220,206 research outputs found
Strength Guided Motion
A methodology and algorithm is presented that generates motions imitating the way humans complete a lifting task under various loading conditions. The path taken depends on natural parameters: the figure geometry, the given load, the final destination, and especially, the strength model of the agent. Additional user controllable parameters of the motion are the comfort of the action and the perceived exertion of the agent. The algorithm uses this information to incrementally compute a motion path of the end effector moving the load. It is therefore instantaneously adaptable to changing force, loading, and strength conditions. Various strategies are used to model human behavior (such as pull back, add additional joints, and jerk) that compute the driving torques as the situation changes. The strength model dictates acceptable kinematic postures. The resulting algorithm offers torque control without the tedious user expression of driving forces under a dynamics model. The algorithm runs in near-realtime and offers an agent-dependent toolkit for fast path prediction. Examples are presented for various lifting tasks, including one- and two-handed lifts, and raising the body from a seated posture
Mechanistic origin of high retained strength in refractory BCC high entropy alloys up to 1900K
The body centered cubic (BCC) high entropy alloys MoNbTaW and MoNbTaVW show
exceptional strength retention up to 1900K. The mechanistic origin of the
retained strength is unknown yet is crucial for finding the best alloys across
the immense space of BCC HEA compositions. Experiments on Nb-Mo, Fe-Si and
Ti-Zr-Nb alloys report decreased mobility of edge dislocations, motivating a
theory of strengthening of edge dislocations in BCC alloys. Unlike pure BCC
metals and dilute alloys that are controlled by screw dislocation motion at low
temperatures, the strength of BCC HEAs can be controlled by edge dislocations,
and especially at high temperatures, due to the barriers created for edge glide
through the random field of solutes. A parameter-free theory for edge motion in
BCC alloys qualitatively and quantitatively captures the strength versus
temperature for the MoNbTaW and MoNbTaVW alloys. A reduced analytic version of
the theory then enables screening over >600,000 compositions in the
Mo-Nb-Ta-V-W family, identifying promising new compositions with high retained
strength and/or reduced mass density. Overall, the theory reveals an unexpected
mechanism responsible for high temperature strength in BCC alloys and paves the
way for theory-guided design of stronger high entropy alloys.Comment: This version corrects the theory and provides more extensive
explanation
Kinetics of self-induced aggregation of Brownian particles: non-Markovian and non-Gaussian features
In this paper we have studied a model for self-induced aggregation in
Brownian particle incorporating the non-Markovian and non-Gaussian character of
the associated random noise process. In this model the time evolution of each
individual is guided by an over-damped Langevin equation of motion with a
non-local drift resulting from the local unbalance distributions of the other
individuals. Our simulation result shows that colored nose can induce the
cluster formation even at large noise strength. Another observation is that
critical noise strength grows very rapidly with increase of noise correlation
time for Gaussian noise than non Gaussian one. However, at long time limit the
cluster number in aggregation process decreases with time following a power
law. The exponent in the power law increases remarkable for switching from
Markovian to non Markovian noise process
Magnetic domain-wall velocity enhancement induced by a transverse magnetic field
Spin dynamics of field-driven domain walls (DWs) guided by Permalloy
nanowires are studied by high-speed magneto-optic polarimetry and numerical
simulations. DW velocities and spin configurations are determined as functions
of longitudinal drive field, transverse bias field, and nanowire width.
Nanowires having cross-sectional dimensions large enough to support vortex wall
structures exhibit regions of drive-field strength (at zero bias field) that
have enhanced DW velocity resulting from coupled vortex structures that
suppress oscillatory motion. Factor of ten enhancements of the DW velocity are
observed above the critical longitudinal drive-field (that marks the onset of
oscillatory DW motion) when a transverse bias field is applied. Nanowires
having smaller cross-sectional dimensions that support transverse wall
structures also exhibit a region of higher mobility above the critical field,
and similar transverse-field induced velocity enhancement but with a smaller
enhancement factor. The bias-field enhancement of DW velocity is explained by
numerical simulations of the spin distribution and dynamics within the
propagating DW that reveal dynamic stabilization of coupled vortex structures
and suppression of oscillatory motion in the nanowire conduit resulting in
uniform DW motion at high speed.Comment: 8 pages, 5 figure
Wetting gradient induced separation of emulsions: A combined experimental and lattice Boltzmann computer simulation study
Guided motion of emulsions is studied via combined experimental and
theoretical investigations. The focus of the work is on basic issues related to
driving forces generated via a step-wise (abrupt) change in wetting properties
of the substrate along a given spatial direction. Experiments on binary
emulsions unambiguously show that selective wettability of the one of the fluid
components (water in our experiments) with respect to the two different parts
of the substrate is sufficient in order to drive the separation process. These
studies are accompanied by approximate analytic arguments as well as lattice
Boltzmann computer simulations, focusing on effects of a wetting gradient on
internal droplet dynamics as well as its relative strength compared to
volumetric forces driving the fluid flow. These theoretical investigations show
qualitatively different dependence of wetting gradient induced forces on
contact angle and liquid volume in the case of an open substrate as opposed to
a planar channel. In particular, for the parameter range of our experiments,
slit geometry is found to give rise to considerably higher separation forces as
compared to open substrate.Comment: 34 pages, 12 figure
Dispersion management using betatron resonances in an ultracold-atom storage ring
Specific velocities of particles circulating in a storage ring can lead to
betatron resonances at which static perturbations of the particles' orbit yield
large transverse (betatron) oscillations. We have observed betatron resonances
in an ultracold-atom storage ring by direct observation of betatron motion.
These resonances caused a near-elimination of the longitudinal dispersion of
atomic beams propagating at resonant velocities, an effect which can improve
the performance of atom interferometric devices. Both the resonant velocities
and the strength of the resonances were varied by deliberate modifications to
the storage ring.Comment: 4 pages, 5 figures. Also available at
http://physics.berkeley.edu/research/ultracol
Transverse and secondary voltages in BSCCO single crystals
Multicontact configuration is one of the most powerful arrangements for
electrical transport measurements applied to study vortex phase transition and
vortex phase dimensionality in strongly anisotropic high-Tc superconducting
materials. In this paper we present electrical transport measurements using a
multiterminal configuration, which prove both the existence of guided vortex
motion in BSCCO single crystals near the transition temperature and that
secondary voltage in zero external magnetic field is induced by thermally
activated vortex loop unbinding. The phase transition between the bound and
unbound states of the vortex loops was found to be below the temperature where
the phase coherence of the superconducting order parameter extends over the
whole volume of the sample. We show experimentally that 3D/2D phase transition
in vortex dimensionality is a length-scale-dependent layer decoupling process
and takes place simultaneously with the 3D/2D phase transition in
superconductivity at the same temperature.Comment: 8 pages, 5 figures, to be published in Physica
Dynamic Vortex Phases and Pinning in Superconductors with Twin Boundaries
We investigate the pinning and driven dynamics of vortices interacting with
twin boundaries using large scale molecular dynamics simulations on samples
with near one million pinning sites. For low applied driving forces, the vortex
lattice orients itself parallel to the twin boundary and we observe the
creation of a flux gradient and vortex free region near the edges of the twin
boundary. For increasing drive, we find evidence for several distinct dynamical
flow phases which we characterize by the density of defects in the vortex
lattice, the microscopic vortex flow patterns, and orientation of the vortex
lattice. We show that these different dynamical phases can be directly related
to microscopically measurable voltage - current V(I) curves and voltage noise.
By conducting a series of simulations for various twin boundary parameters we
derive several vortex dynamic phase diagrams.Comment: 5 figures, to appear in Phys. Rev.
Dispersive response of atoms trapped near the surface of an optical nanofiber with applications to quantum nondemolition measurement and spin squeezing
We study the strong coupling between photons and atoms that can be achieved
in an optical nanofiber geometry when the interaction is dispersive. While the
Purcell enhancement factor for spontaneous emission into the guided mode does
not reach the strong-coupling regime for individual atoms, one can obtain high
cooperativity for ensembles of a few thousand atoms due to the tight
confinement of the guided modes and constructive interference over the entire
chain of trapped atoms. We calculate the dyadic Green's function, which
determines the scattering of light by atoms in the presence of the fiber, and
thus the phase shift and polarization rotation induced on the guided light by
the trapped atoms. The Green's function is related to a full
Heisenberg-Langevin treatment of the dispersive response of the quantized field
to tensor polarizable atoms. We apply our formalism to quantum nondemolition
(QND) measurement of the atoms via polarimetry. We study shot-noise-limited
detection of atom number for atoms in a completely mixed spin state and the
squeezing of projection noise for atoms in clock states. Compared with
squeezing of atomic ensembles in free space, we capitalize on unique features
that arise in the nanofiber geometry including anisotropy of both the intensity
and polarization of the guided modes. We use a first principles stochastic
master equation to model the squeezing as function of time in the presence of
decoherence due to optical pumping. We find a peak metrological squeezing of ~5
dB is achievable with current technology for ~2500 atoms trapped 180 nm from
the surface of a nanofiber with radius a=225 nm.Comment: To be appeared on PR
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