635 research outputs found
Universal Robotic Gripper based on the Jamming of Granular Material
Gripping and holding of objects are key tasks for robotic manipulators. The
development of universal grippers able to pick up unfamiliar objects of widely
varying shape and surface properties remains, however, challenging. Most
current designs are based on the multi-fingered hand, but this approach
introduces hardware and software complexities. These include large numbers of
controllable joints, the need for force sensing if objects are to be handled
securely without crushing them, and the computational overhead to decide how
much stress each finger should apply and where. Here we demonstrate a
completely different approach to a universal gripper. Individual fingers are
replaced by a single mass of granular material that, when pressed onto a target
object, flows around it and conforms to its shape. Upon application of a vacuum
the granular material contracts and hardens quickly to pinch and hold the
object without requiring sensory feedback. We find that volume changes of less
than 0.5% suffice to grip objects reliably and hold them with forces exceeding
many times their weight. We show that the operating principle is the ability of
granular materials to transition between an unjammed, deformable state and a
jammed state with solid-like rigidity. We delineate three separate mechanisms,
friction, suction and interlocking, that contribute to the gripping force.
Using a simple model we relate each of them to the mechanical strength of the
jammed state. This opens up new possibilities for the design of simple, yet
highly adaptive systems that excel at fast gripping of complex objects.Comment: 10 pages, 7 figure
Critical packing in granular shear bands
In a realistic three-dimensional setup, we simulate the slow deformation of
idealized granular media composed of spheres undergoing an axisymmetric
triaxial shear test. We follow the self-organization of the spontaneous strain
localization process leading to a shear band and demonstrate the existence of a
critical packing density inside this failure zone. The asymptotic criticality
arising from the dynamic equilibrium of dilation and compaction is found to be
restricted to the shear band, while the density outside of it keeps the memory
of the initial packing. The critical density of the shear band depends on
friction (and grain geometry) and in the limit of infinite friction it defines
a specific packing state, namely the \emph{dynamic random loose packing}
Differential postural effects of plantar-flexor muscles fatigue under normal, altered and improved vestibular and neck somatosensory conditions
The aim of the present study was to assess the effects of plantar-flexor
muscles fatigue on postural control during quiet standing under normal, altered
and improved vestibular and neck somatosensory conditions. To address this
objective, young male university students were asked to stand upright as still
as possible with their eyes closed in two conditions of No Fatigue and Fatigue
of the plantar-flexor muscles. In Experiment 1 (n=15), the postural task was
executed in two Neutral head and Head tilted backward postures, recognized to
degrade vestibular and neck somatosensory information. In Experiment 2 (n=15),
the postural task was executed in two conditions of No tactile and Tactile
stimulation of the neck provided by the application of strips of adhesive
bandage to the skin over and around the neck. Centre of foot pressure
displacements were recorded using a force platform. Results showed that (1) the
Fatigue condition yielded increased CoP displacements relative to the No
Fatigue condition (Experiment 1 and Experiment 2), (2) this destabilizing
effect was more accentuated in the Head tilted backward posture than Neutral
head posture (Experiment 1) and (3) this destabilizing effect was less
accentuated in the condition of Tactile stimulation than that of No tactile
stimulation of the neck (Experiment 2). In the context of the multisensory
control of balance, these results suggest an increased reliance on vestibular
and neck somatosensory information for controlling posture during quiet
standing in condition of altered ankle neuromuscular function
Optical design and verification of the 4mm receiver for the 20m telescope at Onsala Space Observatory
The work of this research is the design, analysis and verification of the optical performance of a 4 mm receiver channel for the 20 m telescope at Onsala Space Observatory, Onsala, Sweden. The 4 mm (75 GHz) receiver is a newly proposed channel designed to be installed parallel to the existing 3 mm (100 GHz) channel targeting new science at that longer wavelength. Gaussian beam mode analysis is used to produce the fundamental optical design of the system. The design is then analysed more accurately with the physical optics approximation. We report on the comparison of simulation and measurement and verification of the system design
Fluctuations in the coarsening dynamics of the O(N) model: are they similar to those in glassy systems?
We study spatio-temporal fluctuations in the non-equilibrium dynamics of the
d dimensional O(N) in the large N limit. We analyse the invariance of the
dynamic equations for the global correlation and response in the slow ageing
regime under transformations of time. We find that these equations are
invariant under scale transformations. We extend this study to the action in
the dynamic generating functional finding similar results. This model therefore
falls into a different category from glassy problems in which full
time-reparametrisation invariance, a larger symmetry that emcompasses time
scale invariance, is expected to be realised asymptotically. Consequently, the
spatio-temporal fluctuations of the large N O(N) model should follow a
different pattern from that of glassy systems. We compute the fluctuations of
local, as well as spatially separated, two-field composite operators and
responses, and we confront our results with the ones found numerically for the
3d Edwards-Anderson model and kinetically constrained lattice gases. We analyse
the dependence of the fluctuations of the composite operators on the growing
domain length and we compare to what has been found in super-cooled liquids and
glasses. Finally, we show that the development of time-reparametrisation
invariance in glassy systems is intimately related to a well-defined and finite
effective temperature, specified from the modification of the
fluctuation-dissipation theorem out of equilibrium. We then conjecture that the
global asymptotic time-reparametrisation invariance is broken down to time
scale invariance in all coarsening systems.Comment: 57 pages, 5 figure
Herschel observations of extra-ordinary sources: Detection of Hydrogen Fluoride in absorption towards Orion~KL
We report a detection of the fundamental rotational transition of hydrogen
fluoride in absorption towards Orion KL using Herschel/HIFI. After the removal
of contaminating features associated with common molecules ("weeds"), the HF
spectrum shows a P-Cygni profile, with weak redshifted emission and strong
blue-shifted absorption, associated with the low-velocity molecular outflow. We
derive an estimate of 2.9 x 10^13 cm^-2 for the HF column density responsible
for the broad absorption component. Using our best estimate of the H2 column
density within the low-velocity molecular outflow, we obtain a lower limit of
~1.6 x 10^-10 for the HF abundance relative to hydrogen nuclei, corresponding
to 0.6% of the solar abundance of fluorine. This value is close to that
inferred from previous ISO observations of HF J=2--1 absorption towards Sgr B2,
but is in sharp contrast to the lower limit of 6 x 10^-9 derived by Neufeld et
al. (2010) for cold, foreground clouds on the line of sight towards G10.6-0.4.Comment: 5 pages, 3 figures, paper to be published in the Herschel special
issue of A&A letter
Herschel observations of EXtra-Ordinary Sources: The Terahertz spectrum of Orion KL seen at high spectral resolution
We present the first high spectral resolution observations of Orion KL in the
frequency ranges 1573.4 - 1702.8 GHz (band 6b) and 1788.4 - 1906.8 GHz (band
7b) obtained using the HIFI instrument on board the Herschel Space Observatory.
We characterize the main emission lines found in the spectrum, which primarily
arise from a range of components associated with Orion KL including the hot
core, but also see widespread emission from components associated with
molecular outflows traced by H2O, SO2, and OH. We find that the density of
observed emission lines is significantly diminished in these bands compared to
lower frequency Herschel/HIFI bands.Comment: Accepted for publication in the Herschel HIFI special issue of
Astronomy and Astrophysics Letters, 5 pages, 3 figure
The use of implantable cardioverter defibrillators in Iceland: a retrospective population based study
BACKGROUND: Indications for implantable cardioverter defibrillator (ICD) implantation have expanded considerably in recent years, resulting in steadily growing numbers of ICD recipients worldwide. The aim of this study was to review the overall experience with ICDs in Iceland. METHODS: This was a retrospective single centre study set at the University Hospital in Iceland. Data on all ICD implantations in Iceland from the first implantation in 1992 till the end of 2002 was reviewed. RESULTS: Sixty-two patients (71% male) received an ICD during this period. There was an increase in the number of implants by year and the number of new implants in 2001 and 2002 amounted to 56 and 38 per million, respectively. The mean age at implantation was 58 (+/-14) years. Forty patients (65%) had coronary artery disease. The most common indications for ICD implantation were cardiac arrest, 32 (52%) and another 26 (42%) had experienced ventricular tachycardia without cardiac arrest. The most common adverse event was inappropriate shocks. Twenty-eight patients (45%) received therapy from their ICDs, with the majority receiving appropriate therapy. Of the thirteen patients deceased before or during the study period, no case of sudden arrhythmic death was observed. CONCLUSION: This study shows that the experience with ICDs in Iceland is in most respects similar to other Western countries
Reversal of infall in SgrB2(M) revealed by Herschel/HIFI observations of HCN lines at THz frequencies
To investigate the accretion and feedback processes in massive star
formation, we analyze the shapes of emission lines from hot molecular cores,
whose asymmetries trace infall and expansion motions. The high-mass star
forming region SgrB2(M) was observed with Herschel/HIFI (HEXOS key project) in
various lines of HCN and its isotopologues, complemented by APEX data. The
observations are compared to spherically symmetric, centrally heated models
with density power-law gradient and different velocity fields (infall or
infall+expansion), using the radiative transfer code RATRAN. The HCN line
profiles are asymmetric, with the emission peak shifting from blue to red with
increasing J and decreasing line opacity (HCN to HCN). This is most
evident in the HCN 12--11 line at 1062 GHz. These line shapes are reproduced by
a model whose velocity field changes from infall in the outer part to expansion
in the inner part. The qualitative reproduction of the HCN lines suggests that
infall dominates in the colder, outer regions, but expansion dominates in the
warmer, inner regions. We are thus witnessing the onset of feedback in massive
star formation, starting to reverse the infall and finally disrupting the whole
molecular cloud. To obtain our result, the THz lines uniquely covered by HIFI
were critically important.Comment: A&A, HIFI special issue, accepte
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