SIMULATION OF MANUFACTURING SYSTEMS
Averill M. Law Michael G. McComas Averill M. Law & Associates, Inc. P.O. Box 40996 Tucson, Arizona 85717, U.S.A.
ABSTRACT This paper discusses how simulation is used to design new manufacturing systems and to improve the performance ofexisting ones. Topics to be discussed include: manufacturing issues addressed by simulation, simulation software for manufacturing applications, techniques for building valid and credible models, and statistical considerations. A comprehensive example will be given in the conference presentation. 1 INTRODUCTION
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Evaluation of the effect of a new piece of equipment on an existingmanufacturing system Evaluation of capital investments Labor-requirements planning Number of shifts
Performance evaluation • Throughput analysis • Time-in-system analysis • Bottleneck analysis Evaluation of operational procedures • Production scheduling • Inventory policies • Control strategies [e.g., for an automated guided vehicle system (AGVS)] • Reliability analysis (e.g., effect of preventivemaintenance) • Quality-control policies The following are some of performance measures commonly estimated by simulation: • Throughput • Time in system for parts • Times parts spend in queues • Queue sizes • Timeliness of deliveries • Utilization of equipment or personnel 3 SIMULATION SOFTWARE FOR MANUFACTURING APPLICATIONS
One of the largest application areas for simulation modeling is that ofmanufacturing systems, with the first uses dating back to at least the early 1960’s. In this paper we present an overview of the use of simulation in the design and analysis of manufacturing systems. Detailed discussions of simulation, in general, may be found in Banks, Carson, and Nelson (1996) and Law and Kelton (1991). A practical discussion of the steps in a sound simulation study is given inLaw and McComas (1990). This paper is a synopsis of a three-day short course with the same title as this paper, which the first author has given more than fifty times since 1985. 2 MANUFACTURING ISSUES ADDRESSED BY SIMULATION
The following are some of the specific issues that simulation is used to address in manufacturing: The need for and the quantity of equipment and personnel • Number, type,and layout of machines for a particular objective • Requirements for transporters, conveyors, and other support equipment (e.g., pallets and fixtures) • Location and size of inventory buffers • Evaluation of a change in product volume or mix
Most organizations that simulate manufacturing or material-handling systems use a commercial simulation software product, rather than a general-purposeprogramming language (e.g., C). Furthermore, the two most common criteria for selecting simulation software are modeling flexibility (ability to model any system regardless of its complexity or uniqueness) and ease of use.
Law and McComas We now define the major types of simulation software for manufacturing. A simulation language is a software package that is general in nature (in termsof the applications it can address) and where model development is done by “programming.” Traditionally, programming meant the development of a simulation model by writing code, but in recent years there has been a strong movement toward simulation languages that employ a graphical model-building approach. Example of simulation languages are Arena, AweSim!, Extend, GPSS/H, Micro Saint, MODSIM III,SES/workbench, SIMPLE++, SIMSCRIPT II.5, SIMUL8, and SLX. The major advantage of a good simulation language is modeling flexibility, whereas the major disadvantage is that programming expertise is required. A manufacturing-oriented simulation language is one where the modeling constructs are specifically oriented toward manufacturing or material handling. Examples of such software are AutoMod...