Cartographie et spectroscopie des propriétés mécaniques à l'échelle du nanomètre par spectrométrie acoustique locale à fréquence variable

This work aims at completing the development of local mechanical spectroscopy as initiated by F. Oulevey during his thesis. More precisely, the measurement of the amplitude and phase lag of the strain as a function of the applied stress frequency bas to be settled. Parallel to the development of the technical aspects making such an acquisition possible, the model used to analyze the measured quantities is modified in order to describe the studied system more realistically. The local mechanical spectrometer is based on an Atomic Force Microscope (AFM) using an optical method to detect the deflection of the lever. The stress is applied locally on the sample by a transducer fixed at the base of the cantilever. The bandwidth of the microscope' s electronics does not allow one to detect the cantilever movement at frequencies higher than the MHz. Two different approaches are followed to overcome this limit. The first approach exploits the nonlinearity of the contact to down-convert, via mechanical mixing, the high-frequency signals into low-frequency ones. This work demonstrates the feasibility of this technique, as well as its limits, mainly due to the difficulty to analyze confidently the obtained results. The second approach takes advantage of the stroboscopic detection of the movement of the cantilever. The intensity of the laser used to detect the deflection of the cantilever is modulated at a frequency close to the applied stress frequency. The amplitude of the signal recorded at the difference frequency corresponds then to the high-frequency amplitude of the cantilever movement. The frequency range attainable with this method is only limited by the bandwidth of the transducer providing the excitation. The model connecting the measured quantities (strain amplitude and phase lag) to the desired mechanical properties (elasticity and damping) consists in a beam clamped at one end, with a tip attached at some point along its length. The tip itself is connected to the sample via a spring and a dashpot in parallel, representing the normal interaction, and a spring and a dashpot in parallel representing the interaction in the plane of the sample. Combined with the stroboscopic detection, this model enables one to quantitatively determine the elastic modulus of stiff samples. Many results obtained on a wide range of samples exhibiting very diverse mechanical properties are presented. They demonstrate the validity of the stroboscopic approach, which combines simplicity of use and efficiency.

Adaptive frequency tracking and application to biomedical signals

Yann Prudat

The main information of a signal resides in its frequency, its amplitude (or its power) and in their temporal evolution. Thus, a great number of methods for instantaneous frequency estimation have been proposed in the literature. Most of these algorithms use an adaptive filter-based (notch or bandpass filter) structure. In many applications, the information of interest is present in more than one signal. However, to our knowledge no algorithm able to track the frequency on several signals has been presented in the literature. Usually, frequency components are estimated and extracted on each signal independently. Artifacts or noise relative to a specific signal can thus disturb the frequency estimation process. Moreover, low amplitude components present in every signal (but non dominant) will not be estimated. The objective in the first part of this thesis is to develop different methods able to extend existing frequency tracking algorithm in order to improve the quality of the estimate, in terms of estimation variance and robustness with respect to noise. The proposed methods can be applied to algorithms using adaptive filters for the frequency estimation. For the multi-signal frequency tracking extension, these methods use the redundancy of information present in the signals under study. A first approach uses a unique filter for every signal. A set of weights is computed, depending on a measure of the estimate quality, and makes it possible to balance the influence of each signal on the tracking filter update. The second approach consists in using a different adaptive filter for each signal. A set of constraints links the central frequencies of each filter so that they are as similar as possible. Both methods yield frequency estimates more robust with respect to noise and more stable, without any decrease in estimation accuracy. For the harmonic frequency tracking, we propose a method using the information present in the harmonic component to improve the estimate of the fundamental frequency. The proposed methods also permit to extract the signal components corresponding to the estimated frequencies. These components are very useful for subsequent study. In the second part of this thesis, the algorithms developed in the first part are applied to biomedical signals. Two different applications are studied in this work : electrocardiograms and electroencephalograms. Firstly, a frequency tracking algorithm as well as its multi-signal extension are used to predict the success of electrical cardioversion attempts in patients suffering from atrial fibrillation. The instantaneous frequency is estimated using the algorithms and the corresponding signal component is extracted from electrocardiograms recorded prior to the attempt. With a few parameters computed on the estimated frequency and the corresponding signal component, we were able to predict the result of the cardioversion attempt on our database comprising 18 patients with a success rate of 94% for both algorithms. We think that this result can be very useful for helping the clinician to choose the appropriate therapy for atrial fibrillation management. The developed algorithms are also used to track the oscillatory components present in electroencephalograms. The performance of the basic algorithm is illustrated using single-trial electroencephalogram signals from a visual evoked potential experiment. The algorithm is used to track the gamma component (30-50 Hz). It is able to successfully track the spectral component in spite of the fact that large amplitude variations are present in the signal. A complex version of the multi-signal extension is also used to have an algorithm able to track multiple frequency components on multiple signals. The performance of this algorithm is also illustrated with single-trial electroencephalogram signals. It was shown to be able to correctly track up to four frequency components simultaneously. The quality of the estimation is improved using multiple lead signals.

EPFL2009

Hybrid continuous-discrete-time multi-bit delta-sigma A/D converters with auto-ranging algorithm

Sergio Pesenti

In wireless portable applications, a large part of the signal processing is performed in the digital domain. Digital circuits show many advantages. The power consumption and fabrication costs are low even for high levels of complexity. A well established and highly automated design flow allows one to benefit from the constant progress in CMOS technologies. Moreover, digital circuits offer robust and programmable signal processing means and need no external components. Hence, the trend in consumer electronics is to further reduce the part of analog signal processing in the receiver chain of wireless transceivers. Consequently, analog-to-digital converters with higher resolutions and bandwidths are constantly required. The ultimate goal is the direct digitization of radio frequency signals, where the conversion would be performed immediately after the front-end amplifier. ΔΣ-modulation-based converters have proved to be the most suitable to achieve the required performance. Switched-capacitor implementations have been widely used over the last two decades. However, recent publications and books have shown that continuous-time architectures can achieve the same performance with lower power consumption. Most designs found throughout the literature use a single- or few-bit internal quantizer with a high-order modulation. As a result, in order to achieve the resolutions and bandwidths required today, the sampling frequency must exceed 100MHz. This approach leads to non-negligible power consumption in the clock generation. Moreover, the presence of such fast squared signals is not suitable for a system-on-chip comprising radio frequency receivers. In this thesis we propose a low-power strategy relying on a large number of internal levels rather than on a high sampling frequency or modulation order. Besides, a hybrid continuous-discrete-time approach is used to take advantage of the accuracy of switched-capacitor circuits and the low power consumption of continuous-time implementation. The sensitivity to clock jitter brought about by the continuous-time stage is reduced by the use of a large number of levels. An auto-ranging algorithm is developed in this thesis to overcome the limitation of a large-size quantizer under low-voltage supply. Finally, the strategy is applied to a design example addressing typical specifications for a Bluetooth receiver with direct conversion.

EPFL2007

Utilisation de la spectroscopie mécanique dans l'optimisation des traitements thermiques d'aciers au carbone

Ronan Martin

This work is motivated by the industrial need to improve the thermal treatments of carbon steels used for the production of files. It aims at clearly defining the role that carbon plays in hardness and wear resistance of files. Three steel grades with different carbon content have been studied. The samples underwent various austenizing treatments, including treatments under carburizing atmosphere, followed by a quenching process leading to an essentially martensitic microstructure. This microstructure has been studied by mechanical spectroscopy. Several complementary techniques have been used : microscopy, thermopower, calorimetry, measurement of the microhardness and of the file performance. The damping spectra show three relaxation peaks, attributed to the interaction of interstitial atoms with dislocations in martensite. Two of these peaks, M2 and M4, are related with the presence of interstitial carbon. Peak M2 is related with the migration of geometrical kinks along screw dislocations, and peak M4 with the kink-pair formation and migration on screw dislocations. Both peaks are controlled by carbon dragging. This interpretation is supported by the measured activation enthalpies. The amplitude of peak M4 depends on the interstitial-carbon content, which apparently contradicts its physical interpretation. This dependence is attributed to the increase of the quench-induced dislocation density with interstitial-carbon content. The peak amplitude is in fact proportional to the dislocation density. The mechanical spectroscopy measurements are also sensitive to tempering effects. In particular, the precipitation of transition carbides at 380 K is associated with a decrease of the damping background, attributed to the depletion of martensite from interstitial carbon. The decomposition of retained austenite, occurring at 520 K or 670 K depending on the grade, is associated with a modulus anomaly, attributed to a sudden decrease of the dislocation density in martensite. A damping maximum coinciding with this transformation can also be observed at low frequency. This study lead to the use of new thermal treatments under carburizing atmosphere. The used gas mix gives rise to an efficient carbon enrichment of the samples. However, the increase of interstitial-carbon content may also lead to a higher content of retained austenite, thus reducing hardness. Calorimetry measurements allow the quantification of such content, which varies according to the steel grade and the thermal treatment. Martensite hardness and wear resistance of the files clearly increase with the interstitial-carbon concentration, that can be quantified by thermopower measurements, after correction of the effect of retained austenite. Long carburizing treatments can cause the formation of carbides on the sample surface. The presence of carbides in a high-carbon martensite does not seem to improve the mechanical performances, and can even be detrimental. The present study leads to the assessment of an optimal treatment duration of 30 min, which produces a hard martensite with maximum interstitial-carbon content and limits the amount of retained austenite and carbides. This carburizing time seems to produce the saturation of both the content of interstitial carbon at the steel surface and of the amplitude of damping peak M4. The amplitude of this peak seems to be an effective criterion for the evaluation of the ideal carburizing time. Indeed, the peak amplitude increases with the content of interstitial carbon and decreases with the content of retained austenite and carbides. The estimated optimal carburizing time is compatible with production demands. A cryogenic treatment proves to lower efficiently the amount of retained austenite.

EPFL2009

Adaptive frequency tracking and application to biomedical signals

Yann Prudat

EPFL2009

Hybrid continuous-discrete-time multi-bit delta-sigma A/D converters with auto-ranging algorithm

Sergio Pesenti

EPFL2007

Utilisation de la spectroscopie mécanique dans l'optimisation des traitements thermiques d'aciers au carbone

Ronan Martin

EPFL2009