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An improved resonant wireless power transfer system with optimum coil configuration for capsule endoscopy
Accepted Manuscript Title: An improved resonant wireless power transfer system with optimum coil configuration for capsule endoscopy Author: Md. Rubel Basar Mohd Yazed Ahmad Jongman Cho Fatimah Ibrahim PII: DOI: Reference:
S0924-4247(16)30420-4 http://dx.doi.org/doi:10.1016/j.sna.2016.08.035 SNA 9813
To appear in:
Sensors and Actuators A
Received date: Revised date: Accepted date:
29-4-2016 24-8-2016 30-8-2016
Please cite this article as: Md.Rubel Basar, Mohd Yazed Ahmad, Jongman Cho, Fatimah Ibrahim, An improved resonant wireless power transfer system with optimum coil configuration for capsule endoscopy, Sensors and Actuators: A Physical http://dx.doi.org/10.1016/j.sna.2016.08.035 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
An improved resonant wireless power transfer system with optimum coil configuration for capsule endoscopy
Md. Rubel Basara,b, Mohd Yazed Ahmada,b,*, Jongman Chob,c, and Fatimah Ibrahima,b
a
Department of Biomedical Engineering, Faculty of Engineering, University of Malaya, Kuala
Lumpur 50603, Malaysia. b
Center for Innovation in Medical Engineering, Faculty of Engineering, University of Malaya,
Kuala Lumpur 50603, Malaysia. c
Department of Biomedical Engineering, Inje University, Gimhae 621-749, Korea.
Corresponding author: Phone: +603-79677695, Fax: +603-79674579, Email: [email protected], Department of Biomedical Engineering, Faculty of Engineering, University of Malaya, Kuala Lumpur 50603, Malaysia.
Highlights
We developed a mathematical model to enable accurate analysis of WPT system.
The proposed IPTC improves efficiency up to 36.6%, the existing is merely 14.26%.
The proposed IPTC attains 80% field uniformity even with coil diameter of 40 cm.
Current density is analyzed in details with incorporation of ferrite core.
Under stringent guidelines, this technique safely transfers up to 200 mW of power.
Abstract Resonant wireless power transfer is a prominent method of powering a biomedical capsule endoscope, in which the high efficiency of an inductive power link and the electromagnetic safety of biological tissues are greatly important. This paper presents a complete analytical model of a resonant wireless power transfer link, where the efficiency is remarkably increased with the improved power transmission coil configuration. This configuration involves two outer coils and one middle coil. The separation and turn ratios between the outer coils and the middle coil are 0.4 times of the coil radius and 3:2, respectively. An analysis of and comparison with the existing coil configuration show that the proposed configuration relatively attains higher Qfactor, coupling coefficient, and H-field uniformity, which lead to improvement in power transfer efficiency, stability, and electromagnetic safety. The analysis is validated through experiments. With a power receiving coil of ∅11 mm × 8 mm, the proposed power transfer coil attains a minimum efficiency of around 36.6% at the worst position of the power-receiving coil. The attained efficiency was 156% higher than the efficiency of 14.62% obtained by the best of existing coil at the same level of H-field uniformity. Incorporating high permeability core, up to 200 mW of power is transferred with the proposed coil at 300 kHz in accordance with the electromagnetic safety guideline.
Keywords: Capsule endoscopy, inductive coupling, magnetic field uniformity, magnetic resonance, wireless power transfer.
1. Introduction Wireless capsule endoscopy plays a promising role in the early detection of various gastrointestinal (GI) tract diseases, such as gastric cancer, bleeding, and tumors [1, 2]. This technique is noninvasive and painless to the patient in which a tiny capsule can travel even through the complex parts of the GI tract such as the small bowel where typical endoscopes fail to reach [2, 3]. However, powering such capsule is one of its important challenges. Battery usage is often not a good option because of its limited energy storage capacity. Battery recharging or replacement is also impractical in this application [4, 5]. Therefore, a magnetic resonance-based inductively coupled wireless power transfer (WPT) system is a promising solution. This system is typically used to power implantable biomedical devices within a distance of few or tens of millimeters [6, 7]. In this technique, power is transferred from a power transmission coil (PTC) set outside of the human body to a miniaturized power receiving coil (PRC) inside the human body through a linkage of the magnetic field [8]. With perfect tuning, the power transfer efficiency of such system mainly depends on the coupling coefficient (k) between the coils, the coil quality factor (Q), and the load impedance matching with the internal resistance of the PRC [9]. Considering the electromagnetic exposure of the human body tissues, the efficiency of such system is an important concern. Unfortunately, the efficiency of coupling and resultant power transfer is poor in such systems because of the tiny size of the PRC and the large distance between the PTC and the PRC. For example, Jourand et al. [10] developed a WPT system with a PRC of 9 mm diameter. The system was small enough to be embedded in the existing capsule size. The PTC was 75 cm in diameter and can cover the whole abdominal region. The power transfer efficiency attained by this system was as low as
at the 37.5 cm transmission distance. In the last few years, significant
research efforts have been dedicated to the development of an efficient WPT system for capsule endoscopy [8-19]. However, the power transfer efficiency of the existing designs remains low (0.02% to 3.55%) [9, 19, 20]. Recently, Na et al. [19] proposed four coils based resonant WPT system, in which the optimal efficiency tracking theory attained the overall power transfer efficiency of 0.02% at a 7 cm distance. Shi et al. [20] also proposed a portable WPT system for a video capsule endoscope, in which three different PTC configurations were discussed. The optimal configuration achieved a power transfer efficiency of 2.8% at the 20 cm transmission
distance. Liu et al. [21] designed a WPT system for a capsule endoscope. Their design incorporated a new type of PRC, which improved the received power stability. Accordingly, the power transfer efficiency reached 3.51%. Ke et al. [9] developed an analytical model to calculate the quality factor of the PTC. This design attained a power transfer efficiency of 3.55% with an enlarged PTC of 69 cm in diameter. Some other studies on the wireless powered capsule endoscopy are presented in [22, 23]. However, the important performance indices of the WPT system are not discussed in detail. Furthermore, a complete modeling of the WPT system remains limited. The important performance indices, such as Q-factor, coupling coefficient, Hfield uniformity, and efficiency, have not been thoroughly analyzed in the existing design. Moreover, the electromagnetic effect on the biological tissues has not been investigated by incorporating the PRC with a high-permeability ferrite core. On the basis of the summary of existing design limitations, this paper presents a WPT system for the capsule endoscope with a new PTC configuration to achieve higher coupling coefficient, quality factor, and magnetic field uniformity and thus improve the power transfer efficiency, received power stability, and electromagnetic safety. We also provide a complete mathematical model in detail to assess the performance of the proposed PTC and the WPT system. Given the provided model, we analyze the performance of the proposed PTC and compare it with that of the existing PTCs. Finally, we validate the analysis through a rigorous experiment and perform an electromagnetic safety test incorporating a high-permeability ferrite core in the PRC. 2. System overview Fig. 1 shows a schematic of the magnetic resonance-based inductively coupled WPT system for capsule endoscopy. A PTC can be made wearable, and a PRC with a power conversion circuit (rectifier and regulator) can be embedded within the capsule. The PTC is excited by a time-varying current ITC (generated by the power amplifier) to produce a time-varying H-field. A part of the generated H-field interacts with the PRC and induces voltage. Hence, power is being transferred to the load by the linkage of the H-field. In this setup, the power level at the load can be adjusted by exciting the PTC with a higher or lower power. However, transferring a high amount of power with low efficiency does not only waste power but also increases the effect of the reactive electromagnetic field on the nearby tissues. This electromagnetic effect to the nearby body tissues increases with the non-uniformity of H-field generated by the PTC. The H-field is
generally not perfectly uniform in this type of non-radiating (near-field) PTC, whereas the field is much stronger at the positions near the winding of the PTC and weaker in the position a bit away from the PTC winding [16]. This field characteristic results in a very high power level and link efficiency in the strong field regions (near the winding) but a low power level and link efficiency in the weak-field regions (away from the winding) [16, 19]. Consequently, the power received by the PRC becomes unstable when it moves within the non-uniform fields. The received power at the PRC in the weak-field region might be insufficient to power up onboard circuitries. The PTC can be excited with a higher power to boost up the received power level in the weak-field region. However, a high excitation power at the PTC abruptly increases the intensity of the reactive field near the PTC over which the patient body tissues is exposed. Thus, the excitation power at the PTC should be within a limit satisfying the guideline provided by the International Commission on Non-Ionizing Radiation Protection (ICNIRP) and the Standards Coordinating Committee 28 (SCC28) of the IEEE. Additionally, a regulator circuit is required to trim excessive power across the load when the received power is higher than the desired voltage. However, the regulator’s input power (received voltage) must not be very high because excessive power may cause regulator breakdown.
2.1 Modeling of the WPT link efficiency The resonant inductive link can be simplified to a circuit model (Fig. 2) for analysis. In this model, LT and LR denote the effective self-inductance, RT and RR denote the effective series resistance of PTC and PRC, respectively. CT and CR are the tuning capacitors used to resonate both coils. M represents the mutual inductance between the coils. According to circuit theory, the relationship among the applied voltage to the PTC, Vs (t), and current through the coils (IT and IR) can be represented by the following matrix form:
[
]
[
][
]
(1)
At the resonance, the current IT(t) and IR(t) can be calculated using Eq. (1):
I T (t )
( RR RL )VS (t ) , and MVS (t ) I R (t ) 2 2 RT ( RR RL ) M RT ( RR RL ) 2 M 2
(2)
Therefore, the power transfer efficiency can be expressed as follows:
Pout I 2 (t ) RL 2M 2 RL R Pin I T (t )Vs (t ) RT ( RR RL ) 2 M 2 RL RR
(3)
For M k LT LR , k is the coupling coefficient between the coils, and η can be expressed as
k 2QT QR RR RL * 2 RL RR (1 k QT QR ) RR RL
,
(4)
where QT LT / RT and QR LR / RR are the unloaded quality factors of PTC and PRC, respectively. With the condition of maximum power transfer, RL=RR, η can be simplified as
k 2QT QR . 4(1 k 2QT QR / 2)
(5)
2.2 Modeling of the coupling coefficient The coupling coefficient between PTC and PRC can be denoted as
(6)
√
As shown in Fig. 3a, the mutual inductance for a PRC position at (y, z) for a loosely coupled system can be expressed as
(7)
where
is the magnetic flux density within the area enclosed by the PRC (SR), and
effective magnetic permeability of the core materials used in the PRC.
is the
According to Biot–Savart’s Law, the magnetic field generated by the PTC can be calculated at point (y, z) as follows: H z ( y, z ) HYz ( y, z ) ˆj H Zz ( y, z )kˆ
(8)
where 2 IT rT n zSin( ) d H Yz ( y, z ) 3 0 2 2 2 4 2 y z r 2 yrCos( ) r yCos ( ) 2 d 3 IT rT n 2 2 2 H Zz ( y, z ) 4 y z r 2 yrCos( ) 2 0
Therefore, from (6), (7), and (8), the new expression of k can be given as
k ( y, z )
S Cos( ) 2 r yCos ( ) zSin( ) 0 eff R d 3 0 2 2 2 4 LT LR 2 y z r 2 yrCos( )
(9)
2.3 Modeling of the quality factor The equivalent circuit of the PTC is presented in Fig. 3b on the basis of the LCR meter measurement. In this model, the PTC comprises i) self-inductance (L), ii) series AC resistance (Rac) (caused by the skin and proximity effect), and iii) parallel stray capacitance (C). Given that C is very small (in few pico-farad) and Li RaciCi
, the simplified coil impedance can be
written as follows:
Z
Rac L j (1 2 LC) 2 1 2 LC
(10)
The coil model can be simplified with an effective series resistance RES and effective selfinductance Leff based on this expression (Fig. 3c). In that case, RES and Leff can be provided as
RES
L Rac and Leff 2 2 1 2 LC (1 LC )
(11)
Therefore, the quality factor of the coil can be expressed as follows:
QT
L Rac
(1 2 LC )
(12)
By using (5), (9), and (12), the performance of the PTC and the WPT link can be analyzed with the variation of the design parameters. The expression of L, Rac, and C for different coil configurations can be adopted from [7, 24] for this purpose.
3. PTC Configuration We propose a new PTC configuration to improve the H-field uniformity, coupling coefficient, and Q-factor. The performance of the proposed PTC is analyzed using the above mentioned model and compared with that of the three existing PTCs. The configurations of the existing and proposed PTCs are illustrated in the following subsections. 3.1 Configuration of the existing PTCs Three different PTC configurations are found to be used in the existing WPT system for capsule endoscopy. Fig. 4 depicts these PTC configurations. To simplify the analysis, these PTCs are named as i) typical solenoid coil (TSC in Fig. 4a) [11]; ii) typical Helmholtz coil (THC in Fig. 4b) [18, 25]; and iii) segmented solenoid coil (SSC in Fig. 4c) [26]. These PTC configurations mainly varied because of the different coil turn arrangements. The TSC is a loosely wound solenoid coil with n turn and overall height h, where the distance between the contagious turns is h/(n − 1). The THC is configured with two tightly wound solenoid coils connected in series. These coils are separated on a common axis by the distance of their radius (r). The SSC is a number of segments of a tightly wound solenoid coil. Each of the segments is connected to an advanced switching system that can activate and deactivate any particular segment depending on the PRC location. The quality factor and coupling characteristics are two important performance indices used to select any particular PTC configuration. Nonetheless, in the present study, the selection is mainly led by the index of the H-field intensity and uniformity. As such, the TSC and SSC configurations are preferred in the existing studies to increase the
magnetic field intensity, whereas the THC is commonly chosen to enhance the magnetic field uniformity. 3.2 Proposed PTC Among the existing PTC configurations, the generated magnetic field is weak in the THC, whereas the uniformity is poor in the TSC and SSC. In addition, the uniformity of the field in the THC significantly decreased along with the radial (y-axial) distance beyond r/2 (y>r/2). Therefore, we have proposed an improved power transmission coil (IPTC) to improve both Hfield uniformity and intensity at a time (within a large region inside the coil: z≤±h/2 and y≤±3r/4). Fig. 5 shows the IPTC design. In this design, two outer and one middle coils are arranged so that they have the same radius (r). These coils are aligned in parallel. Their centers are fixed at a common axis (Fig. 5a). The separation and turn ratio between the outer coils and the middle coil are optimized considering optimum values of H-field intensity, H-field uniformity and coupling coefficient. These parameters are found to be optimum when the separation between the outer and the middle coils is spaced at 0.4r and their turn ratio is 3:2. In this design, the overall IPTC coil height is 0.8r. The coil height can be enlarged further along the axial distance with the inclusion of additional middle coils to enlarge the area of the uniform Hfield (Fig. 5b).
4. Performance analysis The key performance indices, including the PTC configurations (i.e., H-field intensity and uniformity), coupling characteristics, and Q-factor, are analyzed and compared in this section. 4.1 H-field intensity and uniformity The field distribution in different PTCs is computed using the COMSOL Multiphysics computer simulation software (version 5.1, provided by COMSOL, Inc.) to access the H-field intensity and uniformity. This software works based on partial differential equations. The simulation is initially verified with the H-field intensity calculated using Eq. (8) and adopted from the Biot– Savart’s Law. An identical PTC radius of 20 cm and an excitation RF power of 5 W (at 300 kHz) are used in this simulation. All the PTCs are considered co-centric with the Cartesian coordinate system. The number of turns in each of the PTC is adjusted for an equal effective coil inductance LT to minimize the effect of the tuning capacitor and the transmitting circuit.
Fig. 6 shows the color maps of the simulated H-field distribution in the PTCs. Within the region y≤±3r/4 and z≤±h/2, the maximum and minimum field levels are noted with the coordinates. As shown in Fig. 6, the peak intensity of the field in the TSC and SSC is relatively higher. However, the field intensity abruptly increases with the radial (y-axial) distance and decreases with the axial (z-axial) distance. Therefore, the field intensity varies within a wide range resulting in poor uniformity. In the THC, the field intensity is relatively lower, and the uniformity is quite high within a limited region around the center (y≤±r/2 and z≤±h/4). Beyond this region, the field intensity significantly drops with the radial distance and sharply increases toward the coil winding. Therefore, the uniformity within the large region is poor even in the THC. These problems are overcome with the proposed IPTC. The proposed configuration of our IPTC, indicated in Section 3.2, compensates for the field reduction along the radial distance thus overcomes the excessive increase of the field toward the outer coil winding. Therefore, the variation of the maximum and minimum field intensities is minimized. Consequently, the proposed IPTC attains the three following advantages over the existing PTCs: i) high field intensity, ii) high uniformity, and iii) relatively low peak electromagnetic exposure.
4.2 Coupling characteristics We analyze the coupling characteristics of the existing and proposed PTC configurations by using Eq. (9). A PRC of 5.5 mm radius with high-permeability ferrite core is used in this analysis. The radius of 5.5 mm is chosen to optimally utilize the available space within the existing capsule. The used PRC has the enclosed area SR of 9.5 * 10−5 m2, the inductance LR of 426 µH, and the effective permeability µeff of 210. The PRC is considered to be always perfectly aligned with the PTC. Based on this analysis, the variation of the coupling coefficient caused by the free movement of the PRC within the large region of the PTC (y≤±3r/4 and z≤±h/2) is compared (Fig. 7). The comparison shows that the minimum level of coupling is highest in the IPTC. However, the coupling coefficient attainted by the TSC is close to that of the IPTC around the center region (Fig. 7a). The coupling coefficient of the TSC significantly decreases around the radial line along z = h/2. The coupling coefficient attained by the IPTC along this line is significantly higher than any other PTCs (Fig. 7b). The coupling coefficient attained by THC within the considered large region is always the lowest. This coefficient further reduces along the
radial distance of z = 0. The coupling coefficient of the SSC is moderate and varies within a wide range.
4.3 Quality factor The quality factor of the PTCs (QT) is analyzed using the model given in Eq. (12). In this analysis, the number of turns in the different PTCs has been adjusted for almost the equal value of coil inductance LT ~460 µH. For this coil inductance, the number of turns obtained from the calculation are 30, 26, 22, and 32 for the TSC, THC, SSC, and IPTC, respectively. The variations of the unloaded QT over the frequency are shown in Fig. 8. According to Fig. 8, within the frequency range of 100 kHz - 700 kHz, the maximum QT attained by the IPTC is remarkably higher than that of attained by other PTCs. The maximum QT attained by the IPTC is 533 at 300 kHz. Whereas, the maximum QT obtained by TSC, SSC and THC are 478, 448 and 445 at 262 kHz, 172 kHz and 208 kHz respectively. The maximum QT and the corresponding frequency for a given coil diameter and number of turns depend on Rac, L and C, which vary with a gap between the contagious turns of the coil winding. The tightly winded solenoid coil in the THC and the SSC presents a relatively higher Rac and C, resulting in the lower maximum QT at a lower frequency. The loosely winded solenoid coil in the TSC has Rac, and C, which is relatively lower than that of the THC and the SSC. Therefore, the TSC demonstrates a relatively higher QT at a higher frequency. However, the excessive gap between the turns reduces L by reducing the mutual inductance among the turns. Consequently, QT reduces with an excessive gap between the turns. Therefore, an optimum gap equal to the radius of the wire is used in the IPTC to obtain the maximum QT.
4.4 Variation of the link efficiency and field uniformity with PTC diameter The field uniformity and the link efficiency in the WPT system for capsule endoscopy are expected to be high to minimize the instability of the received power and the electromagnetic exposure effect on the patient body tissues. Unfortunately, the link efficiency and the field uniformity are opposite functions of the PTC diameter. In the existing studies, a PTC diameter has been chosen in between 40 and 75 cm with the medical viewpoint or seeking of a uniform field [10, 27, 28].
The correlation of the link efficiency and the field uniformity with the PTC diameter is illustrated in Fig. 9. The link efficiency is calculated using Eq. (5), whereas the field uniformity is calculated as follows:
H (0,0) H ( y, z ) *100% Uniformity Min1 H (0,0)
(13)
where H(0,0) and H(y,z) are the magnetic field at the center and coordinate (y,z), respectively. As shown in Fig. 9, the WPT link efficiency decreases almost exponentially with the increase of the PTC diameter. The field uniformity increases with the PTC diameter. This increment is abrupt within a certain range of the PTC diameter and then steadies. Therefore, to attain at least 80% field uniformity within the y≤±15 cm and z≤±h/2 regions, possible minimum PTC diameter is 40 cm for the IPTC, 45 cm for the THC, 50 cm for the TSC, and 65 cm for the SSC. Thus ensuring 80% field uniformity with the given coil diameter, the maximum achievable link efficiency can be 36.05% by the IPTC, 9.6% by the THC, 14.26% by the TSC, and 3.8% by the SSC as it is highlighted in the Fig. 9. The THC can attain the highest field uniformity with a larger diameter but may result in the sacrifice of a significant amount of link efficiency. 5. Implementation and experimental validation Two of the PTCs (i.e., SSC and IPTC) are implemented to verify the abovementioned analysis. A twisted round Litz wire with 270 strands of AWG 42 is used to construct the PTC on a lightweight, non-metallic coil frame. The frame is constructed with a diameter of 40 cm and is made of polymethyl methacrylate and polycarbonate sheets. A 1D PRC is implemented to measure the coupling coefficient and link efficiency. The PRC is implemented on a rounded cubic core with 8 mm side length. A ferrite core with high permeability (Feroxcube 3E6 material) is used in the PRC. The details and specifications of the implemented PTC and PRC are provided in Tables I and II, respectively.
5.1 Measurement of k and QT For the implemented PTCs, k and Q are measured with the help of an LCR meter (GW Instek LCR 8101G). The measurement setup is shown in Fig. 10. With this setup, the effective selfinductance of the PTC and the PRC (i.e., LT and LR, respectively) is measured using the LCR meter. The mutual inductance between the PTC and the PRC is measured by varying the PRC coordinates within the PTC. The measured mutual inductance and effective self-inductance of the coils are then converted to k by using Eq. (5). In addition, the Q-factor of the PTCs is directly measured using the LCR meter. The measurement is conducted on a table 1 m above the floor and away from any metallic or conductive object to minimize proximity effect. The measured k and Q are then compared with the calculated values in Figs. 11 and 12, respectively. As shown in Figs. 11 and 12, the measured parameters are in good agreement with the calculated values. However, the measured Q slightly differs from the calculated value because of the manual winding of the PTC.
5.2 Measurement of the link efficiency Fig. 13 shows the experimental setup for measuring the inductive link efficiency. In this experiment, both of the coils are resonated at 300 kHz with the tuning capacitor connected in the series. The PTC is driven by a signal generator. An oscilloscope is used to measure the output voltage across the load (VRL) and the input current through the PTC (IT) by measuring the voltage across a 1 Ω test resistor connected in series with the PTC. The measured power transfer link efficiency (varying the coordinate of the PRC within the PTC) is then calculated from the measured VRL(y, z) and IT as follows:
( y, z )
VRL2 ( y, z ) / RL I T2 RT
(14)
Fig. 14 shows the analytical and measured power transfer efficiency and its variation with the position of the PRC. In comparison with the measured result, the analytical result shows good acceptability of the given efficiency model. In addition, the proposed IPTC configuration
attained the power transfer efficiency significantly higher than that of the existing PTC configuration. The minimum efficiency attained by the IPTC is around 36.5%, whereas that by the SSC is nearly 24%.
6.
Effect of WPT system on the biological tissues The strong reactive magnetic field in the WPT system may exert some potential effects on the
living body tissues. The effects include i) stimulus action caused by the induced current within the tissues and ii) rising temperature caused by the absorption of the electromagnetic energy by the tissues. These parameters are often indexed by the current density (J, A/m−2) and specific absorption rate (SAR, W/kg) [5, 14]. The SAR is not influential at a frequency lower than 10 MHz [14]. Therefore, we use J as the safety analysis index. We obtain J by computer simulation based on partial differential equations. Fig. 15a depicts the simulation setup. In this simulation, a body model 30 cm in diameter is used. This model is built with the materials of multi-layer homogenous body tissues, including the skin, muscles, bones, and intestines, which are the main parts of the abdominal region. The electrical properties of these tissues (i.e., conductivity (σ) and permittivity (ε)) are shown in Fig. 15b for the frequency of 300 kHz which is obtained from [29]. The simulation setup with the IPTC and the distribution of the induced J for the coil excitation current (IT) of 0.5 A are shown in Fig. 15b. The distributed J is relatively higher around the capsule when it is located within the intestine with a relatively higher σ than the skin, muscles, and bones. Moreover, J increases with load reduction (RL), which is kept at 20 Ω in this simulation. The current in the PRC increases with the reducing RL because it is related to I R (t ) MIT (t ) /( RR RL ) . Considering this effect, a limit must be imposed on the allowable IT in the PTCs and the maximum power that can be transferred following the ICNIRP guideline [30] on J. The maximum allowable IT, at which J exceeds the safety limit, and the corresponding output power of different PTC configurations are shown in Fig. 16. Both the IPTC and the THC present a similar amount of maximum Pout for a common PRC location at the center of the PTC. Nevertheless, Pout reduces in the THC for the PRC location in the weak-field region (e.g., y = 0.75 r and z = 0).
7. Effect of biological tissues on WPT system In this application, the PTC and the PRC are working in close proximity of the human body tissues, therefore, an investigation on the effects of biological tissues on the performance of the WPT system is provided in this subsection. In order to investigate this effect, we have used the biological tissues from a slaughtered goat (Capra aegagrus hircus) torso of abdominal region including bone, muscle, fat and intestine. The PRC covered with polyethylene film of 1 mm thickness is placed inside the intestine in abdominal region as indicated in Fig. 17a. Then the efficiency of the WPT system is measured by placing the PRC surrounded with the biological tissues inside the PTC. The PRC is set to be at the center of the PTC with its correctional area perpendicular to the direction of the generated H-field. The measurement setup is shown in the Fig. 17b. The power transfer efficiency obtained from this measurement is 33.18% whereas without the biological tissues (as the measurement setup shown in Fig. 13) the achieved efficiency is 36.60%. The result shows the presence of the biological tissues slightly reduce the power transfer efficiency without much affecting the performance of WPT system. Our finding complies with other similar studies in this area as reported in [16, 19, 20] which indicate slight reduction when the WPT system operating in biological tissue environments at this rage of frequency. 8. Conclusion A complete model of a resonant inductive wireless power transfer system with an improved power transfer coil has been presented for capsule endoscopy. The performances of the improved power transfer coil and the inductive power link have been analyzed in detail by using the provided mathematical model. In comparison with the existing power transfer coil, the proposed coil significantly improves the Q-factor, which is the coupling coefficient increasing the power transfer efficiency. In addition, the improved coil attains a high H-field uniformity even with a smaller diameter. With a power receiving coil of ∅11 mm × 8 mm, the minimum power transfer efficiency attained by the improved coil is 36.05%. The maximum efficiency attained by the existing coils is 14.26% while maintaining a similar H-field uniformity level. An electromagnetic safety analysis is performed with a multi-layer homogenous body model and an incorporation of the high-permeability ferrite core receiving coil. The induced current density is higher around the high-permeability ferrite core than the other regions. A power of 200 mW can be transferred in accordance with the electromagnetic safety guideline.
Acknowledgements This research is financially supported by University of Malaya Research Grant (UMRG: RP009D-13AET and RP022B-14AFR).
Md. Rubel Basar
Md. Rubel Basar received the BSE degree in Electronics and Communication Engineering from Khulna University of Engineering and Technology, Bangladesh and the M.Sc. degree in Communication Engineering from University Malaysia Perlis, Malaysia in 2010 and 2013, respectively. From 2010 to 2012, he was a RF optimization Engineer of GSM network with 3S Network (BD) Ltd., Bangladesh. Currently he is pursuing his Ph.D degree in the Department of Biomedical Engineering, Faculty of Engineering, University of Malaya, Malaysia. His research interests include wireless power transfer system, antenna and RF circuits.
Mohd Yazed Ahmad
Mohd Yazed Ahmad received the B.E. degree from the Department of Electrical Engineering, University of Malaya, Kuala Lumpur, Malaysia, in 2003 and the M.S. degree from the Department of Biomdical Engineering, Faculty of Engineering, University of Malaya, in 2006 and the Ph.D degree from the Center for Health Technologies, School of Electrical, Mechanical and Mechatronic Systems, Faculty of Engineering and Information Technology, University of Technology, Sydney, Australia in 2013. Currently, he is a Senior Lecturer in the Department of Biomedical Engineering, Faculty of Engineering, University of Malaya. His current research interests include localization, positioning, wireless sensors, data fusion, optimization, instrumentation systems using RF, and embedded systems.
Jongman Cho
Jongman Cho is an Associate Professor in the Department of Biomedical Engineering at Inje University, Korea. He received B.E., M.E., and Ph.D degrees at the Department of Electronic Engineering, Inha University, Korea in 1985, 1989, and 1994 respectively. From 1996 to 1998 he was a visiting scholar in the Department of Biomedical Engineering at Rutgers University, New Jersey, U.S.A. His research interests include the application of artificial neural networks, biomedical signal processing, and image processing. Fatimah Ibrahim
Fatimah Ibrahim obtained her Bachelor of Science in Electrical Engineering degree from Marquette University, Wisconsin, USA in 1989, Master of Science in Electronics (Medical Systems) degree from the University of Hertfords-hire, UK, in 1994 and PhD degree in Biomedical Engineering from University of Malaya (UM), Malaysia in 2005 joint funding between UM and Yayasan Sultan Iskandar Johor. She was a Senior lecturer in Universiti Teknologi MARA, from 1990 to 1999. In 1999, she joined the University of Malaya and involved in setting up the pioneer, Department of Biomedical Engineering in Malaysia. She is currently the head of Centre for Innovation in Medical Engineering(CIME) in Faculty of Engineering, University of Malaya. Her research interests are in the detection and monitoring of diseases, physiological modeling and measurement, BIOMEMS, Nanotechnology based biosensor, and artificial intelligence application in biomedicine.
References [1] A. Moglia, A. Menciassi, P. Dario, A. Cuschieri, Capsule endoscopy: progress update and challenges ahead, Nature reviews Gastroenterology & hepatology, 6(2009) 353-62. [2] M.R. Basar, F. Malek, K.M. Juni, M.S. Idris, M.I.M. Saleh, Ingestible Wireless Capsule Technology: A Review of Development and Future Indication, International Journal of Antennas and Propagation, 2012(2012) 1-14. [3] G. Ciuti, A. Menciassi, P. Dario, Capsule Endoscopy: From Current Achievements to Open Challenges, Biomedical Engineering, IEEE Reviews in, 4(2011) 59-72. [4] R. Carta, J. Thoné, R. Puers, A wireless power supply system for robotic capsular endoscopes, Sensors and Actuators A: Physical, 162(2010) 177-83. [5] M.R. Basar, M.Y. Ahmad, J. Cho, F. Ibrahim, Application of Wireless Power Transmission Systems in Wireless Capsule Endoscopy: An Overview, Sensors-Basel, 14(2014) 10929-51. [6] R.F. Xue, K.W. Cheng, M. Je, High-efficiency wireless power transfer for biomedical implants by optimal resonant load transformation, IEEE Transactions on Circuits and Systems I: Regular Papers, 60(2013) 867-74. [7] A.K. RamRakhyani, S. Mirabbasi, M. Chiao, Design and Optimization of Resonance-Based Efficient Wireless Power Delivery Systems for Biomedical Implants, IEEE Trans Biomed Circuits Systs, 5(2011) 48-63. [8] T.J. Sun, X. Xie, G.L. Li, Y.K. Gu, Y.D. Deng, Z.H. Wang, A Two-Hop Wireless Power Transfer System With an Efficiency-Enhanced Power Receiver for Motion-Free Capsule Endoscopy Inspection, IEEE Trans Biomed Eng, 59(2012) 3247-54. [9] Q. Ke, W. Luo, G. Yan, and K. Yang, "Analytical Model and Optimized Design of Power Transmitting Coil for Inductively Coupled Endoscope Robot," IEEE Trans Biomed Eng, 63(2016) 694706. [10] P. Jourand, R. Puers, A Class-E driven inductive power delivery system covering the complete upper body, Sensors and Actuators A: Physical, 183(2012) 132-9. [11] B. Lenaerts, R. Puers, Inductive powering of a freely moving system, Sensors and Actuators A: Physical, 123–124(2005) 522-30. [12] M. Guanying, Y. Guozheng, H. Xiu, Power transmission for gastrointestinal microsystems using inductive coupling, Physiological measurement, 28(2007) 9-18. [13] B. Lenaerts, R. Puers, An inductive power link for a wireless endoscope, Biosensors & bioelectronics, 22(2007) 1390-5. [14] K. Shiba, T. Nagato, T. Tsuji, K. Koshiji, Energy Transmission Transformer for a Wireless Capsule Endoscope: Analysis of Specific Absorption Rate and Current Density in Biological Tissue, IEEE Trans Biomed Eng, 55(2008) 1864-71. [15] R. Carta, M. Sfakiotakis, N. Pateromichelakis, J. Thoné, D.P. Tsakiris, R. Puers, A multi-coil inductive powering system for an endoscopic capsule with vibratory actuation, Sensors and Actuators A: Physical, 172(2011) 253-8. [16] G.B. Pan, W.H. Xin, G.Z. Yan, J.L. Chen, A video wireless capsule endoscopy system powered wirelessly: design, analysis and experiment, Meas Sci Technol, 22(2011). [17] Z. Jia, G. Yan, Y. Shi, B. Zhu, A wireless power transmission system for an active capsule endoscope for colon inspection, Journal of medical engineering & technology, 36(2012) 235-41. [18] W.H. Xin, G.Z. Yan, W.X. Wang, A stable wireless energy transmission system for gastrointestinal microsystems, Journal of medical engineering & technology, 34(2010) 64-70.
[19] K. Na, H. Jang, H. Ma, F. Bien, Tracking Optimal Efficiency of Magnetic Resonance Wireless Power Transfer System for Biomedical Capsule Endoscopy, IEEE Trans Microw Theory Tech, 63(2015) 295-304. [20] Y. Shi, G. Yan, B. Zhu, G. Liu, F. Liu, D.-H. Lee, et al., A portable wireless power transmission system for video capsule endoscopes, Bio-Medical Materials and Engineering, 26(2015) S1721-S30. [21] G. Liu, G. Yan, W. Xu, S. Kuang, Dual-head wireless powered video capsule based on new type of receiving coils, Journal of medical engineering & technology, 39(2015) 246-52. [22] S. He, G.-Z. Yan, Q. Ke, Z.-W. Wang, W.-W. Chen, A wirelessly powered expanding-extending robotic capsule endoscope for human intestine, International Journal of Precision Engineering and Manufacturing, 16(2015) 1075-84. [23] L. Yan, T. Wang, D. Liu, J. Peng, Z. Jiao, C.-Y. Chen, Capsule Robot for Obesity Treatment With Wireless Powering and Communication, IEEE Trans Ind Electron, 62(2015) 1125-33. [24] Z. Yang, W. Liu, E. Basham, Inductor modeling in wireless links for implantable electronics, Magnetics, IEEE Transactions on, 43(2007) 3851-60. [25] M. Basar, M.Y. Ahmad, J. Cho, F. Ibrahim, A wireless power transmission system for robotic capsule endoscopy: Design and optimization, RF and Wireless Technologies for Biomedical and Healthcare Applications (IMWS-Bio), 2014 IEEE MTT-S International Microwave Workshop Series on, IEEE2014, pp. 1-3. [26] H. Yadong, W. Jianfeng, S. Tianjia, X. Xiang, L. Guolin, G. Yingke, et al., An efficiency-enhanced wireless power transfer system with segmented transmitting coils for endoscopic capsule, Circuits and Systems (ISCAS), 2013 IEEE International Symposium on2013, pp. 2279-82. [27] M. Guanying, Y. Guozheng, H. Xiu, Power transmission for gastrointestinal microsystems using inductive coupling, Physiological measurement, 28(2007) 9-18. [28] Q. Ke, W. Luo, G. Yan, K. Yang, Analytical Model and Optimized Design of Power Transmitting Coil for Inductively Coupled Endoscope Robot, Ieee T Bio-Med Eng, 63(2016) 694-706. [29] S. Gabriel, R.W. Lau, C. Gabriel, The dielectric properties of biological tissues: III. Parametric models for the dielectric spectrum of tissues, Physics in Medicine and Biology, 41(1996) 2271-93. [30] I.C.o.N.-I.R. Protection, Guidelines for limiting exposure to time-varying electric and magnetic fields (1 Hz to 100 kHz), Health physics, 99(2010) 818-36.
List of Figure
Fig. 1 Schematic of a WPT system with a PTC fixed around the patient body and a WCE consists of a PRC and a receiver circuit located in the human GI tract.
Fig. 2 Simplified circuit model of the resonant inductively coupled wireless power transfer system.
(b)
(a)
(c)
Fig. 3 (a) Coupling model between PTC-PRC, (b) lumped equivalent circuit of a PTC, and (c) effective lumped equivalent circuit of a PTC.
Fig. 4 Schematic configuration of the existing PTCs: (a) typical solenoid coil (TSC); (b) typical Helmholtz coil (THC); and c) segmented solenoid coil (SSC), the number of segments is denoted by “nos”.
(a)
(b)
Fig. 5 Schematic of the proposed improved power transmission coil (IPTC): (a) basic structure and (b) enlargement of the axial region with additional middle coil. The outer and middle coils are represented by “OC” and “MC,” respectively.
Fig. 6 Color map of the H-field distribution in: a) TSC, b) THC, c) SSC, and d) IPTC. The field is shown on the yz plane at x = 0 for an identical coil radius of 20 cm and inductance of around 460 µH, each of the coil are excited with AC power of 5 W at 300 kHz.
coupling cofficient ( k)
18 x10 16 14
-3
14 x10 13
TSC THC
12 11
12
SSC IPTC
10
10
9
8 0.00
-3
8 0.25
0.50
y/r @ z=0
(a)
0.75
0.00
0.25
0.50
0.75
y/r @ z=h/2
(b)
Fig. 7 Variation of the coupling coefficient in different PTCs along the radial direction: a) at z = 0 and b) at z = h/2. The coupling coefficient is calculated for µeff = 210, SR =
m2, LT ~460 µH, and LR = 426 µH.
Fig. 8 Unloaded quality factor of the PTCs over the frequency obtained for an identical coil radius of 20 cm and inductance of around ~460 µH.
Fig. 9 Variation of the minimum H-field uniformity and power transfer efficiency with the diameter of the PTCs. The maximum efficiency obtained by the different PTCs with ensuring 80% uniformity is indicated.
Fig. 10 Experimental setup for measurement of the coupling coefficient (k) and the quality factor (Q) of the implemented PTCs.
Coupling cofficient (k)
20 x 10
-3
18
14 x 10 13
Calculated Measured
16
12
14
11
IPTC
12 10
IPTC
10 9
SSC 0
-3
3 6 9 12 y distance(cm)@z=0
15
SSC 0
3 6 9 12 15 y distance(cm)@z=h/2
Fig. 11 Comparison of the calculated and the measured coupling coefficient (k) of the implemented PTCs.
550 500 450
QUT
400 350
Calculated, IPTC Measured, IPTC
300 250 200
Calculated, SSC Measured, SSC
150 100k 200k 300k 400k 500k 600k 700k 800k frequency (Hz)
Fig. 12. Comparison of the calculated and the measured Q factor of the implemented PTCs.
Fig. 13 Measurement of power transfer efficiency of the WPT system: a) circuit diagram of the measurement; b) experimental setup.
42
42
Efficiency (%)
39
39
IPTC
36
36
Calculated Measured
33 30 27
33 30 27
SSC
24 21
24 0
3 6 9 12 y distance(cm)@z=0
IPTC
15
21
SSC 0 3 6 9 12 y distance(cm)@z=h/2
15
Fig. 14 Comparison of the measured power transfer efficiency with that of calculated using Eq. (5).
(a) Skin Muscle Bone Intestine
σ 0.14402 0.4069 0.08531 0.6502
ε 6012.6 5226.9 350.43 10337
(b)
(c) Fig. 15 Evaluation of the current density: a) analytical model of the simulation setup with homogeneous multi-layer tissue, IPTC, PRC, and excitation source, b) electrical properties of included biological tissues at 300 kHz , and c) simulated induced current density for 0.5 A of coil current and 146 mW of output power.
400 300
THC
J 2
2
3 A/m at 300 kHz
3 A/m at 300 kHz
6
2
TSC
5 4 3
200
2
100
1
0
0
SSC
500 400 300
2
3 A/m at 300 kHz
IPTC 2
3 A/m at 300 kHz
6
2
600
5 4 3
200
2
100
1
0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Coil current, IT (A)
Current density, J (A/m )
Pout
500
Current density, J (A/m )
Output power, Pout (mW)
Output power,Pout (mW)
600
Coil current, IT (A)
Fig. 16 Output power (Pout) and the induced current density (J) over the coil excitation current IT(t) at the frequency of 300 kHz. The green line indicates the ICNIRP limit of J.
(a)
(b)
Fig. 17 Testing of WPT system’s performance in tissues environment: (a) fresh biological tissues of a slaughtered goat torso with a PRC inside the tissues, and (b) measurement setup in the biological tissue environment.
List of Tables TABLE I SPECIFICATION OF THE IMPLEMENTED POWER TRANSMISSION COIL. Parameters
IPTC
SSC
32 (12 + 8 + 12)
22
Coil diameter
40 cm
40 cm
Wire diameter
1.5 mm
1.5 mm
Wire type
AWG 42 Litz
AWG 42 Litz
wire
wire
Strands
270
270
Rdc (Ω)
0.886
0.623
Leff (µH)
458.37
420.9
RT/RES (Ω)
1.62
2.18
Q-factor
533.30
371.81
Number of
Electrical
Physical
turns
**AC parameters are measured at 300 kHz
TABLE II SPECIFICATION OF THE IMPLEMENTED POWER RECEIVING COIL.
Electrical
Physical
Parameters
Value
Number of turns
130
Coil size
∅: 11.5 mm, h: 8 mm
Core size (mm)
8×8×8
Core materials
Mn–Zn, 3E6
Wire type
AWG 44 Litz wire
Strands
8
Rdc (Ω)
4.48
LR (µH)
437.52
RR (Ω)
7.72
µcore Q-factor
8000 107.1
**AC parameters are measured at 300 kHz
S0924-4247(16)30420-4 http://dx.doi.org/doi:10.1016/j.sna.2016.08.035 SNA 9813
To appear in:
Sensors and Actuators A
Received date: Revised date: Accepted date:
29-4-2016 24-8-2016 30-8-2016
Please cite this article as: Md.Rubel Basar, Mohd Yazed Ahmad, Jongman Cho, Fatimah Ibrahim, An improved resonant wireless power transfer system with optimum coil configuration for capsule endoscopy, Sensors and Actuators: A Physical http://dx.doi.org/10.1016/j.sna.2016.08.035 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
An improved resonant wireless power transfer system with optimum coil configuration for capsule endoscopy
Md. Rubel Basara,b, Mohd Yazed Ahmada,b,*, Jongman Chob,c, and Fatimah Ibrahima,b
a
Department of Biomedical Engineering, Faculty of Engineering, University of Malaya, Kuala
Lumpur 50603, Malaysia. b
Center for Innovation in Medical Engineering, Faculty of Engineering, University of Malaya,
Kuala Lumpur 50603, Malaysia. c
Department of Biomedical Engineering, Inje University, Gimhae 621-749, Korea.
Corresponding author: Phone: +603-79677695, Fax: +603-79674579, Email: [email protected], Department of Biomedical Engineering, Faculty of Engineering, University of Malaya, Kuala Lumpur 50603, Malaysia.
Highlights
We developed a mathematical model to enable accurate analysis of WPT system.
The proposed IPTC improves efficiency up to 36.6%, the existing is merely 14.26%.
The proposed IPTC attains 80% field uniformity even with coil diameter of 40 cm.
Current density is analyzed in details with incorporation of ferrite core.
Under stringent guidelines, this technique safely transfers up to 200 mW of power.
Abstract Resonant wireless power transfer is a prominent method of powering a biomedical capsule endoscope, in which the high efficiency of an inductive power link and the electromagnetic safety of biological tissues are greatly important. This paper presents a complete analytical model of a resonant wireless power transfer link, where the efficiency is remarkably increased with the improved power transmission coil configuration. This configuration involves two outer coils and one middle coil. The separation and turn ratios between the outer coils and the middle coil are 0.4 times of the coil radius and 3:2, respectively. An analysis of and comparison with the existing coil configuration show that the proposed configuration relatively attains higher Qfactor, coupling coefficient, and H-field uniformity, which lead to improvement in power transfer efficiency, stability, and electromagnetic safety. The analysis is validated through experiments. With a power receiving coil of ∅11 mm × 8 mm, the proposed power transfer coil attains a minimum efficiency of around 36.6% at the worst position of the power-receiving coil. The attained efficiency was 156% higher than the efficiency of 14.62% obtained by the best of existing coil at the same level of H-field uniformity. Incorporating high permeability core, up to 200 mW of power is transferred with the proposed coil at 300 kHz in accordance with the electromagnetic safety guideline.
Keywords: Capsule endoscopy, inductive coupling, magnetic field uniformity, magnetic resonance, wireless power transfer.
1. Introduction Wireless capsule endoscopy plays a promising role in the early detection of various gastrointestinal (GI) tract diseases, such as gastric cancer, bleeding, and tumors [1, 2]. This technique is noninvasive and painless to the patient in which a tiny capsule can travel even through the complex parts of the GI tract such as the small bowel where typical endoscopes fail to reach [2, 3]. However, powering such capsule is one of its important challenges. Battery usage is often not a good option because of its limited energy storage capacity. Battery recharging or replacement is also impractical in this application [4, 5]. Therefore, a magnetic resonance-based inductively coupled wireless power transfer (WPT) system is a promising solution. This system is typically used to power implantable biomedical devices within a distance of few or tens of millimeters [6, 7]. In this technique, power is transferred from a power transmission coil (PTC) set outside of the human body to a miniaturized power receiving coil (PRC) inside the human body through a linkage of the magnetic field [8]. With perfect tuning, the power transfer efficiency of such system mainly depends on the coupling coefficient (k) between the coils, the coil quality factor (Q), and the load impedance matching with the internal resistance of the PRC [9]. Considering the electromagnetic exposure of the human body tissues, the efficiency of such system is an important concern. Unfortunately, the efficiency of coupling and resultant power transfer is poor in such systems because of the tiny size of the PRC and the large distance between the PTC and the PRC. For example, Jourand et al. [10] developed a WPT system with a PRC of 9 mm diameter. The system was small enough to be embedded in the existing capsule size. The PTC was 75 cm in diameter and can cover the whole abdominal region. The power transfer efficiency attained by this system was as low as
at the 37.5 cm transmission distance. In the last few years, significant
research efforts have been dedicated to the development of an efficient WPT system for capsule endoscopy [8-19]. However, the power transfer efficiency of the existing designs remains low (0.02% to 3.55%) [9, 19, 20]. Recently, Na et al. [19] proposed four coils based resonant WPT system, in which the optimal efficiency tracking theory attained the overall power transfer efficiency of 0.02% at a 7 cm distance. Shi et al. [20] also proposed a portable WPT system for a video capsule endoscope, in which three different PTC configurations were discussed. The optimal configuration achieved a power transfer efficiency of 2.8% at the 20 cm transmission
distance. Liu et al. [21] designed a WPT system for a capsule endoscope. Their design incorporated a new type of PRC, which improved the received power stability. Accordingly, the power transfer efficiency reached 3.51%. Ke et al. [9] developed an analytical model to calculate the quality factor of the PTC. This design attained a power transfer efficiency of 3.55% with an enlarged PTC of 69 cm in diameter. Some other studies on the wireless powered capsule endoscopy are presented in [22, 23]. However, the important performance indices of the WPT system are not discussed in detail. Furthermore, a complete modeling of the WPT system remains limited. The important performance indices, such as Q-factor, coupling coefficient, Hfield uniformity, and efficiency, have not been thoroughly analyzed in the existing design. Moreover, the electromagnetic effect on the biological tissues has not been investigated by incorporating the PRC with a high-permeability ferrite core. On the basis of the summary of existing design limitations, this paper presents a WPT system for the capsule endoscope with a new PTC configuration to achieve higher coupling coefficient, quality factor, and magnetic field uniformity and thus improve the power transfer efficiency, received power stability, and electromagnetic safety. We also provide a complete mathematical model in detail to assess the performance of the proposed PTC and the WPT system. Given the provided model, we analyze the performance of the proposed PTC and compare it with that of the existing PTCs. Finally, we validate the analysis through a rigorous experiment and perform an electromagnetic safety test incorporating a high-permeability ferrite core in the PRC. 2. System overview Fig. 1 shows a schematic of the magnetic resonance-based inductively coupled WPT system for capsule endoscopy. A PTC can be made wearable, and a PRC with a power conversion circuit (rectifier and regulator) can be embedded within the capsule. The PTC is excited by a time-varying current ITC (generated by the power amplifier) to produce a time-varying H-field. A part of the generated H-field interacts with the PRC and induces voltage. Hence, power is being transferred to the load by the linkage of the H-field. In this setup, the power level at the load can be adjusted by exciting the PTC with a higher or lower power. However, transferring a high amount of power with low efficiency does not only waste power but also increases the effect of the reactive electromagnetic field on the nearby tissues. This electromagnetic effect to the nearby body tissues increases with the non-uniformity of H-field generated by the PTC. The H-field is
generally not perfectly uniform in this type of non-radiating (near-field) PTC, whereas the field is much stronger at the positions near the winding of the PTC and weaker in the position a bit away from the PTC winding [16]. This field characteristic results in a very high power level and link efficiency in the strong field regions (near the winding) but a low power level and link efficiency in the weak-field regions (away from the winding) [16, 19]. Consequently, the power received by the PRC becomes unstable when it moves within the non-uniform fields. The received power at the PRC in the weak-field region might be insufficient to power up onboard circuitries. The PTC can be excited with a higher power to boost up the received power level in the weak-field region. However, a high excitation power at the PTC abruptly increases the intensity of the reactive field near the PTC over which the patient body tissues is exposed. Thus, the excitation power at the PTC should be within a limit satisfying the guideline provided by the International Commission on Non-Ionizing Radiation Protection (ICNIRP) and the Standards Coordinating Committee 28 (SCC28) of the IEEE. Additionally, a regulator circuit is required to trim excessive power across the load when the received power is higher than the desired voltage. However, the regulator’s input power (received voltage) must not be very high because excessive power may cause regulator breakdown.
2.1 Modeling of the WPT link efficiency The resonant inductive link can be simplified to a circuit model (Fig. 2) for analysis. In this model, LT and LR denote the effective self-inductance, RT and RR denote the effective series resistance of PTC and PRC, respectively. CT and CR are the tuning capacitors used to resonate both coils. M represents the mutual inductance between the coils. According to circuit theory, the relationship among the applied voltage to the PTC, Vs (t), and current through the coils (IT and IR) can be represented by the following matrix form:
[
]
[
][
]
(1)
At the resonance, the current IT(t) and IR(t) can be calculated using Eq. (1):
I T (t )
( RR RL )VS (t ) , and MVS (t ) I R (t ) 2 2 RT ( RR RL ) M RT ( RR RL ) 2 M 2
(2)
Therefore, the power transfer efficiency can be expressed as follows:
Pout I 2 (t ) RL 2M 2 RL R Pin I T (t )Vs (t ) RT ( RR RL ) 2 M 2 RL RR
(3)
For M k LT LR , k is the coupling coefficient between the coils, and η can be expressed as
k 2QT QR RR RL * 2 RL RR (1 k QT QR ) RR RL
,
(4)
where QT LT / RT and QR LR / RR are the unloaded quality factors of PTC and PRC, respectively. With the condition of maximum power transfer, RL=RR, η can be simplified as
k 2QT QR . 4(1 k 2QT QR / 2)
(5)
2.2 Modeling of the coupling coefficient The coupling coefficient between PTC and PRC can be denoted as
(6)
√
As shown in Fig. 3a, the mutual inductance for a PRC position at (y, z) for a loosely coupled system can be expressed as
(7)
where
is the magnetic flux density within the area enclosed by the PRC (SR), and
effective magnetic permeability of the core materials used in the PRC.
is the
According to Biot–Savart’s Law, the magnetic field generated by the PTC can be calculated at point (y, z) as follows: H z ( y, z ) HYz ( y, z ) ˆj H Zz ( y, z )kˆ
(8)
where 2 IT rT n zSin( ) d H Yz ( y, z ) 3 0 2 2 2 4 2 y z r 2 yrCos( ) r yCos ( ) 2 d 3 IT rT n 2 2 2 H Zz ( y, z ) 4 y z r 2 yrCos( ) 2 0
Therefore, from (6), (7), and (8), the new expression of k can be given as
k ( y, z )
S Cos( ) 2 r yCos ( ) zSin( ) 0 eff R d 3 0 2 2 2 4 LT LR 2 y z r 2 yrCos( )
(9)
2.3 Modeling of the quality factor The equivalent circuit of the PTC is presented in Fig. 3b on the basis of the LCR meter measurement. In this model, the PTC comprises i) self-inductance (L), ii) series AC resistance (Rac) (caused by the skin and proximity effect), and iii) parallel stray capacitance (C). Given that C is very small (in few pico-farad) and Li RaciCi
, the simplified coil impedance can be
written as follows:
Z
Rac L j (1 2 LC) 2 1 2 LC
(10)
The coil model can be simplified with an effective series resistance RES and effective selfinductance Leff based on this expression (Fig. 3c). In that case, RES and Leff can be provided as
RES
L Rac and Leff 2 2 1 2 LC (1 LC )
(11)
Therefore, the quality factor of the coil can be expressed as follows:
QT
L Rac
(1 2 LC )
(12)
By using (5), (9), and (12), the performance of the PTC and the WPT link can be analyzed with the variation of the design parameters. The expression of L, Rac, and C for different coil configurations can be adopted from [7, 24] for this purpose.
3. PTC Configuration We propose a new PTC configuration to improve the H-field uniformity, coupling coefficient, and Q-factor. The performance of the proposed PTC is analyzed using the above mentioned model and compared with that of the three existing PTCs. The configurations of the existing and proposed PTCs are illustrated in the following subsections. 3.1 Configuration of the existing PTCs Three different PTC configurations are found to be used in the existing WPT system for capsule endoscopy. Fig. 4 depicts these PTC configurations. To simplify the analysis, these PTCs are named as i) typical solenoid coil (TSC in Fig. 4a) [11]; ii) typical Helmholtz coil (THC in Fig. 4b) [18, 25]; and iii) segmented solenoid coil (SSC in Fig. 4c) [26]. These PTC configurations mainly varied because of the different coil turn arrangements. The TSC is a loosely wound solenoid coil with n turn and overall height h, where the distance between the contagious turns is h/(n − 1). The THC is configured with two tightly wound solenoid coils connected in series. These coils are separated on a common axis by the distance of their radius (r). The SSC is a number of segments of a tightly wound solenoid coil. Each of the segments is connected to an advanced switching system that can activate and deactivate any particular segment depending on the PRC location. The quality factor and coupling characteristics are two important performance indices used to select any particular PTC configuration. Nonetheless, in the present study, the selection is mainly led by the index of the H-field intensity and uniformity. As such, the TSC and SSC configurations are preferred in the existing studies to increase the
magnetic field intensity, whereas the THC is commonly chosen to enhance the magnetic field uniformity. 3.2 Proposed PTC Among the existing PTC configurations, the generated magnetic field is weak in the THC, whereas the uniformity is poor in the TSC and SSC. In addition, the uniformity of the field in the THC significantly decreased along with the radial (y-axial) distance beyond r/2 (y>r/2). Therefore, we have proposed an improved power transmission coil (IPTC) to improve both Hfield uniformity and intensity at a time (within a large region inside the coil: z≤±h/2 and y≤±3r/4). Fig. 5 shows the IPTC design. In this design, two outer and one middle coils are arranged so that they have the same radius (r). These coils are aligned in parallel. Their centers are fixed at a common axis (Fig. 5a). The separation and turn ratio between the outer coils and the middle coil are optimized considering optimum values of H-field intensity, H-field uniformity and coupling coefficient. These parameters are found to be optimum when the separation between the outer and the middle coils is spaced at 0.4r and their turn ratio is 3:2. In this design, the overall IPTC coil height is 0.8r. The coil height can be enlarged further along the axial distance with the inclusion of additional middle coils to enlarge the area of the uniform Hfield (Fig. 5b).
4. Performance analysis The key performance indices, including the PTC configurations (i.e., H-field intensity and uniformity), coupling characteristics, and Q-factor, are analyzed and compared in this section. 4.1 H-field intensity and uniformity The field distribution in different PTCs is computed using the COMSOL Multiphysics computer simulation software (version 5.1, provided by COMSOL, Inc.) to access the H-field intensity and uniformity. This software works based on partial differential equations. The simulation is initially verified with the H-field intensity calculated using Eq. (8) and adopted from the Biot– Savart’s Law. An identical PTC radius of 20 cm and an excitation RF power of 5 W (at 300 kHz) are used in this simulation. All the PTCs are considered co-centric with the Cartesian coordinate system. The number of turns in each of the PTC is adjusted for an equal effective coil inductance LT to minimize the effect of the tuning capacitor and the transmitting circuit.
Fig. 6 shows the color maps of the simulated H-field distribution in the PTCs. Within the region y≤±3r/4 and z≤±h/2, the maximum and minimum field levels are noted with the coordinates. As shown in Fig. 6, the peak intensity of the field in the TSC and SSC is relatively higher. However, the field intensity abruptly increases with the radial (y-axial) distance and decreases with the axial (z-axial) distance. Therefore, the field intensity varies within a wide range resulting in poor uniformity. In the THC, the field intensity is relatively lower, and the uniformity is quite high within a limited region around the center (y≤±r/2 and z≤±h/4). Beyond this region, the field intensity significantly drops with the radial distance and sharply increases toward the coil winding. Therefore, the uniformity within the large region is poor even in the THC. These problems are overcome with the proposed IPTC. The proposed configuration of our IPTC, indicated in Section 3.2, compensates for the field reduction along the radial distance thus overcomes the excessive increase of the field toward the outer coil winding. Therefore, the variation of the maximum and minimum field intensities is minimized. Consequently, the proposed IPTC attains the three following advantages over the existing PTCs: i) high field intensity, ii) high uniformity, and iii) relatively low peak electromagnetic exposure.
4.2 Coupling characteristics We analyze the coupling characteristics of the existing and proposed PTC configurations by using Eq. (9). A PRC of 5.5 mm radius with high-permeability ferrite core is used in this analysis. The radius of 5.5 mm is chosen to optimally utilize the available space within the existing capsule. The used PRC has the enclosed area SR of 9.5 * 10−5 m2, the inductance LR of 426 µH, and the effective permeability µeff of 210. The PRC is considered to be always perfectly aligned with the PTC. Based on this analysis, the variation of the coupling coefficient caused by the free movement of the PRC within the large region of the PTC (y≤±3r/4 and z≤±h/2) is compared (Fig. 7). The comparison shows that the minimum level of coupling is highest in the IPTC. However, the coupling coefficient attainted by the TSC is close to that of the IPTC around the center region (Fig. 7a). The coupling coefficient of the TSC significantly decreases around the radial line along z = h/2. The coupling coefficient attained by the IPTC along this line is significantly higher than any other PTCs (Fig. 7b). The coupling coefficient attained by THC within the considered large region is always the lowest. This coefficient further reduces along the
radial distance of z = 0. The coupling coefficient of the SSC is moderate and varies within a wide range.
4.3 Quality factor The quality factor of the PTCs (QT) is analyzed using the model given in Eq. (12). In this analysis, the number of turns in the different PTCs has been adjusted for almost the equal value of coil inductance LT ~460 µH. For this coil inductance, the number of turns obtained from the calculation are 30, 26, 22, and 32 for the TSC, THC, SSC, and IPTC, respectively. The variations of the unloaded QT over the frequency are shown in Fig. 8. According to Fig. 8, within the frequency range of 100 kHz - 700 kHz, the maximum QT attained by the IPTC is remarkably higher than that of attained by other PTCs. The maximum QT attained by the IPTC is 533 at 300 kHz. Whereas, the maximum QT obtained by TSC, SSC and THC are 478, 448 and 445 at 262 kHz, 172 kHz and 208 kHz respectively. The maximum QT and the corresponding frequency for a given coil diameter and number of turns depend on Rac, L and C, which vary with a gap between the contagious turns of the coil winding. The tightly winded solenoid coil in the THC and the SSC presents a relatively higher Rac and C, resulting in the lower maximum QT at a lower frequency. The loosely winded solenoid coil in the TSC has Rac, and C, which is relatively lower than that of the THC and the SSC. Therefore, the TSC demonstrates a relatively higher QT at a higher frequency. However, the excessive gap between the turns reduces L by reducing the mutual inductance among the turns. Consequently, QT reduces with an excessive gap between the turns. Therefore, an optimum gap equal to the radius of the wire is used in the IPTC to obtain the maximum QT.
4.4 Variation of the link efficiency and field uniformity with PTC diameter The field uniformity and the link efficiency in the WPT system for capsule endoscopy are expected to be high to minimize the instability of the received power and the electromagnetic exposure effect on the patient body tissues. Unfortunately, the link efficiency and the field uniformity are opposite functions of the PTC diameter. In the existing studies, a PTC diameter has been chosen in between 40 and 75 cm with the medical viewpoint or seeking of a uniform field [10, 27, 28].
The correlation of the link efficiency and the field uniformity with the PTC diameter is illustrated in Fig. 9. The link efficiency is calculated using Eq. (5), whereas the field uniformity is calculated as follows:
H (0,0) H ( y, z ) *100% Uniformity Min1 H (0,0)
(13)
where H(0,0) and H(y,z) are the magnetic field at the center and coordinate (y,z), respectively. As shown in Fig. 9, the WPT link efficiency decreases almost exponentially with the increase of the PTC diameter. The field uniformity increases with the PTC diameter. This increment is abrupt within a certain range of the PTC diameter and then steadies. Therefore, to attain at least 80% field uniformity within the y≤±15 cm and z≤±h/2 regions, possible minimum PTC diameter is 40 cm for the IPTC, 45 cm for the THC, 50 cm for the TSC, and 65 cm for the SSC. Thus ensuring 80% field uniformity with the given coil diameter, the maximum achievable link efficiency can be 36.05% by the IPTC, 9.6% by the THC, 14.26% by the TSC, and 3.8% by the SSC as it is highlighted in the Fig. 9. The THC can attain the highest field uniformity with a larger diameter but may result in the sacrifice of a significant amount of link efficiency. 5. Implementation and experimental validation Two of the PTCs (i.e., SSC and IPTC) are implemented to verify the abovementioned analysis. A twisted round Litz wire with 270 strands of AWG 42 is used to construct the PTC on a lightweight, non-metallic coil frame. The frame is constructed with a diameter of 40 cm and is made of polymethyl methacrylate and polycarbonate sheets. A 1D PRC is implemented to measure the coupling coefficient and link efficiency. The PRC is implemented on a rounded cubic core with 8 mm side length. A ferrite core with high permeability (Feroxcube 3E6 material) is used in the PRC. The details and specifications of the implemented PTC and PRC are provided in Tables I and II, respectively.
5.1 Measurement of k and QT For the implemented PTCs, k and Q are measured with the help of an LCR meter (GW Instek LCR 8101G). The measurement setup is shown in Fig. 10. With this setup, the effective selfinductance of the PTC and the PRC (i.e., LT and LR, respectively) is measured using the LCR meter. The mutual inductance between the PTC and the PRC is measured by varying the PRC coordinates within the PTC. The measured mutual inductance and effective self-inductance of the coils are then converted to k by using Eq. (5). In addition, the Q-factor of the PTCs is directly measured using the LCR meter. The measurement is conducted on a table 1 m above the floor and away from any metallic or conductive object to minimize proximity effect. The measured k and Q are then compared with the calculated values in Figs. 11 and 12, respectively. As shown in Figs. 11 and 12, the measured parameters are in good agreement with the calculated values. However, the measured Q slightly differs from the calculated value because of the manual winding of the PTC.
5.2 Measurement of the link efficiency Fig. 13 shows the experimental setup for measuring the inductive link efficiency. In this experiment, both of the coils are resonated at 300 kHz with the tuning capacitor connected in the series. The PTC is driven by a signal generator. An oscilloscope is used to measure the output voltage across the load (VRL) and the input current through the PTC (IT) by measuring the voltage across a 1 Ω test resistor connected in series with the PTC. The measured power transfer link efficiency (varying the coordinate of the PRC within the PTC) is then calculated from the measured VRL(y, z) and IT as follows:
( y, z )
VRL2 ( y, z ) / RL I T2 RT
(14)
Fig. 14 shows the analytical and measured power transfer efficiency and its variation with the position of the PRC. In comparison with the measured result, the analytical result shows good acceptability of the given efficiency model. In addition, the proposed IPTC configuration
attained the power transfer efficiency significantly higher than that of the existing PTC configuration. The minimum efficiency attained by the IPTC is around 36.5%, whereas that by the SSC is nearly 24%.
6.
Effect of WPT system on the biological tissues The strong reactive magnetic field in the WPT system may exert some potential effects on the
living body tissues. The effects include i) stimulus action caused by the induced current within the tissues and ii) rising temperature caused by the absorption of the electromagnetic energy by the tissues. These parameters are often indexed by the current density (J, A/m−2) and specific absorption rate (SAR, W/kg) [5, 14]. The SAR is not influential at a frequency lower than 10 MHz [14]. Therefore, we use J as the safety analysis index. We obtain J by computer simulation based on partial differential equations. Fig. 15a depicts the simulation setup. In this simulation, a body model 30 cm in diameter is used. This model is built with the materials of multi-layer homogenous body tissues, including the skin, muscles, bones, and intestines, which are the main parts of the abdominal region. The electrical properties of these tissues (i.e., conductivity (σ) and permittivity (ε)) are shown in Fig. 15b for the frequency of 300 kHz which is obtained from [29]. The simulation setup with the IPTC and the distribution of the induced J for the coil excitation current (IT) of 0.5 A are shown in Fig. 15b. The distributed J is relatively higher around the capsule when it is located within the intestine with a relatively higher σ than the skin, muscles, and bones. Moreover, J increases with load reduction (RL), which is kept at 20 Ω in this simulation. The current in the PRC increases with the reducing RL because it is related to I R (t ) MIT (t ) /( RR RL ) . Considering this effect, a limit must be imposed on the allowable IT in the PTCs and the maximum power that can be transferred following the ICNIRP guideline [30] on J. The maximum allowable IT, at which J exceeds the safety limit, and the corresponding output power of different PTC configurations are shown in Fig. 16. Both the IPTC and the THC present a similar amount of maximum Pout for a common PRC location at the center of the PTC. Nevertheless, Pout reduces in the THC for the PRC location in the weak-field region (e.g., y = 0.75 r and z = 0).
7. Effect of biological tissues on WPT system In this application, the PTC and the PRC are working in close proximity of the human body tissues, therefore, an investigation on the effects of biological tissues on the performance of the WPT system is provided in this subsection. In order to investigate this effect, we have used the biological tissues from a slaughtered goat (Capra aegagrus hircus) torso of abdominal region including bone, muscle, fat and intestine. The PRC covered with polyethylene film of 1 mm thickness is placed inside the intestine in abdominal region as indicated in Fig. 17a. Then the efficiency of the WPT system is measured by placing the PRC surrounded with the biological tissues inside the PTC. The PRC is set to be at the center of the PTC with its correctional area perpendicular to the direction of the generated H-field. The measurement setup is shown in the Fig. 17b. The power transfer efficiency obtained from this measurement is 33.18% whereas without the biological tissues (as the measurement setup shown in Fig. 13) the achieved efficiency is 36.60%. The result shows the presence of the biological tissues slightly reduce the power transfer efficiency without much affecting the performance of WPT system. Our finding complies with other similar studies in this area as reported in [16, 19, 20] which indicate slight reduction when the WPT system operating in biological tissue environments at this rage of frequency. 8. Conclusion A complete model of a resonant inductive wireless power transfer system with an improved power transfer coil has been presented for capsule endoscopy. The performances of the improved power transfer coil and the inductive power link have been analyzed in detail by using the provided mathematical model. In comparison with the existing power transfer coil, the proposed coil significantly improves the Q-factor, which is the coupling coefficient increasing the power transfer efficiency. In addition, the improved coil attains a high H-field uniformity even with a smaller diameter. With a power receiving coil of ∅11 mm × 8 mm, the minimum power transfer efficiency attained by the improved coil is 36.05%. The maximum efficiency attained by the existing coils is 14.26% while maintaining a similar H-field uniformity level. An electromagnetic safety analysis is performed with a multi-layer homogenous body model and an incorporation of the high-permeability ferrite core receiving coil. The induced current density is higher around the high-permeability ferrite core than the other regions. A power of 200 mW can be transferred in accordance with the electromagnetic safety guideline.
Acknowledgements This research is financially supported by University of Malaya Research Grant (UMRG: RP009D-13AET and RP022B-14AFR).
Md. Rubel Basar
Md. Rubel Basar received the BSE degree in Electronics and Communication Engineering from Khulna University of Engineering and Technology, Bangladesh and the M.Sc. degree in Communication Engineering from University Malaysia Perlis, Malaysia in 2010 and 2013, respectively. From 2010 to 2012, he was a RF optimization Engineer of GSM network with 3S Network (BD) Ltd., Bangladesh. Currently he is pursuing his Ph.D degree in the Department of Biomedical Engineering, Faculty of Engineering, University of Malaya, Malaysia. His research interests include wireless power transfer system, antenna and RF circuits.
Mohd Yazed Ahmad
Mohd Yazed Ahmad received the B.E. degree from the Department of Electrical Engineering, University of Malaya, Kuala Lumpur, Malaysia, in 2003 and the M.S. degree from the Department of Biomdical Engineering, Faculty of Engineering, University of Malaya, in 2006 and the Ph.D degree from the Center for Health Technologies, School of Electrical, Mechanical and Mechatronic Systems, Faculty of Engineering and Information Technology, University of Technology, Sydney, Australia in 2013. Currently, he is a Senior Lecturer in the Department of Biomedical Engineering, Faculty of Engineering, University of Malaya. His current research interests include localization, positioning, wireless sensors, data fusion, optimization, instrumentation systems using RF, and embedded systems.
Jongman Cho
Jongman Cho is an Associate Professor in the Department of Biomedical Engineering at Inje University, Korea. He received B.E., M.E., and Ph.D degrees at the Department of Electronic Engineering, Inha University, Korea in 1985, 1989, and 1994 respectively. From 1996 to 1998 he was a visiting scholar in the Department of Biomedical Engineering at Rutgers University, New Jersey, U.S.A. His research interests include the application of artificial neural networks, biomedical signal processing, and image processing. Fatimah Ibrahim
Fatimah Ibrahim obtained her Bachelor of Science in Electrical Engineering degree from Marquette University, Wisconsin, USA in 1989, Master of Science in Electronics (Medical Systems) degree from the University of Hertfords-hire, UK, in 1994 and PhD degree in Biomedical Engineering from University of Malaya (UM), Malaysia in 2005 joint funding between UM and Yayasan Sultan Iskandar Johor. She was a Senior lecturer in Universiti Teknologi MARA, from 1990 to 1999. In 1999, she joined the University of Malaya and involved in setting up the pioneer, Department of Biomedical Engineering in Malaysia. She is currently the head of Centre for Innovation in Medical Engineering(CIME) in Faculty of Engineering, University of Malaya. Her research interests are in the detection and monitoring of diseases, physiological modeling and measurement, BIOMEMS, Nanotechnology based biosensor, and artificial intelligence application in biomedicine.
References [1] A. Moglia, A. Menciassi, P. Dario, A. Cuschieri, Capsule endoscopy: progress update and challenges ahead, Nature reviews Gastroenterology & hepatology, 6(2009) 353-62. [2] M.R. Basar, F. Malek, K.M. Juni, M.S. Idris, M.I.M. Saleh, Ingestible Wireless Capsule Technology: A Review of Development and Future Indication, International Journal of Antennas and Propagation, 2012(2012) 1-14. [3] G. Ciuti, A. Menciassi, P. Dario, Capsule Endoscopy: From Current Achievements to Open Challenges, Biomedical Engineering, IEEE Reviews in, 4(2011) 59-72. [4] R. Carta, J. Thoné, R. Puers, A wireless power supply system for robotic capsular endoscopes, Sensors and Actuators A: Physical, 162(2010) 177-83. [5] M.R. Basar, M.Y. Ahmad, J. Cho, F. Ibrahim, Application of Wireless Power Transmission Systems in Wireless Capsule Endoscopy: An Overview, Sensors-Basel, 14(2014) 10929-51. [6] R.F. Xue, K.W. Cheng, M. Je, High-efficiency wireless power transfer for biomedical implants by optimal resonant load transformation, IEEE Transactions on Circuits and Systems I: Regular Papers, 60(2013) 867-74. [7] A.K. RamRakhyani, S. Mirabbasi, M. Chiao, Design and Optimization of Resonance-Based Efficient Wireless Power Delivery Systems for Biomedical Implants, IEEE Trans Biomed Circuits Systs, 5(2011) 48-63. [8] T.J. Sun, X. Xie, G.L. Li, Y.K. Gu, Y.D. Deng, Z.H. Wang, A Two-Hop Wireless Power Transfer System With an Efficiency-Enhanced Power Receiver for Motion-Free Capsule Endoscopy Inspection, IEEE Trans Biomed Eng, 59(2012) 3247-54. [9] Q. Ke, W. Luo, G. Yan, and K. Yang, "Analytical Model and Optimized Design of Power Transmitting Coil for Inductively Coupled Endoscope Robot," IEEE Trans Biomed Eng, 63(2016) 694706. [10] P. Jourand, R. Puers, A Class-E driven inductive power delivery system covering the complete upper body, Sensors and Actuators A: Physical, 183(2012) 132-9. [11] B. Lenaerts, R. Puers, Inductive powering of a freely moving system, Sensors and Actuators A: Physical, 123–124(2005) 522-30. [12] M. Guanying, Y. Guozheng, H. Xiu, Power transmission for gastrointestinal microsystems using inductive coupling, Physiological measurement, 28(2007) 9-18. [13] B. Lenaerts, R. Puers, An inductive power link for a wireless endoscope, Biosensors & bioelectronics, 22(2007) 1390-5. [14] K. Shiba, T. Nagato, T. Tsuji, K. Koshiji, Energy Transmission Transformer for a Wireless Capsule Endoscope: Analysis of Specific Absorption Rate and Current Density in Biological Tissue, IEEE Trans Biomed Eng, 55(2008) 1864-71. [15] R. Carta, M. Sfakiotakis, N. Pateromichelakis, J. Thoné, D.P. Tsakiris, R. Puers, A multi-coil inductive powering system for an endoscopic capsule with vibratory actuation, Sensors and Actuators A: Physical, 172(2011) 253-8. [16] G.B. Pan, W.H. Xin, G.Z. Yan, J.L. Chen, A video wireless capsule endoscopy system powered wirelessly: design, analysis and experiment, Meas Sci Technol, 22(2011). [17] Z. Jia, G. Yan, Y. Shi, B. Zhu, A wireless power transmission system for an active capsule endoscope for colon inspection, Journal of medical engineering & technology, 36(2012) 235-41. [18] W.H. Xin, G.Z. Yan, W.X. Wang, A stable wireless energy transmission system for gastrointestinal microsystems, Journal of medical engineering & technology, 34(2010) 64-70.
[19] K. Na, H. Jang, H. Ma, F. Bien, Tracking Optimal Efficiency of Magnetic Resonance Wireless Power Transfer System for Biomedical Capsule Endoscopy, IEEE Trans Microw Theory Tech, 63(2015) 295-304. [20] Y. Shi, G. Yan, B. Zhu, G. Liu, F. Liu, D.-H. Lee, et al., A portable wireless power transmission system for video capsule endoscopes, Bio-Medical Materials and Engineering, 26(2015) S1721-S30. [21] G. Liu, G. Yan, W. Xu, S. Kuang, Dual-head wireless powered video capsule based on new type of receiving coils, Journal of medical engineering & technology, 39(2015) 246-52. [22] S. He, G.-Z. Yan, Q. Ke, Z.-W. Wang, W.-W. Chen, A wirelessly powered expanding-extending robotic capsule endoscope for human intestine, International Journal of Precision Engineering and Manufacturing, 16(2015) 1075-84. [23] L. Yan, T. Wang, D. Liu, J. Peng, Z. Jiao, C.-Y. Chen, Capsule Robot for Obesity Treatment With Wireless Powering and Communication, IEEE Trans Ind Electron, 62(2015) 1125-33. [24] Z. Yang, W. Liu, E. Basham, Inductor modeling in wireless links for implantable electronics, Magnetics, IEEE Transactions on, 43(2007) 3851-60. [25] M. Basar, M.Y. Ahmad, J. Cho, F. Ibrahim, A wireless power transmission system for robotic capsule endoscopy: Design and optimization, RF and Wireless Technologies for Biomedical and Healthcare Applications (IMWS-Bio), 2014 IEEE MTT-S International Microwave Workshop Series on, IEEE2014, pp. 1-3. [26] H. Yadong, W. Jianfeng, S. Tianjia, X. Xiang, L. Guolin, G. Yingke, et al., An efficiency-enhanced wireless power transfer system with segmented transmitting coils for endoscopic capsule, Circuits and Systems (ISCAS), 2013 IEEE International Symposium on2013, pp. 2279-82. [27] M. Guanying, Y. Guozheng, H. Xiu, Power transmission for gastrointestinal microsystems using inductive coupling, Physiological measurement, 28(2007) 9-18. [28] Q. Ke, W. Luo, G. Yan, K. Yang, Analytical Model and Optimized Design of Power Transmitting Coil for Inductively Coupled Endoscope Robot, Ieee T Bio-Med Eng, 63(2016) 694-706. [29] S. Gabriel, R.W. Lau, C. Gabriel, The dielectric properties of biological tissues: III. Parametric models for the dielectric spectrum of tissues, Physics in Medicine and Biology, 41(1996) 2271-93. [30] I.C.o.N.-I.R. Protection, Guidelines for limiting exposure to time-varying electric and magnetic fields (1 Hz to 100 kHz), Health physics, 99(2010) 818-36.
List of Figure
Fig. 1 Schematic of a WPT system with a PTC fixed around the patient body and a WCE consists of a PRC and a receiver circuit located in the human GI tract.
Fig. 2 Simplified circuit model of the resonant inductively coupled wireless power transfer system.
(b)
(a)
(c)
Fig. 3 (a) Coupling model between PTC-PRC, (b) lumped equivalent circuit of a PTC, and (c) effective lumped equivalent circuit of a PTC.
Fig. 4 Schematic configuration of the existing PTCs: (a) typical solenoid coil (TSC); (b) typical Helmholtz coil (THC); and c) segmented solenoid coil (SSC), the number of segments is denoted by “nos”.
(a)
(b)
Fig. 5 Schematic of the proposed improved power transmission coil (IPTC): (a) basic structure and (b) enlargement of the axial region with additional middle coil. The outer and middle coils are represented by “OC” and “MC,” respectively.
Fig. 6 Color map of the H-field distribution in: a) TSC, b) THC, c) SSC, and d) IPTC. The field is shown on the yz plane at x = 0 for an identical coil radius of 20 cm and inductance of around 460 µH, each of the coil are excited with AC power of 5 W at 300 kHz.
coupling cofficient ( k)
18 x10 16 14
-3
14 x10 13
TSC THC
12 11
12
SSC IPTC
10
10
9
8 0.00
-3
8 0.25
0.50
y/r @ z=0
(a)
0.75
0.00
0.25
0.50
0.75
y/r @ z=h/2
(b)
Fig. 7 Variation of the coupling coefficient in different PTCs along the radial direction: a) at z = 0 and b) at z = h/2. The coupling coefficient is calculated for µeff = 210, SR =
m2, LT ~460 µH, and LR = 426 µH.
Fig. 8 Unloaded quality factor of the PTCs over the frequency obtained for an identical coil radius of 20 cm and inductance of around ~460 µH.
Fig. 9 Variation of the minimum H-field uniformity and power transfer efficiency with the diameter of the PTCs. The maximum efficiency obtained by the different PTCs with ensuring 80% uniformity is indicated.
Fig. 10 Experimental setup for measurement of the coupling coefficient (k) and the quality factor (Q) of the implemented PTCs.
Coupling cofficient (k)
20 x 10
-3
18
14 x 10 13
Calculated Measured
16
12
14
11
IPTC
12 10
IPTC
10 9
SSC 0
-3
3 6 9 12 y distance(cm)@z=0
15
SSC 0
3 6 9 12 15 y distance(cm)@z=h/2
Fig. 11 Comparison of the calculated and the measured coupling coefficient (k) of the implemented PTCs.
550 500 450
QUT
400 350
Calculated, IPTC Measured, IPTC
300 250 200
Calculated, SSC Measured, SSC
150 100k 200k 300k 400k 500k 600k 700k 800k frequency (Hz)
Fig. 12. Comparison of the calculated and the measured Q factor of the implemented PTCs.
Fig. 13 Measurement of power transfer efficiency of the WPT system: a) circuit diagram of the measurement; b) experimental setup.
42
42
Efficiency (%)
39
39
IPTC
36
36
Calculated Measured
33 30 27
33 30 27
SSC
24 21
24 0
3 6 9 12 y distance(cm)@z=0
IPTC
15
21
SSC 0 3 6 9 12 y distance(cm)@z=h/2
15
Fig. 14 Comparison of the measured power transfer efficiency with that of calculated using Eq. (5).
(a) Skin Muscle Bone Intestine
σ 0.14402 0.4069 0.08531 0.6502
ε 6012.6 5226.9 350.43 10337
(b)
(c) Fig. 15 Evaluation of the current density: a) analytical model of the simulation setup with homogeneous multi-layer tissue, IPTC, PRC, and excitation source, b) electrical properties of included biological tissues at 300 kHz , and c) simulated induced current density for 0.5 A of coil current and 146 mW of output power.
400 300
THC
J 2
2
3 A/m at 300 kHz
3 A/m at 300 kHz
6
2
TSC
5 4 3
200
2
100
1
0
0
SSC
500 400 300
2
3 A/m at 300 kHz
IPTC 2
3 A/m at 300 kHz
6
2
600
5 4 3
200
2
100
1
0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Coil current, IT (A)
Current density, J (A/m )
Pout
500
Current density, J (A/m )
Output power, Pout (mW)
Output power,Pout (mW)
600
Coil current, IT (A)
Fig. 16 Output power (Pout) and the induced current density (J) over the coil excitation current IT(t) at the frequency of 300 kHz. The green line indicates the ICNIRP limit of J.
(a)
(b)
Fig. 17 Testing of WPT system’s performance in tissues environment: (a) fresh biological tissues of a slaughtered goat torso with a PRC inside the tissues, and (b) measurement setup in the biological tissue environment.
List of Tables TABLE I SPECIFICATION OF THE IMPLEMENTED POWER TRANSMISSION COIL. Parameters
IPTC
SSC
32 (12 + 8 + 12)
22
Coil diameter
40 cm
40 cm
Wire diameter
1.5 mm
1.5 mm
Wire type
AWG 42 Litz
AWG 42 Litz
wire
wire
Strands
270
270
Rdc (Ω)
0.886
0.623
Leff (µH)
458.37
420.9
RT/RES (Ω)
1.62
2.18
Q-factor
533.30
371.81
Number of
Electrical
Physical
turns
**AC parameters are measured at 300 kHz
TABLE II SPECIFICATION OF THE IMPLEMENTED POWER RECEIVING COIL.
Electrical
Physical
Parameters
Value
Number of turns
130
Coil size
∅: 11.5 mm, h: 8 mm
Core size (mm)
8×8×8
Core materials
Mn–Zn, 3E6
Wire type
AWG 44 Litz wire
Strands
8
Rdc (Ω)
4.48
LR (µH)
437.52
RR (Ω)
7.72
µcore Q-factor
8000 107.1
**AC parameters are measured at 300 kHz