etd@IISc Collection:
http://hdl.handle.net/2005/20
2017-03-20T07:18:51ZMotion Estimation From Moments Of Projection Data For Dynamic CT
http://hdl.handle.net/2005/2572
Title: Motion Estimation From Moments Of Projection Data For Dynamic CT
Authors: Gokul Deepak, M
Abstract: In X-ray computed tomography, motion of the object (breathing, for example)
while X-ray projections are acquired for tomographic reconstruction leads to mo-
tion artifacts in the reconstructed image. Object motion (such as that of breathing
lungs) during acquisition of a computed tomography scan causes artifacts in the
reconstructed image due to the reason that the source and detectors require a finite
amount of time to rotate around the object while acquiring measurements even as
the object is changing with time. With traditional reconstruction algorithms, the
object is assumed to be stationary while data is acquired. However, in the case of
dynamic tomography, the projection data that is acquired is not consistent, as it is
data measured from an object that is deformed at each view angle of measurement.
In this work, we propose a method for estimation of general (non-rigid) small
motion for dynamic tomography from motion-corrupted projection data. For a
static object, the Helgason-Ludwig consistency conditions impose some structure
on the moments of the projections. However in the case of dynamic object (result-
ing in motion-corrupted projections) this is violated. In the proposed method, we
estimate motion parameters of the general motion model from the moments of the
dynamic projections. The dynamic object can be modeled as f (g(x, t)) where g is
a time-dependent warping function. The non-linear problem of solving a system
involving composition of functions is dealt with in the Fourier transform space
where it simplifies into a problem involving multiplicatively separable functions.
The system is then linearized to solve for object motion. We assume a general
basis function in our model. For numerical simulations, we use polynomial and
B-spline basis functions as special cases of the basis functions.
Simulation is performed by applying known deformations to the Shepp-Logan
phantom, to a head slice of the Visible Human phantom and a thorax slice of the
Zubal phantom. Simulations are performed for projections generated by parallel-
beam and fan-beam geometry. Simulation for fan-beam geometry are performed by
rebinning the motion corrupted fan beam projections to parallel beam projections,
followed by the proposed motion estimation method. Simulation for the Visible
Human phantom and the thorax slice of the Zubal phantom are performed for fan-
beam geometry. Poisson noise is also added to the generated dynamic projections
before motion estimation is performed. To solve the ill-posed problem of motion
estimation by the proposed method, we use a Tikhonov type regularization that
involves minimizing an objective function that is the sum of a data discrepancy
term, a term that penalizes temporal variation of motion, and another term to
penalize large magnitudes of motion.
Using the estimated motion, the original image has been reconstructed from the
motion corrupted projection data, with the knowledge of the underlying motion
which is estimated by the proposed algorithm, by an algebraic technique similar to
the dynamic SART algorithm from the literature. Here, a SART-type coefficient
matrix is computed using ray tracing with rays whose paths are warped according
to the estimated motion. The dynamic image at t = 0 is then reconstructed with
using the computed dynamic SART matrix.2016-09-14T18:30:00ZSpace-Vector-Based Pulse Width Modulation Strategies To Reduce Pulsating Torque In Induction Motor Drives
http://hdl.handle.net/2005/2441
Title: Space-Vector-Based Pulse Width Modulation Strategies To Reduce Pulsating Torque In Induction Motor Drives
Authors: Hari, V S S Pavan Kumar
Abstract: 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.2015-05-26T18:30:00ZA Hierarchical Approach To Music Analysis And Source Separation
http://hdl.handle.net/2005/2460
Title: A Hierarchical Approach To Music Analysis And Source Separation
Authors: Thoshkahna, Balaji
Abstract: Music analysis and source separation have become important and allied areas of research over the last decade. Towards this, analyzing a music signal for important events such as onsets, offsets and transients are important problems. These tasks help in music source separation and transcription. Approaches in source separation too have been making great strides, but most of these techniques are aimed at Western music and fail to perform well for Indian music. The fluid style of instrumentation in Indian music requires a slightly modified approach to analysis and source separation.
We propose an onset detection algorithm that is motivated by the human auditory system. This algorithm has the advantage of having a unified framework for the detection of both onsets and offsets in music signals. This onset detection algorithm is further extended to detect percussive transients. Percussive transients have sharp onsets followed closely by sharp offsets. This characteristic is exploited in the percussive transients detection algorithm. This detection does not lend itself well to the extraction of transients and hence we propose an iterative algorithm to extract all types of transients from a polyphonic music signal. The proposed iterative algorithm is both fast and accurate to extract transients of various strengths. This problem of transient extraction can be extended to the problem of harmonic/percussion sound separation(HPSS), where a music signal is separated into two streams consisting of components mainly from percussion and harmonic instruments. Many algorithms that have been proposed till date deal with HPSS for Western music. But with Indian classical/film music, a different style of instrumentation or singing is seen, including high degree of vibratos or glissando content. This requires new approaches to HPSS. We propose extensions to two existing HPSS techniques, adapting them for Indian music. In both the extensions, we retain the original framework of the algorithm, showing that it is easy to incorporate the changes needed to handle Indian music. We also propose a new HPSS algorithm that is inspired by our transient extraction technique. This algorithm can be considered a generalized extension to our
transient extraction algorithm and showcases our view that HPSS can be considered as an extension to transient analysis. Even the best HPSS techniques have leakages of harmonic components into percussion and this can lead to poor performances in tasks like rhythm analysis. In order to reduce this leakage, we propose a post processing technique on the percussion stream of the HPSS algorithm. The proposed method utilizes signal stitching by exploiting a commonly used model for percussive envelopes. We also developed a vocals extraction algorithm from the harmonic stream of the HPSS algorithm. The vocals extraction follows the popular paradigm of extracting the predominant pitch followed by generation of the vocals signal corresponding to the pitch. We show that HPSS as a pre-processing technique gives an advantage in reducing the interference from percussive sources in the extraction stage. It is also shown that the performance of vocal extraction algorithms improve with the knowledge about locations of the vocal segments. This is shown with the help of an oracle to locate the vocal segments. The use of the oracle greatly reduces the interferences from other dominating sources in the extracted vocals signal.2015-08-05T18:30:00ZDimensioning Of Corona Control Rings For EHV/UHV Line Hardware And Substations
http://hdl.handle.net/2005/2446
Title: Dimensioning Of Corona Control Rings For EHV/UHV Line Hardware And Substations
Authors: Chatterjee, Sreenita
Abstract: High voltage (EHV and UHV) transmission facilitates transfer of large amount of power over long distances. However, due to the inherent geometry, the line and substation hardware of EHV and UHV class generate high electric fields, which results in local ionisation of air called corona discharges. Apart from producing audible noise in the form of frying or hissing sound, corona produces significant electromagnetic interferences in the radio range. The limit for this corona generated Radio Interference (RI) has been stipulated by international standards, which are strictly to be followed.
In line and substation hardware, corona control rings are generally employed to limit or avoid corona. Standard dimensions of corona rings are not available for EHV and UHV class. In most of the cases, their design is based on either a trial and error method or based on empirical extrapolation. Only in certain specific cases, the dimensioning of the rings is carried out using electric field calculations. In any of these approaches, the unavoidable surface abrasions, which can lead to corona, are not considered. There are also efforts to account for nominal surface irregularity by using a surface roughness factor, which is highly heuristic.
In order to address this practically relevant problem, the present work was taken up. The intended exercise requires accurate field computation and a suitable criterion for checking corona onset. For the first part, the Surface Charge Simulation Method is adopted with newly proposed sub-modelling technique. The surface of the toroid is discretised into curvilinear patches with linear approximation for the surface charge density. Owing to its high accuracy, Galerkin’s method of moments formulation is employed. The problem of singularity encountered in the numerical approach is handled using a method based on Duffy’s transformation. The developed codes have also been validated with standard geometries.
After a survey of relevant literature the ‘Critical Avalanche Criteria’ is chosen for its simplicity and applicability to the problem. Through a detailed simulation, the effect of avalanche space charge in reducing the corona onset voltage is found to be around 1.5% and hence it is not considered further.
For utilities not interested in a detailed calculation procedure for dimensioning of corona rings, design curves are developed for circular corona rings of both 400 kV and 765 kV class with surface roughness factor in the range 0.8 – 1.
In the second part of the work, a methodology for dimensioning is developed wherein the inevitable surface abrasion in the form of minute protrusions can be accounted. It is first shown that even though considerable field intensification occurs at the protrusions, such localised modification need not lead to corona. It is shown that by varying the minor radius of the corona ring, it is possible to get a design where the prescribed surface abrasion does not lead to corona onset.
In summary, the present work has successfully developed a reliable methodology for the design of corona rings with prescribed surface abrasions. It involved development of an efficient field computation technique for handling minute surface protrusions and use of appropriate criteria for assessing corona inception. It has also provided design curves for EHV and UHV class corona rings with surface roughness factor specified in the range 0.8 – 1.0.2015-07-12T18:30:00Z