Indian Institute of Science Bangalore
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Structural and functional insights into hybrid AT-less megaenzyme synthase (NRPS) and DNA-MsDps2 complexes using single particle cryo-EM
No full textMicroorganisms, mainly bacteria and fungi, are the producers of structurally diverse, complex organic compounds called as secondary metabolites. These metabolites include polyketides (PKs), non-ribosomal peptides (NRPs), and hybrid PKs/NRPs. The final products from these three classes display different characteristics like antibiotics, antiparasitic agents, antifungals, anticancer drugs, and immunosuppressants. As these products have wide range of potential application in pharmaceuticals, a number of biochemical studies have been carried out to elucidate of their biosynthetic pathways. The biosynthesis of these products is catalysed by large, multi-modular proteins including the NRPSs (non-ribosomal peptide synthases), the PKSs (polyketide synthases) and hybrid NRPS/PKS. Knowledge of the quaternary structure of PKS, NRPS and hybrid NRPS/PKS assembly line enzymes have been topic of interest since it helps not only in elucidating the crosstalk between different domains but also useful in modification of products. It has been demonstrated in the literature that FAS and PKS are homodimeric enzyme complexes, whereas the majority of NRPS are monomeric in nature. Whether NRPS is monomeric or homodimeric in the case of hybrid NRPS/PKS of a modular enzymatic manufacturing line has been questioned. To gain structural insights into the hybrid multidomain NRPS, we focused module-2 (Cy1-Cy2-A-PCP-Ox) of hybrid NRPS/PKS of leinamycin biosynthetic pathway. We performed cryo-EM of module 2 (Cy1-Cy2-A-PCP-Ox) which resulted in a low resolution cryo-EM map of this NRPS perhaps as a result of the linkers present in between the domains. However, to elucidate crucial interdomain interfaces and interactions that occur during different steps of the NRPS catalytic cycle, we undertook truncation studies including the domains (Cy1-Cy2) and (A-PCP-Ox) of the module2 of NRPS. We determined the structure of multidomain constructs (PCP-Cy1-Cy2) and (A-PCP-Ox) at overall resolutions of 5.2 Å and 7 Å respectively. The unravelling of architecture, organization, and mechanism of NRPS module 2 of leinamycin biosynthesis by cryo-EM will help design bioengineering approaches to understand the mechanistic insight into this novel pathway (swapping modules and domains).
DNA-binding protein under starvation (Dps), is a miniature ferritin complex, which plays a vital role in protecting bacterial DNA during starvation for maintaining the integrity of bacteria from hostile conditions. Mycobacterium smegmatis is one such bacteria that express MsDps2, which binds DNA to protect it under oxidative and nutritional stress conditions. Several approaches, including cryo-electron tomography (Cryo-ET), were implemented to identify the structure of the Dps protein that is bound to DNA. However, none of the structures of the Dps-DNA complex was resolved to high resolution to be able to identify the DNA binding residues. In this study, we implemented various biochemical and biophysical studies to characterize the DNA protein interactions of Dps protein. We employed single-particle cryo-EM-based structural analysis of MsDps2-DNA and identify that the region close to N-terminal confers the DNA binding property. Based on cryo-EM data, we performed mutations of several arginine residues proximal to the DNA binding region, which dramatically reduced the MsDps2-DNA interaction. In addition, we propose a model for DNA compaction during lattice formation. We performed single-molecule imaging experiments of MsDps2-DNA interactions that corroborate well with our structural studies. Single molecule imaging also deciphers the mechanism of compaction required for DNA protection
Development and Performance Evaluation of the Flapping Wing with In-situ Piezoceramic Actuator
No full textUnmanned Aerial Vehicles (UAV) are essentially automatic flight vehicles having dimensions, wingspan and airspeed, smaller than the conventional aerial vehicles. UAVs are employed widely in applications such as surveillance over a short distance, acquisition of a local target, detection of hazardous chemicals / biological agent, exploration of a harmful environment, search operations, etc. UAV can be classified into three main types depending on their method of propulsion and lift. These are fixed wing, rotary wing, and flapping wing. Flapping wing UAVs are more suitable for insect scale flights.
Flapping mechanism requires actuators with large stroke periodic (reciprocal) motion at high speed (10-100s of Hz) with large output forces for overcoming the aerodynamic damping. There are several actuation mechanisms applicable to flapping-wing UAVs. The emphasis is on linear actuators, which simplify the mechanical transmission for flapping motions. Most of the prototypes developed so far have employed motor-driven mechanisms to achieve the flapping wing. Unconventional methods such as piezoelectric, thermal, electromagnetic, shape memory, electrostatic, etc. for actuation of flapping wings have also been used. Among the unconventional actuation methods, piezoelectric actuation is the most used mechanism because of its compact size and high-power density.
However, the deflection generated by the piezoelectric actuator is intrinsically very small. Therefore, it is necessary to employ numerous types of motion amplification mechanisms to achieve large deflection. The development of an amplification mechanism is a complex procedure. Hence, several efforts have been made to evaluate different forms of piezoelectric actuators for flapping vehicle application.
The in-situ piezoelectric actuator, employing piezoceramic coating directly on the structure by a simple method, looks promising for flapping wing application as it generates large displacement compared to the displacement produced by the bulk piezoelectric actuators. Piezoceramic coating is light in weight compared to the bulk piezoelectric actuator, as it is a composite material of highly dense piezoceramic material and less dense polymeric material. The current study evaluates the basic characteristics, performance and the aerodynamic behavior of the in-situ piezoceramic actuator for flapping wing applications.
Initially, the basic characteristics of the in-situ piezoceramic actuator were evaluated. Its properties like density, elastic modulus and the transverse piezoelectric coefficient, d31, were evaluated. The density was measured to be 2300 kg/m3. Elastic modulus was measured as 3 GPa. The piezoelectric coefficient, d31 was measured by applying the coating on various substrates such as stainless steel, brass and polymer. Also, the in-situ piezoceramic actuator was applied with different thicknesses 30, 40 and 50 µm on the polymer substrate. The mean value of the measured d31 was found to be -26 pm/V.
The performance of the piezoceramic coating (coated on a flexible substrate) was evaluated for flapping wing application using performance metrics from aerodynamic studies. The performance metrics are the tip velocity,, the dynamic electromechanical coupling factor (EMCF), k12, and the elastic energy per unit mass, . These metrics were obtained and verified using an analytical model. Based on these metrics, the performance of the developed flexible actuator was compared with conventional flexible piezoelectric polyvinylidene difluoride (PVDF) actuator. These metrics were found to be better for the developed actuators than PVDF.
The best suitable wing configuration was selected using FE modelling based on the tip velocity ( ). It was found that the tip velocity depends on the thickness, length and shape of the flapping wing. Wing shapes of the dragonfly’s, tobacco hawkmoth’s and cicada’s forewings were considered. The thickness and length of the in-situ piezoceramic actuator were also varied. The results obtained through FE modelling were verified experimentally. Dragonfly wing was found to give maximum tip velocity ( ). The maximum value for υ of 114.7 mm/s was obtained for a dragonfly wing having in-situ piezoceramic actuator of 30 mm length from the root of the wing and a thickness of 30 µm. The lift force was measured using a load cell measurement set up in the clamped condition for the dragonfly wing.
Insects use a variety of wingbeat kinematics to produce and control aerodynamic forces for their flight. For mimicking an insect flight, the selected actuator needs to be capable of producing insect flight kinematics. Therefore, twisting along with flapping motions of the wings by the in-situ piezoceramic actuators were attempted. Twisting motion of the wing was achieved by actuating two piezoceramic cantilevers type in-situ actuators applied over a wing with sinusoidal signals out of phase with each other by 180°. The fore wing and the hind wing were actuated by sinusoidal signals with a phase difference of 0°, 90°, 180° and 270°. Tip displacements of 4 mm for the fore wing and 3 mm for hind wing were measured. The kinematics of the flapping wing, which has been achieved by other actuators with complex mechanisms can thus be achieved by the simple in-situ piezoceramic actuators. The experiments on the measurement of the lift and kinematics of the flapping wing establishes that the in-situ piezoceramic actuator is a suitable candidate for flapping wing application.
The improvement studies on tip velocity were carried out by implementing in-situ bimorph piezoceramic actuators. Dragonfly wings with the piezoceramic layer of thickness 30 µm were considered. As there was a implication of increase in the mass of the wing, selection of length of piezoceramic layers were carried out using FE modelling. The results obtained through FE modelling were verified experimentally. The maximum tip velocity of 245.1 mm/s is obtained for 25 mm length of both piezoceramic coating layers.
Modified strip theory based on the blade elemental analysis has been used to study the aerodynamic performance of the three types of wing planforms. Lift, thrust and drag forces generated by the three wing forms have been calculated analytically by the model. As flapping wings are operated at the first mode resonance, the first mode resonant frequency, and the tip displacement at resonance from the experimental results were given as input to the aerodynamic model. The effect of variations in aerodynamic parameters such as incident angle, pitching angle and forward speed on the relevant forces were studied for the in-situ piezoceramic actuator actuated wings of three different wing planforms. For all the three wing planforms, it was observed that the lift increased with incident angle and forward speed and it was less affected by increasing the pitching angle. For the increasing forward speed, incident angle and pitching angle, thrust and drag also increased. The modelling results show that the wings produce positive mean lift and condition where the thrust is more than the drag for all the wing planforms.
The overall results show that the in-situ piezoceramic actuator can be employed for flapping wing applications with further efforts
Coding Schemes for Secure and Reliable DNA-Based Data Storage
No full textIn DNA-based data storage, the desired information is encoded into the quaternary
sequence of synthetic DNA molecules, called oligos. We look at secure communication
via information-containing oligos in the usual three-party setting, where Alice and Bob
are legitimate communicators and Eve is an eavesdropper. Our scheme for secure DNAbased
communication is two-fold: First, we store secret data in synthetic DNA molecules
in a properly designed oligo pool of suitably high dilution. The oligo pool comprises
information-containing oligos mixed with background genomic DNA cleaved into strands
of the same length as the useful oligos. Designing oligos for storing the secret data
involves the design of a library of primer pairs and a code book specific to this set
of primers, that satisfy constraints on homopolymer run length, GC content, primerprimer
dissimilarity, and primer-sequence dissimilarity. The differential knowledge of
the designed primer pairs allows Bob to retrieve most of the information-containing
DNA molecules after carrying out sufficient rounds of PCR amplification on the diluted
oligo pool. In contrast, Eve is not able to access the stored information owing to her
lack of any prior knowledge of the primer pairs.
In order to improve upon the security of this scheme, we next develop an index-based
secrecy coding scheme for the resulting wiretap system, where Bob observes a substantially
lower number of erasures in the main channel, as compared to Eve’s channel, which
suffers a large number of erasures (loss of DNA molecules). We show that under the
conditions of a noise-free setting, proper library preparation, and proper measurement of
oligos represented in smaller fractions, this coding scheme achieves the secrecy capacity
of a DNA storage wiretap channel model with strong secrecy.
An error-correcting code for reliable storage of DNA-based data must adhere to the
constraints of GC content and homopolymer run length. In this thesis, we also explore
the constraint of GC balance in the context of insertion and deletion error-correcting
codes for DNA storage systems. We derive an upper bound on the cardinality of single
indel-correcting GC-balanced quaternary codes. This allows us to deduce the minimal
number of redundancy bits required for GC-balancing in such codes. We also develop
a GC-balanced coding scheme for the correction of a single burst of 2 insertion or 2
deletion errors
Fast and Compact Voltage Equalizer for Satellite Applications
No full textLithium-ion batteries have now become an essential constituent of the Electrical Power System
of solar-powered satellites due to their high energy density, wider operating temperature range
, and better radiation tolerance. For the compact realization and better space utilization, the series-parallel connected Li-ion batteries are operated with currents close to the design limit of the cells. This consequently speeds up the increase in the inherent initial imbalance in the individual cell voltages in a series connected stack, demanding fast equalization. Active multicell-to-multicell equalization achieves fast equalization by efficient charge transfer among multiple cells in the series connected stack. PS-MAHB equalizer is a multicell-to-multicell equalizer, with its open-loop control maintaining high equalization current throughout the equalization. Its soft-switched operation and modularization abilities make it an attractive choice for space applications. However, it lacks the necessary protective features and redundancy essential for its use in space applications. Hence, Modified PS-MAHB (MPS-MAHB) equalizer is developed by incorporating necessary protection features and redundancy in the PS-MAHB equalizer. The Failure Mode Impact Analysis of the MPS-MAHB equalizer reveals that during the most likely switch short circuit failure mode, the faulty part of the equalizer is disconnected by the protective device and the circuit redundancy does not let the cell get out of the equalization.
The existing static phase shift-based control of the equalizer causes direct dependency of the
equalization currents on the cell voltages and limits the equalization current levels to lower than
the design equalization current value when the cell voltages are lower. Thus, the control works
with a reduced rate of equalization and causes the under-utilization of the equalizer hardware for
a significant duration of time in the charge-discharge cycle. A dynamic phase shift-based control is proposed to maximize the equalization current through the cells irrespective of the cell voltages to further increase the rate of equalization, and improve the equalizer hardware utilization. In the
simulation, a significant improvement in the equalization rate compared to the static phase shift
control is verified with the proposed dynamic phase shift based-control.
The compact hardware realization of the equalizer hardware and the voltage sensor board addresses the space-volume constraints in satellite applications. The compact realization of 4-cell equalizer modules is achieved by pushing the switching frequency to 1MHz thereby reducing the values of the passive components. The challenges faced during the PCB design of the 4-cell equalizer module are discussed. A non-isolated high-precision op-amp-based voltage sensing scheme is developed to target the equalization band close to 10mV. The concept of an easy-to-design motherboard-based interface is implemented, which does not require any changes in the design of the 4-cell equalizer module and the voltage sensor board, irrespective of the cell connector geometry.
The experimental results verify the operation of the equalizer showing the convergence of cell
voltages from the initial imbalance of 300mV to the band of 10mV. The impact of the non-ideal dynamic response of the Li-ion cell voltage on the voltage-sensing-based control algorithm is discussed along with the necessary modifications brought in the control.ISRO-IISc Space Technology Cell, ISTC/EEE/VJ/43
Dissecting the function of NuMA in cleavage furrow formation and chromatin decondensation at the mitotic exit in animal cells
No full textIn animal cells, the duplicated genetic material is aligned on a microtubule-based structure known as the mitotic spindle during mitosis. At the mitotic exit, the mitotic spindle elongates, and the sister chromatids get separated. The separation of sister chromatids is followed by the cleavage furrow formation and its ingression, which eventually partition the cytoplasmic constituents and genetic material into newly formed daughter cells. How the chromosome separation is coordinated with cleavage furrow formation is incompletely understood. Also, when animal cells enter mitosis, the chromatin gets highly condensed, and the transcription is chiefly paused. However, when cells exit mitosis, the chromatin should get decondensed in a tightly regulated manner to ensure proper landscaping of chromosome territories, which makes it competent enough for DNA-based processes like replication and transcription. The accurate functioning of these processes is critical for the development and for stem cell divisions. In this study, we have linked the function of an evolutionarily conserved protein, nuclear mitotic apparatus (NuMA), in cleavage furrow formation and chromatin decondensation at the mitotic exit. In the first part of my thesis, we have tried to characterize the function of chromatin-localized NuMA in regulating chromatin decondensation. In the second part, we have attempted to provide insight into how spatial localization of NuMA at the plasma membrane coordinates chromosome separation with cleavage furrow formation.
1). NuMA regulates chromatin decondensation at the mitotic exit and nuclear shape in interphase cells
NuMA is a highly abundant (~10^6 copies) protein of interphase nuclei. Few studies hint that nuclear NuMA may have a role in chromatin organization, and it is hypothesized to be a part of the nuclear structural framework. In this regard, the loss of NuMA's function based on antibody-based microinjections was associated with nuclear shape defects. However, since the depletion of NuMA is linked with multiple mitotic abnormalities, it remained unclear whether the nuclear shape defects seen upon NuMA depletion is an indirect effect due to impairment of NuMA's mitotic function or a direct outcome of the absence of NuMA in the nucleus. Further, whether NuMA is bound to chromatin in the nucleus was also unknown. Even if NuMA is bound to chromatin, what mechanisms ensure its release upon mitotic entry was unknown. In this work, by utilizing fluorescence recovery after photobleaching (FRAP) and biochemical analysis, we report that NuMA is transiently bound to chromatin in the nucleus. We show that NuMA, which is bound to DNA, is released in late prophase upon nuclear envelope breakdown (NEBD) by the action of Cdk1-CyclinB kinase. Importantly, we identify evolutionarily conserved sequences rich in basic amino acids, arginine, and lysine, at the C-terminus of NuMA that aid in its direct interaction with DNA. In the absence of such interaction, NuMA becomes significantly mobile in the nucleus. Notably, the expression of the DNA-binding deficient mutant of NuMA delays chromatin decondensation at the mitotic exit. Furthermore, we discovered that DNA binding deficient NuMA polymerizes into high-order structures such as fibrillar networks, which perturbs nuclear shape. The DNA-binding property of NuMA prevents the formation of these higher-order structures and thus helps in maintaining the proper nuclear architecture. Overall, this study links the chromatin binding ability of NuMA with the proper chromatin decondensation at mitotic exit and maintenance of nuclear shape in interphase, independent of its mitotic role.
2). Polarized membrane distribution of NuMA/dynein and Ect2/Cyk4/Mklp1 regulate cleavage furrow formation
Animal cells partition their genetic material and cellular constituents through cytokinesis. The initiation of cytokinesis is regulated by the activation of small GTPase RhoA that helps in myosin II activation and actin polymerization at the equatorial membrane, resulting in cleavage furrow formation. RhoA is spatiotemporally regulated by a heterotetrameric complex known as centralspindlin consisting of a dimer of kinesin-6 member Mklp1 and a dimer of RhoGAP Cyk4. The centralspindlin complex localizes at the spindle midzone and promotes the localization of its downstream effectors RhoGEF Ect2 which directly activates RhoA and regulates cytokinesis. However, how a precise RhoA zone at the equatorial membrane is established and maintained remained unclear.
In anaphase, the mitotic protein NuMA is enriched at the polar membrane via its direct interaction with membrane phospholipids, PtIns(4)P and PtIns(4,5)P2 and is vital for proper spindle elongation by cortically anchoring the dynein/dynactin complex. However, despite the presence of PtIns(4)P and PtIns(4,5)P2 throughout the membrane, the NuMA/dynein complexes are restricted to the polar membrane and are excluded from the equatorial membrane, which is mutually exclusively occupied by RhoA. The mechanism of equatorial membrane exclusion of NuMA/dynein complex and its biological relevance remained unknown. In this work, we uncovered that Ect2, Cyk4, and Mklp1 are critical in restricting NuMA/dynein to the polar cortical region. In the absence of Ect2, Cyk4, or Mklp1, NuMA/dynein complex occupies the equatorial cortex, which impacts proper spindle elongation. Further, we show that Ect2 is in complex with Cyk4 and Mklp1 in anaphase cells. We establish that the membrane localization, but not the spindle midzone localization of the Ect2/Cyk4/Mklp1 complex, is critical for NuMA/dynein exclusion and, thus, for proper spindle elongation. Conversely, we show that polar membrane localization of the NuMA/dynein complex confines RhoA to a narrow zone at the equatorial membrane, which ensures cleavage furrow formation and cytokinesis. Overall our work provides insight into the mechanism that restricts NuMA/dynein and Ect2/Cyk4/Mklp1 to mutually exclusive membrane surfaces, which ensures proper chromatin segregation and cleavage furrow formation in animal cells. This coordination is critical for an error-free cell division program
Investigations on Voltage Control of Stacked DC-link Series Capacitors with a Nine-Level Inverter for an Induction Motor Load
No full textMultilevel Inverters offer several advantages over two-level inverters in
applications involving medium voltage and high power levels. A variety of applications
are now available for MLI technology, ranging from variable speed drives to high
voltage DC (HVDC) applications, power factor correction, and renewable energy sources.
The advantages of MLI include the use of devices with low voltage ratings, low switching
losses, minimal electromagnetic interference, and low dV/dt stress for the solid state
devices. Further, as the number of MLI increases, it o ers a nearly sinusoidal stepped
phase voltage waveform with reduced harmonic content in the phase voltage. The most
commonly used multilevel inverter topologies in literature are the neutral point clamp
(NPC) MLI, flying capacitor (FC) based MLI and cascaded H-bridge (CHB) MLI. Extending
the conventional three-level NPC to higher levels will increase the complexity
of the NP voltage control requirement along with the requirement for a large number of
power diodes. An increase in the number of levels using an FC topology not only increases
the requirement for high voltage electrolytic capacitors, but also increases the complexity
of balancing the capacitors' voltage during PWM switching cycles. CHB-based multilevel
inverters have the disadvantage of requiring a greater number of DC power supplies.
Another type of MLI is a hybrid-MLI, which is constructed by cascading NPC, FC, and
CHB cells. A hybrid MLI serves one of a few purposes, which include a) increasing the
level of inverter with a low switch count, b) reducing the voltage rating for the devices,
and c) implementing di erent control balancing algorithms. In this thesis, Chapter 1
provides an overview of conventional two level inverters and their operation, basic MLIs
and their operation, hybrid MLIs, and the implementation of PWM for MLIs.
When an MLI is supplied from a single DC-link, the four-quadrant operation becomes
much more convenient for motor drive loads. The problem lies in the use of high
voltage devices by a low level MLI. Multiple capacitors stacked in series across a single
DC-link and the resulting stacked MLIs are excellent options for enhancing level of the
inverter and reducing the device voltage requirements. However, the main challenge remains
the same as that of the NPC based inverter - the balancing of DC-link neutral
points. Generally, NP voltage balancing technique fall into three categories: (a) the use
of isolated DC-supplies for DC-link capacitors, (a) the use of external balancing circuits,
and (b) cleverly operated/manipulated switching during PWM based on DC-link capacitor
voltage conditions. Using isolated DC supplies is a conventional old method that
makes a bulky inverter system. DC-link capacitor voltage balancing using external balancing
circuits and manipulated switching during PWM focuses primarily on the average
voltage balancing over a fundamental cycle for DC-link capacitors. In many topologies,
neutral point voltage balancing is addressed by drawing zero average current from each
NP within each 60◦ sector of a fundamental cycle. Therefore, these topologies need large
DC-link capacitance to control the voltage ripple compared to the proposed topology.
In this thesis, the DC-link capacitor voltage balancing is addressed by drawing zero instantaneous
current from the NPs during the PWM operation. The first chapter also
presents a mathematical model of the inverter with 'n' DC-link stacked series capacitors
for instantaneous voltage balancing. Practically, three NPs of four DC-link stacked series
capacitors are balanced using six-phase and three phase IM loads.
On excitation of motor phase terminals with opposite pole voltage, two opposite
phases of a symmetrical six-phase IM generate 180◦ opposite phase currents. These
opposite phases are forced to be connected to a single NP by means of low voltage CHBs.
A generalised MLI for instantaneous NP voltage balancing using a six-phase load is
presented in detail in Chapter 2. This chapter concludes with detailed results on a ninelevel
inverter prototype designed for instantaneously balancing four DC-link capacitor
voltage. The DC-link capacitor voltage balancing algorithm is also tested for steadystate
and transient conditions using the six-phase IM load and the nine-level hybrid
inverter.
The third chapter examines the same concepts as chapter two, but with a three-phase
IM load. The discussion of instantaneous voltage balancing for a general MLI structure
with 'n' DC-link series connected capacitor is continued for a three phase load. In contrast
with chapter 2, in this case all three phases are connected to a single NP or DC-link bus
terminal during the PWM operation by means of low voltage CHBs. It ensures that
iA+iB +iC = 0 for any DC-link NP, and that the DC-link capacitors are instantaneously
balanced. A nine-level hybrid inverter prototype is used to demonstrate the selection of
pole-voltage redundancy, space vector redundancy for NP voltage balancing, and nominal
voltage level control for CHB capacitors. The detailed experimental results for the steady
and transient condition are also presented at the end of this chapter.
There is no discussion in these two chapters above regarding the DC-link capacitor
voltage deviation or disturbance control during steady state or transient PWM operation.
If there is a disturbance in the DC-link NP, the PWM operation will be continued with
that disturbed NP. In the next work of Chapter four, the same nine-level inverter prototype
is used to control DC-link capacitor voltage deviations using the phase currents
of a three-phase load. A control algorithm is provided that regulates the DC-link NP
voltage deviation, instantaneous voltage balancing for already balanced NPs, and thereafter
maintaining the nominal voltage of the CHB capacitors. The results of intentional
charging-discharging and control of DC-link NPs are presented at the end of Chapter 4,
for different loads that cover both low and high power factors load conditions.
The last and final chapter examines the limitations of nine-level inverters, discussed
in chapter 3 for instantaneous NP voltage balancing. An extended linear modulation
range or linear over-modulation is attempted for a three phase IM load. It has been seen
that the linear modulation range (LMR) can be increased to full base speed even for loads
with high power factors. The same topology of instantaneous NP voltage balancing is
also tested under load unbalanced conditions.
The works discussed in Chapters 2 to 6 have been first coded and simulated in
MATLAB. Every control algorithm from each chapter has been tested using a resistive inductive
load (RL load). The load is designed to draw 10 kW of power under all power
factor conditions and frequencies. The hardware experiments share a common controller
platform. A DSP (TMS320F28335) and an FPGA (Xilinx Spartan 3 XC3S400) are used
to implement the control actions in these chapters. The ADC block in DSP sense the
CHB capacitors' voltages and phase currents. The number of samples per fundamental
cycle is chosen such that a minimum 1 kHz and maximum 2 kHz switching frequency
of the inverter is ensured at all modulation indexes. The laboratory prototype is built
using o -the-shelf devices. SKM75GB123D IGBTs for the stacked inverter of each phase,
and IRF260N MOSFET switches for the CHB inverter cells, 4.7 mF, 250 V for CHB
capacitors, and 3.3 mF, 200 V for DC-link capacitors are used to build the prototype.
The advantages of this voltage control technique for NPs are: 1) a single dc link
at the source makes the topology suitable for four-quadrant medium-voltage motor drive
applications; 2) low-voltage series-connected dc-link capacitors can be replaced by battery
cells for EV applications; 3) inherent voltage control of the batteries is possible using the
switching state redundancies and the load phase currents, and no external circuit is
required for voltage balancing of these battery cells. Moreover, the proposed balancing
technique is general in nature and more dc-link series capacitors can added to extend the
level for MLI, which can be realized using low-voltage switching devices
Computational Modeling of two Dimensional Heterostructures for Optoelectronic and Catalytic Applications
No full textTwo-dimensional van der Waals (2D-vdW) materials have attracted significant attention for their unique and excellent properties. The properties of the 2D-vdW materials can be precisely engineered using various techniques for the desired applications. We carried out a study of 2D-vdW materials and their heterostructures for optoelectronic and catalytic applications using state of the art ab-initio modeling such as density functional theory (DFT), many-body perturbation theory (MBPT), and density functional perturbation theory (DFPT). We report the generation of linearly polarized, anisotropic, intra and interlayer excitonic bound states in GeSe/SnS vdW heterostructure using GW and Bethe-Salpeter equation simulations (BSE), addressing the current demand of optical polarizers. A dramatic variation in excitonic binding energy and optical band gap is observed upon applying biaxial strain, which is attributed to the change in effective dielectric constant and band dispersion. Building upon the concept of optical and excitonic properties, we discuss the Z-scheme mechanism in C3N3/C3N4 vdW heterostructure for water splitting catalysts. The spontaneous redox reactions for the water splitting combined with band alignment, presence of higher-order interlayer excitons, fast electron-hole recombination, and high charge mobility facilitate the Z-scheme mechanism compared to the type II mechanism. For optoelectronic applications, the stacking order plays a crucial role in 2D materials. Rhenium disulfide (ReS2) is one of the most potential candidates for optoelectronic properties; however, extremely weak interlayer coupling strength makes it challenging to determine the stacking order in multilayer ReS2. We successfully identify two distinct stacking orders (AA & AB) by the potential energy profile and the vibrational Raman modes. We extend this study to determine the stacking-order-driven optical and excitonic properties. By symmetry analysis, we also explore the origin of extra Raman modes and splitting of Raman modes in multilayer ReS2, which is another debatable topic. The extra modes and the splitting in Raman spectra are attributed to the layer parity-dependent breaking of inversion symmetry. Due to the weak coupling strength between the layers, multilayer ReS2 is designed with a proper doping strategy for the layer-independent deep center defects. The thermodynamic study confirms that S_Re is the best possible deep isolated defect for a single photon emitter. This study highlights the importance of heterostructuring, stacking-order, and strain engineering to study the extraordinary properties of 2D materials and also paves the path to overcome critical challenges in optoelectronic research applications
Bayesian implicit filters for the analysis of numerically stiff structural dynamic state space models
No full textThis thesis reports on combined experimental and computational investigations conducted on problems of state and combined state and parameter estimation applied to vibrating engineering structures. The standard dynamic state space modeling framework is adopted for this purpose, and the analysis is carried out using the Kalman filter (and its variants), particle filters, and Markov chain Monte Carlo (MCMC) samplers. A review of the relevant literature has revealed that these tools have not been applied to situations where the system under study and its numerical representation via the discretized process equation displays numerical stiff behaviour. This numerical stiffness is characterized by the presence of response components with widely separated decay rates and (or) frequencies of oscillations. A computationally efficient treatment of such systems calls for the application of implicit discretization schemes to deduce the discrete process equations from the governing semi-discretized equations of motion resulting from the application of the finite element method. The implicit nature of the process equation, however, poses several challenges in the analysis of the resulting dynamic state space model since most existing methods for this purpose assume explicit process equation models. The present thesis investigates the modifications needed to some of the existing Bayesian filters, such as the Kalman filter, extended Kalman filter, unscented Kalman filter, bootstrap filter, and sequential importance sampling particle filters so that the methods can be employed to allow for implicit process equation models. Some of these tools are then combined with MCMC samplers to tackle problems of combined state and parameter estimation problems. The thesis covers linear and nonlinear dynamical systems and allows for the identification of not only the dynamical system parameters but also the parameters associated with models for the process and measurement noises. The tools developed are applied to a suite of laboratory experimental models, which include shear building frame models containing an inerter element and piecewise geometrical nonlinear features and one-storey and five-storey asymmetric, bending-torsion coupled building frames. These frames are tested on a multi-axes earthquake simulator. Also studied are typical nonlinear dynamical systems such as a limit cycle oscillator, a multi-degree of freedom degrading inelastic frame model, and an elastically mounted pendulum undergoing large amplitude oscillations. The thesis is organized into an introductory chapter, a chapter that provides a review of literature, four contributing chapters, and a chapter that summarizes contributions made and makes a few suggestions for future research. The contributing chapters are sequenced as follows:
(a) Chapter 3 considers problems of state estimation in stiff linear state space model and discusses the application of an implicit Kalman filtering strategy,
(b) Chapter 4 presents the details of the modifications made to the extended Kalman filter, unscented Kalman filter, bootstrap filter, and sequential importance sampling filter and discusses the application of resulting algorithms to tackle problems of state estimation in a set of nonlinear dynamical systems,
(c) Chapter 5 considers the problem of combined state and parameter estimation in linear stiff systems by combining implicit Kalman filter with the general adaptive metropolis algorithm, an MCMC sampler, and
(d) Chapter 6 presents the formulations for the combined state and parameter estimation in nonlinear stiff systems using implicit unscented Kalman filter along with general adaptive Metropolis algorithm. The thesis has around 230 references covering a time window of 1964-2023
Nanomaterials for Magnetism and Catalysis
No full textNanomaterials possess unique physical and chemical properties allowing for advancement and innovation in diverse scientific and technological fields. The remarkable magnetic and catalytic properties of these nanomaterials have prompted research into the design and development of tailored nanostructures. Considerable research efforts have been dedicated to the improve the fundamental understanding of the nanomaterial properties. The synthesis of nanoparticles of metals like Mn, Fe, Co, Zn, etc. in zero oxidation state is quite challenging due to their oxidative instability. Solvated metal atom dispersion method stands out to be an excellent synthetic methodology to realize nanoparticles of reactive metals.1 Additionally, efficient catalysts with enhanced reactivity and selectivity are realized in bimetallic nanostructures combining a noble metal with transition metals like Zn.2 Co-digestive ripening approach offers a solution-based route, allowing for the controlled synthesis of desired bimetallic nanostructures through precise control of synthesis parameters.3 This work emphasizes the crucial role of size, composition, and surface interactions in customizing the magnetic and catalytic properties of nanomaterials.
Mn(0) colloids have been prepared by the SMAD method using hexadecylamine (HDA), toluene and THF as capping agents. Monodispersed nanoparticles of Mn-HDA were obtained through digestive ripening. The magnetic properties of the resulting Mn nanoparticles are investigated, revealing intriguing exchange bias behavior arising from surface oxidation and uncompensated antiferromagnetic spin interactions.4
Furthermore, we employed the co-digestive ripening strategy in conjunction with the SMAD method to synthesize Pd@ZnO core-shell nanocomposites and Pd-Zn alloy nanoparticles. The catalytic performance of the Pd@ZnO nanocatalyst is evaluated for the selective hydrogenation of terminal alkynes, showcasing a sequential hydrogenation process yielding internal alkenes and further hydrogenation to the corresponding alkanes.5 HDA-capped Pd-Zn alloy nanoparticles having a random alloy structure exhibit excellent catalytic activity in the semi-hydrogenation of phenylacetylene, showcasing high selectivity towards styrene. Additionally, acetylene semi-hydrogenation using Pd-Zn alloy has been investigated, demonstrating high selectivity towards ethylene at room temperature.6 The detailed investigation for high selectivity towards ethylene has been performed and the results is presented in the thesis
Small-amplitude Oscillations in Hypersonic Double-cone Flow
No full textUnsteady compressible flows typically pose problems with rich dynamics. The broad concern of this thesis is the shock wave unsteadiness that arises in external high-speed flow over a double-cone body. This unsteadiness is driven by complex interaction between the shock wave and the region of high shear and separation in the external flow. It is well known that the canonical double-cone problem exhibits two different classes of unsteadiness, labeled “pulsations” and “oscillations.” The former is characterized by unsteady shock wave motion over large spatial scales, whereas in the latter the nature of unsteadiness is distinct and occurs at a relatively smaller scale. The detailed mechanisms that sustain pulsations and oscillations are yet to be completely understood. In the present work, experiments were performed in the Roddam Narasimha Hypersonic Wind Tunnel (RNHWT) at Mach 6 to carefully investigate the phenomena of oscillations. Time-resolved schlieren and wall pressure data were obtained for various double-cone models with the second cone angle fixed at 90◦, while the first cone angle and ratio of the slant lengths were varied as parameters. Schlieren data revealed two dissimilar types, or modes, of flow oscillations. Spectral proper orthogonal decomposition (SPOD) analysis performed on experimental data showed the existence of a dominant time scale for the oscillations, and also provided the associated low-rank dynamics. The two different oscillation modes are found to exhibit distinct Strouhal number scaling. Given the direct dependence of shock strength on the flow Mach number (M ), the boundaries of unsteady flow states are expected to show slight changes with M. However, qualitative flow features and the underlying mechanisms that drive unsteadiness are expected to remain the same. Overall, this work reveals new flow behavior and furthers our understanding of the double-cone flow