Développement par le procédé d'extrusion-gonflage de films polymères cellulaires à base de polyéthylène pour des applications piézoélectriques

Cette thèse de doctorat présente une contribution à la fabrication de films polymères cellulaires. Ces matériaux ont récemment fait l’objet d’un vif intérêt aux niveaux académique et industriel grâce à leurs propriétés intéressantes combinant les avantages des polymères et des mousses, et en particulier leur potentiel pour des applications piézoélectriques. En fait, sous l’effet d’un chargement électrique approprié (décharge corona), les films polymères cellulaires peuvent fournir une conversion entre des énergies mécanique et électrique pour être utilisés comme capteurs ou actionneurs. Tout d'abord, une méthode de production de films cellulaires en polyéthylène (PE) combinant le moussage chimique et l'extrusion-gonflage de film a été développée. Ce processus permet d'imposer un étirement biaxial aux échantillons lors de la formation de la structure cellulaire, ce qui favorise l'activité piézoélectrique de l'échantillon. Plusieurs compromis ont été faits pour améliorer la qualité du moussage. Cette optimisation était principalement basée sur une revue de la littérature et des observations directes lors des essais préliminaires. Les propriétés morphologiques ont été présentées et discutées en termes de paramètres de mise en œuvre, à savoir le profil de température, la vitesse de rotation des vis, le débit d'alimentation, le taux d’étirage (TUR), le taux de gonflage (BUR), ainsi que la composition de la matrice. Ces paramètres ont été optimisés pour produire une structure cellulaire homogène présentant des morphologies bien définies et une structure cellulaire bien développée avec des cellules de forme oculaire. Ceci a permis de diminuer le module élastique dans le sens de l'épaisseur et aussi de créer plus de surface spécifique pour la capture de charges conduisant ainsi à améliorer du coefficient piézoélectrique d₃₃. Ensuite, des traitements de pression/température ont été appliqués pour mieux contrôler la morphologie cellulaire des films, afin d’optimiser les propriétés mécaniques et la surface spécifique internede la structure cellulaire. L'étape suivante était le chargement électrique par le procédé corona permettant aux films cellulaires d’acquérir la propriété piézoélectrique. Une amélioration supplémentaire a été obtenue en optimisant le gaz utilisé lors du chargement (azote) et sa pression (15 psi) associés à des conditions de traitement telles que la tension de charge et la distance aiguille-échantillon. Enfin, des traitements thermiques (recuit) et chimiques (acide phosphorique) ont été proposés pour mieux contrôler la microstructure des films et fournir une bonne stabilité temporelle et thermique. Dans l'ensemble, le traitement chimique s'est révélé le plus efficace. À la suite de ces étapes, un échantillon optimisé avec une densité de 450 kg/m³, une épaisseur de 162 μm, un facteur de forme longitudinale (AR-L) de 7.0etun facteur de forme transversale (AR-T) de 4.1 a été fabriqué. Les propriétés piézoélectriques rapportées sont très élevées (même comparées au polypropylène (PP) qui est la polyoléfine la plus utilisée dans ce domaine) avec un coefficient d₃₃ initial de 1315 pC/N se stabilisant après 50 jours à 792 pC/N et une bonne stabilité thermique, car les films restent chargés avec de bons coefficients piézoélectriques (280 pC/N) jusqu’à 80°C. Ces valeurs ont été améliorées par l'application d'un procédé combiné d’inversement de charge et d’empilement de trois couches donnant un d₃₃ initial de 3270 pC/N, un d₃₃ stabilisé de 1580pC/N après 50 jours et une valeurde 641 pC/N à 80°C. Ces films de PE ferroélectrets aux propriétés piézoélectriques importantes peuvent être maintenant exploités pour la production à grande échelle de capteurs et de transducteurs à base d’électret.This doctoral thesis presents a contribution on the fabrication of cellular polymer films. These materials have recently experienced a great interest at academic and industrial levels thanks to their interesting properties combining the advantages of both polymers and foams, in particular their potential for piezoelectric applications. In fact, after charging by an appropriate method (corona discharge), cellular polymers can provide high electrical/mechanical energy conversion to be used as sensors or actuators. Firstly, a method to produce polyethylene (PE) cellular films using extrusion film-blowing was developed. This process allowed to impose biaxial stretching on the samples while foaming, which is believed to enhance the piezoelectric activity of the samples. Several compromises were made to improve the foaming quality. This optimization was mainly based on a literature review and direct observations during preliminary trials. Morphological properties were presented and discussed in terms of processing parameters, namely the temperature profile, screw rotational speed, feeding rate, take-up ratio (TUR), blow-up ratio (BUR), as well as the matrix composition. These parameters were optimized to produce a homogeneous cellular structure with defined morphologies and a well-developed eye-like cellular structure, which is important to decrease the elastic stiffness in the thickness direction and to provide more surface for charge capturing via cell deformation, thus improving the piezoelectric coefficient d₃₃. Then, pressure/temperature treatments were applied to further control the cellular morphology of the films and optimize the mechanical properties and internal specific surface area of the cellular structure.The next step was the electric charging by corona discharge producing cellular films with piezoelectric activity. Further improvement was obtained by optimizing the gas used (nitrogen) and its pressure (15 psi) combined with processing conditions such as the charging voltage and the needle-sample distance. Finally, thermal (annealing) and chemical (phosphoric acid) treatments have been proposed to further control the microstructure of these films and to provide good time and thermal stability. Overall, the chemical treatment was found to be the most efficient. Following these steps, an optimized sample with a density of 450 kg/m³, a thickness of 162 μm, a longitudinal cell aspect ratio (AR-L) of 7.0 and a transversal cell aspect ratio (AR-T) of 4.1 was developed. The piezoelectric properties reported are very high (even compared to PP which is the most used polyolefin in this field) with an initial d₃₃ of 1315 pC/N stabilizing after 50 days at 792 pC/N and a good thermal stability since the films remained charged with good piezoelectric coefficients (280 pC/N) up to 80°C. The values were further improved when a three-layered reverse charging method was appliedgiving an initial d₃₃ of 3270 pC/N, a stabilized d₃₃ of 1580pC/N after 50 days and a value of 641 pC/N at 80°C. These ferroelectret PE films with important piezoelectric properties can now be exploited for the large-scale production of electret-based sensors and transducers

Data and Sensor Fusion Using FMG, sEMG and IMU Sensors for Upper Limb Prosthesis Control

Whether someone is born with a missing limb or an amputation occurs later in life, living with this disability can be extremely challenging. The robotic prosthetic devices available today are capable of giving users more functionality, but the methods available to control these prostheses restrict their use to simple actions, and are part of the reason why users often reject prosthetic technologies. Using multiple myography modalities has been a promising approach to address these control limitations; however, only two myography modalities have been rigorously tested so far, and while the results have shown improvements, they have not been robust enough for out-of-lab use. In this work, a novel multi-modal device that allows data to be collected from three myography modalities was created. Force myography (FMG), surface electromyography (sEMG), and inertial measurement unit (IMU) sensors were integrated into a wearable armband and used to collect signal data while subjects performed gestures important for the activities of daily living. An established machine learning algorithm was used to decipher the signals to predict the user\u27s intent/gesture being held, which could be used to control a prosthetic device. Using all three modalities provided statistically-significant improvements over most other modality combinations, as it provided the most accurate and consistent classification results. This work provides justification for using three sensing modalities and future work is suggested to explore this modality combination to decipher more complex actions and tasks with more sophisticated pattern recognition algorithms

Improving the Robustness of Electromyogram-Pattern Recognition for Prosthetic Control by a Postprocessing Strategy

Electromyogram (EMG) contains rich information for motion decoding. As one of its major applications, EMG-pattern recognition (PR)-based control of prostheses has been proposed and investigated in the field of rehabilitation robotics for decades. These prostheses can offer a higher level of dexterity compared to the commercially available ones. However, limited progress has been made toward clinical application of EMG-PR-based prostheses, due to their unsatisfactory robustness against various interferences during daily use. These interferences may lead to misclassifications of motion intentions, which damage the control performance of EMG-PR-based prostheses. A number of studies have applied methods that undergo a postprocessing stage to determine the current motion outputs, based on previous outputs or other information, which have proved effective in reducing erroneous outputs. In this study, we proposed a postprocessing strategy that locks the outputs during the constant contraction to block out occasional misclassifications, upon detecting the motion onset using a threshold. The strategy was investigated using three different motion onset detectors, namely mean absolute value, Teager–Kaiser energy operator, or mechanomyogram (MMG). Our results indicate that the proposed strategy could suppress erroneous outputs, during rest and constant contractions in particular. In addition, with MMG as the motion onset detector, the strategy was found to produce the most significant improvement in the performance, reducing the total errors up to around 50% (from 22.9 to 11.5%) in comparison to the original classification output in the online test, and it is the most robust against threshold value changes. We speculate that motion onset detectors that are both smooth and responsive would further enhance the efficacy of the proposed postprocessing strategy, which would facilitate the clinical application of EMG-PR-based prosthetic control

Symmetric and Asymmetric Data in Solution Models

This book is a Printed Edition of the Special Issue that covers research on symmetric and asymmetric data that occur in real-life problems. We invited authors to submit their theoretical or experimental research to present engineering and economic problem solution models that deal with symmetry or asymmetry of different data types. The Special Issue gained interest in the research community and received many submissions. After rigorous scientific evaluation by editors and reviewers, seventeen papers were accepted and published. The authors proposed different solution models, mainly covering uncertain data in multicriteria decision-making (MCDM) problems as complex tools to balance the symmetry between goals, risks, and constraints to cope with the complicated problems in engineering or management. Therefore, we invite researchers interested in the topics to read the papers provided in the book