677 research outputs found

    “Caracterización de los sistemas de producción de ovinos de pelo en el suroeste del departamento de Matagalpa 2010”

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    Con el objetivo de caracterizar los sistemas de producción de ovinos de pelo en el territorio suroeste del departamento de Matagalpa 2010. (Sébaco, Ciudad Darío, San Isidro y Matagalpa). Se aplicó una encuesta a 103 productores que poseen ovinos de pelo, la muestra se definió aleatoriamente utilizando la ecuación planteada por Scheaffer (1987), se utilizó la técnica de muestreo de bola de nieve, planteada por Frey et al (2000). Esta investigación permitió conocer las debilidades y oportunidades en los sistemas de explotación de esta especie promisoria para la zona seca del país. Con los resultados obtenidos de las encuestas se procedió ha elaborar una base de datos en el programa SPSS versión 11.5 en español. Encontrando un predominio del sexo femenino como titulares de las explotaciones ovinas, 58.3% cursó educación primaria, el 98% de las explotaciones cuentan con raza pelibuey, el 100% de las explotaciones realizan destete y monta de forma natural, una media de mortalidad de corderos de 1, alimentan a las ovejas con potrero sin división (81.6%), se suministra pasto de corte, pastoreo, leguminosas y se suplementa sal común 49.5%, aplican vacunas contra ántrax y pierna negra (63.1%), desparasitaciones internas y externas (66%), ambos con una frecuencia de 2 veces al año, en el manejo productivo no se lleva control en la actividad ovina (100%), los equipo e instalaciones son rústicas, los corrales ovinos el son elaborados con alambre y/o madera, techado con plástico y/o zinc (49.51%), en cuanto a asistencia técnica el 58.3% ha recibid

    Quantitative Acoustic Emission for Damage Detection in Complex Geometries

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    Acoustic Emission (AE) method is a passive nondestructive evaluation (NDE) method based on the release of elastic waves by active flaws in structures. Since 1950s, the AE method has been applied as a qualitative NDE method to detect discontinuities by real time monitoring. While the AE method is successful in identifying and localizing active flaws, the method has not been fully utilized as a quantitative NDE method due to several challenges: unknown conversion behavior of sensor, complex source mechanism and influence of background noise. In this study, the goal is to address the major challenges of AE in terms of sensor behavior, influence of operational noise, and quantitative AE measurement and contribute our effort to achieve better understandings of acoustic emission. The study has three major components: (i) The multi-physics numerical models of AE sensors are built and validated by experiments. (ii) With the proper design of the laboratory scale test, the fatigue crack growth is simulated, and the corresponding AE signals are collected. Spline geometry of helicopter gearbox system is selected as the test structure to simulate a complex geometry and integrate with the field data obtained from the NAVAIR test facility. (iii) The recorded AE signals are embedded into the streamed signals obtained from the NAVAIR prototype test to evaluate the detectable AE signal in highly noisy operational environment and develop an efficient signal processing method. The purpose is to show the ability to extract fatigue crack signal embedded in noise signal and released from a complex geometry

    Continuous nonsingular terminal sliding mode control for nonlinear systems subject to mismatched terms

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    This paper proposes a continuous nonsingular terminal sliding mode (NTSM) control approach for nonlinear systems subject to mismatched terms in order to achieve finite time exact tracking and disturbance rejection. The controller is constructed using a composite method that utilizes a power integrator and combines the output regulation theory, disturbance observation technique, feedback domination, and sliding mode control technique. The performance analysis demonstrates that the proposed continuous NTSM controller can drive the system output to the desired reference signal in a finite time in the presence of mismatched time-varying disturbances and nonsmoothed nonlinearities in each channel. The finite time Lyapunov theory is utilized to ensure finite-time convergence of the closed-loop system. The simulation results validate the effectiveness of the proposed continuous NTSM controller

    New Insights into Formation of Molecular Sieve SAPO-34 for MTO Reactions

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    Small-pore molecular sieve SAPO-34 is well-known as the most promising and effective catalyst for the conversion of methanol to olefins (MTO). In this paper, we have investigated the formation of SAPO-34 under different dry gel conversion (DGC) conditions using two types of synthesis gels with one containing hydrofluoric acid (HF) and the other not. Particular attention was paid to the Si incorporation and distribution in final SAPO-34 products. The results indicate that under DGC (with HF) conditions SAPO-34 is formed from the transformation of a highly crystalline, layered prephase that is held by covalent bonds. The final SAPO-34 product is a mixture of triclinic and trigonal phases containing six framework Si species. The Si distribution in triclinic and trigonal SAPO-34 is drastically different. The majority of Si species are located in the aluminosilicate domains in trigonal phase. Conversely, under DGC (without HF) conditions, the crystallization of SAPO-34 involves three transformational stages. Initially, a layered, crystalline intermediate forms and then transforms to a semicrystalline phase. The structures of both intermediates are held by weak noncovalent bonding interactions. The final SAPO-34 product is in pure trigonal form and contains two major Si species. Despite the difference in Si distribution, the SAPO-34 products prepared by using different gels have similar bulk Si contents. MTO reaction tests show that before 50 min of time-on-stream the DGC (with HF) product gives lower ethylene and propylene selectivity but higher propane selectivity than the DGC (without HF) product. After 50 min on stream, similar light olefin and propane selectivities are observed over the two SAPO-34 samples

    Directed Assembly of Janus Particles under High Frequency ac-Electric Fields: Effects of Medium Conductivity and Colloidal Surface Chemistry

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    The manipulation and assembly of polystyrene-based Janus particles of varied surface chemistry on one hemispherical particle surface under high frequency nonuniform ac-electric fields is examined experimentally by in situ microscopic observation. Strong effects of ac-field frequency, medium conductivity, and particle surface chemistry on the structure of Janus colloidal assembly are observed. At low medium conductivity, σ<sub>m</sub> from 0.0007 S/m to 0.0153 S/m, pearl chains of Janus particles are observed over the ac-frequency range from 25 kHz to 20 MHz, indicating the dielectrophoresis (DEP)-directed assembly. In contrast, the chaining of Janus particles is disrupted in a certain frequency range at high σ<sub>m</sub> from 0.0153 S/m to 0.116 S/m, suggesting the combining effects of both induced-charge electrophoresis (ICEP) and DEP. The critical transition frequency for the onset of the fractal aggregation at high σ<sub>m</sub> from 0.0153 S/m to 0.116 S/m is experimentally determined, showing a good agreement with the theoretically predicted upper ICEP frequency limit. Additionally, it is demonstrated that by using zwitterionic Janus particles, the assembled structure of Janus particles under ac-fields can be modified by the chemical coatings on each hemispherical surface of Janus particles

    A model-based unmatched disturbance rejection control approach for speed regulation of a converter-driven DC motor using output-feedback

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    The speed regulation problem with only speed measurement is investigated in this paper for a permanent magnet direct current (DC) motor driven by a buck converter. By lumping all unknown matched/unmatched disturbances and uncertainties together, the traditional active disturbance rejection control (ADRC) approach provides an intuitive solution for the problem under consideration. However, for such a higher-order disturbed system, the increase of poles for the extended state observer (ESO) therein will lead to drastically growth of observer gains, which causes severe noise amplification. This paper aims to propose a new model-based disturbance rejection controller for the converter-driven DC motor system using output-feedback. Instead of estimating lumped disturbances directly, a new observer is constructed to estimate the desired steady state of control signal as well as errors between the real states and their desired steady-state responses. Thereafter, a controller with only speed measurement is proposed by utilizing the estimates. The performance of the proposed method is tested through experiments on dSPACE. It is further shown via numerical calculations and experimental results that the poles of the observer within the proposed control approach can be largely increased without significantly increasing magnitude of the observer gains

    Alternative Formulations and Improvements with Valid Inequalities for the Refinery Production Scheduling Problem Involving Operational Transitions

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    Operation mode switching of production units would result in long transitions with fluctuant product yields, production costs, and key product properties. To formulate the remarkable process dynamics, a mixed-integer linear programming (MILP) model involving operational transitions of mode switching was proposed for the refinery production scheduling problem in Shi et al. (Ind. Eng. Chem. Res. 2014, 53 (19), 8155−8170), which can describe transitional behaviors and provide implementable schedules. However, the model is very computationally expensive because it involves a large number of discrete and continuous variables, and the constraints about operational transitions are numerous and complex. In this paper, we study a scheduling problem similar to that in Shi et al. (Ind. Eng. Chem. Res. 2014, 53 (19), 8155−8170) and aim at improving the computational efficiency of MILP models by alternative formulations and valid inequalities. First, we redefine some sets, variables and introduce new variables to propose the basic formulation, with which the nonlinear items are avoided and the variable definitions are simplified. Second, by reformulating constraints to explicitly describe the Fixed Charge Network Flow characteristic of operation mode switching, three Fixed-Charge-Network-Flow-based reformulations are proposed. Third, valid inequalities are developed for lot-sizing relaxations derived from the reformulations. Computational results show that the basic formulation is much smaller-sized and its computational performance is significantly better. The Fixed-Charge-Network-Flow-based reformulations together with the valid inequalities can further reduce by up to more than 95% the computational time while tightening the linear relaxation of the MILP model by more than 45%

    Spatial patterns of farmland conversion in Hang-Jia-Hu region in 1994 and 2003.

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    <p>Spatial patterns of farmland conversion in Hang-Jia-Hu region in 1994 and 2003.</p

    sj-docx-1-jht-10.1177_10963480231199991 – Supplemental material for One Size Fits All? How CSR Communication Influences Donation and Revisit Intention Differently in Local Independent and Chain Restaurants

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    Supplemental material, sj-docx-1-jht-10.1177_10963480231199991 for One Size Fits All? How CSR Communication Influences Donation and Revisit Intention Differently in Local Independent and Chain Restaurants by Michael S. Lin, Jihwan Yeon and Lu Zhang in Journal of Hospitality & Tourism Research</p

    Land cover patterns of Hang-Jia-Hu region in 1994 and 2003 (Data source: [31]).

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    <p>Land cover patterns of Hang-Jia-Hu region in 1994 and 2003 (Data source: [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0125008#pone.0125008.ref031" target="_blank">31</a>]).</p
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