Desired Reliability Research Articles

An (m,k)-Firm Real-Time Aware Fault-Tolerant Mechanism in Wireless Sensor Networks

Many real-time routing mechanisms have been proposed to support the newly developed wireless sensor networks (WSNs) applications such as the transmission and retrieval of multimedia traffic. However, the inherent source constraints of sensor network and instability of wireless communication set quite a problem for the existing routing mechanisms to meet the quality of service (QoS) requirements of some specific QoS-aware applications. Hence, real-time fault-tolerant schemes are highly desired for WSNs to address these challenges. In this paper, we propose an -firm-based real-time fault-tolerant mechanism, which helps routing mechanisms to achieve specific QoS requirement by employing a local status indicator (LSI) at each sensor node to monitor and evaluate the local conditions of node and network. Therefore, specific fault recovery mechanisms could be implemented for ensuring an acceptable QoS performance, according to the evaluated LSI values. By using this fault-tolerant scheme, each node dynamically adjusts its transmission capability to mitigate the performance degradation of real-time service caused by network faults and to maintain the desired reliability and timeliness. Simulation result shows that LSI cannot only help to reduce the effects of congestion, link failure, and void, but also reach higher successful transmission ratio and smaller transmission delay.

Latin hypercube sampling applied to reliability-based multidisciplinary design optimization of a launch vehicle

In this paper, Reliability-Based Multidisciplinary Design Optimization (RBMDO) of a two-stage solid propellant expendable launch vehicle (LV) is investigated. Propulsion, weight, aerodynamics (geometry) and trajectory (performance) disciplines are used in an appropriate combination. Throw weight minimization is chosen as objective function. Design variables for system level optimization are selected from propulsion, geometry and trajectory disciplines. Mission constraints contain the final velocity, the height above ground, and flight path angle. The constraints that appear during the flight are also considered. Assuming a normal distribution for the uncertain variables, Latin Hypercube Sampling (LHS) method selects the sample values for simulation runs which are eventually utilized for calculating probability density function of constraints and their reliability at each design point. Sequential Quadratic Programming (SQP) technique is used to achieve the optimal solution. Although the launch vehicle throw weight is increased negligibly in comparison with deterministic optimization, results show that the reliability-based method satisfied desired reliability of the constraints.

Hybrid systems for decentralized power generation in Oman

The main objective of this study is to determine the optimum size of systems able to fulfil the electrical energy requirements of remote sites located in Hajer Bani (HB) Hameed in the North of Oman, Masirah Island and the Mothorah area in the South of Oman. The methodology applied provides a useful and simple approach for sizing and analysing the hybrid systems using Hybrid Optimization Model for Electric Renewables (HOMER). The aim is to identify a configuration among a set of systems that meet the desired system reliability requirements with the lowest energy cost. The results of the analysis are a list of feasible power supply systems, classified according to their net present cost. Actual hourly load data are taken from these sites and are used in the model, and the meteorological readings are used either from the same site or from a nearby one. A comparison between the costs of electricity generated from renewable energy resources in these locations is done and the payback period for renewable energy components is calculated. Moreover, the effects of the temperature on the photovoltaic cells efficiency and energy cost are considered. The cost of energy was found to be 0.206, 0.361 and 0.327 $/kWh for Masirah Island, Mothorah and HB Hameed, respectively.

Disks from the Perspective of a File System

Most applications do not deal with disks directly, instead storing their data in files in a file system, which protects us from those scoundrel disks. After all, a key task of the file system is to ensure that the file system can always be recovered to a consistent state after an unplanned system crash (for example, a power failure). While a good file system will be able to beat the disks into submission, the required effort can be great and the reduced performance annoying. This article examines the shortcuts that disks take and the hoops that file systems must jump through to get the desired reliability.

Eco-design optimisation of an autonomous hybrid wind–photovoltaic system with battery storage

In this study, a new approach to design an autonomous hybrid wind–photovoltaic (PV)-batteries system is presented in order to assist the designers to take into consideration both the economic and ecological aspects. Primary embodied energy (EE) has been introduced as a new criterion for hybrid systems, designing with the objective to minimise loss of power supply probability (LPSP). For a location, meteorological and load data have been collected and assessed. Then, modelling and primary energy analysis have been achieved for all components of the hybrid system. Finally, an optimal configuration has been carried out using a dynamic model and applying two different algorithms with single and multi-objective optimisation. The methodology has been performed successfully for the sizing of a wind–PV-batteries system to supply at least 95% of the yearly total electric demand of a residential house. The simulation results show that the optimal configuration meets the desired system reliability requirements (LPSP <5%) with the lowest EE. A life cycle cost analysis has been established at the end of the study to demonstrate the importance of economic considerations.

Study on the possibility of increasing the maximum allowable stresses in fibre-reinforced plastics

This article presents the results of a study aimed at investigating the possibility of increasing the maximum allowable stresses in fibre-reinforced plastics compared to current practice. The study consisted of a comparison between the maximum allowable stresses of a group of cross-ply laminates, determined by deterministic analyses with safety and model factors as stated in design rules, and their probabilistic responses estimated with a fracture mechanics based model that accounted for material degradation. The results suggest that for the studied cases, the maximum allowable stresses do not provide the desired reliability; thus, their increase cannot be motivated. More importantly, the investigation shows that a better understanding of the effects of matrix cracks, and therefore, the maximum allowable crack density in a composite laminate, would lead to better and safer composite structures.

HERO: A hierarchical, efficient and reliable routing protocol for wireless sensor and actor networks

It is well known that the use of clusters makes wireless sensor and actor networks more scalable, and it also reduces energy consumption and improves their performance. Most hierarchical routing and clustering protocols are designed to be efficient when the data is sent from the sensor nodes to their cluster-head but not when it is sent the opposite way. In this paper, a simple mechanism to form clusters in an efficient way by using meta-data is proposed. And, furthermore, a novelty multi-hop and fault-tolerance routing protocol able to transport data from sensor nodes to their cluster-head and vice versa in an energy-efficient way is also presented. Moreover, this approach allows developers to establish the desired reliability level (in a quantitative way) between two nodes which are N hops far away from each other. The performance and the energy consumption of our approach is studied by means of a set of experiments which have been carried out by using the COOJA simulator.

Enhancement of Voltage Stability in Distribution Systems by Optimal Placement of Distribution Generator

ABSTRACT This article presents a methodology for optimal placement of DG units in distribution networks to guarantee the enhancement of voltage profile, maximize loadability and reduce distribution system losses. The methodology aims to find the configuration, among a set of system components, which meets the desired system reliability requirements, taking into account the stability limits. Results shown in the article indicate that the proposed formulations can be used to determine which are the best buses, where the addition of small distributed generator units can greatly enhance the voltage stability of the whole system and improve power transfer capability by reducing the system power losses. The proposed methodology is tested and compared with the existing distribution system.

Mechanical properties of BixSb2−xTe3 nanostructured thermoelectric material

Research on thermoelectric (TE) materials has been focused on their transport properties in order to maximize their overall performance. Mechanical properties, which are crucial for system reliability, are often overlooked. The recent development of a new class of high-performance, low-dimension thermoelectric materials calls for a better understanding of their mechanical behavior to achieve the desired system reliability. In the present study we investigate the mechanical behavior of nanostructure bulk TE material p-type BixSb2−xTe3 by means of nanoindentation and 3D finite element analysis. The Young’s modulus of the material was estimated by the Oliver–Pharr (OP) method and by means of numerically assisted nanoindentation analysis yielding comparable values about 40 GPa. Enhanced hardness and yield strength can be predicted for this nanostructured material. Microstructure is studied and correlation with mechanical properties is discussed.

A multiobjective Var/Volt Management System in Smartgrids

This paper presents a methodology for optimal Var/Volt management of Distributed Generation (DG) and FACTS units in power networks. The methodology is based on Genetic Algorithm in order to achieve the desired system reliability requirements taking into account voltage stability limits. Results indicate that the proposed formulation could be used to determine the optimal points in which the incorporation of DG units and connection of FACTS devices would allow a voltage stability enhancement, minimizing, at the same time, real power losses and minimizing the investment cost of the FACTS devices.

A New Multidisciplinary Design Optimization Method Accounting for Discrete and Continuous Variables under Aleatory and Epistemic Uncertainties

Abstract Various uncertainties are inevitable in complex engineered systems and must be carefully treated in design activities. Reliability-Based Multidisciplinary Design Optimization (RBMDO) has been receiving increasing attention in the past decades to facilitate designing fully coupled systems but also achieving a desired reliability considering uncertainty. In this paper, a new formulation of multidisciplinary design optimization, namely RFCDV (random/fuzzy/continuous/discrete variables) Multidisciplinary Design Optimization (RFCDV-MDO), is developed within the framework of Sequential Optimization and Reliability Assessment (SORA) to deal with multidisciplinary design problems in which both aleatory and epistemic uncertainties are present. In addition, a hybrid discrete-continuous algorithm is put forth to efficiently solve problems where both discrete and continuous design variables exist. The effectiveness and computational efficiency of the proposed method are demonstrated via a mathematical proble...

A wide number of trials is required to achieve acceptable reliability for measurement patellar tendon elongation in vivo

The purpose of the current study was to examine the reproducibility of patellar tendon elongation measurements using brightness-mode ultrasonography (BMU) during isometric knee extension contractions. We hypothesized that the measurement of the patellar tendon elongation during only one maximal voluntary isometric knee extension contractions would not provide reliable results and that a wide number of trials is required to achieve acceptable reliability. Ten participants (eight male and two female) performed 10 isometric knee extension contractions on two separate days (5 trials on each day). Using a modified knee brace, the ultrasound probe was firmly adjusted in the sagittal plane overlying the patellar tendon. The registered ultrasound images were analyzed by three different but equally trained observers. The reproducibility was examined by the calculation of the within-day, between-day and overall coefficient of multiple correlations (wCMC, bCMC and oCMC). The Spearman–Brown prophecy formula was use to estimate the required trials to achieve the desired reliability. The wCMC, bCMC and oCMC were in average 0.824, 0.798 and 0.770, respectively, suggesting a rather moderate reproducibility of patellar tendon elongation measurements. For a high reliability (≥0.95) of tendon elongation measurements 5–6 trials are required. Finally, the results revealed an independence of the measurements from days and observers.

Securing Dynamic Distributed Storage Systems Against Eavesdropping and Adversarial Attacks

We address the problem of securing distributed storage systems against eavesdropping and adversarial attacks. An important aspect of these systems is node failures over time, necessitating, thus, a repair mechanism in order to maintain a desired high system reliability. In such dynamic settings, an important security problem is to safeguard the system from an intruder who may come at different time instances during the lifetime of the storage system to observe and possibly alter the data stored on some nodes. In this scenario, we give upper bounds on the maximum amount of information that can be stored safely on the system. For an important operating regime of the distributed storage system, which we call the bandwidth-limited regime, we show that our upper bounds are tight and provide explicit code constructions. Moreover, we provide a way to short list the malicious nodes and expurgate the system.

Stochastic redundancy allocation problem using simulation

Reliability is one of the main parameters of the products considered to be important to survive in the competitive market. Reliability in its simple form means the probability that a failure may not occur in a given period of time that is, the component performs adequately without failure. The subsystems or components in a system are arranged in series or parallel depending on the space constraint. However, the reliability of the system with components arranged in parallel is more than that system for which the components are arranged in series. Redundancy is the method of arranging the components in a subsystem in parallel such that if one component fails then the other component automatically comes into operation. Through redundancy, any desired level of reliability can be obtained, but in doing so, we have to invest money or other material resources to achieve the desired reliability. A designer has to consider the economic views of the organization in designing a system with high reliability. In this paper, a complex system (series-parallel system) is considered with stochastic reliability for its components. For a particular configuration, reliabilities of the components are generated and evaluated by the system reliability, after which simulation is run and repeated for various runs, before finally consolidating the configuration reliability and resource utilization. The simulation is carried for various feasible configurations and each configuration evaluated. The configuration with best reliability within resource restrictions is selected. Key words: Redundancy, allocation, reliability.

Validity and Reliability of the Fear-Avoidance Beliefs Questionnaire (FABQ) in Workers with Upper Extremity Injuries

Fear Avoidance Beliefs (FAB) have been associated with increased pain, dysfunction and difficulty returning to work in Upper Extremity (UE) injures. The FABQ is used to assess FAB, but its measurement properties have not been established in UE. The purpose of this study is to evaluate the reliability and validity of the FABQ to screen UE compensated injured workers for FAB. Consenting workers attending a specialty clinic completed a modified FABQ, QuickDASH (Disability), SPADI Pain Score and von Korff Chronic Pain Grade (Pain), SF-36v2 (General Health), and Work Instability Scale (Job Instability). A sub-sample of workers (n = 48) completed the FABQ 2 weeks later for test-retest reliability. 187 workers; 54.0% male; mean age 45.2 (sd 9.68); 56% were currently working. Mean subscale scores (FABQ-Work [FABQ-W]/FABQ-Physical Activity [FABQ-PA]) were 35/42 and 20/24. Ceiling effects (23%/38%) existed in both subscales. Cronbach's alphas were 0.75/0.78. Test-retest analysis (ICC(2,1)) was lower than desired (0.52/0.59). Construct validation was supported by a moderate correlation between FABQ-W/FABQ-PA and QuickDASH Work Module (0.51/0.42) and WIS (0.46/0.38) in those currently working. Low correlations were found between the subscales measures of pain (SPADI: 0.24/0.23; Chronic Pain Grade: 0.25/0.25), and SF-36 MCS (-0.25/-0.30). Although FAB is an important concept to measure in compensated UE injured workers, the FABQ had limitations in this population as there was a high ceiling effect, and lower than desired reliability for individual discrimination. A priori hypotheses around construct validity were rejected for 16/22 concepts tested.

Optimal sizing of distributed resources in micro grid with loss of power supply probability technology by using breeding particle swarm optimization

The deregulated energy environment, among other effects, has favored a gradual transition from centralized power generation to distributed generation sources connected at the medium voltage or low voltage side of the distribution network. Interconnection of small, modular generations and energy storages of low or medium voltage distribution systems forms a new type of power system, which called the micro grid (MG). This paper has developed a methodology of performing the optimal unit sizing for distributed energy resources in MG. The methodology aims at finding the configuration, among a set of systems components, which meets the desired system reliability requirements, with the minimum cost of energy (COE). Other than solar and wind energies, in this study the biogas power plant and the use of urban waste for producing electricity power are considered. The size of biogas power plant is calculated according to available municipal waste. The micro grid system is connected to the utility and can sale its extra produced power to utility. In first step, accurate mathematical models for characterizing standalone photovoltaic module, wind generator, biogas plant, and battery bank are proposed. In second step, based on a breeding particle swarm optimization algorithm, one optimal sizing method is developed to calculate the optimum system configuration that can achieve the customers required loss of power supply probability with a minimum COE. By this methodology, we can analyze MG

Physical design space for isolated wind-battery system incorporating resource uncertainty

The physical design space of a wind-battery system enables a system designer to understand the trade-offs between different system design variables, the physical limits of the major decision variables, and arrive at an optimum solution subject to an appropriate design objective. An improved methodology for design of isolated wind-battery systems by accounting for the uncertainty associated with wind speed is proposed in this article. Chance-constraint programming technique is used to incorporate resource uncertainty in system sizing; thereby, facilitating system design corresponding to a specified reliability requirement. It is found that the cut-in speed of the wind turbine plays a critical role in delivering desired power supply reliability. A feasible design solution is obtained only when the probabilities associated with the cut-in and the cut-off wind speeds are accounted for along with the specified system reliability requirement. All feasible combination of system variables generated through a time-series simulation are represented on a rotor diameter versus generator rating diagram with minimum battery bank size as parameter. Solutions obtained by the proposed method are validated by sequential Monte Carlo simulations. Optimum system configuration for a given reliability requirement is determined based on minimum cost of energy (US$/kWh). It is shown that wind-battery systems cannot be designed to provide power supply reliability beyond a maximum value.

Stochastic hydro-economic modeling for optimal management of agricultural groundwater nitrate pollution under hydraulic conductivity uncertainty

In decision-making processes, reliability and risk aversion play a decisive role. This paper presents a framework for stochastic optimization of control strategies for groundwater nitrate pollution from agriculture under hydraulic conductivity uncertainty. The main goal is to analyze the influence of uncertainty in the physical parameters of a heterogeneous groundwater diffuse pollution problem on the results of management strategies, and to introduce methods that integrate uncertainty and reliability in order to obtain strategies of spatial allocation of fertilizer use in agriculture. A hydro-economic modeling approach is used for obtaining the allocation of fertilizer reduction that complies with the maximum permissible concentration in groundwater while minimizes agricultural income losses. The model is based upon nonlinear programming and groundwater flow and mass transport numerical simulation, condensed on a pollutant concentration response matrix. The effects of the hydraulic conductivity uncertainty on the allocation of nitrogen reduction among agriculture pollution sources are analyzed using four formulations: Monte Carlo simulation with pre-assumed parameter field, Monte Carlo optimization, stacking management, and mixed-integer stochastic model with predefined reliability. The formulations were tested in an illustrative example for 100 hydraulic conductivity realizations with different variance. The results show a high probability of not meeting the groundwater quality standards when deriving a policy from just a deterministic analysis. To increase the reliability several realizations can be optimized at the same time. By using a mixed-integer stochastic formulation, the desired reliability level of the strategy can be fixed in advance. The approach allows deriving the trade-offs between the reliability of meeting the standard and the net benefits from agricultural production. In a risk-averse decision making, not only the reliability of meeting the standards counts, but also the probability distribution of the maximum pollutant concentrations. A sensitivity analysis was carried out to assess the influence of the variance of the hydraulic conductivity fields on the strategies. The results show that the larger the variance, the greater the range of maximum nitrate concentrations and the worst case (or maximum value) that could be reached for the same level of reliability.

Reliability Analysis of Counterfort Retaining Walls

Traditionally, a constant factor of safety (usually 1.5) is adopted in the design of counterfort retaining walls against instability failure, regardless of the actual uncertainties in the various design variables. This constant factor of safety may not be able to quantify the uncertainties associated with the random variables. This paper presents the stability analysis of a typical counterfort retaining wall, accounting for uncertainties in the ‘design variables’ in the framework of probability theory. The first order reliability method (FORM), second order reliability method (SORM) and Monte Carlo Simulation (MCS) method are used in this study to evaluate the probability of failure associated with the various modes (both geotechnical and structural) of a typical counterfort retaining wall. Sensitivity analysis reveals that the angle of internal friction of the soil, is the most sensitive random variable, which affects all the modes of failure. Plots of reliability index and factor of safety are generated for critical modes of failure, where the constant factor of safety (as recommended in various design codes) is not able to get a desired reliability index/probability of failure.

Performance tradeoffs among percolation-based broadcast protocols in wireless sensor networks

Broadcast of information in wireless sensor networks is an important operation, for example, for code updates, queries and membership information. In this paper, we analyse and experimentally compare the performance of vanilla versions of several well-known broadcast mechanisms: flooding, site percolation, bond percolation and modified bond percolation. While flooding and some percolation-based approaches have been compared in the literature, there is no all-to-all comparison among all schemes. We carry out our comparison for several network topologies defined by node locations: random, grid and clustered. Our analysis is performed at the link layer level, where we use a propagation model based on real experiments from the literature. The link model used is independent of the medium access control (MAC) layer and, therefore, helps us in arriving at the best possible values for the metrics that we compare in our analysis. Our main metrics are bandwidth, energy usage and broadcast latency. Our analytical and experimental results show that, given a desired high reliability for all topologies, flooding has the lowest latency but consumes the most energy per broadcast. For dense networks, site percolation achieves comparable latency and reliability to flooding, while lowering energy consumption. Modified bond percolation further lowers energy consumption compared to site percolation, while basic bond percolation leads to a latency increase. For sparse networks, results are similar to a dense network except that site percolation consumes lower energy than modified bond percolation. We briefly discuss implications for different broadcast applications.