Wireless Power Transfer

Project By:Michael Hall

Under the Supervision of:Prof. Peroulis

3 Credit HoursSpring 2012

This semester was a continuation of work done during the fall of 2010 based on wireless power transfer. The initial steps taken in this project were to construct identical receiving and transmitting half toroid shaped objects in HFSS. The shape seen in figure 1 is the theoretical model of one of the resonators. Using HFSS to analyze this geometry gave results showing the resonant frequency of the objects based on their size. The data can be seen below.

Figure 1: HFSS Model

Table 1: Resonant Frequencies from HFSS

Once these results were obtained for the theoretical model of the half toroid, the models were created out of copper. These models were made at varying sizes and the resonant frequency was then obtained for each model.

Figure 4: Actual Models

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.10

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f(x) = 106.57 x -̂0.90R² = 0.99Fig. 3:Resonant frequency Vs Length Scale

Scale relative to 12 inch length

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t Fre

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f(x) = 102.94 x -̂0.83R² = 0.98

Fig. 2:Resonant frequency Vs Diameter Scale

Scale relative to 4 inch diameter

Res

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t fre

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cy (M

Hz)

After analyzing the sizes and results from the Figure 4 half toroid shapes, it was decided to focus on just one of the sizes. The S(1,1) parameters for the 2 inch diameter toroid were gathered via a network analyzer. The resulting graph can be seen below.

Figure 5: Object tested for Fig. 6 graph

Figure 6: S(1,1) Parameter

This data shows that the above toroid emits power at 600 MHz. Reducing the frequency that this object resonated at would decrease the effect of the shape not being a perfect half toroid. In hopes of increasing the performance of the above half toroid shape, some modifications were made. The two 90 degree copper bends that were used to create the half toroid were welded together instead of taped thus creating a single piece and lowering resistance due to having just a taped junction. Another modification was to add a tunable capacitor to the side of each half toroid. This would allow for the resonant frequency of each half toroid to be tuned to exactly match that of the other half toroid and result in optimum power transfer. The next pictures show the set up for the two shapes and their positions as well as the S(1,1) parameter.

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00 200000000 400000000 600000000 800000000 1E+09

Magnitude (dB)

Frequency (Hz)

S11 of 2 Inch

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Figure 7: Tested receiver and transmitter

Frequency (10^5 Hertz)

Figure 8: S(1,1) parameter showing 160 MHz resonance

The objects from figure 7 were then tested to see how they behaved as a transmitter and a receiver. Results yielded a -10dB power transfer at 2 inches. As the distance increased, the power transferred decreased. This result meant that the object was acting like a one turn LC resonator.

In order to improve on the work done so far, the idea is to create a toroidal inductor that will act like the shapes seen above, but with many turns instead of one. The first steps in the continuation of this project include creating a model in HFSS of a toroidal inductor and testing the magnetic field within it and

comparing that to the magnetic field calculated using the equation:

Equation 1: Maximum magnetic field strength within a toroidal inductor

So far the theoretical magnetic field strength has been calculated for varying toroidal inductors. A model has also been made in HFSS of a 100 turn toroidal inductor. Initial results have been taken and work is still in progress at this point in the project. Figure 9 below shows the toroidal inductor being tested in HFSS, which is where the project currently resides.

Figure 9: HFSS model of a toroidal inductor