### Related Works  
Previously, there were different researches on wireless power transmission. In References [13; 14], continuous inductive coupling in radio-frequency identification (RFID) tags were implemented. In References [15; 16], a wireless power technique based on an inductive coupling mechanism was used for medical implants. Few wireless power systems for robotic applications were developed using a large array of smaller coils driven by microelectromechanical systems (MEMS) switches and organic field effect transistors to selectively transmit wireless power [17]. Some researchers developed efficient wireless power transmission systems by designing a switched-mode direct current/alternating current (DC/AC) inverter based on Class E topology [17; 18; 19; 20; 21]. Recently, there were many researches on power control in wireless power transmission. In References [22; 23; 24; 25], the authors proposed a microcontroller-based power control method. In Reference [26], the authors developed a power control scheme using analog feedback circuits. Some researchers used the closed-loop power control technique using a commercial off-the-shelf (COTS) chipset [27; 28; 29].  
For wireless charging of drones, some authors proposed laser beam systems which deliver the power directly to the drone [30]. Solar energy to support a drone's long flight time was proposed in Reference [31]. Charging docking stations to recharge the drone battery were proposed in Reference [32]. Some authors applied smart contact arrays [33], whereas some proposed a drone charging station [34]. In Reference [35], the authors presented an approach based on optimal designing of the transmitting and receiving coil 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 coil was achieved. In Reference [36], 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 a specific color detection and image-processing scheme. The image-processing algorithm worked by taking the images using a drone camera and converted red/green.blue (RGB) color space to hue/saturation/value (HSV) color space. After applying some filters, the red color was detected and considered as a target. In References [37], the authors presented a positioning system using a binary distance laser sensor and ultrasonic sensors. The system was composed of a charging station comprising a single transmitting coil and a drone equipped with a single receiving coil. The system worked by detecting the position of the receiving coil and aligning the transmitting coil with it for wireless power transmission. The positioning system took almost 5 s to detect and align with the receiving coil. Using sensors, the system complexity increased; it required specific places and areas for installation on the drone and the charging station.