1,945 research outputs found
Keep Rollin' - Whole-Body Motion Control and Planning for Wheeled Quadrupedal Robots
We show dynamic locomotion strategies for wheeled quadrupedal robots, which
combine the advantages of both walking and driving. The developed optimization
framework tightly integrates the additional degrees of freedom introduced by
the wheels. Our approach relies on a zero-moment point based motion
optimization which continuously updates reference trajectories. The reference
motions are tracked by a hierarchical whole-body controller which computes
optimal generalized accelerations and contact forces by solving a sequence of
prioritized tasks including the nonholonomic rolling constraints. Our approach
has been tested on ANYmal, a quadrupedal robot that is fully torque-controlled
including the non-steerable wheels attached to its legs. We conducted
experiments on flat and inclined terrains as well as over steps, whereby we
show that integrating the wheels into the motion control and planning framework
results in intuitive motion trajectories, which enable more robust and dynamic
locomotion compared to other wheeled-legged robots. Moreover, with a speed of 4
m/s and a reduction of the cost of transport by 83 % we prove the superiority
of wheeled-legged robots compared to their legged counterparts.Comment: IEEE Robotics and Automation Letter
The Soft X-ray Spectrum from NGC 1068 Observed with LETGS on Chandra
Using the combined spectral and spatial resolving power of the Low Energy
Transmission Grating (LETGS) on board Chandra, we obtain separate spectra from
the bright central source of NGC 1068 (Primary region), and from a fainter
bright spot 4" to the NE (Secondary region). Both spectra are dominated by line
emission from H- and He-like ions of C through S, and from Fe L-shell ions, but
also include narrow radiative recombination continua, indicating that most of
the soft X-ray emission arises in low-temperature (kT few eV) photoionized
plasma. We confirm the conclusions of Kinkhabwala et al. (2002), based on
XMM-Newton RGS observations, that the entire nuclear spectrum can be explained
by recombination/radiative cascade following photoionization, and radiative
decay following photoexcitation, with no evidence for hot, collisionally
ionized plasma. In addition, this model also provides an excellent fit to the
spectrum of the Secondary region, albeit with radial column densities a factor
of three lower, as would be expected given its distance from the source of the
ionizing continuum. The remarkable overlap and kinematical agreement of the
optical and X-ray line emission, coupled with the need for a distribution of
ionization parameter to explain the X-ray spectra, collectively imply the
presence of a distribution of densities (over a few orders of magnitude) at
each radius in the ionization cone. Relative abundances of all elements are
consistent with Solar abundance, except for N, which is 2-3 times Solar. The
long wavelength spectrum beyond 30 A is rich of L-shell transitions of Mg, Si,
S, and Ar, and M-shell transitions of Fe. The velocity dispersion decreases
with increasing ionization parameter, as deduced from these long wavelength
lines and the Fe-L shell lines.Comment: 12 pages, 11 figures, accepted for publication in Astronomy and
Astrophysic
Euler number of Instanton Moduli space and Seiberg-Witten invariants
We show that a partition function of topological twisted N=4 Yang-Mills
theory is given by Seiberg-Witten invariants on a Riemannian four manifolds
under the condition that the sum of Euler number and signature of the four
manifolds vanish. The partition function is the sum of Euler number of
instanton moduli space when it is possible to apply the vanishing theorem. And
we get a relation of Euler number labeled by the instanton number with
Seiberg-Witten invariants, too. All calculation in this paper is done without
assuming duality.Comment: LaTeX, 34 page
The Ionized Stellar Wind in Vela X-1 During Eclipse
We present a first analysis of a high resolution X-ray spectrum of the
ionized stellar wind of Vela X-1 during eclipse. The data were obtained with
the High Energy Transmission Grating Spectrometer onboard the Chandra X-ray
Observatory. The spectrum is resolved into emission lines with fluxes between
0.02 and 1.04x10^4 ph/cm^2/s. We identify lines from a variety of charge
states, including fluorescence lines from cold material, a warm photoionized
wind. We can exclude signatures from collisionally ionized plasmas. For the
first time we identify fluorescence lines from L-shell ions from lower Z
elements. We also detect radiative recombination continua from a kT = 10 eV
(1.2 x 10^5 K) photoionized optically thin gas. The fluorescence line fluxes
infer the existence of optically thick and clumped matter within or outside the
warm photoionized plasma.Comment: 4 pages, 2 figures, accepted by ApJ letter
Noncommutative Moduli for Multi-Instantons
There exists a recursive algorithm for constructing BPST-type
multi-instantons on commutative R^4. When deformed noncommutatively, however,
it becomes difficult to write down non-singular instanton configurations with
topological charge greater than one in explicit form. We circumvent this
difficulty by allowing for the translational instanton moduli to become
noncommutative as well. This makes possible the ADHM construction of 't Hooft
multi-instanton solutions with everywhere self-dual field strengths on
noncommutative R^4.Comment: 1+9 pages; v2: reference added, published versio
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