Cratering in low-density targets

Cratering in low density targets, and comparisons of various hypervelocity projectile-target combination

Precise control of flexible manipulators

The design and experimental testing of end point position controllers for a very flexible one link lightweight manipulator are summarized. The latest upgraded version of the experimental set up, and the basic differences between conventional joint angle feedback and end point position feedback are described. A general procedure for application of modern control methods to the problem is outlined. The relationship between weighting parameters and the bandwidth and control stiffness of the resulting end point position closed loop system is shown. It is found that joint rate angle feedback in addition to the primary end point position sensor is essential for adequate disturbance rejection capability of the closed loop system. The use of a low order multivariable compensator design computer code; called Sandy is documented. A solution to the problem of control mode switching between position sensor sets is outlined. The proof of concept for endpoint position feedback for a one link flexible manipulator was demonstrated. The bandwidth obtained with the experimental end point position controller is about twice as fast as the beam's first natural cantilevered frequency, and comes within a factor of four of the absolute physical speed limit imposed by the wave propagation time of the beam

Validität des automatisierten Learning Progress Assessments im geschriebenen Englisch für Schüler:innen mit Lernschwierigkeiten

We evaluated the validity of an automated approach to learning progress assessment (aLPA) for English written expression. Participants (n = 105) were students in Grades 2–12 who had parent-identified learning difficulties and received academic tutoring through a community-based organization. Participants completed narrative writing samples in the fall and spring of 1 academic year, and some participants (n = 33) also completed a standardized writing assessment in the spring of the academic year. The narrative writing samples were evaluated using aLPA, four hand-scored written expression curriculum-based measures (WE-CBM), and ratings of writing quality. Results indicated (a) aLPA and WE-CBM scores were highly correlated with ratings of writing quality; (b) aLPA and more complex WE-CBM scores demonstrated acceptable correlations with the standardized writing subtest assessing spelling and grammar, but not the subtest assessing substantive quality; and (c) aLPA scores showed small, statistically significant improvements from fall to spring. These findings provide preliminary evidence that aLPA can be used to efficiently score narrative writing samples for progress monitoring, with some evidence that the aLPA scores can serve as a general indicator of writing skill. The use of automated scoring in aLPA, with performance comparable to WE-CBM hand scoring, may improve scoring feasibility and increase the likelihood that educators implement aLPA for decision-making. (DIPF/Orig.)Wir evaluierten die Validität eines automatischen Ansatzes des Learning Progress Assessments (aLPA) in geschriebener Sprache. Schüler:innen der Klassen 2 bis 12 (n = 105) mit Lernschwierigkeiten, die deren Eltern festgestellt hatten, nahmen an Nachhilfeunterricht, welcher von einer gemeinnützigen Organisation durchgeführt wurde, teil. Die Schüler:innen erstellten im Herbst und Frühjahr eines Schuljahres Schreibproben. Weiterhin nahmen einige Schüler:innen (n = 33) an einer standardisierten Schreibprüfung im Frühjahr teil. Die Schreibproben wurden mit aLPA, vier handkodierten Curriculum-Based Measures (WE-CBM) und hinsichtlich deren Schreibqualität ausgewertet. Unsere Ergebnisse zeigen, (a) aLPA- und WE-CBM-Werte korrelierten hoch mit der Bewertung der Schreibqualität, (b) aLPA- und die komplexeren WE-CBM-Werte zeigten akzeptable Korrelationen mit der standardisierten Rechtschreib- und Grammatikprüfung, jedoch nicht mit tatsächlicher Qualität, und (c) aLPA-Werte zeigten geringe, statistisch signifikante Verbesserungen von Herbst zu Frühjahr. Diese Ergebnisse deuten darauf hin, dass aLPA als effiziente Methode zur Bewertung von Schreibproben verwendet werden kann und der aLPA-Wert als allgemeiner Schreibkompetenzindikator dienen kann. Automatische Bewertung im aLPA, dessen Validität mit der von WE-CBM vergleichbar ist, kann die Bewertung vereinfachen und damit ist es wahrscheinlicher, dass Lehrkräfte aLPA verwenden. (DIPF/Orig.

Connectivity-enhanced diffusion analysis reveals white matter density disruptions in first episode and chronic schizophrenia.

Reduced fractional anisotropy (FA) is a well-established correlate of schizophrenia, but it remains unclear whether these tensor-based differences are the result of axon damage and/or organizational changes and whether the changes are progressive in the adult course of illness. Diffusion MRI data were collected in 81 schizophrenia patients (54 first episode and 27 chronic) and 64 controls. Analysis of FA was combined with "fixel-based" analysis, the latter of which leverages connectivity and crossing-fiber information to assess both fiber bundle density and organizational complexity (i.e., presence and magnitude of off-axis diffusion signal). Compared with controls, patients with schizophrenia displayed clusters of significantly lower FA in the bilateral frontal lobes, right dorsal centrum semiovale, and the left anterior limb of the internal capsule. All FA-based group differences overlapped substantially with regions containing complex fiber architecture. FA within these clusters was positively correlated with principal axis fiber density, but inversely correlated with both secondary/tertiary axis fiber density and voxel-wise fiber complexity. Crossing fiber complexity had the strongest (inverse) association with FA (r = -0.82). When crossing fiber structure was modeled in the MRtrix fixel-based analysis pipeline, patients exhibited significantly lower fiber density compared to controls in the dorsal and posterior corpus callosum (central, postcentral, and forceps major). Findings of lower FA in patients with schizophrenia likely reflect two inversely related signals: reduced density of principal axis fiber tracts and increased off-axis diffusion sources. Whereas the former confirms at least some regions where myelin and or/axon count are lower in schizophrenia, the latter indicates that the FA signal from principal axis fiber coherence is broadly contaminated by macrostructural complexity, and therefore does not necessarily reflect microstructural group differences. These results underline the need to move beyond tensor-based models in favor of acquisition and analysis techniques that can help disambiguate different sources of white matter disruptions associated with schizophrenia

Fractional and unquantized dc voltage generation in THz-driven semiconductor superlattices

We consider the spontaneous creation of a dc voltage across a strongly coupled semiconductor superlattice subjected to THz radiation. We show that the dc voltage may be approximately proportional either to an integer or to a half- integer multiple of the frequency of the applied ac field, depending on the ratio of the characteristic scattering rates of conducting electrons. For the case of an ac field frequency less than the characteristic scattering rates, we demonstrate the generation of an unquantized dc voltage.Comment: 6 pages, 3 figures, RevTEX, EPSF. Revised version v3: corrected typo

The S2 VLBI Correlator: A Correlator for Space VLBI and Geodetic Signal Processing

We describe the design of a correlator system for ground and space-based VLBI. The correlator contains unique signal processing functions: flexible LO frequency switching for bandwidth synthesis; 1 ms dump intervals, multi-rate digital signal-processing techniques to allow correlation of signals at different sample rates; and a digital filter for very high resolution cross-power spectra. It also includes autocorrelation, tone extraction, pulsar gating, signal-statistics accumulation.Comment: 44 pages, 13 figure

Simulation of the evolution of floor covering ceramic tiles during the firing

In the context of the firing of ceramic tiles the problem of simulating the final shape of the body is relevant because several defects can occur and the tile can be rejected if the conditions of the firing are inadequate for the geometry and materials of the tile -- The existing literature on this problem indicates that previous works present limitations in aspects such as not using a model characteristic of ceramics at high temperatures and oversimplifying the problem -- As a response to such shortcomings, this article presents a simulation with a 3-dimensional Norton’s model, which overcomes the difficulties because it is characteristic of ceramics at high temperatures -- The results of our simulated experiments show advantages with respect to the identification of the mechanisms that contribute to the final shape of the body -- Our work is able to divide the history of temperatures in stages where the evolution of the thermal, elastic and creep deformations is simplified and meaningful -- That is achieved because our work found that curvature is the most descriptive parameter of the simulation, the most important contribution of this article -- Future work is to be realized in the creation of a model that takes into account that the shrinkage is dependent on the history of temperatures -- The main shortcoming of the paper is the lack of physical experiments to corroborate the simulation

Object-based task-level control: A hierarchical control architecture for remote operation of space robots

Expanding man's presence in space requires capable, dexterous robots capable of being controlled from the Earth. Traditional 'hand-in-glove' control paradigms require the human operator to directly control virtually every aspect of the robot's operation. While the human provides excellent judgment and perception, human interaction is limited by low bandwidth, delayed communications. These delays make 'hand-in-glove' operation from Earth impractical. In order to alleviate many of the problems inherent to remote operation, Stanford University's Aerospace Robotics Laboratory (ARL) has developed the Object-Based Task-Level Control architecture. Object-Based Task-Level Control (OBTLC) removes the burden of teleoperation from the human operator and enables execution of tasks not possible with current techniques. OBTLC is a hierarchical approach to control where the human operator is able to specify high-level, object-related tasks through an intuitive graphical user interface. Infrequent task-level command replace constant joystick operations, eliminating communications bandwidth and time delay problems. The details of robot control and task execution are handled entirely by the robot and computer control system. The ARL has implemented the OBTLC architecture on a set of Free-Flying Space Robots. The capability of the OBTLC architecture has been demonstrated by controlling the ARL Free-Flying Space Robots from NASA Ames Research Center