Showing papers by "Indian Institute of Science published in 2001"

Improvements to Platt's SMO Algorithm for SVM Classifier Design

Abstract: This article points out an important source of inefficiency in Platt's sequential minimal optimization (SMO) algorithm that is caused by the use of a single threshold value. Using clues from the KKT conditions for the dual problem, two threshold parameters are employed to derive modifications of SMO. These modified algorithms perform significantly faster than the original SMO on all benchmark data sets tried.

Design of folded peptides.

Abstract: The construction of complex protein folds relies on the precise conversion of a linear polypeptide chain into a compact 3-dimensional structure. The interplay of forces that link sequence and folding is intricate and yet to be firmly elucidated. Examination of protein 3-dimensional structures suggests that complex tertiary folds and quaternary associations can be deconstructed into a limited number of secondary structural elements, such as strands, helices, and turns, which are assembled using loosely structured loops (Figure 1). The stability of a specific fold is determined by tertiary interactions between residues which are distant in sequence. De novo design of existing or novel protein folds demands a thorough understanding of the rules that underlie protein structure and stability.

Intraseasonal Oscillations and Interannual Variability of the Indian Summer Monsoon

Abstract: How and to what extent the intraseasonal oscillations (ISOs) influence the seasonal mean and its interannual variability of the Indian summer monsoon is investigated using 42-yr (1956–97) daily circulation data from National Centers for Environmental Prediction–National Center for Atmospheric Research 40-Year Reanalysis and satellite-derived outgoing longwave radiation data for the period of 1974–97 Based on zonal winds at 850 hPa over the Bay of Bengal, a criterion is devised to define “active” and “break” monsoon conditions The underlying spatial structure of a typical ISO cycle in circulation and convection that is invariant over the years is constructed using a composite technique A typical ISO has large-scale horizontal structure similar to the seasonal mean and intensifies (weakens) the mean flow during its active (break) phase A typical active (break) phase is also associated with enhanced (decreased) cyclonic low-level vorticity and convection and anomalous upward (downward) motion in

Formation cross-sections of singlet and triplet excitons in π-conjugated polymers

Abstract: Electroluminescence in organic light-emitting diodes arises from a charge-transfer reaction between the injected positive and negative charges by which they combine to form singlet excitons that subsequently decay radiatively. The quantum yield of this process (the number of photons generated per electron or hole injected) is often thought to have a statistical upper limit of 25 per cent. This is based on the assumption that the formation cross-section of singlet excitons, sigmaS, is approximately the same as that of any one of the three equivalent non-radiative triplet exciton states, sigmaT; that is, sigmaS/sigmaT approximately 1. However, recent experimental and theoretical work suggests that sigmaS/sigmaT may be greater than 1. Here we report direct measurements of sigmaS/sigmaT for a large number of pi-conjugated polymers and oligomers. We have found that there exists a strong systematic, but not monotonic, dependence of sigmaS/sigmaT on the optical gap of the organic materials. We present a detailed physical picture of the charge-transfer reaction for correlated pi-electrons, and quantify this process using exact valence bond calculations. The calculated sigmaS/sigmaT reproduces the experimentally observed trend. The calculations also show that the strong dependence of sigmaS/sigmaT on the optical gap is a signature of the discrete excitonic energy spectrum, in which higher energy excitonic levels participate in the charge recombination process.

Mixed spinel structure in nanocrystalline NiFe 2 O 4

Abstract: Nanocrystalline ${\mathrm{NiFe}}_{2}{\mathrm{O}}_{4}$ spinel has been synthesized with various grain sizes by high-energy ball milling. From the high-field magnetization studies and extended x-ray-absorption fine-structure, and M\"ossbauer measurements in an external magnetic field of 5 T applied parallel to the direction of gamma rays, we could observe that ${\mathrm{Ni}}^{2+}$ ions occupy tetrahedral sites on grain-size reduction due to milling. The ${\mathrm{Fe}}^{3+}$ spins have a canted structure and the canting angle increases with grain-size reduction. It is possible that the core ${\mathrm{Fe}}^{3+}$ spins are also canted because of the magnetocrystalline anisotropy introduced by the occupancy of the ${\mathrm{Ni}}^{2+}$ ions in the tetrahedral sites.

Triclosan offers protection against blood stages of malaria by inhibiting enoyl-ACP reductase of Plasmodium falciparum.

Abstract: The antimicrobial biocide triclosan [5-chloro-2-(2,4-dichlorophenoxy)phenol] potently inhibits the growth of Plasmodium falciparum in vitro and, in a mouse model, Plasmodium berghei in vivo. Inhibition of [14C]acetate and [14C]malonyl-CoA incorporation into fatty acids in vivo and in vitro, respectively, by triclosan implicate FabI as its target. Here we demonstrate that the enoyl-ACP reductase purified from P. falciparum is triclosan sensitive. Also, we present the evidence for the existence of FabI gene in P. falciparum. We establish the existence of the de novo fatty acid biosynthetic pathway in this parasite, and identify a key enzyme of this pathway for the development of new antimalarials.

Simple Synthesis of MoS2 and WS2 Nanotubes

Abstract: Trisulfide molybdenum complexes have been employed to produce nanotubes by simple heating in a stream of hydrogen. Bamboo-like stacking (see Figure) was observed, suggesting that the stimulus for the growth of MoS2 nanotubes is similar to that of their carbon equivalents. The wide experience gained with trisulfide complexes directs this technique towards the preparation of other layered sulfide nanotubes, although other precursors are also shown to be successful.

Direct power control of grid-connected wound rotor induction machine without rotor position sensors

Abstract: A method is presented for fast response control of the torque and flux of a grid connected wound rotor induction machine fed by back to back connected voltage source inverters on the rotor side. It is based on the measurement of active and reactive power on the grid side where voltages and currents are alternating at fixed frequency. The active and reactive powers are made to track references using hysteresis controllers. The method eliminates the need for rotor position sensing and gives excellent dynamic performance, as shown by simulation and experimental results from a variable speed constant frequency induction generator system. It is also capable of starting on the fly. It is thus an attractive sensorless control method for drive as well as generator applications.

An improved Hardy-Sobolev inequality and its application

Abstract: For\Omega \subset $IR^n$,n\geq 2, a bounded domain, and for 1 < p < n, we improve the Hardy-Sobolev inequality, by adding a term with a singular weight of the type \frac{1}{log(1/|x|)}$^2$ . We show that this weight function is optimal in the sense that the inequality fails for any other weight function more singular than this one. Moreover, we show that a series of finite terms can be added to improve the Hardy-Sobolev inequality, which answers a question of Brezis-Vazquez. Finally, we use this result to analyze the behaviour of the first eigenvalue of the operator L\mu\omega := -(div(| abla\upsilon|{p-2} abla\upilson)as \mu increases to \frac{n-p}{p}$^p$ for 1 < p < n.

Design of logical topologies: a linear formulation for wavelength-routed optical networks with no wavelength changers

Abstract: We consider the problem of constructing logical topologies over a wavelength-routed optical network with no wavelength changers. We present a general linear formulation which considers routing traffic demands, and routing and assigning wavelengths to lightpaths, as a combined optimization problem. The formulation also takes into account the maximum number of hops a lightpath is permitted to take, multiple logical links in the logical topology, multiple physical links in the physical topology, and symmetry/asymmetry restrictions in designing logical topologies. The objective is to minimize congestion. We show by examples how equality and inequality logical degree constraints have a bearing on congestion. We prove that, under certain conditions, having equality degree constraints with multiple edges allowed in the design of logical topologies does not affect congestion. This helps in reducing the dimensionality of the search space and hence speeds up the search for an optimal solution of the linear formulation. We solve the linear formulation for small examples and show the tradeoff between congestion, number of wavelengths available and the maximum number of hops a lightpath is allowed to take. For large networks, we solve the linear formulation by relaxing the integer constraints. We develop topology design algorithms for large networks based on rounding the solutions obtained by solving the relaxed problem. Since the whole problem is linearizable, the solution obtained by relaxation of the integer constraints yields a lower bound on congestion. This is useful in comparing the efficiency of our heuristic algorithms. Following Bienstock and Gunluk (1995), we introduce a cutting plane which helps in obtaining better lower bounds on congestion and also enables us to reduce the previously obtained upper bounds on congestion.

Studies on compressive failure features in syntactic foam material

Abstract: Syntactic foam made by mechanical mixing of glass hollow spheres in epoxy resin matrix is characterized for compressive properties in the present study. Volume fraction of hollow spheres in the syntactic foam under investigation is kept at 67.8%. Effect of specimen aspect ratio on failure behavior and stress-strain curve of the material is highlighted. Considerable differences are noted in the macroscopic fracture features of the specimen and the stress-strain curve with the variation in specimen aspect ratio, although compressive yield strength values were within a narrow range. Post compression test scanning electron microscopic observations coupled with the macroscopic observations taken during the test helped in explaining the deviation in specimen behavior and in gathering support for the proposed arguments.

A model for the emergence of cooperation, interdependence, and structure in evolving networks

Abstract: Evolution produces complex and structured networks of interacting components in chemical, biological, and social systems. We describe a simple mathematical model for the evolution of an idealized chemical system to study how a network of cooperative molecular species arises and evolves to become more complex and structured. The network is modeled by a directed weighted graph whose positive and negative links represent “catalytic” and “inhibitory” interactions among the molecular species, and which evolves as the least populated species (typically those that go extinct) are replaced by new ones. A small autocatalytic set, appearing by chance, provides the seed for the spontaneous growth of connectivity and cooperation in the graph. A highly structured chemical organization arises inevitably as the autocatalytic set enlarges and percolates through the network in a short analytically determined timescale. This self organization does not require the presence of self-replicating species. The network also exhibits catastrophes over long timescales triggered by the chance elimination of “keystone” species, followed by recoveries.

Analysis and performance evaluation of a distribution STATCOM for compensating voltage fluctuations

Abstract: Controller design of a STATCOM-based voltage compensator requires a valid analytical model of the system. If phasor algebra is used for modeling, it is difficult to accurately describe the STATCOM behavior during compensation of subcycle transients in the PCC voltage. In this paper, a small-signal model of the system, with a distribution line, is derived. Predictions based on frequency-domain analysis are made, which are validated by experimental results. This model, therefore, can be used for controller design where subcycle voltage transients are to be compensated. It is shown that the voltage controller, so designed, can accomplish voltage sag mitigation. A discussion on the design of DC bus voltage controller-and experimental results showing its performance-is also included.

A simple position-sensorless algorithm for rotor-side field-oriented control of wound-rotor induction machine

Abstract: A simple position-sensorless method for the rotor-side field-oriented control of a wound-rotor induction machine is described in this paper. The algorithm is based on axis transformations. Compared to the previously proposed methods, it is more direct and the dependence on machine parameters is also largely reduced. The algorithm can be started on the fly without the knowledge of the initial rotor position. Operation at synchronous speed, corresponding to zero rotor frequency, is stable, thus making it suitable for variable speed constant frequency operations. Simulation and experimental results show excellent performance of the scheme.

Biomass derived producer gas as a reciprocating engine fuel—an experimental analysis

Abstract: This paper uncovers some of the misconceptions associated with the usage of producer gas, a lower calorific gas as a reciprocating engine fuel This paper particularly addresses the use of producer gas in reciprocating engines at high compression ratio (17 : 1), which hitherto had been restricted to lower compression ratio (up to 12 : 1) This restriction in compression ratio has been mainly attributed to the auto-ignition tendency of the fuel, which appears to be simply a matter of presumption rather than fact The current work clearly indicates the breakdown of this compression ratio barrier and it is shown that the engine runs smoothly at compression ratio of 17 : 1 without any tendency of auto-ignition Experiments have been conducted on multi-cylinder spark ignition engine modified from a production diesel engine at varying compression ratios from 115 : 1 to 17 : 1 by retaining the combustion chamber design As expected, working at a higher compression ratio turned out to be more efficient and also yielded higher brake power A maximum brake power of 175 kWe was obtained at an overall efficiency of 21% at the highest compression ratio The maximum de-rating of power in gas mode was 16% as compared to the normal diesel mode of operation at comparable compression ratio, whereas, the overall efficiency declined by 325% A careful analysis of energy balance revealed excess energy loss to the coolant due to the existing combustion chamber design Addressing the combustion chamber design for producer gas fuel should form a part of future work in improving the overall efficiency

Coherent Intraseasonal Oscillations of Ocean and Atmosphere during the Asian Summer Monsoon

Abstract: The space-time evolution of the ocean and atmosphere associated with 1998-2000 monsoon intraseasonal oscillations (ISO) in the Indian Ocean and west Pacific is studied using validated sea surface temperature (SST) and surface wind speed from the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager, and satellite outgoing longwave radiation. Monsoon ISO consist of alternating episodes of active and suppressed atmospheric convection moving northward in the eastern Indian Ocean and the South China Sea. Negative/positive SST anomalies generated by fluctuations of net heat flux at the ocean surface move northward following regions of active/suppressed convection. Such coherent evolution of SST, surface heat flux and convection suggests that air-sea interaction might be important in monsoon ISO.

Oscillations of Bay of Bengal sea surface temperature during the 1998 Summer Monsoon

Abstract: New measurements from moored buoys in the Bay of Bengal, along with satellite cloud data, reveal strong monsoon intraseasonal oscillations (ISO) during the summer of 1998. The active phase of the monsoon is marked by high surface wind and deep atmospheric convection. The buoy data show that sea surface temperature (SST) in the Bay of Bengal warm pool rises and falls with periods of weeks. These intraseasonal oscillations of SST are not adequately captured in a satellite derived weekly SST analysis. They are a direct response to ISO of net surface heat flux into the ocean, which is negative in the active phase of the monsoon and positive in the quiescent phase. Fresh water from rivers and rain appears to control northern Bay of Bengal SST in late summer by allowing sunlight to escape below a shallow mixed layer.

CMOS op-amp sizing using a geometric programming formulation

Abstract: The problem of CMOS op-amp circuit sizing is addressed here. Given a circuit and its performance specifications, the goal is to automatically determine the device sizes in order to meet the given performance specifications while minimizing a cost function, such as a weighted sum of the active area and power dissipation. The approach is based on the observation that the first order behavior of a MOS transistor in the saturation region is such that the cost and the constraint functions for this optimization problem can be modeled as posynomial in the design variables. The problem is then solved efficiently as a convex optimization problem. Second order effects are then handled by formulating the problem as one of solving a sequence of convex programs. Numerical experiments show that the solutions to the sequence of convex programs converge to the same design point for widely varying initial guesses. This strongly suggests that the approach is capable of determining the globally optimal solution to the problem. Accuracy of performance prediction in the sizing program (implemented in MATLAB) is maintained by using a newly proposed MOS transistor model and verified against detailed SPICE simulation.

A compact manufacturable 76-77-GHz radar module for commercial ACC applications

Abstract: The design and measured results of a single-substrate transceiver module suitable for 76-77-GHz pulsed-Doppler radar applications are presented. Emphasis on ease of manufacture and cost reduction of commercial millimeter-wave systems is employed throughout as a design parameter. The importance of using predictive modeling techniques in understanding the robustness of the circuit design is stressed. Manufacturing techniques that conform to standard high-volume assembly constraints have been used. The packaged transceiver module, including three waveguide ports and intermediate-frequency output, measures 20 mm/spl times/22 mm/spl times/8 mm. The circuit is implemented using discrete GaAs/AlGaAs pseudomorphic high electron mobility transistors (pHEMTs), GaAs Schottky diodes, and varactor diodes, as well as GaAs p-i-n and pHEMT monolithic microwave integrated circuits mounted on a low-cost 127-/spl mu/m-thick glass substrate. A novel microstrip-to-waveguide transition is described to transform the planar microstrip signal into the waveguide launch. The module is integrated with a quasi-optical antenna. The measured performance of both the component parts and the complete radar transceiver module is described.

Space vector PWM control of dual inverter fed open-end winding induction motor drive

Abstract: A space vector PWM technique is developed based on the combination of space vectors from dual inverters feeding the induction motor from both ends (open-end winding without neutral point). A total of 64 voltage space vector combinations are available for PWM voltage control of the inverter fed machine with open-end winding. A space phasor based PWM scheme is proposed with minimum number of switching in a cycle per inverter coupled with equal number of switching for each inverter.

Electrical transport characteristics of Au/n-GaAs Schottky diodes on n-Ge at low temperatures

Abstract: The currenti?½voltage characteristics of Au/n-GaAs Schottky diodes grown by metal-organic vapor-phase epitaxy on Ge substrates were determined in the temperature range 80i?½300 K. The zero-bias barrier height for current transport decreases and the ideality factor increases at low temperatures. The ideality factor was found to show the T0 effect and a higher characteristic energy. The excellent matching between the homogeneous barrier height and the effective barrier height was observed and infer good quality of the GaAs film. No generationi?½recombination current due to deep levels arising during the GaAs/Ge heteroepitaxy was observed in this study. The value of the Richardson constant was found to be 7.04 A K?2 cm?2, which is close to the value used for the determination of the zero-bias barrier height.