Some problems with transient phenomena when simulating unpaced lines

Unlike earlier simulation studies of unpaced production lines which generally concentrated on steady-state behavior, we are concerned with their transient characteristics. This paper (1) presents an approach for studying and monitoring transient behavior, (2) reports some important transient operating characteristics previously overlooked, and (3) demonstrates that the conflicting results reported in some of the earlier simulation studies are due to an inadequate accounting for transient behavior. Thus, this paper illustrates the importance of having a better understanding of transient behavior in unpaced lines, and presents our initial steps towards dealing with it.     

A cost-effective approach to stochastic line balancing with off-line repairs

The layout of production facilities is an important determinant of the productivity potential of a manufacturing enterprise. It is particularly important in the design of assembly lines where the objective is to assign tasks to work stations in such a way as to minimize total variable production costs.Early approaches to the line balancing problem assumed known constant task times and sought a line layout which would produce the desired output with the fewest number of work stations, which is equivalent to minimizing idle time. Studies have shown that task times are random variables, therefore the cost of task incompletion must be considered a part of total production cost. Incompletion cost will be the cost of repairing or completing tasks which cannot be completed within the cycle time after the item has reached the end of the assembly line.This paper describes a methodology for designing approximately minimum cost paced assembly lines under conditions of random task times and off-line repair of incompleted tasks. Task times are assumed to be normally distributed random variables with known means and variances. The methodology consists of heuristically identifying a large number of feasible balances for each of which total costs are computed. The line design with the lowest total is retained as the “best.”In order to evaluate candidate line layouts, a total cost model is developed. Total cost is the sum of normal operating cost—which is simply a function of the number of work stations—and the cost of repairing products containing incompleted tasks. Because this latter cost is a random variable for a given balance, the expected value is used to evaluate a candidate layout. The cost associated with one or more workers exceeding the cycle time is the product of the probability of this happening and the expected cost of off-line repair.The heuristic method for generating feasible balances builds work stations from continually updated lists of precedence satisfying tasks. Qualifying tasks are added to the station as long as the probability of the station exceeding the cycle time remains below a pre-specified threshold. The methodology requires systematically varying this threshold to permit a lowest total cost solution to emerge. The process of generating a large number of balances for a particular threshold is efficient. Evaluating the total costs of the resulting balances takes the majority of the computational time.An experiment was conducted in order to compare the above cost-effective methodology with a purely deterministic approach and a commonly used industrial approximation method for dealing with task time variability. The experiment applied the three methods to four problems from the literature under a variety of repair cost and time variance conditions. In 21 of the 24 cases studied, the stochastic method produced a lower cost balance than the two alternatives. In the remaining 3 cases, the deterministic method also found the lowest cost balance. The stochastic method saved an average of 22.5 percent in total operating cost over the deterministic method and 8.4 percent over the industrial method.The experiment clearly showed the need to explicitly consider task time variability in arriving at a line balance. The stochastic approach of this paper offers large potential savings with no risk of obtaining a less desirable balance and so should be considered for implementation whenever there is a variation in task times. Even for large-scale problems, the computational cost is infinitesimal in the context of assembly line balancing, where very small improvements in productivity can mean substantial increments to profitability.     

Generalized confidence intervals for process capability indices in the one-way random model

The method of generalized confidence intervals is proposed as an alternative method for constructing confidence intervals for process capability indices under the one-way random model for balanced as well as unbalanced data. The generalized lower confidence limits and the coverage probabilities for three commonly used capability indices were studied via simulation, separately for balanced and unbalanced cases. Simulation results showed that the generalized confidence interval procedure is quite satisfactory both in the balanced and unbalanced cases. Examples are provided to illustrate the results.     

Optimal Growth with Public Capital and Public Services

We characterize optimal economic growth in an endogenous growth model in which production requires public capital (a stock) and public services (a flow) in addition to private capital and labor. We analyze the comparative static effects of changes in the fundamental technological and preference parameters of the model on the optimal values of several variables, such as the optimal rate of growth and the optimal allocation of resources among consumption, the provision of public services, and investment in public and private capital. We show that the general optimal path converges in finite time to the balanced growth optimal path. We relate our paper to important contributions to the existing literature by obtaining them as special cases of our model.     

A cost evaluation method for dynamic line balancing

A procedure is presented for calculating stochastic costs, which include operator (labor) and inventory costs, associated with dynamic line balancing. Dynamic line balancing, unlike the traditional methods of assembly and production line balancing, assigns operators to one or more operations, where each operation has a predetermined processing time and is defined as a group of identical parallel stations. Operator costs and inventory costs are stochastic because they are functions of the assignment process employed in balancing the line, which may vary throughout the balancing period, and the required flow rate. Earlier studies focused on the calculation of the required number of stations and demonstrated why the initial and final inventories at the different operations are balanced.The cost minimization method developed in the article can be used to evaluate and compare the assignment of operators to stations for various assignment heuristics. Operator costs and inventory costs are the components of the cost function. The operator costs are based on the operations to which operators are assigned and are calculated for the entire work week regardless of whether an operator is given only a partial assignment which results in idle time. It is assumed that there is no variation in station speeds, no learning curve effect for operators' performance times, and no limit on the number of operators available for assignment. The costs associated with work-in-process inventories are computed on a “value added” basis. There is no charge for finished goods inventory after the last operation or raw material before the first operation.The conditions which must be examined before using the cost evaluation method are yield, input requirements, operator requirements, scheduling requirements and output requirements. Yield reflects the output of good units at any operation. The input requirement accounts for units discarded or in need of reworking. The operator requirements define the calculation of operator-hours per hour, set the minimum number of operators at an operation, and require that the work is completed. The scheduling requirements ensure that operators are either working or idle at all times, and that no operator is assigned to more than one operation at any time. The calculation of the output reflects the yield, station speed, and work assignments at the last operation on the line.An application of the cost evaluation method is discussed in the final section of the article. Using a simple heuristic to assign operators, the conditions for yield, inputs, operators, scheduling, and output are satisfied. The costs are then calculated for operators and inventories.In conclusion, the cost evaluation method for dynamic balancing enables a manager to compare the costs of assigning operators to work stations. Using this method to calculate the operator and inventory costs, a number of different heuristics for assigning operators in dynamic balancing can be evaluated and compared for various configurations of the production line. The least cost solution procedure then can be applied to a real manufacturing situation with similar characteristics.     

Adaptive algorithms for maximizing overall stock return

Deciding which stocks to purchase and how to optimally allocate the total investment among them is a nontrivial task for every investor. In this article, we propose two adaptive techniques that would provide an optimal allocation maximizing the return over the investment period. The first approach is the adaptive power method (PM) which is a modification of the proper orthogonal decomposition method. The adaptive PM uses only the currently available information to compute the optimal allocations, yet its long-term solution approaches the dominant eigen solution, even though that solution would require having a priori knowledge of all stocks’ performance. The second approach is derived from the well-known Least Mean Square (LMS) method, where the optimal allocation can be computed by adaptively steering the overall return toward a desired value. The experimental results have indicated promising gains even when the general market trend is downward.     

企业生产物流系统的建模与仿真

采用Petri网建立了某微型汽车厂总装车间的生产物流模型,并用VisualBasic语言对其生产线进行仿真。从而发现在生产不同的车型时,总装车间的三条流水线的节拍不一致,在生产空调车时,为了满足生产的平衡性,不仅要细化超工位装配的工序,而且需要对车间的人员安排进行调整。     

社会核算矩阵平衡方法研究——最小二乘交叉熵法

针对初始不平衡SAM与真实SAM关系未知的情形,本文提出了最小二乘交叉熵(LSCE)平衡法。基于最小二乘法(LS)、交叉熵法(CE)以及LSCE方法的仿真分析表明,CE与LS的相对稳健性取决于初始不平衡SAM的误差特征:当初始不平衡SAM的交易流量更接近于真实SAM时,LS较优;当初始不平衡SAM的系数矩阵更接近于真实SAM时,CE较优。LSCE方法同时考虑了SAM表流量和系数矩阵信息,故可得到精度介于LS和CE间的平衡SAM表,从而保证了平衡后SAM表的相对精度。     

基于IE技术的笔记本电脑生产线平衡优化

文章通过对A公司笔记本电脑组装生产线进行研究,发现其存在生产线平衡率不高的问题。然后,根据生产线平衡的步骤及方法,运用统计分析的x-σ管制方法和ECRS方法,重新确定了工时,调整了部分作业工序内容,使各工序节拍趋于一致。     

考虑役龄阈值的最佳缓冲区库存维修策略研究

预防性维护PM对于生产系统有重要的作用。由于设备维护时会造成生产停滞,影响企业的经济效益,所以在生产线上下游设备之间建立库存缓冲区,保证生产的持续进行。考虑引入设备役龄阈值,构建费用模型和维护策略模型。以设备役龄阈值和缓冲区库存量为自变量,以最小化周期内总费用为目标,获得最佳的设备役龄阈值和缓冲区库存量。最后运用Matlab仿真软件进行了算例分析,验证了模型的有效性。     

The golden rule of urban transportation investment

The purpose of this paper is to find an optimal taxation rule for transportation investment in an ever growing urban economy. First we dynamize a standard circular-city model with identical residents by introducing population growth and transportation improvements over time. Assuming that utility functions are of a constant-elasticity form and transportation investment is financed by an income tax, we prove the existence, uniqueness and stability of a balanced growth equilibrium for each given tax rate. Then, an optimal tax rate is determined so as to maximize the balanced growth equilibrium level of utility for every resident in the city. It is also shown that our simple rule remains valid in the case of two income classes.     

地铁列车初始布点的鲁棒性研究

针对车辆段内干扰源对地铁列车运行的扰动,以列车连带晚点期望值最小为目标,确定地铁列车平峰时段的始发时刻,分配初始缓冲时间。细分连带晚点,详细描述了连带晚点传播的各种情况,建立了简单情形下地铁列车初始布点的理论模型。经分析,该模型为凸规划的约束极值问题,且证明在小干扰条件下,均衡初始布点的列车时刻表为模型的最优解。     

A closed-form solution for the continuous-time consumption model with endogenous labor income

In this paper, we study the consumption, labor supply, and portfolio decisions of an infinitely lived individual who receives a certain wage rate and income from investment into a risky asset and a risk-free bond. Uncertainty about labor income arises endogenously, because labor supply evolves randomly over time in response to changes in financial wealth. We derive closed-form solutions for optimal consumption, labor supply and investment strategy. We find that deferring the retirement age stimulates optimal consumption over time and discourages optimal labor supply during the working life. We also find explicitly that optimal portfolio allocation becomes more ‘conservative’ when the individual approaches his prescribed retirement age. The effects of risk-aversion coefficients on optimal decisions are examined.     

Optimal monetary policy under incomplete markets and aggregate uncertainty: A long-run perspective

This paper examines the role of monetary policy in an environment with aggregate risk and incomplete markets. In a two-period overlapping-generations model with aggregate uncertainty, optimal monetary policy attains the ex-ante Pareto optimal allocation. This policy aims to stabilize the savings rate in the economy by changing real returns of nominal bonds via variation in expected inflation. Optimal expected inflation is procylical and on average higher than without uncertainty. Simple inflation targeting rules closely approximate the optimal monetary policy.     

要素重置、经济增长与区域非平衡发展

研究目标：要素错配对经济增长和区域非均衡发展的影响。研究方法：运用提出的测算框架将经济增长分解为部门全要素生产率、要素禀赋和资源配置效率,并通过构建反事实框架重点探讨要素错配如何影响区域经济的非平衡发展。研究发现：2000~2013年要素再配置仅能解释中国经济增长的9.2%,中国经济增长主要依赖于要素积累和部门全要素生产率;部门全要素生产率、要素错配和要素禀赋分别可以解释区域非平衡发展的56.5%、30.7%和12.8%;要素完全有效配置将使得人均真实收入年均增加31.4%,但提升空间趋于缩小。研究创新：拓展传统经济增长分解方法并构建反事实框架研究要素错配的经济效应。研究价值：有助于深入理解要素配置在经济增长和区域非平衡发展中的重要性。     

Multi-level and mixed-level <Emphasis Type="Italic">k</Emphasis>-circulant supersaturated designs

Supersaturated designs (SSDs) constitute an important class of fractional factorial designs that could be extremely useful in factor screening experiments. Most of the existing studies have focused on balanced designs. This paper provides a new lower bound for the \(E(f_{NOD})\)-optimality measure of SSDs with general run sizes. This bound is a generalization of existing bounds since it is applicable to both balanced and unbalanced designs. Optimal multi and mixed-level, balanced and nearly balanced SSDs are constructed by applying a k-circulant type methodology. Necessary and sufficient conditions are introduced for the generator vectors, in order to pre-ensure the optimality of the constructed k-circulant SSDs. The provided lower bounds were used to measure the efficiency of the generated designs. The presented methodology leads to a number of new families of improved SSDs, providing tools for directly constructing optimal or nearly-optimal k-circulant designs by just checking the corresponding generator vector.     

Eliciting the socially optimal allocation from responsible agents

We consider designing a mechanism to allocate objects among agents without monetary transfers. There is a socially optimal allocation, which is commonly known by the agents but not observable by the designer. The designer possibly has information about the existence of responsible agents. A responsible agent, when indifferent between his objects at two different allocations, prefers the first allocation to the second if the first allocation is closer to the optimal allocation than the second, in the sense that all the agents who are allocated their optimal objects in the second allocation are allocated their optimal objects also in the first allocation, and there is at least one more agent in the first allocation receiving his optimal object. We show that, if the designer knows that there are at least three responsible agents, even if the identities of the responsible agents are not known, the optimal allocation can be elicited.     

Measure for Measure: Exact F Tests and the Mixed Models Controversy

We consider exact F tests for the hypothesis of null random factor effect in the presence of interaction under the two factor mixed models involved in the mixed models controversy. We show that under the constrained parameter ( CP ) model, even in unbalanced data situations, MSB/MSE (in the usual ANOVA notation) follows an exact F distribution when the null hypothesis holds. We also obtain an exact F test for what is generally (and erroneously) assumed to be an equivalent hypothesis under the unconstrained parameter ( UP ) model. For unbalanced data, such a corresponding test statistic does not coincide with MSB/MSAB (the test statistic advocated for balanced data cases). We compute the power of the exact test under different imbalance patterns and show that although the loss of power increases with the degree of imbalance, it still remains reasonable from a practical point of view.     

On the smoothness of optimal paths II: some local turnpike results

We study the local turnpike property for two classes of infinite-horizon discrete-time deterministic maximization problems which have common applications, e.g., optimal growth theory. We follow a functional-analytic approach and rely on an implicit function theorem for the space of the sequences which converge to zero. We shall assume the existence of an optimal path which is not necessarily a steady-state. Relying on material developped in Blot and Crettez (Decis Econo Finance 27:1–34, 2004), “On the smoothness of optimal paths” Decis Econ Finance, 21:1–34, 2004), we provide conditions under which a variation in the initial conditions (i.e., capital stock and discount rate) yields an optimal solution which converges toward a reference solution when time becomes infinite. We also provide new results on bounded solutions of difference equations. We gratefully thank the editor, Silvano Holzer, and two anonymous referees for remarks and advices on a previous version of this paper.