Application of Markov renewal theory and semi-Markov decision processes in maintenance modeling and optimization of multi-unit systems

In this paper, a condition-based maintenance model for a multi-unit production system is proposed and analyzed using Markov renewal theory. The units of the system are subject to gradual deterioration, and the gradual deterioration process of each unit is described by a three-state continuous time homogeneous Markov chain with two working states and a failure state. The production rate of the system is influenced by the deterioration process and the demand is constant. The states of the units are observable through regular inspections and the decision to perform maintenance depends on the number of units in each state. The objective is to obtain the steady-state characteristics and the formula for the long-run average cost for the controlled system. The optimal policy is obtained using a dynamic programming algorithm. The result is validated using a semi-Markov decision process formulation and the policy iteration algorithm. Moreover, an analytical expression is obtained for the calculation of the mean time to initiate maintenance using the first passage time theory.     

The building evacuation problem with shared information

In this article, the Building Evacuation Problem with Shared Information (BEPSI) is formulated as a mixed integer linear program, where the objective is to determine the set of routes along which to send evacuees (supply) from multiple locations throughout a building (sources) to the exits (sinks) such that the total time until all evacuees reach the exits is minimized. The formulation explicitly incorporates the constraints of shared information in providing online instructions to evacuees, ensuring that evacuees departing from an intermediate or source location at a mutual point in time receive common instructions. Arc travel time and capacity, as well as supply at the nodes, are permitted to vary with time and capacity is assumed to be recaptured over time. The BEPSI is shown to be NP‐hard. An exact technique based on Benders decomposition is proposed for its solution. Computational results from numerical experiments on a real‐world network representing a four‐story building are given. Results of experiments employing Benders cuts generated in solving a given problem instance as initial cuts in addressing an updated problem instance are also provided. © 2008 Wiley Periodicals, Inc. Naval Research Logistics, 2008     

建立雷达超视距探测效能库的一种快速算法

蒸发波导环境中微波雷达经常会出现超视距探测,建立雷达超视距探测效能库有利于微波超视距雷达探测效能的发挥。提出了一种计算雷达超视距探测效能的快速算法,即以蒸发波导高度、总体理查森数和海面风速为变量,先建立雷达超视距探测效能查询表,再利用查表的方法得到各种气象条件下雷达的探测效能,以此作为探测效能库的数据基础。最后将基于此算法得到的雷达探测规律与实际观测进行了比较,表明此算法是可行的。     

流的大小与传输速率相结合的双门限检测算法

提出基于流传输速率与数据量的双门限检测算法。为满足高速网络传输的性能要求,使用Hash表存储流检测的数据结构,将Hash表的冲突处理与基于流速率的缓存替换相结合以实现高效的大流检测,通过限制Hash桶的容量,确保报文的处理性能。真实网络数据的仿真测试结果表明:所提算法在相近的存储开销下,保持了较高的处理性能,准确性优于基于最近最少使用算法的大流检测及其派生算法以及基于统计计数的紧凑型空间节省算法。     

浅海中时谐水平电偶极子在空气中的极低频磁场

为深入分析空气中的极低频磁场,用时谐水平电偶极子等效船舶被主轴调制的水下腐蚀相关电流。根据Maxwell方程组和电磁场边界条件建立浅海条件下电偶极子在空气中的矢量磁位模型,进一步推导了磁场表达式,并利用快速汉克尔变换计算磁场的传播规律。测量了水池碳棒电极和海上钛基电极的磁场,验证了磁场的实用性和模型的有效性。     

采用瞬态球状热斗篷方法的浅层地下目标红外隐身技术

由于浅层地下目标与背景土壤的热物性不同,影响了整个区域表面的温度场分布,容易被敌方探测系统发现并击毁。针对这一问题,考虑太阳辐射、天空辐射以及风速的影响,建立了埋藏地下目标的区域温度分布传热模型,揭示了不同时段埋藏地下目标对区域表面温度场的影响。在变换热力学的基础上,通过坐标变换的方法,推导出球状瞬态热斗篷的导热系数的通解表达式。根据等效介质理论,对设计的热物性参数进行均质化,并通过数值实验方法验证了基于热斗篷地下目标红外隐身技术的可行性。     

雷达侦察效能对防空兵群抗击效率的影响评估

从信息化条件下防空作战的实际需要出发,描述了搜索雷达与火控雷达的侦察效能对群抗击效率影响的基本特点,运用概率论和防空兵射击指挥理论,建立了防空兵群分散指挥和集中指挥时,搜索与火控雷达发现目标概率对群抗击效率影响的数学模型,结合实例计算分析了选择射击指挥方式时搜索雷达发现目标概率的临界值。     

内置频率对Duffing振子微弱二进制相移键控信号盲检测影响

采用Duffing振子实现对微弱二进制相移键控（Binary Phase Shift Keying, BPSK）信号的盲检测时，Duffing系统输出的周期态和混沌态转换之间存在过渡带。针对这一问题，推导出过渡带时长和Duffing系统内置频率之间的关系表达式；指出内置频率越高，过渡带时间越短；仿真实验给出时间频率响应曲线。内置频率的提高，会降低系统检测微弱信号的灵敏度。针对这一问题，推导出周期态下Duffing系统输出幅度作为因变量、内置频率作为自变量的表达式；仿真实验给出幅频响应曲线。针对微弱BPSK信号盲检测，建立变尺度方法和检测阵列相结合的基于S变换提取Duffing系统输出幅度包络的微弱BPSK信号盲检测模型，仿真实验验证了模型方法的有效性。     

非均匀湍流场中螺旋桨噪声研究

针对螺旋桨在湍流中的非定常噪声问题，在均匀湍流下螺旋桨宽频噪声相关性法的基础上，推导了非均匀湍流下螺旋桨宽频噪声的公式。在均匀湍流的研究中使用了十桨叶螺旋桨模型，并将计算结果与试验数据进行了对比，发现吻合较好。对于非均匀湍流中的问题，研究发现湍流的不均匀程度不会对宽频谱波峰的位置产生影响，噪声谱波峰的位置仍然取决于叶频，湍流的不均匀程度只会改变叶频处的波峰高度。最后研究了均匀湍流和非均匀湍流中侧斜角对螺旋桨噪声的影响，发现桨叶侧斜角的存在会降低螺旋桨在湍流中受到的宽频噪声。     

运动目标轨迹分类与识别

运动目标轨迹识别是运动分析中的基本问题,其目的是解释所监视场景中发生的事件,对所监视场景中运动目标轨迹的行为模式进行分析与识别,智能地做出自动分类.对轨迹有效性判断后采用K均值聚类,引入改进的隐马尔可夫模型算法,针对轨迹的复杂程度对各个轨迹模式类建立相应的隐马尔可夫模型,利用训练样本训练模型得到可靠的模型参数,计算测试样本对于各个模型的最大似然概率,选取最大概率值对应的轨迹模式类作为轨迹识别的结果,对两种场景中聚类后的轨迹进行训练与识别,平均识别率较高,实验结果表明该方法是有效的.