Student Paper Design of Radio Frequency Energy Harvesting System for an Unmanned Airplane

Energy harvesting is a promising technique that can be used to drive various sorts of passively powered devices [1]. Viable energy sources include wind, sunlight, thermal energy, radio waves, mechanical vibration and so on. As an energy source existing ubiquitously in our environment, radio frequency (RF) energy harvesting has the potential to be widely applied. In this work, an RF energy harvesting system is described for the purpose of driving an unmanned airplane. Eventually, the airplane will be capable of remote recharge and become a self-powered system. Fig. 1 shows the structure of the whole system. The rectenna is an assembly of hybrid antenna and rectifying diodes. It receives the RF energy from the ambience and converts it to DC signal. Its output is boosted up to a proper voltage to charge the battery by the DC-DC voltage multiplier. Finally, the battery stores the harvested energy and drives the airplane. The rectenna uses low turn-on voltage Schottky diodes for rectification. It is designed based on harmonic balance nonlinear analysis and full-wave electromagnetic field analysis. By using frequencyindependent antenna elements, it eliminates the matching circuits. The fabrication details are given in [2]. To boost the signal up and charge the battery, a switched capacitor DC-DC converter is designed in IBM 8RF 130nm CMOS process. This converter has six stages. It needs to be used with external capacitors and clock signal. Fig. 2 shows the schematic in three stages. The detailed structure and operations are described in [3]. The selectable output provides a maximum of seven times the input voltage level. The energy storage medium in the system is a lightweight flexible thin film battery [4]. It has the highest specific charge storage capacity of any commercially available thin film cell (>20mAh·cm