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.     

Optimale snijsnelheid bij meer-machine-bediening1),2)

In calculating formulas to determine the optimal number of machines assigned to an operator, the ratio of service time to machine operating time is supposed to be a constant for a given type of equipment. Changing the cutting speed on automatic lathes changes the tool life and the unit production time and, therefore, the ratio of service time to machine operating time and the efficiency. In this article a method to determine the optimal cutting speed and the corresponding optimal number of machines assigned to an operator is given. An example is given in which it was possible to attain a saving of f 2200 per lathe per year by assigning 7 lathes to an operator instead of 3. A further saving of f 5500 per lathe per year was calculated by decreasing the cutting speed to three-fourths of its original value and correspondingly assigning 15 to 17 lathes to one operator.     

Scheduling and resource allocation in a food service system

The scheduling of food production is often accomplished informally based upon approximate time requirements stated in recipes and the judgment and experience of a food production manager, administrative dietician, or cook. Such schedules are seldom optimized to least overall duration and consequently contain periods of non-productive time on the part of personnel and resources. In addition, these schedules often attempt to avoid resource conflicts through the early scheduled completion of work activity; this leads to many of the menu items being completed sufficiently in advance of serving time that undesirable changes in the nutritional, organoleptic and microbiological properties of the food can take place.In this paper, a branch and bound algorithm is presented as a solution procedure for the foodservice scheduling problem. The advantage of branch and bound in comparison to a heuristic based scheduling procedure is that it can produce schedules which are optimized to least overall duration from start to finish. The added computational cost of the branch and bound procedure is justified, because most foodservice systems cycle their menus. Consequently, each of a finite number of schedules is reused numerous times over an extended period resulting in long-term productivity gains.Another advantage of the algorithm is that right-shifted or late start schedules may be produced. This is in keeping with our objective of minimizing the delay time between the completion of the food and its being served to the consumer.The paper describes a method by which the process time for each of the various steps in a recipe may be computed as a function of the number of servings being prepared. Although these are normally considered to be linear relationships, the algorithm can easily be modified to accept other types of relationships as well.Perhaps the most important aspect of the this research is that the branch and bound algorithm has been implemented to perform branching operations over two classes of decisions. The first class of decisions involves the selection of which recipe steps or activities are to be scheduled at a certain time, and the second class of decisions involves the choice of which resource class to assign to the activities in those cases where alternative resources are allowed. This dual branching philosophy provides a great deal of flexibility to the decision maker for handling the type of scheduling problems commonly found in practice. The expense of this added flexibility, however, is a substantial increase in the size of the decision tree which must be developed and explored.In order to demonstrate the performance of the algorithm for practical purposes, the lunch menus of a short term, acute case, 300 bed hospital in Syracuse, New York were used to develop production schedules. These menus included a total of 89 different hot food items whose recipes were placed into a menu file in the computer along with the coefficients needed to develop process time estimates as a function of numbers of servings to be prepared. In total, fourteen lunch menus are cycled at the hospital; the number of items appearing on the menus ranges from 8 to 14 hot food items.In the first series of tests, resources were assumed not to be interchangeable. The branch and bound procedure was successful in producing optimal solutions for eleven of the fourteen schedules. The three menus which were not optimally solved were aborted, because the size of the solutions tree grew beyond that which could be stored in 500K bytes of common memory. In spite of this, however, the upper bound solutions given by the aborted problems were found to be very close to lower bound values and therefore may be considered as very good solutions.In the second series of tests, certain resources were allowed to be interchangeable for some of the activities. Specifically, we assumed two labor classes. The first of these are called special cooks who are more experienced personnel. Normally special cooks prepare entree items, but they may be called upon in some cases to perform any activity normally performed by the second labor class- the less experienced cooks—who normally prepare soups, sauces and vegetables. The cooks, on the other hand, are never allowed to prepare menu items which call for special cooks. These groundrules are identical to those currently in practice at the hospital from which the test problems were obtained. Ten of the fourteen menus were selected for this series of tests. In all of the ten cases, the algorithm successfully developed optimal solutions without exceeding the 500K byte common memory limitation. And, in spite of the vastly increased tree size resulting from the dual decision branches, the computation times for these tests were only modestly greater than those of the first set of tests where alternative resources were not considered. The success of the algorithm in solving the dual decision problems resulted in large part from its ability to develop strong upper bounds very early in solution process. In addition, the characteristics of food production scheduling problems are such that the lower bound pruning is very effective especially in the early stages of tree development.It is important to note that the use of an algorithm such as this in a practical setting affords a very friendly user interface. Once the foodservice system's menu items have been placed into a file, the user may easily select any set of these items to include in a given schedule. Only three lines of data input are required. The first line specifies the number of items as well as the zero-hour or serving time. The second line identifies the item numbers of the hot-food items to be scheduled. And the third line specifies the number of servings for each of those items which must be prepared. Kitchen personnel with limited experience can be trained to input the data in less than 15 minutes of instruction time.     

基于eM-Plant的柔性生产调度仿真研究

车间生产调度是流水制造企业运行管理的核心,针对流水车间工件多样化、多机调度平行化的作业特点,在分析系统逻辑流程和运行机理的基础上,用em-plant软件建立仿真模型,通过SimTalk语言程序触发仿真事件和监控仿真状态。分析工件加工时间、到达率及加工工序对调度结果的影响,以零件平均等待时间、资源利用率和置信度为系统评价指标。仿真结果表明,在确定加工工序和加工时间条件下,生产调度方案是合理的。     

Scheduling of re-entrant flow shops

We propose and develop a scheduling system for a very special type of flow shop. This flow shop processes a variety of jobs that are identical from a processing point of view. All jobs have the same routing over the facilities of the shop and require the same amount of processing time at each facility. Individual jobs, though, may differ since they may have different tasks performed upon them at a particular facility. Examples of such shops are flexible machining systems and integrated circuit fabrication processes. In a flexible machining system, all jobs may have the same routing over the facilities, but the actual tasks performed may differ; for instance, a drilling operation may vary in the placement or size of the holes. Similarly, for integrated circuit manufacturing, although all jobs may follow the same routing, the jobs will be differentiated at the photolithographic operations. The photolitho-graphic process establishes patterns upon the silicon wafers where the patterns differ according to the mask that is used.The flow shop that we consider has another important feature, namely the job routing is such that a job may return one or more times to any facility. We say that when a job returns to a facility it reenters the flow at that facility, and consequently we call the shop a re-entrant flow shop. In integrated circuit manufacturing, a particular integrated circuit will return several times to the photolithographic process in order to place several layers of patterns on the wafer. Similarly, in a flexible machining system, a job may have to return to a particular station several times for additional metal-cutting operations.These re-entrant flow shops are usually operated and scheduled as general job shops, ignoring the inherent structure of the shop flow. Viewing such shops as job shops means using myopic scheduling rules to sequence jobs at each facility and usually requires large queues of work-in-process inventory in order to maintain high facility utilization, but at the expense of long throughput times.In this paper we develop a cyclic scheduling method that takes advantage of the flow character of the process. The cycle period is the inverse of the desired production rate (jobs per day). The cyclic schedule is predicated upon the requirement that during each cycle the shop should perform all of the tasks required to complete a job, although possibly on different jobs. In other words, during a cycle period we require each facility to do each task assigned to it exactly once. With this requirement, a cyclic schedule is just the sequencing and timing on each facility of all of the tasks that that facility must perform during each cycle period. This cyclic schedule is to be repeated by each facility each cycle period. The determination of the best cyclic schedule is a very difficult combinatorial optimization problem that we cannot solve optimally for actual operations. Rather, we present a computerized heuristic procedure that seems very effective at producing good schedules. We have found that the throughput time of these schedules is much less than that achievable with myopic sequencing rules as used in a job shop. We are attempting to implement the scheduling system at an integrated circuit fabrication facility.     

A Hybrid Approach to Quantitative Software Project Scheduling Within Agile Frameworks

In this article we focus on the quantitative project scheduling problem in IT companies that apply the agile management philosophy and Scrum, in particular. We combine mathematical programming with an agile project flow using a modified multi‐mode resource constrained project scheduling model for software projects (MRCPS^SP). The proposed approach can be used to generate schedules as benchmarks for agile development iterations. Computational experiments based on real project data indicate that this approach significantly reduces the project cycle time. The approach can be a useful addition to agile project management, especially for software projects with predefined deadlines and budgets.     

A column generation-based approach to solve the preference scheduling problem for nurses with downgrading

The demand for nurses in virtually all western countries has been outpacing the supply for more than a decade. The situation is now at the point where the rules for good practice are being stretched to the limit and patient care is in jeopardy. The purpose of this paper is to present several ideas for maximizing the use of the available staff and to quantify the resultant benefits. Two approaches are investigated for substituting nurses with higher level skills for those with lower level skills when there is sufficient idle time to do so. Idle time is usually due to scheduling constraints and contractual agreements that prevent a hospital from arbitrarily assigning nurses to shifts over the week.When the substitution is skill-related, as it is here, it is often called downgrading. The models that we develop are for preference scheduling, which means that individual preferences are taken into account when constructing monthly rosters. There are several reasons for doing this in today's environment, the most important is the need to boost staff morale and increase retention. The problem is modeled as an integer program and solved with a column generation technique that relies on intelligent heuristics for identifying good candidate schedules. The computations show that high quality solutions, as measured by the reduction in the need for non-unit nurses as well as the degree to which preferences are satisfied, can usually be obtained in a matter of minutes.     

A formal set of algorithms for project scheduling with critical path scheduling/material requirements planning

A hybrid model combining the critical path method (CPM) with material requirements planning (MRP) has been suggested (Aquilano) as a more robust method for scheduling projects and resources. The primary advantage of this technique is that resource acquisition lead times as well as inventory records are integrated into the process of computing the project schedule. This paper presents a set of formal CPM/MRP algorithms that may be used to compute the early and late start schedules as well as the critical sequence. A number of modifications have been incorporated into the CPM/MRP technique to improve the viability of CPM/MRP as a tool for application to actual project scheduling problems. A simple example project is used to demonstrate the CPM/MRP model.The CPM/MRP technique is designed to overcome a basic shortcoming of previously suggested project scheduling methodologies. CPM was initially designed to schedule projects subject to technological constraints only. Later, additional techniques were introduced to consider constraints upon various aspects of resource availability (Davis). None of the suggested techniques attempted to integrate resource acquisition lead time with the generation of requirements for resources. Obviously such a technique would require the integration of inventory records into the scheduling technique.The combination of CPM and MRP provides a possible vehicle for overcoming this drawback in CPM. Both CPM and MRP are linear models that generate schedules based upon precedence relationships. An integrated approach is useful since activities could be scheduled subject to information about the inventory position. An activity may be scheduled as soon as all resources are on hand. It is only delayed by those resources which must be acquired and activities which proceed it in the project network.CPM/MRP also shows promise as an aid to constrained resource scheduling since computations regarding resource availability are an integrated part of the technique. The effect of resource allocation decisions is immediately evident in the MRP-type time phased records.Results of the tests run on short projects of up to 300 activities and resources have shown that the program does work satisfactorily. Execution time for a 300 item network tested was approximately ten seconds on a CYBER 175.     

The effect of lot-sizing on workload variability

Lot-sizing models which group demand requirements for one or more consecutive time periods into a single production run have received considerable attention in recent years. Material Requirements Planning (MRP) systems must, for instance, make a lot-size decision for each planned order release. Existing decision models attempt to minimize the sum of setup plus inventory holding costs. However, lot-sizing tends to increase the work center load variability, and, consequently, the costs associated with changing production levels from period to period should be incorporated into the economic analysis. This study is concerned, first of all, with analytically describing the relationship between dynamic lot-sizing models and workload variability. Secondly, in order to account for production level change costs we propose a simple modification to existing heuristic models. Lastly, we employ a simulation model to empirically extend these results to a typical MRP multiechelon production environment. An example is included to show clearly that with cost premiums for overtime and severance or guaranteed minimum costs for undertime the traditional lot-sizing techniques significantly underestimate actual costs and can lead to very costly policies.Mean, variance and coefficient of variation of period work time requirements are derived as a function of several algorithm characteristics. Average cycle time (number of periods covered by a single batch) is found to be the most influential factor in determining workload variability. Variance grows approximately in proportion to this cycle time with the proportionality constant being the square of average period workload. Cycle time and demand variability also contribute to workload variability. Results indicate that for a given average cycle time, the EOQ method will minimize workload variability. When N products utilize the same work center, the coefficient of load variation will be reduced by a factor of N^{?$\text{1}\text{2}$} unless requirements are positively correlated. Positive correlation would result when products have similar seasons or parent items. In this case grouping such products cannot help reduce variability.In order to incorporate production level change costs into existing heuristics we may simply introduce a term consisting of a penalty factor times average cycle time. The penalty factor represents the costs of period by period production level changes. Several popular heuristics are extended in this fashion, and it is found that solutions are still readily obtainable, requiring only modifications to setup or holding cost parameters.The effects of level change costs are examined via simulation for a specific yet typical environment. It is found that when setup costs are significant, traditional lot-sizing heuristics can provide cost savings and service level improvements as compared to lot-for-lot production. However, whereas for our model the obtainable profit improvement from lot-sizing was 25% in the case of freely variable capacity, actual improvements were only one half as large when reasonable hiring and firing practices and overtime and undertime costs were considered. Consequently, management needs to consider carefully labor costs and work center product relationships when determining a production scheduling method.     

An empirical investigation of scheduling performance criteria

Planning and scheduling significantly influence organizational performance, but literature that pays attention to how organizations could or should organize and assess their planning processes is limited. We extend planning and scheduling theory with a categorization of scheduling performance criteria, based on a three-stage survey research design. Particularly, the results show that, next to schedule quality, the planning process factors timeliness, flexibility, communication, and negotiation are important performance criteria, and especially so in organizations that are faced with high levels of uncertainty. The results suggest that organizational and behavioral aspects of planning and scheduling cannot be mitigated with advanced models and software that solely focus on good schedules. Rather, high quality schedules and high quality scheduling processes need to be facilitated simultaneously to attain high planning and scheduling performance.     

Distribution of run statistics in partially exchangeable processes

Let { X_i} _{i 3 1}{\{ X_{i}\} _{i\geq 1}} be an infinite sequence of recurrent partially exchangeable binary random variables. We study the exact distributions of two run statistics (total number of success runs and the longest success run) in { X_i} _{i 3 1}{\{ X_{i}\} _{i\geq1}} . Since a flexible class of models for binary sequences can be obtained using the concept of partial exchangeability, as a special case of our results one can obtain the distribution of runs in ordinary Markov chains, exchangeable and independent sequences. The results also enable us to study the distribution of runs in particular urn models.     

等概率条件下的转弯型堆垛机作业效率的计算研究

转弯型堆垛机是自动化立体仓库中实现货物自动存取的关键设备,文中对转弯型堆垛机效率问题进行研究。在假定等概率拣取货物的条件下,分别推求转弯型堆垛机的平均单一作业和复合作业循环周期和作业效率的计算公式,并利用华南理工大学仓储综合实验室的基本数据进行了作业效率的实证计算,从而为自动化立体仓库系统的建设设计和有效管理提供依据。     

Evaluation of scheduling techniques for solving flowshop problems with no intermediate storage

The flowshop scheduling problem with no intermediate storage (NIS problem) was studied in this research. This problem, a modification of the classical flowshop scheduling problem, arises when a set of jobs, once started, must be processed with no wait between consecutive machines. By eliminating the need for intermediate storage, reduction of capital investment in work-in-process inventory can be achieved. This approach can be practically applied to a steel mill, in which the metal should be continuously processed in order to maintain high temperature, as well as many other similar processes.To provide insight into selecting an appropriate scheduling technique for solving the NIS problem, six methods were compared in terms of the quality and efficiency of the scheduling solutions they produced. The quality of solution was measured by makespan and the efficiency of solution was measured by the computational time requirements. The six methods examined in this study included: the Gupta algorithm, the Szwarc algorithm, an integer linear programming method, the Campbell et al. algorithm, the Dannenbring rapid access with extensive search algorithm, and a mixed integer linear programming procedure.The problem factors considered in this study were number of jobs, number of machines, and range of processing times. Relatively small-sized problems were tested with up to ten jobs, five machines, and 1–100 processing time units. Six solution techniques were selected and compared, with respect to makespan and computational time requirements, for multiple combinations of the three problem variables.The resulting test data were investigated using graphical procedures and formal statistical analyses. Initially, plots of mean values were used to graphically compare the six solution methods for the two performance criteria. Next, a multivariate analysis of variance study was conducted to investigate the quality and efficiency of the algorithms with respect to the problem factors. Then, a multiple comparison procedure was employed to analyze treatment mean differences among the six solution techniques. Results from the statistical analyses are summarized in this article.It was concluded that the two mathematical programming methods, the integer linear programming procedure and the mixed integer linear programming methods, produced the best performance in terms of makespan. These two methods, however, used a far greater amount of computational time than the other four solution techniques. Producing moderately good results as far as quality of performance, the Gupta and the Szwarc algorithms were comparable with the Campbell et al. and the Dannenbring algorithms in terms of computational efficiency. By comparison, the Campbell et al. and the Dannenbring algorithms produced the poorest performance with respect to the quality of solutions.Certain limitations were imposed for this study. The problem size considered was relatively small and the sample size was also limited to ten problems per cell. In addition, a uniform distribution function was used for generating processing times within certain ranges. These limitations were necessary in order to allow the various scheduling problems to be solved within a reasonable amount of computer time.Finally, some suggestions were provided for future research in the NIS problem area. The integer linear programming method was recommended as a standard of evaluation, owing to its best overall performance. A possible area for future research would involve the improvement of the Gupta and the Szwarc algorithms through the use of backtracking procedures within the branch-and-bound technique, so that they might be competitive with the mathematical programming methods with respect to quality of performance. Other distribution functions could be investigated in terms of the influence of the distribution of processing times on the performance measures.     

高精进口磨床砂轮主轴轴承维修技巧

目前我国许多企业由于加工要求,需要从国外进口机床,那么进口机床的维修就是摆在国内机床维修人员面前的一项关键工作,由其是一些高精度机床,本文就进口高精度磨床的维修介绍了一种简单的维修技巧。     

Elements of manpower forecasting and scheduling

Frequent updates of planning forecasts and scheduling would permit a company to head-off impending layoffs, as well as to take advantage of changes in sales forecasts to eliminate or reduce over- and under-shoot in manning schedules. With the computer to minimize time and cost, such updates are practical.     

Pro/E及PowerMILL软件在叶片零件加工中的应用

文章以自动编程软件技术应用为前提，以风扇叶片零件为例，主要阐明了Pro/E及PowerMILL软件在数控加工过程中的具体应用。简单描述了叶片的Pro/E造型过程，较为详细的阐述了PowerMILL软件针对风扇叶片零件从工艺参数、加工策略到仿真及程序的后置处理的自动编程过程。根据实际加工表明，自动编程软件的使用，提高了产品加工质量的同时，缩短了零件制造周期。     

中型铣刨机噪声现状及噪声源识别

工程机械操作舒适性日渐成为评价一个产品好坏的重要指标,而噪声指标又是舒适性指标的重要组成部分。本文通过对某中型铣刨机进行噪声试验,研究了铣刨机的噪声源分布、频谱特性及噪声传递路径。试验结果表明:操作者位置上的噪声主要是由散热风扇和发动机排气噪声贡献,其能量主要集中在中心频率630Hz以下的中低频,且主要是从地板下方散热出风口传递过来。     

流水型CONWIP生产系统研究

本文研究了流水型CONWIP生产系统中在制品常量、批量和加工次序的问题。以机床加工准备成本及工件库存成本和加工流动时间为优化目标．建立了CONWIP系统整数规划模型，并提出了求解问题的启发式算法。在优化机床准备及库存成本的基础上．得到各产品的生产批量和系统在制品常量。同时，提出了流水型CONWIP生产系统的排序算法（CFA），依此得到产品的较优加工次序。本研究使CONWIP模型求解得到简化且易于应用，并获得满意解。     

基于动态约束的炼钢连铸重调度算法设计

针对炼钢连铸过程中出现的新炉次插入的重调度问题,考虑实际生产中的工艺约束,以最小化开工时间差异度、加工机器差异度的加权和为目标函数建立动态调度模型,采用回溯算法与启发式规则结合的算法。用Matlab7.8.0软件对选取的算例进行仿真,结果表明,此算法能保证重调度结果的实时性和有效性。     

道夫墙板钻孔工装的设计

道夫墙板是梳棉机道夫部件的重要组成部分,属于梳棉机主关键件,其质量好坏对梳棉机整机质量影响较大。文章设计了专用工装进行外圆丝孔加工,分度准确,质量达到了设计要求,而且定位迅速,大大提高了生产效率,解决了生产瓶颈问题。