Suchergebnisse

In the daily life, we are exposed to electromagnetic fields. One of the sources of these fields is the natural environment, but they are also dues to the increasing number of electrical appliances. Thus one is more and more concerned with the issue of the effects of this exposure on human health. The low frequency magnetic fields generate induced currents into the human body. These currents may produce some minor, reversible effects (phosphenes) for J=100mA/m2. On the other hand, the long-term effects of this exposure remain not well known. Basing upon the Principle of Precautions, the governments are upgrading the industrial standards concerning non-ionizing radiations. For low frequency (magnetic) fields, some maximum admissible values are defined for the induced current density into the human body J, and for the flux density B. These limits have been defined on the basis of biological thresholds and simple analytical computations, with some security factors. The knowledge of the sole flux density B is not enough to quantify the induced currents into the human body J, because the magnetic field decreases quickly in proximity of the radiating source (where high exposure is likely to occur), but also owing to the heterogeneity of the human body. Besides, these induced currents J are not measurable in vivo. This Ph.D. thesis is devoted to the development of models and numerical tools to simulate the induced phenomena in the human body, in the frequency range of 50 Hz – 100 kHz. Two main problems are studied: - how to determine the spatial distribution of the magnetic field radiated by an electrical appliance (stray field) - how to compute the induced currents into the human body. To this aim, the influence of several parameters on the distribution of the stray field is first studied, and the different numerical techniques to predict the field radiated by a known source are reviewed. This preliminary study shows that conventional methods are not well adapted to compute the radiated field in the air. Hence, a new 3D model is proposed. This model takes into account only the essential features of the radiating source, and thus it is faster and cheaper than ordinary methods, but still accurate enough. In the proposed model, a system of fictive charges is used to compute the stray field. This model has been validated using some simple geometrical structures (coil + magnetic core + air-gap). The exposure to the field can be also originated by sources, whose inner structure is unknown. Thus, an experimental system has been developed, with the purpose to characterize the stray field with a few measurements. To this aim, several models based on the concept of dipole and multipole have been developed. The problem of estimating the parameters of these models is found to be very ill posed: thus some regularization techniques have been implemented. The feasibility has been shown using simple electrical structures. Finally, a special 3D electromagnetic formulation is presented. This formulation is well adapted to the computation of induced currents into the human body, and has been implemented by the finite elements techniques. The computational domain is bounded to the sole human body: thus, some simple geometrical structures, but also a fine anatomical structure composed of several "materials" (= organs), can be used to describe the human body. Using these tools, several exposure situations have been simulated. Some wire systems, but also more realistic structures, have been chosen as radiating sources. The applications in the normative and industrial context may be very important. ; Ce travail de thèse est consacré au développement de modèles et outils numériques permettant la simulation dans la gamme des fréquences 50 Hz¡100 kHz des phénomènes induits par des champs magnétiques dans le corps humain. Pour cela, il existe deux problématiques principales : - Déterminer la répartition du champ rayonné (ou champ de fuite) par un appareil électrique . - Calculer les courants induits qui en résultent dans le corps humain. Après avoir étudié l'influence de divers paramètres sur la répartition des champs de fuite générés par les systèmes électrotechniques, nous avons recensé les méthodes numériques aptes à calculer ce champ pour des systèmes connus. Cette étude montre que ces méthodes en l'état ne sont pas adaptées au calcul des champs dans l'air, car trop coûteuses. Nous présentons donc un modèle 3D ne prenant en compte que les aspects essentiels de la géométrie du système, beaucoup moins coûteux et suffisamment précis. Dans ce modèle, le champ de fuites est calculé à partir d'une représentation en termes de charges magnétiques fictives. Ce modèle a été validé sur des structures géométriques simples, de type bobine + circuit magnétique + entrefer. L'exposition aux champs en milieu industriel ou domestique est également due à des sources dont la structure est inconnue. Nous avons donc développé un système expérimental, permettant de caractériser le champ de fuites à partir d'un nombre minimum de mesures localisées. Pour cela, nous avons développé divers modèles de la source rayonnante s'appuyant sur les notions de dipôle et multipôles. Nous nous sommes consacrés au divers problèmes d'estimation (influence du choix des mesures, prise en compte du bruit). Le problème étant mal conditionné, des techniques de régularisation ont été mises en oeuvre. La faisabilité a été démontrée sur des structures rayonnantes simples. Enfin, nous présentons une formulation 3D adaptée pour calculer les courants induits dans le corps humain, à partir de la connaissance des champs de fuites. Cette formulation a été implémentée dans un code aux éléments finis, dans lequel le domaine de résolution est limité au seul corps humain : ce code permet de modéliser des structures anatomiques réalistes où les valeurs des conductivités des divers organes sont prises en compte. Différentes situations d'exposition du corps aux rayonnements issus de sources simples filaires, et à des sources réelles ont pu être ainsi simulées, et les applications notamment dans des contextes normatif et industriel sont particulièrement importantes.

In the daily life, we are exposed to electromagnetic fields. One of the sources of these fields is the natural environment, but they are also dues to the increasing number of electrical appliances. Thus one is more and more concerned with the issue of the effects of this exposure on human health. The low frequency magnetic fields generate induced currents into the human body. These currents may produce some minor, reversible effects (phosphenes) for J=100mA/m2. On the other hand, the long-term effects of this exposure remain not well known. Basing upon the Principle of Precautions, the governments are upgrading the industrial standards concerning non-ionizing radiations. For low frequency (magnetic) fields, some maximum admissible values are defined for the induced current density into the human body J, and for the flux density B. These limits have been defined on the basis of biological thresholds and simple analytical computations, with some security factors. The knowledge of the sole flux density B is not enough to quantify the induced currents into the human body J, because the magnetic field decreases quickly in proximity of the radiating source (where high exposure is likely to occur), but also owing to the heterogeneity of the human body. Besides, these induced currents J are not measurable in vivo. This Ph.D. thesis is devoted to the development of models and numerical tools to simulate the induced phenomena in the human body, in the frequency range of 50 Hz – 100 kHz. Two main problems are studied: - how to determine the spatial distribution of the magnetic field radiated by an electrical appliance (stray field) - how to compute the induced currents into the human body. To this aim, the influence of several parameters on the distribution of the stray field is first studied, and the different numerical techniques to predict the field radiated by a known source are reviewed. This preliminary study shows that conventional methods are not well adapted to compute the radiated field in the air. Hence, a new 3D model is proposed. This model takes into account only the essential features of the radiating source, and thus it is faster and cheaper than ordinary methods, but still accurate enough. In the proposed model, a system of fictive charges is used to compute the stray field. This model has been validated using some simple geometrical structures (coil + magnetic core + air-gap). The exposure to the field can be also originated by sources, whose inner structure is unknown. Thus, an experimental system has been developed, with the purpose to characterize the stray field with a few measurements. To this aim, several models based on the concept of dipole and multipole have been developed. The problem of estimating the parameters of these models is found to be very ill posed: thus some regularization techniques have been implemented. The feasibility has been shown using simple electrical structures. Finally, a special 3D electromagnetic formulation is presented. This formulation is well adapted to the computation of induced currents into the human body, and has been implemented by the finite elements techniques. The computational domain is bounded to the sole human body: thus, some simple geometrical structures, but also a fine anatomical structure composed of several "materials" (= organs), can be used to describe the human body. Using these tools, several exposure situations have been simulated. Some wire systems, but also more realistic structures, have been chosen as radiating sources. The applications in the normative and industrial context may be very important. ; Ce travail de thèse est consacré au développement de modèles et outils numériques permettant la simulation dans la gamme des fréquences 50 Hz¡100 kHz des phénomènes induits par des champs magnétiques dans le corps humain. Pour cela, il existe deux problématiques principales : - Déterminer la répartition du champ rayonné (ou champ de fuite) par un appareil électrique . - Calculer les courants induits qui en résultent dans le corps humain. Après avoir étudié l'influence de divers paramètres sur la répartition des champs de fuite générés par les systèmes électrotechniques, nous avons recensé les méthodes numériques aptes à calculer ce champ pour des systèmes connus. Cette étude montre que ces méthodes en l'état ne sont pas adaptées au calcul des champs dans l'air, car trop coûteuses. Nous présentons donc un modèle 3D ne prenant en compte que les aspects essentiels de la géométrie du système, beaucoup moins coûteux et suffisamment précis. Dans ce modèle, le champ de fuites est calculé à partir d'une représentation en termes de charges magnétiques fictives. Ce modèle a été validé sur des structures géométriques simples, de type bobine + circuit magnétique + entrefer. L'exposition aux champs en milieu industriel ou domestique est également due à des sources dont la structure est inconnue. Nous avons donc développé un système expérimental, permettant de caractériser le champ de fuites à partir d'un nombre minimum de mesures localisées. Pour cela, nous avons développé divers modèles de la source rayonnante s'appuyant sur les notions de dipôle et multipôles. Nous nous sommes consacrés au divers problèmes d'estimation (influence du choix des mesures, prise en compte du bruit). Le problème étant mal conditionné, des techniques de régularisation ont été mises en oeuvre. La faisabilité a été démontrée sur des structures rayonnantes simples. Enfin, nous présentons une formulation 3D adaptée pour calculer les courants induits dans le corps humain, à partir de la connaissance des champs de fuites. Cette formulation a été implémentée dans un code aux éléments finis, dans lequel le domaine de résolution est limité au seul corps humain : ce code permet de modéliser des structures anatomiques réalistes où les valeurs des conductivités des divers organes sont prises en compte. Différentes situations d'exposition du corps aux rayonnements issus de sources simples filaires, et à des sources réelles ont pu être ainsi simulées, et les applications notamment dans des contextes normatif et industriel sont particulièrement importantes.

In the daily life, we are exposed to electromagnetic fields. One of the sources of these fields is the natural environment, but they are also dues to the increasing number of electrical appliances. Thus one is more and more concerned with the issue of the effects of this exposure on human health. The low frequency magnetic fields generate induced currents into the human body. These currents may produce some minor, reversible effects (phosphenes) for J=100mA/m2. On the other hand, the long-term effects of this exposure remain not well known. Basing upon the Principle of Precautions, the governments are upgrading the industrial standards concerning non-ionizing radiations. For low frequency (magnetic) fields, some maximum admissible values are defined for the induced current density into the human body J, and for the flux density B. These limits have been defined on the basis of biological thresholds and simple analytical computations, with some security factors. The knowledge of the sole flux density B is not enough to quantify the induced currents into the human body J, because the magnetic field decreases quickly in proximity of the radiating source (where high exposure is likely to occur), but also owing to the heterogeneity of the human body. Besides, these induced currents J are not measurable in vivo. This Ph.D. thesis is devoted to the development of models and numerical tools to simulate the induced phenomena in the human body, in the frequency range of 50 Hz – 100 kHz. Two main problems are studied: - how to determine the spatial distribution of the magnetic field radiated by an electrical appliance (stray field) - how to compute the induced currents into the human body. To this aim, the influence of several parameters on the distribution of the stray field is first studied, and the different numerical techniques to predict the field radiated by a known source are reviewed. This preliminary study shows that conventional methods are not well adapted to compute the radiated field in the air. Hence, a new ...