Chapter 13 Flexible Manufacturing Systems

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Chapter 13Flexible Manufacturing Systems
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Agenda

• Flexibility
• Volume-variety relationships
• Key characteristics of various manufacturing
  systems
• What is an FMS?
• Physical Components of an FMS
• Control components of an FMS
• Operational problems in FMS
• Layout considerations
• Sequencing of robot moves in robotic cells
• Simulation modeling
• FMS benefits

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Flexibility

• Flexibility has multiple dimensions and the
  concept of flexibility still remains vague
• Flexibility can be defined as a collection of
  properties of a manufacturing system that support
  changes in production activities or capabilities
• The flexibility refers to the ability of the
  manufacturing system to respond effectively to
  both internal and external changes by having
  built-in redundancy of versatile equipment

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Types of Flexibility

• Machine flexibility the capability of a machine
  to perform a variety of operations on a variety
  of part types and sizes
• Routing flexibility a parts can be manufactured
  or assembled along alternative routes.
• Process flexibility (mix flexibility) the
  ability to absorb changes in the product mix by
  performing similar operations or producing
  similar products or parts on multipurpose,
  adaptable, CNC machining centers
• Product flexibility (mix-change flexibility) the
  ability to change over to a new set of products
  economically and quickly in response to markets
  or engineering changes or even to operate on a
  make-to-order basis.
• Production flexibility the ability to produce a
  range of products without adding major capital
  equipment, even though new tooling or other
  resources may be required
• Expansion flexibility the ability to change a
  manufacturing system with a view to accommodating
  a changed product envelope
• The product envelope is the range of products
  that can be produced by a manufacturing system at
  moderate cost and time, is determined by process
  envelope
• The process envelope is the hardware and
  software capabilities of a manufacturing system

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Volume-Variety Relationships
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Characteristics of Manufacturing Systems

• Transfer line
• Machines dedicated to produce one or two product
  types. No flexibility
• Maximum utilization and high production volume
• Direct labor involvement is minimum
• Low unit cost of production
• Flexible Manufacturing Module (stand-alone NC
  machines)
• Highest level of flexibility
• Low utilization and low production volume
• Unit cost of production is much higher than for a
  similar product manufactured on a transfer line

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Characteristics of Manufacturing Systems

• Manufacturing cell
• Low to mid volume
• A variety of parts are manufactured in batch mode
• A manufacturing cell is an FMS without central
  control
• More flexible than an FMS but lower production
  rate
• Special manufacturing system
• A fixed-path material-handling system links the
  machines together
• Least flexible catergory of CIM system
• Uses multispindle heads and low-level controller
• High production rate and low unit production cost
• FMS
• Mid-volume, mid-variety
• Permits both sequential and random routing of a
  wide variety of parts
• Higher production rate than a manufacturing cell
  and much higher flexibility than a special
  manufacturing system

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What is an FMS?

• An FMS is an automated, mid-volume, mid-variety,
  central computer-controlled manufacturing system
• In an FMS, families of parts with similar
  characteristics are process GT and CM
• An FMS consists of two subsystems
• Physical subsystem workstations,
  storage-retrieval systems, material-handling
  systems
• Control subsystem hardware and software

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Physical Components of an FMS

• NC machine tools
• Horizontal spindle machine is more flexibility
  than vertical spindle
• Machining centers with NC of movement up to 5
  axes
• Machining centers have the flexibility of
  performing a wide variety of operations.
• Choice of a machining center shape of parts,
  accuracy, weight, compatibility with
  material-handling and storage systems
• Workholding and tooling considerations
• Fixtures must be designed to minimize
  part-handling time
• To reduce buildup error, some fixtures should be
  pinned to the pallets
• High usage of fixtures identification, storage,
  retrieval of fixtures and integration with AS/RS
  and material-handling systems
• Tool magazines disk type, drum type, turret
  type, chain type
• Material-handling equipment robots, conveyors,
  AGV should integrate with machining centers and
  AS/RS
• Inspection equipment Integration of inspection
  equipment with machining centers (CMMs).
• Other components the FMS has deburring and
  cleaning station

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Control Components of an FMS

• Manufacturing a variety of parts involves
  real-time coordination of various subsystems
• Control functions
• Work-order processing and part control system
• Machine-tool control system including inspection
  machines
• Tool management and control system
• Traffic management control system
• Quality control management system
• Maintenance control system
• Management control system
• Interfacing of these subsystems with central
  computer

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Operational Problems in FMS

• Part selection and tool management
• determine a subset of part types form a set of
  part types based on a number of criteria
• Constraints limited availability of tools, tool
  requirements
• The batching approach the selected part types in
  a particular batch are manufactured continuously.
  Remove all tools of the current batch. Models
  (reading)
• The flexible approach new part types are
  introduced if space becomes available in the tool
  magazine ? utilization?

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Operational Problems in FMS

• Tool allocation policies
• Bulk exchange policy each planning horizon, a
  set of tools is mount on the tool magazine to
  determine the batch sizes of parts. No
  replacement
• Tool migration policy similar to the bulk
  exchange policy. However, the tools are replaced
  once the parts are processed to make room for
  tools for processing other parts
• Resident tooling policy a clusters of tools are
  kept permanently at various machine. Tool changes
  occur only when a particular tool reaches the end
  of its scheduled life
• Too sharing policy tools are resident on
  machines based on tool clustering
• Assignment of parts to machines random for the
  bulk exchange and migration policies. The others,
  specific parts are assigned to specific machines
  based on the availability of tooling on those
  groups

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Operational Problems in FMS

• Fixture and pallet selection
• number of pallets(parts required per shift) x
  (average pallet cycle time)/(planned production
  time per shift) x (number of parts per pallet)
• Note round up the result
• Machine grouping and loading, considering part
  and tool assignments group the machines,
  allocate operations and tools required for the
  selected part types among machine groups.
  Allocation criteria
• Balance the assigned machine processing times
• Minimize the number of movements from machine to
  machine
• Balance the workload per machine for a system of
  groups of pooled machines of unequal sizes
• Fill the tool magazines as densely as possible
• Maximize the number of weighted operations
  assigned to the machines

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Example 1

• Consider the following data available from a
  simulation study
• Parts required per shift 20
• Average pallet cycle time 120 min
• Planned production time per shift 480 min
• Number of parts per pallet 1
• Solution apply the formula we have, the number
  of pallets required (20 x 120)/480 5

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Layout Considerations

• The layout of machines to process part families
  in an FMS is determined by the type of material
  handling equipment used
• Types of layouts
• Heuristic algorithm for circular and linear
  single-row machine layouts

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FMS Layouts
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Layout Considerations

• Objective sequence the machines so that the
  material-handling effort is minimized
• Input data
• m number of machines
• fij frequency of trips between all pairs of
  machines, i?j
• cij material-handling cost per unit distance
  between all pairs of machines
• Step 1 From the frequency and cost matrices,
  determine the adjusted flow matrix
• Step 2 determine . Obtain
  the partial solution by connecting i and j. Set
  
• Step 3 determine
• Step 3.1. connect q and p and add q to the
  partial solution
• Step 3.2. delete row p and column p from
• Step 3.3 if p i, set i q otherwise, set
  j q
• Step 4 repeat step 3 until all the machines are
  included in the solution

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Example 2
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Example 2
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Sequencing of Robot Moves in Robotic Cells

• Determine optimal sequence of robot moves to
  minimize the cycle time in a two-machine robotic
  cell

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Sequencing of Robot Moves in Robotic Cells

• Alternative 1
• T1 ? ? ? 3? ? ? ? a ? ?
  ? b 6? 6? a b
• Alternative 2
• T2 ? ? ? 2? w1 ? ? ? 2? ?
  ? ? ?w2
• 6? 8? w1 w2 ? ? w1 w2
• ? ? max 0, b - ?
  max0, a - ? - max0,b- ?
• ? max ?, b max0, a - max?,
  b ? max max?,b, a
• ? max ?, b, a 4? 4? max 2?
  4?, a, b
• Where a, b processing times of machines M1, M2
• ? time for each pickup, load, unload, and
  drop operation
• ? the robot travel time between any pair of
  adjacent locations
• w1, w2 the robot waiting times at M1,M2
• w1 max0, a - 4? - 2? -w2 and w2 max 0, b
  - 2? - 4?
• ? 4? 4?
• ? 2? 4?
• ? ? total time of the robot activities

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Algorithm

• Step 0 calculate ? 2? 4?
• Step 1 ? ? maxa, b, then T2 is optimal.
  Calculate T2 and stop. Otherwise, go to Step 2
• Step 2 if ?gtmaxa,b and 2?? a b, then T2 is
  optimal. Calculate T2 and stop. Otherwise, go to
  Step 3
• Step 3 if ?gtmaxa,b and 2?gt a b, then T1 is
  optimal. Calculate T1 and stop

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Example 13.6
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FMS Benefits

• Flexible manufacturing
• Improve productivity in mid-variety, mid-volume
  systems
• System flexibility
• Responsiveness to short-term problems the NC
  machines and the control systems
• Responsiveness to long-term problems how to cope
  with new products and changing product mixes an
  volumes
• ?
• Reduction in direct labor cost
• Improved operational control through feedback
  control mechanism and reduction in the number of
  uncontrollable variables
• Improved machine utilization
• Reduction inventory