Showing papers by "Indian Institute of Science published in 1998"

Optical Networks: A Practical Perspective

Abstract: This fully updated and expanded second edition of Optical Networks: A Practical Perspective succeeds the first as the authoritative source for information on optical networking technologies and techniques. Written by two of the field's most respected individuals, it covers componentry and transmission in detail but also emphasizes the practical networking issues that affect organizations as they evaluate, deploy, or develop optical solutions.

A unified framework for hybrid control: model and optimal control theory

Abstract: We propose a very general framework that systematizes the notion of a hybrid system, combining differential equations and automata, governed by a hybrid controller that issues continuous-variable commands and makes logical decisions. We first identify the phenomena that arise in real-world hybrid systems. Then, we introduce a mathematical model of hybrid systems as interacting collections of dynamical systems, evolving on continuous-variable state spaces and subject to continuous controls and discrete transitions. The model captures the identified phenomena, subsumes previous models, yet retains enough structure to pose and solve meaningful control problems. We develop a theory for synthesizing hybrid controllers for hybrid plants in all optimal control framework. In particular, we demonstrate the existence of optimal (relaxed) and near-optimal (precise) controls and derive "generalized quasi-variational inequalities" that the associated value function satisfies. We summarize algorithms for solving these inequalities based on a generalized Bellman equation, impulse control, and linear programming.

Obstacle avoidance in a dynamic environment: a collision cone approach

Abstract: A novel collision cone approach is proposed as an aid to collision detection and avoidance between irregularly shaped moving objects with unknown trajectories. It is shown that the collision cone can be effectively used to determine whether collision between a robot and an obstacle (both moving in a dynamic environment) is imminent. No restrictions are placed on the shapes of either the robot or the obstacle, i.e., they can both be of any arbitrary shape. The collision cone concept is developed in a phased manner starting from existing analytical results that enable prediction of collision between two moving point objects. These results are extended to predict collision between a point and a circular object, between a point and an irregularly shaped object, between two circular objects, and finally between two irregularly shaped objects. Using the collision cone approach, several strategies that the robot can follow in order to avoid collision, are presented. A discussion on how the shapes of the robot and obstacles can be approximated in order to reduce computational burden is also presented. A number of examples are given to illustrate both collision prediction and avoidance strategies of the robot.

Processing maps for hot working of titanium alloys

Abstract: In recent years, processing maps are being used to design hot working schedules for making near-net shapes in a wide variety of materials. In this paper, the results obtained on the characterization of hot working behavior of titanium and its alloys using the approach of processing maps are described. In commercial purity α titanium, dynamic recrystallization (DRX) domain occurs at 775°C and 0.001 s−1 with an efficiency of power dissipation [2m/(m+1) where m is the strain rate sensitivity of flow stress] of 43%. The DRX domain shifts to higher strain rates when the interstitial impurity content is lowered. In the near-α and α-β alloys like IMI 685, Ti–6Al–4V, the preform microstructure has a significant influence on the processing maps. For example, in the transformed β (Widmanstatten) preform microstructures, these alloys exhibit a domain of spheroidization at lower temperature and a domain of β superplasticity at higher temperatures, both occurring at slow strain rates. These domains merge at the β transus because of the occurrence of damage processes which lower the tensile ductility. On the other hand, processing maps on alloys with equiaxed preform microstructure exhibit a clear superplasticity domain in the α-β range and the β phase undergoes DRX with a power dissipation efficiency of ≈45–55%. Titanium materials in general, exhibit wide flow instability regimes due to adiabatic shear bands formation at higher strain rates and hence careful process design has to be adopted for successful forging and microstructural control.

Nonlocal Dynamical Correlations of Strongly Interacting Electron Systems

Abstract: We introduce an extension of the dynamical mean-field approximation (DMFA) that retains the causal properties and generality of the DMFA, but allows for systematic inclusion of nonlocal corrections. Our technique maps the problem to a self-consistently embedded cluster. The DMFA (exact result) is recovered as the cluster size goes to 1 (infinity). As a demonstration, we study the Falicov-Kimball model using a variety of cluster sizes. We show that the sum rules are preserved, the spectra are positive definite, and the nonlocal correlations suppress the charge-density wave transition temperature.

Analysis of Subsynchronous Resonance in Power Systems

Abstract: 1. Introduction.- 1.1 General.- 1.2 Definitions of SSR.- 1.3 Interactions with power system controllers.- 1.4 FACTS Controllers.- 1.5 Methods of Analysis of SSR.- 1.6 Chapter outline.- 2. Modelling of Turbine Generator.- 2.1 Introduction.- 2.2 Synchronous machine model.- 2.3 Park's transformation.- 2.4 Per unit quantities.- 2.5 Operational impedances and equivalent circuits.- 2.6 Modelling of excitation control system.- 2.7 Modelling of turbine generator mechanical system.- 2.8 Modelling of turbine and governor.- 2.9 Modelling and analysis of the mechanical and prime mover system.- 2.10 Synchronous generator modelling for transient simulation.- 3. Modelling of the Electric Network.- 3.1 Introduction.- 3.2 Transmission lines.- 3.3 Transformation using ? - ? variables.- 3.4 State equations.- 3.5 Interface between the network and generator.- 3.6 Impedance functions.- 3.7 Simulation of electromagnetic transients.- 4. Analysis of SSR with Fixed Series Compensation.- 4.1 Introduction.- 4.2 Analysis of induction generator effect: frequency scanning method.- 4.3 Analysis of torsional interaction(TI).- 4.4 State equations and eigenvalue analysis.- 4.5 An algorithm for computing torsional modes.- 4.6 Countermeasures for SSR.- 4.7 Torsional oscillations in parallel connected turbine generators.- 5. Interactions with Power System Stabilizer.- 5.1 Introduction.- 5.2 Basic concept in the application of PSS.- 5.3 Design of PSS.- 5.4 Torsional interaction with PSS.- 5.5 A case study.- 6. Interactions with HVDC Converter Control.- 6.1 Introduction.- 6.2 HVDC converters and control.- 6.3 Modelling of HVDC system for study of torsional interactions.- 6.4 Analysis of torsional interactions - A simplified approach.- 6.5 A case study.- 6.6 A simplified damping torque analysis.- 6.7 Control of torsional interaction.- 7. Interactions with Shunt Compensators.- 7.1 Introduction.- 7.2 Static Var Compensator.- 7.3 Torsional Interactions with SVC.- 7.4 Static Condenser(STATCON).- 7.5 Torsional interactions with STATCON.- 7.6 A simplified analysis of torsional interaction with voltage controller.- 8. Interactions with Series Compensators.- 8.1 Introduction.- 8.2 Thyristor Controlled Series Compensator.- 8.3 Modelling of TCSC for SSR studies.- 8.4 Mitigation of SSR with TCSC.- 8.5 Static Synchronous Series Compensator (SSSC).- 8.6 Torsional interactions with SSSC.- Appendices.- A- Data on IEEE Benchmark Models.- A.1 IEEE First Benchmark Model ( FBM ).- A.2 IEEE Second Benchmark Model ( SBM ).- B- Calculation of Initial Conditions.- C- Abbreviations.- References and Bibliography.

Monsoon precipitation in the AMIP runs

Abstract: We present an analysis of the seasonal precipitation associated with the African, Indian and the Australian-Indonesian monsoon and the interannual variation of the Indian monsoon simulated by 30 atmospheric general circulation models undertaken as a special diagnostic subproject of the Atmospheric Model Intercomparison Project (AMIP) The seasonal migration of the major rainbelt observed over the African region, is reasonably well simulated by almost all the models The Asia West Pacific region is more complex because of the presence of warm oceans equatorward of heated continents Whereas some models simulate the observed seasonal migration of the primary rainbelt, in several others this rainbelt remains over the equatorial oceans in all seasons Thus, the models fall into two distinct classes on the basis of the seasonal variation of the major rainbelt over the Asia West Pacific sector, the first (class I) are models with a realistic simulation of the seasonal migration and the major rainbelt over the continent in the boreal summer; and the second (class II) are models with a smaller amplitude of seasonal migration than observed The mean rainfall pattern over the Indian region for July-August (the peak monsoon months) is even more complex because, in addition to the primary rainbelt over the Indian monsoon zone (the monsoon rainbelt) and the secondary one over the equatorial Indian ocean, another zone with significant rainfall occurs over the foothills of Himalayas just north of the monsoon zone Eleven models simulate the monsoon rainbelt reasonably realistically Of these, in the simulations of five belonging to class I, the monsoon rainbelt over India in the summer is a manifestation of the seasonal migration of the planetary scale system However in those belonging to class II it is associated with a more localised system In several models, the oceanic rainbelt dominates the continental one On the whole, the skill in simulation of excess/deficit summer monsoon rainfall over the Indian region is found to be much larger for models of class I than II, particularly for the ENSO associated seasons Thus, the classification based on seasonal mean patterns is found to be useful for interpreting the simulation of interannual variation The mean rainfall pattern of models of class I is closer to the observed and has a higher pattern correlation coefficient than that of class II This supports Sperber and Palmer’s (1996) result of the association of better simulation of interannual variability with better simulation of the mean rainfall pattern The hypothesis, that the skill of simulation of the interannual variation of the all-India monsoon rainfall in association with ENSO depends upon the skill of simulation of the seasonal variation over the Asia West Pacific sector, is supported by a case in which we have two versions of the model where NCEP1 is in class II and NCEP2 is in class I The simulation of the interannual variation of the local response over the central Pacific as well as the all-India monsoon rainfall are good for NCEP2 and poor for NCEP1 Our results suggest that when the model climatology is reasonably close to observations, to achieve a realistic simulation of the interannual variation of all-India monsoon rainfall associated with ENSO, the focus should be on improvement of the simulation of the seasonal variation over the Asia West Pacific sector rather than further improvement of the simulation of the mean rainfall pattern over the Indian region

A Newton-Euler formulation for the inverse dynamics of the Stewart platform manipulator

Abstract: This paper presents an inverse dynamic formulation by the Newton-Euler approach for the Stewart platform manipulator of the most general architecture and models all the dynamic and gravity effects as well as the viscous friction at the joints. It is shown that a proper elimination procedure results in a remarkably economical and fast algorithm for the solution of actuator forces, which makes the method quite suitable for on-line control purposes. In addition, the parallelism inherent in the manipulator and in the modelling makes the algorithm quite efficient in a parallel computing environment, where it can be made as fast as the corresponding formulation for the 6-dof serial manipulator. The formulation has been implemented in a program and has been used for a few trajectories planned for a test manipulator. Results of simulation presented in the paper reveal the nature of the variation of actuator forces in the Stewart platform and justify the dynamic modelling for control.

The Physics of Fluids and Plasmas: An Introduction for Astrophysicists

Abstract: A good working knowledge of fluid mechanics and plasma physics is essential for the modern astrophysicist This graduate textbook provides a clear, pedagogical introduction to these core subjects Assuming an undergraduate background in physics, this book develops fluid mechanics and plasma physics from first principlesThis book is unique because it presents neutral fluids and plasmas in a unified scheme, clearly indicating both their similarities and their differences Also, both the macroscopic (continuum) and microscopic (particle) theories are developed, establishing the connections between them Throughout, key examples from astrophysics are used, though no previous knowledge of astronomy is assumed Exercises are included at the end of chapters to test the reader's understanding This textbook is aimed primarily at astrophysics graduate students It will also be of interest to advanced students in physics and applied mathematics seeking a unified view of fluid mechanics and plasma physics, encompassing both the microscopic and macroscopic theories

Autocatalytic sets and the growth of complexity in an evolutionary model

Abstract: A model of s interacting species is considered with two types of dynamical variables. The fast variables are the populations of the species and slow variables the links of a directed graph that defines the catalytic interactions among them. The graph evolves via mutations of the least fit species. Starting from a sparse random graph, we find that an autocatalytic set inevitably appears and triggers a cascade of exponentially increasing connectivity until it spans the whole graph. The connectivity subsequently saturates in a statistical steady state. The time scales for the appearance of an autocatalytic set in the graph and its growth have a power law dependence on s and the catalytic probability. At the end of the growth period the network is highly nonrandom, being localized on an exponentially small region of graph space for large s.

Estimating regularity in epileptic seizure time-series data

Abstract: The authors apply Ziv-Lempel (LZ) complexity and approximate entropy (ApEn) as measures to quantify the regularity in the various epochs of epileptic seizure time series data. They demonstrate the potential of complexity measures such as LZ and ApEn in quantifying the regularity at different epochs of epileptic seizure time-series data. It is clearly shown that these measures have high values at the beginning and the end of the seizure, and that they decrease during mid-seizure. In fact, the authors observe in the histogram plot that the frequency of the complexity measure in mid-seizure is quite prominent. This gives one an idea about the epoch where one can find more regular patterns. These measures can also be used as relative indices (comparing across state), rather than absolute indices, by using a larger number of subjects to obtain statistical validity in comparing across conditions. The analysis of time series obtained from complex systems, such as the brain, by the above measures provides an alternative easy way to quantify the regularity with finite-length segments (of the order of 1000 samples). The same can be inferred by calculating the correlation dimension and Lyapunov exponent, but the algorithms used to estimate these invariants are susceptible to error due to the finite sample size and are also highly sensitive to noise. The computational complexity of these algorithms is also high. The authors have also applied these measures across the various states of epilepsy.

Effect of buffering on the phosphate-solubilizing ability of microorganisms

Abstract: Native microflora present in the alkaline vertisols and two phosphate-solubilizing bacteria (PSB) isolated from soil using conventional screening media could not release phosphorus from alkaline Indian vertisol soils supplemented with carbon and nitrogen sources. The two PSBs could solubilize both rock phosphate and di-calcium phosphate in unbuffered media but failed to solubilize rock phosphate in buffered media. The organic acids secreted by these PSBs were 20-50 times less than that required to solubilize phosphorus from alkaline soil.

Minireviews: Exploring the Basic Ecological Unit: Ecosystem-like Concepts in Traditional Societies

Abstract: Ancient conceptualizations of ecosystems exist in several Amerindian, Asia-Pacific, European, and African cultures. The rediscovery by scientists of ecosystem-like concepts among traditional peoples has been important in the appreciation of traditional ecological knowledge among ecologists, anthropologists, and interdisciplinary scholars. Two key characteristics of these systems are that (a) the unit of nature is often defined in terms of a geographical boundary, such as a watershed, and (b) abiotic components, plants, animals, and humans within this unit are considered to be interlinked. Many traditional ecological knowledge systems are compatible with the emerging view of ecosystems as unpredictable and uncontrollable, and of ecosystem processes as nonlinear, multiequilibrium, and full of surprises. Traditional knowledge may complement scientific knowledge by providing practical experience in living within ecosystems and responding to ecosystem change. However, the “language” of traditional ecology is different from the scientific and usually includes metaphorical imagery and spiritual expression, signifying differences in context, motive, and conceptual underpinnings.

Electronic structure of electron doped SrTiO 3 : SrTiO 3-δ and Sr 1-x La x TiO 3

Abstract: Electronic structures of electron-doped ${\mathrm{SrTiO}}_{3\ensuremath{-}\ensuremath{\delta}}$ and ${\mathrm{Sr}}_{1\ensuremath{-}x}{\mathrm{La}}_{x}{\mathrm{TiO}}_{3}$ have been investigated within the ab initio band-structure approach using a supercell containing eight basic units. A small amount of electron doping is found to drive ${\mathrm{SrTiO}}_{3}$ metallic, with the Fermi level moving into the conduction band in both the systems. Clustering of oxygen vacancies in the case of ${\mathrm{SrTiO}}_{3\ensuremath{-}\ensuremath{\delta}}$ gives rise to distinct non-rigid-band-like evolution of the electronic structure, trapping doped charge carriers in midgap states; similar effects are not observed for ${\mathrm{Sr}}_{1\ensuremath{-}x}{\mathrm{La}}_{x}{\mathrm{TiO}}_{3}$, explaining the difference in the Hall measurements of these two closely related compounds.

Involvement of p85 in p53-dependent apoptotic response to oxidative stress

Abstract: Reactive oxygen species have damaging effects on cellular components and so trigger defensive responses by the cell and even programmed cell death, although the mechanisms by which mammalian cells transmit signals in response to oxidative damage are unknown. We report here that the protein p85, a regulator of the signalling protein phosphatidyl-3-OH kinase (PI(3)K), participates in the cell death process that is induced in response to oxidative stress and that this role of p85 in apoptosis does not involve PI(3)K. We show that disruption of p85 by homologous recombination impairs the cellular apoptotic response to oxidative stress. Because the protein p53 is required for cell death induced by oxidative damage, we examined the relation between p85 and p53. Using a chimaeric p53 fusion protein with the oestrogen receptor (p53ER) to supply p53 (p53 is induced upon binding of p53ER to oestradiol) in a p53-deficient cell line, we found that p85 is upregulated by p53 and that its involvement in p53-mediated apoptosis is independent of PI(3)K. We propose that p85 acts as a signal transducer in the cellular response to oxidative stress, mediating cell death regulated by p53.

Control design and simulation of unified power flow controller

Abstract: The unified power flow controller (UPFC) is a solid-state controller which can be used to control active and reactive power flows in a power transmission line. In this paper, the authors propose a control strategy for UPFC in which they control real power flow through the line, while regulating magnitudes of the voltages at its two ports. They design a controller for this purpose which uses only local measurements. The control strategy is evaluated using digital simulation for a case study.

A cyclic voltammetric study of the kinetics and mechanism of electrodeposition of manganese dioxide

Abstract: The kinetics and mechanism of electrooxidation of Mn2+ ions to MnO2 (EMD) has been studied in electrolytes comprising MnSO4 and H2SO4 by cyclic voltammetry at 80°C. The voltammogram of a Pt electrode cycled between 0.6 and 1.6V vs SCE exhibits an anodic current peak at about 1.3V vs SCE resulting in the deposition of MnO2 on the electrode, while a cathodic peak appears at 0.8V vs SCE. It is shown that the pair of peaks do not correspond to a single reversible reaction but represent two separate irreversible electrode processes. The cyclic voltammetric peak current for the deposition of EMD is found to be proportional to the square root of Mn2+ ion concentration in the electrolyte and independent of acid concentration. Based on these results, a mechanism for the formation of EMD involving diffusion of Mn2+ ions to the electrode surface, oxidation of Mn2+surface to Mn3+ads, and H2O to OHads as the primary oxidation steps is invoked. Mn3+ads ions dissociate disproportionately into Mn2+ads and Mn4+ads ions at the electrode surface. The Mn2+ads and Mn4+ads ions, respectively, react with OHads and H2O resulting in the formation of EMD.

Dissecting alpha-helices: position-specific analysis of alpha-helices in globular proteins.

Abstract: An analysis of the amino acid distributions at 15 positions, viz., N", N', Ncap, N1, N2, N3, N4, Mid, C4, C3, C2, C1, Ccap, C', and C" in 1,131 alpha-helices reveals that each position has its own unique characteristics. In general, natural helix sequences optimize by identifying the residues to be avoided at a given position and minimizing the occurrence of these avoided residues rather than by maximizing the preferred residues at various positions. Ncap is most selective in its choice of residues, with six amino acids (S, D, T, N, G, and P) being preferred at this position and another 11 (V, I, F, A, K, L, Y, R, E, M, and Q) being strongly avoided. Ser, Asp, and Thr are all more preferred at Ncap position than Asn, whose role at helix N-terminus has been highlighted by earlier analyses. Furthermore, Asn is also found to be almost equally preferred at helix C-terminus and a novel structural motif is identified, involving a hydrogen bond formed by N delta 2 of Asn at Ccap or C1 position, with the backbone carbonyl oxygen four residues inside the helix. His also forms a similar motif at the C-terminus. Pro is the most avoided residue in the main body (N4 to C4 positions) and at C-terminus, including Ccap of an alpha-helix. In 1,131 alpha-helices, no helix contains Pro at C3 or C2 positions. However, Pro is highly favoured at N1 and C'. The doublet X-Pro, with Pro at C' position and extended backbone conformation for the X residue at Ccap, appears to be a common structural motif for termination of alpha-helices, in addition to the Schellman motif. Main body of the helix shows a high preference for aliphatic residues Ala, Leu, Val, and Ile, while these are avoided at helix termini. A propensity scale for amino acids to occur in the middle of helices has been obtained. Comparison of this scale with several previously reported scales shows that this scale correlates best with the experimentally determined values.