▼ The excitation of the switched reluctance motor (SRM) phases needs to be carefully synchronized with rotor position to obtain an acceptable drive performance. Therefore, rotor position sensing is an integral part of the SRM drives control. Additionally, for precise torque and speed control applications, it is necessary to have rotor position information with reasonably good resolution and high degree of accuracy. Sensorless control methods are an attractive approach in which the rotor position is sensed indirectly without a discrete mechanical position sensor. In the past, several methods have been developed to replace the discrete position sensor. Some of these methods are inductance based while others are flux based. Unfortunately, all of them were confined with the estimation to a limited range of speed and for only one quadrant operation. This dissertation develops a four-quadrant sensorless controller for SRM drives functioning over low and high speeds, including zero speed, with a high resolution position information over the entire speed range. This four-quadrant sensorless controller combines two different methods. At zero and low speeds, a pulse injection position estimation method is used to estimate the rotor position in all four quadrants. At high speeds, a sliding mode observer (SMO) based position and speed estimation is used and combined to work with the low speed algorithm. The transition between the algorithms is smooth and transparent to the inner loop torque controller. The accuracy and resolution of the sensorless controller developed in the research has been enhanced through the use of a novel switched reluctance (SR) machine model based on the Fourier series expansion that is accurate and invertible. This model, derived from machine geometry and materials properties, is accurate enough to be used for actual machine representation, and can also be simplified for real time sensorless controller application. The model predicts both the inductance and flux linkage accurately for use in various sensorless control algorithms. The experimental tests performed showed accurate position estimation using the new Fourier model. The error analysis performed in this research demonstrated the superiority of the Fourier model over the models developed in previous research. Advisors/Committee Members: Husain, Iqbal.

▼ When users browse a website, they usually try to accomplish a certain task, such as finding information, buying products, registering for classes, and attending classes on-line. The interaction between the users and the website can give the web engineers insight into the most common user tasks performed on the website. They can learn how most users navigate the website to finish their tasks and what changes can be made to the website structure in order to make the completion of the common tasks easier and faster. Most web servers provide web interaction logs to track the interaction between the users and the website. But such logs are usually designed for debugging purposes and not for the analysis of the website. So there is a need for a deeper conceptual method to analyze the interaction log to reveal information that can be used for enhancing the website structure. In this work, different data mining techniques, along with a rough set learning approach, are presented to enhance website usage. A new active-user-based user identification algorithm was applied to the interaction log to group together records that belong to the same user. The algorithm has a complexity running time of one order faster than other user identification algorithms. Sessions for identified users are found using an ontology-based session identification algorithm, which uses the website ontology in determining the sessions within website users. Different website sessions are then compared using a new Multidimensional Session Comparison Method (MSCM). MSCM takes into consideration other session dimensions, such as pages visited, time spent on the pages and the session length. MSCM compares sessions more precisely than other well known session comparison methods, such as the Sequence Alignment Method (SAM), Multidimensional Sequence Alignment Method (MDSAM), and Path Feature Space. Using the comparison results from the MSCM, sessions are clustered by hierarchal and equivalence classes clustering algorithms. The clustering results are used by the rough set learning method and the centroid method to generate rules that are used for both predicting and describing sessions’ clusters. Rules generated using a rough set learning approach predict and describe clusters better than rules generated using centroid method. Each session cluster is considered one task and the cluster centroid is the navigation path for completing the task. So common tasks along with their navigation path are evaluated, suggestions are then made for the website engineer to enhance the website structure to better serve website users. This work shows how data mining techniques along with rough set learning methods can be used to enhance the website structure for better-to-use websites. Advisors/Committee Members: Durkin, John.

▼ Hydrogen is considered to be the fuel of the next century. Hydrogen can be produced by either water splitting using the solar or nuclear energy or by catalytic cracking and reforming of the fossil fuels. The water splitting process using solar energy and photovoltaics is a clean way to produce hydrogen, but it suffers from very low efficiency. A promising scheme to produce H2 from natural gas involves following steps: (i) partial oxidation and reforming of natural gas to syngas, (ii) water-gas shift reaction to convert CO in the syngas to additional H2, (iii) separation of the H2 from CO2, and (iv) CO2 sequestration. The requirements for the above scheme are (i) a highly active coke resistant catalyst for generation of syngas by direct partial oxidation, (ii) a highly active sulfur tolerant catalyst for the water-gas shift reaction, and (iii) a low cost sorbent with high CO2 adsorption capacity for CO2 sequestration. This dissertation will address the mechanisms of partial oxidation, CO2 adsorption, and water-gas shift catalysis using in-situ IR spectroscopy coupled with mass spectrometry (MS). The results from these studies will lead to a better understanding of the reaction mechanism and design of both the catalyst and sorbent for production of hydrogen with zero emissions. Partial oxidation of methane is studied over Rh/Al2O3 catalyst to elucidate the reaction mechanism for synthesis gas formation. The product lead-lag relationship observed with in situ IR and MS results revealed that syngas is produced via a two-step iv reforming mechanism: the first step involving total oxidation of CH4 to CO2 and H2O and the second step involving the reforming of unconverted methane with CO2 and H2O to form syngas. Furthermore, the Rh on the catalyst surface remains predominantly in the partially oxidized state (Rhä+ and Rh0). For the water-gas shift reaction, addition of Re to the Ni/CeO2 catalyst enhanced the water gas shift activity by a factor of three. The activity of the Ni-Re/CeO2 catalyst was reduced by only 20% in the presence of sulfur compared to a 50% reduction with the Ni/CeO2 catalyst. These results show that Re not only promotes the water-gas shift reaction but also enhances the sulfur tolerance of the Ni/CeO2 catalyst. Novel amine based solid sorbents have been developed to capture CO2 reversibly using temperature-swing adsorption process. The IR study shows that CO2 adsorbs on amine grafted SBA-15 to form carbonates and bicarbonates. Comparison of monoamine and diamine-grafted SBA-15 showed that diamine grafted SBA-15 provides almost twice the active sites for CO2 adsorption. The adsorption of SO2 on the amine-grafted SBA-15 revealed that SO2 adsorbs irreversibly and the sorbent cannot be regenerated under normal operating conditions. Results of these studies can be used to enhance the overall conversion of CH4 to H2 thus lowering the cost of H2 product. Advisors/Committee Members: Chuang, Steven S. C.

▼ This research involved the development of a polarimetric imaging algorithm used for selective detection of back scattered photons transmitted through a medium. The novelty of the algorithm was the use of Stokes parameters and Mueller matrix methods to enhance turbid media imaging. This in turn may replace time-gated imaging, optical heterodyne, and second-harmonic generation techniques. The first part consisted of exploring polarimetric phenomenology of 1-D and 2-D signals in a variety of experimental scenarios. Corner cube reflectors and arrays, photodiode windows, and camera zoom lenses were used as targets. Their polarization signatures, mainly reflected signal, optical cross section, line spread function (LSF) and modulation transfer function (MTF), were studied by applying Mueller matrix formalism. The second part consisted of exploring the applicability of multi-fusional multispectral active imaging principles. This was aimed at enhancing image quality under different harsh imaging conditions using arithmetic manipulations of images at more than one wavelength. Aluminum strips, bar patterns, and computer chips were used as underwater targets and interrogated with more than one wavelength of laser. The images were analyzed using the Mueller matrix formalism. The resulting polarized images, and multi-wavelength subtraction images, were studied for contrast differences. As a result of the study, we noted a few improvements. There was a visible increase in signal level when applying the degree of linear polarization (DOLP) algorithm. Since significant results were established using polarimetric signals, the polarimetric algorithm was used to test the impact on the size of a target’s cross section. Using the range equation, we were able to improve the back scattered cross section of a corner cube and a camera zoom lens. DOLP was also able to improve the contrast of images taken with the lens focused, blurry, and out of focus. Similar results were observed for the LSF and MTF. For underwater imaging, polarimetric intensities improved even though scattered intensities decreased with increasing water cloudiness. However, the increase was not uniform. When comparing images of two separate wavelengths, it was noted that the contrast of the DOLP images was not significantly better than the non-polarimetric images. The image of the first Stokes parameter of each wavelength managed to improve contrast two fold or better. The subtraction of back scattered images also improved contrast. Advisors/Committee Members: Giakos, George C.

▼ The polymer industry has become one of the fastest growing areas in the materials industry for several decades, and would be continue to do so in the foreseeable future. However, due to growing environmental and health concern, the polymer manufacturers have faced increasing pressure to apply environmentally benign technologies in order to accommodate tightened environmental regulations. In the process of searching for clean and low emission polymerization techniques, supercritical fluid technology and sonochemistry have attracted more and more interest because of their unique advantages over conventional techniques. The present study is to expand our knowledge of polymer synthesis processes involving supercritical fluid, sonochemistry and microemulsion technologies. This study included three affiliated projects as supercritical dispersion polymerization, ultrasonically initiated polymerization in near-critical environment and ultrasound assisted microemulsion polymerization in aqueous solution. The success of projects will significantly broaden the application potential for these advanced chemical processes in both conventional and unconventional systems. In the study of dispersion polymerization in scCO2, a new PDMS macromonomer has been successfully applied as surfactant to stabilize the polymerization process. The polymerization results indicated that the conversion is increasing with the increasing of stabilizer concentration, and the particle morphology become more uniform at the same time. In the study of ultrasound irradiation in high-pressure medium, it has been confirmed that sonication alone could initiate the polymerization process. The monomer: CO2 ratio and ultrasound irradiation time appeared to have impact on the molecular weight and its distribution of the polymeric products. Discrete morphology from SEM image suggested that the polymer particles could be stabilized without surfactant during the polymerization process. In the study of ultrasound assisted microemulsion polymerization, ultrasound irradiation has been proved crucial to achieve a stable microemulsion polymerization. The kinetics study suggested that sonication could facilitate the polymerization dramatically. The ultrasound irradiation has also been proved to be an effective approach to control the particle size and its distribution of resultant polymer solution. Advisors/Committee Members: Cheung, Henry Michael.

▼ Smart materials are increasingly used in structural control of vibration and wave propagation. Most of existing studies have focused on the vibration control using smart materials in the form of patches or films, and ring-type has seldom been used. There is not much research on control of cylindrical shells using tunable materials. To meet this need, the present study develops theoretical models for adaptively–passive and active control of vibration and wave propagation in cylindrical shells using smart materials. One unique characteristic of shape memory alloy (SMA), i.e., the controllable elastic modulus with respect to temperature, is adopted in adaptively–passive control of vibration; with the capability of providing line circumferential distributed control forces due to their property of piezoelectricity in piezoelectric ceramic materials (e.g., PZT), the ring-type actuators are proposed to actively control the forced vibration response. The cylindrical shells both in vacuo and filled with fluid are investigated, and two different problems are considered: one is the wave propagation and transmission, and the other is the forced vibration response from external excitation. With the controllable elastic modulus of SMA, SMA wall joint has the capability of controlling the vibration source with wide-band frequencies or with a time-varying frequency. With the solution of the characteristics of the free wave propagation from the dispersive equation, the vibration response and characteristics of reflection/transmission from incident wave are investigated by using the wave approach and the method of residues. Numerical simulation indicates that the SMA wall joint has the potential to solve the problem of pass-band, and the transmission loss is more than 20dB for all frequency ranges providing a proper temperature. This SMA wall joint is also adopted to adaptively control the forced vibration response from external excitation. Parametric study demonstrates that the SMA joint has the capability of controlling the forced vibration of the shell excited by external excitation, and increasing damping ratio in the SMA joint does not mean to improve its vibration control performance. With the property of piezoelectricity, the piezoelectric ceramic material is able to function as both sensors and actuators. With the capability to offer more control authority by providing active control line force/moment, the ring-type PZT actuator (being modeled as a line circumferential distributed control force) is adopted to actively control the vibration in cylindrical shell. The transmission loss of this active control method is obtained by using the theory of residues. Simulation results demonstrate that it is possible to achieve a vibration reduction of 20 dB for the shells both in vacuo and filled with fluid by using only one control force. In summary, the present study illustrates the effectiveness and capabilities of smart materials (e.g., SMA and PZT) on control of vibration and wave propagation in cylindrical shells, and the proposed theoretical models provide better understanding of vibration and wave propagation behaviors of cylindrical shells with smart materials and can be used to guide design and analysis of smart cylindrical shell structures. Advisors/Committee Members: Qiao, Pizhong.

▼ In this dissertation, the system performance of multiple access schemes in UltraWideband (UWB) communications is evaluated in a multipath and multiuser fading environment. Three multiple access schemes, namely Time Hopping (TH), Direct Sequence (DS)and hybrid Direct Sequence-Time Hopping (DS-TH) are investigated. The TH multiple access has been well studied in Radar communication systems. This research extends the previous studies by applying three pulse modulations techniques, including pulse position modulation, pulse shift keying and pulse amplitude modulation. The idea of the DS multiple access schemes is also generalized from the well-known Direct Sequence-Code Division Multiple Access (DS-CDMA) cellular systems to UWB radios. It is shown that the DS multiple access has the potential to reach higher data transmission rates and that TH techniques are more resistant to fading. The DS-TH multiple access schemes are proposed by combining the advantages of both TH and DS multiple access schemes. Results show that the DS-TH ultra wideband achieves better system performance while maintaining the required data transmission rate and multiple access capacity. The system performance is illustrated and examined in terms of the signal to noise plus interference ratio, bit error rate and outage probability. Advisors/Committee Members: Ugweje, Okechukwu C.