778 research outputs found
Gait learning for soft microrobots controlled by light fields
Soft microrobots based on photoresponsive materials and controlled by light
fields can generate a variety of different gaits. This inherent flexibility can
be exploited to maximize their locomotion performance in a given environment
and used to adapt them to changing conditions. Albeit, because of the lack of
accurate locomotion models, and given the intrinsic variability among
microrobots, analytical control design is not possible. Common data-driven
approaches, on the other hand, require running prohibitive numbers of
experiments and lead to very sample-specific results. Here we propose a
probabilistic learning approach for light-controlled soft microrobots based on
Bayesian Optimization (BO) and Gaussian Processes (GPs). The proposed approach
results in a learning scheme that is data-efficient, enabling gait optimization
with a limited experimental budget, and robust against differences among
microrobot samples. These features are obtained by designing the learning
scheme through the comparison of different GP priors and BO settings on a
semi-synthetic data set. The developed learning scheme is validated in
microrobot experiments, resulting in a 115% improvement in a microrobot's
locomotion performance with an experimental budget of only 20 tests. These
encouraging results lead the way toward self-adaptive microrobotic systems
based on light-controlled soft microrobots and probabilistic learning control.Comment: 8 pages, 7 figures, to appear in the proceedings of the IEEE/RSJ
International Conference on Intelligent Robots and Systems 201
On Controller Tuning with Time-Varying Bayesian Optimization
Changing conditions or environments can cause system dynamics to vary over
time. To ensure optimal control performance, controllers should adapt to these
changes. When the underlying cause and time of change is unknown, we need to
rely on online data for this adaptation. In this paper, we will use
time-varying Bayesian optimization (TVBO) to tune controllers online in
changing environments using appropriate prior knowledge on the control
objective and its changes. Two properties are characteristic of many online
controller tuning problems: First, they exhibit incremental and lasting changes
in the objective due to changes to the system dynamics, e.g., through wear and
tear. Second, the optimization problem is convex in the tuning parameters.
Current TVBO methods do not explicitly account for these properties, resulting
in poor tuning performance and many unstable controllers through
over-exploration of the parameter space. We propose a novel TVBO forgetting
strategy using Uncertainty-Injection (UI), which incorporates the assumption of
incremental and lasting changes. The control objective is modeled as a
spatio-temporal Gaussian process (GP) with UI through a Wiener process in the
temporal domain. Further, we explicitly model the convexity assumptions in the
spatial dimension through GP models with linear inequality constraints. In
numerical experiments, we show that our model outperforms the state-of-the-art
method in TVBO, exhibiting reduced regret and fewer unstable parameter
configurations.Comment: To appear in the proceedings of the 61st IEEE Conference on Decision
and Contro
Experience Transfer for Robust Direct Data-Driven Control
Learning-based control uses data to design efficient controllers for specific
systems. When multiple systems are involved, experience transfer usually
focuses on data availability and controller performance yet neglects robustness
to variations between systems. In contrast, this letter explores experience
transfer from a robustness perspective. We leverage the transfer to design
controllers that are robust not only to the uncertainty regarding an individual
agent's model but also to the choice of agent in a fleet. Experience transfer
enables the design of safe and robust controllers that work out of the box for
all systems in a heterogeneous fleet. Our approach combines scenario
optimization and recent formulations for direct data-driven control without the
need to estimate a model of the system or determine uncertainty bounds for its
parameters. We demonstrate the benefits of our data-driven robustification
method through a numerical case study and obtain learned controllers that
generalize well from a small number of open-loop trajectories in a quadcopter
simulation
Event-Triggered Time-Varying Bayesian Optimization
We consider the problem of sequentially optimizing a time-varying objective
function using time-varying Bayesian optimization (TVBO). Here, the key
challenge is the exploration-exploitation trade-off under time variations.
Current approaches to TVBO require prior knowledge of a constant rate of
change. However, in practice, the rate of change is usually unknown. We propose
an event-triggered algorithm, ET-GP-UCB, that treats the optimization problem
as static until it detects changes in the objective function online and then
resets the dataset. This allows the algorithm to adapt to realized temporal
changes without the need for prior knowledge. The event-trigger is based on
probabilistic uniform error bounds used in Gaussian process regression. We
provide regret bounds for ET-GP-UCB and show in numerical experiments that it
outperforms state-of-the-art algorithms on synthetic and real-world data.
Furthermore, these results demonstrate that ET-GP-UCB is readily applicable to
various settings without tuning hyperparameters
Multi-Arm Bin-Picking in Real-Time: A Combined Task and Motion Planning Approach
Automated bin-picking is a prerequisite for fully automated manufacturing and
warehouses. To successfully pick an item from an unstructured bin the robot
needs to first detect possible grasps for the objects, decide on the object to
remove and consequently plan and execute a feasible trajectory to retrieve the
chosen object. Over the last years significant progress has been made towards
solving these problems. However, when multiple robot arms are cooperating the
decision and planning problems become exponentially harder. We propose an
integrated multi-arm bin-picking pipeline (IMAPIP), and demonstrate that it is
able to reliably pick objects from a bin in real-time using multiple robot
arms. IMAPIP solves the multi-arm bin-picking task first at high-level using a
geometry-aware policy integrated in a combined task and motion planning
framework. We then plan motions consistent with this policy using the BIT*
algorithm on the motion planning level. We show that this integrated solution
enables robot arm cooperation. In our experiments, we show the proposed
geometry-aware policy outperforms a baseline by increasing bin-picking time by
28\% using two robot arms. The policy is robust to changes in the position of
the bin and number of objects. We also show that IMAPIP to successfully scale
up to four robot arms working in close proximity.Comment: 8 page
Direct evidence for the emergence of a pressure induced nodal superconducting gap in the iron-based superconductor Ba_0.65Rb_0.35Fe_2As_2
Identifying the superconducting (SC) gap structure of the iron-based
high-temperature superconductors (Fe-HTS's) remains a key issue for the
understanding of superconductivity in these materials. In contrast to other
unconventional superconductors, in the Fe-HTS's both -wave and extended
s-wave pairing symmetries are close in energy, with the latter believed to be
generally favored over the former. Probing the proximity between these very
different SC states and identifying experimental parameters that can tune them,
are of central interest. Here we report high-pressure muon spin rotation
experiments on the temperature-dependent magnetic penetration depth (lambda) in
the optimally doped Fe-HTS Ba_0.65Rb_0.35Fe_2As_2. At ambient pressure this
material is known to be a nodeless s-wave superconductor. Upon pressure a
strong decrease of (lambda) is observed, while the SC transition temperature
remains nearly constant. More importantly, the low-temperature behavior of
(1/lambda^{2}) changes from exponential saturation at zero pressure to a
power-law with increasing pressure, providing unambiguous evidence that
hydrostatic pressure promotes nodal SC gaps. Comparison to microscopic models
favors a d-wave over a nodal s^{+-}-wave pairing as the origin of the nodes.
Our results provide a new route of understanding the complex topology of the SC
gap in Fe-HTS's.Comment: 33 pages and 12 figures (including supplementary information
Measurement of charm production at central rapidity in proton-proton collisions at TeV
The -differential production cross sections of the prompt (B
feed-down subtracted) charmed mesons D, D, and D in the rapidity
range , and for transverse momentum GeV/, were
measured in proton-proton collisions at TeV with the ALICE
detector at the Large Hadron Collider. The analysis exploited the hadronic
decays DK, DK, DD, and their charge conjugates, and was performed on a
nb event sample collected in 2011 with a
minimum-bias trigger. The total charm production cross section at TeV and at 7 TeV was evaluated by extrapolating to the full phase space
the -differential production cross sections at TeV
and our previous measurements at TeV. The results were compared
to existing measurements and to perturbative-QCD calculations. The fraction of
cdbar D mesons produced in a vector state was also determined.Comment: 20 pages, 5 captioned figures, 4 tables, authors from page 15,
published version, figures at
http://aliceinfo.cern.ch/ArtSubmission/node/307
Forward-central two-particle correlations in p-Pb collisions at root s(NN)=5.02 TeV
Two-particle angular correlations between trigger particles in the forward pseudorapidity range (2.5 2GeV/c. (C) 2015 CERN for the benefit of the ALICE Collaboration. Published by Elsevier B. V.Peer reviewe
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