229 research outputs found
Linear magnetic flux amplifier
By measuring the critical current versus the applied magnetic field
of an Al superconducting loop enclosing a soft Permalloy magnetic
dot, we demonstrate that it is feasible to design a linear magnetic flux
amplifier for applications in superconducting quantum interference devices. The
selected dimensions of a single-domain Permalloy dot provide that the
preferential orientation of the magnetization is rotated from the perpendicular
direction. By increasing an applied magnetic field, the magnetization of the
dot coherently rotates towards the out-of-plane direction, thus providing a
flux gain and an enhancement of the sensitivity. As a result of a pronounced
shape anisotropy, the flux gain generated by the dot can be tuned by adjusting
the dimensions of the dot.Comment: to appear in Applied Physics Letter
High precision motion control of parallel robots with imperfections and manufacturing tolerances
This work attempts to achieve precise motion control using parallel robots with manufacturing tolerances and inaccuracies by migrating the measurements from their joint space to task space in order to decrease control system’s sensitivity to any
kinematical uncertainty rather than calibrating the parallel plant. The problem of dynamical model uncertainties and its effect on the derivation of the control law is also addressed in this work through disturbance estimation and compensation. Eventually, both task space measurement and disturbance estimation are combined to formulate a control framework that is unsensitive to either kinematical and dynamical system uncertainties
Little-Parks effect in a superconducting loop with magnetic dot
We have studied the nucleation of superconductivity in a mesoscopic Al loop,
enclosing magnetic dot with perpendicular magnetization. The superconducting
phase boundary Tc(B), determined from transport measurements, is asymmetric
with respect to the polarity of an applied magnetic field. The maximum critical
temperature has been found for a finite applied magnetic field, which is
antiparallel to the magnetization of the dot. Theoretical phase boundary shows
a good agreement with the experimental data.Comment: to be published in Phys. Rev. B - Brief Report
Super-Arrhenius dynamics for sub-critical crack growth in disordered brittle media
Taking into account stress fluctuations due to thermal noise, we study
thermally activated irreversible crack growth in disordered media. The
influence of material disorder on sub-critical growth of a single crack in
two-dimensional brittle elastic material is described through the introduction
of a rupture threshold distribution. We derive analytical predictions for crack
growth velocity and material lifetime in agreement with direct numerical
calculations. It is claimed that crack growth process is inhibited by disorder:
velocity decreases and lifetime increases with disorder. More precisely,
lifetime is shown to follow a super-Arrhenius law, with an effective
temperature theta - theta_d, where theta is related to the thermodynamical
temperature and theta_d to the disorder variance.Comment: Submitted to Europhysics Letter
Thermal activation of rupture and slow crack growth in a model of homogenous brittle materials
Slow crack growth in a model of homogenous brittle elastic material is
described as a thermal activation process where stress fluctuations allow to
overcome a breaking threshold through a series of irreversible steps. We study
the case of a single crack in a flat sheet for which analytical predictions can
be made, and compare them with results from the equivalent problem of a 2D
spring network. Good statistical agreement is obtained for the crack growth
profile and final rupture time. The specific scaling of the energy barrier with
stress intensity factor appears as a consequence of irreversibility. In
addition, the model brings out a characteristic growth length whose physical
meaning could be tested experimentally.Comment: To be published in : Europhysics Letter
Optimal motion control and vibration suppression of flexible systems with inaccessible outputs
This work addresses the optimal control problem
of dynamical systems with inaccessible outputs. A case in which
dynamical system outputs cannot be measured or inaccessible.
This contradicts with the nature of the optimal controllers which can be considered without any loss of generality as state feedback control laws for systems with linear dynamics. Therefore, this work attempts to estimate dynamical system states through a novel state observer that does not require injecting the dynamical system outputs onto the observer structure during its design. A linear quadratic optimal control law is then realized based on the
estimated states which allows controlling motion along with active vibration suppression of this class of dynamical systems with inaccessible outputs. Validity of the proposed control framework is evaluated experimentally
Subcritical crack growth in fibrous materials
We present experiments on the slow growth of a single crack in a fax paper
sheet submitted to a constant force . We find that statistically averaged
crack growth curves can be described by only two parameters : the mean rupture
time and a characteristic growth length . We propose a model
based on a thermally activated rupture process that takes into account the
microstructure of cellulose fibers. The model is able to reproduce the shape of
the growth curve, the dependence of on as well as the effect of
temperature on the rupture time . We find that the length scale at which
rupture occurs in this model is consistently close to the diameter of cellulose
microfibrils
Two new topologically ordered glass phases of smectics confined in anisotropic random media
We show that smectic liquid crystals confined in_anisotropic_ porous
structures such as e.g.,_strained_ aerogel or aerosil exhibit two new glassy
phases. The strain both ensures the stability of these phases and determines
their nature. One type of strain induces an ``XY Bragg glass'', while the other
creates a novel, triaxially anisotropic ``m=1 Bragg glass''. The latter
exhibits anomalous elasticity, characterized by exponents that we calculate to
high precision. We predict the phase diagram for the system, and numerous other
experimental observables.Comment: 4 RevTeX pgs, 2 eps figures, submitted to Phys. Rev. Let
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