9,984 research outputs found
Bipedal Hopping: Reduced-order Model Embedding via Optimization-based Control
This paper presents the design and validation of controlling hopping on the
3D bipedal robot Cassie. A spring-mass model is identified from the kinematics
and compliance of the robot. The spring stiffness and damping are encapsulated
by the leg length, thus actuating the leg length can create and control hopping
behaviors. Trajectory optimization via direct collocation is performed on the
spring-mass model to plan jumping and landing motions. The leg length
trajectories are utilized as desired outputs to synthesize a control Lyapunov
function based quadratic program (CLF-QP). Centroidal angular momentum, taking
as an addition output in the CLF-QP, is also stabilized in the jumping phase to
prevent whole body rotation in the underactuated flight phase. The solution to
the CLF-QP is a nonlinear feedback control law that achieves dynamic jumping
behaviors on bipedal robots with compliance. The framework presented in this
paper is verified experimentally on the bipedal robot Cassie.Comment: 8 pages, 7 figures, accepted by IROS 201
A microscopic model for solidification
We present a novel picture of a non isothermal solidification process
starting from a molecular level, where the microscopic origin of the basic
mechanisms and of the instabilities characterizing the approach to equilibrium
is rendered more apparent than in existing approaches based on coarse grained
free energy functionals \`a la Landau.
The system is composed by a lattice of Potts spins, which change their state
according to the stochastic dynamics proposed some time ago by Creutz. Such a
method is extended to include the presence of latent heat and thermal
conduction.
Not only the model agrees with previous continuum treatments, but it allows
to introduce in a consistent fashion the microscopic stochastic fluctuations.
These play an important role in nucleating the growing solid phase in the melt.
The approach is also very satisfactory from the quantitative point of view
since the relevant growth regimes are fully characterized in terms of scaling
exponents.Comment: 7 pages Latex +3 figures.p
Quantum storage of polarization qubits in birefringent and anisotropically absorbing materials
Storage of quantum information encoded into true single photons is an
essential constituent of long-distance quantum communication based on quantum
repeaters and of optical quantum information processing. The storage of
photonic polarization qubits is, however, complicated by the fact that many
materials are birefringent and have polarization-dependent absorption. Here we
present and demonstrate a simple scheme that allows compensating for these
polarization effects. The scheme is demonstrated using a solid-state quantum
memory implemented with an ensemble of rare-earth ions doped into a biaxial
yttrium orthosilicate () crystal. Heralded single photons generated
from a filtered spontaneous parametric downconversion source are stored, and
quantum state tomography of the retrieved polarization state reveals an average
fidelity of , which is significantly higher than what is
achievable with a measure-and-prepare strategy.Comment: 7 pages, 3 figures, 1 table, corrected typos and added ref. 3
Sample variance in the local measurements of the Hubble constant
The current tension between the Hubble constant measured
from local distance indicators and from cosmic microwave background is one of
the most highly debated issues in cosmology, as it possibly indicates new
physics or unknown systematics. In this work, we explore whether this tension
can be alleviated by the sample variance in the local measurements, which use a
small fraction of the Hubble volume. We use a large-volume cosmological
-body simulation to model the local measurements and to quantify the
variance due to local density fluctuations and sample selection. We explicitly
take into account the inhomogeneous spatial distribution of type Ia supernovae.
Despite the faithful modelling of the observations, our results confirm
previous findings that sample variance in the local Hubble constant measurements is small; we find $\sigma(H_0^{\rm loc})=0.31\,{\rm km\
s^{-1}Mpc^{-1}}\sim6\,{\rm km\
s^{-1}Mpc^{-1}}H_0H_0\sim
150 \,\rm Mpc(\delta\simeq -0.8)\Lambda\LambdaH_0$
measurements even after taking into account the inhomogeneous selection of type
Ia supernovae.Comment: 10 pages, 6 figures, 1 table; main result in Figure 3; replaced to
match published versio
Machine Learning tools for global PDF fits
The use of machine learning algorithms in theoretical and experimental
high-energy physics has experienced an impressive progress in recent years,
with applications from trigger selection to jet substructure classification and
detector simulation among many others. In this contribution, we review the
machine learning tools used in the NNPDF family of global QCD analyses. These
include multi-layer feed-forward neural networks for the model-independent
parametrisation of parton distributions and fragmentation functions, genetic
and covariance matrix adaptation algorithms for training and optimisation, and
closure testing for the systematic validation of the fitting methodology.Comment: 12 pages, 9 figures, to appear in the proceedings of the XXIIIth
Quark Confinement and the Hadron Spectrum conference, 1-6 August 2018,
University of Maynooth, Irelan
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