Showing papers by "Indian Institute of Science published in 1982"

Algebraic design techniques for reliable stabilization

Abstract: In this paper we study two problems in feedback stabilization. The first is the simultaneous stabilization problem, which can be stated as follows. Given plants G_{0}, G_{1},..., G_{l} , does there exist a single compensator C that stabilizes all of them? The second is that of stabilization by a stable compensator, or more generally, a "least unstable" compensator. Given a plant G , we would like to know whether or not there exists a stable compensator C that stabilizes G ; if not, what is the smallest number of right half-place poles (counted according to their McMillan degree) that any stabilizing compensator must have? We show that the two problems are equivalent in the following sense. The problem of simultaneously stabilizing l + 1 plants can be reduced to the problem of simultaneously stabilizing l plants using a stable compensator, which in turn can be stated as the following purely algebraic problem. Given 2l matrices A_{1}, ..., A_{l}, B_{1}, ..., B_{l} , where A_{i}, B_{i} are right-coprime for all i , does there exist a matrix M such that A_{i} + MB_{i} , is unimodular for all i? Conversely, the problem of simultaneously stabilizing l plants using a stable compensator can be formulated as one of simultaneously stabilizing l + 1 plants. The problem of determining whether or not there exists an M such that A + BM is unimodular, given a right-coprime pair ( A, B ), turns out to be a special case of a question concerning a matrix division algorithm in a proper Euclidean domain. We give an answer to this question, and we believe this result might be of some independent interest. We show that, given two n \times m plants G_{0} and G_{1} we can generically stabilize them simultaneously provided either n or m is greater than one. In contrast, simultaneous stabilizability, of two single-input-single-output plants, g 0 and g 1 , is not generic.

Analysis of Paschen Curves for air, N2 and SF6 Using the Townsend Breakdown Equation

Abstract: It has been shown that it is possible to extend the validity of the Townsend breakdown criterion for evaluating the breakdown voltages in the complete pd range in which Paschen curves are available. Evaluation of the breakdown voltages for air (pd=0.0133 to 1400 kPa · cm), N2(pd=0.0313 to 1400 kPa · cm) and SF6 (pd=0.3000 to 1200 kPa · cm) has been done and in most cases the computed values are accurate to ±3% of the measured values. The computations show that it is also possible to estimate the secondary ionization coefficient ? in the pd ranges mentioned above.

Grain boundary strengthening in strongly textured magnesium produced by hot rolling

Abstract: The grain size dependence of the yield stress in hot rolled 99.87 pct magnesium sheets and rods was measured in the temperature range 77 K to 420 K. Hot rolling produced strong basal textures and, for a given grain size, the hot rolled material has a higher strength than extruded material. The yield strength-grain size relation in the above temperature range follows the Hall-Petch equation, and the temperature dependencies of the Hall-Petch constants σ0 and k are in support of the theory of Armstrong for hcp metals that the intercept σ0 is related to the critical resolved shear stress (CRSS) for basal slip (easy slip) and the slope k is related to the CRSS for prismatic slip (difficult slip) occurring near the grain boundaries. In the hot rolled magnesium, σ0 is larger and k is smaller than in extruded material, observations which are shown to result from strong unfavorable basal and favorable {1010} textures, respectively. Texture affects the Hall-Petch constants through its effect on the orientation factors relating them to the CRSS for the individual slip systems controlling them.

Photodimerization of coumarin in aqueous and micellar media

Abstract: The photochemical and photophysical processes of many organic compounds are a function of the environment in which they are present. In this connection we have chosen to investigate the environmental perturbations on the photodimerization of coumarin,l and the results of our study in aqueous and micellar media are presented in this paper. Coumarin has historically been the subject of intense photochemical and spectroscopic interest, mainly as a consequence of its importance in biological systems. Coumarin has been chosen for our investigation as its fascinating photochemical behavior has been fairly well explored,2 and therefore the environmental influence, which is the subject of our concern, would be easily understandable.

Nondiffusive quantum transport in a dynamically disordered medium

Abstract: For a dynamically disordered continuum it is found that the exact quantum mechanical mean square displacement $〈{x}^{2}(t)〉\ensuremath{\sim}{t}^{3}$, for $t\ensuremath{\rightarrow}\ensuremath{\infty}$. A Gaussian white-noise spectrum is assumed for the random potential. The result differs qualitatively from the diffusive behavior well known for the one-band lattice Hamiltonian, and is understandable in terms of the momentum cutoff inherent in the lattice, simulating a "momentum bath."

Structure of the deoxytetranucleotide d-pApTpApT and a sequence-dependent model for poly(dA-dT)

Abstract: Synopsis The x-ray structure of the hydrated ammonium salt of the deoxytetranucleotide dpApTpApT was determined by Patterson and direct methods at a resolution of 1 A. The crystal structure contains right-handed double-helical segments formed by complementary Watson-Crick-type hydrogen bonding between the adenine and thymine bases of neighboring molecules. The minihelix contains two base pairs. The chains are antiparallel. The A-T and T-A sequences have different phosphodiester conformations. The deoxyribose-pucker and the sugar-base orientation alternate along the chain depending on the nature of the base (3’-endo for purine and 2’-endo for pyrimidine). The extended structure is stabilized by base-base, base-sugar, and hydrogen-bond interactions. The minihelix of two base pairs provides starting coordinates for model-building studies of the dA-dT polymer. A B-DNAtype polymer structure is described, which has sequence-dependent alternations of both the deoxyribose pucker and the phosphate diester bridge conformation. Such sequence-dependent DNA structures, if present locally in regions such as operator sequences, could facilitate sequence-specific interactions. The crystal study also suggests possible geometrical parameters for the replication fork.

Aggregation of apolar peptides in organic solvents. Concentration dependence of 1H-nmr parameters for peptide NH groups in 310 helical decapeptide fragment of suzukacillin

Abstract: Peptide NH chemical shifts and their temperature dependences have been monitored as a function of concentration for the decapeptide, Boc-Aib-Pro-Val-Aib-Val-Ala-Aib-Ala-Aib-Aib-OMe in CDCl3 (0.001-0.06M) and (CD3)2SO (0.001-0.03M). The chemical shifts and temperature coefficients for all nine NH groups show no significant concentration dependence in (CD3)2SO. Seven NH groups yield low values of temperature coefficients over the entire range, while one yields an intermediate value. In CDCl3, the Aib(1) NH group shows a large concentration dependence of both chemical shift and temperature coefficient, in contrast to the other eight NH groups. The data suggest that in (CD3)2SO, the peptide adopts a 310 helical conformation and is monomeric over the entire concentration range. In CDCl3, the 310 helical peptide associates at a concentration of 0.01M, with the Aib(1) NH involved in an intermolecular hydrogen bond. Association does not disrupt the intramolecular hydrogen-bonding pattern in the decapeptide.

Pregnancy suppression by active immunization against gestation-specific riboflavin carrier protein.

Abstract: A riboflavin carrier protein isolated from chickens cross-reacts with a gestation-specific rodent carrier for riboflavin. Active immunization of female rats of proved fertility with the purified chicken carrier protein completely yet reversibly suppressed early pregnancy without impairing implantation per se. Concurrently there were no discernible adverse effects on maternal health in terms of weight gain, vitamin status, and fertility.

Proline-containing β-turns in peptides and proteins. II. physicochemical studies on tripeptides with the Pro-Gly sequence

Abstract: Amino acids are known to differ in their individual preferences for each of the four positions of the ?-turn conformation formed by tetrapeptide segments. Proline and glycine show relatively high preferences for positions 2 and 3, respectively, of the \beta -turn. Using tripeptides of the type N-acetyl-Pro-Gly-X-OH, where X = Gly, Ala, Leu, Ile, and Phe, we have sought to study the influence of the 4th residue X on the stability of the \beta -turn conformation in these tripeptides. Our nmr and CD results show that the \beta -turn stability is quite significantly governed by the nature of the amino acid residue at this position in the following order: Leu > Ala > Ile, Gly > Phe.

Interatomic Auger transitions in transition-metal oxides

Abstract: Interatomic L3(M)M23(M)V(O) and L3(M)V(O)V(O) Auger transitions of some transition-metal oxides are reported for the first time. The interatomic mode of decay becomes progressively more dominant (relative to the intra-atomic mode) as the metal d level gets depleted or as the oxidation state of the metal increases. The usefulness of interatomic Auger transitions in studying oxidation of metals has been examined.

Biodegradable Polymers: Prospects, Problems, and Progress

Abstract: At different times in the history of polymer science specific subjects have come to center stage; for intense investigation because they represent new and important intellectual challenges as well as technological opportunities. In past years the chief incentive for the scientific study of biodegradation of polymers was concern with the problem of preventing or retarding attack on such products by microorganisms, insects, rodents, and other animals. Many research efforts in synthetic polymer chemistry were directed toward the synthesis of polymers resistant to biodegradation. The paramount feature sought by polymer chemists and engineers was stability, and the production of such materials has been very impressive. This is still a very important factor in many applications such as paints, protective coatings, textiles, electrical insulation, and plastic films and sheets used for many applications such as upholstery and floor covering. Fortunately, most synthetic polymers are resistant to biologica...

Synthesis and Characterisation of Metal Hydrazine Nitrate, Azide and Perchlorate Complexes

Abstract: Metal hydrazine nitrate complexes of the type M(N2H4)Nn (NO3)2 where M = Mg, n = 2; M = Mn, Fe, Co, Ni, Zn and Cd and n = 3; metal dihydrazine azide complexes of the type M(N2H4)2 (N3)2 where M = Mg, Co, Ni and Zn; and Mg(N2H4)2 (C1O4)2 have been prepared by dissolving the respective metal powders in the solution of corresponding ammonium salts (NO3, N3 and C1O4) in hydrazine hydrate. These hydrazine complexes were also prepared by the conventional method involving the addition of alcoholic hydrazine hydrate to the aqueous solution of metal salts. The hydrazine complexes have been characterised by chemical analysis, infrared spectra and differential thermal analysis (DTA). Impact sensitivities of hydrazine complexes were determined by the drop weight method. The reactivity of these hydrazine complexes does not change with the method of preparation.

Satellites in the X-ray photoelectron spectra of transition-metal and rare-earth compounds

Abstract: An attempt has been made at synthesis and in resolving some of the uncertainties related to the assignments of charge-transfer satellites in the X-ray photoelectron spectra of transition-metal and rare-earth compounds. New satellites are reported in the ligand core-hole spectra as well as in the metal core-level spectra of oxides of second- and third-row transition metals including rare earths. Satellites in the ligand levels and the metal levels tend to be mutually exclusive, a behaviour that can be understood on the basis of metal-ligand overlap. Systematics in the intensities and energy separations of satellites in the first-row transition-metal compounds have been examined in order to gain an insight into the nature of these satellites. A simple model involving the sudden approximation has been employed to explain the observed systematics in intensities of satellites appearing next to metal and ligand core levels on the basis of metal-ligand overlap.