3,507 research outputs found
Thermal Recovery of Multi-Limbed Robots with Electric Actuators
The problem of finding thermally minimizing configurations of a humanoid robot to recover its actuators from unsafe thermal states is addressed. A first-order, data-driven, effort based, thermal model of the robots actuators is devised, which is used to predict future thermal states. Given this predictive capability, a map between configurations and future temperatures is formulated to find what configurations, subject to valid contact constraints, can be taken now to minimize future thermal states. Effectively, this approach is a realization of a contact-constrained thermal inverse-kinematics (IK) process. Experimental validation of the proposed approach is performed on the NASA Valkyrie robot hardware
A neuromorphic systems approach to in-memory computing with non-ideal memristive devices: From mitigation to exploitation
Memristive devices represent a promising technology for building neuromorphic
electronic systems. In addition to their compactness and non-volatility
features, they are characterized by computationally relevant physical
properties, such as state-dependence, non-linear conductance changes, and
intrinsic variability in both their switching threshold and conductance values,
that make them ideal devices for emulating the bio-physics of real synapses. In
this paper we present a spiking neural network architecture that supports the
use of memristive devices as synaptic elements, and propose mixed-signal
analog-digital interfacing circuits which mitigate the effect of variability in
their conductance values and exploit their variability in the switching
threshold, for implementing stochastic learning. The effect of device
variability is mitigated by using pairs of memristive devices configured in a
complementary push-pull mechanism and interfaced to a current-mode normalizer
circuit. The stochastic learning mechanism is obtained by mapping the desired
change in synaptic weight into a corresponding switching probability that is
derived from the intrinsic stochastic behavior of memristive devices. We
demonstrate the features of the CMOS circuits and apply the architecture
proposed to a standard neural network hand-written digit classification
benchmark based on the MNIST data-set. We evaluate the performance of the
approach proposed on this benchmark using behavioral-level spiking neural
network simulation, showing both the effect of the reduction in conductance
variability produced by the current-mode normalizer circuit, and the increase
in performance as a function of the number of memristive devices used in each
synapse.Comment: 13 pages, 12 figures, accepted for Faraday Discussion
The propagation of uncertainties in stellar population synthesis modeling III: model calibration, comparison, and evaluation
Stellar population synthesis (SPS) provides the link between the stellar and
dust content of galaxies and their observed spectral energy distributions. In
the present work we perform a comprehensive calibration of our own flexible SPS
(FSPS) model against a suite of data. Several public SPS models are
intercompared, including the models of Bruzual & Charlot (BC03), Maraston (M05)
and FSPS. The relative strengths and weaknesses of these models are evaluated,
with the following conclusions: 1) The FSPS and BC03 models compare favorably
with MC data at all ages, whereas M05 colors are too red and the age-dependence
is incorrect; 2) All models yield similar optical and near-IR colors for old
metal-poor systems, and yet they all provide poor fits to the integrated J-K
and V-K colors of both MW and M31 star clusters; 4) All models predict ugr
colors too red, D4000 strengths too strong and Hdelta strengths too weak
compared to massive red sequence galaxies, under the assumption that such
galaxies are composed solely of old metal-rich stars; 5) FSPS and, to a lesser
extent, BC03 can reproduce the optical and near-IR colors of post-starburst
galaxies, while M05 cannot. Reasons for these discrepancies are explored. The
failure at predicting the ugr colors, D4000, and Hdelta strengths can be
explained by some combination of a minority population of metal-poor stars,
young stars, blue straggler and/or blue horizontal branch stars, but not by
appealing to inadequacies in either theoretical stellar atmospheres or
canonical evolutionary phases (e.g., the main sequence turn-off). We emphasize
that due to a lack of calibrating star cluster data in regions of the
metallicity-age plane relevant for galaxies, all of these models continue to
suffer from serious uncertainties that are difficult to quantify. (ABRIDGED)Comment: 26 pages, 16 figures, submitted to ApJ. The FSPS code can be
downloaded at http://www.astro.princeton.edu/~cconroy/SPS
Double diffusivity model under stochastic forcing
The "double diffusivity" model was proposed in the late 1970s, and reworked in the early 1980s, as a continuum counterpart to existing discrete models of diffusion corresponding to high diffusivity paths, such as grain boundaries and dislocation lines. It was later rejuvenated in the 1990s to interpret experimental results on diffusion in polycrystalline and nanocrystalline specimens where grain boundaries and triple grain boundary junctions act as high diffusivity paths. Technically, the model pans out as a system of coupled Fick-type diffusion equations to represent "regular" and "high" diffusivity paths with "source terms" accounting for the mass exchange between the two paths. The model remit was extended by analogy to describe flow in porous media with double porosity, as well as to model heat conduction in media with two nonequilibrium local temperature baths, e.g., ion and electron baths. Uncoupling of the two partial differential equations leads to a higher-ordered diffusion equation, solutions of which could be obtained in terms of classical diffusion equation solutions. Similar equations could also be derived within an "internal length" gradient (ILG) mechanics formulation applied to diffusion problems, i.e., by introducing nonlocal effects, together with inertia and viscosity, in a mechanics based formulation of diffusion theory. While being remarkably successful in studies related to various aspects of transport in inhomogeneous media with deterministic microstructures and nanostructures, its implications in the presence of stochasticity have not yet been considered. This issue becomes particularly important in the case of diffusion in nanopolycrystals whose deterministic ILG-based theoretical calculations predict a relaxation time that is only about one-tenth of the actual experimentally verified time scale. This article provides the "missing link" in this estimation by adding a vital element in the ILG structure, that of stochasticity, that takes into account all boundary layer fluctuations. Our stochastic-ILG diffusion calculation confirms rapprochement between theory and experiment, thereby benchmarking a new generation of gradient-based continuum models that conform closer to real-life fluctuating environments
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