3,789 research outputs found
The use of precession modulation for nutation control in spin-stabilized spacecraft
The relations which determine the nutation effects induced in a spinning spacecraft by periodic precession thrust pulses are derived analytically. By utilizing the idea that nutation need only be observed just before each precession thrust pulse, a difficult continuous-time derivation is replaced by a simple discrete-time derivation using z-transforms. The analytic results obtained are used to develop two types of modulated precession control laws which use the precession maneuver to concurrently control nutation. Results are illustrated by digital simulation of an actual spacecraft configuration
Quasiperiodic graphs: structural design, scaling and entropic properties
A novel class of graphs, here named quasiperiodic, are constructed via
application of the Horizontal Visibility algorithm to the time series generated
along the quasiperiodic route to chaos. We show how the hierarchy of
mode-locked regions represented by the Farey tree is inherited by their
associated graphs. We are able to establish, via Renormalization Group (RG)
theory, the architecture of the quasiperiodic graphs produced by irrational
winding numbers with pure periodic continued fraction. And finally, we
demonstrate that the RG fixed-point degree distributions are recovered via
optimization of a suitably defined graph entropy
Can Community Policing Increase Residents\u27 Informal Social Control? Testing the Impact of the Chicago Alternative Policing Strategy
This study examines whether community policing can build informal social control. Specifically, this paper assesses the impact of the Chicago Alternative Policing Strategy (CAPS) in Chicago neighborhoods. The data for this research are drawn from both the Community Survey of the Project on Human Development in Chicago Neighborhoods (PHDCN) and the CAPS Prototype Panel Survey. Bivariate and multivariate methods are used to analyze data gathered from 8782 residents nested within 343 neighborhood clusters. Initially, community policing was found to increase informal social control, but this effect was rendered non-significant after controlling for theoretically and empirically relevant variables. Several social (dis)organization variables, as well as satisfaction with policing services, yielded significant effects in a multilevel regression model. Further analysis found that the community policing effect on informal social control was mediated through satisfaction with the police. These findings indicate indirect support for the ability of community policing to build informal social control and suggest that general satisfaction with the police is important to neighborhood crime control strategies
WOODLAND POND SALAMANDER ABUNDANCE IN RELATION TO FOREST MANAGEMENT AND ENVIRONMENTAL CONDITIONS IN NORTHERN WISCONSIN
Woodland ponds are important landscape features that help sustain populations of amphibians that require this aquatic habitat for successful reproduction. Species abundance patterns often reflect site-specific differences in hydrology, physical characteristics, and surrounding vegetation. Large-scale processes such as changing land cover and environmental conditions are other potential drivers influencing amphibian populations in the Upper Midwest, but little information exists on the combined effects of these factors. We used Blue-spotted (Ambystoma laterale Hallowell) and Spotted Salamander (A. maculatum Shaw) monitoring data collected at the same woodland ponds thirteen years apart to determine if changing environmental conditions and vegetation cover in surrounding landscapes influenced salamander movement phenology and abundance. Four woodland ponds in northern Wisconsin were sampled for salamanders in April 1992-1994 and 2005-2007. While Bluespotted Salamanders were more abundant than Spotted Salamanders in all ponds, there was no change in the numbers of either species over the years. However, peak numbers of Blue-spotted Salamanders occurred 11.7 days earlier (range: 9-14 days) in the 2000s compared to the 1990s; Spotted Salamanders occurred 9.5 days earlier (range: 3 - 13 days). Air and water temperatures (April 13- 24) increased, on average, 4.8 oC and 3.7 oC, respectively, between the decades regardless of pond. There were no discernible changes in canopy openness in surrounding forests between decades that would have warmed the water sooner (i.e., more light penetration). Our finding that salamander breeding phenology can vary by roughly 10 days in Wisconsin contributes to growing evidence that amphibian populations have responded to changing climate conditions by shifting life-cycle events. Managers can use this information to adjust monitoring programs and forest management activities in the surrounding landscape to avoid vulnerable amphibian movement periods. Considering direct and indirect stressors such as changing habitat and environmental conditions simultaneously to better understand trends in space and time can help improve monitoring programs for this taxa, which is at major risk of continued declines
A 100 kJ Pulse Unit for Electromagnetic Forming of Large Area Sheet Metals
Magnetoforming of tube or sheet metal parts can significantly extend the range of geometries conceivable with state-of-the-art forming methods. A major advantage is the considerably higher forming speed of the process achievable by using a magnetic piston without inertia. Key for this technology is the development of reliable, long-lifetime, high current pulse power generators able to deliver tens to hundreds of kiloamps of peak current to a mainly inductive load which is highly variable in time during the forming process. A high-current, high energy pulse generator for electromagnetic forming of large area sheet metal has been developed and was taken into operation. Design criteria were reliability and safety for all possible load cases, including short circuits and short-circuiting loads under operation, at nominal peak currents of up to 200 kA and peak pulse energies of up to 100 kJ. In order to comply with the safety requirements, an all-solid-state design has been chosen using high power semiconductor switches for pulse forming instead of ignitrons or spark gaps. Due to constraints concerning space and manageability, the coupling between the load and the pulse forming unit is achieved via a semi-rigid bundle of high voltage cables, allowing an adjustment of the carrier of the forming coil while being electrically connected to each other. We report on the development of a pulse generator for peak currents of 50 kA to up to 200 kA at a pulse width of typically around 100 µs, depending on the load parameters. In order to meet lifetime requirements suitable for industrial applications, the short circuit handling capability of peak currents of up to 450 kA is a major issue in the pulse generator design. A modular 3-branch design of parallel capacitor banks has been adopted to achieve the requirements concerning reliability, lifetime, and short circuit handling. The prototype pulse generator is based upon off-the-shelf devices, including high-current semiconductor switches. First operating results of the commissioning phase of the installation are reported
Ambiguities in recurrence-based complex network representations of time series
Recently, different approaches have been proposed for studying basic
properties of time series from a complex network perspective. In this work, the
corresponding potentials and limitations of networks based on recurrences in
phase space are investigated in some detail. We discuss the main requirements
that permit a feasible system-theoretic interpretation of network topology in
terms of dynamically invariant phase-space properties. Possible artifacts
induced by disregarding these requirements are pointed out and systematically
studied. Finally, a rigorous interpretation of the clustering coefficient and
the betweenness centrality in terms of invariant objects is proposed
Statistical Mechanics and Information-Theoretic Perspectives on Complexity in the Earth System
Peer reviewedPublisher PD
Nutation control during precession of a spin-stabilized spacecraft
The effects of precession thrust pulses and energy dissipation upon nutation of a spin-stabilized spacecraft are studied. Methods for controlling nutation during a precession maneuver are proposed and examined. A precession modulation control law is developed which uses precession thrust pulses to control nutation. Digital simulations show that precession control with separate nutation control is the fastest precessing system; however, the precession modulation method is only fractionally slower while not requiring a separate nutation control system
The relationship between macroalgae taxa and human disturbance on central Pacific coral reefs
Climate change and human disturbance threatens coral reefs across the Pacific, yet there is little consensus on what characterizes a “healthy” reef. Benthic cover, particularly low coral cover and high macroalgae cover, are often used as an indicator of reef degradation, despite uncertainty about the typical algal community compositions associated with either near-pristine or damaged reefs. In this study, we examine differences in coral and algal community compositions and their response to human disturbance and past heat stress, by analysing 25 sites along a gradient of human disturbance in Majuro and Arno Atolls of the Republic of the Marshall Islands. Our results show that total macroalgae cover indicators of reef degradation may mask the influence of local human disturbance, with different taxa responding to disturbance differently. Identifying macroalgae to a lower taxonomic level (e.g. the genus level) is critical for a more accurate measure of Pacific coral reef health
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