Faculty of Informatics - Papers

Experimental characterisation of prototype HTS SMES device

Christopher J. Hawley — Mon, 30 Jan 2012 19:30:02 PST

Preliminary Design of a High Tc Superconducting Persistent Mode Current Switch

Christopher J. Hawley — Mon, 30 Jan 2012 19:29:52 PST

Persistent Mode Current Switches (PMCS) are made from coils of superconductor that in theory will provide zero conduction losses when closed. By causing a section of the superconductor to leave its superconducting state and become resistive, the switch is effectively opened. There are currently 3 methods to open a PMCS; 1) applying heat, 2) applying a magnetic field or 3) application of current above the critical current (Ic) of the tape. The first two methods are not suitable for power applications as the switching time is too slow. Recent work at the University of Wollongong and in literature has shown that by careful design, a switch using High Temperature Superconductors (HTS) and applying a high frequency current pulse can potentially achieve a switching time of less than 50μs.

Use of Cryoelectronics to Reduce Power Losses

Christopher J. Hawley — Mon, 30 Jan 2012 19:29:43 PST

The operational characteristics of power electronics operating at ambient temperatures are well known. Less well known are the characteristics of these devices when operating at cryogenic temperatures. This emerging field is known as cryoelectronics. The primary driver for operation at reduced temperatures is the promise of a resultant reduction in device operation losses. The operating characteristics of Metal Oxide Silicon Field Effect Transistors (MOSFETs) and Insulated Gate Bipolar Transistors (IGBTs) at Liquid Nitrogen (LN) temperature have been experimentally analysed. The results have been used to evaluate the viability of using cryoelectronics in medium - high power applications, considering trade-offs between device costs, cryocooler power requirements and device power ratings.

Power system applications of superconducting magnetic energy storage systems

X D. Xue — Thu, 05 Jan 2012 21:54:52 PST

This study overviewed current researches on power system applications of SMES systems. Some key schematic diagrams of applications were given, too. Furthermore, the authors tried to present a few valuable suggestions for future studies of SMES applications to power systems.

A study of the status and future of superconducting magnetic energy storage in power systems

X D. Xue — Thu, 05 Jan 2012 21:54:44 PST

Superconducting magnetic energy storage (SMES) systems offering flexible, reliable, and fast acting power compensation are applicable to power systems to improve power system stabilities and to advance power qualities. The authors have summarized researches on SMES applications to power systems. Furthermore, various SMES applications to power systems have been described briefly and some crucial schematic diagrams and equations are given. In addition, this study presents valuable suggestions for future studies of SMES applications to power systems. Hence, this paper is helpful for co-researchers who want to know about the status of SMES applications to power systems.

Unified mathematical modelling of steady-state and dynamic voltage-current characteristics for PEM fuel cells

X D. Xue — Thu, 05 Jan 2012 21:54:37 PST

In this study, a unified mathematical modelling technique for computing the steady-state and dynamic voltage-current (V-I) characteristics of PEM fuel cell stacks is developed. The proposed modelling method is based on the least squares technique and a set of electrochemical equations representing the PEM fuel cells. Three PEM fuel cell systems are considered for validating the proposed model. Furthermore, the authors investigated load current optimization by using the proposed method, in order to maximize the power output. Hence, this study provides a valuable approach for optimization of operating points of fuel cells and design of power conditioning units, simulators, and system controllers. (c) 2006 Elsevier Ltd. All rights reserved.

Automatic classification of ground-penetrating-radar signals for railway-ballast assessment

Wenbin Shao — Mon, 19 Dec 2011 20:58:53 PST

The ground-penetrating radar (GPR) has been widely used in many applications. However, the processing and interpretation of the acquired signals remain challenging tasks since an experienced user is required to manage the entire operation. In this paper, we present an automatic classification system to assess railway-ballast conditions. It is based on the extraction of magnitude spectra at salient frequencies and their classification using support vector machines. The system is evaluated on real-world railway GPR data. The experimental results show that the proposed method efficiently represents the GPR signal using a small number of coefficients and achieves a high classification rate when distinguishing GPR signals reflected by ballasts of different conditions.

Sparse signal decomposition for ground penetrating radar

Wenbin Shao — Tue, 13 Dec 2011 23:55:06 PST

In this paper, we present an adaptive approach for sparse signal decomposition, in which each GPR trace is decomposed into elementary waves automatically. A sparse feature vector is extracted from the decomposition and used for classification of railway ballast. The experimental results show that the proposed approach can represent the GPR signals efficiently, and effective features can be extracted for pattern classification.

A Dixmier-Douady theorem for Fell algebras

Astrid An Huef — Mon, 12 Dec 2011 22:27:39 PST

We generalise the Dixmier–Douady classification of continuous-trace C*-algebras to Fell algebras. To do so, we show that C*-diagonals in Fell algebras are precisely abelian subalgebras with the extension property, and use this to prove that every Fell algebra is Morita equivalent to one containing a diagonal subalgebra. We then use the machinery of twisted groupoid C*-algebras and equivariant sheaf cohomology to define an analogue of the Dixmier–Douady invariant for Fell algebras, and to prove our classification theorem.

Using computers to support children as authors: an examination of three cases

Lisa K. Kervin — Sun, 11 Dec 2011 19:27:34 PST

The changing nature of literacy is well documented within the literature, challenging educators to examine their pedagogies in light of the needs of learners in the current climate. The development and creation of non‐linear texts within primary classrooms is of current interest as they reflect the types of texts accessed and created by users of technology and afford children opportunities to make connections between their in and out of school literacy experiences. The authors observed children over extended periods as they collaborated with teachers and peers to author non‐linear texts for a range of purposes. Three case studies will be reported, providing examples of how three cohorts of primary school children (one lower primary, one middle primary, one upper primary) responded to the challenge to create texts using Information and Communication Technology and the specific learning observed. What is interesting about these cases is the process that the children engaged with as they planned, constructed and evaluated their texts. This paper describes this process and the implications it presents for what we understand about the pedagogy of writing, the creation of text and the opportunities for providing classroom literacy experiences aimed at supporting and fostering student learning.

Automatic Classification of Ground-Penetrating-Radar Signals for Railway-Ballast Assessment

Wenbin Shao — Thu, 08 Dec 2011 23:06:23 PST

Hecke algebras from groups acting on trees and HNN extensions

Udo Baumgartner — Sun, 04 Dec 2011 16:50:23 PST

We study Hecke algebras of groups acting on trees with respect to geometrically defined subgroups. In particular, we consider Hecke algebras of groups of automorphisms of locally finite trees with respect to vertex and edge stabilizers and the stabilizer of an end relative to a vertex stabilizer, assuming that the actions are sufficiently transitive.We focus on identifying the structure of the resulting Hecke algebras, give explicit multiplication tables of the canonical generators and determine whether the Hecke algebra has a universal C!-completion. The paper unifies algebraic and analytic approaches by focusing on the common geometric thread. The results have implications for the general theory of totally disconnected locally compact groups.

When does QP yield the exact solution to constrained NMPC?

José Mare — Sun, 04 Dec 2011 16:50:13 PST

It is well known that the optimal control sequence for a linear system with a quadratic cost and linear inequality constraints over a finite optimisation horizon can be computed by means of a quadratic programme (QP). The aim of this article is to investigate when the optimal control sequence for a non-linear single-input single-output system also can be computed via QP. Our main contribution is to show that the optimal control sequence for non-linear systems, with a quadratic cost and linear inequality constraints can be computed in exact form via QP provided the optimisation horizon is no larger than a critical quantity that we name the ¿input¿ output linear horizon¿. The results do not require any linearisation technique and are applicable to general non- linear systems, provided their input¿output linear horizon is larger than their relative degree.

Contraction groups in complete Kac-Moody groups

Udo Baumgartner — Sun, 04 Dec 2011 16:50:01 PST

Let G be an abstract Kac-Moody group over a finite field and overlineG the closure of the image of G in the automorphism group of its positive building. We show that if the Dynkin diagram associated to G is irreducible and neither of spherical nor of affine type, then the contraction groups of elements in overline G which are not topologically periodic are not closed. (In those groups there always exist elements which are not topologically periodic.)

Affine Kac-Moody groups of types II and III

Jacqueline Ramagge — Sun, 04 Dec 2011 16:49:32 PST

We announce a theorum which states that, over certain fields, affine Kac-Moody groups of types II and III arise as the fixed point subgroups under particular automorphism of affine Kac_moody groups obtained from a simply laced extended Cartan matrices (and hence of type I) of higher rank. Thus our result extends a theorum on Kac-Moody algebras to corresponding groups. A detailed proof of this result will appear in the J. Algebra.

On certain fixed point subgroups of affine Kac-Moody groups

Jacqueline Ramagge — Sun, 04 Dec 2011 16:49:23 PST

In this paper we give a detailed description of the fixed point subgroups of certain Kac-Moody groups under particular automorphisms. We consider Kac-Moody groups arising from simply laced extended Cartan matrices and their fixed point subgroups under automorphisms which contain a non-trivial field automorphism constituent as well as a non-trivial graph automorphism constituent. We show that such a situation satisfies all the conditions of a theorem of J-Y. Hée "Torsion de groupes munis dune donnée radicelle, Théorème (4.5)" which concludes that the fixed point subgroup has a B , N )-pair and a system of root subgroups. This paper contains some of the results in the authors doctoral thesis "On Some Twisted Kac-Moody Groups".

A realization of certain affine Kac-Moody groups of types II and III

Jacqueline Ramagge — Sun, 04 Dec 2011 16:49:14 PST

The aim of this paper is, in the notation of Kac, to extend results about Kac-Moody algebras to corresponding groups by proving that certain affine Kac-Moody groups of types II and III arise as the fixed point subgroups of affine Kac-Moody groups of type I of higher rank under particular automorphisms. We prove an analogue of a theorem of Hée which enables us to deduce some results about the fixed point subgroups of Kac-Moody groups arising from simply-laced extended Cartan matrices under automorphisms which are the product of a graph and a diagonal automorphism. We then prove that the groups obtained in this way are in fact isomorphic to Kac-Moody groups arising from affine Cartan matrices which are not of extended type. This paper contains the main results in the author′s doctoral thesis.

Triangle buildings and actions of type III 1/q2

Jacqueline Ramagge — Sun, 04 Dec 2011 16:49:04 PST

We study certain group actions on triangle buildings and their boundaries and some von Neumann algebras which can be constructed from them. In particular, for buildings of orderq⩾3 certain natural actions on the boundary are hyperfinite of type III_1/q².

Factors from trees

Jacqueline Ramagge — Sun, 04 Dec 2011 16:48:56 PST

We construct factors of type III,/n for n E N, n > 2, from group actions on homogeneous trees and their boundaries. Our result is a discrete analogue of a result of R.J Spatzier, where the hyperfinite factor of type III1 is constructed from a group action on the boundary of the universal cover of a manifold.

Factors from buildings

Jacqueline Ramagge — Sun, 04 Dec 2011 16:48:47 PST