Modelo dinámico de un sistema de control de par por cuatro giróscopos

The dynamic model of a Four Control Moment Gyroscope (4-CMG) is traditionally obtained after computing the derivative of the angular momentum equation. Although this approach leads to a simple dynamic model, new models have been introduced due to terms not taken into account during the computation of the angular momentum equation. In this paper, a new dynamic model for a 4-CMG based on the Newton-Euler algorithm, which is well accepted in Robotics, was developed. This new approach produces a complete dynamic model. © The authors; licensee Universidad Nacional de Colombia

Resumen de los trabajos realizados en fitomejoramiento en el cultivo de la soya.

Soya-Soja- Glycine ma

An experimental study of process variables in turning operations of Ti 6Al 4V and Cr Co spherical prostheses

[EN] Ti 6Al 4V and Cr Co alloys are extensively used in manufacturing prostheses due to their biocompatibility, high strength-to-weight ratio and high resistance to corrosion and wear. However, machining operations involving Ti 6Al 4V and Cr Co alloys face a series of difficulties related to their low machinability which complicate the process of controlling the quality levels required in these parts. The main objective of this paper is to study the influence of cutting parameters, machine tool control accuracy and metrology procedures on surface roughness parameters and form errors in contouring operations of Ti 6Al 4V and Cr Co workpieces. The machining performance of the two biocompatible materials is compared, focusing the study on part quality at low feed per revolution and the stochastic nature of plastic deformations at this regime. The results showed a better surface roughness control for Ti 6Al 4V, whereas for Cr Co alloys, the performance presents high variability. In the case of form errors (sphericity), contouring errors and metrology procedures are important factors to be considered for quality assurance. In addition, the study analyses the correlation of the machining performance with different sensor signals acquired from a low cost non-intrusive multi-sensor, showing a high correlation of signals from acoustic emission sensors and accelerometers in the machining of spherical features on Ti 6Al 4V parts. The findings of this research work can be taken into account when designing prostheses components and planning their manufacturing processes.This work was partially supported by Fundacio Caixa-Castello Bancaixa under the research project INV-2009-39. The authors are grateful to Miguel Angel Aymerich and Arcadi Sanz, who assisted in the experimental part. The authors would also like to extend their acknowledgments to Lafitt Company for its collaboration. Additional support was provided by Tecnologico de Monterrey through the Research Chair in Mechatronics and Intelligent Machines.Abellán Nebot, JV.; Siller, H.; Vila, C.; Rodríguez, C. (2012). An experimental study of process variables in turning operations of Ti 6Al 4V and Cr Co spherical prostheses. International Journal of Advanced Manufacturing Technology. 63(9-12):887-902. doi:10.1007/s00170-012-3955-0S887902639-12Balazic M, Kopac J, Jackson MJ, Ahmed W (2007) Review: titanium and titanium alloy applications in medicine. Int J Nano Biomater 1:3–34Long M, Rack HJ (1998) Titanium alloys in total joint replacement—a materials science perspective. Biomaterials 19:1621–1639Ohkubo C, Watanabe I, Ford JP, Nakajima H, Hosoi T, Okabe T (2000) The machinability of cast titanium and Ti–6Al–4 V. Biomaterials 21:421–428Yang X, Liu CR (1999) Machining titanium and its alloys. Mater Sci Technol 3:107–139Barry J, Byrne G, Lennon D (2001) Observations on chip formation and acoustic emission in machining Ti–6Al–4 V alloy. Int J Mach Tools Manuf 41:1055–1070Ezugwu EO (2005) Key improvements in the machining of difficult-to-cut aerospace alloys. Int J Mach Tools Manuf 45:1353–1367Ezugwu EO, Da Silva RB, Bonney J, Machado AR (2005) Evaluation of the performance of CBN tools when turning Ti–6Al–4 V. Int J Mach Tools Manuf 45:1009–1014Aspinwall DK, Dewes RC, Mantle AL (2005) The machining of gamma-TiAl intermetallic alloys. CIRP Ann 54:99–104López de Lacalle LN, Pérez-Bilbatua J, Sánchez JA, Llorente JI, Gutierrez A, Albóniga J (2000) Using high pressure coolant in the drilling and turning of low machinability alloys. Int J Adv Manuf Technol 16:85–91Aydin AK (1991) Evaluation of finishing and polishing techniques on surface roughness of chromium–cobalt castings. 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Int J Mach Tools Manuf 48:275–288Gadelmawla ES, Koura MM, Maksoud TMA, Elewa IM, Soliman HH (2002) Roughness parameters. J Mater Process Technol 123:133–145Stephenson DA, Agapiou JS (1997) Metal cutting theory and practice. Marcel Dekker, New YorkRamesh R, Mannan MA, Poo AN (2000) Error compensation in machine-tools—a review. Part I: geometric, cutting-force induced and fixture-dependent errors. Int J Mach Tools Manuf 40:1235–1256Ramesh R, Mannan MA, Poo AN (2000) Error compensation in machine-tools—a review. Part II: thermal errors. Int J Mach Tools Manuf 40:1257–1284López de Lacalle LN, Lamikiz A (2009) Machine-tools for high performance machining. Springer, LondonRamesh R, Mannan MA, Poo AN (2005) Tracking and contour error control in CNC servo systems. Int J Mach Tools Manuf 45:301–326Liang M, Mgwatu M, Zuo M (2001) Integration of cutting parameter selection and tool adjustment decisions for multipass turning. Int J Adv Manuf Technol 17:861–869Feng CXJ, Wang X (2002) Development of empirical models for surface roughness prediction in finish turning. Int J Adv Manuf Technol 20:348–356Benardos PG, Vosniakos GC (2003) Predicting surface roughness in machining: a review. Int J Mach Tools Manuf 43:833–844Schwenke H, Knapp W, Haitjema H, Weckenmann A, Schmitt R, Delbressine F (2008) Geometric error measurement and compensation of machines—an update. CIRP Ann 57:660–675Siller H, Rodriguez CA, Ahuett H (2006) Cycle time prediction in high-speed milling operations for sculptured surface finishing. J Mater Process Tech 174:355–362Liu K, Melkote SN (2006) Effect of plastic side flow on surface roughness in micro-turning processes. Int J Mach Tools Manuf 46:1778–1785Grzesik W (1996) A revised model for predicting surface roughness in turning. Wear 194:143–148Boothroyd G, Knight WA (1989) Fundamentals of machining and machine-tools. 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Turnover and stability in the deep sea: Benthic foraminifera as tracers of Paleogene global change

Benthic foraminifera are the most common meiofaunal unicellular deep-sea biota, forming skeletons used as proxies for past climate change. We aim to increase understanding of past non-analog oceans and ecosystems by evaluating deep-sea benthic foraminiferal responses to global environmental changes over latest Cretaceous through Oligocene times (67–23 million years ago). Earth suffered an asteroid impact at the end of the Cretaceous (~instantaneous; 66 Ma), episodes of rapid global warming during the Paleocene-Eocene Thermal Maximum (PETM; ~56 Ma) and other hyperthermals (millennial timescales), followed by gradual, but punctuated cooling (timescales of hundred thousands of years) from a world without polar ice sheets to a world with a large Antarctic ice sheet. Here we present the first compilation of quantitative data on deep-sea foraminifera at sites in all the world''s oceans, aiming to build a first unique, uniform database that allows comparison of deep-sea faunal turnover across the uppermost Cretaceous through Paleogene. We document variability in space and time of benthic foraminiferal diversity: lack of extinction at the asteroid impact even though other marine and terrestrial groups suffered mass extinction; major extinction at the PETM followed by recovery and diversification; and gradual but fundamental turnover during gradual cooling and increase in polar ice volume (possibly linked to changes in the oceanic carbon cycle). High latitude cooling from ~45 Ma on, i.e., after the end of the Early Eocene Climate Optimum (53.2–49.2 Ma), may have made the middle Eocene a critical period of several millions of years of faunal turnover and establishment of latitudinal diversity gradients. This compilation thus illuminates the penetration of global change at very different rates into the largest and one of the most stable habitats on Earth, the deep sea with its highly diverse biota

Updating a Paleogene magnetobiochronological time scale through graphical integration

All studies focused on the evaluation of paleoecological variability over geological time must be linked to a specific age or time interval, which can be defined using different time scales (biostratigraphic, chronostratigraphic, geochronological or orbital). Therefore, integrated time scales are essential to allow comparisons of data from different locations and/or to assess evolutionary and other events through time. Here we use a new method to update a Paleogene magnetobiochronological time scale, with the following contributions: • The update of the Paleogene magnetobiochronological scale was made by graphical correlation with new age models and adding calcareous nannoplankton and planktonic foraminiferal biozones from different authors. • An excel file structure was proposed to plot any kind of data in MATLAB software, as long as they are associated with some of the scales shown in our updated version of Paleogene magnetobiochronology. • The excel file structure facilitates the analysis of long-term trends of taxonomic groups throughout the Paleogene, and of their evolution in a period characterized by intense climate variability

Underlying SUSY in a generalized Jaynes–Cummings model

We present a general qubit-boson interaction Hamiltonian that describes the Jaynes–Cummings model and its extensions as a single Hamiltonian class. Our model includes non-linear processes for both the free qubit and boson field as well as non-linear, multi-boson excitation exchange between them. It shows an underlying algebra with supersymmetric quantum mechanics features allowing an operator based diagonalization that simplifies the calculations of observables. As a practical example, we show the evolution of the population inversion and the boson quadratures for an initial state consisting of the qubit in the ground state interacting with a coherent field for a selection of cases covering the standard Jaynes–Cummings model and some of its extensions including Stark shift, Kerr-like, intensity dependent coupling, multi-boson exchange and algebraic deformations

Determinants of food insecurity among smallholder farmer households in Central America: recurrent versus extreme weather-driven events

To ensure food security among rural communities under a changing climate, policymakers need information on the prevalence and determinants of food insecurity, the role of extreme weather events in exacerbating food insecurity,and the strategies that farmers use to cope with food insecurity. Using household surveys in Guatemala and Honduras, we explore the prevalence of food insecurity among smallholder farmers on both a recurrent (seasonal)and episodic (resulting from extreme weather events) basis, analyze the factors associated with both types of food insecurity, and document farmer coping strategies. Of the 439 households surveyed, 56% experienced recurrent foodinsecurity, 36% experienced episodic food insecurity due to extreme weather events, and 24% experienced both types. Food insecurity among smallholder farmers was correlated with sociodemographic factors (e.g., age, education,migration) and asset ownership. The factors affecting food insecurity differed between type and prevalence of food insecurity. Our results highlight the urgent need for policies and programs to help smallholder farmers improve theiroverall food security and resilience to extreme weather shocks. Such policies should focus on enhancing farmer education levels, securing land tenure, empowering women, promoting generational knowledge exchange, and providing emergency food support in the lean season or following extreme weather events