436 research outputs found
On A Parabolic Equation in MEMS with An External Pressure
The parabolic problem on a
bounded domain of with Dirichlet boundary condition models the
microelectromechanical systems(MEMS) device with an external pressure term. In
this paper, we classify the behavior of the solution to this equation. We first
show that under certain initial conditions, there exists critical constants
and such that when , , there exists a global solution, while for or , the solution quenches in finite time. The
estimate of voltage , quenching time and pressure term
are investigated. The quenching set is proved to be a compact subset
of with an additional condition, provided is a
convex bounded set. In particular, if is radially symmetric, then the
origin is the only quenching point. Furthermore, we not only derive the
two-side bound estimate for the quenching solution, but also study the
asymptotic behavior of the quenching solution in finite time.Comment: 35 pages, 1 figure. arXiv admin note: substantial text overlap with
arXiv:1402.0066 by other author
Multi-Phase Multi-Objective Dexterous Manipulation with Adaptive Hierarchical Curriculum
Dexterous manipulation tasks usually have multiple objectives, and the
priorities of these objectives may vary at different phases of a manipulation
task. Varying priority makes a robot hardly or even failed to learn an optimal
policy with a deep reinforcement learning (DRL) method. To solve this problem,
we develop a novel Adaptive Hierarchical Reward Mechanism (AHRM) to guide the
DRL agent to learn manipulation tasks with multiple prioritized objectives. The
AHRM can determine the objective priorities during the learning process and
update the reward hierarchy to adapt to the changing objective priorities at
different phases. The proposed method is validated in a multi-objective
manipulation task with a JACO robot arm in which the robot needs to manipulate
a target with obstacles surrounded. The simulation and physical experiment
results show that the proposed method improved robot learning in task
performance and learning efficiency.Comment: Accepted by the Journal of Intelligent & Robotic System
Stable In-hand Manipulation with Finger Specific Multi-agent Shadow Reward
Deep Reinforcement Learning has shown its capability to solve the high
degrees of freedom in control and the complex interaction with the object in
the multi-finger dexterous in-hand manipulation tasks. Current DRL approaches
prefer sparse rewards to dense rewards for the ease of training but lack
behavior constraints during the manipulation process, leading to aggressive and
unstable policies that are insufficient for safety-critical in-hand
manipulation tasks. Dense rewards can regulate the policy to learn stable
manipulation behaviors with continuous reward constraints but are hard to
empirically define and slow to converge optimally. This work proposes the
Finger-specific Multi-agent Shadow Reward (FMSR) method to determine the stable
manipulation constraints in the form of dense reward based on the state-action
occupancy measure, a general utility of DRL that is approximated during the
learning process. Information Sharing (IS) across neighboring agents enables
consensus training to accelerate the convergence. The methods are evaluated in
two in-hand manipulation tasks on the Shadow Hand. The results show FMSR+IS
converges faster in training, achieving a higher task success rate and better
manipulation stability than conventional dense reward. The comparison indicates
FMSR+IS achieves a comparable success rate even with the behavior constraint
but much better manipulation stability than the policy trained with a sparse
reward
Curriculum-based Sensing Reduction in Simulation to Real-World Transfer for In-hand Manipulation
Simulation to Real-World Transfer allows affordable and fast training of
learning-based robots for manipulation tasks using Deep Reinforcement Learning
methods. Currently, Sim2Real uses Asymmetric Actor-Critic approaches to reduce
the rich idealized features in simulation to the accessible ones in the real
world. However, the feature reduction from the simulation to the real world is
conducted through an empirically defined one-step curtail. Small feature
reduction does not sufficiently remove the actor's features, which may still
cause difficulty setting up the physical system, while large feature reduction
may cause difficulty and inefficiency in training. To address this issue, we
proposed Curriculum-based Sensing Reduction to enable the actor to start with
the same rich feature space as the critic and then get rid of the
hard-to-extract features step-by-step for higher training performance and
better adaptation for real-world feature space. The reduced features are
replaced with random signals from a Deep Random Generator to remove the
dependency between the output and the removed features and avoid creating new
dependencies. The methods are evaluated on the Allegro robot hand in a
real-world in-hand manipulation task. The results show that our methods have
faster training and higher task performance than baselines and can solve
real-world tasks when selected tactile features are reduced
A Novel Center-based Deep Contrastive Metric Learning Method for the Detection of Polymicrogyria in Pediatric Brain MRI
Polymicrogyria (PMG) is a disorder of cortical organization mainly seen in
children, which can be associated with seizures, developmental delay and motor
weakness. PMG is typically diagnosed on magnetic resonance imaging (MRI) but
some cases can be challenging to detect even for experienced radiologists. In
this study, we create an open pediatric MRI dataset (PPMR) with PMG and
controls from the Children's Hospital of Eastern Ontario (CHEO), Ottawa,
Canada. The differences between PMG MRIs and control MRIs are subtle and the
true distribution of the features of the disease is unknown. This makes
automatic detection of cases of potential PMG in MRI difficult. We propose an
anomaly detection method based on a novel center-based deep contrastive metric
learning loss function (cDCM) which enables the automatic detection of cases of
potential PMG. Additionally, based on our proposed loss function, we customize
a deep learning model structure that integrates dilated convolution,
squeeze-and-excitation blocks and feature fusion for our PPMR dataset. Despite
working with a small and imbalanced dataset our method achieves 92.01% recall
at 55.04% precision. This will facilitate a computer aided tool for
radiologists to select potential PMG MRIs. To the best of our knowledge, this
research is the first to apply machine learning techniques to identify PMG from
MRI only.Comment: 24 pages, 13 figure
Late Gas accretion onto Primordial Minihalos: a Model for Leo T, Dark Galaxies and Extragalactic High-Velocity Clouds
In this letter we revisit the idea of reionization feedback on dwarf galaxy
formation. We show that primordial minihalos with v_cir<20 km/s stop accreting
gas after reionization, as it is usually assumed, but in virtue of their
increasing concentration and the decreasing temperature of the intergalactic
medium as redshift decreases below z=3, they have a late phase of gas accretion
and possibly star formation. We expect that pre-reionization fossils that
evolved on the outskirts of the Milky Way or in isolation show a bimodal star
formation history with 12 Gyr old and <10 Gyr old population of stars. Leo T
fits with this scenario. Another prediction of the model is the possible
existence of a population of gas rich minihalos that never formed stars. More
work is needed to understand whether a subset of compact high-velocity clouds
can be identified as such objects or whether an undiscovered population exists
in the voids between galaxies.Comment: 5 pages, 4 figures, accepted version MNRAS 392, L4
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