Zili Wang

Parallel algorithm for finding modules of large-scale coherent fault trees

Abstract The computation of the probability of the top event or minimal cut sets of fault trees is known as intractable NP-hard problems. Modularization can be used to reduce the computational cost of basic operations on fault trees efficiently. The idea of the linear time algorithm, as a very efficient and compact modules detecting algorithm, is visiting the nodes one by one with top-down depth-first left-most traversal of the tree. So the efficiency of the linear time algorithm is limited by nodes visiting time successively and serially, especially when confronting large-scale fault trees. Aiming at improving the efficiency of modularizing large-scale fault trees, this paper proposes a new parallel method to find all possible modules. Firstly, we transform the fault tree into a directed acyclic graph (DAG) and treat the terminal basic nodes as entries of the algorithm. And then, according to the proposed rules in this paper, we traverse the graph bottom-up from the terminal nodes and mark the internal nodes in a parallel way. Therefore, we can compare all internal nodes and decide which nodes are modules. Eventually, an experiment is carried out to compare the linear and parallel algorithm, and the result shows that the proposed parallel algorithm is efficient on handling large-scale fault trees.

An Alternative Lifetime Model for White Light Emitting Diodes under Thermal–Electrical Stresses

The lifetime prediction using accelerated degradation test (ADT) method has become a main issue for white light emitting diodes applications. This paper proposes a novel lifetime model for light emitting diodes (LEDs) under thermal and electrical stresses, where the junction temperature and driving current are deemed the input parameters for lifetime prediction. The features of LEDs’ lifetime and the law of lumen depreciation under dual stresses are combined to build the lifetime model. The adoption of thermal and electrical stresses overcomes the limitation of single stress, and junction temperature in accelerated degradation test as thermal stress is more reliable than ambient temperature in conventional ADT. Furthermore, verifying applications and cases studies are discussed to prove the practicability and generality of the proposed lifetime model. In addition, the lifetime model reveals that electrical stress is equally significant to the thermal stress in the degradation of LEDs, and therefore should not be ignored in the investigation on lumen decay of LEDs products.

The effect of reactant consumption on thermal explosion of battery by using kinetic-based simulation approach

Evaluating thermal safety of pyrotechnic composition is an important topic in the process of design, production, storage and transportation of pyrotechnic products. The ‘battery’ is a typical kind of pyrotechnic products which is combined with several single cylindrical pyrotechnics side by side. In this paper, the method of kinetic-based simulation has been applied to analyze the effect of reactant consumption on thermal explosion of battery. The reactant parameters and establishment of model and algorithm are the key points of this method. The physical and chemical parameters of black powder were obtained by experimental data. A three-dimensional non-steady-state thermal explosion model of battery with reactant consumption and corresponding algorithm and program are novel and first developed, followed by analysis of the history and distribution of temperature and concentration. Several suggestions for thermal safety design were proposed by analyzing parameters. Further, an example is calculated to clarify that chemical system is more safety relatively when considering reactant consumption. The results can provide basic data and theoretical guidance for thermal safety design and management of battery.

Fleet-level selective maintenance problem under a phased mission scheme with short breaks: A heuristic sequential game approach

Abstract Selective maintenance is the most widely used strategy for identifying and performing the maintenance actions necessary for fleet mission success. A fleet of equipment is usually required to perform phased missions with short scheduled breaks. In this case, a selective maintenance model should be extended for frequency-based maintenance optimization. We research the problem considering the application of condition-based maintenance (CBM). The problem is formulated with the objective of reducing the repair frequency and cost. The constraint is the reliability of the phased mission, and the variables are the remaining useful lifetimes (RUL) of all the key subsystems. The equipment can be classified into three echelons based on the health status before each wave of a mission, and a heuristic game framework with state backtracking is proposed for the three echelons to solve the problem. The flowchart and heuristic rules of the game framework are given, and the game algorithms for the second and third echelons are presented. The second echelon algorithm aims to select the dispatched equipment for the current wave and minimize maintenance, and the third echelon algorithm aims to ensure that sufficient equipment is available for the next wave by performing necessary maintenance. Finally, we present two types of strategy adjustment methods with state backtracking to turn infeasible solutions into feasible solutions and to optimize feasible solutions. To verify the capacity of the proposed method, a case involving a fleet of 12 aircraft is analyzed for a three-mission scheme, and the aircraft repair times and costs are reduced by the method.

Thermal effects of packaging material and structure on high power white LEDs

The heat dissipation is a crucial factor for the reliability of high power white LEDs(HPLEDs). In this paper, a novel thermal pad structure is presented for HPLEDs, a theoretic thermal transfer model is built to analyze the thermal transferring in LED component. Besides, the thermal simulations of the LED component with the typical or the novel thermal pad are conducted, the temperature distributions are obtained to assess the thermal performance of the LED components. Moreover, the material of the substrate is changed into FR4 to investigate the advantage of the novel thermal pad structure in LED component. The thermal conductivity gap between the thermal pad and the packaging material is recognized as the most important factor on the selection of thermal pad structure.

Compound poisson process based reliability analysis method for performance degradation of GaAs space solar cells

GaAs space solar cells always suffered from the complex and hostile space environment. Space solar cell reliability is crucial factor to ensure the normal operation of space satellites. A method of modeling the degradation of GaAs space solar cells is proposed here. The method is to find one crucial performance parameter to depict the degradation process. By applying compound Poisson process and general stress-strength interference model, the reliability of GaAs space solar cell can be calculated. According to the 15-years observation data of particle radiation, high-energy electron radiation experiment on the GaAs space solar cells was developed to verify the method. The method is more effective to assess the GaAs space solar cell reliability compared to the existing degradation mechanism modeling. And reliability can be calculated through the attenuation of output power which was easy to detect and record.

Modeling method of SysML-based reliability block diagram

In allusion to the impossibility of conducting both a specific products reliability modeling and performance modeling at the same time as well as the fact that the reliability design of a product being out of line with performance design, this study defines a modeling method of function model based on SysML, which allows the systems engineers to build system function model using IBD to depict the relationships of system static function and generate reliability block diagram. An algorithm based on system function matrix is also proposed. This work enables an automatic reliability block diagram analysis during the system performance design phase. Such a method promotes the effectiveness and reliability of products as well as the consistency that a reliability design may be involved with performance design.