An introduction to Factory Physics

1. An introduction to Factory Physics

2. Main Objectives • Provide an analytical characterization of the operation of production systems and their performance, based on queueing-theoretic models • Derive qualitative insights on the attributes and factors that shape the behavior and performance of these systems. • Focus primarily on flow lines, since they are the main layout used in the context of high-volume, repetitive manufacturing. • Also, flow line dynamics are easier to trace and analyze, and therefore, more enlightening in terms of qualitative and quantitative insights. • However, many of the derived insights and results are extensible to more complex environments either directly or through some appropriate decomposition.

3. A conceptual characterization of the considered environment • Flow line: A sequence of workstations supporting the production of a single part type. • Each workstation consists of one or more identical servers executing one particular stage of the entire production process. • The part processing time at each workstation follows some general distribution which must be defined in such a way that accounts for the various detractors affecting the station operations; these detractors will include machine downtime, lack of consumables, operator unavailability, experienced set-up times, preventive maintenance, etc. • Finished parts could constitute end items or raw material for some other downstream process. • The operation of the line workstations can be decoupled through the installation of some buffering capacity between them. • Some performance measures of interest: line capacity (i.e., maximum sustainable production rate or throughput), line cycle time, average Work-In-Porcess (WIP) accumulated at different stations, expected utilization of the station servers.

4. Production Authorization Mechanisms • The issue here is to what extent part production is triggered from actual orders or from forecasted demand. • In a “produce-to-stock” scheme, a certain amount of end-item inventory is maintained in an effort to serve the experienced demand with zero lead time. • “Produce-to-stock” operation is most appropriate for highly commoditized and standardized items. • A key performance measure for “produce-to-stock” production systems is the fill rate, i.e. the percentage of the experienced demand that is actually met from stock. • In a “produce-to-order” scheme, end items are produced in response to particular orders. • “Produce-to-order” operation is more appropriate for (highly) customized items. • A key performance measure for “produce-to-order” production is the attained service level, i.e. the percentage of orders that are served within the quoted lead time. • In practice, many systems are a hybrid scheme consisting of some “produce-to-stock” and some “produce-to-order” components. • In particular, today’s mass customization is supported by an “assemble-to-order” scheme where end-items are assembled to order from a number of sub-assemblies that are produced to stock.

5. Shop-Floor / Line Control Mechanisms • Mechanisms that control the part release and advancement through the line. • They are broadly distinguished into “push” and “pull” mechanisms. • A “push” mechanism releases material into the line according to a target production rate, and material is advanced to downstream stations as early as possible. • Typical instantiations of “push” systems are: the asynchronous transfer line and the synchronous transfer line. • A “pull” system controls the part release and advancement in the line taking into consideration the status of the various workstations in the line. • Typical instantiations of “pull” systems are: the KANBAN and the CONWIP (controlled) production lines. • In general, “pull” systems reduce congestion, since they take into consideration the actual shop-floor status in their decision making, but the same mechanisms will also make them more inert in case of shifts in the production level. • Both mechanisms are effectively implementable in a “produce-to-stock” or “produce-to-order” context.

6. Asynchronous Transfer Lines W1 W2 W3 TH TH TH TH B1 M1 B2 M2 B3 M3 • Some important issues: • What is the maximum throughput that is sustainable through this line? • What is the expected cycle time through the line? • What is the expected WIP at the different stations of the line? • What is the expected utilization of the different machines? • How does the adopted batch size affect the performance of the line? • How do different detractors, like machine breakdowns, setups, and maintenance, affect the performance of the line?

7. Synchronous Transfer Lines • The key issue (Assembly Line Balancing – ALB) • Given • a set of tasks to be supported by the line stations • each possessing a nominal processing time, • a number of precedence constraints among these tasks, • and a target throughput, • determine • a partitioning of these tasks to a number of stations that observes the aforementioned specifications, while it minimizes the resulting number of stations (and therefore, the resulting labor cost).

8. Station 1 Station 2 Station 3 KANBAN-based production lines • Some important issues: • What is the throughput attainable by a certain selection of KANBAN levels? • What is the resulting cycle time? • How do we select the KANBAN levels that will attain a desired production rate? • How do we introduce the various operational detractors into the model?

9. FGI Station 1 Station 2 Station 3 CONWIP-based production lines • Some important issues: • Same as those for the KANBAN model, plus • How can we compare the performance of such a system to that of an asynchronous transfer line and/or a KANBAN-based system?

10. Plan for this part of the course • Modeling and Performance Analysis of Asynchronous Transfer Lines through a Series of G/G/m queues • Investigating the effect of blocking • Design of Asynchronous Transfer Lines • Design of Synchronous Transfer Lines • Modeling the impact of operational detractors • Employing factory physics in line diagnostics • Modeling and Performance Analysis of CONWIP-based production lines through Closed Queueing Networks • Designing batching policies based on factory physics • A comparison of the various “push” and “pull”-based production systems

11. Reading Assignment This part of the course is based on • Chapters 7-10 and • Chapter 18 of your textbook, plus on • any other material reported or quoted in class.