Distance Variation - Results

other hand, the rectangular coils show much different behavior than their counterparts, having a rising amplitude up until the 3.5 cm mark, and only decreasing afterwards.

Figure 5.26: Signal Amplitude Behavior with Horizontal Distance Variation for the Three Sets of Coils.

In figure 5.27, it is possible to see a variation of the efficiency of the power transfer when it comes to signal amplitude. This figure comprises both vertical and horizontal distance variations from the oval coil testing results, seen in section 5.2.2. The value of peak-to-peak amplitude when the receiver is coupled to the transmitter at 0 cm was considered 100 % efficiency. The values of distance and amplitude were presented on tables A.13 and A.14, were then compared with the maximum amplitude to see how the efficiency behaved.

Figure 5.27: Variation in amplitude (mV) with vertical and horizontal distance for a Flat Oval Coil.

The results from the oval coil experiments show a larger signal amplitude variation with vertical distance in comparison with horizontal distance. At a vertical distance of 0.5 cm, the signal amplitude is already at 60 to 40 % of the maximum registered efficiency. On the other hand, in the horizontal test, the signal amplitude only drops below 20 % at around 1 cm. These results show the high volatility and instability of this wireless power system. In order to prevent a decrease in efficiency, the transmitting and receiving coils must either be physically coupled or at a very short distance from each other at all times during the power transfer.

Looking at figure 5.28, it is presented the same variation of efficiency as in figure 5.27, but for the conical coils. The values for the vertical and horizontal distance variation test used on this figure are listed in tables A.15 and A.16.

Figure 5.28: Variation in amplitude (mV) with vertical and horizontal distance for a Conical Coil.

In this case, the behavior of the signal amplitude is very similar, in terms of horizontal distance variation, to the one of the flat oval coils, but there is a clear difference when it comes to vertical distance. At 0.5 cm of vertical distance variation, the efficiency is still within 60 to 40 % of maximum signal amplitude, and it only falls bellow 20 % at around 2.5 cm. These results show a clear improvement in terms of magnetic field reach from the conical coil when compared with the flat oval coils.

The graphic presented in figure 5.29 presents the signal amplitude behavior from the vertical and horizontal distance variation experiments with rectangular coils. The values of distance and amplitude are presented in tables A.17 and A.18.

Figure 5.29: Variation in amplitude (mV) with vertical and horizontal distance for a rectangular Coil.

Efficiency is maintained above 80 % at around 2 cm on the vertical scale and 4 cm horizontally, not dropping below 40 % until 6 cm is reached. This distinct set of results comes as proof of how the rectangular coils differ from the other two sets of coils in terms of magnetic field reach.

5.3 Testing the ”Sleeping” Coils’ Influence on the Wireless Power Transfer

Both the transmitters used in the experiments had three coils, which allowed for the detection and charging of a receiving coil in each and any of these coils. As described in section 2.5.1, theQi Commu-nication Protocols have different phases in order to allow the detection, configuration, and calibration of a wireless power system, as well as prevention mechanisms that detect foreign objects that might damage the equipment. The Ping Phase marks the phase when a transmitter coil is searching for the receiver.

When a receiver is detected by one of the coils, this one enters the next phase until it reaches a power transfer contract and, ultimately, enters the Power Transfer Phase. Only one of the three coils can estab-lish a connection with a receiver, while the others stay in the Ping Phase. The objective of this test is to understand the influence on the wireless power transfer of the coils that remain in the Ping Phase. For this, two tests were conducted using the ASHATA Transmitter Module and the Zerone Wireless Power receiver, referenced in sections 3.2.3 and 3.2.4, respectively. For the first test, a wireless power transfer

was established between the transmitter, with three coils, and the receiver, using the middle coil of the transmitter as the transmitting coil. In similarity with the experiments in section 5.2.2, an oval coil was used to take measurements of the variation in signal amplitude with vertical distance variation. For the second test, the same experiment was conducted using the transmitter with only one coil as the other ones were removed. As seen in figures 5.30 and 5.31, the amplitude of the signal is 1.46 times greater at its peak when using only one coil. This can be explained by the fact that, when using 3 coils, the ones that are not in an established connection with the receiver are constantly emitting a ping signal in order to detect other objects.

Figure 5.30: Behavior of signal amplitude (mV)

with vertical distance using 3 coils Transmitter. Figure 5.31: Behavior of signal amplitude (mV) with vertical distance using 1 coil Transmitter.

By comparing the amplitude curve of both figures with the curve from function _r¹2, it is possible to see that the presence of the other coils is altering the normal behavior of the power transfer. With these results, it is possible to say that a system comprised of just one coil in the transmitting end is capable of transmitting power with higher maximum signal amplitude. On the other hand, using a three-coil transmitter allows for a slower decrease in efficiency with distance.