109 research outputs found
System for Measuring Flexing of a Large Spaceborne Structure
An optoelectronic metrology system is used for determining the attitude and flexing of a large spaceborne radar antenna or similar structure. The measurements are needed for accurate pointing of the antenna and correction and control of the phase of the radar signal wavefront. The system includes a dual-field-of-view star tracker; a laser ranging unit (LRU) and a position-sensitive-detector (PSD)-based camera mounted on an optical bench; and fiducial targets at various locations on the structure. The fiducial targets are illuminated in sequence by laser light coupled via optical fibers. The LRU and the PSD provide measurements of the position of each fiducial target in a reference frame attached to the optical bench. During routine operation, the star tracker utilizes one field of view and functions conventionally to determine the orientation of the optical bench. During operation in a calibration mode, the star tracker also utilizes its second field of view, which includes stars that are imaged alongside some of the fiducial targets in the PSD; in this mode, the PSD measurements are traceable to star measurements
Parameter Identification by Deep Learning of a Material Model for Granular Media
Classical physical modelling with associated numerical simulation
(model-based), and prognostic methods based on the analysis of large amounts of
data (data-driven) are the two most common methods used for the mapping of
complex physical processes. In recent years, the efficient combination of these
approaches has become increasingly important. Continuum mechanics in the core
consists of conservation equations that -- in addition to the always necessary
specification of the process conditions -- can be supplemented by
phenomenological material models. The latter are an idealized image of the
specific material behavior that can be determined experimentally, empirically,
and based on a wealth of expert knowledge. The more complex the material, the
more difficult the calibration is. This situation forms the starting point for
this work's hybrid data-driven and model-based approach for mapping a complex
physical process in continuum mechanics. Specifically, we use data generated
from a classical physical model by the MESHFREE software to train a Principal
Component Analysis-based neural network (PCA-NN) for the task of parameter
identification of the material model parameters. The obtained results highlight
the potential of deep-learning-based hybrid models for determining parameters,
which are the key to characterizing materials occurring naturally, and their
use in industrial applications (e.g. the interaction of vehicles with sand).Comment: arXiv admin note: text overlap with arXiv:2212.0313
Digital Averaging Phasemeter for Heterodyne Interferometry
A digital averaging phasemeter has been built for measuring the difference between the phases of the unknown and reference heterodyne signals in a heterodyne laser interferometer. This phasemeter performs well enough to enable interferometric measurements of distance with accuracy of the order of 100 pm and with the ability to track distance as it changes at a speed of as much as 50 cm/s. This phasemeter is unique in that it is a single, integral system capable of performing three major functions that, heretofore, have been performed by separate systems: (1) measurement of the fractional-cycle phase difference, (2) counting of multiple cycles of phase change, and (3) averaging of phase measurements over multiple cycles for improved resolution. This phasemeter also offers the advantage of making repeated measurements at a high rate: the phase is measured on every heterodyne cycle. Thus, for example, in measuring the relative phase of two signals having a heterodyne frequency of 10 kHz, the phasemeter would accumulate 10,000 measurements per second. At this high measurement rate, an accurate average phase determination can be made more quickly than is possible at a lower rate
Elliptic operators with discontinuous coefficients in meshfree GFDM
peer reviewedIn phase change simulations, material properties such as density, viscosity, or thermal conductivity may exhibit jump discontinuities, possibly of several orders of magnitude. These jump discontinuities represent interfaces between the phases, and they emerge naturally during the simulation; thus, their exact location is generally unknown a priori. Our goal is to simulate phase change processes with a meshfree generalized finite difference method in a monolithic model without distinguishing between the different phases. There, the material properties mentioned above appear as coefficients inside elliptic operators in divergence form and the jumps must be treated adequately by the numerical method. We present a numerical method for discretizing elliptic operators with discontinuous coefficients without the need for a domain decomposition or tracking of interfaces. Our method facilitates the construction of diagonally dominant diffusion operators that lead to M‐matrices for the discrete Poisson's equation, and thus, satisfy the discrete maximum principle. We demonstrate the applicability of the new method for the case of smooth diffusivity and discontinuous diffusivity. We show that the method is first‐order accurate for discontinuous diffusion problems and provides second‐order and fourth‐order convergence for continuous diffusion coefficients
A Multi-wavelength Differential Imaging Experiment for the High Contrast Imaging Testbed
We discuss the results of a multi-wavelength differential imaging lab
experiment with the High Contrast Imaging Testbed (HCIT) at the Jet Propulsion
Laboratory. The HCIT combines a Lyot coronagraph with a Xinetics deformable
mirror in a vacuum environment to simulate a space telescope in order to test
technologies and algorithms for a future exoplanet coronagraph mission. At
present, ground based telescopes have achieved significant attenuation of
speckle noise using the technique of spectral differential imaging (SDI). We
test whether ground-based SDI can be generalized to a non-simultaneous spectral
differential imaging technique (NSDI) for a space mission. In our lab
experiment, a series of 5 filter images centered around the O2(A) absorption
feature at 0.762 um were acquired at nominal contrast values of 10^-6, 10^-7,
10^-8, and 10^-9. Outside the dark hole, single differences of images improve
contrast by a factor of ~6. Inside the dark hole, we found significant speckle
chromatism as a function of wavelength offset from the nulling wavelength,
leading to a contrast degradation by a factor of 7.2 across the entire ~80 nm
bandwidth. This effect likely stems from the chromatic behavior of the current
occulter. New, less chromatic occulters are currently in development; we expect
that these new occulters will resolve the speckle chromatism issue.Comment: 24 pages, 8 figures, 3 tables, accepted by PAS
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