Toward battery-free flight: Duty cycled recharging of small drones

Constrained battery life on current Unmanned Aerial Vehicles (drones) limits the time they can operate and distance they can travel. We address this challenge by harvesting solar power to enable duty-cycled operation on a palm-sized drone. We present a scaling analysis that suggests that more solar power can be collected per unit mass of the drone as scale reduces, favoring small drones. By charging from the sun, the drone can operate for more than a single charging cycle, enabling extended mission time, and long-distance travel. To realize this, we design a high efficiency charging circuit and introduce two innovations. The first is a photovoltaic array that passively folds down while in flight to reduce air drag and automatically opens during landing due to the ground effect. The second is a sensor system and controller that autonomously finds suitable charging sites that are flat and well-lit. The drone can be fully charged in 3 hrs using the solar array and charging circuit with an average efficiency of 90.84%. Each charge enables a 4.7 min flight, allowing the drone to travel up to 1.2 km in a day. We also discuss how this platform could be used to take periodic measurements for smart agriculture or wildlife tracking, rapidly deploy wireless networks, or deploy microrobots in the future.