## 3 Designs of Wirelessly-Powered Cage Systems
### Wirelessly-Powered Cage Systems with a Scalable Tx Coil Array  
With the configuration of the scalable Tx coil array, the wirelessly-powered cage systems introduced in this section have scalable designs [11; 17; 31]. Each Tx coil in the array has its optimum diameter matched with the Rx coil at a given Rx-Tx separation distance. Within the array, the Tx coil closest to the animal, which is often in the best position to power the mobile unit, is activated. Several different methods have been used to dynamically and automatically activate the Tx coil that is in the optimum alignment with the Rx coil. The inductive link configuration in Figure 16a includes a single Rx coil embedded in the mobile device and two overlapping arrays of planar Tx coils placed at the bottom of the cage [31]. Every four coils are connected to a single driver module. The 50% overlap in both X and Y directions eliminates dead spots on the power transmission side. Rx tracking is performed by the impedance tracking technique. Particularly, the proximity of the Rx coil will induce a reflected impedance adding onto the Tx coil, causing a reduction in the current drawn from the driver. The Tx power controller detects the impedance variation of each Tx coil by sensing its supply current variation so that the Tx coil closest to the Rx can be identified as the one with supply current reduction. In Figure 16b, the wirelessly-power cage system includes a resonance-based inductive link with its configuration adjustable [11]. The stationary unit includes a three-layer array of overlapping hex-PSCs that tile the floor of the experimental arena, Tx coils on the third layer driven by PAs and Tx resonators on the first and second layers. The Tx coil can directly link to the mobile unit by forming a three-coil inductive link if the nearest to the Rx is a Tx coil. Otherwise, a four-coil inductive link will be formed if a Tx resonator is closest to the Rx. The animal location tracking is performed via an array of magnetic sensors under the stationary unit and a magnetic tracer in the mobile device. This system is equipped with CLPC, which can enhance the robustness of the wireless power delivery against the Tx-Rx distance variation. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Publications** & [28] & [29] & [30] & [18] \\ \hline Inductive link & 2-coil & 2-coil & 2-coil & 2-coil \\ Frequency & 6.78 MHz & 13.56 MHz & 120 kHz & 13.56 MHz \\ WPT coverage, & \(40\times 24\times 4\) cm\({}^{3}\) & \(40\times 40\times 20\) cm\({}^{3}\) & diameter of 8 inch & \(34\times 18\) cm\({}^{2}\) \\ length \(\times\) width \(\times\) height & \(24.5\times 13\times 16\) mm\({}^{3}\) & \(10\times 10\times 10\) mm\({}^{3}\) & \(<\)1 cm\({}^{3}\) & \(12\times 12\) mm\({}^{2}\) \\ PTE & 3.8–7.5\% & \(>\)0.56\% & N/A & 17\% \\ PDL & 100 mW & 8.5 mW & 2 W & 1.7 mW \\ CLPC & \(\times\) & \(\times\) & \(\times\) & \(\surd\) \\ Scalability & \(\times\) & \(\times\) & \(\times\) & \(\surd\) \\ Compatible to racks & \(\surd\) & \(\surd\) & \(\surd\) & \(\times\) \\ Coil size matching & \(\times\) & \(\times\) & \(\times\) & \(\surd\) \\ Vertical misalignment Resilience & \(\surd\) & \(\surd\) & \(\times\) & \(\times\) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Benchmarking of the wirelessly-powered cage systems with a single Tx coil configuration. Figure 16c shows another wirelessly-powered cage design based on a 13.56 MHz resonance-based four-coil inductive link [17]. The power Tx includes a Tx coil powered by a PA and two arrays of Tx resonators connected in parallel and tiled at the top and bottom surfaces of the chamber to form uniform wireless power distribution in 3D. The top and bottom resonator arrays have more or less contribution to the power transmission, depending on the location of the mobile device in the z-direction. The distance between the surfaces is set for ensuring almost constant power delivery to the mobile device in the z-direction.