FMS - Flexible Manufacturing Systems
Integrated
Methodology for the Design of FMS – Flexible
Manufacturing Systems
1.0 INTRODUCTION
Generating a modelling approach for the design of
Flexible Manufacturing Systems (FMS) using a concurrent
engineering approach assumes independent operation
of four modules:
¤ Automation
and Robotics
¤ Command and Control
¤ Production Planning and Plant Layout
¤ Maintenance Planning and Logistics.
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FMS - DESIGN AND CONCURRENT ENGINEERING
A FMS is defined as an integrated and automated
production system containing:
(a) Flexible process equipment, normally
automated machines with numeric
control and equipped with quick
tool change ability,
(b) Material handling equipment including
transfer lines or conveyor belts,
forklifts, elevators, automated
guided vehicles (AGVS) as well as
automated storage and inventory-handling
systems such as automated
storage and retrieval systems (ASRS),
(c) Sophisticated computerised communication
and control systems integrating process
and material
handling equipment, and
(d) A modern maintenance support
structure that can bring the system
quickly back to normal after equipment
failure. The design of such facilities is a time-consuming
multidisciplinary effort with several production-related
objectives that may include the minimisation of the
transfer cycle duration, work-in-progress and other
inventory, set-up times and the amount of fixed investment
required.
Other
operational objectives such as the maximisation
of flexibility, reactivity (or the ability
to handle contingencies), availability and
productivity
should also be taken into account in particular
for FMS designed to do batch jobs, small
and medium-sized
series in addition to mass production volumes.
Design
costs are significant as they include system
engineering, project preparation and test,
installation, personnel
training in addition to direct and indirect
operating costs after installation.
Flexibility is a particular important design objective
implying that the same production line
can be used for different products, either
sequentially or simultaneously
without major transformation costs. Stigler first introduced
the concept in 1939 as the slope in the production
cost function. A system is said to be flexible if the
production cost function is almost flat for a given
volume interval, meaning that a slight increase in
volume will increase productions costs very little.
A
graphic representation is provided below
showing that technology II is much more flexible
than technology
I in the production interval VI - V2. In
today's literature, this concept is known
as volume flexibility but the
concept has been extended to cover other
areas as well.
Browne
proposed a taxonomy of flexibility that is often
used as a reference6.
Combining that taxonomy with the one generated
by Sethi and Sethi (1990) (7) flexibility
may be define with regards to:
¤ Volume
¤ Product
mix (allowing simultaneous processing of different products),
¤ Parts
(can be added to or eliminated from a production line or equipment),
¤ Routing
(alternative production paths available in case of breakdown or sudden
changes in demand patterns),
¤ Product
design changes,
¤ Process
sequence changes,
¤ Machine
tools and
¤ Expansion
of the overall system.
The
advice is to prioritise these types of flexibility
and to define the required type of flexibility
for the specific system under consideration (8).
The
main economic advantage of such systems is the
capability to manufacture parts and products
economically and in small volumes with the ability
to respond
to market changes, quality problems, design
changes,
scheduling conflicts with a low break-even
volume, low supervisory costs and low reject
levels. The
disadvantages are well known: high initial
acquisition costs and dependence on highly skilled
maintenance
and computer programming personnel (9).
The concept has been applied to a wide range of
manufacturing industries processes requiring shape
and form changing (machining, metal forming, plastic
moulding and forming, wood and fibre processing),
chemical transformation (plastics, pharmaceutical),
assembly (where robots have had a major impact in
design), information (to monitor and control manufacturing,
co-ordination and decision-making) and transportation
(raw materials, in-process goods, finished products
and other resources).
The process determination implies a choice of technology
(labour, machines, energy sources and other inputs)
for which the most important criteria are feasibility
and cost.
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The
choice of technology requires close link with
product design,
a function that is concerned with
functional and aesthetic requirements necessary
to meet actual or potential market needs at an
acceptable rate of return (10).
Design
for Manufacturing methods have been developed
as
an alternative to
decrease total development time and improve
the consideration of life cycle issues
during product design11.
Concurrent engineering methods suggest that product
and
process development should proceed in parallel,
if possible.
The ISO 9000 standard for quality assurance
also stresses the coherence needed between
product and
process design processes. The argument
is particularly important given the short average
product life
and the relatively long product development
cycles observed
in modern industry.
In
process design, the choice of equipment, the
set or processing steps and their sequence will
determine
the material flow through the future plant,
volumes
and physical placement of raw materials,
in-process and finished good inventory as well
as bottlenecks
and congested areas, most of which are useful
information for layout decisions. Graphs are
widely used internationally
to represent alternative choices in FMS process
design (12).
Equipment choice is particularly important in FMS
design and the decision criteria should include total
investment, maintainability, future obsolescence,
labour skill requirements, quality consistency, tools
requirements, output rate and overall flexibility.
There is often a choice between general and special
purpose equipment, the former designed to accommodate
a wide range of transformations.
The complete set of criteria to evaluate process
design choices should include technological feasibility,
financial considerations, training requirements for
operators and maintenance personnel, compatibility
with existing facilities, raw material requirements,
equipment size and weight as well as other physical
requirements (safety, temperature, water, waste,
etc.), maintainability and spare part requirements.
In
general, economic pressures on the manufacturing
industry require quick response to new markets
and products, all subject to uncertain demand
patterns
in a very competitive global environment.
The trends are towards (13):
a) Increasing global competition and pressure for
lower costs,
b) Demanding customers with rapidly changing expectations;
c) Accelerating technical evolution;
d) Pressures to shorten lead times and
to decrease work-in-progress throughout
the production
system; e) Shorter product life
cycles and longer R&D
cycles,
f) Demand for increased production system flexibility
and overall efficiency.
Effective
facility layout alone can reduce material handling
costs by 10% to 30%(14).
In addition, plant layout can reduce initial
investment costs,
work-in-progress inventories and manufacturing
lead times15;.
Projected future research needs points out towards
concurrent consideration of layout and production
system design
issues (16).
Serial
engineering leads to unacceptable delays due
to the
sequential nature of main design activities
and necessary corrections (17).
Local decisions made by various experts
are isolated in time, space
and
function. Concurrent engineering on the
other hand is "a systematic approach to the
integrated, concurrent design of products and their
related processes,
including manufacture and support".
This approach is intended to cause the
developers, from the outset,
to consider all elements of the product
life cycle from conception through disposal,
including quality,
cost, schedule and user requirements (18).
It is a common-sense approach to product
and process design as well as
support.
The
clear specification of product requirements through
its life cycle from
the start of conception
can lead to significant cost reductions
both in design and production as well
as shorten the development
process.
Concurrent
engineering approaches integrate the functions
of
product design, process planning, installation,
production and final distribution as
well as the experts from various functions such
as company
designers,
R&D specialists, industrial engineers,
experts in automation and robotics, production
planners and
schedulers, marketing managers and others.
As a consequence, the simultaneity of
the activities integrates the
views from various sectors and functions
to reduce the total design time significantly.
There is particular
interest in functions that increase product
quality and performance, reduce product
manufacturing or
investment costs as well as reduce lead-time
for design and manufacturing (19).
In process design, concurrent engineering means
the integration of conceptual design, concept
optimisation,
factory design, detailed design (CAD,
CAM and CIM plus 3D Simulation), detailed
process plans including
NC programs and machine tooling.
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