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In this paper, we consider acyclic networks of queues as a model to support the design of a dynamic production system. Each service station in the network represents a manufacturing or assembly operation. Only one type of product is produced by the system, but there exist several distinct production processes for manufacturing this product, each one corresponding with a directed path in the network of queues. In each network node, the number of servers in the corresponding service station is either one or infinity. The service time in each station is either exponentially distributed or belongs to a special class of Coxian distribution. Only in the source node, the service system may be modeled by an
queue. The transport times between every pair of service stations are independent random variables with exponential distributions. In method proposed in this paper, the network of queues is transformed into an equivalent stochastic network. Next, we develop a method for approximating the distribution function of the length of the shortest path of the transformed stochastic network, from the source to the sink node. Hence, the method leads to determining the distribution function of the time required to complete a product in this system (called the manufacturing lead time). This is done through solving a system of linear differential equations with non-constant coefficients, which is obtained from a related continuous-time Markov process. The results are verified by simulation.This complete issue was revised and published online in November 2004. The previous version contained a false date.
Correspondence to: Mohammad Modarres 相似文献
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为了进行对蛋白质相互作用网络的拓扑分析,应用最短路径技术对蛋白质相互作用数据库(DIP)中包括酵母在内的7个物种的8个蛋白质相互作用网络进行了研究,包括对网络直径、特征路径长度、连通效率、顶点介数与顶点度的相关性以及高介数边和长间隔边在网络连通中的作用的研究.分析发现,这些网络对随机移除一定数量的蛋白质顶点(或边)具有很好的健壮性,但对高介数顶点(或边)的确定性移除却相当脆弱,而且按顺序移除2%高介数顶点所引起的网络连通效率下降明显大于随机移除10%顶点所引起的网络连通效率变化;所研究的7个物种的网络都存在不同比例的边缺失替代路径,绝大多数网络在移除一定比例的长间隔边后网络连通效率下降. 相似文献
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The problem of searching for all minimal paths (MPs) in a network obtained by modifying the original network, e.g. for network expansion or reinforcement, is discussed and solved in this study. The existing best-known method to solve this problem was a straightforward approach. It needed extensive comparison and verification, and failed to solve some special but important cases. Therefore, a more efficient, intuitive and generalized method to search for all MPs without an extensive research procedure is proposed. In this presentation, first we develop an intuitive algorithm based upon the reformation of all MPs in the original network to search for all MPs in a modified network. Next, the computational complexity of the proposed algorithm is analyzed and compared with the existing methods. Finally, examples illustrate how all MPs are generated in a modified network based upon the reformation of all of the MPs in the corresponding original network. 相似文献
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提出一种以障碍物和机器人位置和速度等信息为基础的障碍躲避方法,重点探讨了确定性环境下障碍躲避和转弯过程中机器人应遵循的合理路径问题,并通过证明指出了给定环境条件下实现转弯和障碍躲避的最佳路径。仿真实验表明,该结论简单有效,便于实施,具有较高的应用价值。 相似文献
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Roughing tool path of panel machining, which is a bottleneck of spacecraft production, should be optimised rapidly to shorten process time. This problem has a large solution space, and surface quality should be taken into account. The decision variables are cavity machining order, feed point and cutting direction of each cavity. Our problem is presented as an asymmetric general travelling salesman problem (AGTSP). A cluster optimisation-based hybrid max–tmin ant system (CO-HMMAS) is proposed, which solves two sub-problems as a whole. The oriented pheromone and dynamic heuristic information calculating methods are designed. We analyse the differences between one-stage and two-stage AGTSP local search heuristics and combine CO-HMMAS with them properly. An improved Global 3-opt heuristic suitable for both symmetric and asymmetric cases is proposed with sharply reduced time complexity. Comparison experiments verified that, two-stage local search heuristics decrease solution error significantly and rapidly when the error is great, and one-stage ones improve a near-optimal solution costing much more computing time. Benchmarks tests show that, CO-HMMAS outperforms the state-of-the-art algorithm on several technical indexes. Experiments on typical panels reveal that all algorithm improvements are effective, and CO-HMMAS can obtain a better tool path than the best algorithm within less CPU time. 相似文献
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基于路段时间窗考虑备选路径的AGV路径规划 总被引:1,自引:0,他引:1
针对自动化集装箱码头基于卸箱任务的自动导引车(automated guided vehicle,AGV)路径规划问题,结合最优路径数学模型、路径搜索方法和时间窗,提出了一种基于路段时间窗的AGV路径规划方法。首先,在给AGV下派任务的基础上,用最优路径数学模型为AGV规划出最短路径;其次,用路径搜索方法搜索AGV的备选路径,在路径长度相同的情况下,按照路径中转折次数确定备选路径优先级,转折次数少的备选路径优先级高;最后,在各AGV最短路径下,设置各个路段的时间窗,时间窗无重叠则表明AGV无冲突,对于时间窗重叠的路段,采用在原路径上插入时间窗或者在备选路径上插入时间窗的方法,再进行时间窗重叠测试,若还存在重叠的,则继续调整至最终实现多AGV的无冲突路径规划。为了验证方法的有效性,以8台AGV分区同时工作为例,用实例证明所提出的路径规划方法的避碰效果。结果显示该方法能为多台同时工作的AGV规划出一条无冲突优化路径,并且用时较短;在试验中发现选择在备选路径上插入时间窗的方法效果更好。研究表明所提方法能有效实现AGV的避碰,提高AGV利用率和自动化集装箱码头的运作效率。 相似文献
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室内地面清洁机器人路径规划 总被引:7,自引:1,他引:7
梁喜凤 《中国计量学院学报》2006,17(1):64-68
采用栅格法建立环境地图模型,利用沿边走的路径规划算法进行室内地面清洁机器人无障碍与避障路径规划与仿真试验.结果表明,在静态结构化环境中,清洁机器人能够沿内螺旋式“回”字型路径完成清扫任务,并成功避开障碍物和边界,自动定位和转向,清洁完毕机器人能够自动返回到初始位置.这说明沿边走的内螺旋式“回”字型路径规划方式是可行的. 相似文献
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The optimal route between a given origin and destination needs to be computed in a fast and efficient manner in dynamic route guidance systems. Conventional routing algorithms have been found to be inadequate when applied directly to large road networks. Algorithms based on the concept of hierarchical abstraction make use of the knowledge about the road network to reduce search and provide near-optimal solutions. A generic procedure is presented for organising a given road network as a multiple-layer hierarchy. An efficient hierarchical routing algorithm is proposed, which breaks down the route search into a number of individual searches in small sub-networks. The algorithm incorporates a heuristic layer-switching technique to improve its performance without compromising the accuracy. The hierarchical routing algorithm was tested on the road network of Singapore and the solutions were found to be comparable to the optimal least-cost paths. 相似文献
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Sung-Ho Youn Byung-June Mun Joun Ho Lee Byeong Koo Kim Hyun Chul Choi 《Journal of Modern Optics》2013,60(3):257-262
In general, light propagating an inhomogeneous liquid crystal (LC) cell can be modeled as ‘bundle rays’ because the LC cell consists of many birefringence layers. In order to calculate the optical path of the propagating light in the inhomogeneous LC cell, we multidimensionally calculated the wavevector, k, and the Poynting vector, S, of an ordinary and an extraordinary ray at LC grid interfaces, which are isotropic to a uniaxial medium and a uniaxial-to-uniaxial medium, by using the phase matching method. Furthermore, we also investigated the transmission coefficients and transmittance of the ordinary and the extraordinary rays as a function of difference of the optical axes of the facing birefringence medium at the interface to obtain the significant rays in the LC cell. Finally, we could calculate the exact path of the significant rays in the inhomogeneous LC cell, and compared the ray path in an electrically controlled birefringence (ECB) mode and a twisted nematic (TN) LC mode. 相似文献
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Transportation planning models are typically used to estimate future traffic patterns, peak period traffic, travel time, and various environmental or other related traffic flow characteristics. Unfortunately, traffic safety is seldom, if ever, explicitly considered proactively during the transportation planning process. This omission is attributed to various factors, including the lack of available tools needed to estimate the number of crashes during this process. To help fill this void, the research on which this paper is based aimed, as a primary objective, to develop a tool that would allow the estimation of crashes on digital or coded urban transportation networks during the planning process. The secondary objective of the research was to describe how the predictive models should be applied on these networks and explain the important issues and limitations surrounding their application. To accomplish these objectives, safety performance functions specifically created for this work were applied to two sample digital networks created with the help of EMME/2, a software package widely used in transportation planning. The results showed that it is possible to predict crashes on digital transportation networks, but confirmed the reality that the accuracy of the predictions is directly related to the precision of the traffic flow estimates. The crash predictions are also sensitive to how the digital network is coded, and it is shown how appropriate adjustments can be made. 相似文献
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奇异位形严重影响并联机器人机构的性能,有必要在获得机构奇异位形分布规律基础上进一步探讨机构的奇异规避问题.主要研究六自由度Gough-Stewart并联机构的奇异轨迹几何性质和无奇异路径规划.构建机构位置奇异轨迹方程,并指出机构位于一系列动平面上的位置奇异轨迹均为具有明显几何性质的二次曲线.基于上述奇异轨迹几何性质,当给定起始点和目标点时,给出机构无奇异运动路径存在与否的一般性判别方法;若存在无奇异运动路径,进一步给出机构位于动平面上的无奇异路径规划具体实现方法;以数值实例验证了上述方法的有效性.研究成果对并联机器人机构的奇异规避问题研究具有重要的理论意义和实际参考价值. 相似文献
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Freeform surfaces have been widely used in various engineering applications. Increasing requirements for the accuracy of freeform surfaces have led to significant challenges for the manufacturing of these surfaces. A method for manufacturing of freeform surfaces is introduced in this paper by integrating inspection and tool path generation to improve manufacturing quality while reducing manufacturing efforts. Inspection is conducted by comparing the digitised manufactured surface with the design surface to identify the error regions. In this new inspection technique, the areas on the manufactured surface that are beyond the design tolerance boundaries are used as the objective function during the localisation process, in order to minimise post-inspection machining efforts. The tool path generation methods are then selected based on the geometric characteristics of the identified error regions, for creating tool paths to remove the errors. Computational efficiency, machining efficiency, and quality are considered in this integrated method. 相似文献
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An efficient method for reliability evaluation of multistate networks given all minimal path vectors
The multistate networks under consideration consist of a source node, a sink node, and some independent failure-prone components in between the nodes. The components can work at different levels of capacity. For such a network, we are interested in evaluating the probability that the flow from the source node to the sink node is equal to or greater than a demanded flow of d units. A general method for reliability evaluation of such multistate networks is using minimal path (cut) vectors. A minimal path vector to system state d is called a d-MP. Approaches for generating all d-MPs have been reported. Given that all d-MPs have been found, the issue becomes how to evaluate the probability of the union of the events that the component state vector is greater than or equal to at least one of the d-MPs. There is a need for a more efficient method of determining the probability of this union of events. In this paper, we report an efficient recursive algorithm for this union probability evaluation based on the Sum of Disjoint Products (SDP) principle, and name it the Recursive Sum of Disjoint Products (RSDP) algorithm. The basic idea is that, based on the SDP principle and a specially defined “maximum” operator, “⊕”, the probability of a union with L vectors can be calculated via calculating the probabilities of several unions with L-1 vectors or less. The correctness of RSDP is illustrated. The efficiency of this algorithm is investigated by comparing it with an existing algorithm that is generally accepted to be efficient. It is found that RSDP is more efficient than the existing algorithm when the number of components of a system is not too small. RSDP provides us with an efficient, systematic and simple approach for evaluating multistate network reliability given all d-MPs. 相似文献
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A simple minimal path method for estimating the weighted multi-commodity multistate unreliable networks reliability 总被引:1,自引:0,他引:1
The weighted multicommodity multistate unreliable network (WMMUN) is a novel network composed of multistate unreliable components (arcs and nodes) capable of transmitting different types of commodities in which capacity weight varies with components. It is an extension of the multistate network. The current method for evaluating the directed WMMUN reliability has been derived from minimal cut (MC) based algorithm. The existing best-known method needed extensive comparison and verification, and failed to find the real directed WMMUN reliability. A very simple algorithm based on minimal paths (MPs) is developed for the WMMUN reliability problem. The correctness and computational complexity of the proposed algorithm will be analyzed and proven. An example is given to illustrate how the WMMUN reliability is evaluated using the proposed algorithm. The relationships among all different versions of MPs are also clarified. 相似文献