## 3 Designs of Wirelessly-Powered Cage Systems
### Wirelessly-Powered Cage Systems for Omnidirectional Power Transmission
However, the Tx coil configuration causes two \"cold corners\" formed by opposite magnetic fields and two \"hot corners\" formed by constructive magnetic fields in the cage. In the \"cold corners\", the triple orthogonal Rx coils still cannot ensure omnidirectional powering. Besides, the Rx coils block the access to the electronics inside the mobile device, making it difficult to repair or to have electrode feed-through. The triple orthogonal Tx coils together with a single Rx coil is another coil configuration offering the omnidirectional powering. If the three Tx coils are driven by the same current, a constant magnetic field in a fixed direction will be generated. For omnidirectional powering, the three Tx coil currents should be driven appropriately different from each other in order to generate a rotating magnetic field. To achieve this objective, there are three possible modulation methods in general: (1) phase-domain modulation (PDM); (2) amplitude-domain modulation (ADM); and (3) frequency-domain modulation (FDM). Given the simplicity of implementing system control, the PDM method is widely used. In Figure 18c, the PDM method is used. More specifically, the three Tx coils excited with three-phase currents with a peak magnitude of 250 mA and 120\({}^{\circ}\) displacement are wound on the outer surface of a glass bowl [36]. The magnetic field vectors rotate periodically in all directions. Three loads hung inside the glass bowl with the Rx coil facing in different directions. Each load consists of a planar Rx coil, a rectifier, and four LEDs. It can be seen that the LEDs in all three loads are on, confirming the omnidirectional nature of the wireless power transmission. The third viable coil configuration offering omnidirectional powering is the double orthogonal Tx coils together with double orthogonal Rx coils. The two orthogonal Tx coils should be driven by two nonidentical currents to form a rotating magnetic field. Figure 18d shows an example of such a coil configuration [22]. Instead of using loop coils, both the Tx and Rx coil, operating at 280 kHz, have crossed-dipole coil structure. The crossed-dipole Tx and Rx coils are composed of a dual set of two orthogonal coils, wire-wounded around ferromagnetic cores. The currents of the two Tx coils have the same magnitude with a 90\({}^{\circ}\) phase difference from each other in order to generate a rotating magnetic field. The magnetic field produced can always pass through the crossed-dipole Rx coils at any orientations, to allow for the omnidirectional power receiving. Additionally, the physical dimensions of Tx and Rx are reduced from volume to plane, which is crucial for practical applications, where volumetric coil structure is highly prohibited. Table 5 compares the features of the abovementioned systems for omnidirectional power transmission. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Publications** & [34] & [14] & [35] & [36] & [22] \\ \hline Inductive link & Cavity & 4-coil & 2-coil & 2-coil & 2-coil \\ Frequency & 346.6 MHz & 13.56 MHz & 6.78 MHz & 530 kHz & 280 kHz \\ WPT coverage, & 61 \(\times\) 61 \(\times\) 30 cm\({}^{3}\) & 46 \(\times\) 24 \(\times\) 20 cm\({}^{3}\) & 30 \(\times\) 30 \(\times\) 30 cm\({}^{3}\) & N/A & 1 m\({}^{3}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Benchmarking of the wirelessly-powered cage systems offering omnidirectional power transmission.