9,303 research outputs found
Toward Controllable Hydraulic Coupling of Joints in a Wearable Robot
In this paper, we develop theoretical foundations for a new class of rehabilitation robot: body powered devices that route power between a user’s joints. By harvesting power from a healthy joint to assist an impaired joint, novel bimanual and self-assist therapies are enabled. This approach complements existing robotic therapies aimed at promoting recovery of motor function after neurological injury.
We employ hydraulic transmissions for routing power, or equivalently for coupling the motions of a user’s joints. Fluid power routed through flexible tubing imposes constraints within a limb or between homologous joints across the body. Variable transmissions allow constraints to be steered on the fly, and simple valve switching realizes free space and locked motion.
We examine two methods for realizing variable hydraulic transmissions: using valves to switch among redundant cylinders (digital hydraulics) or using an intervening electromechanical link. For both methods, we present a rigorous mathematical framework for describing and controlling the resulting constraints. Theoretical developments are supported by experiments using a prototype fluid-power exoskeleton
Effects of image charges, interfacial charge discreteness, and surface roughness on the zeta potential of spherical electric double layers
We investigate the effects of image charges, interfacial charge discreteness,
and surface roughness on spherical electric double layers in electrolyte
solutions with divalent counter-ions in the setting of the primitive model. By
using Monte Carlo simulations and the image charge method, the zeta potential
profile and the integrated charge distribution function are computed for
varying surface charge strengths and salt concentrations. Systematic
comparisons were carried out between three distinct models for interfacial
charges: 1) SURF1 with uniform surface charges, 2) SURF2 with discrete point
charges on the interface, and 3) SURF3 with discrete interfacial charges and
finite excluded volume. By comparing the integrated charge distribution
function (ICDF) and potential profile, we argue that the potential at the
distance of one ion diameter from the macroion surface is a suitable location
to define the zeta potential. In SURF2 model, we find that image charge effects
strongly enhance charge inversion for monovalent interfacial charges, and
strongly suppress charge inversion for multivalent interfacial charges. For
SURF3, the image charge effect becomes much smaller. Finally, with image
charges in action, we find that excluded volumes (in SURF3) suppress charge
inversion for monovalent interfacial charges and enhance charge inversion for
multivalent interfacial charges. Overall, our results demonstrate that all
these aspects, i.e., image charges, interfacial charge discreteness, their
excluding volumes have significant impacts on the zeta potential, and thus the
structure of electric double layers.Comment: 11 pages, 10 figures, some errors are change
Destruction of long-range antiferromagnetic order by hole doping
We study the renormalization of the staggered magnetization of a
two-dimensional antiferromagnet as a function of hole doping, in the framework
of the t-J model. It is shown that the motion of holes generates decay of spin
waves into ''particle-hole'' pairs, which causes the destruction of the
long-range magnetic order at a small hole concentration. This effect is mainly
determined by the coherent motion of holes. The value obtained for the critical
hole concentration, of a few percent, is consistent with experimental data for
the doped copper oxide high-Tc superconductors.Comment: 12 pages, 2 figure
Autocorrelated process control: Geometric Brownian Motion approach versus Box-Jenkins approach
Existing of autocorrelation will bring a significant effect on the performance and
accuracy of process control if the problem does not handle carefully. When dealing with
autocorrelated process, Box-Jenkins method will be preferred because of the popularity.
However, the computation of Box-Jenkins method is too complicated and challenging which
cause of time-consuming. Therefore, an alternative method which known as Geometric
Brownian Motion (GBM) is introduced to monitor the autocorrelated process. One real case of
furnace temperature data is conducted to compare the performance of Box-Jenkins and GBM
methods in monitoring autocorrelation process. Both methods give the same results in terms of
model accuracy and monitoring process control. Yet, GBM is superior compared to Box-Jenkins
method due to its simplicity and practically with shorter computational time
A sol-gel method for growing superconducting MgB2 films
In this paper we report a new sol-gel method for the fabrication of MgB2
films. Polycrystalline MgB2 films were prepared by spin-coating a precursor
solution of Mg(BH_4)_2 diethyl ether on (001)Al2O3 substrates followed with
annealing in Mg vapor. In comparison with the MgB2 films grown by other
techniques, our films show medium qualities including a superconducting
transition temperature of Tc ~ 37 K, a critical current density of Jc(5 K, 0 T)
~ 5 {\times} 10^6 A cm^{-2}, and a critical field of H_{c2}(0) ~ 19 T. Such a
sol-gel technique shows potential in the commercial fabrication of practically
used MgB2 films as well as MgB2 wires and tapes.Comment: 8 pages, 5 figure
Gossamer Superconductivity near Antiferromagnetic Mott Insulator in Layered Organic Conductors
Layered organic superconductors are on the verge of the Mott insulator. We
use Gutzwiller variational method to study a Hubbard model including a spin
exchange coupling term. The ground state is found to be a Gossamer
superconductor at small on-site Coulomb repulsion U and an antiferromagnetic
Mott insulator at large U, separated by a first order phase transition. Our
theory is qualitatively consistent with major experiments reported in organic
superconductors.Comment: 5 pages, 3 figure
Strongly Ideal Robust Weyl Semimetals in Cubic Symmetry with Spatial-Inversion Breaking
We show that compounds in a family that possess time-reversal symmetry and
share a non-centrosymmetric cubic structure with the space group F-43m (No.
216) host robust ideal Weyl semi-metal fermions with desirable topologically
protected features. The candidates in this family are compounds with different
chemical formulas AB2, ABC, ABC2, and ABCD and their Fermi levels are
predominantly populated by nontrivial Weyl fermions. Symmetry of the system
requires that the Weyl nodes with opposite chirality are well separated in
momentum space. Adjacent Weyl points have the same chirality, thus these Weyl
nodes would not annihilate each other with respect to lattice perturbations. As
Fermi arcs and surface states connect Weyl nodes with opposite chirality, the
large separation of the latter in momentum space guarantees the appearance of
very long arcs and surface states. This work demonstrates the use of system
symmetry by first-principles calculations as a powerful recipe for discovering
new Weyl semi-metals with attractive features whose protected fermions may be
candidates of many applications.Comment: 15 pages, 4 figure
Nonequilibrium relaxation of the two-dimensional Ising model: Series-expansion and Monte Carlo studies
We study the critical relaxation of the two-dimensional Ising model from a
fully ordered configuration by series expansion in time t and by Monte Carlo
simulation. Both the magnetization (m) and energy series are obtained up to
12-th order. An accurate estimate from series analysis for the dynamical
critical exponent z is difficult but compatible with 2.2. We also use Monte
Carlo simulation to determine an effective exponent, z_eff(t) = - {1/8} d ln t
/d ln m, directly from a ratio of three-spin correlation to m. Extrapolation to
t = infinity leads to an estimate z = 2.169 +/- 0.003.Comment: 9 pages including 2 figure
Spin wave theory for antiferromagnetic XXZ spin model on a triangle lattice in the presence of an external magnetic field
Spin wave theory is applied to a quantum antiferromagnetic XXZ model on a
triangle lattice in the presence of an in-plane magnetic field. The effect of
the field is found to enhance the quantum fluctuation and to reduce the
sublattice magnetization at the intermediate field strength in the anisotropic
case. The possible implication to the field driven quantum phase transition
from a spin solid to a spin liquid is discussed.Comment: 5 pages,4 figure
Dual E627K and D701N mutations in the PB2 protein of A(H7N9) influenza virus increased its virulence in mammalian models
The ongoing avian H7N9 influenza outbreaks in China have caused significant human fatal cases and the virus is becoming established in poultry. Mutations with potential to increase mammalian adaptation have occurred in the polymerase basic protein 2 (PB2) and other viral genes. Here we found that dual 627K and 701N mutations could readily occur during transmission of the virus among ferrets via direct physical contact, and these mutations conferred higher polymerase activity and improved viral replication in mammalian cells, and enhanced virulence in mice. Special attention needs to be paid to patients with such mutations, as these may serve as an indicator of higher virus replication and increased pathogenicity.published_or_final_versio
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