2,162 research outputs found
Memristor models for machine learning
In the quest for alternatives to traditional CMOS, it is being suggested that
digital computing efficiency and power can be improved by matching the
precision to the application. Many applications do not need the high precision
that is being used today. In particular, large gains in area- and power
efficiency could be achieved by dedicated analog realizations of approximate
computing engines. In this work, we explore the use of memristor networks for
analog approximate computation, based on a machine learning framework called
reservoir computing. Most experimental investigations on the dynamics of
memristors focus on their nonvolatile behavior. Hence, the volatility that is
present in the developed technologies is usually unwanted and it is not
included in simulation models. In contrast, in reservoir computing, volatility
is not only desirable but necessary. Therefore, in this work, we propose two
different ways to incorporate it into memristor simulation models. The first is
an extension of Strukov's model and the second is an equivalent Wiener model
approximation. We analyze and compare the dynamical properties of these models
and discuss their implications for the memory and the nonlinear processing
capacity of memristor networks. Our results indicate that device variability,
increasingly causing problems in traditional computer design, is an asset in
the context of reservoir computing. We conclude that, although both models
could lead to useful memristor based reservoir computing systems, their
computational performance will differ. Therefore, experimental modeling
research is required for the development of accurate volatile memristor models.Comment: 4 figures, no tables. Submitted to neural computatio
Mass Hierarchy and Vacuum Energy
A hierarchically small weak scale does not generally coincide with enhanced
symmetry, but it may still be exceptional with respect to vacuum energy. By
analyzing the classical vacuum energy as a function of parameters such as the
Higgs mass, we show how near-criticality, i.e. fine-tuning, corresponds
universally to boundaries where the vacuum energy transitions from exactly flat
to concave down. In the presence of quantum corrections, these boundary regions
can easily be perturbed to become maxima of the vacuum energy. After
introducing a dynamical scalar field which scans the Higgs sector
parameters, we propose several possible mechanisms by which this field could be
localized to the maximum. One possibility is that the potential has many
vacua, with those near the maximum vacuum energy expanding faster during a long
period of cosmic inflation and hence dominating the volume of the Universe.
Alternately, we describe scenarios in which vacua near the maximum could be
anthropically favored, due to selection of the late-time cosmological constant
or dark matter density. Independent of these specific approaches, the physical
value of the weak scale in our proposal is generated naturally and dynamically
from loops of heavy states coupled to the Higgs. These states are predicted to
be a loop factor heavier than in models without this mechanism, avoiding
tension with experimental null results.Comment: 45 pages, 10 figures. v2: Additional discussion of inflationary
cosmology scenarios, added reference
Probing a spin transfer controlled magnetic nanowire with a single nitrogen-vacancy spin in bulk diamond
The point-like nature and exquisite magnetic field sensitivity of the
nitrogen vacancy (NV) center in diamond can provide information about the inner
workings of magnetic nanocircuits in complement with traditional transport
techniques. Here we use a single NV in bulk diamond to probe the stray field of
a ferromagnetic nanowire controlled by spin transfer (ST) torques. We first
report an unambiguous measurement of ST tuned, parametrically driven,
large-amplitude magnetic oscillations. At the same time, we demonstrate that
such magnetic oscillations alone can directly drive NV spin transitions,
providing a potential new means of control. Finally, we use the NV as a local
noise thermometer, observing strong ST damping of the stray field noise,
consistent with magnetic cooling from room temperature to 150 K.Comment: 6 pages, 5 figures, plus supplementary informatio
Towards Bayesian Data Compression
In order to handle large data sets omnipresent in modern science, efficient
compression algorithms are necessary. Here, a Bayesian data compression (BDC)
algorithm that adapts to the specific measurement situation is derived in the
context of signal reconstruction. BDC compresses a data set under conservation
of its posterior structure with minimal information loss given the prior
knowledge on the signal, the quantity of interest. Its basic form is valid for
Gaussian priors and likelihoods. For constant noise standard deviation, basic
BDC becomes equivalent to a Bayesian analog of principal component analysis.
Using Metric Gaussian Variational Inference, BDC generalizes to non-linear
settings. In its current form, BDC requires the storage of effective instrument
response functions for the compressed data and corresponding noise encoding the
posterior covariance structure. Their memory demand counteract the compression
gain. In order to improve this, sparsity of the compressed responses can be
obtained by separating the data into patches and compressing them separately.
The applicability of BDC is demonstrated by applying it to synthetic data and
radio astronomical data. Still the algorithm needs further improvement as the
computation time of the compression and subsequent inference exceeds the time
of the inference with the original data.Comment: 39 pages, 15 figures, 1 table, for code, see
https://gitlab.mpcdf.mpg.de/jharthki/bd
Investigations on the properties and estimation of earth response operators from EM sounding data
Incl. 3 reprints at backAvailable from British Library Document Supply Centre- DSC:D82993 / BLDSC - British Library Document Supply CentreSIGLEGBUnited Kingdo
Deep Electromagnetic Studies from Land, Sea, and Space: Progress Status in the Past 10Years
This review paper summarizes advances in deep electromagnetic studies of the Earth in the past decade. The paper reports progress in data interpretation, with special emphasis on three-dimensional and quasi one-dimensional developments, and results. The results obtained from data of different origin—geomagnetic observatories, long-period magnetotelluric experiments, submarines cables, and from low-Earth orbiting geomagnetic satellite missions—are described. Both frequency-domain and time-domain approaches are addressed. Perspectives for the future are also discusse
Optimal control and robust estimation for ocean wave energy converters
This thesis deals with the optimal control of wave energy converters and some associated
observer design problems. The first part of the thesis will investigate model
predictive control of an ocean wave energy converter to maximize extracted power.
A generic heaving converter that can have both linear dampers and active elements
as a power take-off system is considered and an efficient optimal control algorithm
is developed for use within a receding horizon control framework. The optimal
control is also characterized analytically. A direct transcription of the optimal control
problem is also considered as a general nonlinear program. A variation of
the projected gradient optimization scheme is formulated and shown to be feasible
and computationally inexpensive compared to a standard nonlinear program solver.
Since the system model is bilinear and the cost function is not convex quadratic, the
resulting optimization problem is shown not to be a quadratic program. Results are
compared with other methods like optimal latching to demonstrate the improvement
in absorbed power under irregular sea condition simulations.
In the second part, robust estimation of the radiation forces and states inherent in
the optimal control of wave energy converters is considered. Motivated by this, low
order H∞ observer design for bilinear systems with input constraints is investigated
and numerically tractable methods for design are developed. A bilinear Luenberger
type observer is formulated and the resulting synthesis problem reformulated as that
for a linear parameter varying system. A bilinear matrix inequality problem is then
solved to find nominal and robust quadratically stable observers. The performance
of these observers is compared with that of an extended Kalman filter. The robustness
of the observers to parameter uncertainty and to variation in the radiation
subsystem model order is also investigated.
This thesis also explores the numerical integration of bilinear control systems with
zero-order hold on the control inputs. Making use of exponential integrators, exact
to high accuracy integration is proposed for such systems. New a priori bounds
are derived on the computational complexity of integrating bilinear systems with a
given error tolerance. Employing our new bounds on computational complexity, we
propose a direct exponential integrator to solve bilinear ODEs via the solution of
sparse linear systems of equations. Based on this, a novel sparse direct collocation
of bilinear systems for optimal control is proposed. These integration schemes are
also used within the indirect optimal control method discussed in the first part.Open Acces
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