IEEE CONTROL SYSTEMS SOCIETY TECHNICAL COMMITTEE
ON DISCRETE EVENT SYSTEMS

    Ryan J. Leduc
    Chair, IEEE CSS Technical Committee on DES
    Dept. of Computing and Software
    McMaster University
    1280 Main Street West
    Hamilton, Ontario
    Canada L8S 4K1

    Phone: (905) 525-9140 Ext. 27962
    Fax: (905) 524-0340
    e-mail: leduc@mcmaster.ca
    WWW: http://www.cas.mcmaster.ca/~leduc/

DESTC Web Page: http://www.cas.mcmaster.ca/destc/

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Contents:

1. Editorial

2. Announcements
 2.1 PhD: Clemson University, USA
 2.2 PhD, PostDoc: University of Zaragoza, Spain

3. Conferences
 3.1 17th MEDITERRANEAN CONFERENCE ON CONTROL AND AUTOMATION, MAKEDONIA
     PALACE HOTEL, THESSALONIKI, GR, June 24 - 26, 2009
 3.2 7th IEEE International Conference on Control and Automation,
     Christchurch, New Zealand, December 9 - 11, 2009

4. Journals
 4.1 Selections from IEEE Transactions on Automatic Control, Volume: 53,
      Issue: 5, June 2008
 4.2 Selections from IEEE Transactions on Automatic Control, Volume: 53,
      Issue: 6, July 2008
 4.3 Selections from IEEE Transactions on Automatic Control, Volume: 53,
      Issue: 7, August 2008
 4.4 Selections from IEEE Transactions on Software Engineering, Volume:
     34, Issue: 3, May 2008
 4.5 Selections from IEEE Transactions on Systems, Man and Cybernetics, Part
     A, Volume: 38, Issue: 1, January 2008
 4.6 Selections from IEEE Transactions on Systems, Man and Cybernetics, Part
     A,  Volume: 38,  Issue: 3, May 2008

Welcome to the newsletter of the IEEE Control Systems Technical Committee on Discrete Event Systems!

See http://www.cas.mcmaster.ca/destc/ for information on the DESTC.

Personal note from the editor:

Welcome to the September 2008 newsletter,

Ryan

Contributed by: Ryan Leduc <leduc aT mcaster dOt ca>

PHD: CLEMSON UNIVERSITY, USA

A Research Assistantship position will be available for prospective
graduate students interested in pursuing a PhD degree at the Department
of Mechanical Engineering, in the Automotive Engineering program, at
Clemson University, in the area of vehicle fault diagnosis and prognosis.

The interested applicants are expected to have good mathematical skills
and strong background in modern control theory. Basic knowledge of
stochastic processes, signal processes, quantized systems, discrete
event systems, automatons and state machines is desirable. Particularly
relevant areas of research include observer design, robust diagnostic
schemes, automatic thresholds, and fault tolerant control.

In addition, familiarity with computer programming
languages such as Matlab is desirable. A Master's degree in
mechanical engineering, electrical engineering, or system and
control is required.

The RA position is starting from Spring 2009 and will be open until it
is filled. The interested applicants should contact Professor Pierluigi
Pisu (pisup@clemson.edu) for more information and apply online for the
graduate school admission at
http://www.grad.clemson.edu/ProspectiveStudents.php.
For more information regarding the Clemson University Automotive
Engineering Program, interested applicants should contact Tameka Boyce
at TBOYCE@exchange.clemson.edu or call at +1-864-656-0999.

Contributed by: Ryan Leduc <leduc aT mcaster dOt ca>

PHD, POSTDOC: UNIVERSITY OF ZARAGOZA, SPAIN

PhD position with a grant for 4 years.
PostDoc position for one year (and can be extended to a second one).

Systems Engineering and Discrete Event Systems Group
(http://webdiis.unizar.es/GISED)
University of Zaragoza (Spain)

The activities of the group are related to the modelling, analysis and
formal synthesis of some classes of artificial systems that are
interesting from atechnical point of view (production, computer,
logistic, coordination, etc.).Usually more adequately modelled by means
of discrete event systems formalisms (automata, Petri nets, process
algebra, queuing networks...), the state explosion problem puts a limit
to most of the analysis or synthesis techniques. Relaxations based on
fluidification appear as a possible way to overcome these difficulties
when largely populated systems or systems with high cadence are considered.

More specifically, the project will be mainly focused on the formalism
of continuous and hybrid Petri nets, defined by relaxing the
integrality constraint in a similar way as in fluid queuing networks.

Activities will be mainly related to the following aspects:

* Modelling (fluidisability of "­discrete event"­ net models)
* Analysis and verification (both structural and model checking)
* Optimization of design (placement and size of buffers, equipment
  selection, ...)
* Operation (centralized and modular control, scheduling)
* Observers design
* Discretization of "optimal" continuous trajectories
* Development of adequate computer tools for the modelling, analysis 
  and synthesis.

--------------
PhD position
--------------
The main task for the accepted candidate will be to conduct research
towards his/her PhD. The candidate is expected to complete the degree
within 4 years.

Applicants must have MSc (or equivalent) degree in Automatic Control,
Computer Science, Mathematics, Electrical/Electronics or Communications
Engineering. Some knowledge on formal models on computer science,
automatic control or operational research will be especially welcome.

The salary will be around 1200 EUR/month, with an incremental raise in
the following years. Health insurance will also be provided.

The candidate could start on October-November 2008 or January 2009.

--------------
PostDoc position
--------------
The position will be for one year (and can be extended to a second one).

A PhD-degree in automatic control, computer science, or operations
research is required. Knowledge on Petri nets, hybrid or piecewise
linear systems will be especially welcome.

The salary will be around 2000 EUR/month (in case, with an incremental
raise in the following year). Health insurance will also be provided.

The candidate could start on October-November 2008 or January 2009.

Contact: Applicants should send by email their CVs before 8th September
2008 to:

Prof. Manuel Silva (silva@unizar.es)
with copy to Dr. Cristian Mahulea (cmahulea@unizar.es)

Departamento de Informatica e Ingenieria de Sistemas, and
Instituto de Investigacion en Ingenieria de Aragon (I3A)
Universidad de Zaragoza

Maria de Luna, 1
50018 ZARAGOZA (Spain)

Contributed by: Ryan Leduc <leduc aT mcaster dOt ca>

17TH MEDITERRANEAN CONFERENCE ON CONTROL AND AUTOMATION
MAKEDONIA PALACE HOTEL, THESSALONIKI, GR
June 24 - 26, 2009

                          Call for Papers
                          ---------------

January 15, 2009: Contributed, papers, invited session proposals,
Workshop / Tutorial proposals, due.

The 17th Mediterranean Conference on Control and Automation, MED'09,
will be held in Thessaloniki, Greece. Thessaloniki, the Cultural
Capital of Europe 1997, is a beautiful port city on the Aegean Sea. It
is the centre of the Greek North and the country' s second largest
city. The conference venue is the seafront Makedonia Palace Hotel. King
Cassander of Macedonia founded Thessaloniki in 315 B.C. and named the
new city after his wife, Thessaloniki, the sister of Alexander the
Great. Throughout its long history, the city has received several
designations which are indicative of Thessaloniki's grandeur and
glorious past: Capital of Macedonia, Joint-administrative capital of
the Byzantine Empire. Thessaloniki has witnessed in the 2300 years of
its life the might and glory of the Macedonians, the power of Rome and
the grandeur of Byzantium. Its position and the contribution it has
made have helped it play a major role in world history. Standing as
undeniable proof of that role are its many antiquities, monuments and
other impressive archaeological finds, representing every historical
period. Within fifty kilometers of Thessaloniki are the beautiful
beaches of Halkidiki and several sites of exceptional historical
importance. There is Olympus, the mountain of the gods and seat of
ancient Greek mythology. Athos, "the Holy Mountain," is a sanctuary for
a unique monastic community, safeguarding orthodox Christian tradition
and priceless historical treasures. Pella, the ancient capital of the
Macedonian Kingdom, features the palace of Alexander the Great, with
its exquisite mosaics. Vergina became known worldwide when golden urns
containing the remains of Phillip II and his wife - the parents of
Alexander the Great - were unearthed during excavation of royal
Macedonian tombs. The numerous historical sites, as well as the
plethora of natural beauty spots and endless emerald-green shores less
than an hour away, establish Thessaloniki as a fascinating journey into
art, civilization and relaxation. The conference, through its technical
program, will provide a unique opportunity for the academic and
industrial community to address new challenges, share solutions and
discuss future research directions. A broad range of topics is
proposed, following current trends of combining control/systems theory
with software/communication technologies. For up-to-date information on
MED'09, visit http://www.med09.org

General Chairs:
Vassilios Petridis, Automation & Robotics Lab, Dept. of Electrical &
Computer Engineering, Aristotle University of Thessaloniki, Greece,
petridis@eng.auth.gr

Frank L. Lewis, Automation & Robotics Research Institute, The
University of Texas at Arlington, lewis@uta.edu

web site: click here

Contributed by: Ryan Leduc <leduc aT mcaster dOt ca>

7TH IEEE INTERNATIONAL CONFERENCE ON CONTROL AND AUTOMATION
Christchurch, New Zealand
December 9 - 11, 2009

CALL FOR PAPERS

The 7th IEEE International Conference on Control and Automation
(ICCA'09) will be held on December 9 to 11, 2009, in Christchurch, New
Zealand. The conference is jointly organized by the IEEE Control
Systems Chapter, Singapore and IEEE New Zealand South Section, and
technically sponsored by IEEE Control Systems Society, Technical
Committee of Control Theory, Chinese Association of Automation,
University of Canterbury, New Zealand and North China University
of Electric Power. It is to create a forum for scientists and
practicing engineers throughout the world to present the latest
research findings and ideas in the areas of control and automation.

The city of Christchurch is famous for natural beauty and a wealth of
things to do. Christchurch has a lively entertainment scene, strong
cultural heritage. In 1996, Christchurch was acknowledged as the
outstanding garden city from 620 international entries and in 1997, was
judged Overall Winner of Major Cities in the Nations in Bloom
International Competition to become 'Garden City of the World'!

IEEE ICCA Proceedings are included in ISI Proceedings (ISTP), EI
Compendex and IEEE Xplore.

Topics of interest include but not limited to:

- Modeling of Complex Systems
- Linear Systems
- Robust and H-infinity Control
- Nonlinear Systems and Control
- Fuzzy and Neural Systems
- Estimation and Identification
- Fault Detection
- Process Control & Instrumentation
- Motion Control
- Flexible Manufacturing Systems
- Integrated Manufacturing
- Factory Modeling and Automation
- Petri-Nets and Applications
- Micro and Nano Systems
- Sensor networks and networked control
- Optimal Control
- Discrete Event Systems
- Adaptive Control
- Learning Systems
- Intelligent and AI Based Control
- Real-time Systems
- Sensor/data fusion
- Robotics
- Automated Guided Vehicles
- Control Education
- Control Applications
- Process Automation
- Man-machine Interactions
- Smart Structures
- Multi-agent systems

Submission of Papers:

Authors should submit the full version of their manuscripts online
through the conference web site at http://www.ieee-icca.org (or
http://hdd.ece.nus.edu.sg/~icca09/). General inquiries should be
addressed directly to Program Chair, Professor Cishen Zhang, at Nanyang
Technological Uni-versity (e-mail: icca2009@ntu.edu.sg). Proposals for
invited sessions in the related areas are also solicited and should be
submitted through email to Invited Session Chair, Professor
Gary Feng at City University of Hong Kong (e-mail: megfeng@cityu.edu.hk).

All materials must be written in English, and a paper should be
submitted only if you intend to pre-sent the paper at the conference.
The manuscript to be submitted to the conference should contain
sufficient details including key concepts and novel features of the
work. It should include the title, authors, mailing addresses,
affiliations, telephone and fax numbers and email addresses.

Important Dates:

Submission of Manuscripts: April 1, 2009
Notification of Acceptance: July 1, 2009
Submission of Final Papers: September 1, 2009
Conference: December 9-11, 2009

Conference General Chairs: Lihua Xie, Singapore; Xiaoqi Chen, New
Zealand; Jizhen Liu, China

Conference Program Chairs: Cishen Zhang, Singapore; Wenhui Wang,
New Zealand; Zhengtao Ding, UK

web site: click here

Contributed by: Ryan Leduc <leduc aT mcaster dOt ca>

SELECTIONS FROM IEEE TRANSACTIONS ON AUTOMATIC CONTROL
VOLUME: 53, ISSUE: 5
JUNE, 2008

1) Safe Diagnosability of Stochastic Discrete Event Systems

Fuchun Liu; Daowen Qiu

Abstract:

Recently, safe diagnosability of discrete event systems (DESs) was
investigated by Paoli and Lafortune, which was viewed as the first
necessary step of fault-tolerant supervision. In this paper, we
consider the problem of safe diagnosability in the framework of
stochastic discrete event systems (SDESs). We define the notion of safe
diagnosability for stochastic automata, in which fault detection occurs
before any given forbidden string in the failed mode of system is
executed. The relationship between diagnosability and safe
diagnosability for SDESs is analyzed. In particular, a necessary and
sufficient condition for safe diagnosability of SDESs is presented by
constructing the recognizer of illegal language and the safe diagnoser.
Some examples are described to illustrate the results.

2) Modeling and Analysis of Hybrid Supervisory Systems: A Petri Net
Approach

Villani, E.; Miyagi, P. E.; Valette, R.

Abstract:

Not available.

web site: click here

Contributed by: Ryan Leduc <leduc aT mcaster dOt ca>

SELECTIONS FROM IEEE TRANSACTIONS ON AUTOMATIC CONTROL
VOLUME: 53, ISSUE: 6
JULY, 2008

1) Supervisory Control Architecture for Discrete-Event Systems

Feng, L.; Wonham, W. M.

Abstract:

A flexible decentralized and hierarchical architecture is presented to
reduce computational effort in designing optimal nonblocking
supervisors for discrete-event systems (DES). We organize a DES into
modular subsystems that embody internal interacting dependencies.
Verification of, and coordination among modular subsystems are achieved
through their model abstractions. Sufficient conditions are presented
to guarantee that coordinators and modular supervisors result in
maximally permissive and nonblocking control. A medium-sized example
demonstrates the computational effectiveness of our approach.

2) Modular Implementation of Robust Supervisory Controllers for
Discrete Event Systems

Economakos, C.; Koumboulis, F. N.

Abstract:

In this paper, the problem of designing robust supervisory controllers
for uncertain discrete event systems is studied. It is assumed that
both the plant behavior and the design specifications can be
represented by regular languages. It is also assumed that the unknown
model of the uncertain plant belongs to a finite set of possibilities.
To solve the problem, a modular approach based on a general recursive
robust control scheme is developed. The contribution of the present
paper consists in deriving a maximally permissive modular robust
supervisory controller for the case of prefix-closed languages, having
the advantage of linear complexity in the number of models as opposed
to the exponential complexity of the worst-case scenario in the direct
implementation of standard supervisors for augmented systems.

web site: click here

Contributed by: Ryan Leduc <leduc aT mcaster dOt ca>

SELECTIONS FROM IEEE TRANSACTIONS ON AUTOMATIC CONTROL
VOLUME: 53, ISSUE: 7
AUGUST, 2008

1) Optimal Nonblocking Directed Control of Discrete Event Systems

Huang, J.; Kumar, R.

Abstract:

For the control of discrete event systems, the notion of directed
control refines that of supervisory control. A directed controller is
one that selects at most one controllable event to be enabled at any
state (without disabling any uncontrollable event), which is in fact
how a discrete event control is implemented. In contrast, a supervisory
controller computes a maximal allowable set of controllable events at
each state, leaving undecided exactly which controllable event should
be enabled. In previous works, we developed a framework for the
computation of optimal directed controllers and a polynomial synthesis
algorithm for acyclic plants. In this paper, we present a novel
synthesis approach for general plants, i.e., plants with or without
cycles, thus providing a complete solution to the optimal directed
control problem. The complexity of the approach remains polynomial in
the size of plant.

2) Optimal Model Predictive Control of Timed Continuous Petri Nets

Mahulea, C.; Giua, A.; Recalde, L.; Seatzu, C.; Silva, M.

Abstract:

This paper addresses the optimal control problem of timed continuous
Petri nets under infinite servers semantics. In particular, our goal is
to find a control input optimizing a certain cost function that permits
the evolution from an initial marking (state) to a desired
steady-state. The solution we propose is based on a particular
discrete-time representation of the controlled continuous Petri net
system, as a certain linear constrained system. An upper bound on the
sample period is given in order to preserve important information of
the timed continuous net, in particular the positiveness of the
markings. The reachability space of the sampled system in relation to
autonomous continuous Petri nets is also studied. Based on the
resulting linear constrained model, the optimal control problem is
studied through model predictive control (MPC). Implicit and explicit
procedures are presented together with a comparison between the two
schemes. Stability of the closed-loop system is also studied.

web site: click here

Contributed by: Ryan Leduc <leduc aT mcaster dOt ca>

SELECTIONS FROM IEEE TRANSACTIONS ON SOFTWARE ENGINEERING
VOLUME: 34, ISSUE: 3
MAY, 2008

1) Characterizing Communication Channel Deadlocks in Sequence Diagrams

Mitchell, B.

Abstract: 

UML sequence diagrams (SDs) are a mainstay of requirements
specifications for communication protocols. Mauw and Reniers' algebraic
(MRA) semantics formally specifies a behavior for these SDs that
guarantees deadlock-free processes. Practitioners commonly use
communication semantics that differ from MRA, which may result in
deadlocks, for example, FIFO, token ring, etc. We define a process
algebra that is an extension of the MRA semantics for regular SDs. Our
algebra can describe several commonly used communication semantics.
Regular SDs are constructed from concurrent message flows via
iteration, branching, and sequential composition. Their behavior is
defined in terms of a set of partial orders on the events in the SD.
Such partial orders are known as causal orders. We define partial order
theoretic properties of a causal order that are particular kinds of
race condition. We prove that any of the common communication semantics
that we list either guarantees deadlock-free SDs or can result in a
deadlock if and only if a causal order of an SD contains one of these
types of race condition. This describes a complete classification of
deadlocks as specific types of race condition.

web site: click here

Contributed by: Ryan Leduc <leduc aT mcaster dOt ca>

SELECTIONS FROM IEEE TRANSACTIONS ON SYSTEMS, MAN AND CYBERNETICS, PART
A
VOLUME: 38, ISSUE: 1
JANUARY, 2008

1) Resource-Oriented Petri Net for Deadlock Avoidance in Flexible
Assembly Systems

Naiqi Wu; MengChu Zhou; ZhiWu Li

Abstract: 

In many flexible assembly systems, base components are transported with
pallets; parts to be mounted onto the base ones are transported by
trays with no pallets. When an assembly operation is performed by using
some parts in a tray but not all, the tray with the remaining parts
still occupies a buffer space. In this way, an assembly/disassembly
material flow is formed. In such a material flow, deadlock can occur
both in the base component and part flow. Furthermore, the assembly
operations can also result in a deadlock. Thus, it is a great challenge
to tackle deadlocks in such processes. This paper models them using
resource-oriented Petri nets. Based on the models, a deadlock control
policy is proposed and proved to be computationally efficient and less
conservative than the existing policies in the literature. An
industrial case study is used to show the results.

2) Verification of Knowledge-Based Systems Using Predicate/Transition Nets

Shen, V.R.L.; Juang, T.T.-Y.

Abstract: 

As expert-system technology gains broader acceptance, the need to build
and maintain large-scale knowledge-based systems (KBSs) will assume
greater importance. Traditional approaches to KBS verification
generally contain no predicate/transition (PrT) net models, thus making
them slow for the large-scale KBS with chained errors. This paper
proposes an attractive alternative to KBS verification, in which the
KBS is modeled as a PrT-net model. Then, the least fixpoint semantics
of the PrT-net model can be introduced into the KBS for the purpose of
speeding up the computations of the KBSs. The significance of this
paper is that seven propositions are formulated to detect errors of
redundancy, subsumption, unnecessary condition, circularity,
inconsistency, dead end, and unreachable goal. Thus, the performance of
a computer-aided-design tool for KBSs can be improved to some extent.
Meanwhile, specification languages, including Programming in Logic,
Frame-and-Rule-Oriented Requirements Specification Language, and the
like, are suitable to this approach.

3) Control of Elementary and Dependent Siphons in Petri Nets and Their
Application

ZhiWu Li; MengChu Zhou

Abstract: 

The importance of siphons is well recognized in the analysis and
control of deadlocks in a Petri net. To minimize the number of siphons
that have to be explicitly controlled, siphons in a net are divided in
a net into elementary and dependent ones. The concepts of token-rich,
token-poor, and equivalent siphons are newly presented. More general
conditions under which a dependent siphon can be always marked are
established. The existence of dependent siphons in a Petri net is
investigated. An algorithm is developed to find the set of elementary
siphons in a net system for deadlock control purposes. The application
of the proposed elementary siphon concept to the existing deadlock
control policies is discussed. A few different-sized manufacturing
examples are used to demonstrate the advantages of elementary
siphon-based policies. The significant value of the proposed theory via
a particular deadlock control policy is shown. Finally, some
interesting and open problems are discussed.

4) An Efficient Algorithm for Finding Minimal Overconstrained
Subsystems for Model-Based Diagnosis

Krysander, M.; Aslund, J.; Nyberg, M.

Abstract: 

In model-based diagnosis, diagnostic system construction is based on a
model of the technical system to be diagnosed. To handle large
differential algebraic imemodels and to achieve fault isolation, a
common strategy is to pick out small overconstrained parts of the model
and to test these separately against measured signals. In this paper, a
new algorithm for computing all minimal overconstrained subsystems in a
model is proposed. For complexity comparison, previous algorithms are
recalled. It is shown that the time complexity under certain conditions
is much better for the new algorithm. This is illustrated using a truck
engine model.

5) Designs of Bisimilar Petri Net Controllers With Fault Tolerance
Capabilities

Lingxi Li; Hadjicostis, C.N.; Sreenivas, R.S.

This paper proposes an approach for providing tolerance against faults
that may compromise the functionality of a given controller modeled by
a Petri net. The method is based on embedding the given Petri net
controller into a larger (redundant) Petri net controller that retains
the original functionality and properties, and uses additional places,
connections, and tokens to impose invariant conditions that allow the
systematic detection and identification of faults via linear parity
checks. In particular, this paper considers two types of redundant
Petri net controllers: 1) nonseparate redundant Petri net controllers
have the same functionality as the given Petri net controller and allow
for fault detection and identification, but do not necessarily retain
the given controller intact; and 2) separate redundant Petri net
controllers are a special case of the nonseparate redundant controllers
that retain the given Petri net controller intact but enhance it with
additional places to enable fault detection and identification. The
work in this paper obtains complete characterizations of both types of
redundant controllers along with necessary and sufficient conditions
for them to be bisimulation equivalent to the given original Petri net
controller. In addition, this paper discusses how each type of
redundant controllers can be designed to have desirable fault detection
and identification capabilities. When the bisimulation equivalence
requirement is not directly enforced, nonseparate redundant controllers
can potentially have advantages over separate ones (e.g., they can use
fewer connections to detect and identify the same number of faults). An
example of a Petri net controller for a production cell and its fault
tolerance capabilities using separate and nonseparate embeddings is
used to illustrate the approach.

web site: click here

Contributed by: Ryan Leduc <leduc aT mcaster dOt ca>

SELECTIONS FROM IEEE TRANSACTIONS ON SYSTEMS, MAN AND CYBERNETICS, PART
A
VOLUME: 38, ISSUE: 3
MAY, 2008

1) A Petri Net Approach to Support Resource Assignment in Project
Management

Yen-Liang Chen; Ping-Yu Hsu; Yuan-Bin Chang

Abstract: 

Petri nets have long been used in modeling and simulating project
execution because of their great capability to describe concurrent
activities and simulate the evolvement of processes. Although a number
of extended Petri net models have been proposed to model and simulate
resource sharing and activity dependence in projects, none of them has
ever included a resource assignment mechanism into their models.
Because resource assignments influence how limited resources are
allocated among conflicting activities, they may heavily affect the
availability of resources and the execution of projects. Therefore, a
model without considering resource-sharing and resource assignment
strategies may lead to a misunderstanding about project scheduling,
resource consumption behaviors, and estimated project time.
Accordingly, this paper proposes a new extended Petri net model that
can describe how resources are shared and assigned among concurrent
activities of multiple projects. The proposed model is named as
resource assignment Petri net (RAPN), which extends an object
composition Petri net with new places, transitions, attributes, and
firing rules to model resource-sharing and resource assignment
strategies. Finally, we prove that RAPN can correctly model the
resource consumption behaviors of projects and can correctly compute
the total elapsed time of projects.

2) Using Shared-Resource Capacity for Robust Control of Failure-Prone
Manufacturing Systems

Shengyong Wang; Song Foh Chew; Lawley, M.A.

Abstract: 

Deadlock-free resource allocation has been an active area of research
in flexible manufacturing. Most researchers have assumed that allocated
resources do not fail, and thus, little research has addressed the
discrete-event supervision of manufacturing systems that are subject to
resource failure. In our previous work, we developed supervisory
controllers to ensure robust deadlock-free operation for systems with
unreliable resources. These controllers guarantee that parts requiring
failed resources do not block the production of parts that are not
requiring failed resources. This previous work assumes that parts
requiring failed resources can be advanced into failure-dependent (FD)
buffer space (buffer space exclusively dedicated to parts requiring
unreliable resources). Supervisors admit only states for which a
sequence of such part advancements is feasible. The research presented
in this paper relaxes this assumption because, in some systems,
providing FD buffer space might be too expensive or it might be
desirable to load the system more heavily with FD parts. In this paper,
we concentrate on distributing parts requiring failed resources
throughout the buffer space of shared resources so that these
distributed parts do not block the production of part types that are
not requiring failed resources. The approach presented here requires no
state enumeration and is polynomial in stable measures of system size.
We also present results from simulation experiments that compare system
performance under these new policies with system performance under our
previously published supervisors. These results show that our new
policies allow better performance if the required part mixes favor FD
part types. The systems of interest are single-unit resource allocation
systems.

3) Distributed Diagnosis Under Bounded-Delay Communication of
Immediately Forwarded Local Observations

Wenbin Qiu; Kumar, R.

Abstract: 

We study distributed failure diagnosis under a -bounded communication
delay, where each local site transmits its observations to other sites
immediately after each observation and the transmitted observation is
received within at most more event executions of the plant. A notion of
diagnosability is introduced so that any failure can be diagnosed
within a bounded delay of its occurrence by one of the local sites
using its own observations and the -bounded delayed observations
received from other local sites. The local sites communicate among each
other using an ldquoimmediate observation passing (iop)rdquo protocol,
forwarding any observation immediately up on its occurrence. We
construct models for the -bounded communication delay and use them to
extend the system and nonfault specification models for capturing the
effect of bounded-delay communication. By using the extended system and
specification models, the distributed diagnosis problem under the
immediate observation passing protocol is then converted to a
decentralized diagnosis problem of our previous work, where the results
are applied for verifying diagnosability and for synthesizing local
diagnosers. Methods by which complexity of testing diagnosability and
of online diagnosis can be reduced are presented. Finally, we compare
the notions of diagnosability, codiagnosability, and diagnosability.

4) On Siphon Computation for Deadlock Control in a Class of Petri Nets

ZhiWu Li; MengChu Zhou

Abstract: 

As a structural object, siphons are well recognized in the analysis and
control of deadlocks in resource allocation systems modeled with Petri
nets. Many deadlock prevention policies characterize the deadlock
behavior of the systems in terms of siphons and utilize this
characterization to avoid deadlocks. This paper develops a novel
methodology to find interesting siphons for deadlock control purposes
in a class of Petri nets, i.e., a system of simple sequential processes
with resources . Resource circuits in an are first detected, from
which, in general, a small portion of emptiable minimal siphons can be
derived. The remaining emptiable ones can be found by their
composition. A polynomial-time algorithm for finding the set of
elementary siphons is proposed, which avoids complete siphon
enumeration. It is shown that a dependent siphon can always be
controlled by properly supervising its elementary siphons. A
computationally efficient deadlock control policy is accordingly
developed. Experimental study shows the efficiency of the proposed
siphon computation approach.

web site: click here