• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 1161
  • 291
  • 244
  • 221
  • 109
  • 103
  • 30
  • 28
  • 28
  • 28
  • 28
  • 28
  • 28
  • 24
  • 22
  • Tagged with
  • 2908
  • 337
  • 315
  • 280
  • 246
  • 183
  • 159
  • 152
  • 151
  • 149
  • 142
  • 140
  • 127
  • 126
  • 121
  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1081

Utvärdering av en FPGA för rymdbruk / Evaluation of an FPGA for space applications

Gustavsson, Per, Håkansson, Pär January 2005 (has links)
A new FPGA suitable for space applications has just reached the market. To investigate whether there are any possible flaws or limitations similar to those previously seen on FPGAs, an evaluation has to be done. This master thesis contains the evaluation of this new radhard FPGA with focus on possible design limitations and package related electrical phenomena.Areas evaluated: Ground-/VDD bounce, Cross talk, Rise time sensitivit, Power cycling, Power consumption, Place and route tool, Radiation hardnessThis report contains all steps in the evaluation. From method to measurements, comparisons, theory, results and conclusions. In the evaluation work, special effort has been made to develop designs that really stress the FPGA to find potential problems. All problems found are dealt with in this report.Results: Ground-/VDD bounce measurements showed that devices using a fast slew rate resulted in TTL-level violation. However, by separating sensitive signals and SSOs in different I/O banks it is possible to work around the problem. Cross talk measurements has shown that the phenomena causes problems when using a long rise time input with toggling outputs placed next to the signal. Power cycling did not result in any alarming inrush currents. Regular power up showed an unwanted behaviour with pulses on all I/Os right before power on reset kicked in. When comparing the tool value with measurements regarding power consumption it was clear that it differed as much as 40-50%. The FPGA consumes 40-50% more power than what the power calculator tool estimates.
1082

Conceptual design of miniature vegetation cutter for demining activities in difficult terrain – an evaluation : Intended for the Chouf Mountains, Lebanon

Sjölander, Emmily, Risén, Hanna January 2009 (has links)
A conceptual design of a miniature vegetation cutter for use in minefields in southern Lebanon has been developed and the authors have evaluated its mechanical properties focusing on the stress in welded joints using the CAD software Pro/Engineer Wildfire 4.0. The conceptual design has been developed in the field in close cooperation with field staff from MAG Lebanon. The requirements from the field specify that the cutter should have a cutting range of 80 cm, be equipped with adjustable covers, weigh less than 200 kg, be fitted to a commercial hydraulic excavator, and have the ability to cut vegetation, bushes and small trees including (olive) trees with a diameter up to 10 cm. The miniature cutter is to be manufactured in the field and fitted to a commercial hydraulic ex­cava­tor (Caterpillar 301.6C). It consists of a rotor on which eight cutting blades, alternatively chains, are attached in a helix formation. The cutter is protected by adjustable covers. When in operation, the rotor spins at 750 revolutions per minute whereby the blades cut through the shrubs and bushes in the cutter’s path. The blades are mounted in a T-shape on arms, which are fastened to the rotor by a pin joint between two brackets (each) on the rotor. Blades and arms are to be welded together, as are the brackets to the rotor. These welded joints are the primary focus of the report. 3D CAD models have been created and analysed in PTC Pro/Engineer Wildfire 4.0 to ascertain that the stress in the joints will not exceed the yield strength of the weld consumables, which should be 500 MPa. Ideally, the stress in the joints would be half the yield strength. Type of bearings and a hydraulic motor have been selected for the cutter. Based on the specifica­tions of the hydraulic motor an approximation of the forces acting on the weld joints in the case of an accidental stop (e.g. collision with a rock) has been calculated, and entered into the CAD software. Also, an approximation of the size (diameter) of branches the cutter would be able to tear apart in the case of branches getting stuck has been calculated and shown to be about 14.6 mm. Based on this, it is estimated that the cutter should be used only in areas where the shrubbery is of 20-30 mm in diameter, maximum. Considering this, and the relative light weight of the cutter, it is not likely that the cutter will be able to cut through the larger olive trees as requested, but it is considered that the tool still could be a valuable asset for mine clearing in Lebanon. In order to cut through thicker trees, it would be necessary to increase the power supply to the cutting system as well as the sturdiness of the cutting parts. Finding the required power and technical solutions for this demands further research which does not fit within the time frame for this report. A preliminary weight approximation shows that the cutter will weigh roughly 170 kg, which falls below the limit of 200 kg and leaving some room for the bearings to be added. The results from the stress analyses show that the stress in the welded joints falls well below the yield limit of 500 MPa, but not below 250 MPa. Still, the stress in all the welded joints is shown to be less than 300 MPa or at 40 % of the limit, which may still be acceptable. The end user will have to decide whether this is an acceptable safety margin before manufacturing the cutter and if it is not, measures will need to be taken to reinforce the weld joints and try to minimise the stress concentration in them.
1083

Construction and evaluation of a magnetoresistive ground penetrating radar system

Blomqvist, Mikael January 2011 (has links)
This Master Thesis examines the possibility to apply a magnetometer developed by the Ångstöm space technology center to a small magnetic ground penetrating radar system with dimension in the order of one dm³. The magnetometer is broadband (DC-1GHz) and miniaturized. Loop antennas are used to transmit the signal.    A series of experiments have been performed in order to characterize the system, mainly examining the ability to determine distance to a target, using continuous sine wave signals and pulse trains. Standing wave patterns are formed between antenna and target and can be used for determining distance in the continuous case. When using a pulse train, the echo from the target could not be resolved using the current experiment set up, distance could therefore not be determined.
1084

Abiotic Stresses to Vegetation Re-establishment in a Cutover Bog Contaminated with Seawater

Montemayor, Marilou B. January 2006 (has links)
Part of a cutover bog in Pokesudie Island, New Brunswick, Canada was contaminated with seawater and was still largely devoid of vegetation 5 years after the event and was consequently chosen for study. The study area consisted of rectangular fields with cambered surface that sloped down (2%) to the drainage ditches on both sides. Across this slope zones were created: Up-, Mid- and Low- areas on either side of the centerline of fields. Two field experiments were conducted to determine abiotic stresses to plant re-establishment in terms of hydrology and peat characteristics along this cambered surface. The general objective was to identify microsites or zones that could be suitable to the introduction of wetland halophytes <em>Juncus balticus</em> Willd. and <em>Spartina pectinata</em> Link obtained from nearby salt marshes. <br /><br /> In the first experiment, cylindrical <em>J. balticus</em> sods were transplanted into the Up- and Low- areas, at 1, 3, 5, 10 and 20 d of incubation (in May 2005) with measurements made on the Outer and Inner annular sod sections, replicated over 4 blocks. Moisture (% dry weight basis (dwb)) reached maximum values 1 day after transplantation, 84±0. 05 for Outer and 103±0. 07 for Inner sod section. Salinity (dS m<sup>-1</sup>) in sods due to ingress of sodium (Na<sup>+</sup> ) and chloride (Cl<sup>-</sup>) reached values of the surrounding peat 3 days after transplantation, 3. 52±1. 06 for Inner sod section and 4. 11±0. 99 for Outer sod section in Up-areas, and 1. 76±0. 24 for Inner sod section and 2. 57±0. 28 for Outer sod section in Low-areas. Maximum decrease in pH was at 5 days after transplantation, in Outer sod section in the Up-areas (from 5. 89 to 4. 88±0. 14) which was much higher than the pH range of 3-4 of the surrounding peat. This was due to the buffering capacity of calcium (Ca<sup>2+</sup>) and magnesium (Mg<sup>2+</sup>) in sods which did not change in concentration after 20 days of incubation. Therefore, Inner sod sections were less affected by the surrounding peat compared to the Outer sod sections, suggesting that a larger sod volume may alleviate stressful conditions for a longer time at transplantation and consequently allow greater time for adaptation. <br /><br /> In the second experiment, <em>J. balticus</em> and <em>S. pectinata</em> were transplanted on the 3 Locations Up-, Mid- and Low- areas, replicated over 10 blocks; and peat characteristics were measured at Depths 0-5, 5-10, 10-15 and 15-20 cm 5 times during the study period May-August 2005. Survival of <em>J. balticus</em> was poorest (27. 5±8. 3 %) in the Low-areas compared to 68. 5±8. 9 % in the Up- and 58. 5±8. 7% in the Mid- areas. <em>S. pectinata</em> survival was very good at all Locations (89±5. 3, 91. 6±3. 1 and 84. 2±4. 4 for Up-, Mid- and Low- areas, respectively) having better adaptation to early season waterlogged conditions. Waterlogged conditions resulted from a perched water table during the early part of the growing season (May-June) and were alleviated only upon the complete thaw of the frozen peat layer on 8 July. Thereafter, important changes in hydrology and peat characteristics occurred: water table depths decreased from -8. 5±1. 7 and -1. 6±1. 2 cm on 26 May, to -51. 5±2. 5 and -40. 7±2. 4 cm by 9 August in Up- and Low-areas, respectively; redox potentials at 12 cm depth increased from 26 June (190. 9±8, 175±10. 8 and 109. 2±29. 4 mV) to 9 August (282. 8±8, 302. 8±14. 3 and 312. 3±29. 6 mV) in the Up-, Mid- and Low-areas, respectively which showed that anaerobic conditions were maintained throughout the study period; decreased moisture content from 1256. 8±61. 9, 1667. 4±126. 3 and 1728. 6±153 on 30 May, to 851. 7±21. 2, 874. 6±47 and 1008. 2±57. 5 % dwb on 25 July) which caused increased dry bulk density (from 0. 07±0. 002, 0. 06±0. 003 and 0. 07±0. 01 to 0. 09±0. 003, 0. 09±0. 005 and 0. 08±0. 004) in the Up-, Mid- and Low-areas, respectively; and increased electrical conductivity (salinity) especially on the 0-5cm surface (from 1. 9±0. 13, 1. 8±0. 31 and 1. 5±0. 29 to 18±1. 9, 17. 5±1. 1 and 12. 2±1 dS m<sup>-1</sup>) which also caused decreased pH (from 3. 5±0. 04, 3. 5±0. 08 and 3. 6±0. 01 to 2. 85±0. 04, 2. 85±0. 01 and 2. 9±0. 03) in the Up-, Mid- and Low-areas, respectively. Therefore, spring flooding followed by high surface salinity in summer precludes plant establishment by seeding and explains the current lack of spontaneous revegetation. Waterlogged conditions were of greater magnitude and duration at lower elevation areas unfavourable to <em>J. balticus</em> survival but salinity levels were high in the Up- and Mid-areas. <br /><br /> In the subsequent part of the second experiment, plants of <em>J. balticus</em> and <em>S. pectinata</em> grown in the study area and those collected from marshes were divided into above- and below- ground parts and accumulation of salt ions in plant tissues were determined to understand the species' salt-tolerance mechanism, as well as the accumulation of potentially toxic levels of iron (Fe) and manganese (Mn). Both plant species had similar accumulations (mmol kg<sup>-1</sup> dry wt,) of Na<sup>+</sup> (474. 3±41 and 468. 3±31. 7, respectively) and Cl<sup>-</sup> (314. 9±21. 9 and 310. 5±27. 5, respectively) in the above-ground parts but differed in how they managed Na<sup>+</sup>. <em>J. balticus</em> accumulated more Na<sup>+</sup> in below-ground parts (659. 3±88. 7) and had limited transport to the above-ground parts, while <em>S. pectinata</em> accumulated and excreted Na<sup>+</sup> in the above-ground parts and had less accumulation in the below-ground parts (397. 4±25. 1). <em>S. pectinata</em> maintained (313. 1±23. 8 in marsh <em>vs. </em> 292. 4±26. 2 in bog) and <em>J. balticus</em> increased (84. 2±1. 2 in marsh <em>vs. </em> 531. 2±38. 6 in bog) K<sup>+</sup>-selectivity in the shoots, a key requirement for survival in saline conditions. Compared with their respective marsh plants, <em>S. pectinata</em> had more salinity-tolerance than <em>J. balticus</em> primarily through its maintenance of Ca<sup>2+</sup> (21. 5±1. 7 in marsh <em>vs. </em> 35. 6±3. 8 in bog) compared to a decrease in <em>J. balticus</em> (144. 7±12. 5 in marsh <em>vs. </em> 41±3. 7 in bog). Furthermore, Fe and Mn uptake in both species decreased but reached critical Fe-deficiency levels (1. 1±0. 1 mmol kg<sup>-1</sup> dry wt,) only in <em>S. pectinata</em> grown in drier areas. <br /><br /> It is concluded that local conditions of waterlogging (especially in lower elevation areas) and high salinity and low pH (notably in the upper elevation areas) were favourable to the survival of <em>S. pectinata</em> in all areas and <em>J. balticus</em> only in upper elevation areas. Sod transplanting may alleviate the acidity problem and depending on sod volume may delay the effects of harsh conditions of the cutover bog. However, long-term survival and growth of both species in drier areas may be constrained by deficiency in calcium in <em>J. balticus</em> and iron in <em>S. pectinata</em>.
1085

Doubling and Desire

Zepf, Diana January 2010 (has links)
This thesis proposes that an investigation into the phenomenon of doubling may engage architecture with a type of desire that has deep rooted connections with the complexities of human nature, with the very human condition of desiring to know who/what/where/when/how we are. It proposes that an experience of doubling is suggestive of a specific kind of affective space that tests this relationship, expanding into the interval we have formed between our body, its being and space. The proposal is to explore the material, spatial, and psychological characteristics of such a phenomenon - to understand the virtual space created through this doubling and its architectonic characteristics. The design ambition of this thesis is to construct an architectural fiction that engages with this doubling. If architecture has the capacity to embody the ambitions and anxieties of society, the work produced attempts to invoke, through choreographed doublings manifested by the movement of figure and light through constructions in time, that human condition of desire that is concerned with finding/defining itself in the unknown, not to provide an answer for what the unknown is, but to engage with its enigmatic nature. By engaging in the protean dynamics of doubling and desire, this thesis attempts to poeticize the interval between the body and its built environment.
1086

Path integral Langevin dynamics of complex molecular systems: from low-temperature quantum clusters to biomolecules

Ing, Christopher 22 October 2011 (has links)
This thesis presents an implementation of path integral molecular dynamics (PIMD) for sampling equilibrium and dynamical properties within the molecular modelling toolkit (MMTK) [J. Comp. Chem. 21, 79 (2000)], an open source Python package. Rigorous simulation using this code serves to benchmark this implementation as well as the robust- ness of the path integral Langevin equation as a thermostat [J. Chem. Phys. 133, 124104 (2010)]. PIMD is used to calculate equilibrium properties for clusters of HeN-CO2 at low- temperatures, with comparison to experimental and exact results. We characterize the convergence of structural and energetic properties as a function of path-integral discretiza- tion error. The radial and angular distribution of these clusters is studied as a function of size in the absence of rotation and bosonic exchange. These distributions are subsequently used to calculate vibrational shifts of CO2. This result is compared to high-accuracy path integral Monte Carlo simulations which include rotational and exchange effects. These sim- ulations indicate that the neglect of rotational degrees of freedom leads to an unphysical localization of helium atoms and incorrect vibrational shifts when compared to experiment. Approximate real-time quantum dynamics is presented for doped helium clusters using the ring-polymer molecular dynamics (RPMD) method. The accuracy of RPMD is tested iii for low-temperature simulations and compared to exact results. Preliminary calculation of the dynamics of the helium solvated CO2 dopant with respect to the center of mass of the cluster is presented. The effect of a cartesian integrator versus a normal-mode integrator for quantum dynamics is addressed. The path integral ground-state method is applied in order to calculate T = 0 properties. A convergence study of the ground-state energy of the quantum harmonic oscillator with respect to sampling time and path discretization is shown. As a final application of this implementation, a sugar in a periodic water box is simulated at T = 300K. The calculation of rotamer populations and a dipole autocorrelation indicate negligible change with the inclusion of quantum effects. This work offers a comprehensive foundation from which to base future PIMD centered research.
1087

Biomechanical assessment of locomotion in two rodent models of nervous system injury

Bennett, Sean W, 04 January 2010 (has links)
The adaptation of inverse dynamics was performed to quantitatively examine the subtle locomotor changes, previously undetectable, in rodent locomotion following nervous system injury. The first experiment performed an injury with known effects, a unilateral lesion of the medial and lateral branches of the left tibial nerve of Long-Evans rats, and measured the resulting data via inverse dynamics. Special effort was made to account for skin movement artefacts using a global optimization method for marker digitization. The second experiment attempted to apply this technique to Long-Evans rats with spinal hemisections at spinal level T-10. After the peripheral nerve injury to the tibial nerve branches, the main findings were that ankle joint still produces an extensor moment and positive power without the active contraction of the gastrocnemius m. It is possible that this phenomenon is due to passive contractile elements of the muscle and tendon. In addition, the knee and hip of the lesion leg stiffen, resulting in substantial reductions in moment generation and nearly total losses of both negative and positive power production. There were also compensations made by the opposite hindlimb and contralateral forelimb. The spinal cord hemisection produced subtle, complicated intra and interlimb changes in both joint moment and joint power analysis that could not be seen by looking at joint angles alone.
1088

"Har du kontrollen?" : En fallstudie om positionsöverlämningar på ATCC Stockholm

Nilsson, Lisa January 2010 (has links)
Flygledare övervakar alla flygplan som färdas världen över genom att se till attflygplanen håller separationsgränserna. Flygledarna jobbar i pass på cirka en timme ochbyts sedan av, en så kallad positionsöverlämning. Det är viktigt att den som tar överpositionen får all information den behöver för att kunna fortsätta leda flygplanen på ett säkert sätt och eventuellt vidta åtgärder för att säkerställa en välordnad flygtrafik. Incidentrapporter visar att ett oproportionerligt stort antal incidenter sker 5- 15minuter efter en positionsöverlämning. På grund av detta vill LFV undersöka hur positionsöverlämningarna går till för att senare kvalitetssäkra dem. Syftet med denna rapport har varit att beskriva positionsöverlämningar på ATCC Stockholm. Resultatet visar att flygledarna till stor del använder sig av en memorerad checklista som inte skiljer sig mycket från den fysiska de har framför sig och följer därmed ett generellt mönster. Det visar även att det svåra kan vara att upprätthålla uppmärksamheten under alla överlämningarna. Resultaten ger LFV en utgångspunkt till att börja med kvalitetssäkringen. Ur ett akademiskt perspektiv visar resultaten en tillämpning på hurde teorier som används i studien kan appliceras i kontexten positionsöverlämningar mellan flygledare.
1089

Coherent design of uninhabited aerial vehicle operations and control stations

Gonzalez Castro, Luis Nicolas 22 May 2006 (has links)
This work presents the application of a cognitive engineering design method to the design of operational procedures and ground control station interfaces for uninhabited aerial vehicles (UAVs). Designing for UAV systems presents novel challenges, both in terms of selecting and presenting adequate information for effective teleoperation, and in creating operational procedures and ground control station interfaces that are robust to a range of UAV platforms and missions. Creating a coherent set of operating procedures, automatic functions and operator interfaces requires a systematic design approach that considers the system and the mission at different levels of abstraction and integrates the different element of the system. Several models are developed through the application of this cognitive engineering method. An analysis of the work of operating a UAV creates an abstraction decomposition space (ADS) model. The ADS helps identify the control tasks needed to operate the system. A strategies analysis then identifies methods for implementing these control tasks. The distribution of activities and roles between the human and automated components in the system is then considered in a social organization and cooperation analysis. These insights are applied to the design of coherent sets of operational procedures, ground control station interfaces and automatic functions for a specific UAV in support of a continuous target surveillance (CTS) mission. The importance of the coherence provided by the selected design method in the design of UAV operational procedures and ground control station interfaces is analyzed through a human in the loop simulation experiment for this mission. The results of the simulation experiment indicate that UAV controllers using coherently designed elements achieve significantly higher mission performance and experience lower workloads than those that when using incoherently matched elements.
1090

Improved Performance of a Radio Frequency Identification Tag Antenna on a Metal Ground Plane

Prothro, Joel Thomas 18 May 2007 (has links)
Simulation and experiments quantify the effect of moving a horizontal dipole antenna close to a metal ground plane. Solutions to the radiation problems are offered.

Page generated in 0.0579 seconds