142,830 research outputs found
Circuit measures hysteresis loop areas at 30 Hz
Analog circuit measures hysteresis loop areas as a function of time during fatigue testing of specimens subjected to sinusoidal tension-compression stresses at a frequency of Hz. When the sinusoidal stress signal is multiplied by the strain signal, the dc signal is proportional to hysteresis loop area
A Novel Rate Control Algorithm for Onboard Predictive Coding of Multispectral and Hyperspectral Images
Predictive coding is attractive for compression onboard of spacecrafts thanks
to its low computational complexity, modest memory requirements and the ability
to accurately control quality on a pixel-by-pixel basis. Traditionally,
predictive compression focused on the lossless and near-lossless modes of
operation where the maximum error can be bounded but the rate of the compressed
image is variable. Rate control is considered a challenging problem for
predictive encoders due to the dependencies between quantization and prediction
in the feedback loop, and the lack of a signal representation that packs the
signal's energy into few coefficients. In this paper, we show that it is
possible to design a rate control scheme intended for onboard implementation.
In particular, we propose a general framework to select quantizers in each
spatial and spectral region of an image so as to achieve the desired target
rate while minimizing distortion. The rate control algorithm allows to achieve
lossy, near-lossless compression, and any in-between type of compression, e.g.,
lossy compression with a near-lossless constraint. While this framework is
independent of the specific predictor used, in order to show its performance,
in this paper we tailor it to the predictor adopted by the CCSDS-123 lossless
compression standard, obtaining an extension that allows to perform lossless,
near-lossless and lossy compression in a single package. We show that the rate
controller has excellent performance in terms of accuracy in the output rate,
rate-distortion characteristics and is extremely competitive with respect to
state-of-the-art transform coding
Performance Investigation on Electrochemical Compressor with Ammonia
To improve the efficiency of the compressor in the vapor compression system, an electrochemical compressor (EC) was suggested and its performance was investigated. The EC compresses refrigerant by charging an external voltage to the membrane electrolyte assembly (MEA). Ammonia is used as a working fluid due to its high thermodynamic characteristics. This paper mainly focuses on two topics: thermodynamic compression limit at the open loop and the EC performance at the closed loop. The thermodynamic compression limit of the EC at open loop verifies the voltage and compression ratio relationship based on the Nernst equation. The current density of the MEA was investigated with voltage and refrigerant charge. The pressure lift across the EC was measured as well.
Terminal-shock and restart control of a Mach 2.5, mixed compression inlet coupled to a turbofan engine
Results of an experimental program conducted on a mixed-compression inlet coupled to a turbofan engine are presented. Open-loop frequency response data are presented that show the response of shock position (as measured by an average inlet static pressure) to sinusoidal airflow disturbances produced at the compressor face station. Also presented are results showing the effect of different passive terminations (a choke plate or a long duct) on the characteristics of the inlet. Transfer functions obtained by using experimental data are presented and compared to the experimental data. Closed-loop frequency response of shock position (with a proportional-plus-integral controller) is presented. In addition, transient data are presented that show the unstart-restart characteristics of the inlet
Coronal Electron Distribution in Solar Flares: Drift-Kinetic Model
Using a model of particle acceleration and transport in solar flares, we
investigate the height distribution of coronal electrons by focusing on the
energy-dependent pitch-angle scattering. When pitch-angle scattering is not
included, the peak heights of loop-top electrons are constant, regardless of
their energy, owing to the continuous acceleration and compression of the
electrons via shrinkage of magnetic loops. On the other hand, under pitch-angle
scattering, the electron heights are energy dependent; intermediate energy
electrons are at a higher altitude, whereas lower and higher energy electrons
are at lower altitudes. This implies that the intermediate energy electrons are
inhibited to follow the shrinking field lines to lower altitudes because
pitch-angle scattering causes efficient precipitation of these electrons into
the footpoint and their subsequent loss from the loop. This result is
qualitatively consistent with the position of the above-the-loop-top hard X-ray
(HXR) source that is located above coronal HXR loops emitted by lower energy
electrons and microwaves emitted by higher energy electrons. Quantitative
agreement with observations might be achieved by considering primary
acceleration before the onset of loop shrinkage and additional pitch-angle
scattering via wave-particle interactions.Comment: 18 pages, 6 figures, accepted by Ap
Development of a refrigeration system for lunar surface and spacecraft applications
An evaluation of refrigeration devices suitable for potential lunar surface and spacecraft applications was performed. The following conclusions were reached: (1) the vapor compression system is the best overall refrigeration system for lunar surface and spacecraft applications and the single phase radiator system is generally preferred for earth orbit applications, (2) the vapor compression cycle may have some application for simultaneous heating and cooling, (3) a Stirling cycle refrigerator was selected for the manned cabin of the space shuttle, and (4) significant increases in payload heat rejection can be obtained by a kit vapor compression refrigerator added to the shuttle R-21 loop. The following recommendations were made: (1) a Stirling cycle refrigerator may be used for food freezer and biomedical sample storage, (2) the best system for a food freezer/experiments compartment for an earth orbit space station has not been determined, (3) a deployed radiator system can be designed for large heat loads in earth orbit
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