of redundancy. Failures in the reliquefaction system may require flaring of the BOG, and the associated cost is compared with the cost of introducing redundancy and the cost of onboard maintenance. A model for maintenance optimization is developed and exemplified on a main unit of the reliquefaction system. Reliability and maintenance cost data for reliquefaction systems on LNG ships are very scarce. The input data have been collected from the best available data sources and adjusted by expert judgement. A tailor-made computer program has been developed and may be supplied to interested readers.     

Tolerance optimization of a mobile phone camera lens system

In the manufacturing process for the lens system of a mobile phone camera, various types of assembly and manufacturing tolerances, such as tilt and decenter, should be appropriately allocated. Because these tolerances affect manufacturing cost and the expected optical performance, it is necessary to choose a systematic design methodology for determining optimal tolerances. In order to determine the tolerances that minimize production cost while satisfying the reliability constraints on important optical performance indices, we propose a tolerance design procedure for a lens system. A tolerance analysis is carried out using Latin hypercube sampling for evaluating the expected optical performance. The tolerance optimization is carried out using a function-based sequential approximate optimization technique that can reduce the computational burden and smooth numerical noise occurring in the optimization process. Using the proposed design approach, the optimal production cost was decreased by 28.3% compared to the initial cost while satisfying all the constraints on the expected optical performance. We believe that the tolerance analysis and design procedure presented in this study can be applied to the tolerance optimization of other systems.     

Soft computing approach for reliability optimization: State-of-the-art survey

In the broadest sense, reliability is a measure of performance of systems. As systems have grown more complex, the consequences of their unreliable behavior have become severe in terms of cost, effort, lives, etc., and the interest in assessing system reliability and the need for improving the reliability of products and systems have become very important. Most solution methods for reliability optimization assume that systems have redundancy components in series and/or parallel systems and alternative designs are available. Reliability optimization problems concentrate on optimal allocation of redundancy components and optimal selection of alternative designs to meet system requirement. In the past two decades, numerous reliability optimization techniques have been proposed. Generally, these techniques can be classified as linear programming, dynamic programming, integer programming, geometric programming, heuristic method, Lagrangean multiplier method and so on. A Genetic Algorithm (GA), as a soft computing approach, is a powerful tool for solving various reliability optimization problems. In this paper, we briefly survey GA-based approach for various reliability optimization problems, such as reliability optimization of redundant system, reliability optimization with alternative design, reliability optimization with time-dependent reliability, reliability optimization with interval coefficients, bicriteria reliability optimization, and reliability optimization with fuzzy goals. We also introduce the hybrid approaches for combining GA with fuzzy logic, neural network and other conventional search techniques. Finally, we have some experiments with an example of various reliability optimization problems using hybrid GA approach.     

Optimization of nanoelectronic systems' reliability under massive defect density using cascaded R-fold modular redundancy

The theoretical analysis of R-fold modular redundancy, cascaded R-fold modular redundancy and NAND multiplexing is presented and these fault-tolerant techniques are compared in terms of resistance to massive levels of defect density. Optimal cluster size analysis and redundancy optimization of the cascaded R-fold modular redundancy technique has been performed for the first time in the context of a large-scale system. The optimal window of application of each fault-tolerant technique with respect to defect density is presented as a way to find the optimum design trade-off between the reliability and power/area. Building viable systems consisting of components with high defect densities in future nanoscale technologies will have a high cost in power/area, regardless of the fault-tolerant techniques used.     

Design fault tolerance

Typical software fault tolerance techniques are modeled on successful hardware fault tolerance techniques. The software fault tolerance techniques rely on design redundancy to tolerate residual design faults in the software; the hardware fault tolerance techniques rely on component redundancy to tolerate physical degradation in the hardware. Investigations of design redundant software have revealed difficulties in adapting the hardware strategy to software.We survey three categories of issues: (1) practical issues in the implementation of design-redundant software, (2) economic considerations for the development and maintenance of multiple software implementations, and (3) assessment difficulties in measuring and predicting the performance of design-redundant software. All of these issues should be considered by would- be developers of design-redundant software to justify use of the technique.     

Reliability measurement of mass storage system for onboard instrumentation

Advances in spaceborne vehicular technology have made possible the long-life duration of the mission in harsh cosmic environments. Reliability and data integrity are the commonly emphasized requirements of spaceborne solid-state mass storage systems, because faults due to the harsh cosmic environments, such as extreme radiation, can be experienced throughout the mission. Acceptable dependability for these instruments has been achieved by using redundancy and repair. Reconfiguration (repair) of memory arrays using spare memory lines is the most common technique for reliability enhancement of memories with faults. Faulty cells in memory arrays are known to show spatial locality. This physical phenomenon is referred to as fault clustering . This paper initially investigates a quadrat-based fault model for memory arrays under clustered faults to establish a reliable foundation of measurement. Then, lifelong dependability of a fault-tolerant spaceborne memory system with hierarchical active redundancy, which consists of spare columns in each memory module and redundant memory modules, is measured in terms of the reliability (i.e., the conditional probability that the system performs correctly throughout the mission) and mean-time-to-failure (i.e., the expected time that a system will operate before it fails). Finally, minimal column redundancy search technique for the fault-tolerant memory system is proposed and verified through a series of parametric simulations. Thereby, design and fabrication of cost-effective and highly reliable fault-tolerant onboard mass storage system can be realized for dependable instrumentation.     

A unified methodology for reliability assessment of systems with active redundancy

The reliability of engineering systems is usually improved by the inclusion of redundant components in the design. Often, the redundant components must all contribute actively to the operation of the system. Examples include structures, water and power distribution systems, and communication networks. For these systems, the failure of each successive component defines a different topological configuration for the system. A reliable system should perform adequately in as many of these configurations as possible. Consequently, the reliability of a system with active redundancy depends on two factors: the probability, considering component failures, that a functional system topology is maintained; and the probability of adequate system performance in each functional configuration. To date, no single reliability measure exists which combines both of these factors, but such a measure would be useful for comparison of alternative redundant designs. Current methods for reliability assessment have been tailored to the purposes of individual engineering disciplines and reflect the inherent physical properties of specific types of systems. However, an increasing need for reliability analysis of large, complex, multidisciplinary systems requires a more general and unified approach. In this paper, we propose a unified, model-based methodology for reliability-based design which provides a single, second moment reliability index for systems with active redundancy. The reliability index of a redundant pipe network is calculated as an illustrative example.     

A heuristic for solving the redundancy allocation problem for multi-state series-parallel systems

The redundancy allocation problem is formulated with the objective of minimizing design cost, when the system exhibits a multi-state reliability behavior, given system-level performance constraints. When the multi-state nature of the system is considered, traditional solution methodologies are no longer valid. This study considers a multi-state series-parallel system (MSPS) with capacitated binary components that can provide different multi-state system performance levels. The different demand levels, which must be supplied during the system-operating period, result in the multi-state nature of the system. The new solution methodology offers several distinct benefits compared to traditional formulations of the MSPS redundancy allocation problem. For some systems, recognizing that different component versions yield different system performance is critical so that the overall system reliability estimation and associated design models the true system reliability behavior more realistically. The MSPS design problem, solved in this study, has been previously analyzed using genetic algorithms (GAs) and the universal generating function. The specific problem being addressed is one where there are multiple component choices, but once a component selection is made, only the same component type can be used to provide redundancy. This is the first time that the MSPS design problem has been addressed without using GAs. The heuristic offers more efficient and straightforward analyses. Solutions to three different problem types are obtained illustrating the simplicity and ease of application of the heuristic without compromising the intended optimization needs.     

Defect and Transient Fault-Tolerant System Design for Hybrid CMOS/Nanodevice Digital Memories

Targeting on the future fault-prone hybrid CMOS/nanodevice digital memories, this paper presents two fault-tolerance design approaches that integrally address the tolerance for defects and transient faults. These two approaches share several key features, including the use of a group of Bose-Chaudhuri-Hocquenghem (BCH) codes for both defect tolerance and transient fault tolerance, and integration of BCH code selection and dynamic logical-to-physical address mapping. The first approach is straightforward and easy to implement but suffers from a rapid drop of achievable storage capacity as defect densities and/or transient fault rates increase, while the second approach can achieve much higher storage capacity under high defect densities and/or transient fault rates at the cost of higher implementation complexity and longer memory access latency. Based on extensive computer simulations and BCH decoder circuit design, we have demonstrated the effectiveness of the presented approaches under a wide range of defect densities and transient fault rates, while taking into account of the fault-tolerance storage overhead and BCH decoder implementation cost in CMOS domain