450 research outputs found
Robotic manipulator inspired by human fingers based on tendon-driven soft grasping
Die menschliche Hand ist in der Lage, verschiedene Greif- und Manipulationsaufgaben auszufĂŒhren und kann als einer der geschicktesten und vielseitigsten Effektoren angesehen werden.
In dieser Arbeit wurde ein Soft Robotic-Greifer entwickelt, der auf den Erkenntnissen aus der Literatur zur Taxonomie der menschlichen GreiffĂ€higkeiten und den biomechanischen Synergien der menschlichen Hand basiert. Im Bereich der RoboterhĂ€nde sind sehnengetriebene, unteraktuierte Strukturen weit verbreitet. Inspiriert von der Anatomie der menschlichen Hand, bieten sie durch ihre FlexibilitĂ€t passive AdaptivitĂ€t und Robustheit. Es wurde ein Sensorsystem implementiert, bestehend aus Force Sensing Resistors (FSRs), Biegungssensoren und einem Stromsensor, wodurch das System charakterisiert werden kann. Die Kraftsensoren wurden in die Fingerkuppen integriert. In Anlehnung an die menschliche Haut wurden AbgĂŒsse aus Silikonkautschuk an den Fingerballen verwendet. Diese versprechen eine erhöhte Reibung und bessere AdaptivitĂ€t zum gegriffenen Objekt. Um den entwickelten Greifer zu evaluieren, wurden erste Tests durchgefĂŒhrt. ZunĂ€chst wurde die FunktionalitĂ€t der Sensoren, wie z.B. der als FSRs ausgewĂ€hlten Kraftsensoren, getestet. Im weiteren Verlauf wurden die GreiffĂ€higkeiten des Greifers durch Manipulation verschiedener Objekte getestet. Basierend auf den Erkenntnissen aus den praktischen Versuchen kann festgestellt werden, dass der entwickelte Greifer ein hohes MaĂ an Geschicklichkeit aufweist. Auch die AdaptivitĂ€t ist dank der verwendeten mechanischen Komponenten gewĂ€hrleistet. Mittels der Sensorik ist es möglich, den Greifprozess zu kontrollieren. Die Ergebnisse zeigen aber auch, dass z. B. die interne Systemreibung die Verlustleistung des Systems stark beeinflusst.The human hand is able to perform various grasping and manipulation tasks, and can be seen as one of the most dexterous and versatile effectors known. The prehensile capabilities of the hand have already been analyzed, categorized and summarized in a taxonomy in numerous studies. In addition to the taxonomies, research on the biomechanical synergies of the human hand led to the following conceptions: The adduction/abduction movement is independent of the flexion/extension movement. Furthermore, the thumb is rather independent in its mobility from the other fingers, while those move synchronously within their corresponding joints. Lastly, the consideration of the synergies provides that the proximal and distal interphalangeal joints of a human finger are more intensely coordinated than those of the metacarpal joints. In this work, a soft robotic gripper was developed based on the knowledge from the literature on the taxonomy of human gripping abilities and the biomechanical synergies of the human hand. In the domain of robotic hands, tendon-driven underactuated structures are widely used. Inspired by the tensegrity structure of the human hand, they offer passive adaptivity and robustness through their flexibility. A sensor system was implemented, consisting of FSRs, flex sensors and a current sensor, thus the system parameters can be characterized continously. The force sensors were integrated into the fingertips. Molds of silicone rubber were used as finger pads to provide higher friction and better adaptivity to the grasped object on the contact areas of the finger, to mimic human skin. Initial tests were carried out to evaluate the gripper. First, the functionality of the sensors, such as the force sensors selected as FSRs, was tested. In the further course, the gripping capabilities of the gripper were tested by manipulation of various different objects. Based on the findings from the practical experiments, it may be stated that the gripper has a high degree of dexterity. Thanks to the mechanical components used, adaptivity is guaranteed as well. By means of the sensor system it is possible to control the gripping processes. However, the results also showed that, for example, the internal system friction dominates the systemâs power dissipation
Fitting orbits to tidal streams with proper motions
The Galaxy's stellar halo seems to be a tangle of disrupted systems that have
been tidally stretched out into streams. Each stream approximately delineates
an orbit in the Galactic force-field. In the first paper in this series we
showed that all six phase-space coordinates of each point on an orbit can be
reconstructed from the orbit's path across the sky and measurements of the
line-of-sight velocity along the orbit. In this paper we complement this
finding by showing that the orbit can also be reconstructed if we know proper
motions along the orbit rather than the radial velocities. We also show that
accurate proper motions of stream stars would enable distances to be determined
to points on the stream that are independent of any assumption about the
Galaxy's gravitational potential. Such "Galactic parallaxes" would be as
fundamental as conventional trigonometric parallaxes, but measureable to
distances ~70 times further.Comment: 5 pages, 2 figures, submitted to MNRAS Letter
Four New Stellar Debris Streams in the Galactic Halo
We report on the detection of four new stellar debris streams and a new dwarf
galaxy candidate in the Sloan Digital Sky Survey. Three of the streams, ranging
between 3 and 15 kpc in distance and spanning between 37 and 84 degrees on the
sky, are very narrow and are most probably tidal streams originating in extant
or disrupted globular clusters. The fourth stream is much broader, roughly 45
kpc distant, at least 53 degrees in length, and is most likely the tidal debris
from a dwarf galaxy. As the streams each span multiple constellations, we
extend tradition and designate them the Acheron, Cocytos, Lethe, and Styx
streams. At the same distance and apparently embedded in the Styx stream is a
~1 kpc-wide concentration of stars with a similar color-magnitude distribution
which we designate Bootes III. Given its very low surface density, its location
within the stream, and its apparently disturbed morphology, we argue that
Bootes III may be the progenitor of Styx and in possibly the final throes of
tidal dissolution. While the current data do not permit strong constraints,
preliminary orbit estimates for the streams do not point to any likely
progenitors among the known globular clusters and dwarf galaxies.Comment: 10 pages, accepted for publication in the Astrophysical Journa
Search for Extratidal Features Around 17 Globular Clusters in the Sloan Digital Sky Survey
The dynamical evolution of a single globular cluster and also of the entire
Galactic globular cluster system has been studied theoretically in detail. In
particular, simulations show how the 'lost' stars are distributed in tidal
tails emerging from the clusters. We investigate the distribution of Galactic
globular cluster stars on the sky to identify such features like tidal tails.
The Sloan Digital Sky Survey provides consistent photometry of a large part of
the sky to study the projected two dimensional structure of the 17 globular
clusters in its survey area. We use a color-magnitude weighted counting
algorithm to map (potential) cluster member stars on the sky. We recover the
already known tidal tails of Pal 5 and NGC 5466. For NGC 4147 we have found a
two arm morphology. Possible indications of tidal tails are also seen around
NGC 5053 and NGC 7078, supporting earlier suggestions. Moreover, we find
potential tails around NGC 5904 and Pal 14. Especially for the Palomar clusters
than Pal 5, deeper data are needed in order to confirm or to rule out the
existence of tails. For many of the remaining clusters in our sample we observe
a pronounced extratidal halo, which is particularly large for NGC 7006 and Pal
1. In some cases, the extratidal halos may be associated with the stream of the
Sagittarius dwarf spheroidal galaxy (e.g.,NGC 4147, NGC 5024, NGC 5053).Comment: Accepted by A&A, 24 pages, 24 figure
The luminosity function of Palomar 5 and its tidal tails
We present the main sequence luminosity function of the tidally disrupted
globular cluster Palomar 5 and its tidal tails. For this work we analyzed
imaging data obtained with the Wide Field Camera at the INT (La Palma) and data
from the Wide Field Imager at the MPG/ESO 2.2 m telescope at La Silla down to a
limiting magnitude of approximately 24.5 mag in B. Our results indicate that
preferentially fainter stars were removed from the cluster so that the LF of
the cluster's main body exhibits a significant degree of flattening compared to
other GCs. This is attributed to its advanced dynamical evolution. The LF of
the tails is, in turn, enhanced with faint, low-mass stars, which we interpret
as a consequence of mass segregation in the cluster.Comment: 4 pages, 3 figures, to be published in the proceedings of the
conference "Satellites and tidal streams" held at La Palma, Canary Islands,
May 26 - 30, 200
Why is the mass function of NGC 6218 flat?
We have used the FORS-1 camera on the VLT to study the main sequence (MS) of
the globular cluster NGC 6218 in the V and R bands. The observations cover an
area of 3.4 x 3.4 around the cluster centre and probe the stellar population
out to the cluster's half-mass radius (r_h ~ 2.2). The colour-magnitude diagram
(CMD) that we derive in this way reveals a narrow and well defined MS extending
down to the 5 sigma detection limit at V~25, or about 6 magnitudes below the
turn-off, corresponding to stars of ~ 0.25 Msolar. The luminosity function (LF)
obtained with these data shows a marked radial gradient, in that the ratio of
lower- and higher-mass stars increases monotonically with radius. The mass
function (MF) measured at the half-mass radius, and as such representative of
the clusters global properties, is surprisingly flat. Over the range 0.4 - 0.8
Msolar, the number of stars per unit mass follows a power-law distribution of
the type dN/dm \propto m^{0}, where, for comparison, Salpeter's IMF would be
dN/dm \propto m^{-2.35}. We expect that such a flat MF does not represent the
cluster's IMF but is the result of severe tidal stripping of the stars from the
cluster due to its interaction with the Galaxy's gravitational field. Our
results cannot be reconciled with the predictions of recent theoretical models
that imply a relatively insignificant loss of stars from NGC 6218 as measured
by its expected very long time to disruption. They are more consistent with the
orbital parameters based on the Hipparcos reference system that imply a much
higher degree of interaction of this cluster with the Galaxy than assumed by
those models. Our results indicate that, if the orbit of a cluster is known,
the slope of its MF could be useful in discriminating between the various
models of the Galactic potential.Comment: 11 pages, 7 figures, accepted for publication in Astronomy and
Astrophysic
Constraining the Milky Way potential with a 6-D phase-space map of the GD-1 stellar stream
The narrow GD-1 stream of stars, spanning 60 deg on the sky at a distance of
~10 kpc from the Sun and ~15 kpc from the Galactic center, is presumed to be
debris from a tidally disrupted star cluster that traces out a test-particle
orbit in the Milky Way halo. We combine SDSS photometry, USNO-B astrometry, and
SDSS and Calar Alto spectroscopy to construct a complete, empirical
6-dimensional phase-space map of the stream. We find that an eccentric orbit in
a flattened isothermal potential describes this phase-space map well. Even
after marginalizing over the stream orbital parameters and the distance from
the Sun to the Galactic center, the orbital fit to GD-1 places strong
constraints on the circular velocity at the Sun's radius V_c=224 \pm 13 km/s
and total potential flattening q_\Phi=0.87^{+0.07}_{-0.04}. When we drop any
informative priors on V_c the GD-1 constraint becomes V_c=221 \pm 18 km/s. Our
6-D map of GD-1 therefore yields the best current constraint on V_c and the
only strong constraint on q_\Phi at Galactocentric radii near R~15 kpc. Much,
if not all, of the total potential flattening may be attributed to the mass in
the stellar disk, so the GD-1 constraints on the flattening of the halo itself
are weak: q_{\Phi,halo}>0.89 at 90% confidence. The greatest uncertainty in the
6-D map and the orbital analysis stems from the photometric distances, which
will be obviated by Gaia.Comment: 16 pages, 18 figures; accepted to ApJ; full resolution version is
available at http://www.ast.cam.ac.uk/~koposov/files/gd1_fullres.pd
On the structure of tidal tails
We examine the longitudinal distribution of the stars escaping from a cluster
along tidal tails. Using both theory and simulations, we show that, even in the
case of a star cluster in a circular galactic orbit, when the tide is steady,
the distribution exhibits maxima at a distance of many tidal radii from the
cluster.Comment: 6 pages, 4 figures, accepted for publication in MNRA
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