Theresa M. Roeder

A Simulation-Based Closed Queueing Network Approximation of Semiconductor Automated Material Handling Systems

Queueing networks have been used to model material handling in flexible manufacturing systems. We explore the use of a closed queueing network model to approximate an intrabay automated material handling system (AMHS) in semiconductor manufacturing. A simulation-based approach is proposed to obtain estimates of system performance measures. Current simulation models in the industry are very complex, require significant development time, and take a long time to run. The proposed simulation-based queueing network approximation can be used as an easy and fast alternative or as a precursor to a full-fledged modeling exercise depending on the type of decision being made and the level of granularity required in the output. Using a simple intrabay configuration, we compare estimates from the approximation approach and a more detailed modeling approach to characterize the performance of the approximation. In addition, a simulation information model is used to compare AMHS models based on the amount and types of information needed to model the system, and to obtain desired output. This classification aids modelers in determining the level of detail to incorporate in a model based on the objectives of the simulation study.

Information models for queueing system simulation

When planning simulations of large-scale systems, it is important to anticipate what information is required to model the system and obtain desired output. This can be done without tying the study to a specific simulation package or language. It is valuable to do so to avoid unnecessarily long development and execution times. In this article, we offer a simulation information model (SIM) designed to help organize system information in the early stages of a project. (It can also be used to analyze existing models.) The SIM allows complexity analysis of the system to be performed, and may lead to a better selection of simulation language. The SIM is illustrated using two examples, and its relationship to current formalisms is discussed.

Estimating expected completion times with probabilistic job routing

A common problem in production environments is the need to estimate the remaining time in system for work-in-progress jobs. Simulation can be used to obtain the estimates. However, when the future path of a job is uncertain (due to stochastic events such as rework), using simulation to estimate the remaining cycle time of a job at step k can be imprecise; traditional confidence intervals on the estimated remaining cycle times may be too large to be of practical significance. We propose a response surface methodology-based approach to estimating conditional confidence intervals on the remaining cycle times as jobs progress through the system and more information is obtained on them. This method will provide more useful and accurate estimates of remaining cycle times at various stages of the process flow. Further, we outline two different simulation approaches for estimating the response surfaces used to generate the confidence intervals

Using Simulation to Model Improvements in Pediatric Bed Placement in an Acute Care Hospital

OBJECTIVE The objective of this project was to use an interdisciplinary approach to analyze strategies through simulation technology for improving patient flow in a pediatric hospital. BACKGROUND Various statistics have been offered on the number of children admitted annually to hospitals. For administrators, particularly in smaller systems, the financial burden of equipping and staffing pediatric units often outweighs the moral desire to maintain a pediatric unit as a viable option for patients and pediatricians. METHODS Discrete event simulation was used to model current operations of a pediatric unit. Cost analysis was conducted using simulation reflecting various percentages of patients being referred to a discharge holding area (DHA) upon discharge and of the use of all private rooms. RESULTS Both DHA and private rooms resulted in increased patient volumes. CONCLUSIONS Administrators should consider the use of a DHA and/or private rooms to ease the census strains of pediatric units and the resultant revenue of this service.

Perspectives on teaching simulation in a college of business

In this paper, we explore the challenges and opportunities we face teaching computer simulation at a business school. While our students tend not to be as technically savvy as most engineering students, at times limiting the technical complexity of what we can cover in our courses, we are able to use simulation as a tool to analyze and discuss business problems in-depth. We explore the differences both between business and non-business simulation courses, as well as those between our graduate and undergraduate courses.

Simulating student flow through a college of business for policy and structural change analysis

Under the pressure of sharp budget cuts and external demands for better performance, public institutions of higher education must examine how they can facilitate student graduation even as institutional resources diminish. This paper describes a computer model simulating the movement of undergraduates through a large, public college of business. The model allows changes in curriculum policy, prerequisite structure, and staffing capacity to be tested prior to implementation. Outcome measures focus primarily on the expected time to degree of two types of students who enter the university, first-time freshmen and upper division transfers, along with their respective 6-year and 4-year graduation rates. The validated model is used to experiment with both actual and potential scenarios facing the college and gauge their possible impact.

Using simulation as a teaching tool in an introductory operations management course

We discuss the use of two online simulations as a part of our core undergraduate and graduate business operations classes. We have found the games, Littlefield Technologies, to be of pedagogical value, as they can engage students in the material due to the competitive nature of the game, as well as the ability to see consequences of their actions. Students are able to apply what they have learned in the class to a more realistic scenario than textbook problems. Despite the additional work and challenges associated with ensuring all students register on time and engaging everyone, we feel the games are worthwhile. We provide some strategies for facilitating the smooth and successful execution of the simulations.

Schedule-Based Group Assignment Using Constraint Programming

Group work is an important aspect of the educational process in many disciplines, especially business. The literature suggests there are pedagogical advantages to instructor-assigned teams. In this paper, we present a constraint programming approach to assigning students to groups based on their availability. We compare solutions created using several objectives and find that no single objective dominates for all performance measures. Using the model produces group assignments of higher quality and much more quickly than those produced manually.

Case Article---Embedding a Sustainability Module Into Quantitative Business Courses

Business schools face pressure to incorporate sustainability and ethics into curricula, not only by offering single-themed classes but also by including these topics in existing classes. We embed a stand-alone module, sustainability and supply chain management, into two quantitative courses, the graduate core operations class and an undergraduate concentration class in management science. We develop this module to provide students the background and tools to analyze energy usage and resultant greenhouse gas GHG emissions for a product or process. Students research a supply chain, use carbon auditing software, and make recommendations for improvement. To answer the research question of whether students perceive the module as useful and objective we perform an exploratory survey N = 76. We find that students are engaged and consider delivery as unbiased. In short, this module adds value to the learning experience and can be integrated into classes without extensive instructor preparation. Readers may freely download all supporting materials for use in their own courses. More generally, our approach could prove useful in developing modules on other sustainability subtopics to place into quantitative courses, so as to support the relationship between analytical and qualitative approaches to decision making, especially when those decisions involve ethical or environmental issues. Case Teaching Note: Interested Instructors please see the Instructor Materials page for access to the restricted materials. To maintain the integrity and usefulness of cases published in ITE, unapproved distribution of the case teaching notes and other restricted materials to any other party is prohibited.

On gradient estimation of scheduling for multi-cluster tools with general robot moving times

This paper introduces three methods to deriving gradient estimators of system performance of multi-cluster tools. A multi-cluster tool is composed of multiple single cluster tools connected non-cyclically through intermediate buffer modules. Each single cluster has a separate robot for transporting wafers within the cluster and in and out of the cluster. The coordination among these robots poses a challenge in system-performance analysis and optimal scheduling. This paper focuses on two-cluster tools and develops three approaches (formula-based, linear programming-based, and regression-based) for estimating the gradient of the throughput or any activity times with respect to all timing parameters.