Valerio De Santis

Human Exposure to Close-Range Resonant Wireless Power Transfer Systems as a Function of Design Parameters

In this study, human exposure to close-range wireless power transfer (WPT) systems operating in the frequency range 0.1-10 MHz with coil diameters up to 150 mm are investigated. Approximation formulae, which include scaling factors derived from numerical simulations that take variations of complex human anatomies into consideration, are proposed to conservatively estimate human exposure with respect to the most authoritative exposure guidelines. The approximation has been verified for two precommercial prototype WPT systems, the first of which, a 5-W system operating at 100 kHz, has been evaluated in this study; the second system been verified was reported in a separate study and operates at 6.78 MHz with a nominal current of 5.4 A rms. Based on the results obtained, the optimal operational frequency range for WPT with respect to compliance with exposure safety guidelines is revealed to be ca. 1-2.5 MHz. In summary, this study provides novel and insightful information for the design of an exposure-compliant close-range magnetic resonant WPT system.

Analysis of human brain exposure to low‐frequency magnetic fields: A numerical assessment of spatially averaged electric fields and exposure limits

Compliance with the established exposure limits for the electric field (E-field) induced in the human brain due to low-frequency magnetic field (B-field) induction is demonstrated by numerical dosimetry. The objective of this study is to investigate the dependency of dosimetric compliance assessments on the applied methodology and segmentations. The dependency of the discretization uncertainty (i.e., staircasing and field singularity) on the spatially averaged peak E-field values is first determined using canonical and anatomical models. Because spatial averaging with a grid size of 0.5 mm or smaller sufficiently reduces the impact of artifacts regardless of tissue size, it is a superior approach to other proposed methods such as the 99th percentile or smearing of conductivity contrast. Through a canonical model, it is demonstrated that under the same uniform B-field exposure condition, the peak spatially averaged E-fields in a heterogeneous model can be significantly underestimated by a homogeneous model. The frequency scaling technique is found to introduce substantial error if the relative change in tissue conductivity is significant in the investigated frequency range. Lastly, the peak induced E-fields in the brain tissues of five high-resolution anatomically realistic models exposed to a uniform B-field at ICNIRP and IEEE reference levels in the frequency range of 10 Hz to 100 kHz show that the reference levels are not always compliant with the basic restrictions. Based on the results of this study, a revision is recommended for the guidelines/standards to achieve technically sound exposure limits that can be applied without ambiguity.

EMF Safety and Thermal Aspects in a Pacemaker Equipped With a Wireless Power Transfer System Working at Low Frequency

A wireless power transfer (WPT) system based on magnetic resonant coupling is applied to a pacemaker for recharge its battery. The primary coil is assumed to be on-body, while the secondary coil is in-body. Three different configurations of the secondary coil are hereby investigated placing it inside the titanium case of the pacemaker, on the top surface of the case, or being part of the top surface case. The operational frequency is fixed to be at a relatively low frequency (20 kHz) in order to allow field penetration through the case and to limit the electric and magnetic field safety and thermal increase issues. For each examined configuration, these aspects are investigated by numerical and experimental techniques. The obtained results demonstrate the feasibility of the proposed solutions highlighting their advantages and disadvantages.

Safety Assessment of UWB Radio Systems for Body Area Network by the

The paper deals with the numerical prediction of the specific absorption (SA) of ultra wideband (UWB) radio systems for wireless body area network (BAN). The electro-magnetic analysis is performed by a frequency-dependent finite difference time domain (FD2TD) method here proposed with a new formulation based on a total current density approach. A first order Debye approximation is used to model the frequency-dependent properties of the human body in the frequency range of the UWB signals. The proposed method permits to assess the specific absorption and power loss in the human body exposed to an UWB pulsed source. Different numerical models of the human bodies are finally considered in order to investigate safety aspects.

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This paper deals with contact currents that may occur when the human body is in contact with two electrodes at different electrical potentials, e.g., an electrical/electronic device and the floor. Actually, any device must comply not only with electromagnetic compatibility and safety requirements, but also with specific electromagnetic field exposure recommendations in order to prevent health hazards for the occupational and general public population. Since the contact currents depend on the applied voltage and on the human body impedance, this last parameter has been measured for several configurations in a broadband frequency range, from 40 Hz to 110 MHz. From the measurement results, a new equivalent circuit of the human body impedance is derived by using a vector-fitting procedure. This equivalent circuit is very easy and can be adopted for compliance tests against contact currents.

Method

An equivalent circuit is proposed to model frequency selective surfaces (FSSs) made of optically transparent metal patches. The field penetration through a unit cell of a simple FSS is modeled circuitally by a shunt admittance composed by inductance and capacitance, while the losses in the parts covered by metal thin film are modeled by resistances. The circuit results are validated by full-wave numerical simulations for different test configurations. The circuit model permits fast simulations adequate for FSS design.

Assessment of Human Body Impedance for Safety Requirements Against Contact Currents for Frequencies up to 110 MHz

This article discusses technical issues related to compliance assessment of ICNIRP 2010 basic restrictions. Several difficulties are identified in this study when assessing the spatial average and 99th percentile value of the electric field. These issues are mainly attributed to the lack of clarity in the guideline specifications, which leads to inadequate or irreproducible results. Effects on compliance results due to such ambiguous procedures are hereby investigated, with particular focus on technical issues rather than biological ones. Examples spanning from simple canonical test cases to realistic applications have been selected to highlight the strong variability in dosimetry results. Based on our findings, revisiting the ICNIRP 2010 guidelines is strongly recommended, and proposed alternative solutions are outlined.

Equivalent Circuit Modeling of Frequency-Selective Surfaces Based on Nanostructured Transparent Thin Films

In this paper, open issues pertaining to the modeling of skin, i.e., a potential target tissue for peripheral nerve stimulation due to low-frequency magnetic field exposure, are addressed. First, an equivalent conductivity for a single-layer skin model is derived using a multi-layer skin structure. Unlike previous works, where the conductivity of the stratum corneum or of a weighted average between several skin layers were employed, the conductivity value of the dermis is found to conservatively estimate the peak electric field induced in the stratum basale (i.e., where Merkel nerve endings start to emerge). Then, the induced fields inside a high-resolution anatomical model using the proposed skin conductivity are compared with the basic restrictions provided by the existing guidelines and safety standards. The analysis of the obtained results shows that the relationship between the basic restrictions and the reference or maximum permissible exposure levels recommended by these safety standards is not always consistent.

On the issues related to compliance assessment of ICNIRP 2010 basic restrictions.

The paper deals with the numerical prediction of the magnetic shielding of a coated aperture in a perfectly conductive planar shield. The single layer or multilayer thin film resistive coating is characterized and homogenized by using the transmission line approach. The electromagnetic analysis is performed by two methods: 1) a lumped circuit approach with numerically calculated parameters; 2) a finite element method (FEM) approach using the impedance network boundary conditions (INBCs). The results obtained by the two methods in some test configurations are validated and discussed.

An equivalent skin conductivity model for low-frequency magnetic field dosimetry

In this study, the safety compliance for nonuniform field exposures is discussed using coupling factor concepts. The coupling factor, which is defined in the International Electrotechnical Commission 62311 standard, is extended to consider the effects of harmonics and also to apply to the specific absorption rate (for frequencies up to 30 MHz). The proposed compliance procedure is applied to and demonstrated for a prototype wireless power transfer (WPT) system with induction coupling operating at the fundamental frequency in 140 kHz band. First, measurements confirm that the perturbation of the external magnetic field strength and S11 parameter of a one-loop antenna by a human-equivalent phantom are sufficiently small, suggesting the applicability of the magneto-quasi-static approximation to frequencies up to 30 MHz. Then, the frequency characteristics of the coupling factor are derived for the WPT system. For the prototype system that is not optimized for commercial usage, the maximum allowable transmitting power is relaxed by a factor of 23 with the proposed procedure. The contribution of the harmonics decreased the allowable transmitting power by 39%, indicating their importance for safety compliance.