An introduction to Factory Physics

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An introduction to Factory Physics
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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.

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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.

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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, todays 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.

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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.

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Asynchronous Transfer Lines
W1
W2
W3
TH
TH
TH
TH
B1
B2
B3
M1
M2
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?

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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).

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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?

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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?

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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

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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.