3.3 Receiver coil design

In practical applications, it is almost always beneficial to reduce the size of an implantable device, whereas the WPT system requires a match of the resonant frequencies of the primary and secondary coils. We have designed a new structure of the implantable coil with a miniaturized size while having enough turns to match the resonant frequency of the primary coil. Figure 20 illustrates our design in which

Figure 16.

Z-component of the magnetic field distribution with different separations between the evaluation plane and the transmitter consisting of seven coils. (a) 3.25 cm, (b) 7.25 cm, (c) 10.25 cm, and (d) 20.05 cm.

Wireless Power Transfer for Miniature Implantable Biomedical Devices DOI: http://dx.doi.org/10.5772/intechopen.89120

Figure 17. HFSS simulation. (a) 3D model of the transmitter mat; (b) Dimensions of each PSC.

Figure 18.

respectively, above the power mat (i.e., the X-Y plane). Color indicates the magnitude of the magnetic field in the z-direction. It can be seen that, at z = 8 cm (Figure 18a), the magnitude of the magnetic field was the highest (peak) at the center of each coil, and the lowest (valley) at the junction of three coils. When the distance to the HPT mat increased to 20 cm, a smoother magnetic field distribution

(Figure 18b). In order to evaluate the evenness of distribution quantitatively, the coefficient of variation (COV) was utilized which was defined as the standard deviation of the field values divided by the mean. Thus, a smaller value of the COV indicates a more uniform distribution. Figure 19 shows the COVs of the magnetic field in the z-direction above the HPT mat at distances from 5 to 40 cm. It can be observed that the COV achieves a value <10% when the distance is larger than the

In practical applications, it is almost always beneficial to reduce the size of an implantable device, whereas the WPT system requires a match of the resonant frequencies of the primary and secondary coils. We have designed a new structure of the implantable coil with a miniaturized size while having enough turns to match the resonant frequency of the primary coil. Figure 20 illustrates our design in which

Z-component of the magnetic field distribution with different separations between the evaluation plane and the

transmitter consisting of seven coils. (a) 3.25 cm, (b) 7.25 cm, (c) 10.25 cm, and (d) 20.05 cm.

was observed, but approaching to the field generated by a large spiral coil

size of the transmitter coil.

Recent Wireless Power Transfer Technologies

3.3 Receiver coil design

Figure 16.

84

Distribution of the z-component of the magnetic field in a plane at (a) 8 cm and (b) 20 cm above the HPT mat at the resonant frequency of 85.2 MHz.

Figure 19.

Variation in coefficient of variation (COV) of vertical field distribution as a function of distance above the HPT mat at the resonant frequency of 85.2 MHz.

#### Figure 20.

(a) Three sub-coils winded in proper directions are combined and connected to form a single coil shown in (b).

coils serve as both implant exterior housing and power receiving elements. It consists of two planar sub-coils and one helical sub-coil. The sub-coils are combined into a single coil within a shallow box assembly. By choosing different geometric designs for the sub-coils, different shaped boxes can be obtained. In practical implementation, the exterior of the box must be covered by a biocompatible material for biological safety.

By measuring the induced voltage in the small receiver coil, the variation of the

<sup>∂</sup><sup>t</sup> ¼ �<sup>N</sup> <sup>∂</sup>BzS

When the measuring coil is sufficiently small and the system is driven by sinusoidal input, the induced voltage V in the measuring coil is proportional to the local Bz. As our objective function cancels out the constants relating these two values, we can directly evaluate the cost function using V instead of Bz and compare with the calculation results. As shown in Figure 22, these measurement locations were chosen because the magnetic field distribution at any interior PSC in a regular

mat can be approximated by the central PSC in each single seven-coil cell

In order to compare the measured voltage with the calculated Bz, we need to normalize both to the same scale, as they are proportional to each other. Figure 23 shows that the measured data matches the calculated ones very well, except for that the measured data tends to be larger than the calculated ones, which is because the receiver can capture a small portion of horizontal field in additional to the field in the vertical direction. When the measured data are normalized to the center value where the horizontal component is almost zero, the other positions will have larger

In our WPT system design (Figure 13), the separation between the primary and secondary coils includes a distance between the mat and the floor. This distance can be adjusted to achieve both a high WPT performance and an acceptably uniform magnetic field distribution. At different separations, we measured the peak-to-peak values of induced RF voltages in the load coil when the system was powered by a sinusoidal wave at the resonant frequency (approximately 26.6 MHz). Our experiments show that, for our particular system design, the separations of approximately 10 cm and 8 cm between the primary and secondary coils provide a good compromise between performance and magnetic field distribution. In order to visualize this distribution, we interpolated the 21 measured values and plotted the results in Figure 24. At a separation of 10 cm, the measured 21 voltage values were in the range between 1.12 and 1.64 V, whereas the mean and standard deviation were

<sup>∂</sup><sup>t</sup> <sup>∝</sup>Bz (16)

<sup>V</sup> ¼ � <sup>∂</sup><sup>φ</sup>

Wireless Power Transfer for Miniature Implantable Biomedical Devices

DOI: http://dx.doi.org/10.5772/intechopen.89120

vertical magnetic field is given by

Sketch of a seven-resonator mat and the test points.

(Figure 15).

87

Figure 22.

field than the expected one.

## 3.4 Experiment tests

In order to study the performance of the mat-based WPT system experimentally, we constructed a prototype mat-based system shown in Figure 21, where a transmitter consists of seven circular spiral coils arranged in a hexagonal form. Each PSC was 13.2 cm in diameter, 2.9 mm in trace width, and 1.6 mm in trace spacing. On the reverse side of each PSC, several conductor strips were utilized to form distributed capacitances with respect to the coil on the front side. By changing the numbers of these strips, the resonant frequencies of the PSCs became adjustable. The frequencies were adjusted to 29.453 0.072 MHz with a Q-factor of approximately 100. The prototype of a receiver coil includes three coils and is 25 mm in diameter, 7 mm in height, and 3.39 g in weight. The resonant frequency and Q factor were measured to be 29.075 MHz and 61, respectively.

Figure 21. Experimental platform for measuring magnetic field distribution.

Wireless Power Transfer for Miniature Implantable Biomedical Devices DOI: http://dx.doi.org/10.5772/intechopen.89120

Figure 22. Sketch of a seven-resonator mat and the test points.

coils serve as both implant exterior housing and power receiving elements. It consists of two planar sub-coils and one helical sub-coil. The sub-coils are combined into a single coil within a shallow box assembly. By choosing different geometric designs for the sub-coils, different shaped boxes can be obtained. In practical implementation, the exterior of the box must be covered by a biocompatible mate-

(a) Three sub-coils winded in proper directions are combined and connected to form a single coil shown in (b).

In order to study the performance of the mat-based WPT system experimentally, we constructed a prototype mat-based system shown in Figure 21, where a transmitter consists of seven circular spiral coils arranged in a hexagonal form. Each PSC was 13.2 cm in diameter, 2.9 mm in trace width, and 1.6 mm in trace spacing. On the reverse side of each PSC, several conductor strips were utilized to form distributed capacitances with respect to the coil on the front side. By changing the numbers of these strips, the resonant frequencies of the PSCs became adjustable. The frequencies were adjusted to 29.453 0.072 MHz with a Q-factor of approximately 100. The prototype of a receiver coil includes three coils and is 25 mm in diameter, 7 mm in height, and 3.39 g in weight. The resonant frequency and Q

factor were measured to be 29.075 MHz and 61, respectively.

Experimental platform for measuring magnetic field distribution.

rial for biological safety.

Recent Wireless Power Transfer Technologies

Figure 20.

Figure 21.

86

3.4 Experiment tests

By measuring the induced voltage in the small receiver coil, the variation of the vertical magnetic field is given by

$$V = -\frac{\partial \rho}{\partial t} = -N \frac{\partial \overline{B}\_x S}{\partial t} \propto B\_x \tag{16}$$

When the measuring coil is sufficiently small and the system is driven by sinusoidal input, the induced voltage V in the measuring coil is proportional to the local Bz. As our objective function cancels out the constants relating these two values, we can directly evaluate the cost function using V instead of Bz and compare with the calculation results. As shown in Figure 22, these measurement locations were chosen because the magnetic field distribution at any interior PSC in a regular mat can be approximated by the central PSC in each single seven-coil cell (Figure 15).

In order to compare the measured voltage with the calculated Bz, we need to normalize both to the same scale, as they are proportional to each other. Figure 23 shows that the measured data matches the calculated ones very well, except for that the measured data tends to be larger than the calculated ones, which is because the receiver can capture a small portion of horizontal field in additional to the field in the vertical direction. When the measured data are normalized to the center value where the horizontal component is almost zero, the other positions will have larger field than the expected one.

In our WPT system design (Figure 13), the separation between the primary and secondary coils includes a distance between the mat and the floor. This distance can be adjusted to achieve both a high WPT performance and an acceptably uniform magnetic field distribution. At different separations, we measured the peak-to-peak values of induced RF voltages in the load coil when the system was powered by a sinusoidal wave at the resonant frequency (approximately 26.6 MHz). Our experiments show that, for our particular system design, the separations of approximately 10 cm and 8 cm between the primary and secondary coils provide a good compromise between performance and magnetic field distribution. In order to visualize this distribution, we interpolated the 21 measured values and plotted the results in Figure 24. At a separation of 10 cm, the measured 21 voltage values were in the range between 1.12 and 1.64 V, whereas the mean and standard deviation were

higher coupling factor than the conventional solenoid coil when a lateral or angular misalignment exists. In addition, the DH coil achieves the largest coupling factor and energy transfer efficiency when the axis of the DH coil is in parallel with the plane of the planar spiral transmitter coil. Our computer simulation and experiments under lateral and angular misalignments have been conducted and their

Wireless Power Transfer for Miniature Implantable Biomedical Devices

In order to support biomedical studies using the animal model, we have designed a new power mat, enabling wireless power delivery to miniaturized moving targets. The power mat contains a single or multiple transmitter cells and each cell consists of seven hexagonally packed PSCs. We have conducted theoretical, computational and experimental studies on the special WPT system to meet the challenge of distributing the electromagnetic field evenly over the power mat. We have analyzed the HPT cell using the CMT. Formulas have been derived relating the received power to the inputs and system parameters. Then, we utilize computer simulation to study the evenness of the magnetic field distribution over the power mat at different distances between the power mat and the floor of the animal cage. Finally, we constructed a prototype system, measured its magnetic field distribution and verified that our design has met the challenge. We have also presented a new design of the receiver coil consisting of three serially connected sub-coils. This new design of the receiver coil allows it to capture the most magnetic flux produced by the transmitter, facilitates a match of resonant frequencies of the transmitter and

, Wenyan Jia3

1 School of Artificial Intelligence and Automation, Huazhong University of Science

2 Department of Electrical Engineering, Shanghai Jiao Tong University, Shanghai,

3 Department of Electrical and Computer Engineering, University of Pittsburgh,

4 Department of Neurosurgery, University of Pittsburgh, Pittsburgh, USA

5 Department of Bioengineering, University of Pittsburgh, Pittsburgh, USA

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

, Zhi-Hong Mao3,5

results have verified our analytical results.

DOI: http://dx.doi.org/10.5772/intechopen.89120

receiver, and reduces the volume of the implant.

Qi Xu1,4, Tianfeng Wang2,4, Shitong Mao3

\*Address all correspondence to: drsun@pitt.edu

provided the original work is properly cited.

and Technology, Wuhan, China

Author details

China

89

Pittsburgh, USA

and Mingui Sun3,4,5\*

Figure 23. Comparison of measured and calculated vertical magnetic field over the prototype mat.

Figure 24.

Voltage distribution computed by interpolating the 21 measured voltage values across the load coil at a constant primary and secondary coil separation of (a) 8 cm and (b) 10 cm.

1.23 and 0.11 V, respectively. The relatively small standard deviation indicates a nearly uniform magnetic field distribution, as observed in Figure 24. The results indicate that a flat magnetic field can be achieved by our power mat design and that this design is effective for WPT to biomedical implants in freely moving animals.
