## 4 Experiments and Results
### Comparative Study  
A comparative study of the proposed and previously presented solutions for drone imperfect landing (misalignment issues) is presented in this section. The system is compared with three solutions presented in Reference [37; 38; 39]. In Reference [37], the authors presented an approach based on the optimal design of the transmitting and receiving coils with the goal of becoming less sensitive to the misalignment of coils. The system was composed of a charging station with multiple arrays of primary or transmitting coils with a specifically designed secondary or receiving coil. The receiving coil was designed to perfectly fit in the landing skid of the drone. The authors proposed a transmitting coil overlapping scheme to entirely cover the charging area on the charging station. By calculating the impedance of the multiple transmitting coils and choosing the transmitting coil with maximum impedance, power transmission between the transmitting and receiving coils was achieved. Figure 16: Load power response for different misalignment cases. Figure 17: Trajectories of hill climbing algorithm. This system works well for a specific type of drone with some specific dimensions; however, if the size and dimension of the drone (physical size) changes, a whole new design of the charging station is required, and the transmitting coils will need to be readjusted again with some proper overlapping to cover the charging area properly. On the other hand, our proposed system has non-overlapped multiple transmitting coils which can be moved in four directions. Furthermore, the proposed system utilizes the feature of backscattering which generally happens in wireless communication in order to identify the most closely coupled receiving coil among them. Even though the closely coupled receiving coil is detected, a further fine alignment of the centroids of the transmitting coil and the receiving coil is required to obtain efficient wireless power transmission by moving multiple transmitting coils. The proposed system is autonomous and totally free from the physical dimensions of the drone; it just needs to detect a receiving coil. This coil could be placed at any part of the drone where it can come in contact with any of the multiple transmitting coils placed on the charging station. The drone can land at any part of the charging station and, within just 1 to 2 s (min to max, depending on the distance of misalignment), the coils would be properly aligned with a strong coupling factor. In Reference [33], the impedance of the transmitting coils was monitored, which requires some current and voltage sensors to provide the measured value at every instance of charging operation. It increases the complexity of the charging station and requires more electronic components. On the other hand, in our case, the voltages of the transmitting coils are enough to ensure proper alignment between coils. Regarding power transfer capabilities and efficiency, in Reference [37], the authors presented results based on the efficiency of wireless power transmission. The results showed that the efficiency of the system changed and decreased when there was misalignment between the coils. The efficiency met the minimum level requirement of 75% in all cases; however, upon misalignment, there was a reduction in efficiency from 88% (normal; no misalignment) to 83% (100-mm misalignment) and 78% (200-mm misalignment). In our system, the efficiency will always be the same (normal; no misalignment) because there is no misalignment and power transmission is done after ensuring this. In Reference [38], the authors presented a target detection technique based on image processing. After landing, the center of the coil was aligned with the transmitting coil using some specific color detection and image-processing scheme. The image-processing algorithm worked by taking the images using a drone camera and converted RGB color space to HSV color space. After applying some filters, the red color was detected and considered as a target.