Stability results for constrained dynamical systems

Differential-Algebraic Equations (DAE) provide an appropriate framework to model and analyse dynamic systems with constraints. This framework facilitates modelling of the system behaviour through natural physical variables of the system, while preserving the topological constraints of the system. The main purpose of this dissertation is to investigate stability properties of two important classes of DAEs. We consider some special cases of Linear Time Invariant (LTI) DAEs with control inputs and outputs, and also a special class of Linear switched DAEs. In the first part of the thesis, we consider LTI systems, where we focus on two properties: passivity and a generalization of passivity and small gain theorems called mixed property. These properties play an important role in the control design of large-scale interconnected systems. An important bottleneck for a design based on the aforementioned properties is their verification. Hence we intend to develop easily verifiable conditions to check passivity and mixedness of Single Input Single Output (SISO) and Multiple Input Multiple Output (MIMO) DAEs. For linear switched DAEs, we focus on the Lyapunov stability and this problem forms the basis for the second part of the thesis. In this part, we try to find conditions under which there exists a common Lyapunov function for all modes of the switched system, thus guaranteeing exponential stability of the switched system. These results are primarily developed for continuous-time systems. However, simulation and control design of a dynamic system requires a discrete-time representation of the system that we are interested in. Thus, it is critical to establish whether discrete-time systems, inherit fundamental properties of the continuous-time systems from which they are derived. Hence, the third part of our thesis is dedicated to the problems of preserving passivity, mixedness and Lyapunov stability under discretization. In this part, we examine several existing discretization methods and find conditions under which they preserve the stability properties discussed in the thesis

Stability results for constrained dynamical systems

Differential-Algebraic Equations (DAE) provide an appropriate framework to model and analyse dynamic systems with constraints. This framework facilitates modelling of the system behaviour through natural physical variables of the system, while preserving the topological constraints of the system. The main purpose of this dissertation is to investigate stability properties of two important classes of DAEs. We consider some special cases of Linear Time Invariant (LTI) DAEs with control inputs and outputs, and also a special class of Linear switched DAEs. In the first part of the thesis, we consider LTI systems, where we focus on two properties: passivity and a generalization of passivity and small gain theorems called mixed property. These properties play an important role in the control design of large-scale interconnected systems. An important bottleneck for a design based on the aforementioned properties is their verification. Hence we intend to develop easily verifiable conditions to check passivity and mixedness of Single Input Single Output (SISO) and Multiple Input Multiple Output (MIMO) DAEs. For linear switched DAEs, we focus on the Lyapunov stability and this problem forms the basis for the second part of the thesis. In this part, we try to find conditions under which there exists a common Lyapunov function for all modes of the switched system, thus guaranteeing exponential stability of the switched system. These results are primarily developed for continuous-time systems. However, simulation and control design of a dynamic system requires a discrete-time representation of the system that we are interested in. Thus, it is critical to establish whether discrete-time systems, inherit fundamental properties of the continuous-time systems from which they are derived. Hence, the third part of our thesis is dedicated to the problems of preserving passivity, mixedness and Lyapunov stability under discretization. In this part, we examine several existing discretization methods and find conditions under which they preserve the stability properties discussed in the thesis

Once an Other, always an Other: Contemporary discursive representations of the Asian Other in Aotearoa/New Zealand

Developments in the theorising of representation and the constitutive nature of language have encouraged an increased scholarly interest in the discursive construction of social identities, relations, and realities. This includes a growing body of literature internationally that focuses on the construction of social groups positioned as Others. However, critical research in this area is more limited in the domestic setting. In Aotearoa/New Zealand, the contemporary construction of social identities is embedded within a specific socio-political and historical context, including a particular colonial context. This context is fundamental to the ways in which social relations between the white settler Self and various Other groups have been, and continue to be, constituted. In this thesis, I have explored the discursive representation of Asian identity in dominant institutional discourses in Aotearoa/New Zealand, with a particular focus on the construction of the Asian as Other. Using critical discourse analysis, contemporary newspaper and parliamentary texts were examined to identify content areas, discursive strategies, and lexical choices involved in the representation of the Asian Other by elite institutions in Aotearoa/New Zealand. Through this process, several recurring manifestations of Asian Otherness were recognised, namely those of Asians as threat, Asian as impermanent, Asian as commodity, and Asian as victim. These representations of the Asian Other embody continuities and contradictions. They function to contribute to contemporary understandings and positionings of Asian individuals and collectives, to the ongoing construction of the Self in Aotearoa/New Zealand, and to the broader national narrative

Sensor-based formation control using a generalised rigidity framework and passivity techniques

The research in this thesis addresses the subject of sensor-based formation control for a network of autonomous agents. The task of formation control involves the stabilisation of the agents to a desired set of relative states, with the possible additional objective of manoeuvring the agents while maintaining this formation. Although the formation control challenge has been widely studied in the literature, many existing control strategies are based on full state information, and give little consideration to the sensor modalities available for the task. The focus of this thesis lies in the use of a generic arrangement of partial state measurements as can commonly be acquired by onboard sensors; for example, time-of-flight sensors can be used to measure the distances between vehicles, and onboard cameras can provide the bearing from one vehicle to each of the others. Particular aspects of the problem that are addressed in this thesis include (i) ways of modelling the formation control task, (ii) methods of analysing the system's behaviour, and (iii) the design of a formation control scheme based on generic arrangements of sensors that provide only partial position information. A key contribution in this thesis is a generalisation of the classical notion of rigidity, which considers the use of distance constraints between agents in R^2 or R^3 to specify a rigid body (or formation). This enables the concept of rigidity to be applied to agent networks involving a variety of (possibly non-Euclidean) state-spaces, with a generic set of state constraints that may, for example, include bearings between agents as well as distances. I demonstrate that this framework is very well-suited for modelling a wide variety of formation control problems (addressing goal (i) above), and I extend several fundamental results from classical rigidity theory in order to provide significant insight for system analysis (addressing goal (ii) above). To design a formation control scheme that uses generic partial position measurements (addressing goal (iii) above), I employ a modular passivity-based approach that is developed using the bondgraph modelling formalism. I illustrate how adaptive compensation can be incorporated into this design approach in order to account for the unknown position information that is not available from the onboard sensors. Although formation control is the subject of this thesis, it should be noted that the rigidity-based and passivity-based frameworks developed here are quite general and may be applied to a wide range of other problems

Étude de la sublimation du chrome lors de l’oxydation haute température de l’alliage AISI 441 et recherche de solutions de protection

Ferritic stainless steels are the most attractive materials that are able to fulfil SOFC(Solid Oxide Fuel Cell) interconnect properties. However, in cathodic SOFC condition, Crvolatilisation from ferritic steels leads to degradations of interconnect and cathode materialsand shorter service lifetime of SOFC. The objectives of this work are (i) to study Crvolatilisation from AISI 441 ferritic stainless steel and (ii) to find the way out to suppress Crvolatilisation. The Cr volatilisation protective methods used in this work were coating withMn-Co spinel by an electroplating method and preoxidation in the condition of Ar or CO/CO2at either 250°C or 850°C, for 3 h. Cr volatilisation experiments were performed in 5%H2O inO2 at 800°C for 96 h. According to the results, it can be concluded that the preoxidation ofthe AISI 441 in Ar or CO/CO2 at low temperature (250°C) or in the CO/CO2 at a highertemperature (850°C) are treatments that lead to lowering the sublimation rate of the Cr andimprovement in the oxidation resistance of this alloy in the cathodic compartment of theSOFC.Les aciers inoxydables ferritiques sont actuellement les meilleurs candidats pour répondre au cahier des charges des matériaux destinés aux interconnexions de piles à combustibles à oxyde solide (SOFC acronyme anglo-saxon pour Solid Oxide Fuel Cell). Cependant, du coté du compartiment cathodique de ces piles, le phénomène de sublimation du chrome à haute température conduit à des dégradations importantes, réduisant ainsi la durée de vie en service des SOFC. Les objectifs de ce travail de thèse sont (i) d'étudier le phénomène de sublimation du chrome sur l'acier inoxydable ferritique AISI 441 et (ii) de rechercher le ou les moyen(s) d'atténuer cette même sublimation. Sur le deuxième point, des méthodes de protection ont été mises en œuvre, en particulier avec le dépôt de films minces de spinelle Mn-Co obtenues via un procédé de galvanoplastie ou bien encore en procédant à des pré-oxydations des alliages dans des atmosphères contrôlées de type argon impur ou mélanges gazeux équimolaires CO / CO2 (250 ° C et 850 ° C, pour une durée totale de 3 h).Concernant l'étude de la sublimation du chrome, des essais d'oxydation simulant les conditions de service ont été conduites dans des mélanges gazeux : 5% de O2 dans H2O à 800 °C pendant 96 h. A vitesse de gaz oxydants faible, de 1 à 3 cm / s, la sublimation du chrome est limitée par un phénomène de diffusion dans la phase gazeuse de l'espèce volatile oxo-hydroxyde de chrome. A vitesse plus élevée, de 3 à 10 cm / s, la sublimation du chrome semble limitée par un phénomène interfacial. La vitesse de sublimation apparait indépendante de la rugosité de surface des échantillons, elle même variant selon la préparation des échantillons. Du point de vue de la caractérisation morphologique des couches d'oxydation thermiques obtenues, sur les surfaces de plus forte rugosité (surface industrielle), nous avons noté le développement de nodules riches en titane et en niobium avec une partie externe et une autre interne. Dans le substrat, des phases de lave de type Fe2Nb ont été observées le long des joints de grain métalliques.Dans une deuxième partie, la sublimation du chrome a été étudiée sur échantillons revêtus ou pré-oxydés. Les films minces de spinelle Mn-Co ne conduisent pas à l'abaissement de la vitesse de sublimation du chrome et sont donc pas protecteurs. L'adhérence de ces revêtements est de mauvaise qualité. Les couches présentent de nombreuses fissures. La pré-oxydation conduit quant à elle à une réduction considérable (jusqu'à un facteur 10) de la vitesse de sublimation du chrome (sauf dans le cas de la pré-oxydation dans l'argon à 850 °C). La formation d'un film riche en fer obtenu à basse température (250 °C) peut expliquer cette réduction par l'établissement d'une barrière de diffusion. Cette même réduction est cependant surprenante sur les films riches en chrome obtenus par pré-oxydation à 850 °C dans CO / CO2. Nous proposons dans ce travail une interprétation originale basée sur les différences de nature semi-conductrice des films de chromine formées à haute pression d'oxygène (proche de l'atmosphère) et identifiés comme étant de type p par rapport à ceux obtenus à basse pression (dans CO/CO2) qui sont connu pour être de type n. Après l'oxydation à haute température de 96 heures, tous les échantillons préalablement pré-oxydés ont été observés comme étant composés d'oxyde de chrome et d'un spinelle Mn-Cr.Au terme de ce travail de thèse, il peut être conclu que la pré-oxydation de l'alliage 441 à basse température (250 °C) dans l'argon ou le mélange CO/CO2 ou encore à plus haute température (850 °C) dans le mélange CO/CO2 sont des traitements qui conduisent à l'abaissement de la vitesse de sublimation du chrome et par voie de conséquence à l'amélioration de la tenue de cet alliage en condition d'oxydation dans le compartiment cathodique des SOFC

Governing the constructs of life: what constitutes ‘good’ governance?

This thesis explores contrasting perspectives on what constitutes 'good governance' for human embryonic stem cell (hESC) research. It asks whether there are systematic differences between perspectives of UK and US policy actors and what kinds of patterns are discernible. Biomedical technologies like hESCs generate complex interactions between public values, institutional interests, societal expectations and technological uncertainties. These pose serious governance challenges. Under such conditions, diverse aspects and implications of risk, ambiguity and uncertainty come into focus. We need appraisal processes that address these issues by combining quantitative and qualitative dimensions to 'open up' divergent governance framings. The research framework employed here uses and further develops one such elicitation and analysis process called Multicriteria Mapping (MCM). MCM combines qualitative sensitivity with quantitative precision, while also aiding transparency and reflexivity in documenting and understanding diverse stakeholder perspectives. We therefore address 'good' governance both as an analytical subject and as a rationale for testing a novel form of appraisal. The analysis discerns systematic patterns in perspectives on good governance across national contexts and between stakeholders, identifying several points of convergence and divergence. We examine underlying rationales behind individual perspectives, obtaining empirical support for recent theoretical arguments concerning technology appraisal and democratic deliberation. We find national policy literatures make greater use of moral and ethical language to frame governance challenges, by comparison with stakeholders' emphasis on institutional and socio-political factors. This suggests a more critical and cautious stance is needed towards the legitimatory language of 'bioethics' in policy making. Finally, we explore some of the normative implications for governance of culturally sensitive and scientifically uncertain issues. By providing reflexive explanations of factors influencing perspectives of policy actors, this thesis makes a number of interlinked theoretical, methodological, empirical and normative contributions to understanding of how good governance of biomedical technologies is and should be conducted

The stoics on nature and truth

First, this thesis outhnes part of the thought of some pre-Socratic thinkers, particularly Heraclitus. In doing this, I explore the historical provenance of certain ideas which came to be important in Stoicism. It then moves on to look at the Stoic view of 'physics', including some comparison with Epicurus and Aristotle, and with a focus on the concept of the continuum. The third chapter attempts to synthesise a common problem arising from a belief in the continuum, namely a problem of indeterminacy. In the fourth chapter, certain characterisations of Stoic epistemology are considered, along with an overview of recent interpretations of the Stoic theory of impressions. It concludes with the thought that at certain crucial points - such as whether impressions themselves are to be thought of as true and false - the Stoic position is underdetermined with respect to the evidence. Pursuing this thought into the fifth chapter, we see the evidence as being equivalently consistent with a 'two-tier’ theory of perception, where impressions themselves are understood as neither true nor false in any sense, but iu which 'the true' arises as a result of the transformative effect of reason. This theory is shown to connect with verbalisation through the 'rational impression'. This leads to the suggestion that the Stoics had a linguistic diagnosis for some problems in philosophy, arrived at by their reflections on ambiguity and etymology. In the final chapter, an account of intersubjectivity is explored, which preserves for the Stoics the claim that their truth has an objective character and is thus appropriate for a 'dogmatic' philosophy