A new method for the placement of buffers in serial production lines

The placement of buffers in a production line is an old and well-studied problem in industrial engineering/operations research that is still relevant today. Decisions regarding the amount and placement of buffers in a production line occur repeatedly in system design. In this paper we document a new buffer placement method for serial production lines that operate under a variety of assumptions. The method uses information generated in a production line simulation, whose conceptual representation of job flow and workstation interaction can be described with a network, to place buffers in order to maximise throughput. This buffer placement method is very efficient and can be implemented in a spreadsheet. We demonstrate the effectiveness of the method by comparing our results against those produced by a genetic algorithm for buffer placement. Experiments are conducted on a variety of test cases. This new buffer placement optimisation method will permit designers to quickly and effectively evaluate many design alternatives and thus improve final production system performance.     

Flow-shop problems with intermediate buffers

In this paper the following extension of the classical flow-shop problem is considered: Between each two consecutive machines a buffer of limited capacity is given. After finishing processing on a machine, a job either directly has to be processed on the following machine or it has to be stored in the buffer between these machines. If the buffer is completely occupied the job may wait on its current machine but blocks this machine for other jobs. The objective is to determine a feasible schedule minimizing the makespan. To model such a problem setting, a variation of the classical disjunctive graph model for shop problems is extended. A tabu search procedure is described where neighborhoods based on an extension of the classical block approach theorem are used. Computational results for extended flow-shop benchmark instances are presented. Corespondence to: Silvia HeitmannThe authors are grateful to Tim Nieberg for implementing the tabu search procedure proposed in this paper.S. Heitmann     

漏电保护器测试仪的几种校准方法

文章介绍了漏电保护器的工作原理,对漏电保护器测试仪动作电、流跳闸时间和回路阻抗等主要参数,分析几种校准方法,并对各种方法的优缺点作了评价,同时还简单地介绍了各测试参数的不确定度的来源和评定结果。     

An improved decomposition method for the analysis of production lines with unreliable machines and finite buffers

We consider production lines consisting of a series of machines separated by finite buffers. The processing time of each machine is deterministic and all the machines have the same processing time. All machines are subject to failures. As is usually the case for production systems we assume that the failures are operation-dependent (Buzacott and Hanifin 1978, Dallery and Gershwin 1992). Moreover, we assume that the times to failure and the times to repair are exponentially distributed. To analyze such systems, a decomposition method was proposed by Gershwin (1987). The computational efficiency of this method was later significantly improved by the introduction of the so-called DDX algorithm [Dallery et al. 1988, 1989). In general, this method provides fairly accurate results. There are however cases for which the accuracy of this decomposition method may not be so good. This is the case when the reliability parameters (mean times to failure and mean times to repair) of the different machines have different orders of magnitude. Such a situation may be encountered in real production lines. The purpose of this paper is to propose an improvement of Gershwin's original decomposition method that provides accurate results even in the above mentioned situation. The basic difference between the decomposition method presented in this paper with that of Gershwin is that the times to repair of the equivalent machines are modeled as generalized exponential distributions (Kouvatsos 1986) instead of exponential distributions. This allows us to use a two-moment approximation instead of a one- moment approximation of the repair time distributions of these equivalent machines.     

Job-shop scheduling with limited capacity buffers

In this paper we investigate job-shop problems where limited capacity buffers to store jobs in non-processing periods are present. In such a problem setting, after finishing processing on a machine, a job either directly has to be processed on the following machine or it has to be stored in a prespecified buffer. If the buffer is completely occupied the job may wait on its current machine but blocks this machine for other jobs. Besides a general buffer model, also specific configurations are considered. The aim of this paper is to find a compact representation of solutions for the jobshop problem with buffers. In contrast to the classical job-shop problem, where a solution may be given by the sequences of the jobs on the machines, now also the buffers have to be incorporated in the solution representation. In a first part, two such representations are proposed, one which is achieved by adapting the alternative graph model and a second which is based on the disjunctive graph model. In a second part, it is investigated whether the given solution representation can be simplified for specific buffer configurations. For the general buffer configuration it is shown that an incorporation of the buffers in the solution representation is necessary, whereas for specific buffer configurations possible simplifications are presented.     

An efficient analytical method for performance evaluation of transfer lines with unreliable machines and finite transfer-delay buffers

Buffers are widely adopted in transfer lines to reduce the fluctuations caused by the imbalances of systems or machine failures. This paper presents an efficient analytical method to evaluate the performance of transfer lines with unreliable machines and finite transfer-delay buffers. Firstly, the buffers with transfer delays are transformed equivalently into a series of perfect machines and buffers without transfer delays. Correspondingly, the initial transfer line is replaced by an equivalent transfer line with more machines and zero-transfer-delay buffers. Since in the equivalent transfer line the orders of magnitude of machines’ reliability parameters (mean times between failures and mean times to repair) are not at the same level, an advanced decomposition method is introduced to analyse the equivalent transfer line, using the general-exponential distributions instead of the exponential distributions to approximate the repair time distributions of the fictitious machines. Finally, extensive simulation and numerical cases are carried out to verify the performance of the developed method.     

Planning of buffers within mixed-model lines

Summary This paper deals with the planning of buffers within a mixed-model line, which is typical of vehicle industries. The positioning of the buffers will be of special interest and will be discussed by an exact method and three heuristics. In addition the determining of buffer sizes will briefly be addressed. All aspects are considered concerning their influence on scheduling criteria. We will demonstrate that the placing of buffers leads to an improvement in job sequencing problems. In particular a balanced capacity usage will be achieved.     

Strain relaxation in metamorphic InAlAs buffers

The strain relaxation in a metamorphic In_xAl_1−xAs buffer placed on top of an AlAs/GaAs superlattice (SL) was analyzed using high-resolution X-ray diffraction and compared to a reference sample containing no SL. Pole figures were constructed to characterize the strain relaxation and twist in the metamorphic buffer layers, AlAs/GaAs SL and GaAs substrate. Lattice mismatch induced strain within such heterostructure causes tilt angles of the layers inside the buffer to rotate around the surface normal. The strong disorder observed in the AlAs/GaAs superlattice supports our previous finding that an AlAs/GaAs SL in the virtual substrate is important for strain relaxation on the substrate side.     

An improved decomposition method for evaluating the performance of transfer lines with unreliable machines and finite buffers

In this paper, we consider transfer lines consisting of a series of machines separated by finite buffers. The processing time of each machine is deterministic and may be not identical. All machines are prone to operation-dependent failures, and the times between failures and the times to repair are assumed to be exponentially distributed. Many analytical methods have been developed to evaluate the performance of such lines. In general, these methods provide fairly accurate results. However, in some real cases where the orders of magnitude of machines’ reliability parameters (mean times between failures and mean times to repair) are not at the same level, the accuracy of these existing methods may not be good enough. The purpose of this paper is to propose an improved decomposition method that performs well even in the situation above. We use generalised exponential distributions instead of exponential distributions to approximate the repair-time distributions of the fictitious machines, and a new ADDX algorithm is developed to calculate the performance parameters such as the production rate and the average buffer levels. Numerical results indicate that the improved decomposition method provides more accurate results and converges in most cases. It is feasible and valid to evaluate the performance of transfer lines with unreliable machines and finite buffers.     

An algorithm analog-to-digital converter using unity-gain buffers

An algorithmic stage for bipolar 1-b analog-to-digital (A/D) conversion using a unity-gain buffer is proposed. Cyclic and pipeline A/D converter architectures using this stage iteratively or in cascade are also described. Error analysis and SPICE simulations show that a conversion accuracy higher than 8-b and a conversion rate up to 10 Mb/s are attainable with presently available 3-μm CMOS technologies. Video frequency operation is also possible with finer linewidths. The component requirement is minimum, and thus it is best suited for an analog interface in application-specific integrated circuits (ASIC). A prototype cyclid A/D converter built using discrete components confirms the principles of operation     

一种基于代理服务的位置隐私保护方法

针对无线移动网络中通过代理用户进行位置服务时,不可信赖的代理用户可能带来隐私泄露和区域匿名查询造成通信代价过高等问题,提出了kAgPrivacy方法.首先,查询用户通过用户之间协作形成匿名组,然后将匿名组形成的泛化区域代替真实单一位置发送给代理用户进行位置服务.这样既避免用户和位置服务商的直接联系,也避免了代理用户的不可信性造成的隐私泄露问题.同时,采用增量近邻查询,保证了查询结果准确性;采用Voronoi图技术进行查询返回结果过滤,可减少系统通信开销.最后通过和其它算法比较,验证了本文方法的有效性.     

Dielectric cell response in highly conductive buffers

We present a novel method for the identification of live and dead T-cells, dynamically flowing within highly conductive buffers. This technique discriminates between live and dead (heat treated) cells on the basis of dielectric properties variations. The key advantage of this technique lies in its operational simplicity, since cells do not have to be resuspended in isotonic low conductivity media. Herein, we demonstrate that at 40 MHz, we are able to statistically distinguish between live and dead cell populations.     

How lean can lean buffers be?

This paper provides a quantitative characterization of the smallest, i.e., lean, buffer capacity necessary and sufficient to attain a desired throughput in serial production lines with identical exponential machines. The development is carried out in terms of normalized buffer capacity and production line efficiency. The smallest normalized buffer capacity required to ensure the desired line efficiency is referred to as the Lean Level of Buffering (LLB). Exact formulas for the LLB in two- and three-machine lines are presented and an approximate expression for the LLB in lines with more than three machines is derived. Along with these analytical results, several qualitative insights into the nature of lean buffering in serial production lines are presented.     

Scheduling flexible flow lines with no in-process buffers

An heuristic algorithm is proposed for scheduling a flexible flow line with no intermediate buffers. The line is made up of several processing stages in series, where each stage has one or more identical parallel machines. In the line different part types can be manufactured simultaneously, each of which is processed by at most one machine in every stage. Intermediate queues of parts waiting between the stages for their next operations are not allowed. The problem objective is to minimize the makespan of the schedule for a set of part types selected for processing. The algorithm proposed is a part-by-part heuristic, in which during every iteration a complete processing schedule is determined for one part type selected for loading into the line. The selection of the part type and its complete schedule are based on the cumulative partial schedule obtained for all parts selected so far. The decisions in every iteration are made using a local optimization procedure aimed at minimizing total blocking and waiting time of the machines along the route of the selected part type. The efficiency of the algorithm is tested on several groups of random test problems     

Development of hybrid ROCOF and RPV method for anti-islanding protection

ABSTRACT

Distributed generation (DG) must have the capability to quickly and accurately detect islanding operations under various operating conditions. An ideal islanding detection method should be without a non-detection zone (NDZ) and exempt from deteriorating power quality. A novel Anti-islanding protection technology that is a hybrid of rate of change of frequency (ROCOF) and reactive power variation (RPV) is proposed in this paper. The method can differentiate between a DG that falls into the NDZ for ROCOF and one that does not, and the RPV method will be initiated when the DG is within the NDZ. The amplitude of the reactive power variable can be accurately calculated using the proposed method. Thus, the degree of deterioration of power quality and power consumption can be effectively limited. The method has been implemented on a 3 kW DG to prove its feasibility. The experimental results demonstrated that in the proposed method, the DG was able to use the minimal amplitude of RPV to detect islanding in the NDZ. The breaker tripping time under various operating conditions conformed to the criteria specified in IEEE Std. 1547.     

Static and dynamic analysis of composite shear walls by the boundary element method

Summary A boundary element solution for the static and dynamic analysis of composite shear walls is developed. The composite shear wall consists of a matrix material surrounding a finite number of inclusions (in-fill walls) with different elastic constants firmly bonded to it. The problem is formulated in terms of the displacement components and all kinds of boundary conditions are encountered. Several numerical examples are worked out to demonstrate and attest the efficiency of the method. The results, whenever possible, are compared with those obtained from analytical or other numerical solutions. The case of homogeneous shear wall with or without holes is included as a special case.With 8 FiguresPresented at the 1st Nat. Cong. on Mechanics of the Hellinic Society for Theoretical and Applied Mechanics (HSTAM), Athens, June 1986.