The equivalence of duplicate automated serial workstations and two-workstation tandem systems and its use in serial production line analysis

In automated serial production systems it is sometimes necessary to maintain two identical workstations in series, with jobs receiving processing on only one of these two workstations. This is required because a single workstation may not have sufficient capacity to support the target throughput rate and due to cost, space, and sequencing reasons, the workstations are not arranged in parallel. We show how these duplicate automated serial workstations can be approximated by a two-workstation tandem system with zero buffer. The parameters of the tandem system are simple functions of the duplicate workstation parameters. In isolation we show that the tandem system is equivalent to the duplicate workstation system. When part of a larger serial production system, the equivalence no longer holds however, simulation testing shows that a two-workstation tandem system approximation to the duplicate workstation system is very accurate. This extends the analysis capabilities of analytical tools and simulations that are tailored for serial production systems, but do not explicitly model duplicate workstations.     

The effects of skewness and kurtosis of processing times in unpaced lines

Numerous empirical studies have shown that, in unpaced lines, the stations' processing times have a variety of shapes (i.e., skewness and kurtosis))65. Thispaper investigates how the line's utilization factor is affected by these skewness and kurtosis measures. We found that positive skewness has a significant negative effect on utilization, and that the magnitude of this effect is greater if the line has more stations or less buffer capacity, or if the processing-times' cv is larger. Kurtosis also affects utilization significantly, but the effect may be negative or positive, depending upon the processing-times' skewness and the number of stations in the line. Our findings imply that when simulating long lines with small buffer capacities, it may be necessary to model more accurately the actual processing times. Also, the common practice of using normal distributions to approximate the typically positively-skewed processing times often leads to an overestimation of the actual utilization rate.     

ANALYTICAL TOOLS FOR ASSEMBLY LINE DESIGN AND SEQUENCING

Many complex assembly lines, such as those in the automobile industry, have dozens or hundreds of stations that are affected by customer-selected options on the jobs being assembled. The various options often require significantly different amounts of processing time, and the role of assembly line sequencing in this context is to smooth out the flow of work to each station. However, most assembly line sequencing algorithms developed for such situations cannot consider so many stations or options effectively. In this paper, we develop an analytical method to compute a criticality index for each station, which can be used to determine which stations are most important to include in an assembly line sequencing algorithm. We report computational results using actual industry data which indicates that substantial improvements can be obtained by selecting stations based upon this criticality index.     

Spc of a near zero-defect process subject to random shocks

In this paper we study the problem of monitoring and control of a type of process in which long series with no non-conformities are observed together with occasional samples containing a large number of non-conformities. We call this a near zero-defect process subject to random shocks. Such processes occur often in practice, and a model is proposed for the identification of real non-random variations of process characteristics. Based on the statistical analysis carried out for this model, a procedure for decision-making in the control of this type of process is suggested, and analysis of some actual cases presented.     

The formation of subfamilies for machine loading of flow-line cells

Cellular manufacturing constitutes a key element of contemporary approaches to manufacturing such as just-in-time and flexible manufacturing systems. In some settings, it may not be possible to release all members of a family to a cell at the same time due to tooling, level of the in-process inventory and limited machine buffer capacity constraints. When such constraints exist, one approach is to partition each family into subfamilies.

In this research, we consider the effective partitioning of a family of parts into subfamilies in cyclically scheduled cells involving unidirectional material flow between machines laid out in a loop and where capacity constraints on tooling are present. The criterion for forming subfamilies is the minimization of the total machine idle time. This criterion maximizes the rate of output of the cell as well as maximizing machine utilization. A batching approach to part and tool change is utilized and all tool magazine set-ups are assumed lo occur off-line while the cell is in operation.

We present a model for the formation of subfamilies using the above criterion and develop four heuristic approaches based on it. We then examine their effectiveness by conducting computational experiments over a wide range of realistic situations generated by varying the characteristics of the operating environment, tooling requirements and tool magazine capacity.     

Scheduling of wafer test processes in semiconductor manufacturing

This research focuses on solving a common wafer test scheduling problem in semiconductor manufacturing. During wafer testing, a series of test processes are conducted on wafers using computer-controlled test stations at various temperatures. The test processes are conducted in a specified order on a wafer lot, resulting in precedence constraints for the schedule. Furthermore, the assignment of the wafer lots to test stations and the sequence in which they are processed affects the time required to set up the test operations. Thus, the set-up times are sequence dependent. Four heuristics are developed to solve the test scheduling problem with the aim of minimizing the makespan required to test all wafers on a set of test stations. The heuristics generate a sorted list of wafer lots as a dispatching sequence and then schedule the wafer lots on test stations in order of appearance on the list. An experimental analysis and two case studies are presented to validate the proposed solution approaches. In the case studies, the heuristics are applied to actual data from a semiconductor manufacturing facility. For both case studies, the proposed solution approaches decrease the makespan by 23–45% compared with the makespan of the actual schedule executed in the manufacturing facility.     

Free-transfer lines with unreliable assembly stations and small buffers

This paper describes the arrangement in series of assembly stations that are linked by storage places. The distribution of the stations and of the number of places in the stocks affects the efficiency of the assembly. The results of the continuous model show that grouping stations by two or three does not improve the effectiveness of the line when the number of places is limited. On the other hand, the discrete synchronous model proved that it is profitable to group stations when a low capacity of storage is experienced; these results have been confirmed using a simulation software which processes the components in a discrete manner.     

Optimizing the yard layout in container terminals

The main activities of container terminals are to load outbound containers on to vessels, discharge inbound containers from vessels, and store those containers in the yard before loading (or after discharging) them. This study proposes a method for determining an optimal layout of container yards taking into consideration the storage space requirements and throughput capacities of yard cranes and transporters. Two types of yard layout are under consideration: a layout where blocks are laid out parallel to the quay and transfer points are located beside a bay per block, and a layout whose blocks are laid out perpendicular to the quay and transfer points are located at both ends of each block. Various cost factors are used for optimizing the yard layout, which include the construction cost of the ground space, the fixed overhead cost of yard cranes, and the operating costs of yard cranes and transporters. Sensitivity analysis is performed to investigate the effect of various design parameters on the optimal layout of the yard. The two types of yard layout are compared with each other, and the results of this study are compared against those for real-world yard layouts from the perspective of throughput capacity and storage space capacity.     

Production control policy for tandem workstations with constant service times and queue time constraints

Queue time constraints are commonly imposed to ensure product quality in contemporary production systems. We study the performance of two single stations with deterministic service times and a predetermined time window in between, where both stations suffer time-based pre-emptive breakdowns. To improve productivity, achieving higher capacity and lower rework rate are the two main objectives. While higher capacity requires a higher work-in-process (WIP)-level threshold, a lower rework rate requires a smaller one. To quantify the trade-off between the two objectives, an analytical model is derived. The model is then used to derive the WIP-level threshold control policy for a time-constrained system. We also show that system capacity diminishes with the decrease in WIP-level thresholds.     

Similarity among tandem mass spectra from proteomic experiments: detection,significance, and utility

Liquid chromatography paired with tandem mass spectrometry is a standard technique for identifying peptides from complex protein mixtures. Most fragment ion spectra acquired by this technique are unique, but some are repeated. Similarities among the spectra from 1D and 2D liquid chromatography experiments were calculated by the dot product algorithm. Similar spectra were grouped, and the degree of duplication was calculated for each sample. In 1D liquid chromatography data from 1D gel bands, 18% of the fragment ion spectra were duplicates. A six-cycle 2D liquid chromatographic separation of more than 200 proteins produced 28% duplicate spectra. A rat hippocampal homogenate analyzed by a 12-cycle 2D liquid chromatographic separation contained 25% duplicate spectra. Removal of these duplicate spectra, however, resulted in fewer peptides being successfully identified by SEQUEST. We propose a modification for peptide identification algorithms that would improve their performance and accuracy by explicitly recognizing and making use of spectral similarity.     

More Accurate Prediction Intervals for Exponential Smoothing with Covariates with Applications in Electrical Load Forecasting and Sales Forecasting

The most widely used prediction intervals in empirical time series analysis are of plug‐in type; that is, the empirical estimates of model parameters are inserted into formulae for prediction intervals that are obtained from a theoretical analysis of the time series model. Several authors have pointed out that such model‐based prediction intervals are too narrow, that is, that the actual confidence level is smaller than the nominal confidence level. The reason is that the uncertainty contained in the parameter estimates is not taken into account in the prediction interval. We investigate this problem for exponential smoothing under covariates with additive trend and additive season. We determine alternative prediction intervals by analysing a linearisation of the underlying model with linear model methods. Two simulation studies based on electricity load data and on sales data confirm the reservations about the plug‐in prediction intervals, whereas the intervals based on the linearisation approach are significantly better, and perform very well, with actual confidence levels close to the nominal. Copyright © 2014 John Wiley & Sons, Ltd.     

Method for designing error-compensating phase-calculation algorithms for phase-shifting interferometry

We propose a general approach to eliminating some error source effects in phase-calculation algorithms for phase-shifting interferometry. We express the actual phase shift in a convenient form that takes the errors into account and develop in series the detected phase from a generic algorithm. Setting to zero the terms of the series that involve unwanted errors leads to a set of linear equations for the algorithm coefficients, which can thus be found. By using this approach, one could develop an algorithm series for an individual interferometer based on relevant concerns about the main error sources in it and eliminate the error source effects to any desired order. Two examples of algorithm series, to eliminate distorted phase shifts caused by the geometric effect in an interferometer with a spherical Fizeau cavity and to eliminate vibration effects, are discussed.     

Performance evaluation and capacity planning in a metallurgical job-shop system using open queueing network models

In this paper, we apply performance evaluation and capacity allocation models to support decisions in the design (or redesign) and planning of a job-shop queueing network of a metallurgical plant. Approximate parametric decomposition methods are used to evaluate system performance measures, such as the expected work-in-process (WIP) and production leadtimes. Based on these methods, optimisation models are then applied for the allocation (or reallocation) of capacity to the stations of the job-shop network. These models are also used to generate approximate trade-off curves between capacity investment and WIP or leadtime, which are valuable for a production manager to estimate how much capacity should be allocated to the stations to reach some targeted performance measures. These curves are also useful for the sensitivity analysis of the solutions to changes in the input parameters, such as the variability of the product demands, the mix of the production and the throughput rate of the network.     

The operating characteristics of‘ single’ and ‘ double’ non-mechanical flow line systems

Two types of ‘ series queue ’ type flow-lines are simulated. Both are balanced, mechanically unpaced systems with provision for interstage buffer stocks. The effects of buffer capacity, service-time variability and number of stations on the output efficiency of both types of line are examined and comparisons are made.     

PHC桩的竖向极限承载力的预测

基于理论与实测分析,提出了用逐次线性概似法加上残差修正,利用数量较少的沉降观测数据来预测PHC桩(超高强预应力管桩)的竖向极限承载力.所附的大量工程实例的分析表明,这个方法精度较高,可操作性较强.     

Performance of infra-plant overtime policies

Production quotas smooth production in order to synchronize inter-plant production networks. To meet quotas, plants work overtime whenever they're behind. In addition to this 'mandatory' overtime required to satisfy the quota of finished product at the end of the line, many plants run selected stations within the plant overtime to fill critical buffers within the plant. We call this intra-plant overtime 'preventive' overtime because by spending a little overtime this shift on critical stations, the plant can avoid massive overtime next shift. We describe this real production control problem currently facing industry. The problem arises because of (often underestimated or ignored) production variability. We exhibit actual production data that shows the degree of variability often inherent in current manufacturing processes. We describe four policies and compare them using simulation.     

Nuclear energy: current situation and prospects to 2020

For close to half a century nuclear fission has been providing reliable supplies of electricity to the UK, with virtually no emissions of carbon dioxide. Over that period, the UK nuclear industry has avoided the emission of over one and a half billion tonnes of CO2. Yet no nuclear plant has been built in the UK for over two decades even though many of the stations in our current fleet are now within a decade or so of the end of their lifetime. Without new plants being ordered soon, the UK's nuclear capacity will decline dramatically, from 23% today to 3% post-2020--just as considerations of supply security and climate change are becoming increasingly important. Elsewhere in the world, many countries such as China, India, Japan, South Korea, Finland and France are building new stations. Other countries such as the USA, South Africa, and some nations that currently do not have nuclear stations (such as Indonesia and Poland) are making preparations for future nuclear stations. Globally capacity factors for nuclear plants are higher than they have ever been, averaging around 85% and with the best stations achieving well over 90%. Lifetime can be 60 years. That the economics of such stations compete well with other technologies is well founded and easily verifiable--especially in the face of rising fossil fuel prices and the pricing in of costs for CO2 emissions--both of which stand to improve the economics of nuclear energy still further. Waste volumes arising from modern plants are just a fraction of those of some earlier stations, and the technologies are in place to deal with them safely and effectively. Following recent reviews and international developments, there is growing confidence that internationally available competitive designs of nuclear plant will provide part of the solution to the UK's long-term energy needs.     

两类排队网络的逼近研究

本文给出了两类排队网络。一类是容量有限的队列网络,我们证明了在高负荷下,标准化的队长过程弱收敛于半鞅反射的布朗运动：另一类是带有反馈的多类顾客多服务台队列网络,我们获得了队列网络中负荷过程的扩散逼近。     

A new method for online monitoring when grinding Ti-6Al-4V alloy

This study proposes a grinding process from a control perspective to improve surface quality and precision in the machining process and speed up intelligent production. This method provides a cognitive decision-making process. On this basis, a new approach was developed to measure the amount of grinding wheel wear (GWW) and predict surface roughness (SR) in accordance with the compressed air measuring head and hybrid algorithms fuzzy neural networks (ANFIS)–Gaussian regression function (GPR) and Taguchi empirical analysis. A series of experiments were conducted in various processing conditions. The results showed the efficiency of the grinding process in measuring the amount of GWW and predicting the SR of the Ti–6Al–4V alloy accurately. The proposed model is able to predict SR values with 99.69% precision and 98% confidence interval. This study laid the foundation for monitoring GWW and SR in actual industrial environments.     

下沉式冷却塔对周边环境影响的模拟分析

本文通过采用k-ε双方程紊流模型对某城市下沉式冷却塔运行时的压力场、速度场和温度场进行模拟，对下沉式冷却塔运行时对周边环境的影响进行了定量分析和评价，对同类型工程具有借鉴意义。