Showing papers by "National University of La Plata published in 1993"

Variable structure control of an SRM drive

Abstract: The applications of a variable-structure system (VSS) to the control of a switched reluctance motor (SRM) drive is presented. After reviewing the operation of an SRM drive, a VSS-based scheme is formulated to control the drive speed. The scheme is then designed and tested by simulation. The results show that the VSS control is effective in reducing the torque ripple of the motor, compensating for the nonlinear torque characteristics, and making the drive insensitive to parameter variations and disturbances. >

Los roedores de la Formación Collon Cura (Mioceno Medio), y la Ignimbrita Pilcaniyeu, Cañadon del Tordillo, Neuquen

Abstract: THE RODENTS OF COLLON CURA FORMATION (MIDDLE MIOCENE) AND THE PILCANIYEU IGNIMBRITE. CANADON DEL TORDILLO, NEUQUEN. Collon Cura Formation has been identified in this area by the presence of Pilcaniyeu Ignimbrite (Middle Member). The Upper and Lower Members of this formation, where fossils were found, are mainly reworked pyroclastic rocks, probably deposited simultaneously with the ignimbrite. Pedogenetic features are common, especially in the Lower Member, where basal strata show slightly different features. The following taxa are described: Neosteiromys ? tordillense gen. et sp. nov. (Erethizontidae); Maruchito trilofodonte gen. et sp. nov., Protacaremys denisae sp. nov.. Acarechimys sp. (Echimydae) and Dasyproctidae indet. They provide a better characterization of the Colloncuran s.s. which is restricted to the fauna of Collon Cura Formation associated to the Pilcaniyeu Ignimbrite. Based on the rodents, the following sequence of faunas is proposed: Santacrucian, Frisian S.S., Colloncuran s.s. and Mayoan. The Mayoan rodents have affinities with those of the Chasicoan, while the rodents of the other faunas are more related to each other. Our study suggests a hiatus within the Friasean Land-Marnmal Age. Rodents found at this locality, together with the presence of primates, sedimentological features, and other geological evidences for other regions of Patagonia, are indicative of warmer and more humid climatic conditions than previously postulated for the Friasian in Patagonia.

Stability of frozen starch pastes : effect of freezing, storage and xanthan gum addition

Abstract: . Starch pastes act as protective systems of the solid elements in precooked frozen foods, minimizing their dehydration and chemical changes during storage. the effect of xanthan gum (0.3 % w/w) on corn starch and wheat flour pastes (10 % w/w), frozen at different freezing rates and stored between -5 and -20C, was analyzed. Freezing modified the quality attributes of the starch pastes increasing exudate production, structure deterioration and rheological changes compared to the unfrozen samples. Starch pastes showed a pseudoplastic behavior and apparent viscosities decreased in frozen samples. High freezing rates led to smaller ice crystals (indirect microscopic observation) and the absence of starch retrogradation. Starch retrogradation and ice recrystallization, both contributed to the deterioration of the frozen paste during storage (spongy structure, marked decrease of apparent viscosity and increase of syneresis). the addition of xanthan gum minimized spongy structure formation, exudate production and rheological changes; however, no protective effect was observed on ice crystal sizes and on amylopectin retrogradation by Differential Scanning Calorimetry.

An introduction to population approaches for optimization and hierarchical objective functions: a discussion on the role of tabu search

Abstract: Population approaches suitable for global combinatorial optimization are discussed in this paper. They are composed of a number of distinguishable individuals called "agents", each one using a particular optimization strategy. Periods of independent search follow phases on which the population is restarted from new configurations. Due to its intrinsic parallelism and the asynchronicity of the method, it is particularly suitable for parallel computers. Results on two test problems are presented in this paper. The individual search optimization strategies for each agent have been chosen having the basic characteristics of tabu search. This has been done in order to avoid mixing the hypothesized properties of these population approaches with those of more elaborate tabu search strategies, but remarking on its main characteristics. A set of four test problem "landscapes" is discussed and their use to improve and benchmark the results by using tabu search as the individual optimization strategy within a population heuristic is suggested and explored. The application of tabu search to new problem areas, like molecular biology, is also investigated.

The fracture energy (G F) of high-strength concretes

Abstract: This paper presents results for the fracture energy of concrete (GF) obtained from a wide range of high-strength concretes. Strength levels up to 100 MPa, aggregate type and aggregate surface texture were included as variables. The determination ofGF was performed according to the recommendation of the RILEM 50-FMC Committee. Compressive and tensile strengths and the modulus of elasticity are also presented. Measured values ofGF are compared with those proposed in the last CEB Model Code.

Disturbance torque estimation in a sensorless DC drive

Abstract: The estimation of the disturbance torque in a sensorless DC drive is carried out by extending the classical observer theory. Three estimation schemes are formulated according to the representation of the disturbance torque and processing of the observer states. In addition to the disturbance torque, all the schemes deliver an estimation of the motor speed. Steady-state accuracy and dynamics of the schemes are analyzed in nominal conditions, identifying the scheme with the best performance. The effects of variations in the motor parameters are also analyzed, finding that a proper modeling of the motor makes the steady-state estimation of the disturbance torque insensitive to any variation. As a test, the schemes are applied to a sensorless DC drive for both compensating for the disturbance torque and closing the speed loop. The responses obtained with the best-performance scheme are reported. >

"populismo" y "neopopulismo" en america latina

Abstract: _ Y INGUN ANALISIS de la realidad latinoamericana del siglo xx, y sobre todo de la evolucion historica de las ideologias y proyectos politicos en el continena te, puede dejar de otorgar un lugar central al "populismo". Sin embargo, es dificil encontrar una reflexion reciente sobre dicho tema que no haga referencia a la diversidad y amplitud de significados que ha adquirido el concepto y a la falta de precision en su uso que de ello resulta. La vaguedad del termino se manifiesta, en primer lugar, en la larga lista de "casos" que se incluyen generalmente bajo la denominacion de "populistas"; por otra parte, designa indistintamente movimientos, gobiernos, "tipos de Estado", "modelos de acumulacion", "estilos politicos", ideologias, etc. Para agravar la confusion, en trabajos recientes comienza a utilizarse la expresion "neopopulismo", mientras que desde hace ya tiempo se viene hablando de la "crisis del Estado populista": estas referencias reproducen y potencian la imprecision senalada, ya que no parten de una mejor definicion del fenomeno al que ahora se considera muerto o en vias de ser resucitado. E1 objetivo de este trabajo es realizar un intento de ordenamiento y una revision critica de las distintas definiciones e interpretaciones producidas hasta el momento en torno al "populismo" en America Latina.' No pretendemos construir una "nueva definicion superadora", sino aportar algunas reflexiones que permitan un uso mas productivo de los conceptos que se proponen dar cuenta de una vision global y comparada de la historia latinoamericana del presente siglo. Partimos de la idea de que no se trata de definir ontologicamente "que es" el populismo, sino de construir categorias generales que revelen una verdadera utilidad cienttfica a la

Synergistic Effects in the Inhibition of Copper Corrosion

Abstract: Benzotriazole (BTA), thiourea (TU), and potassium ethylxanthate (KEX), behave as copper (Cu) corrosion inhibitors under certain conditions. These chemicals have been investigated to establish whether they provide synergistic effects. The Cu corrosion inhibition was followed through changes in electrochemical characteristics. Cu specimens were tested at 25°C in two aggressive media, 0.1 M NaCl and 1 M NaClO4, using the linear potential sweep technique at 0.001 V s−1 and by scanning electron microscopy of Cu specimens subjected to potentiodynamic and potentiostatic routines. A comparative behavior of the different substances for Cu was presented in the 6.6 ≤ pH ≤ 11 range. For KEX-BTA mixtures, synergistic inhibition effects were found in 0.1 M NaCl (7 ≤ pH ≤ 11). The apparent synergistic inhibition was explained tentatively by an increase in the compactness of the polymer-like passivating layer of KEX-Cu, which formed in the presence of BTA and Cl− ions.

Immunohistochemical investigation of tyrosine-hydroxylase in the islets of Langerhans of adult mice, rats and guinea pigs.

Abstract: As an indirect way to establish whether catecholamines are actually elaborated in the islets of Langerhans of adult albino and pigmented mice and guinea pigs, or albino rats, the presence of tyrosine-hydroxylase was immunohistochemically investigated using a highly specific polyclonal serum against the enzyme. The antiserum was applied to paraffin sections of pancreases fixed either in acrolein or Bouin's fluid. Such sections were then treated with the avidin-biotin method. Tyrosine-hydroxylase was found exclusively in the beta cells of rats and in those of the two strains of mice. Results in the guinea pig varied according to whether the animals were pigmented or albino. In pigmented specimens, the enzyme was detected in beta and non-beta cells, whereas in albino animals it was undetectable in any of the islet cells. All these observations were performed in material fixed in acrolein. Bouin's fluid resulted a rather poor fixative for the detection of the enzyme in the islets. The results are compared with those of other authors who investigated the presence of monoamines in the islets of Langerhans. Since tyrosine-hydroxylase is a specific marker for catecholamine-synthesizing cells, it is concluded that insular cells have the ability to elaborate these substances.

Spectrophotometric Method for Fungal Pectinesterase Activity Determination

Abstract: A continuous spectrophotometric assay modified for fungal pectinesterases is reported. This method is based on that developed by Hagerman and Austin for plant pectinesterases. The amount of carboxylic groups released during the assay is calculated from the decrease in absorbance of a pH indicator added to the reaction mixture with a pKi value within the pH range of maximum enzyme activity. Bromocresol green was chosen for our modified method as fungal pectinesterases have an optimum activity at acid pHs (4.6 to 5.1) which are within the pH range of color change (3.8 to 5.4) for this dye. Our methodology was tested by employing a commercial pectinase product as the source of pectinesterases. Statistical analysis using the Student's t-test showed good agreement between the reference titrimetric method and that reported herein.

Quercetin and the mutagenicity of wines

Abstract: Various studies have shown the mutagenicity of red wine. The major mutagens identified in red wine have been flavonoids, i.e. rutin and its aglycone quercetin. Besides flavonoids, however, it has recently been reported that H2O2 may account for the mutagenicity of red wine in the L-Arabinose resistance test. In the present study we report on the role of flavonoids in the mutagenicity of red wine in the Ames assay. Different wines from Portugal and Spain have been tested after concentration in XAD-2 columns in strains TA98 and TA104 of Salmonella typhimurium concurrently with the determination of the respective content of quercetin by HPLC. A similar approach was used for pilot scale productions of red wines. In all cases quercetin could be demonstrated as the major mutagen in red wines. The levels of quercetin in finished wines and during the wine-making process showed a good fit with the levels of mutagenicity detected. Catalase had no effect whatsoever on the mutagenicity of wines in both TA98 and TA104. These results do not rule out a role for H2O2 in the mutagenicity of wines, detected in other genetic end-points, because H2O2 can be formed from the auto-oxidation of quercetin.

Darstellung und Eigenschaften von Vanadyl(IV)-Saccharinat

Abstract: The synthesis of a new vanadyl(IV)/saccharin complex is reported. Its bonding characteristics differ considerably from all known divalent metal-saccharinates. In this complex a VO(OH)+ moiety is coordinated to a saccharin molecule through its carbonyl oxygen and to a saccharinate ion through its deprotonated nitrogen. Two water molecules complete the coordination sphere. X-ray powder diagrams, infrared- and electronic absorption-spectra were recorded and analyzed for the characterization of the compound and its coordination properties. Besides, the magnetic susceptibility and the thermal behaviour were also investigated.

Hyperfine interactions of 111Cd in Ga2O3.

Abstract: Perturbed-angular-correlation measurements were carried out after implantation of 111 In into amorphous and polycrystalline powder samples of α- and β-Ga 2 O 3 , and single crystals of β-Ga 2 O 3 . After annealing of the radiation damage the quadrupole hyperfine interactions of 111 Cd were determined. In both α- and β-crystalline phases, three electric-quadrupole interactions were observed. In addition, the orientations of the electric-field gradients in β-Ga 2 O 3 at the probe sites were determined. In β-Ga 2 O 3 preferential substitutional In occupation of the octahedral sites was found

Caracteristicas del magmatismo post-orogenico finibrasiliano en el uruguay: formaciones sierra de ríos y sierra de animas

Abstract: Se realizaron estudios petrograficos, geoquimicos y geocronologicos sobre rocas volcanicas y sub-volcanicas de dos unidades finibrasilianas del denominado Cinturon Cuchilla Dionisio en la R.O.Uruguay. La aplicacion de isocronas Rb-Sr permitio definir edades en el deslinde Precambrico-Cambrico y Cambrico: 1. Formacion Sierra de RIOS, 575 ± 14 Ma (Ri = 0,71046 ± 0,0069); y 2. Formacion Sierra de Animas, 520 ± 5 Ma (Ri = 0,70655 ± 0,00010). Las caracteristicas de composicion quimica son afines a rocas de origen cortical y sugieren vinculacion a etapas distensivas post-orogenicas y a megatranscurrencias.

Sexual dimorphism in schoolchildren of the Villa IAPI neighborhood (Quilmes, Buenos Aires, Argentina)

Abstract: Villa IAPI is a poor neighborhood of about 5,000 inhabitants in the overpopulated conurbano bonaerense at the outskirts of the city of Buenos Aires. Most of the adult male residents are construction workers; the rest are temporary workers, underemployed, or unemployed. The inhabitants of Villa IAPI suffer the effects of many adverse socio-economic conditions, including poor nutrition, deficient sanitation, and inadequate medical care. Seven anthropometric variables were measured on 765 children from 6 to 14 years old to test for the presence of an altered pattern of sexual dimorphism. It was found that there were practically no sex differences in standing height and upper-arm muscle circumference. In some age groups, there was a weak but significant sexual dimorphism in body weight and sitting height. The greatest and most persistent dimorphism was found in head circumference, and in triceps and subscapular skinfolds. In all except two age groups, head circumference in males was significantly greater than in females. The other dimorphic variables (body weight, sitting height, and triceps and subscapular skinfolds) showed the opposite relationship. Essentially, females showed increments in subcutaneous fat, while reduced growth in muscle and bone was evident in males. The hypothesis of "better female canalization" can explain the altered dimorphic pattern found in the malnourished Villa IAPI population.