843 research outputs found

    On the accuracy of modelling the dynamics of large space structures

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    Proposed space missions will require large scale, light weight, space based structural systems. Large space structure technology (LSST) systems will have to accommodate (among others): ocean data systems; electronic mail systems; large multibeam antenna systems; and, space based solar power systems. The structures are to be delivered into orbit by the space shuttle. Because of their inherent size, modelling techniques and scaling algorithms must be developed so that system performance can be predicted accurately prior to launch and assembly. When the size and weight-to-area ratio of proposed LSST systems dictate that the entire system be considered flexible, there are two basic modeling methods which can be used. The first is a continuum approach, a mathematical formulation for predicting the motion of a general orbiting flexible body, in which elastic deformations are considered small compared with characteristic body dimensions. This approach is based on an a priori knowledge of the frequencies and shape functions of all modes included within the system model. Alternatively, finite element techniques can be used to model the entire structure as a system of lumped masses connected by a series of (restoring) springs and possibly dampers. In addition, a computational algorithm was developed to evaluate the coefficients of the various coupling terms in the equations of motion as applied to the finite element model of the Hoop/Column

    How Large Is the Private Sector in Africa? Evidence from National Accounts and Labor Markets

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    In recent years, the private sector has been recognized as a key engine of Africa's economic development. Yet, the most simple and fundamental question remains unanswered: how large is the African private sector? We present novel estimates of the size of the private sector in 50 African countries derived from the analysis of national accounts and labor market data. Our results point to a relatively large size of the African private sector. National account data shows that this accounts for about 2/3 of total investments, 4/5 of total consumption and 3/4 of total credit. In relative terms, large private sector countries are concentrated in Western Africa (Cote d'Ivoire, Guinea, Niger, Senegal and Togo), Central Africa (Cameroun, Republic of Congo) and Eastern Africa (Kenya, Sudan, Uganda and Tanzania), with the addition of Mauritius. Countries with small private sectors include a sample of oil-exporters (Algeria, Angola, Equatorial Guinea, Libya and Nigeria), some of the poorest countries in the continent (Burundi, Burkina Faso, Guinea Bissau, Mali and Sao Tome e Principe), Zambia and Botswana. Over the last ten years, the size of the private sector has been contracting significantly in oil exporting countries, although the variation in its size does not appear to be significantly correlated with growth performance. Labor market data reinforces the idea of a large private sector, which provides about 90% of total employment opportunities. However, most of this labor is informal and characterized by low productivity: permanent wage jobs in the private sector account on average for only 10% of total employment (a share similar to that provided by public administration and state owned enterprises). South Africa is the notable exception, with formal wage employment in the private sector representing 46% of total employment. Finally, we find evidence of negative private sector earning premiums, suggesting that market distortions abound. These are likely to prevent the efficient allocation of human resources, and to reduce the overall productivity of the African economies.private sector size, private sector development, private consumption, private investment, national accounts, private sector employment, private sector earnings, labor markets, Africa

    Importance of carbon solubility and wetting properties of nickel nanoparticles for single wall nanotube growth

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    Optimized growth of Single Wall Carbon Nanotubes requires a full knowledge of the actual state of the catalyst nanoparticle and its interface with the tube. Using Tight Binding based atomistic computer simulations, we calculate carbon adsorption isotherms on nanoparticles of nickel, a typical catalyst, and show that carbon solubility increases for smaller nanoparticles that are either molten or surface molten under experimental conditions. Increasing carbon content favors the dewetting of Ni nanoparticles with respect to sp2 carbon walls, a necessary property to limit catalyst encapsulation and deactivation. Grand Canonical Monte Carlo simulations of the growth of tube embryos show that wetting properties of the nanoparticles, controlled by carbon solubility, are of fundamental importance to enable the growth, shedding a new light on the growth mechanisms

    The dynamics and control of large flexible space structures, 8

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    A development of the in plane open loop rotational equations of motion for the proposed Spacecraft Control Laboratory Experiment (SCOLE) in orbit configuration is presented based on an Eulerian formulation. The mast is considered to be a flexible beam connected to the (rigid) shuttle and the reflector. Frequencies and mode shapes are obtained for the mast vibrational appendage modes (assumed to be decoupled) for different boundary conditions based on continuum approaches and also preliminary results are obtained using a finite element representation of the mast reflector system. The linearized rotational in plane equation is characterized by periodic coefficients and open loop system stability can be examined with an application of the Floquet theorem. Numerical results are presented to illustrate the potential instability associated with actuator time delays even for delays which represent only a small fraction of the natural period of oscillation of the modes contained in the open loop model of the system. When plant and measurement noise effects are added to the previously designed deterministic model of the hoop column system, it is seen that both the system transient and steady state performance are degraded. Mission requirements can be satisfied by appropriate assignment of cost function weighting elements and changes in the ratio of plant noise to measurement noise

    The dynamics and control of large flexible space structures, part 11

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    A mathematical model is developed to predict the dynamics of the proposed Spacecraft Control Laboratory Experiment during the stationkeeping phase. The Shuttle and reflector are assumed to be rigid, while the mass connecting the Shuttle to the reflector is assumed to be flexible with elastic deformations small as compared with its length. It is seen that in the presence of gravity-gradient torques, the system assumes a new equilibrium position primarily due to the offset in the mass attachment point to the reflector from the reflector's mass center. Control is assumed to be provided through the Shuttle's three torquers and throught six actuators located by painrs at two points on the mass and at the reflector mass center. Numerical results confirm the robustness of an LQR derived control strategy during stationkeeping with maximum control efforts significantly below saturation levels. The linear regulator theory is also used to derive control laws for the linearized model of the rigidized SCOLE configuration where the mast flexibility is not included. It is seen that this same type of control strategy can be applied for the rapid single axis slewing of the SCOLE through amplitudes as large as 20 degrees. These results provide a definite trade-off between the slightly larger slewing times with the considerable reduction in over-all control effort as compared with the results of the two point boundary value problem application of Pontryagin's Maximum Principle

    The dynamics and control of large flexible space structures X, part 1

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    The effect of delay in the control system input on the stability of a continuously acting controller which is designed without considering the delay is studied. The stability analysis of a second order plant is studied analytically and verified numerically. For this example it is found that the system becomes unstable for a delay which is equivalent to only 16 percent of its natural period of motion. It is also observed that even a small amount of natural damping in the system can increase the amount of delay that can be tolerated before the onset of instability. The delay problem is formulated in the discrete time domain and an analysis procedure suggested. The maximum principle from optimal control theory is applied to minimize the time required for the slewing of a general rigid spacecraft. The slewing motion need not be restricted to a single axis maneuver. The minimum slewing time is calculated based on a quasi-linearization algorithm for the resulting two point boundary value problem. Numerical examples based on the rigidized in-orbit model of the SCOLE also include the more general reflector line-of-sight slewing maneuvers

    Efficacite agronomique du compost a base de la biomasse du « neem » et de l’anacarde sur des cultures maraicheres dans la zone des Niayes au Senegal

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    Dans la zone des Niayes au Sénégal, les sols sont pauvres en matières organiques et en éléments nutritifs nécessaires aux plantes. Les engrais chimiques utilisés pour améliorer les rendements ont engendré une forte pollution des sols et de la  nappe phréatique. L’objectif de cette étude est d’évaluer la valeur agronomique du compost fabriqué à base des feuilles de neem et d’anacarde, de fiente de volaille et de la cendre de bois. Le compost obtenu présente un rapport C/N de 15,49 et des teneurs en N - P - K de 9,1, 14,8 et 7,3 mg/kg, respectivement. Les différentes doses du compost testées  sur  des  cultures  maraichères  ont  révélé  des  performances  agronomiques  meilleures par rapport aux témoins. La dose T3 (30 t/ha) a induit les meilleurs rendements pour la tomate (27,213 t/ha) et l’oignon (105,263 t/ha). Pour le chou et la pomme de terre, la dose T1 (10 t/ha) a permis d’obtenir les meilleurs rendements (144,533 t/ha et 55,163 t/ha, respectivement). Par contre pour le poivron, la dose T2 (20 t/ha) a favorisé les meilleurs rendements (32,534 t/ha) avec un poids moyen (81,748g) et un nombre de fruits/récolte considérablement plus élevé. Ce compost pourrait constituer une alternative réelle à l’utilisation abusive de l’engrais minéral dans la zone agricole des Niayes.Mots clés : Compost, Neem, Feuilles d’anacarde, Maraichage, Sénéga

    The dynamics and control of large flexible space structures, part 7

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    A preliminary Eulerian formulation of the in-plane dynamics of the proposed spacecraft control laboratory experiment configuration is undertaken when the mast is treated as a cantilever type beam and the reflector as a lumped mass at the end of the beam. Frequency and mode shapes are obtained for the open loop model of the beam system and the stability of closed loop control systems is analyzed by both frequency and time domain techniques. Environmental disturbances due to solar radiation pressure are incorporated into models of controlled large flexible orbiting platforms. Thermally induced deformations of simple beam and platform type structures are modelled and expressions developed for the disturbance torques resulting from the interaction of solar radiation pressure. Noise effects in the deterministic model of the hoop/column antenna system are found to cause a degradation in system performance. Appropriate changes in the ratio of plant noise to the measurement noise and/or changes in the control weighting matrix elements can improve transient and steady state performance

    La lutte contre les schistosomoses en Afrique de l'Ouest

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    Deuxième endémie parasitaire du Mali, les schistosomoses sont irrégulièrement réparties dans le pays. #Schistosoma haematobium atteint 2 millions et demi de sujets et est assez largement répandu. #S. mansoni est focalisé dans les régions rizicoles du centre du pays, le pays dogon et la région de Kayes. La lutte contre les schistosomoses a commencé dans les années soixante-dix par des programmes localisés. Ces programmes ont été réunis en un programme national de lutte en 1982. La décentralisation et l'intégration des activités aux structures nationales de santé a été décidée en 1986. La mise en oeuvre du programme national de lutte a débuté aussitôt après par la formation des équipes périphériques et l'équipement des structures sanitaires. Les stratégies de base retenues sont le dépistage des cas et la chimiothérapie à la fois dans les centres de santé et les écoles. L'éducation pour la santé, l'assainissement et l'approvisionnement en eau potable sont associés. Une surveillance épidémiologique est organisée. Trois phases successives ont conduit le programme national de lutte à être intégré à l'ensemble des activités des structures de santé périphériques. Les difficultés rencontrées sont évoquées. (Résumé d'auteur

    The dynamics and control of large flexible space structures-V

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    A general survey of the progress made in the areas of mathematical modelling of the system dynamics, structural analysis, development of control algorithms, and simulation of environmental disturbances is presented. The use of graph theory techniques is employed to examine the effects of inherent damping associated with LSST systems on the number and locations of the required control actuators. A mathematical model of the forces and moments induced on a flexible orbiting beam due to solar radiation pressure is developed and typical steady state open loop responses obtained for the case when rotations and vibrations are limited to occur within the orbit plane. A preliminary controls analysis based on a truncated (13 mode) finite element model of the 122m. Hoop/Column antenna indicates that a minimum of six appropriately placed actuators is required for controllability. An algorithm to evaluate the coefficients which describe coupling between the rigid rotational and flexible modes and also intramodal coupling was developed and numerical evaluation based on the finite element model of Hoop/Column system is currently in progress
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