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Search Results: 1 - 10 of 64 matches for " Aage Skullestad "
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Modeling and control of a gravity gradient stabilised satellite
Aage Skullestad
Modeling, Identification and Control , 1999, DOI: 10.4173/mic.1999.1.1
Abstract: This paper describes attitude control, i.e., 3-axes stabilisation and pointing, of a proposed Norwegian small gravity gradient stabilized satellite to be launched into low earth orbit. Generally, a gravity gradient stabilised system has limited stability and pointing capabilities, and wheels and/or magnetic coils are added in order to improve the attitude control. The best attitude accuracy is achieved using wheels, which can give accuracies down to less than one degree, but wheels increase the complexity and cost of the satellite. Magnetic coils allow cheaper satellites, and are an attractive solution to small, inexpensive satellites in low earth orbits and may provide an attitude control accuracy of a few degrees. Scientific measurements often require accurate attitude control in one or two axes only. Combining wheel and coil control may, in these cases, provide the best solutions. The simulation results are based on a linearised mathematical model of the satellite.
Identification of Vibration Parameters in a Spacecraft using Kalman Filters, Numerical Search and Phase-Locked Loops
Aage Skullestad
Modeling, Identification and Control , 2000, DOI: 10.4173/mic.2000.2.1
Abstract: Communication in space requires antennas that are accurately pointed. Often antennas are mounted on lightly damped space structures, that are easily set into vibration, which decreases the pointing accuracy of the antennas. Knowing the vibration frequencies simplifies the control tasks and may also improve the pointing accuracy. This paper describes identification of vibration frequencies in a space structure, based on frequency weighted Kalman filters, a Gauss-Newton numerical search method and phase-locked loops.
Control of a gravity gradient stabilised satellite using fuzzy logic
Aage Skullestad,Kjetil Olsen,Stein Rennehvammen,H?vard Fl?ystad
Modeling, Identification and Control , 2001, DOI: 10.4173/mic.2001.3.2
Abstract: This paper describes attitude control of a small gravity gradient stabilised satellite. A gravity gradient stabilised satellite has limited stability and pointing capabilities, and magnetic coils are added in order to improve the accuracy of the attitude control. The magnetic coils are controlled using a fuzzy logic controller, based on a combination of membership functions and rules. The control of the pitch axis is separated from the roll and azimuth axes and excellent pitch angle accuracy is achieved. The roll and azimuth axes are controlled using a common magnetic coil, that has a non-linear and time-varying torque characteristic and, therefore, accurate roll and azimuth angular control become much more difficult to achieve. However, combining one roll controller and two azimuth controllers result in an acceptable roll and azimuth angular accuracy after a few orbital periods.
Identification of Vibration Parameters in a Spacecraft using Nonparametric and Parametric Methods
?ge Skullestad
Modeling, Identification and Control , 1996, DOI: 10.4173/mic.1996.4.1
Abstract: This article describes identification of vibration frequencies in a spacecraft. A mathematical model of a simple spacecraft is built in MATLAG code, and after a theoretical overview, this spacecraft model is simulated, using a torque excitation, to generate time series measurements. These measurements are subsequently used in the identification experiments. Parametric methods, here represented by AR, ARX, ARMA and ARMAX, are the main concern, but nonparametric methods are also discussed."
Preben Sepstrup: The Electronic Dilemma of TV Advertising. Documentation, the Reaction of Business, Cultural consequences and Consumer Implications. Working paper no. 2
Aage Erhardtsen
MedieKultur : Journal of Media and Communication Research , 1987,
Abstract:
R yking og hjerte-kard delighet
Aage Tverdal
Norsk Epidemiologi , 2009,
Abstract: SAMMENDRAG I en oppf lgingsstudie av 44 374 menn og kvinner i alder 35-49 r var d deligheten av hjerteinfarkt tre til fem ganger h yere blant sigarettr ykere enn blant aldri sigarettr ykere. Hos sigarettr ykere kte d deligheten med kende antall sigaretter. D deligheten av hjerteinfarkt blant kvinner som r ykte 20 sigaretter daglig var omtrent den samme som blant menn som aldri hadde r ykt sigaretter. Hos menn ga r yking av annet enn sigaretter omtrent samme overd delighet som r yking av sigaretter. For menn som ogs m tte til en unders kelse tre til fem r senere var infarktd deligheten hos dem som var r ykere ved en av unders kelsene og ikke-r ykere ved den andre, mellom niv et for de som begge ganger enten var r ykere eller ikke-r ykere. For menn var infarktd deligheten hos de som hadde sluttet r yke for mer enn 5 r siden litt h yere enn hos aldri r ykere, mens den hos kvinner var klart lavere. D deligheten av hjerneslag for sigarettr ykere l to til fire ganger h yere enn den for aldri sigarettr ykere. For menn var det ingen sammenheng mellom antall sigaretter og slagd delighet, mens det var en sammenheng for hjertefriske kvinner. Sigarettr yking ga ogs en overd delighet av andre kardiovaskul re sykdommer. For menn kom denne f rst til syne ved 20+ sigaretter. Tverdal A.
D delighet av hjerteinfarkt i fylker og helseregioner i perioden 1951-94
Aage Tverdal
Norsk Epidemiologi , 2009,
Abstract: SAMMENDRAG Basert p data fra Statistisk sentralbyr er d deligheten av hjerteinfarkt i fylkene studert. D deligheten i alder 40-69 r kte markert i alle fylker mellom 1951-55 og 1971-75 for deretter avta jevnt fram til 1991- 94. For menn er ingen av fylkene kommet ned p niv et i 1951-55, mens for kvinner er d deligheten i 1991-94 lavere i fire fylker. Endringene varierte i st rrelse mellom fylkene slik at rangeringen av fylkene etter d delighet er forskjellig i f rste og siste periode. Det var prosentvis en svakere kning i d deligheten for kvinner enn for menn fra 1951-55 til 1971-75 og ogs en svakere nedgang fra 1971-75 til 1991-94. Den n v rende tendens er meget gunstig i alle helseregioner. Tverdal A. Mortality from myocardial infarction in Norwegian counties and health regions 1951-94. Nor J Epidemiol ENGLISH SUMMARY Data from Statistics Norway on mortality from myocardial infarction in Norwegian counties have been studied. The mortality in the age group 40-69 years increased distinctly between 1951-55 and 1971-75 in all counties followed by a gradual decrease until 1991-94. For men none of the counties have yet reached the level of 1951-55, whilst for women the mortality is now lower in four of the counties. The changes varied in magnitude between the counties so that the ranking of the counties by mortality differed between the first and the last period. In per cent the increase in mortality was weaker for women than for men from 1951-55 to 1971-75, and there was also a weaker decline from 1971-75 to 1991-94 in women.
Alcohol in Greenland 1951–2010: consumption, mortality, prices
Hans Aage
International Journal of Circumpolar Health , 2012, DOI: 10.3402/ijch.v71i0.18444
Abstract: Background. Fluctuations in alcohol consumption in Greenland have been extreme since alcohol became available to the Greenland Inuit in the 1950s, increasing from low levels in the 1950s to very high levels in the 1980s – about twice as high as alcohol consumption in Denmark. Since then, consumption has declined, and current consumption is slightly below alcohol consumption in Denmark, while alcohol prices are far above Danish prices. Objective. Description of historical trends and possible causal connections of alcohol prices, alcohol consumption and alcohol-related mortality in Greenland 1951–2010 as a background for the evaluation of the impact of various types of policy. Design. Time series for Greenland 1951–2010 for alcohol prices, consumption and mortality are compiled, and variation and correlations are discussed in relation to various policies aimed at limiting alcohol consumption. Corresponding time series for Denmark 1906–2010 are presented for comparison. Results. The trends in alcohol prices and consumption followed each other rather closely until the 1990s in Greenland and the 1980s in Denmark. At this time, consumption stabilised while prices decreased further, but the effect of prices upon consumption is strong, also in recent years. A trend in Greenlandic mortality similar to consumption is discernible, but not significant. Among alcohol-related deaths cirrhosis of the liver is less prevalent whilst accidents are more prevalent than in Denmark. Conclusions. The effect of alcohol excise taxes and rationing upon consumption is evident. The stabilisation and subsequent decline in consumption since the mid-1990s, while alcohol prices decreased persistently, does not preclude continued effects of prices. On the contrary, price effects have been neutralised by other stronger causes. Whether these are government anti-alcohol campaigns or a cultural change is not clear.
Betydning og grammatik – i et semiotisk perspektiv
Per Aage Brandt
Tidsskrift for Sprogforskning , 2004,
Abstract: Grammar is directly rooted in human phenomenology, in the sense that we all immediately experience the salient and meaningful compositional events in the syntactic structure of currently heard or read texts. Since we have this introspective access to syntax and can grasp the grammatical properties, grammatical analysis can be based on experiential phenomenal composition. I intend to show some basic principles of such an analysis and of a possible graphic form of representation that can work in a pedagogical settting. The structure that appears through long term use of phenomenological and ‘stemmatic’ analysis of sentence syntax points to the existence of a generic and canonical grammar underlying specific construction grammars; this canonical format is, I suggest to think, rooted in human cognition and its semantic conceptual formats. The article finally presents a text sample from a German newspaper, transcribed according to the stemmatic principles.
Nogle problemer i empiriske TV-receptionsunders gelser
Niels Aage Nielsen
Massekultur & Medier , 1984,
Abstract: Nogle problemer i empiriske TV-receptionsunders gelser
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