101 research outputs found
Nonlinear position and stiffness Backstepping controller for a two Degrees of Freedom pneumatic robot
This paper presents an architecture of a 2 Degrees of Freedom pneumatic robot which can be used as a haptic interface. To improve the haptic rendering of this device, a nonlinear position and stiffness controller without force measurement based on a Backstepping synthesis is presented. Thus, the robot can follow a targeted trajectory in Cartesian position with a variable compliant behavior when disturbance forces are applied. An appropriate tuning methodology of the closed-loop stiffness and closed-loop damping of the robot is given to obtain a desired disturbance response. The models, the synthesis and the stability analysis of this controller are described in this paper. Two models are presented in this paper, the first one is an accurate simulation model which describes the mechanical behavior of the robot, the thermodynamics phenomena in the pneumatic actuators, and the servovalves characteristics. The second model is the model used to synthesize the controller. This control model is obtained by simplifying the simulation model to obtain a MIMO strict feedback form. Finally, some simulation and experimental results are given and the controller performances are discussed and compared with a classical linear impedance controller
On the magnetic fields generated by experimental dynamos
We review the results obtained by three successful fluid dynamo experiments
and discuss what has been learnt from them about the effect of turbulence on
the dynamo threshold and saturation. We then discuss several questions that are
still open and propose experiments that could be performed to answer some of
them.Comment: 40 pages, 13 figure
Multifractal stationary random measures and multifractal random walks with log-infinitely divisible scaling laws
We define a large class of continuous time multifractal random measures and
processes with arbitrary log-infinitely divisible exact or asymptotic scaling
law. These processes generalize within a unified framework both the recently
defined log-normal Multifractal Random Walk (MRW) [Bacry-Delour-Muzy] and the
log-Poisson "product of cynlindrical pulses" [Barral-Mandelbrot]. Our
construction is based on some ``continuous stochastic multiplication'' from
coarse to fine scales that can be seen as a continuous interpolation of
discrete multiplicative cascades. We prove the stochastic convergence of the
defined processes and study their main statistical properties. The question of
genericity (universality) of limit multifractal processes is addressed within
this new framework. We finally provide some methods for numerical simulations
and discuss some specific examples.Comment: 24 pages, 4 figure
Comparing the performance of FA, DFA and DMA using different synthetic long-range correlated time series
Notwithstanding the significant efforts to develop estimators of long-range
correlations (LRC) and to compare their performance, no clear consensus exists
on what is the best method and under which conditions. In addition, synthetic
tests suggest that the performance of LRC estimators varies when using
different generators of LRC time series. Here, we compare the performances of
four estimators [Fluctuation Analysis (FA), Detrended Fluctuation Analysis
(DFA), Backward Detrending Moving Average (BDMA), and centred Detrending Moving
Average (CDMA)]. We use three different generators [Fractional Gaussian Noises,
and two ways of generating Fractional Brownian Motions]. We find that CDMA has
the best performance and DFA is only slightly worse in some situations, while
FA performs the worst. In addition, CDMA and DFA are less sensitive to the
scaling range than FA. Hence, CDMA and DFA remain "The Methods of Choice" in
determining the Hurst index of time series.Comment: 6 pages (including 3 figures) + 3 supplementary figure
Wavelets techniques for pointwise anti-Holderian irregularity
In this paper, we introduce a notion of weak pointwise Holder regularity,
starting from the de nition of the pointwise anti-Holder irregularity. Using
this concept, a weak spectrum of singularities can be de ned as for the usual
pointwise Holder regularity. We build a class of wavelet series satisfying the
multifractal formalism and thus show the optimality of the upper bound. We also
show that the weak spectrum of singularities is disconnected from the casual
one (denoted here strong spectrum of singularities) by exhibiting a
multifractal function made of Davenport series whose weak spectrum di ers from
the strong one
Renormalization flow for extreme value statistics of random variables raised to a varying power
Using a renormalization approach, we study the asymptotic limit distribution
of the maximum value in a set of independent and identically distributed random
variables raised to a power q(n) that varies monotonically with the sample size
n. Under these conditions, a non-standard class of max-stable limit
distributions, which mirror the classical ones, emerges. Furthermore a
transition mechanism between the classical and the non-standard limit
distributions is brought to light. If q(n) grows slower than a characteristic
function q*(n), the standard limit distributions are recovered, while if q(n)
behaves asymptotically as k.q*(n), non-standard limit distributions emerge.Comment: 21 pages, 1 figure,final version, to appear in Journal of Physics
“Savages Who Speak French”: Folklore, Primitivism and Morals in Robert Hertz
Hertz's analysis of the Alpine cult of Saint Besse apparently marks a break from his studies of death, sin and the left to folkloric studies. This analysis helps one to understand the personality of Robert Hertz. His sociological curiosity about folklore reveals his ambiguous position in social sciences at the beginning of the twentieth century. His text appears to be a variation from the Durkheimian norm, but another reading could suggest that Hertz continued and went beyond Durkheimian thought to something between sociology of the modern world and engaged socialism. Through this study, Hertz linked his political ideals, his work in ethnology and his desire for social involvement. The cult of Saint Besse perpetuated as much religious tradition as local identity. The Alpine people were presented in the text as wilful perpetuators of an ideal social order, whose loss for his contemporary city dwellers Hertz feared. The alpine Other, marked by a material and moral backwardness, represented for activist and socialist Hertz one of the paths of balanced social organization that stabilized the identity of a group across time if it fit rather well into the folkloric stereotypes of the beginning of the twentieth century. Finally, by linking events in Herz's life (e.g., the accidental Alpine death of his father), this article suggests that the legend of Saint Besse embodied several recurring motifs in Hertz' career: the accidental deaths of saint and father by falls, the military role of the saint and of Hertz himself
Multi-scale polarisation phenomena
Multi-scale methods that separate different time or spatial scales are among the most powerful techniques in physics, especially in applications that study nonlinear systems with noise. When the time scales (noise and perturbation) are of the same order, the scales separation becomes impossible. Thus, the multi-scale approach has to be modified to characterise a variety of noise-induced phenomena. Here, based on stochastic modelling and analytical study, we demonstrate in terms of the fluctuation-induced phenomena and Hurst R/S analysis metrics that the matching scales of random birefringence and pump–signal states of polarisation interaction in a fibre Raman amplifier results in a new random birefringence-mediated phenomenon, which is similar to stochastic anti-resonance. The observed phenomenon, apart from the fundamental interest, provides a base for advancing multi-scale methods with application to different coupled nonlinear systems ranging from lasers (multimode, mode-locked, random, etc.) to nanostructures (light-mediated conformation of molecules and chemical reactions, Brownian motors, etc.)
Multistable syllables as enacted percepts: a source of an asymmetric bias in the verbal transformation effect
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