TY - JOUR
AU1 - Ahn,, Jangyong
AU2 - Hong,, Seon-Eui
AU3 - Kim,, Haerim
AU4 - Chun,, Yangbae
AU5 - Choi,, Hyung-Do
AU6 - Kim,, Kibeom
AU7 - Andrés,, Brito
AU8 - Choi,, Junsung
AU9 - Ahn,, Seungyoung
AB - Abstract Recently, a wireless charging system (WCS) for drones has been extensively studied, although standards for compliance testing of a WCS for drones have yet to be established. In this study, we propose methods for human exposure assessments of a WCS for drones by comprehensively considering the various positions of the system and the postures of human body models. The electromagnetic fields from a WCS are modeled and the internal quantities of the human body models, consisting of current density, internal electric field and specific absorption rate, are calculated. The incident fields around the WCS and the internal quantities are analyzed at 140 kHz, which is the operating frequency of the WCS applied. Results of an exposure assessment based on the confirmed worst-case scenario are presented. In addition, the internal quantities depending on the human body models and the material characteristics of the simplified models are also discussed using four different anatomical and simplified human body models. INTRODUCTION The use of unmanned aerial vehicles, also referred to as drones, is rapidly spreading to various applications. However, because drones use a built-in lithium-ion battery as an energy storage element, the battery limits their power consumption and operating time. To solve the problem, a charging system using wireless power transfer (WPT) technology was developed to allow the battery to be charged automatically when the drones land on their base station(1, 1). However, since the wireless charging system (WCS) generates electromagnetic fields (EMFs), concerns arise regarding adverse health effects(3). To address the safety issues generated by the use of the WCS for drones, human exposure to EMFs should be assessed and ascertained to be in compliance with international safety guidelines and standards(4–7). Compliance testing for WCSs is generally conducted against basic restrictions or reference levels. According to human safety standards(4, 4), at low frequencies, below 100 kHz, the dominant factors for an assessment are instantaneous fields or induced current density in the tissues of the body, which can cause electrostimulation. However, at higher frequencies, over 10 MHz, the heating effect dominates electromagnetic exposure. To comply with a system frequency lying between 100 kHz and 10 MHz, both the electrostimulation and thermal effects should be satisfied. According to the technical report by the International Electrotechnical Commission (IEC)(8), four steps for evaluating the effect of direct exposure to a WPT system based on exposure safety guidelines are introduced, among which, the step that is most appropriate for the exposure situation can be applied. The two simplest ways to confirm the compliance of a system are an evaluation based on: (1) the transmit power or current, or (2) an evaluation of the incident fields against the reference levels. However, due to the reference levels are derived under uniform EMF exposure in a human body model, an evaluation of the incident fields may be too conservative for localized exposure to a WCS. In such a case, a third step, utilizing a coupling factor or the generic gradient source model method can be applied to the evaluation as a corrective method for dealing with the non-uniformity of the incident fields. Finally, evaluation of the internal quantities (J, E and SAR) against basic restrictions can be the last (fourth) step. In this paper, exposure-assessment methods for testing the compliance of a WCS for drones under the existing safety guidelines are proposed. For the WCS for drones, a standardized method for achieving EMF compliance has yet to be established. Compliance standards(8–11) for WPT systems, such as those used in electric vehicles (EVs), mobile devices and typical desktop applications, can be applied to the WCS for drones. Thus, the results of using such evaluation methods to a WCS for drones were analyzed. Furthermore, various scenarios of applying a WCS for drones are considered to evaluate the incident fields, induced current density, internal electric field and specific absorption rate (SAR) against the basic restrictions. Finally, different human body models are compared and discussed for compliance testing based on numerical analyses. MODELING AND METHODS WCS for drones A WCS was used for a DJI Spreading Wings S1000 Plus drone(12). The WCS operates at a resonant frequency of 140 kHz and has maximum transmit power of 200 W with the system efficiency of ~60%. The WCS has an optimum receiver structure for expanding the chargeable region, as shown in a previous study(13). Transmission coils are separated into a transmitting coil (Tx-coil) and receiving coils (Rx-coils). The Tx-coil is a five-turn single-layer coil, and the diameter of the wire is 3 mm. The two identical Rx-coils consist of 13-turn coils, and the diameter of the wires is 1.5 mm. The measured inductances are 33.6 uH for the Tx-coil, and 99.8 and 99.1 uH for the Rx-coils, respectively. A ferrite and a metal plate are located under the Tx-coil for guiding and shielding the EMFs. The structure and geometry of the WCS for the drone are shown in Figure 1. Figure 1 Open in new tabDownload slide Geometry of the WCS for drones: (a) side-view and (b) top-view. Figure 1 Open in new tabDownload slide Geometry of the WCS for drones: (a) side-view and (b) top-view. Computational methods The commercial electromagnetic simulation software Sim4Life(14) is used for the WCS modeling and human exposure calculations. The magneto-quasi-static (MQS) approximation is applicable to a WCS for drones with a resonant frequency of 140 kHz(15). Thus, a two-step computational process based on an MQS approximation is conducted. First, in the absence of a human body, the external magnetic field or magneto-static vector potential is evaluated within the space where the human body will be located. A field analysis inside the human body is then calculated using the extracted external magnetic field as a source(16). To evaluate the internal fields of a human body, anatomical human body models and simplified models are used, as shown in Figure 2. The anatomical models, Duke (a 34-year-old male) and Ella (a 26-year-old female)(17), are from Information Technologies in Society (IT’IS), and simplified models were taken from the IEC standard, namely, a uniform human body model (dx = dy = 0.35 m, dz = 1.528 m) and a cuboid model (dx = dy = 0.4 m, dz = 1.8 m)(18). The dielectric parameters of the tissue are extracted from the IT’IS database(19, 19). In this study, all four human body models are represented with a uniform cubic grid of 2 × 2 × 2 mm. Figure 2 Open in new tabDownload slide Human body models: (a, b) anatomical models Duke and Ella, (c, d) simplified models uniform human body model, and a cuboid model. Figure 2 Open in new tabDownload slide Human body models: (a, b) anatomical models Duke and Ella, (c, d) simplified models uniform human body model, and a cuboid model. Compliance procedure for the WCS According to the compliance procedures described in various standards(8–11), human exposure assessments testing of a WPT system for use by an EV and a typical WPT for application by a consumer device are introduced. Methods used to evaluate the incident magnetic field against the reference levels are defined in different ways depending on the device applications. For the scenarios of a human body model standing around an EV, the maximum magnetic field strength in the area is measured at 20 cm from the system and compared to the reference level. If it exceeds the limit, a spatially averaged magnetic field within the space occupied by the human body model or an averaged magnetic field strength measured at heights of 0.5, 1.0 and 1.5 m above the ground is considered. In the case of portable devices such as typical desktop applications, the measurement should be made from all sides of the system with 10 cm from the center of the probe to the edge of the device. For both applications, if the incident field exceeds the reference level, evaluation methods against the basic restrictions are applied. To evaluate the incident fields from a WCS for drones against the reference levels, several methods from the standards have been applied. The WCS for drones can be located at various positions depending on the user applications, namely, on the ground, on a table or on the top of a vehicle. In this study, WPT systems located at various heights are considered; in addition, at a specific height, namely, the worst-case scenario for a human body, several postures are also compared. Based on the analysis from these studies, the results from different human body models are compared. Coupling factor calculation of WCS for drones In compliance testing procedures, the concept of a coupling factor was originally introduced for non-uniform exposure situations(18, 18). For localized exposure scenarios when the human body is located in the vicinity of a WCS for drones, the coupling factor is applied for a direct comparison to the reference levels. The coupling factor is defined by the ratio of the maximum values of the induced current density (Jmax) to the incident magnetic flux density (Bmax) times the ratio of the reference level value of the incident magnetic flux density (Blim) to the basic restriction value of the induced current density (Jlim). The concept of a coupling factor has been expanded through different definitions using the maximum values of an internal electric field (Emax) and the local SAR (SARmax)(21). $$\begin{equation} {a}_c=\frac{J_{\mathrm{max}}}{B_{\mathrm{max}}} \times \frac{B_{\mathrm{lim}}}{J_{\mathrm{lim}}\ } \end{equation}$$ (1) $$\begin{equation} {a}_{c1}=\frac{E_{\mathrm{max}}}{H_{\mathrm{max}}} \times \frac{H_{\mathrm{lim}}}{E_{\mathrm{lim}}} \end{equation}$$ (2) $$\begin{equation} {a}_{c2}=\frac{\sqrt{\mathrm{SA}{\mathrm{R}}_{\mathrm{max}}}}{H_{\mathrm{max}}} \times \frac{H_{\mathrm{lim}}}{\sqrt{\mathrm{SA}{\mathrm{R}}_{\mathrm{lim}}}} \end{equation}$$ (3) Here, Bmax and Hmax are the maximum spatial levels measured with isotropic sensors, which have a measuring area of 3 ± 0.3 cm2 in accordance with the prescribed standards(18, 18). The values of Jmax, Emax and SARmax refer to the maximum current density averaged over 1 cm2, the internal electric field in a 2 × 2 × 2 mm3 cube, and 10 g local SAR, respectively, as defined in the safety guidelines(4, 4). In equations (1) and (3), Blim, and Hlim are the reference levels, and Jlim and SARlim are the basic restrictions for general public exposure, respectively(4). In equation (2), Hlim is the reference level, and Elim is the basic restriction for general public exposure, both of which are defined according to the safety guidelines(5). The measured Bmax (or Hmax) can be replaced with the computed Bmax,sim (or Hmax,sim) through a simulation. COMPLIANCE TESTING RESULTS Magnetic field strength from the WCS The first step in the human exposure assessment of the WCS is an evaluation based on the transmit power, which is not applicable to a high-power WCS for drones. If the WCS is not in compliance with the low-power exception condition, evaluations of the incident fields against the reference levels are needed as a second step. Unlike a WCS for EVs, which is embedded underground, the WCS for drones can be in various locations. Therefore, it is difficult to apply a three-point measurement method, which is the average magnetic field strength measured at 0.5, 1.0 and 1.5 m above the ground. Another method is an evaluation of the spatial average value of the magnetic field on a space occupied by a human body at 200 mm, which was introduced in ‘Modeling and Methods’ section. For this system, the magnetic field strength is 5.09 and 5.21 A m–1 in the applied measurement and simulation, respectively, whereas the spatial average is 3.08 A m–1 at a distance of 200 mm. This evaluation method is too loosely defined, resulting in most drones satisfying the compliance test. Therefore, the magnetic field strength is applicable to a WCS for drones to evaluate the incident field against the reference level. Here, the magnetic field strength is the 99th percentile used to compensate for the computational error with an isotropic field sensor(5), which has a measurement area of 100 ± 5 cm2 in the standards(18, 18). The measured magnetic fields generated by the WCS are compared with the simulation results shown in Figure 3. In the measurements, the Tx-coil has an inductance of 33.6 uH and the Rx-coils have inductances of 99.8 and 99.1 μH, which are <1 and 7% differences compare to the simulation results, respectively. As a result of measuring the magnetic field strength generated by a 140 kHz WCS for drones, it was confirmed that the simulation results are in accordance with the measurement results with errors of up to 10%. Figure 3 Open in new tabDownload slide Comparison between the measured and simulated magnetic field strength around a WCS: (a) x–z and (b) y–z planes. Figure 3 Open in new tabDownload slide Comparison between the measured and simulated magnetic field strength around a WCS: (a) x–z and (b) y–z planes. As shown in Table 1, to satisfy the incident field evaluation against the reference level provided in the guidelines, the magnetic field should not exceed 5 A m–1 (4). Therefore, the reference level evaluation is satisfied at more than 200 mm. To evaluate the system within this distance, a basic restriction evaluation must be conducted. Table 1 ICNIRP guidelines for the general public at 140 kHz. . Reference levels . Basic restrictions . Electric field strength (V m–1) . Magnetic flux density (μT) . Magnetic field strength (A m–1) . Current density (mA m–2) . Localized SAR for head/trunk (W kg–1) . Localized SAR for limbs (W kg–1) . Internal electric field (V m–1) . ICNIRP 1998 87 6.25 5 280 2 4 — ICNIRP 2010 83 27 21 — — — 18.9 . Reference levels . Basic restrictions . Electric field strength (V m–1) . Magnetic flux density (μT) . Magnetic field strength (A m–1) . Current density (mA m–2) . Localized SAR for head/trunk (W kg–1) . Localized SAR for limbs (W kg–1) . Internal electric field (V m–1) . ICNIRP 1998 87 6.25 5 280 2 4 — ICNIRP 2010 83 27 21 — — — 18.9 Open in new tab Table 1 ICNIRP guidelines for the general public at 140 kHz. . Reference levels . Basic restrictions . Electric field strength (V m–1) . Magnetic flux density (μT) . Magnetic field strength (A m–1) . Current density (mA m–2) . Localized SAR for head/trunk (W kg–1) . Localized SAR for limbs (W kg–1) . Internal electric field (V m–1) . ICNIRP 1998 87 6.25 5 280 2 4 — ICNIRP 2010 83 27 21 — — — 18.9 . Reference levels . Basic restrictions . Electric field strength (V m–1) . Magnetic flux density (μT) . Magnetic field strength (A m–1) . Current density (mA m–2) . Localized SAR for head/trunk (W kg–1) . Localized SAR for limbs (W kg–1) . Internal electric field (V m–1) . ICNIRP 1998 87 6.25 5 280 2 4 — ICNIRP 2010 83 27 21 — — — 18.9 Open in new tab Figure 4 Open in new tabDownload slide Various exposure scenarios where the WCS is in (a) different heights at a distance (d = 100 mm), and (b) different distances (d = 1, 10, 30, 50 and 100 mm). Figure 4 Open in new tabDownload slide Various exposure scenarios where the WCS is in (a) different heights at a distance (d = 100 mm), and (b) different distances (d = 1, 10, 30, 50 and 100 mm). Evaluation results of the incident fields against the basic restrictions using coupling factors The WCS for drones has various configurations depending on their location, and different exposure assessment results occur for each scenario. Therefore, to evaluate the incident fields against the basic restrictions depending on position, the coupling factors are extracted at each position to find the worst-case conditions. Because the magnetic field varies around the system, the human body is positioned where the highest magnetic field strength is measured around the WCS. The six positions are at heights of ~0, 0.2, 0.5, 0.9, 1.3 and 1.6 m, which are the ground, ankle, knee, waist, chest and face positions, respectively, as shown in Figure 4a; in addition, the extracted coupling factors at each position are shown in Table 2. Table 2 Coupling factors depending on the heights. Height (m) . |${a}_c$| . |${a}_{c1}$| . |${a}_{c2}$| . 0 0.088 0.145 0.001 0.2 0.183 0.308 0.001 0.5 0.339 0.542 0.002 0.9 0.331 0.545 0.003 1.3 0.311 0.394 0.003 1.6 0.218 0.161 0.001 Height (m) . |${a}_c$| . |${a}_{c1}$| . |${a}_{c2}$| . 0 0.088 0.145 0.001 0.2 0.183 0.308 0.001 0.5 0.339 0.542 0.002 0.9 0.331 0.545 0.003 1.3 0.311 0.394 0.003 1.6 0.218 0.161 0.001 Open in new tab Table 2 Coupling factors depending on the heights. Height (m) . |${a}_c$| . |${a}_{c1}$| . |${a}_{c2}$| . 0 0.088 0.145 0.001 0.2 0.183 0.308 0.001 0.5 0.339 0.542 0.002 0.9 0.331 0.545 0.003 1.3 0.311 0.394 0.003 1.6 0.218 0.161 0.001 Height (m) . |${a}_c$| . |${a}_{c1}$| . |${a}_{c2}$| . 0 0.088 0.145 0.001 0.2 0.183 0.308 0.001 0.5 0.339 0.542 0.002 0.9 0.331 0.545 0.003 1.3 0.311 0.394 0.003 1.6 0.218 0.161 0.001 Open in new tab Coupling factors calculated at each height using equations (1–3) confirmed that the system placed at a height of 0.9 m, which is located at the center of the human body, is the worst-case scenario. Therefore, the coupling factors depending on the distance are extracted at the 0.9 m height, as shown in Table 3. The calculated coupling factors are multiplied by the magnetic field and directly compared with the reference levels shown in Figure 5. In the evaluation results using the coupling factors, the magnetic field strength may exceed the reference levels depending on the position or distance. In such cases, the fourth-assessment step, which is an evaluation of the internal quantities against the basic restrictions, should be conducted. Table 3 Coupling factors depending on the distances. Distance (mm) . |${a}_c$| . |${a}_{c1}$| . |${a}_{c2}$| . 1 0.026 0.034 0.0002 10 0.113 0.162 0.0009 30 0.211 0.335 0.0018 50 0.267 0.433 0.0023 100 0.331 0.545 0.0029 Distance (mm) . |${a}_c$| . |${a}_{c1}$| . |${a}_{c2}$| . 1 0.026 0.034 0.0002 10 0.113 0.162 0.0009 30 0.211 0.335 0.0018 50 0.267 0.433 0.0023 100 0.331 0.545 0.0029 Open in new tab Table 3 Coupling factors depending on the distances. Distance (mm) . |${a}_c$| . |${a}_{c1}$| . |${a}_{c2}$| . 1 0.026 0.034 0.0002 10 0.113 0.162 0.0009 30 0.211 0.335 0.0018 50 0.267 0.433 0.0023 100 0.331 0.545 0.0029 Distance (mm) . |${a}_c$| . |${a}_{c1}$| . |${a}_{c2}$| . 1 0.026 0.034 0.0002 10 0.113 0.162 0.0009 30 0.211 0.335 0.0018 50 0.267 0.433 0.0023 100 0.331 0.545 0.0029 Open in new tab Various exposure scenarios can exist depending on the various postures of the human body, even at the same height (h = 0.9 m) and distance (d = 100 mm). Figure 6 shows the possible exposure scenarios for different human postures from the WCS for drones, in which a person is standing in front of the system, has a backward or side position, has a hand-on posture, or has hands behind the body while looking straight ahead. Figure 5 Open in new tabDownload slide Variations in the coupling factor according to the various scenarios depending on the (a) height and (b) distance. Figure 5 Open in new tabDownload slide Variations in the coupling factor according to the various scenarios depending on the (a) height and (b) distance. Figure 6 Open in new tabDownload slide Various exposure scenarios in which the WCS is at the (a) front, (b) back, (c) side, (d) front with a hand-on position and (e) front with a hand behind the back posture (h = 0.9 m and d = 100 mm). Figure 6 Open in new tabDownload slide Various exposure scenarios in which the WCS is at the (a) front, (b) back, (c) side, (d) front with a hand-on position and (e) front with a hand behind the back posture (h = 0.9 m and d = 100 mm). The coupling factors for each scenario are shown in Table 4. In the case of |${a}_c$|⁠, the coupling factor when the human body model is on the rear side is larger than on the front side, and |${a}_{c1}$| and |${a}_{c2}$| are higher when the human body is standing in front of the system. However, for the hand-on and hands behind the back postures, the coupling factors are similar to those of the standing in front posture because of the maximum values of the induced current density, internal electric field and SAR appear on the body rather than on the hand. When the human body is standing at the side, the coupling factors are much smaller compared with the other scenarios except for the SAR. In conclusion, it is sufficient to consider only the case in which the system is located at the front and back of the human body. Evaluation results of the induced electric field and SAR against the basic restrictions As shown in ‘Compliance Testing Results’ section, the evaluations of the WCS for drones when applying the coupling factors are not in compliance with the reference levels for some of the conditions. Therefore, the last step, evaluations of the induced current density, electric field and SAR against the basic restrictions are conducted and analyzed. During the step, evaluations are carried out depending on the height and distance shown in Figure 7. To evaluate the induced current density, the internal electric field and the SAR against the basic restrictions when the WCS is located at a height of 0.9 m, which is the center of the body, is applied as the worst-case scenario. Depending on the distance from the WCS to the human body, the induced current density does not satisfy the standard at within 10 mm, whereas the internal electric field and SAR comply with the standard regardless of the distance. Figure 7 Open in new tabDownload slide Open in new tabDownload slide Evaluation of the (a) induced current density, (b) electric field, and (c) SAR against the basic restrictions depending on the position. Figure 7 Open in new tabDownload slide Open in new tabDownload slide Evaluation of the (a) induced current density, (b) electric field, and (c) SAR against the basic restrictions depending on the position. As the evaluation results indicate, the current density is the dominant exposure factor at this frequency band, and thus the electric field and SAR are satisfied under a situation when the current density satisfies the standards. Because the compliance of the basic restriction is also proportional to the transmitted power, the maximum transmitted power at each distance can be calculated. The maximum permissible transmitting power based on the distance is as follows: 199 W at 10 mm, 240 W at 30 mm, 284 W at 50 mm and 397 W at 100 mm. Evaluations depending on the human body models In the IEC standard, simplified models are presented for a numerical evaluation of the induced quantities in a human body(18). Therefore, the anatomical models and the simplified models are compared for the analysis. The anatomical human models use hundreds of tissues, whereas the simplified models only have a single representative material. To analyze the results according to the electrical characteristics of the tissues in the anatomical models, maximum values of the internal quantities in each tissue and the electrical characteristics of the tissues are shown in Tables 5 and 6, respectively. Because of the current density, which is the dominant factor, is maximal in tissues with high conductivity in the anatomical models, applying a representative conductivity value is important. To determine the conductivity of the simplified models, we calculated the conductivity from the results of the current density and internal electric field of the Duke model shown in Figure 7. Since the conductivity can be calculated as a ratio of the current density to the electric field, the conductivity is calculated at each height, and a value of 0.22 S m–1 is then obtained by taking the average at all heights. Although there are slight variations in the value depending on the height, all values are within a range of ~±10% from the average value of 0.22 S m–1. The coupling factors according to the human body models are shown in Figure 8. Although the current density and SAR are not significantly different, the internal electric field has more than a 10-fold variation. Table 4 Coupling factors depending on the posture. Posture . |${a}_c$| . |${a}_{c1}$| . |${a}_{c2}$| . Front 0.331 0.545 0.0029 Back 0.365 0.516 0.0027 Side 0.149 0.166 0.0029 Hand-on 0.322 0.516 0.0026 Hands behind 0.321 0.540 0.0027 Posture . |${a}_c$| . |${a}_{c1}$| . |${a}_{c2}$| . Front 0.331 0.545 0.0029 Back 0.365 0.516 0.0027 Side 0.149 0.166 0.0029 Hand-on 0.322 0.516 0.0026 Hands behind 0.321 0.540 0.0027 Open in new tab Table 4 Coupling factors depending on the posture. Posture . |${a}_c$| . |${a}_{c1}$| . |${a}_{c2}$| . Front 0.331 0.545 0.0029 Back 0.365 0.516 0.0027 Side 0.149 0.166 0.0029 Hand-on 0.322 0.516 0.0026 Hands behind 0.321 0.540 0.0027 Posture . |${a}_c$| . |${a}_{c1}$| . |${a}_{c2}$| . Front 0.331 0.545 0.0029 Back 0.365 0.516 0.0027 Side 0.149 0.166 0.0029 Hand-on 0.322 0.516 0.0026 Hands behind 0.321 0.540 0.0027 Open in new tab Table 5 Maximum values in each tissue for the basic restriction evaluation based on the anatomical models. Anatomical model . Tissue . Current density (mA m–2) . Internal electric field (V m–1) . SAR (W kg–1) . Duke Max. 261.1 6.872 0.00155 Small intestine lumen 261.1 0.580 0.00012 Muscle 256.9 0.185 0.00155 Skin 148.9 6.872 0.00003 Subcutaneous fat (SAT) 213.0 4.064 0.00079 Ella Max. 206.7 11.528 0.00165 Small intestine lumen 206.7 0.456 0.00007 Muscle 198.2 1.225 0.00051 Skin 101.3 11.528 0.00008 SAT 134.2 5.880 0.00165 Anatomical model . Tissue . Current density (mA m–2) . Internal electric field (V m–1) . SAR (W kg–1) . Duke Max. 261.1 6.872 0.00155 Small intestine lumen 261.1 0.580 0.00012 Muscle 256.9 0.185 0.00155 Skin 148.9 6.872 0.00003 Subcutaneous fat (SAT) 213.0 4.064 0.00079 Ella Max. 206.7 11.528 0.00165 Small intestine lumen 206.7 0.456 0.00007 Muscle 198.2 1.225 0.00051 Skin 101.3 11.528 0.00008 SAT 134.2 5.880 0.00165 Open in new tab Table 5 Maximum values in each tissue for the basic restriction evaluation based on the anatomical models. Anatomical model . Tissue . Current density (mA m–2) . Internal electric field (V m–1) . SAR (W kg–1) . Duke Max. 261.1 6.872 0.00155 Small intestine lumen 261.1 0.580 0.00012 Muscle 256.9 0.185 0.00155 Skin 148.9 6.872 0.00003 Subcutaneous fat (SAT) 213.0 4.064 0.00079 Ella Max. 206.7 11.528 0.00165 Small intestine lumen 206.7 0.456 0.00007 Muscle 198.2 1.225 0.00051 Skin 101.3 11.528 0.00008 SAT 134.2 5.880 0.00165 Anatomical model . Tissue . Current density (mA m–2) . Internal electric field (V m–1) . SAR (W kg–1) . Duke Max. 261.1 6.872 0.00155 Small intestine lumen 261.1 0.580 0.00012 Muscle 256.9 0.185 0.00155 Skin 148.9 6.872 0.00003 Subcutaneous fat (SAT) 213.0 4.064 0.00079 Ella Max. 206.7 11.528 0.00165 Small intestine lumen 206.7 0.456 0.00007 Muscle 198.2 1.225 0.00051 Skin 101.3 11.528 0.00008 SAT 134.2 5.880 0.00165 Open in new tab Table 6 Electrical properties of the tissue at 140 kHz. Tissue . Mass density, |$\rho$| (kg m–3) . Conductivity, |$\sigma$| (S m–1) . Relative permittivity, |$\varepsilon$| . Small intestine lumen 1045.2 0.37 7277.17 Muscle 1090.4 0.37 7277.17 Skin 1109 0.00066 1113.39 SAT 911 0.043 85.17 Tissue . Mass density, |$\rho$| (kg m–3) . Conductivity, |$\sigma$| (S m–1) . Relative permittivity, |$\varepsilon$| . Small intestine lumen 1045.2 0.37 7277.17 Muscle 1090.4 0.37 7277.17 Skin 1109 0.00066 1113.39 SAT 911 0.043 85.17 Open in new tab Table 6 Electrical properties of the tissue at 140 kHz. Tissue . Mass density, |$\rho$| (kg m–3) . Conductivity, |$\sigma$| (S m–1) . Relative permittivity, |$\varepsilon$| . Small intestine lumen 1045.2 0.37 7277.17 Muscle 1090.4 0.37 7277.17 Skin 1109 0.00066 1113.39 SAT 911 0.043 85.17 Tissue . Mass density, |$\rho$| (kg m–3) . Conductivity, |$\sigma$| (S m–1) . Relative permittivity, |$\varepsilon$| . Small intestine lumen 1045.2 0.37 7277.17 Muscle 1090.4 0.37 7277.17 Skin 1109 0.00066 1113.39 SAT 911 0.043 85.17 Open in new tab Figure 8 Open in new tabDownload slide Open in new tabDownload slide Coupling factors extracted from the different human body models: (a) ac, (b) ac1 and (c) ac2. Figure 8 Open in new tabDownload slide Open in new tabDownload slide Coupling factors extracted from the different human body models: (a) ac, (b) ac1 and (c) ac2. DISCUSSION To analyze the effect of the conductivity, we compare the coupling factors depending on three conductivity values: the calculated value, 0.22 S m–1; two-thirds of the conductivity of the muscles(16), 0.247 S m–1 and that of the muscles, 0.37 S m–1, at an operating frequency of 140 kHz. As shown in Table 7, the value of ac is directly proportional to the conductivity in the simplified models. In addition, there is a positive correlation with the conductivity in the SAR. Table 7 Coupling factors according to the conductivity of the simplified models. Simplified model . Material . Conductivity (S m–1) . |${a}_c$| . |${a}_{c1}$| . |${a}_{c2}$| . Uniform body model Calculated 0.22 0.216 0.070 0.0023 2/3 of muscle 0.247 0.242 0.070 0.0024 Muscle 0.37 0.363 0.070 0.0030 Cuboid model Calculated 0.22 0.294 0.085 0.0035 2/3 of muscle 0.247 0.330 0.085 0.0037 Muscle 0.37 0.494 0.085 0.0045 Simplified model . Material . Conductivity (S m–1) . |${a}_c$| . |${a}_{c1}$| . |${a}_{c2}$| . Uniform body model Calculated 0.22 0.216 0.070 0.0023 2/3 of muscle 0.247 0.242 0.070 0.0024 Muscle 0.37 0.363 0.070 0.0030 Cuboid model Calculated 0.22 0.294 0.085 0.0035 2/3 of muscle 0.247 0.330 0.085 0.0037 Muscle 0.37 0.494 0.085 0.0045 Open in new tab Table 7 Coupling factors according to the conductivity of the simplified models. Simplified model . Material . Conductivity (S m–1) . |${a}_c$| . |${a}_{c1}$| . |${a}_{c2}$| . Uniform body model Calculated 0.22 0.216 0.070 0.0023 2/3 of muscle 0.247 0.242 0.070 0.0024 Muscle 0.37 0.363 0.070 0.0030 Cuboid model Calculated 0.22 0.294 0.085 0.0035 2/3 of muscle 0.247 0.330 0.085 0.0037 Muscle 0.37 0.494 0.085 0.0045 Simplified model . Material . Conductivity (S m–1) . |${a}_c$| . |${a}_{c1}$| . |${a}_{c2}$| . Uniform body model Calculated 0.22 0.216 0.070 0.0023 2/3 of muscle 0.247 0.242 0.070 0.0024 Muscle 0.37 0.363 0.070 0.0030 Cuboid model Calculated 0.22 0.294 0.085 0.0035 2/3 of muscle 0.247 0.330 0.085 0.0037 Muscle 0.37 0.494 0.085 0.0045 Open in new tab Moreover, owing to the complex structure of the anatomical models, the maximum points in the anatomical models appear only in an extremely small space, unlike with the simplified models, as shown in Figure 9. Figure 9 Open in new tabDownload slide Comparison between the basic restriction assessment results (Duke, Ella, uniform body and cuboid model): (a) current density, (b) internal electric field and (c) 10 g spatial peak SAR distribution. Figure 9 Open in new tabDownload slide Comparison between the basic restriction assessment results (Duke, Ella, uniform body and cuboid model): (a) current density, (b) internal electric field and (c) 10 g spatial peak SAR distribution. The results show that the electromagnetic exposure assessment of an identical WPT system can have different results depending on the human body models. The internal field strength is much higher in the anatomical model, and the induced current density and SAR differ depending on the conductivity of the simplified model. Further research on the material and correction factors is needed to compensate for the differences in the results between the anatomical and simplified models. CONCLUSION In this paper, exposure-assessment methods for testing the compliance of a WCS for drones are proposed and analyzed under various cases using several human body models and exposure scenarios. Because of an evaluation method for the WCS for drones has yet to be established, the compliance standards for EVs and portable devices can be applied to the WCS for drones. The exclusionary clause of low-power devices does not apply to a WCS for drones, which is a high-power device; thus, evaluations against the reference levels or basic restrictions should be carried out. To evaluate the incident fields against the reference levels of the WCS for drones, the maximum field strength is suggested rather than a three-point measurement method or the spatial average on a space occupied by a human body. In addition, various scenarios for the internal quantities in a human body were analyzed. The worst-exposure conditions are observed when the drones are positioned in front or in back of the human body at a center height, and both conditions should be considered for a careful exposure evaluation. To analyze the internal quantities of the human body, the evaluation results are also compared based on the human body models used in the evaluations. For compliance testing, several simplified models have been provided in the IEC standard(22), although an analysis can now be conducted using anatomical models owing to the development of powerful computational tools. Thus, we compared four different models, namely, two anatomical human body models and two simplified models, using coupling factors. Even when the system is placed at the same position with respect to the human body models, the coupling factor varies depending on the human body models. To extract the coupling factors, the maximum values are used, which are unpredictable with the current simplified model owing to the complicated organizational structure and physical shape of the anatomical human body models applied. Therefore, to use the simplified model, the maximum value appearing within the local region of the anatomical model should be considered. In the future, the material properties of the simplified models and the correction factor between a simplified model and an anatomical model will be investigated. ACKNOWLEDGEMENT This work was supported by Electronics and Telecommunications Research Institute (ETRI) grant funded by ICT R&D program of MSIT/IITP [2019-0-00102, A Study on Public Health and Safety in a Complex EMF Environment]. We would like to thank ZMT for providing free license of Sim4Life used in this study. REFERENCES 1. Choi , C. H. , Jang , H. J. , Lim , S. G. , Lim , H. C. , Cho , S. H. and Gaponov , I. Automatic wireless drone charging station creating essential environment for continuous drone operation international conference on control . Autom. Inform. Sci. (ICCAIS). ( 2016 ). OpenURL Placeholder Text WorldCat 2. Kim , S.-W. , Cho , I.-K. and Hong , S.-Y. Design of transmitting coil for wireless charging system to expand charging area for drone applications . Microw. Opt. Technol. Lett. 60 , 1179 – 1183 ( 2017 ). Google Scholar Crossref Search ADS WorldCat 3. Song , C. , Kim , H. , Kim , Y. , Kim , D. , Jeong , S. , Cho , Y. , Lee , S. , Ahn , S. and Kim , J. EMI reduction methods in wireless power transfer system for drone electrical charger using tightly coupled three-phase resonant magnetic field . IEEE Trans. Ind. Electron. 65 , 6839 – 6849 ( 2018 ). Google Scholar Crossref Search ADS WorldCat 4. International Commission on Non-Ionizing Radiation Protection (CNIRP) . Guidelines for limiting exposure to time-varying electric, magnetic, and electromagnetic fields (up to 300 GHz) . Health Phys. 74 , 494 – 521 ( 1998 ). PubMed OpenURL Placeholder Text WorldCat 5. International Commission on Non-Ionizing Radiation Protection (CNIRP) . Guidelines for limiting exposure to time-varying electric and magnetic fields (1 Hz to 100 kHz) . Health Phys. 99 , 818 – 836 ( 2010 ). PubMed OpenURL Placeholder Text WorldCat 6. IEEE C95.6-2002 . IEEE standard for safety levels with respect to human exposure to electromagnetic fields, 0-3 kHz . 7. IEEE C95.1-2005 . IEEE standard for safety levels with respect to human exposure to radio frequency electromagnetic fields, 3 kHz-300 GHz. 8. International Electrotechnical Commission (IEC). IEC TR 62905 . Exposure Assessment Methods for Wireless Power Transfer Systems ( 2018 ). 9. International Electrotechnical Commission (IEC) . IEC 61980-1:2015 . Electric vehicle wireless power transfer systems (WPT), part 1: General requirements . Federal Communications Commission (FCC) . 10. RF exposure considerations for low power consumer wireless power transfer applications 680106 D01 RF Exposure Wireless Charging Apps v02 DR02-41372. ( 2013 ). 11. Radio Standards Specification (RSS) . RSS-216. Wireless Power Transfer Devices . RSS-216, issue 2 , Canada ( 2016 ). 12. DJI http://www.dji.com/spreading-wings-s1000-plus ( 2014 ). 13. Kim , S.-W. , Cho , I.-K. and Hong , S.-Y. Comparison of charging region differences according to receiver structure in drone wireless charging system . International Conference on Information and Communication Technology Convergence (ICTC) ( 2017 ). 14. Sim4Life https://www.zmt.swiss ( 2018 ). 15. Laakso , I. , Tsuchida , S. , Hirata , A. and Kamimura , Y. Evaluation of SAR in a human body model due to wireless power transmission in the 10 MHz band . Phys. Med. Biol. 57 , 4991 – 5002 ( 2012 ). Google Scholar Crossref Search ADS PubMed WorldCat 16. Hirata , A. , Ito , F. and Laakso , I. Confirmation of quasi-static approximation in SAR evaluation for a wireless power transfer system . Phys. Med. Biol. 58 , N241 ( 2013 ). Google Scholar Crossref Search ADS PubMed WorldCat 17. Gosselin , M.-C. et al. Development of a new generation of high-resolution anatomical models for medical device evaluation: The virtual population 3.0 . Phys. Med. Biol. 59 , 5287 – 5303 ( 2014 ). Google Scholar Crossref Search ADS PubMed WorldCat 18. International Electrotechnical Commission (IEC). IEC 62311:2007 . Assessment of electronic and electrical equipment related to human exposure restrictions for electromagnetic fields (0 Hz – 300 GHz) . 19. Hasgall , P. A. , Di Gennaro , F. , Baumgartner , C. , Neufeld , E. , Lloyd , B. , Gosselin , M. C. , Payne , D. , Klingenböck , A. and Kuster , N. IT’IS Database for Thermal and Electromagnetic Parameters of Biological Tissues, v4.0 ed . ( 2018 ). 20. Gabriel , S. , Lau , R. W. and Gabriel , C. The dielectric properties of biological tissues: III. Parametric models for the dielectric spectrum of tissues . Phys. Med. Biol. 41 , 2271 – 2293 ( 1996 ). Google Scholar Crossref Search ADS PubMed WorldCat 21. Sunohara , T. , Hirata , A. , Laakso , I. , De Santis , V. and Onishi , T. Evaluation of nonuniform field exposures with coupling factors . Phys. Med. Biol. 60 , 8129 – 8140 ( 2015 ). Google Scholar Crossref Search ADS PubMed WorldCat 22. International Electrotechnical Commission (IEC) . IEC 62233:2005 . Measurement methods for electromagnetic fields of household appliances and similar apparatus with regard to human exposure . Author notes J. Ahn and S. Hong contributed equally. © The Author(s) 2020. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oup.com This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model)
TI - COMPLIANCE TESTING FOR HUMAN BODY MODEL EXPOSURE TO ELECTROMAGNETIC FIELDS FROM A HIGH-POWER WIRELESS CHARGING SYSTEM FOR DRONES
JF - Radiation Protection Dosimetry
DO - 10.1093/rpd/ncaa008
DA - 2020-07-07
UR - https://www.deepdyve.com/lp/oxford-university-press/compliance-testing-for-human-body-model-exposure-to-electromagnetic-YNRBkVzsjB
DP - DeepDyve
ER -