Showing papers by "Instituto Politécnico Nacional published in 1989"
TL;DR: In this paper, internal reflection spectroscopy is used for the study of the structural gradient from the outer skin into the core in a commercial polyethylene film. And it is shown that the skin is highly crystalline and that an approximate amount of 40% amorphous material is found in the core of the film.
141 citations
TL;DR: The obra resulta de gran importancia para la identificación de materiales en diversos estados de la republica mexicana as mentioned in this paper, e.g., Mexico.
Abstract: La obra resulta de gran importancia para la identificacion de materiales en diversos estados de la republica mexicana.
122 citations
TL;DR: The effect of training on the EMG pattern and on the subjects' mechanical performances was investigated by comparing a population of untrained subjects with one of highly trained gymnasts, indicating that a long term adaptation had taken place.
Abstract: Fast backward trunk movements are accompanied by hip, knee and ankle rotation which compensate for the backward shift of the center of gravity. The electromyographic pattern associated with the performance of these movements and the associated synergies consists of a fairly synchronous activation of the prime mover (erectores spinae) and the muscles situated at the back of the leg (hamstring, calf muscles). This pattern is called the “non anticipated pattern”. The effect of training on the EMG pattern and on the subjects' mechanical performances was investigated by comparing a population of untrained subjects with one of highly trained gymnasts. A new EMG pattern was observed in the highly trained gymnasts, the “distally anticipated pattern” consisting of an early activation of the gastrocnemius, and in some subjects also of the hamstring, indicating that a long term adaptation had taken place. Performances expressed as a ratio between the displacement of the center of gravity projection onto the ground and the velocity of the movement were clearly better in the gymnasts. Short term adaptation was found to occur in the gymnasts and not in the untrained group when the movement was performed while standing on a narrow support. A suppression of the distal gastrocnemius burst occurred in the gymnasts from the first trial under the constrained standing condition whereas no change occurred in the untrained group. The flexibility of the EMG patterns associated with axial movements occurring either spontaneously or as a result of long or short term training is discussed.
121 citations
14 May 1989
TL;DR: Two adaptive controllers that globally achieve the impedance objective for the general nonlinear dynamic model are presented and include a nonlinear feedback of positions, velocities, and end-effector applied forces.
Abstract: Impedance control specifications for robot manipulators are given in terms of a desired motion trajectory and a desired dynamic relationship between position errors and interaction forces. An adaptive implementation is proposed as an alternative to reduce the design sensitivity due to model-manipulator parameter mismatch. Two adaptive controllers that globally achieve the impedance objective for the general nonlinear dynamic model are presented. The controller structures consist of a nonlinear feedback of positions, velocities, and end-effector applied forces. Computer simulations were carried out to demonstrate stability and performance control. >
115 citations
TL;DR: The influx of Ca2+ and its subsequent intracellular increase are required for the acrosome reaction of sea urchin sperm to occur and the influx of Mn2+ increased after a lag time of 5 s, which could be explained by the activation of two different Ca2- channels by egg jelly.
84 citations
TL;DR: A careful comparison of the theoretical and experimental results of the plane-wave Gaussian models of optical bistability and of the experimental investigations of the single-mode instability induced on a beam of sodium atoms in a cavity by an external driving field is provided.
Abstract: This paper gives a detailed description of the plane-wave Gaussian models of optical bistability and of the experimental investigations of the single-mode instability induced on a beam of sodium atoms in a cavity by an external driving field. Our goal is to provide a careful comparison of the theoretical and experimental results. We focus on the shape of the instability domain, the nature and behavior of the spontaneous output oscillations produced by the instability, and their frequency dependence on the control parameters. We carry out this comparison using both the plane-wave and Gaussian models. Our analysis shows that the latter is in good quantitative agreement with the experimental results. We investigate in detail the main assumption of the Gaussian model, namely, that the internal cavity field retains the same radial profile as the input field.
83 citations
TL;DR: In this article, the analysis of bifurcation phenomena, specifically strain localization (onset of a strain rate discontinuity), in small deformation elastoplasticity, is discussed.
Abstract: This note contains some remarks on the analysis of bifurcation phenomena, specifically strain localization (onset of a strain rate discontinuity), in small-deformation elastoplasticity. Nonassociative flow rules are allowed for to cover constitutive models frequently adopted for frictional (and softening) materials such as concrete. The conventional derivation of the localization criterion resting on an incrementally linear “comparison material” is critically reviewed and compared to the criterion resulting from “actual” nonlinear plastic flow laws.
68 citations
TL;DR: Amaranthus hypochondriacus Mercado type was isolated from a new amaranth variety with outstanding agronomic traits as discussed by the authors, and the physicochemical and functional properties were comparable to a maize starch sample.
Abstract: Starch was isolated from a new amaranth variety (Amaranthus hypochondriacus Mercado type) with outstanding agronomic traits. Some of its physicochemical and functional properties were comparable to a maize starch sample. After annealing, both starches exhibited little change in gelatinization peak temperature as assessed by differential scanning calorimetry. It is suggested that this behaviour might be due to the high amylopectin content of the amaranth starch and to a previous annealing effect undergone by the commercial sample during the isolation procedure. Amaranth starch granules showed by scanning electron microscopy a polygonal shape.
59 citations
TL;DR: The predictions for the semileptonic decays of charm baryons are given in two quark models: The nonrelativistic and the MIT bag models and it is found that several processes have large branching ratios.
Abstract: Predictions for the semileptonic decays of charm baryons are given in two quark models: The nonrelativistic and the MIT bag models. We find that several processes have large branching ratios, of the order of few percent; these are very large in comparison with the strange-hyperon semileptonic-decay branching ratios. We also notice that the {ital q}{sup 2} dependence overwhelms other corrections and should be considered in the future with more emphasis. Finally, the predictions found for the {Lambda}{sub {ital c}}{sup +} decay---when compared with the corresponding experimental value---leave ample room for the existence of a fourth generation.
55 citations
TL;DR: The number of (random) patterns required for random testing of RAMs (random-access memories), when classical fault models including pattern-sensitive faults are considered is determined, and Markov chains are a powerful tool for this purpose.
Abstract: The number of (random) patterns required for random testing of RAMs (random-access memories), when classical fault models including pattern-sensitive faults are considered is determined. Markov chains are a powerful tool for this purpose. Single faults are considered first, and the influence of different parameters is analyzed. Double faults are then considered and arguments are presented to extend the results to all multiple-coupling faults. Those results are compared to the optimal or best-known number of test patterns required when deterministic testing is considered, for the same fault models. >
53 citations
TL;DR: In this paper, a simple and mild procedure based on photochemically induced decarboxylation of carboxylic acids by iodosobenzene diacetate to obtain the substitution of bases by nucleophilic alkyl radicals was reported.
TL;DR: In this article, the authors considered a homogeneously broadened ring laser with spherical mirrors, where the atomic line is resonant with three degenerate transverse modes of the resonator.
TL;DR: In this paper, unpromoted and caesium-promoted ZnCrO and MnCrO catalysts have been characterized and tested in the direct synthesis of methanol and hig
TL;DR: Amaranth seeds were germinated at a water activity of about 0.92 for up to 72 hr and showed a pinky color which appeared very attractive for various food uses.
Abstract: Amaranth seeds were germinated at a water activity of about 0.92 for up to 72 hr. Crude protein, true protein and crude fiber were found to increase and fat content to decrease. For 48 hr of germination reactive lysine values did not change. At 72 hr, a slight decrease was observed in lysine and in vitro protein digestibility was similar to the control. During germination Hunter color parameters L were lowered and a and b were enhanced. Germinated seeds showed a pinky color which appeared very attractive for various food uses.
TL;DR: In this paper, the direct alkylation of p -toluidine, as a model of aromatic amines, with 1,3-, 1,4, 1,5- and 1,6-ditosylates, -dibromides or -dimesylates in HMPA as solvent affords the corresponding cyclic amines.
TL;DR: It is shown that recruitment and destruction of inflammatory cells following intestinal amebic invasion may take place in the absence of bacterial multiplication, providing histological support to the hypothesis that lysis of PMNs by trophozoites participates in the genesis of amebi intestinal lesions.
Abstract: The morphological features of early intestinal ulcerations induced in rodents with axenic cultures of Entamoeba histolytica were studied by scanning and transmission electron microscopy. Amebas did not attach to the luminal surface of the mucosa except at interglandular regions, where parasites penetrated apparently through pseudopodial movement. Once in the lamina propria, trophozoites multiplied and destroyed mucosal components. Damage extended laterally through the mucosa, but progression to deeper layers of the intestinal wall was prevented by the muscularis mucosae, which acted as a partial barrier. This was eventually breached at focal points where amebas invaded the submucosa. Electron microscopy clearly showed the lysis of abundant polymorphonuclear neutrophil leukocytes (PMNs) by the amebas at the periphery of intestinal ulcerations as well as the lack of bacteria at these sites. This study demonstrates that recruitment and destruction of inflammatory cells following intestinal amebic invasion may take place in the absence of bacterial multiplication. The observations provide histological support to the hypothesis that lysis of PMNs by trophozoites participates in the genesis of amebic intestinal lesions.
TL;DR: The possibility of disorted configurations in free standing nematic liquid crystal films is considered in this article, where the authors consider the possibility of configurations distordues in the context of Frank elastic theory and show that the stable configurations are always the undistorted ones compatible with boundary conditions.
Abstract: The possibility of existence of disorted configurations in free standing nematic liquid crystal films is considered. In the framework of Frank elastic theory the stable configurations are always the undistorted ones, compatible with boundary conditions. When the mixed splay-bend elastic term with elastic constant k 13 is considered and the Nehring-Saupe free energy density used to analyse t On considere la possibilite d'existence de configurations distordues dans des films de cristaux liquides nematiques. Dans le cadre de la theorie de l'elasticite de Frank, les configurations stables sont celles qui ne sont pas distordues et compatibles avec les conditions aux limites. Quand on tient compte du terme de torsion contenant la constante K 13 et qu'on utilise l'energie libre de Nehring-Saupe dans l'analyse de la stabilite des configurations nematiques, il apparait que la structure deformee pourrait etre plus stable que la structure non distordue si k 13 verifie une relation particuliere. Ce resultat, independant de l'epaisseur de l'echantillon, semble justifie par quelques resultats experimentaux. Nous montrons que ce resultat etrange est du a une simplification de la theorie elastique dans laquelle on neglige les termes qui dependent du carre des derivees deuxieme de l'orientation moyenne des molecules du cristal nematique. Dans cet article, nous tenons compte de ces termes habituellement negliges. Nous montrons que les configurations distordues peuvent rester stables par rapport a la configuration non distordue seulement si la constante elastique k 13 est suffisamment grande et si l'epaisseur de l'echantillon est inferieure a une valeur critique. De plus, nous determinons l'ordre de grandeur du rapport entre la nouvelle constante elastique, liee au terme elastique et qui depend de la derivee du deuxieme ordre, et la constante elastique habituelle, en utilisant une theorie simple quasi microscopique
TL;DR: The results show that, at levels of water below 0.04% (v/v), the F1-ATPase is in a state that does not carry out catalysis and possesses high thermostability, and as the water content is increased, the enzyme acquires the progressive flexibility that is required forCatalysis and for undergoing rapid thermal denaturation.
Abstract: Soluble mitochondrial F1-ATPase from bovine heart can be transferred to systems composed of a nonpolar solvent (toluene), phospholipid, and water at concentrations between 0.02 and 0.05% (volume of water per volume toluene). In these systems, F1 becomes resistant to cold denaturation and acquires a remarkable thermostability; Le., its half-life at 70 OC is more than 24 h. Thermostability is due to the low content of water, since increases of water concentration bring about a progressive decrease in ther- mostability. At 0.04% water, the enzyme fails to catalyze a single splitting of ATP per enzyme. Gradual increases in water concentration up to 2.5% result in a progressive increase of hydrolytic activity. However, even at 2.5% water, the activity is orders of magnitude lower than in totally aqueous media. At various concentrations of water (0.1-2.5% v/v) and Mg-ATP, it was found that water affects the V,,,, but not the K,. The results show that, at levels of water below 0.04% (v/v), the enzyme is in a state that does not carry out catalysis and possesses high thermostability. As the water content is increased, the enzyme acquires the progressive flexibility that is required for catalysis and for undergoing rapid thermal denaturation.
TL;DR: In this article, the mechanism involved in the synthesis of methanol and higher alcohols from syngas has been investigated by combined FT-IR spectroscopy and TPSR techniques.
TL;DR: Oleanolic, ursolic, 2α-hydroxyursolic and maslinic acids and the new 2epi-tormentic acid (2β,3β,19α-trihydroxy-urs-12-en-28-oic acid) were isolated from the aerial parts of Cunila lythrifolia.
TL;DR: In the presence of olefins and aromatics, inter-and intramolecular free radical chain or oxidative additions are observed as discussed by the authors. But the results are limited.
TL;DR: The results indicate that in intact muscle fibers Ca2+ channels inactivate in a voltage-dependent manner through a mechanism that does not requireCa2+ entry into the cell, the effect being more prominent for short pulses.
Abstract: Inactivation of slow Ca2+ channels was studied in intact twitch skeletal muscle fibers of the frog by using the three-microelectrode voltage-clamp technique. Hypertonic sucrose solutions were used to abolish contraction. The rate constant of decay of the slow Ca2+ current (ICa) remained practically unchanged when the recording solution containing 10 mM Ca2+ was replaced by a Ca2+-buffered solution (126 mM Ca-maleate). The rate constant of decay of ICa monotonically increased with depolarization although the corresponding time integral of ICa followed a bell-shaped function. The replacement of Ca2+ by Ba2+ did not result in a slowing of the rate of decay of the inward current nor did it reduce the degree of steady-state inactivation. The voltage dependence of the steady-state inactivation curve was steeper in the presence of Ba2+. In two-pulse experiments with large conditioning depolarizations ICa inactivation remained unchanged although Ca2+ influx during the prepulse greatly decreased. Dantrolene (12 microM) increased mechanical threshold at all pulse durations tested, the effect being more prominent for short pulses. Dantrolene did not significantly modify ICa decay and the voltage dependence of inactivation. These results indicate that in intact muscle fibers Ca2+ channels inactivate in a voltage-dependent manner through a mechanism that does not require Ca2+ entry into the cell.
TL;DR: To establish if the different clinical outcome of giardiasis in children could be due, at least partially, to strain differences, isolates from asymptomatic and symptomatic cases studied in Mexico City were cultured under axenic conditions and the isoenzyme electrophoretic patterns of 10 different enzymes were compared.
Abstract: Infection of the small intestine of humans with the parasitic protozoon Giardia lamblia may have an asymptomatic course, or it may produce acute or chronic diarrhoea. In order to establish if the different clinical outcome of giardiasis in children could be due, at least partially, to strain differences, 19 isolates from asymptomatic and symptomatic cases studied in Mexico City were cultured under axenic conditions and the isoenzyme electrophoretic patterns of 10 different enzymes were compared. Strains from carriers and from symptomatic cases of giardiasis were equally amenable to isolation and axenization. Isoenzyme electrophoresis demonstrated remarkable homogeneity in 7 enzyme patterns for all 19 isolates, except for phosphoglucomutase, for which 3 different zymodemes were found. Therefore, these isolates of G. lamblia, obtained from a single geographical location, tended to be genetically homogeneous. In addition, there were no consistent zymodeme differences between isolates from symptomatic and asymptomatic human infections.
TL;DR: In this paper, two varieties of black and light yellow seeds were stored for 135 days at 40°C and 80% rh. Texture measurements produced force-time deformation curves of two peaks for the fresh samples and of three peaks plus a substantial increase in Instron hardness for the aged beans.
TL;DR: In this article, the authors analyze the problem of single-mode optical bistability using a model which neglects the intensity-dependent Stark shift and demonstrate the existence of a phase instability which arises in bad-cavity conditions in the lower branch of the steady-state curve.
TL;DR: In this article, the spontaneous formation of transverse spatial patterns due to diffraction in lasers can be described by a phase diffusion equation of the Kuramoto-Sivashinsky kind, and the origin of the pattern formation lies in spatial instabilities of the phase of the complex slowly varying laser field amplitude.
TL;DR: In this paper, the slip phenomenon through capillaries has been analyzed experimentally using solutions of polyacrylamide and xanthan gum, which present macromolecules with different conformations in flow.
Abstract: The slip phenomenon through capillaries has been analyzed experimentally using solutions of polyacrylamide and xanthan gum These polymeric solutions present macromolecules with different conformations in flow Attention was given to the influence of molecular conformation of those polymeric solutions upon the slip velocity Considerable variations on the slip velocity were observed when the ionic strength of the xanthan solution was increased and also when the material of the capillaries was changed Measured magnitudes were much larger in the xanthan solutions than those observed in the polyacrylamide solution
TL;DR: In this article, the α-β transition of NiMoO4 and the crystallization of pure NiO are used to stabilize the high-temperature phase of NiO4 at room temperature by forming a solid solution with NiO.
TL;DR: In this paper, it was shown that the spectrum of two-photon optical bistability exhibits the onset of a large squeezing on a frequency region on the order of the cavity linewidth.