1,256 research outputs found
Planning for Decentralized Control of Multiple Robots Under Uncertainty
We describe a probabilistic framework for synthesizing control policies for
general multi-robot systems, given environment and sensor models and a cost
function. Decentralized, partially observable Markov decision processes
(Dec-POMDPs) are a general model of decision processes where a team of agents
must cooperate to optimize some objective (specified by a shared reward or cost
function) in the presence of uncertainty, but where communication limitations
mean that the agents cannot share their state, so execution must proceed in a
decentralized fashion. While Dec-POMDPs are typically intractable to solve for
real-world problems, recent research on the use of macro-actions in Dec-POMDPs
has significantly increased the size of problem that can be practically solved
as a Dec-POMDP. We describe this general model, and show how, in contrast to
most existing methods that are specialized to a particular problem class, it
can synthesize control policies that use whatever opportunities for
coordination are present in the problem, while balancing off uncertainty in
outcomes, sensor information, and information about other agents. We use three
variations on a warehouse task to show that a single planner of this type can
generate cooperative behavior using task allocation, direct communication, and
signaling, as appropriate
Real-Time Predictive Modeling and Robust Avoidance of Pedestrians with Uncertain, Changing Intentions
To plan safe trajectories in urban environments, autonomous vehicles must be
able to quickly assess the future intentions of dynamic agents. Pedestrians are
particularly challenging to model, as their motion patterns are often uncertain
and/or unknown a priori. This paper presents a novel changepoint detection and
clustering algorithm that, when coupled with offline unsupervised learning of a
Gaussian process mixture model (DPGP), enables quick detection of changes in
intent and online learning of motion patterns not seen in prior training data.
The resulting long-term movement predictions demonstrate improved accuracy
relative to offline learning alone, in terms of both intent and trajectory
prediction. By embedding these predictions within a chance-constrained motion
planner, trajectories which are probabilistically safe to pedestrian motions
can be identified in real-time. Hardware experiments demonstrate that this
approach can accurately predict pedestrian motion patterns from onboard
sensor/perception data and facilitate robust navigation within a dynamic
environment.Comment: Submitted to 2014 International Workshop on the Algorithmic
Foundations of Robotic
S-matrix approach to quantum gases in the unitary limit II: the three-dimensional case
A new analytic treatment of three-dimensional homogeneous Bose and Fermi
gases in the unitary limit of negative infinite scattering length is presented,
based on the S-matrix approach to statistical mechanics we recently developed.
The unitary limit occurs at a fixed point of the renormalization group with
dynamical exponent z=2 where the S-matrix equals -1. For fermions we find T_c
/T_F is approximately 0.1. For bosons we present evidence that the gas does not
collapse, but rather has a critical point that is a strongly interacting form
of Bose-Einstein condensation. This bosonic critical point occurs at n lambda^3
approximately 1.3 where n is the density and lambda the thermal wavelength,
which is lower than the ideal gas value of 2.61.Comment: 26 pages, 16 figure
Resolved Spectra of the Narrow-Line Region in NGC 1068. III. Physical Conditions in the Emission-Line Gas
The physical conditions in the inner narrow line region (NLR) of the Seyfert
2 galaxy NGC 1068 are examined using HST/STIS ultraviolet and optical spectra
and photoionization models. 1) The emission-line gas in the blueshifted
northeast quadrant is photoionized by the hidden central source out to 100 pc,
at which point we find evidence of another source of ionizing radiation, which
may be due to fast (1000 km/s) shocks resulting from the interaction of the
emission-line knots and the interstellar medium. 2) The gas in the redshifted
northeast quadrant is photoionized by continuum radiation that has been heavily
absorbed by gas within 30 pc of the central source. We find no strong evidence
of the effects of shocks in this component. 3) The redshifted emission-line gas
in the southwest quadrant is photoionized by unabsorbed continuum from the
central source, similar to that in the inner 100 pc of the blueshifted
northeast quadrant. Finally, 4) the emission-line spectrum of the blueshifted
southwest quadrant appears to be the superposition of highly ionized, tenuous
component within the ionization cone and gas outside the cone, the latter
photoionized by scattered continuum radiation. There are several implications
of this complicated physical scenario. First, the hidden active nucleus is the
dominant source of ionizing radiation in the inner NLR. The absorption of
continuum radiation along the line-of-sight to the redshifted northeast
quadrant may result from the intersection of the ionization cone and the plane
of the host galaxy. Finally, the evidence for shock-induced continuum radiation
at the point where the emission-line knots begin to decelerate indicates that
the deceleration is due to the interaction of emission-line knots with slower
moving gas.Comment: 53 pages, Latex, includes 4 figures (postscript), two additional
tables in Latex landscape format, to appear in the Astrophysical Journa
Probabilistically Safe Avoidance of Dynamic Obstacles with Uncertain Motion Patterns
This paper presents a real-time path planning algorithm which can guarantee
probabilistic feasibility for autonomous robots subject to process noise and an
uncertain environment, including dynamic obstacles with uncertain motion
patterns. The key contribution of the work is the
integration of a novel method for modeling dynamic obstacles with uncertain future
trajectories. The method, denoted as RR-GP, uses a learned motion pattern model
of the dynamic obstacles to make long-term predictions of their future paths. This is done by combining the
flexibility of Gaussian processes (GP) with the efficiency of RRT-Reach,
a sampling-based reachability computation method which ensures dynamic
feasibility. This prediction model is then utilized within chance-constrained rapidly-exploring random
trees (CC-RRT), which uses chance constraints to explicitly achieve probabilistic
constraint satisfaction while maintaining the computational
benefits of sampling-based algorithms. With RR-GP embedded in the CC-RRT framework, theoretical guarantees
can be demonstrated for linear systems subject to Gaussian uncertainty,
though the extension to nonlinear systems is also considered. Simulation results
show that the resulting approach can be used in real-time to efficiently and
accurately execute safe paths
Virial expansion coefficients in the harmonic approximation
The virial expansion method is applied within a harmonic approximation to an
interacting N-body system of identical fermions. We compute the canonical
partition functions for two and three particles to get the two lowest orders in
the expansion. The energy spectrum is carefully interpolated to reproduce
ground state properties at low temperature and the non-interacting large
temperature limit of constant virial coefficients. This resembles the smearing
of shell effects in finite systems with increasing temperature. Numerical
results are discussed for the second and third virial coefficients as function
of dimension, temperature, interaction, and the transition temperature between
low and high energy limits.Comment: 11 pages, 7 figures, published versio
Rapidly changing subglacial hydrological pathways at a tidewater glacier revealed through simultaneous observations of water pressure, supraglacial lakes, meltwater plumes and surface velocities
This work was funded by the Conoco Phillips-Lundin Northern Area Program through the CRIOS project (Calving Rates and Impact On Sea level, http://www.researchinsvalbard.no/project/7037). Penelope How is supported by a NERC PhD studentship.Subglacial hydrological processes at tidewater glaciers remain poorly understood due to the difficulty in obtaining direct measurements and lack of empirical verification for modelling approaches. Here, we investigate the subglacial hydrology of Kronebreen, a fast-flowing tidewater glacier in Svalbard during the 2014 melt season. We combine observations of borehole water pressure, supraglacial lake drainage, surface velocities and plume activity with modelled run-off and water routing to develop a conceptual model that thoroughly encapsulates subglacial drainage at a tidewater glacier. Simultaneous measurements suggest that an early-season episode of subglacial flushing took place during our observation period, and a stable efficient drainage system effectively transported subglacial water through the northern region of the glacier tongue. Drainage pathways through the central and southern regions of the glacier tongue were disrupted throughout the following melt season. Periodic plume activity at the terminus appears to be a signal for modulated subglacial pulsing, i.e. an internally driven storage and release of subglacial meltwater that operates independently of marine influences. This storage is a key control on ice flow in the 2014 melt season. Evidence from this work and previous studies strongly suggests that long-term changes in ice flow at Kronebreen are controlled by the location of efficient/inefficient drainage and the position of regions where water is stored and released.Publisher PDFPeer reviewe
Effectiveness of mitigation measures to reduce interactions between commercial fishing gear and whales
Effectiveness of mitigation measures to reduce interactions between commercial fishing gear and whales.
Objectives:
1. Start to collect additional information required to determine the spatial and temporal extent of migrating whales and how this overlaps with commercial fishing gear.
2. Examine the effectiveness of potential gear modifications to the float rigs of fishing pots/traps to reduce their likelihood of entangling whales
Titania-doped tantala/silica coatings for gravitational-wave detection
Reducing thermal noise from optical coatings is crucial to reaching the required sensitivity in next generation interferometric gravitational-wave detectors. Here we show that adding TiO2 to Ta2O5 in Ta2O5/SiO2 coatings reduces the internal friction and in addition present data confirming it reduces thermal noise. We also show that TiO2-doped Ta2O5/SiO2 coatings are close to satisfying the optical absorption requirements of second generation gravitational-wave detectors
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