Joseph Lardies

Identification of modal parameters using the wavelet transform

The wavelet transform is used as a time-frequency representation for the determination of modal parameters such as natural frequencies, damping ratios and mode shapes of a vibrating system. It is shown that using a particular form of the son wavelet function, results are improved compared to those obtained with the traditionally Morlet wavelet function. The accuracy of this new technique is confirmed by applying it to a numerical example and to ambient vibration measurements of a tower excited by wind.

Natural frequencies and modal damping ratios identification of civil structures from ambient vibration data

Damping is a mechanism that dissipates vibration energy in dynamic systems and plays a key role in dynamic response prediction, vibration control as well as in structural health monitoring during service. In this paper a time domain and a time-scale domain approaches are used for damping estimation of engineering structures, using ambient response data only. The use of tests under ambient vibration is increasingly popular today because they allow to measure the structural response in service. In this paper we consider two engineering structures excited by ambient forces. The first structure is the 310 m tall TV tower recently constructed in the city of Nanjing in China. The second example 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 cable is carried out using accelerometers. From output data only, the modal parameter are extracted using a subspace method and the wavelet transform method.

Indoor noise exposure at home: a field study in the family of urban schoolchildren.

This article aims at evaluating indoor noise levels at home and investigating the factors that may influence their variability. An 8-day noise measurement campaign was conducted in the homes of 44 schoolchildren attending the public primary schools of Besançon (France). The presence of the inhabitants in the dwelling and the noisy events occurring indoors and outdoors was daily collected using a time-location-activity diary (TLAD); 902 time periods were analyzed. The indoor noise level increased significantly with the outdoor noise level, along with the duration of the presence or level of activity of the inhabitants at home. However, this effect may vary according to the period of day and the day of the week. Moreover, a significant part of the day and evening indoor noise level variability was explained when considering the TLAD variables: 46% and 45% in the bedroom, 54% and 39% in the main room, respectively. Our results highlight the complexity of the indoor environment in the dwellings of children living in an urban area. Combining the inhabitant presence and indoor noise source descriptors with outdoor noise levels and other dwelling or inhabitant characteristics could improve large-scale epidemiological studies. However, additional efforts are still needed, particularly during the night period.

Dynamic and acoustic modeling of capacitive micromachined ultrasonic transducers

The Capacitive Micromachined Ultrasonic Transducers (CMUTs) are a promising alternative to piezoelectric ultrasound transducers. They are constituted by a very large number of silicon membranes electrostatically actuated. We propose an original method to calculate the acoustic pressure emitted by CMUTs by taking into account explicitly the dynamics of each membrane of the network. The electrostatic forces are linearized, the displacements of the membranes are projected on a mechanical modal base and the acoustic pressures are computed via the Rayleigh integral. The method has been validated by comparison with ANSYS for a single cell and a small hexagonal network of 7 cells. Application on a 2D CMUT network shows the interest of such a calculation method.

Nonlinear Static and Dynamic Behavior of an Imperfect Circular Microplate Under Electrostatic Actuation

In this article, we investigate the nonlinear static and dynamic behavior of a clamped circular microplate in presence of imperfections. By taking in account the von Karman nonlinearity, the geometrical imperfections lead to a significant delay in static pull-in occurrence. Numerical simulations are performed in the frequency domain to study the dynamic behavior under primary resonance. A parametric analysis is conducted with respect to actuation voltages and initial deflection in order to capture the competition between hardening and softening behavior. Interestingly, we show that a geometric imperfection can change the type of nonlinear response from softening to hardening. In practice, the imperfection can be functionalized to enhance the performances of capacitive micromachined ultrasonic transducers.Copyright

Modal Parameter Identification from Output Data Only: Equivalent Approaches

The problem of modal parameter identification from output data only is presented. To identify the modal parameters different algorithms are presented: the block Hankel matrix and its shifted version and the block observability and block controllability matrices and their shifted version. These algorithms are derived from properties of the subspace approach. It is shown in the paper that these algorithms give the same results even in the noisy data case. Numerical and experimental results are presented showing the effectiveness of the procedure. In particular a microsystem constituted of a perforated microplate is analysed.

A COMPUTATIONAL MODEL FOR LARGE-AMPLITUDE NONLINEAR VIBRATION OF CIRCULAR CMUTS UNDER PRIMARY RESONANCE

A computational multiphysics model for an electrically actuated clamped circular plate is developed. The Galerkin approach is used in order to transform the continuous model into a system having finite degrees of freedom. The discretized system is solved using the harmonic balance method (HBM) coupled with the asymptotic numerical method (ANM). The effects of the electrostatic excitation on the CMUT frequency responses are investigated for two set of design parameters. This model is an effective tool for MEMS designers to enhance the performances of CMUTs in term of generated acoustic power.

Improvement of Compressor Blade Vibrations Spectral Analysis From Tip Timing Data: Aliasing Reduction

Tip-timing is a technique for measuring rotating blades vibrations in operation. Its concept exists since the early 70s but it has been more experimented in the last decade through improvements in hardware and software capabilities. It consists of a set of sensors mounted on stator casings that record blade passing times. Then, from this measurement, blade vibrations can be estimated.Tip-timing technique presents several advantages compared to usual mean of measurement: strain gages. Indeed, installation is easier, it is non-intrusive and all the blades can be monitored.However, resulting sampling depends on physical configuration i.e. number of sensors. In practice, as the number of sensors is limited, sampling rate is low in relation to the physical observed frequencies and do not respect the Shannon criterion. Thus, it generates important aliasing effects in spectrum, which makes the analysis difficult. In fact, such measurements aim to lead to pseudo-blind analysis, especially for asynchronous vibrations, when there is no hypothesis of underneath structural model.This main problem of aliasing is partially softened by using a minimum variance spectral estimator that iteratively reduces non-physical content in spectrum, but pseudo-blind analysis remains complicated.This paper presents a comparison of several methods based on different multisampling averaging concepts for reducing aliasing. Multisampling averaging consists in averaging spectrums from different sampling patterns of the same original signal, so that only stable physical content remains in the final spectrum. This study is presented on different industrial test cases of blade vibrations.Copyright

Influence of the shape of the membrane of computed cMUT using FEA/BEM analysis

The possibility to excite and detect acoustic waves in fluids using capacitive micro‐machined ultrasonic transducers (cMUT) built on silicon using clean room techniques offers attractive opportunities for manufacturing high quality low cost imaging probes. CMUTs developed for acoustic imaging exploit the first flexural mode of thin and stiff membranes, leading to bandwidth larger than 100%. These transducers can be accurately designed using mixed finite element analysis/boundary element methods (FEA/BEM). Periodic FEA particularly allows for the simulation of devices exhibiting complicated shape interfaces and involving materials of different nature. BEM also are particularly well‐suited to provide an accurate description of any stacked medium assuming flat interfaces for the radiation area and the layer interface for 2 and 3D structures as well. In this work, we have analysed the influence of the shape of the MUT membrane on the spectral response of the transducer. 3D Computations have been conducted con...

Performance of high‐resolution sensor array processing algorithms in the localization of acoustic sources

The localization of noise sources from a specified direction may often be accomplished with an array of sensors. One commonly used processor consists of delay and add networks: a conventional beamformer, however its spectrum suffers from the Rayleigh resolution and its performance is highly degraded, specially in lower frequency range. In the communication, the performance of some typical high‐resolution sensor array processing algorithms: Minimum Variance, MUSIC, Mini‐Norm algorithms are investigated for wideband source location. Their performances are compared with a new source localization algorithm which is based on a sparse representation of sensor measurements with an overcomplete basis composed of samples from the array manifold. The key of the method is the use of the SVD for data reduction and the formulation of a joint multiple‐sample sparse representation problem in the signal subspace domain. Increased resolution and improved robustness to noise is obtained with this algorithm applied to vario...