Wang Z.,CEA Grenoble |
Devel M.,CNRS Femto ST Institute
Physical Review B - Condensed Matter and Materials Physics | Year: 2011
We study the mechanism of wrinkling of suspended graphene, by means of atomistic simulations. We argue that the structural instability under edge compression is the essential physical reason for the formation of periodic ripples in graphene. The ripple wavelength and out-of-plane amplitude are found to obey 1/4-power scaling laws with respect to edge compression. Our results also show that parallel displacement of the clamped boundaries can induce periodic ripples, with oscillation amplitude roughly proportional to the 1/4 power of edge displacement. © 2011 American Physical Society.
Rakotondrabe M.,CNRS Femto ST Institute |
Ivan I.A.,University Valahia of Targoviste
IEEE Transactions on Automation Science and Engineering | Year: 2011
A new microgripper dedicated to micromanipulation and microassembly tasks is presented in this paper. Based on a new actuator, called thermo-piezoelectric actuator, the microgripper presents both a high range and a high positioning resolution. The principle of the microgripper is based on the combination of the thermal actuation (for the coarse positioning) and the piezoelectric actuation (for the fine positioning). In order to improve the performances of the microgripper, its actuators are modeled and a control law for both the position and the manipulation force is synthesized afterwards. A new control scheme adapted for the actuators of the hybrid thermo-piezoelectric microgripper is therefore proposed. To prove the interest of the developed microgripper and of the proposed control scheme, the control of a pick-and-release task using this microgripper is carried out. The experimental results confirm their efficiency and demonstrate that the new microgripper and the control law are well suited for micromanipulation and microassembly applications. © 2011 IEEE.
Boyer P.,CNRS Femto ST Institute
Journal of the Optical Society of America A: Optics and Image Science, and Vision | Year: 2013
We extend the differential theory to anisotropic cylindrical structures with an arbitrary cross section. Two cases have to be distinguished. When the anisotropic cylinders do not contain the origin, the scattering matrix of the device is calculated from the extended differential theory with the help of the scattering matrix propagation algorithm. The fields outside the cylinders are described by Fourier-Bessel expansions. When the origin is located in one cylinder, the fields inside the cylinder are expressed from a semi-analytical theory related to a homogeneous anisotropic medium. In this second case, the formalism of the scattering matrix propagation algorithm is not exactly the same and requires suitable change. The numerical results are in good agreement with the ones obtained for the diffraction by one circular cylinder. The theory is then applied on the diffraction by an elliptical cylinder. © 2013 Optical Society of America.
Gouriveau R.,CNRS Femto ST Institute |
Zerhouni N.,CNRS Femto ST Institute
IEEE Transactions on Reliability | Year: 2012
Prognostics and Health Management aims at estimating the remaining useful life of a system (RUL) , i.e. the remaining time before a failure occurs. It benefits thereby from an increasing interest: prognostic estimates (and related decision-making processes) enable increasing availability and safety of industrial equipment while reducing costs. However, prognostics is generally based on a prediction step which, in the context of data-driven approaches as considered in this paper, can be hard to achieve because future outcomes are in essence difficult to estimate. Also, a prognostic system must perform sufficient long term estimates, whereas many works focus on short term predictions. Following that, the aim of this paper is to formalize and discuss the connexionist-systems-based approaches to ensure multi-step ahead predictions for prognostics. Five approaches are pointed out: the Iterative, Direct, DirRec, Parallel, and MISMO approaches. Conclusions of the paper are based, on one side, on a literature review; and on the other side, on simulations among 111 time series prediction problems, and among a real engine fault prognostics application. These experiments are performed using the exTS (evolving extended Takagi-Sugeno system). As for comparison purpose, three types of performances measures are used: prediction accuracy, complexity (computational time), and implementation requirements. Results show that all three criteria are never optimized at the same time (same experiment), and best practices for prognostics application are finally pointed out. © 2012 IEEE.
Lardies J.,CNRS Femto ST Institute |
Minh-Ngi T.,CNRS Femto ST Institute
Mechanical Systems and Signal Processing | Year: 2011
Stay cables are one of the most critical structural components in modern cable-stayed bridges and the cable tension plays an important role in the construction, control and monitoring of cable-stayed bridges. We propose a time domain and a timefrequency domain approaches for modal parameter identification of stay cables using output-only measurements. The time domain approach uses the subspace algorithm which is improved with a new modal coherence indicator. The timefrequency approach uses the wavelet transform of signals which is improved with a new analyzing wavelet. The wavelet transform is applied to the free response of ambient vibration which is obtained using the random decrement technique. Two experiments of stay cables are presented. The first experiment concerns a stay cable in laboratory where the external load is applied through an impact hammer and the vibratory signals are acquired through four accelerometers. The second experiment concerns the Jinma cable-stayed bridge that connects Guangzhou and Zhaoqing in China. It is a single tower, double row cable-stayed bridge supported by 112 stay cables. Ambient vibration of each stay cable is carried out using accelerometers. From output-only measurements, the modal parameters of stay cables are extracted. Once the eigenfrequencies and the damping coefficients are obtained, the cable forces and the Scruton number are derived. In a continuous monitoring and modal analysis process, the tension forces and Scruton numbers could be used to assess the health of stay cables in cable-stayed bridges. © 2010 Elsevier Ltd. All rights reserved.
Rakotondrabe M.,CNRS Femto ST Institute
Proceedings of the American Control Conference | Year: 2012
This paper presents a new approach to compensate the static hysteresis in smart material based actuators that is modeled by the Prandtl-Ishlinskii approach. The proposed approach allows a simplicity and ease of implementation. Furthermore, as soon as the direct model is identified and obtained, the compensator is directly derived. The experimental results on piezoactuators show its efficiency and prove its interest for the precise control of microactuators without the use of sensors. In particular, we experimentally show that the hysteresis of the studied actuator which was initially 23% was reduced to less than 2.5% for the considered working frequency. © 2012 AACC American Automatic Control Council).
Rakotondrabe M.,CNRS Femto ST Institute
Proceedings of the American Control Conference | Year: 2011
This paper presents the performances inclusion on time and frequency domains of SISO stable interval systems. We demonstrate that an interval transfer function included in another interval transfer function will have its performances also included in those of the second one. While the results may be intuitive, the paper provides an analytical demonstration by using interval arithmetic and related tools. These results are of great interest for robust performances analysis and for controller design in parametric uncertain systems. © 2011 AACC American Automatic Control Council.
Pureur V.,CNRS Femto ST Institute |
Dudley J.M.,CNRS Femto ST Institute
Optics Letters | Year: 2010
We study nonlinear spectral broadening and supercontinuum generation mechanisms in two-dimensional solidcore photonic bandgap fibers. Using rigorous frequency-domain numerical simulations, we determine how the spectral characteristics are influenced by the strong frequency dependence of the effective area, dispersion, and confinement losses. We also investigate soliton stabilization and the conditions under which efficient nonlinear spectral energy transfer is possible across high attenuation between adjacent bandgaps. Our results provide insight into recent experiments. © 2010 Optical Society of America.
Baida F.I.,CNRS Femto ST Institute
Optics Express | Year: 2010
The control and localization of light at sub-wavelength scale are theoretically demonstrated with a very simple sub-wavelength dimension structure. This is demonstrated through a peculiar structure that can support localized modes which are not linked to any plasmon resonance. It is based on the acronym "FEMTO" that is designed using 26 sub-wavelength rectangular apertures engraved into perfectly conducting metal screen. A polarization-sensitive guided mode through these nano-apertures is at the origin of the light localization. Consequently, sub-wavelength light spots can be achieved with very simple structures illuminated by temporally shaped plane waves. Three parameters are temporally controlled for this purpose: the polarization, the wavelength and the amplitude of the incident beam. It is also demonstrated that replacing the perfect conductor by a real metal with dispersion leads to accentuate both the light confinement and its localization. These results open the path to the conception of optical nano-structures dedicated to sub-wavelength light addressing. © 2010 Optical Society of America.
Coen S.,University of Auckland |
Randle H.G.,University of Auckland |
Sylvestre T.,CNRS Femto ST Institute |
Erkintalo M.,University of Auckland
Optics Letters | Year: 2013
A generalized Lugiato-Lefever equation is numerically solved with a Newton-Raphson method to model Kerr frequency combs. We obtain excellent agreement with past experiments, even for an octave-spanning comb. Simulations are much faster than with any other technique despite including more modes than ever before. Our study reveals that Kerr combs are associated with temporal cavity solitons and dispersive waves, and opens up new avenues for the understanding of Kerr-comb formation. © 2012 Optical Society of America.