Stochastic unpaced line design: Review and further experimental results

Previous design studies of unpaced assembly lines that exhibit stochastic task times indicate that an unbalanced allocation of task times results in optimal output rates. In this article, we present a comprehensive review of the previous literature on this topic and discuss the results of simulation experiments that test the bowl distribution for unbalancing unpaced lines. The simulation experiment was designed to test the bowl distribution in more realistic environments than previously tested and illustrates that a balanced line configuration is as good as or better than an unbalanced line configuration when task times are modeled with more typical values of variance.Stochastic unpaced assembly line research employs both simulation and analytical approaches to test the allocation of buffer capacity and task times to work stations. Analytical models are utilized to investigate simple line designs with exponential or Erlang task time distributions. Simulation is used for longer lines and for normal task time distributions. From the review of the previous research using both approaches, we note five major findings: 1) unbalancing task time allocation is optimal when task time variation is large; 2) unbalanced allocation of buffer storage capacity improves line output rate when task time variation is large; 3) output rate of an unpaced line decreases as the number of sequential workstations increases; 4) output rate increases as more buffer storage capacity is available; and 5) output rate decreases as the task time variation increases.Most of the previous research on unpaced lines investigated lines with few workstations and large task time variation. Empirical research by Dudley (6) suggests that variation of task times in practice is much less than variations employed in previous unpaced line studies. We present the results from simulation experiments that model longer unpaced lines with lower levels of task time variance of the magnitude that is likely to occur in practice.The results of our simulation experiments verify the benefits of using the bowl distribution for task time allocation when line lengths are short and task times experience large variance. However, when line lengths are extended or task time variation is reduced, the use of the bowl distribution for unbalancing the line degrades the line's efficiency. In these situations, the optimal task time allocation is a balanced line.Two important implications for managers follow from the results of our experiments: 1) that unpaced line output rate is relatively insensitive to moderate variations from optimal task time allocations when buffer storage is limited; and 2) that perfectly balanced line designs are optimal for most cases in practice.     

Currently practiced formulations for the assembly line balance problem

New formulations for the assembly line balance problem are proposed based on interviews and surveys of practicing engineers. These formulations are the basis for a model in goal programming form. A branch-and-bound algorithm is developed which can solve the model for an optimum solution. Computational studies show that computer run time is very modest for moderate size problems.Interviews and a survey of practicing engineers were used to develop a list of goals or constraints germane to the assembly line balance problem. These included minimizing working areas or employees, making sure tasks assigned to a station do not exceed the cycle time, and adhering to sequence constraints. These constraints are included in most traditional models. However, additional goals were mentioned. These included avoiding changes in workload assigned to a work area, adhering to layout requirements of the plant, making combinations of tasks interesting, avoiding the combination of physiologically demanding tasks, etc.The additional goals above are incorporated in a new formulation for the assembly line balance problem. This formulation is in goal programming form. The goal programming model attempts to minimize deviations from goals. If deviations are necessary, lowest ranked goals are violated first. The objective function of the model is based on an ordinal ranking of goals only. The survey mentioned above showed that engineers did not find it difficult to rank the importance of goals.The proposed goal programming model is a mixed integer linear program. Previous studies have shown that cutting plane and implicit enumeration techniques are inferior to branch-and-bound algorithms. A branch-and-bound method, called GoalOriented Algorithm for Line Balance (GOAL), is developed to solve the formulations proposed in the paper.GOAL was computationally tested with 50 problems. These problems varied in size from 4 to 35 tasks. One of the problems was an engine cradle assembly problem encountered by an automobile manufacturer. Computer run times appeared reasonable. For example, the engine cradle problem 35 tasks) required only 16.3 seconds CPU on the DEC PDF at California State University, Northridge. GOAL's execution time appeared linearly proportional to the number of tasks required for most problems.     

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.     

电动三轮车装配流水线的平衡分析与改进

在生产线平衡原理及改善方法的基础上,以电动三轮车装配流水线为研究对象,运用工作研究的方法分析工艺流程,测定流水线各工位的作业时间,找出瓶颈工位;运用"5W1H"提问技术和ECRS(取消、合并、重排、简化)原则对流水线进行分析与改进。通过对比分析,改进后该流水线大大节省了空间和人力,工位减少6个,工人减少11个,与此同时,平衡率提高27.6%。     

汽车发动机预装线装配平衡改进研究

为提高已经投产的汽车总装生产线的生产平衡率,研究发动机生产线生产平衡改进研究。以现有生产节拍和各个工位操作工时作为基础,运用生产线平衡的基本概念,对现有生产线进行优化。首先确定优化目标,通过工序同期化优化,减少生产岗位,从而提高部分岗位负荷,提高生产平衡率,为企业解决实际问题,节约人力、场地等资源。     

Modelling of repairable systems with various degrees of repair

The availability of a stochastic repairable system depends on the failure behaviour and on repair strategies. In this paper, we deal with a general repair model for a system using auxiliary counting processes and corresponding intensities which include various degrees of repair (between minimal repair and perfect repair). For determining the model parameters we need estimators depending on failure times and repair times: maximum likelihood (ML) estimator and Bayes estimators are considered. Special results are obtained by the use of Weibull-type intensities and random observation times.     

基于时间和成本的加工装配线物流平衡方法

本文提出一种基于时间和成本的加工装配线上物流的规划方法，在对现有的生产装配线重新分配工件的同时保证最大效益的改善，执行花费／效益分析，同时该平衡方法借助于数据库和电子表格的联合应用，可以在短时间内完成。文中以哈锅阀门股份有限公司生产线的改造为例，详细介绍了该方法的使用。计算结果表明，这种物流平衡方法提供了企业一种效率高，费用低的现有加工装配线改造方法。     

Coordinating debris cleanup operations in post disaster road networks

We propose a constructive heuristic that generates roadside debris cleanup plans for a limited number of equipment in the post-disaster road recovery planning problem. Travel times between cleanup tasks are not pre-fixed but depend on the blockage status of the entire road network at the time of travel. We develop a novel mathematical model that maximizes cumulative network accessibility throughout the cleanup operation and minimizes makespan. We propose several practical and robust task selection rules that favor one or both goals that are tested on realistic size road networks with deterministic and stochastic debris cleanup times.     

生产物流平衡的持续优化在硬盘磁头制造业的应用

以广东某硬盘磁头代工企业生产线为研究对象，运用装配线平整技术，以生产物流平衡为目标分析了设备能力、任务分配、工装改善以及生产计划排配等方面存在的问题：提出并实施了改善方棠。保证了生产线的物流负荷平衡。优化方案的实施使企业生产能力提高并有效地降低了生产线在制堆积的问题，在制品周转从3．2天缩短到2．5天，生产现场的环境也得到了改善。     

机械装配的工艺基础分析

机器装配的基本任务是在一定的生产条件下,以高生产率和较低的成本装配出高质量的产品。文章主要讨论分析了装配方法以及装配工艺规程的制定。     

客专大站到发线运用随机机会约束模型与算法研究

在考虑到发线运用过程中不确定性因素的基础上，研究旅客列车停站时间和接续时间随机变动情况下客专大站到发线运用优化问题，以提高车站作业效率和方便旅客乘降为优化目标，建立了到发线运用的随机机会约束规划模型。通过将模型中的随机机会约束转化为相应的确定性等价形式，从而将随机规划模型转化为确定性模型，并设计了解决该问题的蚁群算法。算例表明该随机机会约束规划模型能取得可靠性更高的到发线运用计划。     

云环境下物流任务分配研究

云是未来经济社会发展的必然趋势,云环境为物流资源集成和任务发布提供了信息平台,将物流任务以最经济的方式分配给物流资源。文中根据云环境和物流的特点,将零散物流任务中同类型的活动打包为整合物流任务,实现规模经济;在任务分配过程中,以成本最低、时间最短为目标,考虑了云物流特有的信息传递成本,构建云物流任务模型,为云环境在物流行业的应用做了尝试性研究。     

Might randomization in queue discipline be useful when waiting cost is a concave function of waiting time?

This paper suggests that introducing randomization in queue discipline might be welfare enhancing in certain queues for which the cost of waiting is a concave function of waiting time. Concavity can make increased variability in waiting times good not bad for aggregate customer welfare. Such concavity may occur if the costs of waiting asymptotically approach some maximum or if the customer incurs a fixed cost if there is any wait at all. As examples, cost might asymptotically approach a maximum for patients seeking organ transplants who will not live beyond a certain threshold time, and fixed costs could pertain for knowledge workers seeking a piece of information that is required to proceed with their current task, so any delay creates a “set up charge” associated with switching tasks.     

某减速器装配线平衡的精益改造研究

从装配线平衡和精益生产的概念出发,以某企业减速器装配生产线为实例,介绍了精益生产理论在改造装配线平衡问题上的应用。考虑装配生产现场,用鱼刺图分析了影响装配线平衡的各种制约因素,并系统的应用全员"5S"活动,制定标准作业时间,连续流改造以及团队互助协作等精益手段对原装配生产线进行改造。改造结果显示,装配线各工作站作业负荷趋于均衡,整体生产效率得以提高,改造效果明显。     

A continuous-time sequential testing problem

A Bayesian testing problem for a simple hypothesis against a simple alternative is considered. However the observations on which the test is based are not immediately available, but occur atn i.i.d. random times. Assuming linear cost of time until a decision the task is to construct and to characterize an optimal stopping strategy for a time-sequential testing problem and to compute the Bayes risk. The paper provides a solution to this problem.     

Promoting cost efficiency and uniformity in parcel delivery centre locations and service areas: a GIS-based analysis

This paper provides a systematic framework to determine local delivery centre (LDC) locations and service areas to reduce delivery costs and balance the allocation of parcel delivery loads to promote sustainable LDC development. Based on a case study of a parcel distribution company in Korea, this study proposes two strategies for LDC service area delineation within each terminal service boundary that either minimises the total delivery time or balances the allocation of delivery loads. This research contributes to improving cost efficiency and balancing delivery load allocations in planning LDC locations and service areas that can potentially promote the long-term mutual sustainable development of parcel distribution companies and their local delivery counterparts.     

基于FLEXSIM的某生产线物流仿真优化

通过收集某企业挖掘机装配流水线的工艺路线、工序设备处理率、生产节拍和生产线平衡等方面数据,利用FLEXSIM物流仿真软件构建装配流水线仿真模型,比较装配流水线优化前后指标,发现优化后的装配线具有生产节拍时间缩短、瓶颈工序缓解、生产线产能平衡等优点。     

Fixed and Random Effects in Stochastic Frontier Models

Received stochastic frontier analyses with panel data have relied on traditional fixed and random effects models. We propose extensions that circumvent two shortcomings of these approaches. The conventional panel data estimators assume that technical or cost inefficiency is time invariant. Second, the fixed and random effects estimators force any time invariant cross unit heterogeneity into the same term that is being used to capture the inefficiency. Inefficiency measures in these models may be picking up heterogeneity in addition to or even instead of inefficiency. A fixed effects model is extended to the stochastic frontier model using results that specifically employ the nonlinear specification. The random effects model is reformulated as a special case of the random parameters model. The techniques are illustrated in applications to the U.S. banking industry and a cross country comparison of the efficiency of health care delivery.JEL classification: C1, C4     

Sequential clinical scheduling with patient no-show: The impact of pre-defined slot structures

This paper develops a sequential scheduling algorithm for consultation periods not divided into slots. Patients call a scheduler and request appointments with a specified provider. The scheduler provides the patient with an appointment time before the call terminates. In making the appointment, the scheduler cannot alter the appointments of previously scheduled patients. Service times are random and each scheduled patient has a probability of “no-showing” for the appointment. Each arriving patient generates a given amount of revenue, and costs are incurred from patient waiting and provider overtime. The scheduling method selects the calling patient's appointment time by minimizing the total expected cost. We prove that total expected cost is a convex function of appointment times and that the expected profit of the schedule is unimodal, which provides a unique stopping criterion for the scheduling algorithm. Computational studies compare this approach with no-show based sequential scheduling methods for out-patient clinics where a predefined slot structure is assumed. The new method yields higher expected profit and less overtime than when service periods are pre-divided into slots. Because slot scheduling is ingrained in healthcare, we use the model to design slot structures that account for no-show and service time variation.     

基于DELMIA的汽车装配线建模与仿真

汽车装配线直接决定了汽车生产的效率,因此,对汽车装配线进行建模与仿真,优化装配流程十分必要。基于DELMIA的DPM(Digital Process for Manufacturing)模块,对汽车装配线的装配序列规划、装配干涉以及装配路径规划进行研究,按规划的工艺流程对总装线进行模拟仿真,分析装配线的平衡率,通过仿真结果验证该装配线的可达性、可行性以及装配线的人因工效性。文中研究工作对优化及改善汽车装配过程,缩短工艺规划时间,实现汽车装配线的流水化具有一定指导意义和应用价值。