A Review of Different Approaches to the FMS Loading Problem A. Grieco, Q. Semeraro, T. Tolio

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A Review of Different Approaches to the FMS
Loading ProblemA. Grieco, Q. Semeraro, T. Tolio

• Salih ÖZTOP
• 20202801
• 07.04.2003

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Context
1) Introduction 2) Analysis of Loading Problem 3)
Literature Review
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1. Introduction

• Aim of the paper
• Address the different factors affecting the
  loading problem in Flexible Manufacturing
  Systems(FMS)
• Survey previous studies addressing some of these
  problems
• Give some future research directions

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1. Introduction

• First article Short-term production planning
  problems in FMSs, Stecke and Solberg (1981)
• Few of them focused on loading problem

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2. Analysis of Loading Problem

• Elements affecting the loading problem
• Characteristics of the FMS
• Characteristics of the plant where FMS operates
• Production Planning Hierarchy (interface of
  loading module with upper and lower level of the
  management hierarchy)

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2.1 Characteristics of the FMS

• Machines
• Control System
• Tools
• Tool Handling System
• Parts, Pallets and Fixtures

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2.1.1 Machines
M1 First FMSs different machines with
different capabilities New FMSs identical
machines Reasons for this evaluation -More
versatile machining centers due to increased
number of control axes, increased spindle power
and speed. Parallel machine FMSs vs. General
FMSs M2 Grouping some identical machines Ex.
For higher accuracy assign jobs to this
group. M3 Under tight part position tolerances,
two or more operations are done on the same
machine
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2.1.2 Control System

• Control System types
• A Supervisor controls and coordinates different
  machines, CNCs and PLCs
• Computerized Numerical Controls (CNCs) controls
  specific device
• Programmable Logic Controllers (PLCs) controls
  specific device

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2.1.2 Control System
Lack of standardization in those controllers
causes some problems -Integration of hardwares
of different companies is difficult -Integrating
FMS with other systems in the same
company -Changing control hierarchy -Integrating
new sensors in an existing control system Some
projects continue to standardize control systems
such as OSACA,OMAC-TEAMAPI,OSEC...
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2.1.2 Control System
Since part programs in CNCs are seen as a whole
indivisible program, following problems
arise. P1 All tools required in the program are
requested at the beginning P2 A tool assigned to
a part program is busy for the whole part program
duration P3 Whole part program should be
executed on the same machine P4 Sequence of
operations are given P5 No alternative operation
is considered
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2.1.3 Tools and Tool Handling System
T1 Tool life issues Tools subject to wear and
need to be reconditioned T2 Number of tool
copies Expensive, only few copies of that tool
type is available TH1 Tool magazine
capacity Finite capacity. Large tool magazine
size ? high tool seek time?high spindle idle
time TH2 Tool transport system Automatic tool
transport system that can exchange the tools in
the tool magazines of different machines and tool
store.
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2.1.4 Parts, pallets, and Fixtures
Parts should be positioned correctly before
machining operations and transpotation activities
therefore they require some fixtures and
pallets Parts are mounted on fixtures and
fixtures are mounted on pallets
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2.1.4 Parts, pallets, and Fixtures
F1 Part Loading In general parts are manually
loaded to the fixtures But now automated
load/unload robots are present. F2 Fixture Cycle
Time After usage of the fixture, it can need to
be cleaned or inspected before going back to the
load/unload station. F3 Parts requiring more
than one fixture To change the position of the
part, new fixtures are needed. Single fixture is
not enough to complete all the machine work on
that part F4 More than one part type on the same
fixture When required part amount is small, one
fixture type can be used for different parts
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2.2 Characteristics of the plant where FMS
operates
E1 Characteristics of the Shifts FMSs are
automated, but they require people for
loading/unloading, monitoring, placing parts to
fixtures etc. Unpersonned shifts are an important
constraint E2 Tool Room Management Tool room
give service to all of the plant not just the
FMS. E3 Preventive Maintenance In preventive
maintenance times whole FMS or some of its
machines can be unavailable. E4 Downstream
Assembly Operations If the parts produced in FMS
are components of other assembled products, then
the part ratios should be considered as a
constraint.
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2.3 Production Planning Hierarchy
Loading model must fit into the
production-planning hierarchy The
characteristics of the model affected by the
higher or lower levels of the hierarchy
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2.3.1 Higher Level
H1 Requests are handled periodically or
continuously Generally periodic handling of
request is used but in small firms continuous
case is mostly used due to the dynamic structure
of this firms H2 Due Dates/ Release Dates If due
dates are decided at the higher levels, they are
used as an input to the loading level. If they
are not decided at higher level, loading problem
should be modeled differently H3 Priorities In
real life some parts have higher priority than
the others. Ex. Parts of strategic
clients Contracts can cause some priority
restrictions
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2.3.2 Lower Level
Lower levels are affected by the loading level
decisions. Most of the details of the lower
levels are not very important L1Unforeseen
Events Unforeseen events of little importance
such as tool breakages are dealt at lower levels.
If the problem causes a capacity decrease then a
new loading plan is needed quickly. L2 Limits
of the Lower Levels Lower levels can create
constraints that are not directly related to FMS
but related to the management software. Ex. FMS
includes a tool transport system but the
management software cannot handle this tool
scheduling.
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3.1 Literature Review
Articles formalize the loading problem as -Mixed
integer programming problem -0-1 programming
problem In solution cases they slightly modify
the models with relaxation and suggest some
heuristics to overcome the computational problems
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3.2 Types of Loading Problems in Literature
Parallel(identical) vs. General FMSs Parallel
systems are special form of general case.Since
there are several parallel FMSs, they take
interest of researchers
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3.2.2 Tool Management Strategy
1) Batching (Tool Dominant) Most common
one Configuration of each tool magazine is frozen
for a given length of time. In that period
machines can only use the tools at the tool
magazine 2) Flexible (Part Dominant) Continuos
evaluation of the tool magazine configuration. 3)
Hybrid Continuous evaluation with some limitations
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3.2.2 Tool Management Strategy
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3.3.1 Objective Functions
1) Directly connected to the firms goals Cost
minimization Flow time/WIP minimization Minimizati
on of number of late part types Minimization of
makespan 2) Not connected to the firms
goals Workload balance among work centers Load
minimization of some sub systems Maximization of
number of alternative routings
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3.3.1 Objective Functions
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3.3.2 Constraints
1) Assignment Constraints They limit the
assignment of parts to machines due to
technological constraints and to reduce the
computational complexity by reducing the solution
space 2) Capacity Constraints Tool magazine
capacity, limited number of fixtures/pallets,
limited number tools for each tool type,
available machining time 3) Management
Constraints Workload balancing among machines Due
date restrictions Part priorities
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3.3.2 Constraints
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4. Future Research Directions
1) Test Problems To compare different algorithms
to similar problems a common test problem data
collection should be formed 2) Clarify the
characteristics of production planning
hierarchy 3) Due date requirements 4) Unforeseen
events Can make the loading planning
impractical 5) Tool life management with
unpersonned shifts 6) Integrate production and
process planning Same part can be obtained by
different operations 7) Tool selection and
operation sequence relaxation Same job can be
done by another tool and with different operation
sequences
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5. Questions Answers
Thank you