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This dissertation presents work conducted at the Center for Power Electronics Systems (CPES) at Virginia Polytechnic Institute and State University. Chapter 1 introduces the concept of impedance measurement, and discusses previous work on this topic. This chapter also addresses issues associated with impedance measurement. Chapter 2 introduces the analyzer architecture and the proposed algorithm. The algorithm involves locking on to the voltage vector at the point of common coupling between the analyzer and the system via a PLL to establish a D-Q frame. A series of sweeps are performed, injecting at least two independent angles in the D-Q plane, acquiring D- and Q-axis voltages and currents for each axis of injection at the point of interest. Chapter 3 discusses the analyzer hardware and the criteria for selection. The hardware built ranges from large-scale power level hardware to communication hardware implementing a universal serial bus. An eight-layer PCB was constructed implementing analog signal conditioning and conversion to and from digital signals with high resolution. The PCB interfaces with the existing Universal Controller hardware. Chapter 4 discusses the analyzer software. Software was written in C++, VHDL, and Matlab to implement the measurement process. This chapter also provides a description of the software architecture and individual components. Chapter 5 discusses the application of the analyzer to various examples. A dynamic model of the analyzer is constructed, considering all components of the measurement system. Congruence with predicted results is demonstrated for three-phase balanced linear impedance networks, which can be directly derived based on stationary impedance measurements. Other impedances measured include a voltage source inverter, Vienna rectifier, six-pulse rectifier and an autotransformer-rectifier unit. / Ph. D.

The design of controllers for structural systems, particularly those associated with large space structures, has received a considerable amount of attention in the past few years. The usual approach to designing these controllers is to apply modern control theory to a reduced linear system obtained from finite element analysis or from a truncated modal analysis. In most of these designs, the sensor signal must be processed to separate out the contributions from each mode so that it may be sent to the appropriate actuators. The analysis presented here, on the other hand, obtains exact solutions for a selected set of sensor and actuator positions for simple structural elements. Sensor signals are fed back directly to the actuators with appropriate gains. The method of analysis is that of classical control theory using Laplace transforms and the associated open and closed-loop transfer functions. Single-input-single-output feedback control is applied to various flexible cable and beam configurations. Root-loci for various values of gain are constructed and the system characteristics and the global system stability are determined. Although the procedure outlined above can be carried out for basic structural elements, more complex structures and control configurations are synthesized using the dynamic stiffness matrix method. With this method, the exact relationships of the basic elements can be combined to allow analysis of multi-input-multi-output control of more complex structures. Using this approach, examples for flexible cable and beam configurations are presented. It was found that exact solutions can be obtained using a finite number of sensors and actuators. It was also determined that a single co-located sensor-actuator at the boundary of a fixed-free cable or beam can control all the vibrational modes of the cable or beam. Also, pure signals from a perfect sensor can be used without any additional signal processing. The multi-input-multi-output investigation demonstrates that, even without cross-gain feedback, there is interaction between the sets of co-located sensor-actuator pairs. It appears that this interactive effect needs to be included in any multi-input-multi-output control design. By starting with fundamental elements of beams and cables, it was shown that reasonably sophisticated systems can be modeled. Finally, considerable insight is offered by analyzing the control of flexible structures using exact transfer function relationships. / Ph. D.

[pt] Este trabalho propõe uma técnica chamada de Opacidade 3D para visualização volumétrica de dados sísmicos. O grande desafio da visualização volumétrica é definir uma função de transferência multidimensional que melhor se adapte ao dado que se deseja visualizar. Será apresentada uma função de transferência que utiliza três tabelas de cores 1D para compor a uma tabela de cores 3D. O trabalho de Silva[30] sobre opacidade 2D serviu de motivação para o desenvolvimento da técnica de opacidade 3D e ao longo deste trabalho são feitas comparações entre ambos. São apresentados exemplos reproduzindo a opacidade 2D e outros mostrando como a técnica proposta pode auxiliar no estudo de determinados eventos sísmicos. / [en] This work proposes a 3D opacity technique for the volume rendering of seismic data. The greater challenge of volume rendering is to define a multidimensional transfer function better adapted to the data to be visualized. This work presents a transfer function that uses three 1D color tables to compose a 3D color table. The work from Silva[30] about 2D opacity has served as a motivation for the development of the 3D opacity technique and, hence, some comparisons are made between them. Some examples are presented in order to reproduce the 2D opacity technique and to show how the proposed technique can improve the visualization of specific seismic events.

Improving agricultural water management is important for conserving water during dry seasons, using limited water resources in the most efficient way, and minimizing environmental risks (e.g., leaching, surface runoff). The understanding of water movement in different zones of agricultural production fields is crucial to developing an effective irrigation strategy. This work centered on optimizing field water management by characterizing the spatial patterns of soil hydraulic properties. Soil hydraulic conductivity was measured across different zones in a farmer’s field, and its spatial variability was investigated by using geostatistical techniques. Since direct measurement of hydraulic conductivity is time-consuming and arduous, pedo-transfer functions (PTFs) have been developed to estimate hydraulic conductivity indirectly through more easily measurable soil properties. Due to ignoring soil structural information and spatial covariance between soil variables, PTFs often perform unsatisfactorily when field-scale estimations of hydraulic conductivity are needed. The performance of PTFs in estimating hydraulic conductivity in the field was therefore critically evaluated. Due to the presence of structural macro-pores, saturated hydraulic conductivity (Ks) showed high spatial heterogeneity, and this variability was not captured by texture-dominated PTF estimates. However, the general spatial pattern of near-saturated hydraulic conductivity can still be reasonably generated by PTF estimates. Therefore, the hydraulic conductivity maps based on PTF estimates should be evaluated carefully and handled with caution. Recognizing the significant contribution of macro-pores to saturated water flow, PTFs were further improved by including soil macro-porosity and were proven to perform much better in estimating Ks compared with established PTFs tested in this study. Additionally, the spatial relationship between hydraulic conductivity and its potential influencing factors were further quantified by the state-space approach. State-space models outperformed current PTFs and effectively described the spatial characteristics of hydraulic conductivity in the studied field. These findings provided a basis for modeling water/solute transport in the vadose zone, and sitespecific water management.

<p>Locomotion is one of the most important behaviours and requires interaction between sensors at various levels of the nervous system and the limb muscles of an animal. The basic neural rhythm for locomotion in mammals has been shown to arise from local neural networks residing in the spinal cord and these networks are known as central pattern generators (CPGs). However, during the locomotion, these centres are constantly interacting with the sensory feedback signals coming from muscles, joints and peripheral skin receptors in order to adapt the stepping to varying environmental conditions. Conceptual models of mammalian locomotion have been constructed using</p><p>mathematical models of locomotor subsystems based on the abundance of neurophysiological evidence obtained primarily in the cat. Several aspects of locomotor control using the cat as an animal model have been investigated employing computer simulations and here we use the same approach to address number of questions or/and hypotheses related to rhythmic locomotion in quadrupeds. Some of the involve questions are, role of mechanical linkage during deafferented walking, finding inherent stabilities/instabilities of muscle-joint interactions during normal walking, estimating phase dependent controlability of muscle action over joints.</p><p>This thesis presents the basics of a biologically realistic model of mammalian locomotion and summarises methodological approaches in modelling quadruped locomotor subsystems such as CPGs, limb muscles and sensory pathways. In the first appended article, we extensively discuss the construction details of the three-dimensional computer simulator for the study of the hind leg neuro-musculo-skeletal-control system and its interactions during normal walking of the cat. The simulator with the walking model is programmed in Python scripting language with other supported open source libraries such as Open Dynamics Engine (ODE) for simulating body dynamics and OpenGL for three dimensional graphical representation. We have examined the</p><p>functionality of the simulator and the walking model by simulating deafferented walking. It was possible to obtain a realistic stepping in the hind legs even without sensory feedback to the two controllers (CPGs) for each leg. We conclude that the mechanical linkages between the legs also play a major role in producing alternating gait.</p><p>The use of simulations of walking in the cat for gaining insights into more complex interactions between the environment and the neuro-muscular-skeletal system is important especially for questions where a direct neurophysiological experiment can not be performed on a real walking animal. For instance, it is experimentally hard to isolate individual mechanisms without disrupting the natural walking pattern. In the second article, we introduce a different approach where we use the walking model to identify what control is necessary to maintain stability in the musculo-skeletal system. We show that the actions of most of the hindlimb muscles over the joints have an inherent stability during stepping, even without the involvement of proprioceptive feedback mechanisms. In addition, we observe that muscles generating movements in the ankle joint of the hind leg must be controlled by neural mechanisms, which may involve supraspinal structures, over the whole step cycle.</p>

Direct Volume Rendering (DVR) is a technique for creating images directly from a representation of a function defined over a three-dimensional domain. The technique has many application fields, such as scientific visualization and medical imaging. A striking property of the data sets produced within these fields is their ever increasing size and complexity. Despite the advancements of computing resources these data sets seem to grow at even faster rates causing severe bottlenecks in terms of data transfer bandwidths, memory capacity and processing requirements in the rendering pipeline. This thesis focuses on efficient methods for DVR of large data sets. At the core of the work lies a level-of-detail scheme that reduces the amount of data to process and handle, while optimizing the level-of-detail selection so that high visual quality is maintained. A set of techniques for domain knowledge encoding which significantly improves assessment and prediction of visual significance for blocks in a volume are introduced. A complete pipeline for DVR is presented that uses the data reduction achieved by the level-of-detail selection to minimize the data requirements in all stages. This leads to reduction of disk I/O as well as host and graphics memory. The data reduction is also exploited to improve the rendering performance in graphics hardware, employing adaptive sampling both within the volume and within the rendered image. The developed techniques have been applied in particular to medical visualization of large data sets on commodity desktop computers using consumer graphics processors. The specific application of virtual autopsies has received much interest, and several developed data classification schemes and rendering techniques have been motivated by this application. The results are, however, general and applicable in many fields and significant performance and quality improvements over previous techniques are shown. / On the defence date the status of article IX was Accepted.

This thesis investigates sea-level changes in order to test the hypothesis that the main contribution to early 20th century sea-level rise was Northern Hemisphere land-based ice melt. Multiproxy sea-level reconstructions were established for the Falkland Islands, a location where models suggest sea-level rise from Northern Hemisphere ice melt produces the largest signal. The Falklands reconstruction indicated sea levels in the early 20th century accelerated compared to the long-term rate, synchronous with accelerations observed globally. The magnitude of the acceleration in the Falklands reconstruction was greater than Northern Hemisphere rates, consistent with the spatial pattern from a Northern Hemisphere melt source, but likely less than in New Zealand and Australia. It is therefore not possible rule out other contributions to the observed sea-level acceleration. The Falklands reconstruction indicated a rapid sea-level jump around 8.4 ka BP, synchronous with a jump observed in the Northern Hemisphere, which has been attributed to the sudden drainage of Laurentide proglacial lake Agassiz-Ojibway associated with the 8.2 ka BP climatic downturn. A maximum estimate of 0.89 ± 0.22 m for this jump in the Falklands is considerably less than estimates from Northern Hemisphere records. This difference could indicate additional contributions from the Southern Hemisphere are being recorded in the Northern Hemisphere signal. This thesis also focused on developing testate amoebae as sea-level indicators. In the Falklands, testate amoebae transfer functions were able to reconstruct sea level with precision (±0.08 m) comparable to diatoms (±0.07 m). However, preservation issues were indicated in the fossil testate amoebae assemblages which limits their use as tools for sea-level reconstruction. In addition, contemporary distributions of salt-marsh testate amoebae were investigated over one annual cycle. Seasonal variations in the live assemblages were observed to be asynchronous between taxa. Variations in the death assemblages were also observed which were correlated with variations in the live assemblages. This observation suggests the commonly applied assumption in palaeoenvironmental studies that analysing the death population negates temporal bias is invalid. Further research is required to investigate the impact these observed variations have on reconstructive performance.

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