Factory Models for Manufacturing Systems Engineering

We review MIT research in manufacturing systems engineering, and we describe current and possible future research activities in this area. This includes advances in decomposition techniques, optimization, token-based control systems analysis, multiple part types, inspection location, data collection and several other topics.Singapore-MIT Alliance (SMA

Modeling waste production into two-machine–one-buffer transfer lines

This article focuses on analytical models of two-machine one-buffer Markov lines including waste production. The aim is to compute the probability of producing good parts, referred to as effective efficiency, when waste production is related to stoppages of the first machine. This problem is common in industrial fields where parts are generated by a continuous process; e.g., in high-speed beverage packaging lines. Two innovative models including waste production are presented: the WP-Basic Model extends the model of a basic two-machine-one-buffer transfer line; the WP-RP Model extends the model of a two-machine-one-buffer transfer line with a restart policy operating on the first machine (i.e., when the first machine is blocked because the buffer is filled, it is not allowed to resume production until the buffer becomes empty). The two existing models are improved by distinguishing, at any time step the first machine is operational, whether it is producing a good or a bad part. The probabilities of the system being in any feasible state are analytically derived for both the WP-Basic Model and the WP-RP Model. Then, the obtained probabilities are used to determine the performance measures of interest; i.e., waste probability and effective efficiency. Finally, some numerical results are provided to illustrate the effectiveness of the WP-Basic Model and the WP-RP Model

Modeling and Analysis of Manufacturing Systems with Multiple-Loop Structures

Kanban and Constant Work-In-Process (CONWIP) control methods are designed to impose tight controls over inventory, while providing a satisfactory production rate. This paper generalizes systems with kanban or CONWIP control as assembly/disassembly networks with multiple-loop structures. We present a stochastic mathematical model which integrates the information control flows into material flows. Graph theory is used to analyze the multiple-loop structures. An efficient analytical algorithm is developed for evaluating the expected production rate and inventory levels. The performance of the algorithm is reported in terms of accuracy, reliability and speed.Singapore-MIT Alliance (SMA

On Production and Subcontracting Strategies for Manufacturers with Limited Capacity and Backlog-Dependent Demand

We study a manufacturing firm that builds a product to stock to meet a random demand. If there is a positive surplus of finished goods, the customers make their purchases without delay and leave. If there is a backlog, the customers are sensitive to the quoted lead time and some choose not to order if they feel that the lead time is excessive. A set of subcontractors, who have different costs and capacities, are available to supplement the firm's own production capacity. We derive a feedback policy that determines the production rate and the rate at which the subcontractors are requested to deliver products. The performance of the system when it is managed according to this policy is evaluated. The subcontractors represent a set of capacity options, and we calculate the values of these options

Scheduling Manufacturing Systems With Work-in-Process Inventory Control: Single-Part-Type Systems

In this paper, a real-time feedback control algorithm is developed for scheduling single-part-type production lines in which there are three important classes of activities: operations, failures, and starvation or blockage. The scheduling objectives are to keep the actual production as close to the demand as possible, and to keep the level of work-in-process inventory as low as possible. By relating the starvation and blockage to the system capacity, the buffer sizes and the target buffer levels are chosen according to the demands and machine parameters. The processing time for each operation is deterministic. Failure and repair times are random. Whenever a machine fails or is starved or blocked, the scheduling system recalculates short term production rates. To begin with, we study a very simple case, a two machine and one part type system, to get insight into the buffer effects and production control policies. Using the relationship between system capacity and starvation or blockage, we find desirable buffer levels and buffer sizes. The production control policy is determined to meet the system performance requirements concerning low WIP inventory and tardiness. The results from the simple case are extended to N-machine, one-part-type systems

Quality and Quantity Modeling of a Production Line

During the past three decades, the success of the Toyota Production System has spurred research in the area of manufacturing systems engineering. Two research fields, productivity and quality, have been extensively studied and reported separately both in the manufacturing systems research literature and the practitioner, but there is a lack of research in the intersection of these areas. In addition to that, most studies on the relationship among manufacturing system design, quality and productivity are based on anecdotal evidence or qualitative reasoning that lack sound scientific quantitative foundations. This study tries to establish a scientific foundation to investigate how production system design and operation influence productivity and product quality by developing conceptual and computational models and performing experiments. By doing so, this study will show an important part of the way to produce high quality products with minimum cost.Singapore-MIT Alliance (SMA

Modeling and Analysis of Re-entrant Production Systems

This paper presents a model and analysis of a re-entrant production line with finite buffers and unreliable machines. Semiconductor device and liquid crystal display (LCD) fabrication processes are characterized as a re-entrant process, in which a similar sequence of processing step is repeated several times. This re-entrant behavior of material flow with the stochastic nature of the system caused by machine failures or demand changes makes the system difficult predict and analyze. The decomposition method analyzes the behavior of the manufacturing systems by decomposing a long transfer line into small analytically tractable components, called two-machine line building blocks. Existing decomposition methods are limited to an in-linear production system without re-entrant flow. Since many manufacturing systems, particularly semiconductor and LCD production lines, consist of re-entrant flow paths, it is essential that models be developed to reflect this. The purpose of this paper is to present mathematical formulations and algorithms to analyze the material behavior of the re-entrant production system using the decomposition method. In developing equations for the two-machine building blocks for the re-entrant production line, we modify the existing decomposition model that has been created for the multiple-part type line. Two main performance measures are evaluated with the developed mathematical model: production rate and average inventory levels for each buffer space in the system. The qualitative behavior of the re-entrant production line under different machine parameters and demand scenarios is also described.Singapore-MIT Alliance (SMA

Modeling and Analysis of Two-Part Type Manufacturing Systems

This paper presents a model and analysis of a synchronous tandem flow line that produces different part types on unreliable machines. The machines operate according to a static priority rule, operating on the highest priority part whenever possible, and operating on lower priority parts only when unable to produce those with higher priorities. We develop a new decomposition method to analyze the behavior of the manufacturing system by decomposing the long production line into small analytically tractable components. As a first step in modeling a production line with more than one part type, we restrict ourselves to the case where there are two part types. Detailed modeling and derivations are presented with a small two-part-type production line that consists of two processing machines and two demand machines. Then, a generalized longer flow line is analyzed. Furthermore, estimates for performance measures, such as average buffer levels and production rates, are presented and compared to extensive discrete event simulation. The quantitative behavior of the two-part type processing line under different demand scenarios is also provided.Singapore-MIT Alliance (SMA

Setting Real Time WIP Levels in Production Lines

A new algorithm is developed for setting WIP level in production lines. It is a pull policy that determines when to authorize a job to be processed. Dynamic information in the system such as machine failures and repairs, and demand variations are taken into consideration to provide an efficient method to dynamically determine the WIP level. Two types of production lines are examined in this paper. The first is production lines which have few random interruptions and the second production line is exposed to frequent interruptions. Experiment results from these two types of production lines illustrate that the new algorithm generates higher service level with lower WIP compared to the CONWIP policy. In addition, the superiority of the new algorithm is more significant when the production line is exposed to frequent random interruptions.Singapore-MIT Alliance (SMA