Difficulties in Fault-Tree Synthesis for Process Plant

This paper identifies a number of related difficulties, some of which are still unsolved. Attention is drawn to failings in the type of pressure-flow model commonly used in the literature. Difficulties also exist when published algorithms are applied to control loops. These are illustrated for simple and cascade control applications and discussed in some detail. Eight general conclusions are: 1. The concept of 2-way flow of information in failure models is important in certain situations, e.g., fluid flow. 2. The accuracy of failure models is generally low. This reflects the fact that much of the effort expended in systematic failure analyses has been heavily oriented towards algorithms. 3. Models used in failure analyses do not have to be comprehensive. Only the credible set of events is needed. 4. No always-satisfactory algorithm has been published for fault-tree synthesis where control loops are encountered. 5. The control loop problem is inextricably interlinked with the general difficulty that fault-tree methodology is primarily oriented to binary systems where the time dimension can be ignored. 6. Fault-tree methodology uses simple models to approximate system failures. If these failures are complex then fault trees might not be suitable. The results of analyses involving complex failures must be treated with great care. 7. When fault-tree methodology is not completely suitable one ought to consider using a different technique altogether. The cause-consequence diagram might be appropriate since it can be used to study failure modes where time is important. 8.     

A cognitive model‐based approach for autonomic fault management in OpenFlow networks

Autonomic network management is an approach to the management of complex networks and services that incorporates the detection, diagnosis and reconfiguration, as well as optimization, of their performance. A control loop is fundamental as it facilitates the capture of the current state of the networks and the reconfiguration of network elements without human intervention. For new networking architectures such as software‐defined networking and OpenFlow networks, in which the control plane is moved onto a centralized controller, an efficient control loop and decision making are more crucial. In this paper, we propose a cognitive control loop based on a cognitive model for efficient problem resolving and accurate decision making. In contrast to existing control loops, the proposed control loop provides reactive, deliberative and reflective loops for managing systems based on analysis of current status. In order to validate the proposed control loop, we applied it to fault management in OpenFlow networks and found that the protection mechanism provides fast recovery from single failures in OpenFlow networks, but it cannot cover multiple‐failure cases. We therefore also propose a fast flow setup (FFS) algorithm for our control loop to manage multiple‐failure scenarios. The proposed control loop adaptively uses protection and FFS based on analysis of failure situations. We evaluate the proposed control loop and the FFS algorithm by conducting failure recovery experiments and comparing its recovery time to those of existing methods. Copyright © 2013 John Wiley & Sons, Ltd.     

梅逊公式在采样控制系统中的应用扩展

在分析采样控制系统时，求脉冲传递函数是关键之一。在一些文献中所介绍的脉冲传递函数求解方法显得较为复杂。本文将应用于连续控制系统求传递函数的梅逊公式，应用扩展到采样控制系统，给出了一种要样控制系统闭环脉冲传递函数的简便方法。     

Delay times in fault tree analysis

In reliability analysis of technical systems the failure frequency of cut sets alone is often insufficient to assess the probability of an undesired event. In many systems an undesired event occurs after failures of system elements, however not before a limited time period is over. Since system elements can be repaired the delay times can have a crucial effect on the probability of an undesired event.Here a method is developed to consider as many different delay times as desired in calculating the probability of the fault tree top event.     

On-line hazard aversion: a Bayes approach

A Bayes theorem and fault tree (BFT) method is developed for online hazard aversion in process systems. BFT requires as input a prediction of the measurement patterns resulting from system operating nodes, equipment malfunctions and disturbances, and the prior rates of equipment malfunctions and disturbances. The Bayes theorem is used to estimate the posterior probability of each fault candidate, given the online measurements, producing a list of fault candidates ranked by their posterior probabilities. A fault tree is then used to estimate the current top event probability, given the online measurements in which basic event probabilities are the posterior probabilities of the faults. The future safety of the process is measured by safety meters which compare the estimated top event probability at given times over a time horizon. The estimated current top event probability and the safety meters indicate process safety, providing the basis for an unplanned shutdown, maintenance, or corrective action decision. A negative feedback process control loop illustrates the accuracy of BFT and lays the groundwork for applying BFT to more complicated process systems     

Failure Concepts in Reliability Theory

The reduction and control of failures in electronics systems is essential due to: 1) the need for maximum safety of personnel on exotic missions; 2) the long unattended life required of space systems; 3) the high dollar costs to buy additional systems and support additional maintenance personnel required to compensate for the unreliability and system down-time. As practiced by advanced organizations, efforts to reduce parts failures by design controls and conventional statistical reliability methods are a necessary condition of reliability, but not a sufficient one. For clarity and background, present reliability and failure rate theory and practices are reviewed briefly and some limitations pointed out. The physics and the mechanisms of actual failures are discussed and it is pointed out that failures follow the laws of cause and effect; that they are therefore intrinsically predictable. The concept of random failures is valid in a statistical sense but it should not discourage efforts to predict and prevent individual failure by all means open to us. A powerful method of failure prevention is a thorough system test prior to use when the test is geared to look for failures where they can be detected. Lack of precision in terminology is part of the problem of failure control; we commonly intermingle references to failures by source, by cause, by end-effect, and by symptom. An improved organization and definition of failures is developed through which the methods, means, and limitations of failure detection, prediction, and prevention can be established.     

Optimisation of K-Out-Of-N systems subject to common-cause failures with repair provision

This paper considers K-Out-Of-N systems that is subject to two modes of failures, Common cause failures (CCFs) and Random failures, We assure that the system is repaired after each system failure. We develop an optimality criterion leading to the optimal number of units N^* for a K-Out-Of-N system, based on cost considerations. Numerical examples are included to illustrate the procedure.     

Two Algorithms for Computing Reliability

This paper developes two algorithms for finding the time-dependent reliability of any active-redundant system in terms of component failure probabilities. These algorithms apply to non-loaded, continuously-operating systems in which drift-type failures are neglected. The first algorithm is used when each component can fail in only one mode. A procedure, based on the Quine-McCluskey technique from switching circuit theory, is stated in conjunction with this algorithm which permits rapid simplification of the reliability expression. Two failure modes are permitted in the second algorithm. The relation between the expressions resulting from the algorithms and lifetime random variables is given so that the algorithms do not depend on specific component failure distributions.     

基于动态错误图的容错计算机系统可靠性分析(英文)

Dependability analysis is an important step in designing and analyzing safety computer systems and protection systems.Introducing multi-processor and virtual machine increases the system faults' complexity,diversity and dynamic,in particular for software-induced failures,with an impact on the overall dependability.Moreover,it is very different for safety system to operate successfully at any active phase,since there is a huge difference in failure rate between hardware-induced and softwareinduced failures.To handle these difficulties and achieve accurate dependability evaluation,consistently reflecting the construct it measures,a new formalism derived from dynamic fault graphs(DFG) is developed in this paper.DFG exploits the concept of system event as fault state sequences to represent dynamic behaviors,which allows us to execute probabilistic measures at each timestamp when change occurs.The approach automatically combines the reliability analysis with the system dynamics.In this paper,we describe how to use the proposed methodology drives to the overall system dependability analysis through the phases of modeling,structural discovery and probability analysis,which is also discussed using an example of a virtual computing system.     

Reliability growth modeling for in-service electronic systems considering latent failure modes

A latent failure mode is a type of failure that may not occur until the system has operated in the field for a certain period of time. Predicting latent failures is often difficult, but it has a great importance for reliability management in terms of system maintenance and warranty services. This paper proposes a stochastic model to predict the reliability growth for field or in-service electronic systems considering latent failures. The proposed model can be applied to electronics industries where extended in-house reliability testing cannot be implemented due to the tight design schedule. Based on the new method, the product management can proactively implement corrective actions against key failure modes using relevant engineering resources. A discussion between the effectiveness of corrective actions and the associated cost is also provided. Finally, field failure data collected from a fleet of automatic test equipment are used to demonstrate the applicability and performance of the model.     

Reliability of Some Modularly Redundant Systems

A mathematical model is established for the reliability of modularly redundant systems with unequal failure rates for the operating and standby units. The failure modes include failures of the active and standby units, three types of switch failures, and failures on system recovery. System reliability is considered for cases of both similar and dissimilar units, and for various restrictions on the failure parameters. It is shown that the most important failure parameters are those related to catastrophic failures, and that putting more reliable units as basic units, which operate initially, is important when switches and recovery are imperfect.     

卫星激光测距系统控制分系统设计及实现方法

卫星激光测距系统是一套集光、机、电为一体的精密复杂系统。控制分系统主要通过精确控制激光器分系统、望远镜分系统、折轴光学系统实现测距信号的发射、接收和测距数据处理。是保证系统具有高自动化程度和高测距成功率的关键技术。使用 FPGA 设计制作控制信号板,产生激光点火信号和距离门信号,实现精确控制激光器、单光子雪崩二极管和时间计数器。 本文对系统的整个工作流程进行了仔细分析,设计出软件的各个功能模块。分别对 1m 望远镜、 激光器做了控制试验,用真实卫星白天测距数据做了数据处理试验,得到了很好的效果。     

Output Impedance Design of Parallel-Connected UPS Inverters With Wireless Load-Sharing Control

This paper deals with the design of the output impedance of uninterruptible power system (UPS) inverters with parallel-connection capability. In order to avoid the need for any communication among modules, the power-sharing control loops are based on the$P/Q$droop method. Since in these systems the power-sharing accuracy is highly sensitive to the inverters output impedance, novel control loops to achieve both stable output impedance and proper power balance are proposed. In this sense, a novel wireless controller is designed by using three nested loops: 1) the inner loop is performed by using feedback linearization control techniques, providing a good quality output voltage waveform; 2) the intermediate loop enforces the output impedance of the inverter, achieving good harmonic power sharing while maintaining low output voltage total harmonic distortion; and 3) the outer loop calculates the output active and reactive powers and adjusts the output impedance value and the output voltage frequency during the load transients, obtaining excellent power sharing without deviations in either the frequency or the amplitude of the output voltage. Simulation and experimental results are reported from a parallel-connected UPS system sharing linear and nonlinear loads.     

Scattering analysis of conducting bodies of revolution usingfictitious currents and point-matching

In this paper, a fictitious current model using point-matching is introduced for the electromagnetic scattering problem of a conducting body of revolution. To take advantage of the rotational symmetry of a scatterer, test points and source points are assumed to have φ-dependence of e^jnφ and are grouped into test loops and source loops, respectively. The number of sample points in each loop is determined in proportion to the length of the loop. Thus, the range of orders to be considered in each loop is dependent on the length of the loop. The field components of higher order modes which can arise due to aliasing are removed using appropriate approximations. Since the current sources and the boundary condition testing are of point-form, the numerical evaluation of the matrix elements is simple and fast. Numerical results are compared with other methods, and the numerical efficiencies obtained with this method are discussed     

Prognostic methodology for deep submicron semiconductor failuremodes

Semiconductor reliability issues are beginning to emerge as a major impediment to long term reliability of critical systems such as Internet routers, ATM machines, and Automotive/Aerospace fly-by-wire systems. Semiconductors have certain defined failure modes that can contribute to end-of life failures. These modes include time-dependent dielectric breakdown of the gate oxide (TDDB), hot carrier damage, and metal migration. All of these common failure modes are far worse at geometries below 0.25 μm. Fortunately, there are methods proposed that counteract these common failure modes. This paper surveys the problems involved, and recommends a methodology for the inclusion of pre-calibrated prognostic cells that can be co-located with a host circuit to provide an “early-warning” of a system failure, so that appropriate corrective action can be taken     

三电平PWM整流器双闭环系统的设计与仿真

三电平PWM整流器多采用电压控制外环和电流控制内环组成的双闲环控制系统。电压外环的作用是根据直流电压Udc的大小决定三电平PWM整流器输出功率的大小和方向以及三相电流的给定信号。电流内环的作用是使整流器的实际输入电流能够跟踪电流给定,实现单位功率因数或功率因数可变。文中主要研究了三电平PWM整流器的系统设计,并进行了仿真。结果表明,所设计的双闭环系统具有良好的抗扰动性能,动态响应也得到了明显的改善。     

A position control differential drive wheeled mobile robot

For more accurate path tracking of a four-wheeled two-degrees-of-freedom mobile robot (WMR), a position control algorithm is proposed with two separated feedback loops, a velocity feedback loop and a position feedback loop. In the most conventional position control system of a WMR, internal error is mainly considered, while external error has, as yet, hardly been treated, although it plays an important role in accurate position control. This external error is caused by unexpected environmental situations. The proposed control algorithm is designed to compensate for both internal error and external error. This algorithm makes it possible to accurately follow the designed trajectory     

Cost Minimization with HPDFG and Data Mining for Heterogeneous DSP

Cost minimization and execution-time reduction have become the most important issues in today’s real-time embedded system. Meanwhile, for the DSP (Digital Signal Processing) applications running on embedded system, loops inside them are the most critical part for performance optimization. To optimize the loop iteration patterns, we need to schedule the loop execution order. Due to the uncertainties within the execution time of tasks, we model varied execution times of tasks as random variables and propose a novel data graph model, called HPDFG (Heterogeneous Probabilistic Data-Flow Graph) to model DSP applications on embedded systems. A novel algorithm, LSHAPE, is proposed to minimize the cost and satisfy the timing constraints. First of all, we use the data mining methods to estimate the probabilistic distribution of the execution time variables. Second, we rotate the loops in the application to explore different possible execution patterns. Finally, we combine the list-scheduling and the dynamic programming to generate a near-optimal task allocation and a core-mode assignment. Experimental results demonstrate the effectiveness of our algorithm. Our approach can handle loops efficiently.     

Fuzzy fault-tree analysis using failure possibility

In conventional probability-based fault-tree analysis, the vague property in many systems, especially, the man-machine system is often ignored. To overcome this disadvantage, the fuzzy set theory is applied to fault-tree analysis. The failure possibility defined by a triangular fuzzy number on the interval [0,1] is used to characterize the possible deviation of the basic events. The fuzzy operations according to the extension principle is used to calculate the failure possibility of the top event. Furthermore, a fuzzy importance index is proposed to demonstrate the contribution of a basic event to the safety improvement of the top event in a fuzzy environment.     

A Second-Order Moments Method for Uncertainty Analysis

The system moments approach is extended to combine random variables for large systems. A computer code, COSMOS, has been developed to propagate uncertainties for systems which contain up to 100 components. Extension of the method to very large systems is possible by modular evaluation. An example illustrates the method for evaluating the distribution of the top event unavailability for a fault tree. Advantages of the method include small required execution times and the avoidance of random number generation. Application of the method to Boolean sum-of-products representations of fault trees provides results which are consistent with the Monte Carlo method since, for this special case, higher order derivatives are zero. Serious limitations to the method include: unquantifiable accuracy for functions whose higher derivatives are non-zero (as is often the case), inability to model s-dependent failures as the moments equations assume s-independence, and difficulty in improving the accuracy of the method (compared with Monte Carlo evaluation which may be arbitrarily improved).