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organization paradigm: the Virtual Enterprise, which originates from joining
different resources, such as workers, machines, and competencies, coming from
different part of the world. Creating a Distributed Manufacturing System is one
of the best solutions for surviving in the present economic situation. Managing
this kind of systems can result very complicate. Discrete Event Simulation can
help in the analysis of complex problems.
Simulation is the illustration of a system with its dynamic processes in an
experimentation capable model in order to find solutions which are portable
into reality (definition by VDI – guideline 3633). The term “simulation” is
normally used to describe the process of executing a modelling program with
user‐selected parameters and input data for simulating the future evolution of a
system under prescribed conditions. Before a system can be simulated a model
must be defined by abstracting the description of the structure and the
behaviour from the real system.
Usually, the simulation is performed on the base of a unique whole model,
which runs on a computer. This traditional method is not able to represent the
actual organization and manufacturing systems, which are geographically
distributed. The recent emphasis on distributed simulations is now changing
the traditional approach to simulation programs; different computing platforms
interact with each other over a network. This can offer a convenient way of
combining existing software to represent more complex operating realities such
as in the distributed manufacturing.
The emphasis on distributed simulation is often put on the re‐usability and
interoperability of models. Thus, standards become important to ensure model
compatibility. The use of the High Level Architecture can be very helpful in
designing the simulation models for Distributed Manufacturing. This
architecture has been developed in the nineties for military purposes, becoming
a IEEE and OMG standard and applicable for civil use. Despite the potential
advantages and the existence of an international standard, distributed
simulation has not widely utilized yet in manufacturing. The reasons are
typically related to the necessity of hard programming, standard software
interfaces and lost of data and piracy risks.
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1.2 Objective and Structure
The work presented in this thesis intends to design a software layer for the
integration of a discrete event simulation software within a HLA adapter for
use in a distributed simulation environment of manufacturing systems. The
motivation of the work is the followings. Traditional simulations are already
widely adopted in manufacturing organizations. Nevertheless, those
organizations are not capable and don’t have the willingness to change their
way to simulate in the view of a distributed environment. The solution
proposed allows a transparent integration of a traditional simulation tool in a
distributed environment. Transparency means that for the integration process it
is necessary to adopt mechanisms that expose the simulation model in the
distributed environment without redesigning the entire model. In this way, no
further costs nor training are necessary for the distributed simulation.
On the base of a previous work, after defining the requirements for the
integration architecture, a first component of the HLA adapter has been
developed. This layer, called General Adapter or Core Adapter simplifies the
utilization of the HLA standard and the corresponding Runtime Infrastructure
(RTI). Furthermore, it gives warranties against lost of data e facilitates the
integration process. The second layer allows the connection of the first layer to
the simulation software and facilitates the design of the simulation models in a
distributed environment. The common simulation objects included in the
simulators are integrated with entities which permits interactions with the
entire simulation model.
The remainder of the thesis is organized as follows:
• Chapter 2. presents a brief overview on Distributed Manufacturing
Systems. These systems are classified in several categories, remarking
the characteristics and behaviours of each one: Fractal Factory, Bionic
Manufacturing System, Holonic Manufacturing System. Real cases
and relative problems are discussed.
• The Distributed Simulation is outlined in Chapter 3. After describing
how classical simulation tools work, some examples of Distributed
Simulation Systems are presented and their advantages and issues
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evidenced. In particular, the Parallel Distributed Simulation and the
Advanced Distributed Simulation are discussed.
• Chapter 4. includes a description of the HLA standard conception
process and its rules, components and the interface specification.
Advantages and issues concerned with the use of this integration
architecture are evidenced, highlighting the critical points for the final
users.
• In Chapter 5. the design and development process of the HLA adapter
for the software integration is described. The description of each
object in the adapter architecture is performed.
• Finally, in Chapter 6. conclusions, problems and possible solutions for
further researches are drawn.
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2. Distributed Manufacturing
Recently, the worldwide social and economic conditions evolved more and
more rapidly, with the fashion of the fast technological changes. This evolution
happened without any appropriate infrastructure, capable to support the
resulting development. The recent market conditions are the evidence of this
situation. Operating in these conditions is very difficult for the enterprises,
which need to drastically reorganize themselves and sometimes are forced to
give up. The response to this situation is given by new organization paradigms,
which give the agility needed for rapidly changing to market pressures. The
most representative paradigm is the Distributed Manufacturing System.
This chapter gives a brief overview on Distributed Manufacturing Systems.
The Fractal Factory, Bionic Manufacturing System, Holonic Manufacturing
System are described, remarking the characteristics and features of each one.
Real cases and relative problems are discussed.
2.1 Introduction
In modern manufacturing organizations it is a common intention the practice
of directly control all the business processes; this leads to the necessity of
maintaining a great amount of knowledge and competencies. This influences
the development of Information Systems inside the organization, leading to the
creation of monolithic architectures with rigid control structures. A typical
example of this approach is represented by the Computer Integrated
Manufacturing (CIM); this is characterized by a centralized coordination, low
flexibility and adaptability, but also efficiency in intensive and repetitive
processes.
Recent researches in manufacturing and business in general, try to solve
problems deriving from the creation of this kind of systems, introducing new
approaches for the organization and the IT architecture, mainly based on
distributed systems. On the base of different opinions, four common trends are
observed in today’s manufacturing context:
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• Increasing product variety over time;
• Mass production to Mass customization transition;
• Increasing technological complexity;
• Market globalization.
The distributed systems allow the introduction in the manufacturing systems
of new different approaches, such as knowledge supply chains, rapid
realization of products and processes, pervasive modelling and simulation,
adaptative and reactive information systems, flexible workforce, extended
enterprise collaboration, enterprise integration.
For an effective transition from monolithic systems to the new distributed
systems, innovative organization paradigms arise in order to respond to the
global changes of the markets adopting the aforementioned approaches.
Among all, the Distributed Manufacturing proved his effective potentials in the
reaction to new market requirements.
2.2 The evolution of organization paradigms
Organization paradigm is a set of definitions and rules for modelling a
system in order to reach a specific set of goals. Generally, new organization
paradigms arise from new research ideas which identify success characteristics
of an organization or enterprise, such as in the case of the Taylor’s theory. In
other cases, they simply describe the features of a successful organization; for
instance, the Lean Production.
During time, the replacement of new organization paradigms has become
faster and faster; on the one hand, for effectively reacting to new market
requirements; on the other hand, by simply following the fashion without any
real reason. New paradigms inherit, modify and extend the heritage of theories
and concepts of predecessors. Recently, the most successful paradigms are
Computer Integrated Manufacturing, Lean Production, Agile Manufacturing,
Distributed Manufacturing and the Virtual Enterprise.
The Computer Integrated Manufacturing (CIM) is a manufacturing system
based on the computer integration concept. In this type of system the computer
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integration can be seen as the link among different workstation which crosses
cells, departments and even enterprises. In this way, it is possible to obtain
rapid communications and the possibility of exchanging and reusing
information and data in the organization. This paradigm arises from the
necessity to improve enterprise efficiency in large organization with a great
amount of computerized systems (e.g. machine tools, handling systems,
computers) which normally work stand‐alone.
The Lean Production arises in response to the success of Japanese industries,
where techniques such as the Just In Time (JIT) dramatically improved the
efficiency and competitiveness. The Lean Production proved soon to be
winning in the automotive industry, quickly extending the benefits to other
manufacturing areas. The basic concept of this paradigm is that the
organization must be “lean”, which means “doing everything with fewer
resources”. Idle times, huge buffers, long lead times, must be eliminated by
means of JIT, concurrent engineering, total quality management, cost reduction,
improvement of the relations with suppliers and customers.
The Agile Manufacturing is a business concept which leads to the
development of agility characteristics, that is rapidity, readiness and proactive
approach, in order to quickly respond to market changes maximizing the
utilization of knowledge resources. Knowledge and competence must be
exploited and transformed in new processes, products, and services, also
improving the state of the art. It turns out the importance of creating enterprises
from the synthesis of core competencies coming from different enterprises
connect by joint ventures.
The Distributed Manufacturing is a new paradigm which arises from being
aware that every organization is naturally provided with a distributed
decentralized architecture, done that each component, resource, tool, process is
temporally and geographically distributed. The Distributed Manufacturing
represent an extension and evolution of the CIM concept, keeping its distance
from it in the development and evolution of the basic concepts. This
organization paradigm has multiple faces, done that it comprises different
paradigms.
One of the most recent organization paradigms is the Virtual Enterprise. It
inherits the supply chain management concept from other paradigms, such as
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the Distributed Manufacturing, originating the Integrated and Extended
Enterprise concepts. The Virtual Enterprise represents the extreme concept of
crossing the physical enterprises boundaries. Actually, in this paradigm the
organization is virtual, temporary and strictly related to the integration of
advanced information systems.
In order to facilitate the understanding, the Figure 2.1 shows the evolution of
the organization paradigms, remarking the mutual influences. It is interesting
to notice that the temporal evolution takes place along the diagonal of the
graphic represented in the figure, thought that another evolution process
originates from the CIM and proceeds throughout the information technology
systems.
Figure 2.1 ‐ Organization paradigm evolution
2.3 Distributed Manufacturing Systems
Today’s market is characterized by globalization and high competitiveness.
Enterprises must be aware of momentary opportunities and quickly and
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properly react to customers’ demands. Starting from this situation, Distributed
Manufacturing Systems (DMS) try to offer suitable solutions. The following
considerations can be stated:
• Increasing product diversity over time expands risks and costs
associated with the development and implementation of new product
and processes, to levels that are sometimes prohibitive. However,
distributing responsibilities by multiple entities, risks and costs
became acceptable and market opportunity can be achieved;
• Escalating technological complexity enforces enterprise to acquire
knowledge in non‐fundamental domains, which implies increased
time‐to‐market periods. However, distributing competencies by
different enterprises, each one maintains its core competency while
achieving competitiveness in the market;
• Market globalization virtually increases both enterprise opportunities
and risks. Each enterprise has to operate in the global market with
globally based enterprises supplying global products. However,
developing relations and partnerships with such enterprises,
decreases challenges and risks while benefiting from a wider market.
Different management approaches have been adopted, in relation to different
levels of partnership, trust and dependency between enterprises, such as:
• Supply Chain Management, characterised by rudimentary
relationship between supplied and supplier, tasks and technological
competencies distribution, but centralising strategies and risks;
• Extended Enterprise, where entities develop more durable, coupled
and mutual intervening relation, sharing technological and strategic
efforts. Yet, supplied entity maintains a dominant position over
suppliers;
• Virtual Enterprise, is a very dynamic and restructuring organisation,
where supplier and supplied are undifferentiated and no dominant
position exists.
Even if the previous description relates to inter‐organizational context, the
same characteristics and behaviours (distribution, decentralisation, autonomy
and dependency) are also suggested in an intra‐organizational context. A
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simple example is represented by workgroups that emphasise self‐
competencies while combining efforts for a global response to external
requirements.
The DMS is an abstract concept (i.e., a class of systems) characterised by a set
of common features and behaviours, with several specific characterisations (i.e.,
instantiations), named organisational paradigms.
2.3.1 DMS’ Properties
DMS are characterised by several properties and behaviours. Such features
relate both to the overall system and each composing entity. Basic properties
include:
• Autonomy: an entity is said to be autonomous if it has the ability to
operate independently of the rest of the system and it possesses some
kind of control over its actions and internal state. I.e. autonomy is the
ability of an entity to create and control the execution of its own plans
and/or strategies, instead of being commanded by other entity (e.g., a
master/slave relationship);
• Distribution: a system is said to be distributed if different entities
operate in the system;
• Decentralization: it means that an operation/competency can be
carried out by multiple entities. One single system can be
simultaneously centralised and decentralised. I.e. (de) centralisation
refers to operations not to the system itself;
• Dynamism: refers to changes in manufacturing systemʹs structure and
behaviour during operation. This express different competencies,
responsibilities and relations between entities;
• Reaction: an entity is said to be reactive if it adjust its plans according
to its perceptions.
Other properties deserve a deeper description and as such will be presented
in the following subsections.
Flexibility
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Flexibility is the ability the system exhibits during operation that allows it to
change processes easily and rapidly in a predefined set of possibilities each one
specified as a routine procedure, defined ahead of time so that the needs to
manage it are in place.
The flexibility of manufacturing systems is related to physical flexible
machinery. Flexible is intended in the sense that the machines (or cells) are able
to execute several operations. In addition, they can quickly change among
different production plans according to the partʹs type to manufacture at a
given point in time. The concept of Flexible Manufacturing System (FMS) is
very popular for companies who produce small lots of each product, mixing
different lots in the production flow. One of the main problems in achieving
flexibility is related to transportation. Since a product will need to pass through
several workstations in order to be manufactured and different products will
have different routes, the transport links between workstations should be as
ʺfreeʺ as possible.
Adaptability
Adaptability is the manufacturing system ability to be maintained easily and
rapidly, in order to respond to manufacturing requirements, based on its
shopfloor constraints. Adaptability refers to production facilities
reconfiguration and scalability, workforce that has the incentive and flexibility
to respond creatively to customer needs and thus requires flexibility.
A system is said to be adaptable if it can continue to operate in the face of
disturbances changing its structure, properties and behaviours accordingly to
new situations it encounters during its lʺife‐timeʺ. A disturbance is any event
not previously and formerly specified (e.g. machine breakdown or a new type
of product to manufacture). However, it is very hard to predict every
disturbance that may happen.
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Figure 2.2 –Cost/Time plot for adaptable & non‐adaptable systems
Figure 2.2 shows the expected cost for developing, installing and running an
adaptable and a nʺon‐adaptableʺ system. It is evident that in the development
phase the cost of an adaptable system is higher than a rigid one. The main
reason for this is that the programming effort for fault–tolerance,
reconfiguration, etc. is greater than the one that is necessary if the problem is
simplified by not adding this functionality. On the second sector (installation)
the cost of the adaptable system is still higher than the non‐adaptable system.
The effort for configuring all the components and extra‐coding necessary for
additional information feedback from the hardware is the main reason for this
higher cost. On the third stage (exploitation), it is expected that the adaptable
system will give rise to lower costs by handling disturbances with minimum
human intervention, without stopping the production or causing great delays
and long waiting queues. Since this is the longest phase (the system is supposed
to be ʺup‐and‐runningʺ for several years), the higher costs of the initial phases
are attenuated.
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