Multi-Agent System for Flexible Manufacturing
Systems Management
Krzysztof Cetnarowicz, Jarosław Koźlak
Institute of Computer Science
AGH - University of Mining and Metallurgy
Al. Mickiewicza 30, 30-059 Krakow, Poland
e-mail:[email protected]
Abstract. The paper focuses on the application of a multi-agent system for a
management process. The presented system is working on the structure of graph,
where nodes represent decision modules and edges – technological processes.
Kinds of agents working in such environment are presented and an overview of
the interaction protocols which can be used in such a system is given.
1 Introduction
The development of production systems introduces new tasks, such as optimal production systems designing, efficient and safe production systems structure management
or management of optimal functioning of a production process. On the other hand,
proper management of a production system structure is very important for the enterprise rentability and common problem. The contemporary production system becomes
more and more complex and must beflexible for modifications. It makes a management
process very difficult, inefficient and expensive.
It seems that these tasks should be supported in their realization by computer systems, or even totally managed by them. It may be realized when management systems
are provided with models of a production process realized as decentralized systems.
Attempts at realizing such systems improving the process of production systems functioning can be found in [7].
An optimal movement of processed objects in the production system depends on a
proper structure of the production system layout and real time routing, which can make
use of the current production system structure in an optimal way. A proper choice of the
production system layout structure has a basic influence on the effectiveness of activity.
Consequently, it seems that multi-agent systems can be used as a model for a more
complex computer management by means of effective object (parts) routing, but the
evolution of the production system layout must be steered dynamically, thus adapting
its structure to the current needs.
2 Principles of functioning of decentralized system managing
computer network
We may determine two goals of the computer management system for the production
process [13], [1], [9], [5] (fig. 1):
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– management in real time to obtain the optimal functioning of the production process,
– management of the production process structure (layout) to be adapted to the production realization.
Fig. 1. Schema of a computer system (with multiagent model) for production process management
To develop a multi-agent managing system for the production process we have to
take a particular point of view to consider the production process as a graph environment
for the agents population. In general, the production system may be represented as a
network with boxes representing technological operations such as: milling machine,
lathers, drills, grinder, etc. with a common operation: transport of parts fig. 2. The parts
enter the system, are treated and leave the system at the output.
Fig. 2. Schema of the production system layout consisting of milling machine, lathers, drills, heat
treat, grinders, gear cutting and assembly as an output with circulating parts 1, 2, 3, 4.
So the environment of that production system could be considered as a directed
graph created in the following way (fig. 3):
– The production system is composed of a technological operation and edges representing the operation, that follows.
– The transport of parts between operations is considered as a technological operation.
– A node is placed between every technological operation.
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– The whole production system layout may be mapped to a graph called a precedence
diagram.
– The edges represent technological operations, the nodes - decision nodes linking
following operations and enables to take decision to switch the way of a part and
select a proper path in the system.
– The part is treated in the system following a properly chosen path in the graph.
Fig. 3. Schema of the production system layout - a, corresponding precedence diagram - b.
The optimal treatment of a given part may be considered from two points of view:
– point of view of the part: use the cheapest or the quickest way of operation execution with a given quality standard,
– point of view of the system: prepare the best adaptation of the system to perform
demanded operations (be competitive) or use the cheapest way of the system modification to adapt it to the treated parts needs (demand).
The optimal management may be expressed in two ways:
– Production management system functioning is based on the essential principle that
defines conditions of a sending treated parts between the nodes following the optimal path (that is created in real time) of the part treatment.
– The production system layout must be dynamically adapted to the treatment needs
in an optimal way.
These two points are dependent between them and must be considered as an integrity during the management process.
The main principle of the multiagent management system functioning is that the
management process bases on the marked oriented decisions methods. To make a possible application of the marked oriented management, it is necessary that every part
pay for the technological operation executed on it. It means that a part pays for transfer
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through every edge between the nodes. Therefore a part is considered as a special agent
(Part Agent - PA). Its task is to go by the graph from the source node to the destination
node in given conditions and follow a path corresponding to the demanded technological operations [2].
The conditions (the quickest, the cheapest treatment, etc.) are known to a given
agent PA and are a basis for realizing a routing process of choosing dynamically the
proper path.
The payment may be done
– directly:
For the purpose, each node (exactly - output port on an edge) has an account
opened. Appropriate sums may be paid into the account by the treated part,
Each processed part which is to be treated in the system has its account and
transfers an appropriate payment for sending to an account of a sending port
for the treatment by the corresponding edge.
Agents authorized to use the founds accumulated on accounts may perform all
operations on the accounts with the intermediary of network operations.
– indirectly:
In a simplified case, a sending port obtains a generalized payment in a form
of some number of (stipulated) points. The number of points collected makes
evidence on activity of a given port, and what follows - a given operation (treatment) corresponding to the edge in a graph.
The points collected may then be changed into cash and transferred from the
funds, (e.g. centrally fixed), planned to be spent on production system functioning or development.
The points collected may then be used as a basis to take decisions concerning
the optimal functioning and development of production system.
The payments make a basis for market oriented decisions related to modifications
(extension, decrease) of certain parts of a production system. Decisions are undertaken
and then realized by the multi-agent system managing the production system.
3 Multi-agent system managing the production system
The production system is dynamically mapped into the model (of the form of graph).
The model is managed by the multiagent system. The multi-agent system consists of
two main parts: a population of different kinds of agents and the environment in which
the agents work. The environment is determined by the model of the production system, and may be considered as a particular graph (Fig. 3). The model is created and
dynamically updated to take into consideration the circulation of the treated parts in the
system. There are the following types of agents in the system:
– Input Part Agent (IPA) that provides the interface between the external reality
and production system and creates the Part Agent.
– Path Agent (PthA) that provides the proper path selection for the treated part.
– Path Messenger Agent (PMA) that collects and transfers information around the
graph about available operations (treatments) in the system.
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– Port Agent (PoA) that makes possible for agents to leave a given node to reach the
neighboring one by the selected edge, and to be treated by an operation represented
by the edge.
– Part Agent (PA) that represents a given treated part in the system, and provides a
transfer of a given part from the beginning to the end following the proper path (i.
e. sequence of operations) in the system.
– Linker Agent (LA) that collects necessary information around the graph (or its
part) to enable detection of a problem of traffic in the graph (such as overloaded
operations, waiting lines for operations etc.).
– Investor Agent (IA) that enables to undertake decisions with investment in the
graph (extension or reduction of connections, new operations etc.) to modify the
production system layout.
– Investor Proxy Agent (IPxA) that participates in negotiations concerning investments in the graph.
Fig. 4. Schema of the graph node where the agent undertake an action to establish new itinerary
continuation
The real time optimal system management consists of a properly composed path
for the treated part navigation in the production system graph. Using its own required
sequence of operations Part Agent goes across the graph from the beginning node to
the ending node. At every node it enters the nodemanager where cooperating with
PthA, PMA, LA agents it obtains information about the possibilities of the path continuation. Accordingly to its need (cheapest, quickest) it selects the best continuation
of the itinerary and goes to the appropriate port where it pays (cooperation with PoA
agent) for entering the following edge in the graph (which means that the corresponding
operation will be executed on the part)
The decisions concerning investments in the production process layout (which results in the layout modifications) are settled as a result of negotiations among Investors
and Investor Proxy agents (IA, IPxA). We can consider that the edge linking node A
with the node E is overloaded (it means that the technological operations - drills between A and E has insufficient efficiency rate). A scenario of the action undertaken
by agents to establish a new connection (or to raise operation rate) may consists of the
following steps (fig. 5):
– Port at the node A linking it with the node B is overloaded and the agent AoP
corresponding to the port creates the LA agent that is sent to the node B .
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Fig. 5. Schema of the graph where the agent undertake action to establish new edges (operations,
treatments)
– At the node B the AL agent verifies the traffic linking the node B with the node C .
– If the traffic from the node B to the node C going via the node B is too heavy,
the agent Linker goes back to the node A with a proposition to create a direct link
between the nodes A and C , or looks for the next overloaded node (for example
node E ). This proposition is paid to the Investor agent at the node A.
– Investor agent at the node A builds a financial project of the enterprise, and sends
the Investor-Proxy agent to the node E to negotiate its participation in the project.
– If the negotiations are completed successfully the agent Investor-Proxy realizes the
connection (direct or complex) placing order for the appropriate service with the
appropriate company.
4 Interactions in the multi-agent system for the production system
management
An interaction protocol is a set of rules of interaction that describes what action each
agent can take at each time. In this chapter typical interaction protocols used in the
multi-agent systems are presented, then a presentation is given how these protocols can
be applied to the interactions in the system for the production process management,
which was presented in the previous chapter.
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4.1 The interactions protocols used in the multi-agent systems.
Contract net. Its idea is based on the mechanism of contracting, used in business [12].
There are two roles of the agents in the protocol: manager – agent, which orders the
realization of the task. and contractors – the agents – potential executors of the task.
Voting In some situations the agents can hold a vote on certain decisions. The result of
the voting is obligatory for all agents participating in the voting. In [11] some protocols
of voting are presented: plurality protocol, binary protocol and Bord’s protocol.
Auctions The exchange of goods between the agents may be realized by auction. The
agents participating in the auction can perform one of the followings roles: auctioneer
or participant of the auction. There are many different protocols of auctions which are
associated with particular strategies of the agents. On the basis of an overview from [10]
we can enumerate: English (first-price open cry) auction, Vickrey auction (second-price
sealed-bid), all-pay auction.
Market interactions The interactions based on the market approach have several advantages, such as well analyzed (in economic science) theory and a low cost of communication. There are two kinds of agents in the system: producers and consumers. Producers
transform the goods using technologies they have and the consumers put demands for
the goods needed and their decisions are undertaken on the basis of their utility function
[15, 4]. The market approach was used for creation of systems strictly connected with
the management of the graph or network:
– the system for analysis of the distributed multi-commodity flow problem [14],
– finding the optimal localization of the mirrors of services in the Internet [8],
– system of routing of the packages in the Internet based on market mechanisms [6].
4.2 The types of the interactions in the multi-agent system for the management
of the product ion process.
Introduction This sub-chapter will analyze what interactions protocols can be chosen
for particular tasks of the production management system. We concentrate on the following problems:
– choosing the Port by PA agent, which enables the best quality of the connection
(using the criterion based on the cost and/or the performance of the treatment),
– gaining information about the state of the graph,
– adding or removing the links to/from the graph by Investor and Investor Proxy.
Choosing the Port by Part Agent The process of choosing the Port by the Part Agent
may be realized using the following interactions protocols:
– Contract Net – PA agent informs all Ports on the Node that it wants to get a defined
treatment (which means that it is looking for a particular node in the graph). The
Ports inform what price it has to pay for the available treatment, and what are other
properties of the treatment (time elapsed, quality etc.) The PA agent chooses the
best from its point of view of treatment – it means the corresponding Port.
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– voting – The PA agents vote which treatment is the best and the corresponding edge
gets a privilege to be selected for such treatment.
– auction – If the production system is heavily loaded (overloaded), the PA agents
can take part in the auctions of treatment offered by the Ports (i.e. edges).
The opposite situation is also possible – the agent PA demands to buy the treatment
offered by one of the Ports at a given node. There are more than one available ports
offering similar kinds of treatment and the Ports participate in the auction offering
lower and lower prices and better treatment. The PA agent chooses the lowest price,
the best treatment. This algorithm may be used if the production system is currently
not loaded.
– market – The complex negotiation of the cost of the treatment between PA agents
considered as consumers and Ports playing a role of producers can be implemented
using a market model.
Collecting the information about the current state of the network. Linker Agent can
gain the information about the state of the network using the following interaction protocols:
– Contract Net – LA announces that it is ready to buy information about the state
of the network. The group of the nodes (Node Managers) can send their proposals
concerning the quality and the costs of the possessed information.
– Market – LA and Port Agents participate in the exchange of the information about
the structure of the network. The prices of the information are set by the market
methods.
The modification of the structure of the production process structure (graph) - adding
or removing the operations (edges) The modification of the structure of the network by
adding or removing the edges representing technological operations can be performed
using the following interaction protocols:
– Auction – Agents Investors take part in the auction of adding a new edge to the
graph or selling the one by the Investor who actually owns it. It is also possible to
increase or reduce the efficiency, productivity or quality of a given operation (edge).
– Market – The Investors can buy or sell technological operations (edges).
5 Simulation of the multi-agent system for the production system
layout (graph management)
The multi-agent system proposed for the production system layout management has
been modelled and verified by simulation (fig. 6).
It has a limited function – provides only the migration of the Part Agents using the
Contract Net Protocol. Other tasks, such as collecting information about the structure
of the network and changing the structure of the network (adding/removing nodes) are
not realized.
The following interaction among the agents has been realized:
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Fig. 6. Schema of the examined production system layout - graph corresponding to the system: a
- at the beginning of simulation, b - at the end of simulation after the improvement of the system
layout (extension of operations - corresponding edges)
– Part Agent when arriving at a given node tries to reach the destination node by the
edge with demanded operation at the lowest costs.
– At the node a number of options of transfer continuation at different prices are
proposed to the Part Agent.
– Part Agent accepts the solution, taking into consideration the price of the following
operation versus its own resources and the quality of the treatment versus the time it
can waste for all the operations from the beginning to the end of the part treatment.
– Part Agent pays the Port Agent for the treatment. With the resources saved the
Port Agents (and Node Manager) undertake modification of the production system
layout (graph).
The simulation of the system was performed on the computer network composed of
6 nodes with edges representing technological operations.
Technology needs and pricing policy favour the following edges of the graph:
(B,F), (B,G), (C,G), (D,G), (E,F), (E,G), (F,G). The initial state of the network with
the same productivity rate of all edges (technological operations) is shown in fig. 6a.
After a number of operations execution and transfers of parts in the system the nodes
using collected funds modify the parameters of the network. The result of the system
investment is presented in fig. 6b. After the improvement (changes of the efficiency of
operations corresponding to the edges) realized by the system the overloaded operations
(edges)were reinforced (thick lines in the fig. 6b).
6 Conclusion
The proposed system of a automatic management of production system enables both
optimal real-time management of the production system functioning, and optimal, selfacting adaptation of the network structure to users’ needs, also in real-time [3]. Both
aspects of optimal management, considered together may bring very interesting results.
The application of the above mentioned system will lower costs of production system
exploitation. In the future it will enable the realization of optimal management of production systems layout for large, extensive production systems. The proposed system
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enables flexible production systems management (of graph structure) with the use of
economic criteria of optimization. The system takes advantage of possibilities given by
the decentralized, multi-agent system, that enables making local decisions, taking into
account local, and if necessary and possible - a global point of view.
The presented system may be easily extended to uniform, optimal management of
network, including not only hardware management but also other resources, e. g. network software.
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