Modeling, Stability Analysis, and Testing of a Hybrid Docking Simulator

A hybrid docking simulator is a hardware-in-the-loop (HIL) simulator that includes a hardware element within a numerical simulation loop. One of the goals of performing a HIL simulation at the European Proximity Operation Simulator (EPOS) is the verification and validation of the docking phase in an on-orbit servicing mission.....Comment: 30 papge

Analytical and experimental stability investigation of a hardware-in-the-loop satellite docking simulator

The European Proximity Operation Simulator (EPOS) of the DLR-German Aerospace Center is a robotics-based simulator that aims at validating and verifying a satellite docking phase. The generic concept features a robotics tracking system working in closed loop with a force/torque feedback signal. Inherent delays in the tracking system combined with typical high stiffness at contact challenge the stability of the closed-loop system. The proposed concept of operations is hybrid: the feedback signal is a superposition of a measured value and of a virtual value that can be tuned in order to guarantee a desired behavior. This paper is concerned with an analytical study of the system's closed-loop stability, and with an experimental validation of the hybrid concept of operations in one dimension (1D). The robotics simulator is modeled as a second-order loop-delay system and closed-form expressions for the critical delay and associated frequency are derived as a function of the satellites' mass and the contact dynamics stiffness and damping parameters. A numerical illustration sheds light on the impact of the parameters on the stability regions. A first-order Pade approximation provides additional means of stability investigation. Experiments were performed and tests results are described for varying values of the mass and the damping coefficients. The empirical determination of instability is based on the coefficient of restitution and on the observed energy. There is a very good agreement between the critical damping values predicted by the analysis and observed during the tests...Comment: 16 page

The reasons for the collapse of the Tacoma Narrows Bridge and the lessons for the classroom

On 7 November 1940, a historical event occurred for suspension bridge construction and aerodynamic engineering around suspension bridges. Engineers investigating the event concluded the bridge collapsed due to high winds but did not explain how. Later lab tests by other engineers and scientists demonstrated that the collapse happened either due to forced oscillations with Resonance or aeroelastic flutter. Forced oscillations with Resonance treated the bridge as an object being periodically pushed by the winds in Resonance with its natural frequency. And aeroelastic flutter treats the bridge as a wingspan in a fluid stream where the winds would alternate the pushing of the bridge span as it enters above and under the plate. Due to historical similarities, some believed the collapse occurred due to Resonance. However, later articles would discuss the aeroelastic flutter and criticize the resonance argument. One of these articles would be written by Billah and Scanlan, criticizing the use of the bridge as an example of Resonance in physics books and showing an alternative interpretation of the collapse. After discussing the collapse with an expert in aerodynamics on bridges from the University of Stavanger, we were informed that the Billah and Scanlan article is considered the modern explanation by the professional community. However, there are still physics books today that still misrepresent the circumstances around the collapse. We, as teaching students, agree with the Billah and Scanlan article and the opinion of the professional community that the collapse was most likely due to aeroelastic flutter. And that the collapse being represented as Resonance simplifies and misrepresents a more complicated and comprehensive problem around the Tacoma Narrows Bridge.On 7 November 1940, a historical event occurred for suspension bridge construction and aerodynamic engineering around suspension bridges. Engineers investigating the event concluded the bridge collapsed due to high winds but did not explain how. Later lab tests by other engineers and scientists demonstrated that the collapse happened either due to forced oscillations with Resonance or aeroelastic flutter. Forced oscillations with Resonance treated the bridge as an object being periodically pushed by the winds in Resonance with its natural frequency. And aeroelastic flutter treats the bridge as a wingspan in a fluid stream where the winds would alternate the pushing of the bridge span as it enters above and under the plate. Due to historical similarities, some believed the collapse occurred due to Resonance. However, later articles would discuss the aeroelastic flutter and criticize the resonance argument. One of these articles would be written by Billah and Scanlan, criticizing the use of the bridge as an example of Resonance in physics books and showing an alternative interpretation of the collapse. After discussing the collapse with an expert in aerodynamics on bridges from the University of Stavanger, we were informed that the Billah and Scanlan article is considered the modern explanation by the professional community. However, there are still physics books today that still misrepresent the circumstances around the collapse. We, as teaching students, agree with the Billah and Scanlan article and the opinion of the professional community that the collapse was most likely due to aeroelastic flutter. And that the collapse being represented as Resonance simplifies and misrepresents a more complicated and comprehensive problem around the Tacoma Narrows Bridge

Studies on the radiation preservation of fish I. The Effect on Certain Vitamins in Fresh Fillets of Cod and Dogfish and in Smoked Fillets of Cod and Herring

The effect of radiopasteurization (0.3 Mrad) and radiosterilization (3.0 Mrad) on certain vitamins in canned fresh fillets of cod and dogfish and in vacuum packed smoked cod and herring, were studied. The effect of post-irradiation storage of the samples at appropriate temperatures on the vitamin contents was determined. Some differences between vitamin sensitivity to irradiation and storage among the various fish samples studied were noted. Thiamine was found to be most sensitive to both irradiation and storage of non irradiated samples. Irradiated samples lost little or no thiamine upon storage. Riboflavin was more sensitive to storage than to irradiation. Niacin and vitamin B12 were not appreciably affected by irradiation or storage. Vitamin A appeared to be more radiosensitive in smoked than in fresh fish. Thus a sample of dogfish retained 50 percent of the vitamin A content after irradiation with 3.0 Mrad. Vitamin E was not affected by the radiation doses employed in the fish samples investigated

FLEET: Butterfly Estimation from a Bipartite Graph Stream

We consider space-efficient single-pass estimation of the number of butterflies, a fundamental bipartite graph motif, from a massive bipartite graph stream where each edge represents a connection between entities in two different partitions. We present a space lower bound for any streaming algorithm that can estimate the number of butterflies accurately, as well as FLEET, a suite of algorithms for accurately estimating the number of butterflies in the graph stream. Estimates returned by the algorithms come with provable guarantees on the approximation error, and experiments show good tradeoffs between the space used and the accuracy of approximation. We also present space-efficient algorithms for estimating the number of butterflies within a sliding window of the most recent elements in the stream. While there is a significant body of work on counting subgraphs such as triangles in a unipartite graph stream, our work seems to be one of the few to tackle the case of bipartite graph streams.Comment: This is the author's version of the work. It is posted here by permission of ACM for your personal use. Not for redistribution. The definitive version was published in Seyed-Vahid Sanei-Mehri, Yu Zhang, Ahmet Erdem Sariyuce and Srikanta Tirthapura. "FLEET: Butterfly Estimation from a Bipartite Graph Stream". The 28th ACM International Conference on Information and Knowledge Managemen

Interpreting Attoclock Measurements of Tunnelling Times

Resolving in time the dynamics of light absorption by atoms and molecules, and the electronic rearrangement this induces, is among the most challenging goals of attosecond spectroscopy. The attoclock is an elegant approach to this problem, which encodes ionization times in the strong-field regime. However, the accurate reconstruction of these times from experimental data presents a formidable theoretical challenge. Here, we solve this problem by combining analytical theory with ab-initio numerical simulations. We apply our theory to numerical attoclock experiments on the hydrogen atom to extract ionization time delays and analyse their nature. Strong field ionization is often viewed as optical tunnelling through the barrier created by the field and the core potential. We show that, in the hydrogen atom, optical tunnelling is instantaneous. By calibrating the attoclock using the hydrogen atom, our method opens the way to identify possible delays associated with multielectron dynamics during strong-field ionization.Comment: 33 pages, 10 figures, 3 appendixe

Nef Decreases HIV-1 Sensitivity to Neutralizing Antibodies that Target the Membrane-proximal External Region of TMgp41

Primate lentivirus nef is required for sustained virus replication in vivo and accelerated progression to AIDS. While exploring the mechanism by which Nef increases the infectivity of cell-free virions, we investigated a functional link between Nef and Env. Since we failed to detect an effect of Nef on the quantity of virion-associated Env, we searched for qualitative changes by examining whether Nef alters HIV-1 sensitivity to agents that target distinct features of Env. Nef conferred as much as 50-fold resistance to 2F5 and 4E10, two potent neutralizing monoclonal antibodies (nAbs) that target the membrane proximal external region (MPER) of TMgp41. In contrast, Nef had no effect on HIV-1 neutralization by MPER-specific nAb Z13e1, by the peptide inhibitor T20, nor by a panel of nAbs and other reagents targeting gp120. Resistance to neutralization by 2F5 and 4E10 was observed with Nef from a diverse range of HIV-1 and SIV isolates, as well as with HIV-1 virions bearing Env from CCR5- and CXCR4-tropic viruses, clade B and C viruses, or primary isolates. Functional analysis of a panel of Nef mutants revealed that this activity requires Nef myristoylation but that it is genetically separable from other Nef functions such as the ability to enhance virus infectivity and to downregulate CD4. Glycosylated-Gag from MoMLV substituted for Nef in conferring resistance to 2F5 and 4E10, indicating that this activity is conserved in a retrovirus that does not encode Nef. Given the reported membrane-dependence of MPER-recognition by 2F5 and 4E10, in contrast to the membrane-independence of Z13e1, the data here is consistent with a model in which Nef alters MPER recognition in the context of the virion membrane. Indeed, Nef and Glycosylated-Gag decreased the efficiency of virion capture by 2F5 and 4E10, but not by other nAbs. These studies demonstrate that Nef protects lentiviruses from one of the most broadly-acting classes of neutralizing antibodies. This newly discovered activity for Nef has important implications for anti-HIV-1 immunity and AIDS pathogenesis

HIV-1 assembly in macrophages

The molecular mechanisms involved in the assembly of newly synthesized Human Immunodeficiency Virus (HIV) particles are poorly understood. Most of the work on HIV-1 assembly has been performed in T cells in which viral particle budding and assembly take place at the plasma membrane. In contrast, few studies have been performed on macrophages, the other major target of HIV-1. Infected macrophages represent a viral reservoir and probably play a key role in HIV-1 physiopathology. Indeed macrophages retain infectious particles for long periods of time, keeping them protected from anti-viral immune response or drug treatments. Here, we present an overview of what is known about HIV-1 assembly in macrophages as compared to T lymphocytes or cell lines

Mechanisms of seawater acclimation in a primitive, anadromous fish, the green sturgeon

Relatively little is known about salinity acclimation in the primitive groups of fishes. To test whether physiological preparative changes occur and to investigate the mechanisms of salinity acclimation, anadromous green sturgeon, Acipenser medirostris (Chondrostei) of three different ages (100, 170, and 533 dph) were acclimated for 7 weeks to three different salinities (<3, 10, and 33 ppt). Gill, kidney, pyloric caeca, and spiral intestine tissues were assayed for Na+, K+-ATPase activity; and gills were analyzed for mitochondria-rich cell (MRC) size, abundance, localization and Na+, K+-ATPase content. Kidneys were analyzed for Na+, K+-ATPase localization and the gastro-intestinal tract (GIT) was assessed for changes in ion and base content. Na+, K+-ATPase activities increased in the gills and decreased in the kidneys with increasing salinity. Gill MRCs increased in size and decreased in relative abundance with fish size/age. Gill MRC Na+, K+-ATPase content (e.g., ion-pumping capacity) was proportional to MRC size, indicating greater abilities to regulate ions with size/age. Developmental/ontogenetic changes were seen in the rapid increases in gill MRC size and lamellar length between 100 and 170 dph. Na+, K+-ATPase activities increased fourfold in the pyloric caeca in 33 ppt, presumably due to increased salt and water absorption as indicated by GIT fluids, solids, and ion concentrations. In contrast to teleosts, a greater proportion of base (HCO3− and 2CO32−) was found in intestinal precipitates than fluids. Green sturgeon osmo- and ionoregulate with similar mechanisms to more-derived teleosts, indicating the importance of these mechanisms during the evolution of fishes, although salinity acclimation may be more dependent on body size