29,939 research outputs found
Active Estimation of 3D Lines in Spherical Coordinates
Straight lines are common features in human made environments, which makes
them a frequently explored feature for control applications. Many control
schemes, like Visual Servoing, require the 3D parameters of the features to be
estimated. In order to obtain the 3D structure of lines, a nonlinear observer
is proposed. However, to guarantee convergence, the dynamical system must be
coupled with an algebraic equation. This is achieved by using spherical
coordinates to represent the line's moment vector, and a change of basis, which
allows to introduce the algebraic constraint directly on the system's dynamics.
Finally, a control law that attempts to optimize the convergence behavior of
the observer is presented. The approach is validated in simulation, and with a
real robotic platform with a camera onboard.Comment: Accepted in 2019 American Control Conference (ACC) (Final Version
A multi-spacecraft view of a giant filament eruption during 26/27 September 2009
We analyze multi-spacecraft observations of a giant filament eruption that
occurred during 26 and 27 September 2009. The filament eruption was associated
with a relatively slow coronal mass ejection (CME). The filament consisted of a
large and a small part, both parts erupted nearly simultaneously. Here we focus
on the eruption associated with the larger part of the filament. The STEREO
satellites were separated by about 117 degree during this event, so we
additionally used SoHO/EIT and CORONAS/TESIS observations as a third eye (Earth
view) to aid our measurements. We measure the plane-of-sky trajectory of the
filament as seen from STEREO-A and TESIS view-points. Using a simple
trigonometric relation, we then use these measurements to estimate the true
direction of propagation of the filament which allows us to derive the true
R=R_sun v/s time profile of the filament apex. Furthermore, we develop a new
tomographic method that can potentially provide a more robust three-dimensional
reconstruction by exploiting multiple simultaneous views. We apply this method
also to investigate the 3D evolution of the top part of filament. We expect
this method to be useful when SDO and STEREO observations are combined. We then
analyze the kinematics of the eruptive filament during its rapid acceleration
phase by fitting different functional forms to the height-time data derived
from the two methods. We find that, for both methods, an exponential function
fits the rise profile of the filament slightly better than parabolic or cubic
functions. Finally, we confront these results with the predictions of
theoretical eruption models.Comment: 16 pages, 9 figures, to appear in Astrophysical Journa
Active Estimation of Distance in a Robotic Vision System that Replicates Human Eye Movement
Many visual cues, both binocular and monocular, provide 3D information. When an agent moves with respect to a scene, an important cue is the different motion of objects located at various distances. While a motion parallax is evident for large translations of the agent, in most head/eye systems a small parallax occurs also during rotations of the cameras. A similar parallax is present also in the human eye. During a relocation of gaze, the shift in the retinal projection of an object depends not only on the amplitude of the movement, but also on the distance of the object with respect to the observer. This study proposes a method for estimating distance on the basis of the parallax that emerges from rotations of a camera. A pan/tilt system specifically designed to reproduce the oculomotor parallax present in the human eye was used to replicate the oculomotor strategy by which humans scan visual scenes. We show that the oculomotor parallax provides accurate estimation of distance during sequences of eye movements. In a system that actively scans a visual scene, challenging tasks such as image segmentation and figure/ground segregation greatly benefit from this cue.National Science Foundation (BIC-0432104, CCF-0130851
A fast multipole method for stellar dynamics
The approximate computation of all gravitational forces between
interacting particles via the fast multipole method (FMM) can be made as
accurate as direct summation, but requires less than
operations. FMM groups particles into spatially bounded cells and uses
cell-cell interactions to approximate the force at any position within the sink
cell by a Taylor expansion obtained from the multipole expansion of the source
cell. By employing a novel estimate for the errors incurred in this process, I
minimise the computational effort required for a given accuracy and obtain a
well-behaved distribution of force errors. For relative force errors of
, the computational costs exhibit an empirical scaling of . My implementation (running on a 16 core node) out-performs a
GPU-based direct summation with comparable force errors for .Comment: 21 pages, 15 figures, accepted for publication in Journal for
Computational Astrophysics and Cosmolog
Role of hydrodynamic flows in chemically driven droplet division
We study the hydrodynamics and shape changes of chemically active droplets.
In non-spherical droplets, surface tension generates hydrodynamic flows that
drive liquid droplets into a spherical shape. Here we show that spherical
droplets that are maintained away from thermodynamic equilibrium by chemical
reactions may not remain spherical but can undergo a shape instability which
can lead to spontaneous droplet division. In this case chemical activity acts
against surface tension and tension-induced hydrodynamic flows. By combining
low Reynolds-number hydrodynamics with phase separation dynamics and chemical
reaction kinetics we determine stability diagrams of spherical droplets as a
function of dimensionless viscosity and reaction parameters. We determine
concentration and flow fields inside and outside the droplets during shape
changes and division. Our work shows that hydrodynamic flows tends to stabilize
spherical shapes but that droplet division occurs for sufficiently strong
chemical driving, sufficiently large droplet viscosity or sufficiently small
surface tension. Active droplets could provide simple models for prebiotic
protocells that are able to proliferate. Our work captures the key
hydrodynamics of droplet division that could be observable in chemically active
colloidal droplets
Estimation of width and inclination of a filament sheet using He II 304 A observations by STEREO/EUVI
The STEREO mission has been providing stereoscopic view of the filament
eruptions in EUV wavelengths. The most extended view during filament eruptions
is seen in He II 304 \AA observations, as the filament spine appears darker and
sharper. The projected filament width appears differently when viewed from
different angles by STEREO satellites. Here, we present a method for estimating
the width and inclination of the filament sheet using He II 304 \AA\
observations by STEREO-A and B satellites from the two viewpoints. The width of
the filament sheet, when measured from its feet to its apex, gives estimate of
filament height above the chromosphere.Comment: 9 pages, 2 figures, in Annales Geophysica
Global axis shape of magnetic clouds deduced from the distribution of their local axis orientation
Coronal mass ejections (CMEs) are routinely tracked with imagers in the
interplanetary space while magnetic clouds (MCs) properties are measured
locally by spacecraft. However, both imager and insitu data do not provide
direct estimation on the global flux rope properties. The main aim of this
study is to constrain the global shape of the flux rope axis from local
measurements, and to compare the results from in-situ data with imager
observations. We perform a statistical analysis of the set of MCs observed by
WIND spacecraft over 15 years in the vicinity of Earth. With the hypothesis of
having a sample of MCs with a uniform distribution of spacecraft crossing along
their axis, we show that a mean axis shape can be derived from the distribution
of the axis orientation. In complement, while heliospheric imagers do not
typically observe MCs but only their sheath region, we analyze one event where
the flux-rope axis can be estimated from the STEREO imagers. From the analysis
of a set of theoretical models, we show that the distribution of the local axis
orientation is strongly affected by the global axis shape. Next, we derive the
mean axis shape from the integration of the observed orientation distribution.
This shape is robust as it is mostly determined from the global shape of the
distribution. Moreover, we find no dependence on the flux-rope inclination on
the ecliptic. Finally, the derived shape is fully consistent with the one
derived from heliospheric imager observations of the June 2008 event. We have
derived a mean shape of MC axis which only depends on one free parameter, the
angular separation of the legs (as viewed from the Sun). This mean shape can be
used in various contexts such as the study of high energy particles or space
weather forecast.Comment: 13 pages, 12 figure
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