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Voltage source inverter (VSI) is used to control the speed of an induction motor by applying AC voltage of variable amplitude and frequency. The semiconductor switches in a VSI are turned on and off in an appropriate fashion to vary the output voltage of the VSI. Various pulse width modulation (PWM) methods are available to generate the gating signals for the switches. The process of PWM ensures proper fundamental voltage, but introduces harmonics at the output of the VSI. Ripple in the developed torque of the induction motor, also known as pulsating torque, is a prominent consequence of the harmonic content. The harmonic voltages, impressed by the VSI on the motor, differ from one PWM method to another. Space-vector-based approach to PWM facilitates a large number of switching patterns or switching sequences to operate the switches in a VSI. The switching sequences can be classified as conventional, bus-clamping and advanced bus-clamping sequences. The conventional sequence switches each phase once in a half-carrier cycle or sub-cycle, as in case of sine-triangle PWM, third harmonic injection PWM and conventional space vector PWM (CSVPWM). The bus-clamping sequences clamp a phase to one of the DC terminals of the VSI in certain regions of the fundamental cycle; these are employed by discontinuous PWM (DPWM) methods. Popular DPWM methods include 30 degree clamp PWM, wherein a phase is clamped during the middle 30 degree duration of each quarter cycle, and 60 degree clamp PWM which clamps a phase in the middle 60 degree duration of each half cycle. Advanced bus-clamping PWM (ABCPWM) involves switching sequences that switch a phase twice in a sub-cycle besides clamping another phase. Unlike CSVPWM and BCPWM, the PWM waveforms corresponding to ABCPWM methods cannot be generated by comparison of three modulating signals against a common carrier. The process of modulation in ABCPWM is analyzed from a per-phase perspective, and a computationally efficient methodology to realize the sequences is derived. This methodology simplifies simulation and digital implementation of ABCPWM techniques. Further, a quick-simulation tool is developed to simulate motor drives, operated with a wide range of PWM methods. This tool is used for validation of various analytical results before experimental investigations. The switching sequences differ in terms of the harmonic voltages applied on the machine. The harmonic currents and, in turn, the torque ripple are different for different switching sequences. Analytical expression for the instantaneous torque ripple is derived for the various switching sequences. These analytical expressions are used to predict the torque ripple, corresponding to different switching sequences, at various operating conditions. These are verified through numerical simulations and experiments. Further, the spectral properties are studied for the torque ripple waveforms, pertaining to conventional space vector PWM (CSVPWM), 30 degree clamp PWM, 60 degree clamp PWM and ABCPWM methods. Based on analytical, simulation and experimental results, the magnitude of the dominant torque harmonic with an ABCPWM method is shown to be significantly lower than that with CSVPWM. Also, this ABCPWM method results in lower RMS torque ripple than the BCPWM methods at any speed and CSVPWM at high speeds of the motor. Design of hybrid PWM methods to reduce the RMS torque ripple is described. A hybrid PWM method to reduce the RMS torque ripple is proposed. The proposed method results in a dominant torque harmonic of magnitude lower than those due to CSVPWM and ABCPWM. The peak-to-peak torque in each sub-cycle is analyzed for different switching sequences. Another hybrid PWM is proposed to reduce the peak-to-peak torque ripple in each sub-cycle. Both the proposed hybrid PWM methods reduce the torque ripple, without increasing the total harmonic distortion (THD) in line current, compared to CSVPWM. CSVPWM divides the zero vector time equally between the two zero states of a VSI. The zero vector time can optimally be divided to minimize the RMS torque ripple in each sub-cycle. It is shown that such an optimal division of zero vector time is the same as addition of third harmonic of magnitude 0.25 times the fundamental magnitude to the three-phase sinusoidal modulating signals. ABCPWM applies an active state twice in a sub-cycle, with the active vector time divided equally. Optimal division of active vector time in ABCPWM to minimize the RMS torque ripple is evaluated, both theoretically and experimentally. Compared to CSVPWM, this optimal PWM is shown to reduce the RMS torque ripple significantly over a wide range of speed. The various PWM schemes are implemented on ALTERA CycloneII field programmable gate array (FPGA)-based digital control platform along with sensorless vector control and torque estimation algorithms. The controller generates the gating signals for a 10kVA IGBT-based two-level VSI connected to a 5hp, 400V, 4-pole, 50Hz squirrel-cage induction motor. The induction motor is coupled to a 230V, 3kW separately-excited DC generator.

Replay of spike sequences can be seen during sharp wave – ripple population burst activity in the hippocampus. It is thought that this activity, which occurs during rest and sleep, is involved in memory consolidation. The cellular mechanisms underlying the initiation of these replay events are not well understood. To investigate this, a hippocampal slice model, showing spontaneous sharp wave – ripple activity, and a combination of planar multi-electrode array recordings and whole-cell patch-clamp recordings of anatomically identified hippocampal neurons were used. Firstly, the spatial and temporal profile of sharp waves in vitro was analysed in detail. Sharp waves were generated by changing subpopulations of pyramidal neurons in the CA3 region and had characteristics similar to those found in vivo. Secondly, four major receptor types present in hippocampal CA3, namely NMDA, AMPA, GABAA and GABAB receptors, were investigated for their involvement in sharp wave generation. Surprisingly, not only AMPA receptor-mediated events, but also phasic GABAA receptor-mediated inhibition, were necessary for sharp wave generation. Thirdly, single perisomatic-targeting interneurons were activated. This experiment showed that induced spiking activity of an individual perisomatic-targeting interneuron can both suppress and subsequently enhance local sharp wave generation. Spiking activity of other neuron types (i.e. pyramidal neurons, dendritic-targeting interneurons and interneuron-selective interneurons) had no significant effect on sharp wave incidence. Finally, it is suggested that this post-inhibitory enhancement of sharp wave generation can be mediated by a transient increase in the ratio of excitation to inhibition in the local network. In conclusion, these results suggest a new role for perisomatic-targeting interneurons in controlling the local initiation of sharp waves by selectively suppressing and subsequently enhancing recruitment of a subpopulation of pyramidal neurons. These results further imply that interneurons may play an integral part in the local information processing that takes place in the hippocampal network.

With the technical advances in neuroscience experiments in the past few decades, we have seen a massive expansion in our ability to record neural activity. These advances enable neuroscientists to analyze more complex neural coding and communication properties, and at the same time, raise new challenges for analyzing neural spiking data, which keeps growing in scale, dimension, and complexity. This thesis proposes several new statistical methods that advance statistical analysis approaches for neural spiking data, including sequential Monte Carlo (SMC) methods for efficient estimation of neural dynamics from membrane potential threshold crossings, state-space models using multimodal observation processes, and goodness-of-fit analysis methods for neural marked point process models. In a first project, we derive a set of iterative formulas that enable us to simulate trajectories from stochastic, dynamic neural spiking models that are consistent with a set of spike time observations. We develop a SMC method to simultaneously estimate the parameters of the model and the unobserved dynamic variables from spike train data. We investigate the performance of this approach on a leaky integrate-and-fire model. In another project, we define a semi-latent state-space model to estimate information related to the phenomenon of hippocampal replay. Replay is a recently discovered phenomenon where patterns of hippocampal spiking activity that typically occur during exploration of an environment are reactivated when an animal is at rest. This reactivation is accompanied by high frequency oscillations in hippocampal local field potentials. However, methods to define replay mathematically remain undeveloped. In this project, we construct a novel state-space model that enables us to identify whether replay is occurring, and if so to estimate the movement trajectories consistent with the observed neural activity, and to categorize the content of each event. The state-space model integrates information from the spiking activity from the hippocampal population, the rhythms in the local field potential, and the rat's movement behavior. Finally, we develop a new, general time-rescaling theorem for marked point processes, and use this to develop a general goodness-of-fit framework for neural population spiking models. We investigate this approach through simulation and a real data application.

A finalidade inicial deste estudo é desenvolver uma metodologia de análise da ondulação de torque de um motor de relutância chaveado especial bifásico com 4 pólos no estator e 2 no rotor (MRC 4/2) através de dados experimentais de aceleração. De forma a validar o método proposto, são apresentados resultados experimentais de aceleração obtidos com dois rotores: o rotor de referência e o rotor otimizado. Uma vez alcançados os objetivos de otimização do torque através da modificação da geometria do rotor, implementou-se um estudo complementar de modo a explorar a vibração e a ondulação de torque, do ponto de vista do acionamento. Desta forma, desenvolveu-se uma plataforma digital de testes, onde os dados de aceleração podem ser monitorados no domínio da frequência, e os parâmetros de controle do acionamento, tais como, a velocidade, os ângulos de ligamento e condução são flexíveis e podem ser alterados através de programa. A plataforma digital foi concebida a partir de um kit de desenvolvimento (eZdsp LF2407A) associado a uma interface de controle serial, desenvolvida a partir de um aplicativo LabView, instalado num microcomputador. / The initial purpose of this study is to develop a methodology of analysis of the torque ripple of a two-phase special SRM 4/2 using acceleration experimental data. In order to validate the proposed method, acceleration experimental results are presented, these results were obtained with two rotors: the reference rotor and the optimized rotor. Once the torque optimization objectives were reached by modifying the rotor geometry, a complementary study from the point of view of motor drive vibration and torque ripple optimization was carried out. Thus, a test digital setup was developed, where the acceleration data could be monitored, in the frequency domain, and the drive control parameters, such as speed, turn-on and dwell angles, and phase current were flexible and could be altered by program. The test digital setup was conceived using a development kit (eZdsp LF2407A) associated with a serial control interface developed on a LabView application, installed in a microcomputer.

A finalidade inicial deste estudo é desenvolver uma metodologia de análise da ondulação de torque de um motor de relutância chaveado especial bifásico com 4 pólos no estator e 2 no rotor (MRC 4/2) através de dados experimentais de aceleração. De forma a validar o método proposto, são apresentados resultados experimentais de aceleração obtidos com dois rotores: o rotor de referência e o rotor otimizado. Uma vez alcançados os objetivos de otimização do torque através da modificação da geometria do rotor, implementou-se um estudo complementar de modo a explorar a vibração e a ondulação de torque, do ponto de vista do acionamento. Desta forma, desenvolveu-se uma plataforma digital de testes, onde os dados de aceleração podem ser monitorados no domínio da frequência, e os parâmetros de controle do acionamento, tais como, a velocidade, os ângulos de ligamento e condução são flexíveis e podem ser alterados através de programa. A plataforma digital foi concebida a partir de um kit de desenvolvimento (eZdsp LF2407A) associado a uma interface de controle serial, desenvolvida a partir de um aplicativo LabView, instalado num microcomputador. / The initial purpose of this study is to develop a methodology of analysis of the torque ripple of a two-phase special SRM 4/2 using acceleration experimental data. In order to validate the proposed method, acceleration experimental results are presented, these results were obtained with two rotors: the reference rotor and the optimized rotor. Once the torque optimization objectives were reached by modifying the rotor geometry, a complementary study from the point of view of motor drive vibration and torque ripple optimization was carried out. Thus, a test digital setup was developed, where the acceleration data could be monitored, in the frequency domain, and the drive control parameters, such as speed, turn-on and dwell angles, and phase current were flexible and could be altered by program. The test digital setup was conceived using a development kit (eZdsp LF2407A) associated with a serial control interface developed on a LabView application, installed in a microcomputer.

In this work the roughness and topography evolution of optical materials sputtered with low energy ion beam (≤1 keV) has been investigated. These materials (bulk or thin layer) are used in the manufacturing of mirrors for scientific (ground or space) instruments or for other optical applications. In the first part of the work, the roughness evolution of optical surfaces under sputtering has been investigated in the frame of the industrial process known as Ion Beam Figuring. This technique consists in removing shape errors on optical surfaces with a low energy ion beam (≤1 keV). One disadvantage of this process is a potential increase of roughness for surfaces under treatment. The roughness evolution of some materials relevant to the optical industry has been accurately characterized as function of etching depth down to 5 µm. These sputtering experiments have been carried out at normal incidence, mainly with argon ions (but also in a lesser extent with krypton and xenon ions), ion current density of ~1 mA/cm2 and ion beam energy ranging from 200 eV to 1000 eV. The roughness evolution under sputtering is low for materials with amorphous (glass, electroless nickel), monocrystalline (silicon) or even polycrystalline structure (CVD silicon carbide, PVD gold or nickel film), whereas it is considerably more important for some other metallic materials such as electroplated nickel and aluminium. This work has shown small differences in the roughness evolution of CVD silicon carbide as function of the ion beam energy. The roughness increase is faster at low ion energy (<500 eV) than at higher ion energy (650-1000 eV). The grain structure of this material is less revealed at higher energy, which is supposed to be due to a larger amorphization of the sputtered layer in this case. The influence of the ion mass on CVD silicon carbide and gold films on nickel substrates has been also illustrated. Our measurements have been also compared to scaling laws. Various growth and roughness exponents have been found, sometimes rather different from those foreseen by the KPZ equation. In the second part, we focus on periodically modulated structures (ripples) which developed on many solids when sputtered by an off-normal ion bombardment. In this work, we first observed these ripples on gold films deposited on electroplated nickel (materials used as reflective surfaces for X-ray space telescope) sputtered at grazing incidence. We studied the influence of sputtering parameters (ion beam incidence angle, energy and flux) on the characteristics of ripples induced on gold and silver thin film (~0.2 µm). Ion-induced ripples have also been observed on CdS, an interesting semiconductor crystal for optical applications. The ripples orientation and dimensions (spatial wavelengths from 0.13 µm to 0.29 µm) have been confronted to the Bradley-Harper (B-H) linear model. We used the SRIM software to evaluate the deposited energy and the surface tension coefficient distributions. Our results can be in great part explained by the current theories (Bradley-Harper, Makeev) on morphology of ion-sputtered surfaces. These results can be summarized hereunder: Clear development of ripples for angle of incidence equal or higher than 60° on gold film and 70° on silver film. In this work the ripples wave vector is always perpendicular to the ion beam direction for all angles, whereas the change in ripple orientation beyond a critical angle is usually reported in literature. This is a due to the different shape of the energy distribution function for our sputtering conditions. Different regimes for roughness and topography evolution (grains, ripples) have been observed in function of the angle of incidence. 3 different areas can be distinguished, as predicted by Makeev non-linear model. The diminution of ripple wavelength with ion energy shows that thermal diffusion is the main relaxation mechanism.

In order to understand flow characteristics inside a positive displacement pump, every point in the flow field must be carefully observed. Such observations are difficult, expensive and usually time consuming to achieve with physical testing. During tests one can observe flow characteristics only at the locations where the instrument device is attached, not the whole flow domain. This thesis mainly focuses on the evaluation of design and performance characteristics of a positive displacement triplex diaphragm pump. For this purpose not only numerical investigations but also experimental studies were conducted using a positive displacement pump which is supplied by the pump manufacturer and is available in the fluid mechanics laboratory of Middle East Technical University. The effect of valve characteristics on the pump efficiency such as valve spring stiffness, valve displacement, mass of the check valves, and diaphragm shape are investigated in this thesis by using CFD (Computational Fluid Dynamics) technique. The pump performance is analyzed in terms of its volumetric and hydraulic efficiencies. The effect of the valve closure delay is also discussed. After the CFD and experimental results of the current pump model are compared and it is seen that they are in close agreement with each other, parametric studies are performed in computer environment. From analysis results it is observed that using stiffer springs reduces valve closing time and tend to decrease flow reversal effects. Secondly, using heavier check valves increases valve closing and opening times and also increases the stresses on the components of the pump with the increased pressure drop through discharge valve. As a result of this condition, hydraulic and volumetric efficiency reduce. Thirdly, with the longer valve displacement arrangement, more time is required for opening and closing of the check valves therefore efficiency of the pump reduces.

An auxiliary winding with an associated capacitor is added on the single-stage power factor corrector (PFC) based on fly-back conversion to reduce the ripple on the dc output voltage. The associated capacitor takes out partial energy at every switching cycle from the fly-back conversion and releases the stored energy to the load at the valley of the rectified line voltage. The negative effect of such an approach is that the converter does not draw a current from the AC line at the lower voltage near zero crossing, leading to deterioration in the power factor. This thesis analyzes how an auxiliary winding affects the voltage of the associated capacitor, which in turn changes the cut-in angle of the input current and thus the power factor of the AC source. To facilitate the implementation, the fly-back converter is operated at the boundary conduction mode (BCM). A design example is given for the 24 V, 48 W load, based on the derived equations. The laboratory circuit is built and tested to verify the computer simulations and analytical predictions. The experimental results confirm the circuit analyses on the converter performances.

An investigation was conducted to determine and compare the effect of cell current pulsing and "self-oxidation" in increasing the CO tolerance of a PEM fuel cell. The most effective pulsing parameter values were also determined. Current pulsing involves periodically demanding positive current pulses from the fuel cell to create an anode over-potential, while "self-oxidation" or sustained potential oscillations is achieved when the anode catalyst becomes so saturated with CO that the anode over-potential increases to a value at which CO is oxidized from the catalyst surface. The CO tolerance of a fuel cell system with a Pt-Ru anode was tested using 50 and 496 ppm CO in the anode fuel. The performance of the system declined with an increase in CO concentration. Current pulses of various amplitude, frequency, and duty cycle were applied to the cell while CO was present in the anode fuel. With 50 ppm CO in the anode fuel, the most effective pulse in increasing CO tolerance while maintaining normal cell operation was 1.0 A/cm2, 0.25 Hz, and a 5% duty cycle. A pulse (120 Hz, 50% duty cycle) similar to the ripple current often generated when converting DC to single-phase 60 Hz AC had a positive effect on the CO tolerance of the system, but at frequencies that high, the pulse duration was not long enough to completely oxidize the CO from the catalyst surface. With 496 ppm CO in the anode fuel, a pulse of 1.0 A/cm2, 0.5 Hz, and a 20% duty cycle proved most effective. When the cell was exposed to 496 ppm CO, without employing pulsing, "self-oxidation" occurred and CO was periodically oxidized from the catalyst surface. However, pulsing allowed the cell to operate at the desired voltage and power a higher percentage of the time than "self-oxidation"; hence, pulsing was more effective.