Range anxiety isn’t limited to electric cars. As people and companies explore wider applications for battery-powered drones, their limited endurance remains a critical challenge. Electric motors are simple and reliable, but batteries still lack the energy density necessary for long-duration missions. 

If drones could fly longer, they could be more effective at missions like providing continuous internet connectivity over a rural area, tracking progress of a wildfire, ormonitoring an international border. But deploying recharging stations on the ground comes with significant drawbacks. 

AAE associate professor Ran Dai has a different idea, borrowed straight from the Air Force.

“If a drone needs extra charge, going back to the charging station would require a long-term break from the mission. It is better if we can send another drone with charging capability to charge a smaller drone in the air,” she says.

As the smaller "worker" drone lowers itself into the docking funnel, the larger "supply" drone hovers in place, aligning itself with markings on the bottom of the smaller drone. [Alan Cesar/Purdue University]

Dai has developed a guidance, navigation and control (GNC) system that allows drones to dock autonomously in the air. She has paired this with the idea for a larger network of charging drones, all operating in unison to maintain a continuous, large-scale mission.

It began on the ground

Dai’s previous work, funded by the NSF CAREER grant she earned in 2015, planted the seeds for this approach. With that grant, she designed an autonomous, ground-based, multi-vehicle sys-tem that could handle a long-duration mission. A mobile charging vehicle, fitted with solar panels, would support the missions’ energy needs. It had charging stations for smaller, more agile “worker” vehicles, which would detach and travel to hard-to-reach locations.

Dai’s team built the larger charging vehicle and customized a few smaller, off-the-shelf wheeled drones to serve as the workers, then programmed GNC systems for both. The robots communicate in a connected network, so the smaller robots could conduct their mission and return to dock and charge when they’d finished.

In 2018, her research team also published a paper in IEEE exploring the idea of adding solar panels as range extenders to an unmanned aerial vehicle (UAV). Through simulation and real-world experimentation, they investigated the panel’s ef-fects on the quadcopter UAV’s aerodynamics and its GNC systems.

From left: Associate Professor Ran Dai and her students: Yooseung Choi, Victor Ene, Megan Collins, Clifford Gamble, Aayush Iyengar and Abhishek Kini. [Alan Cesar/Purdue University]

Dai took the lessons from each of those projects and earned an NSF Foundational Research in Robotics grant to explore a new paradigm: extending a UAV’s mission through a solar-powered vertical take-off and landing (VTOL) aircraft that can provide in-air charging.

Combining optics, altitude and GPS

The main challenge with in-air recharging is sim-ply bringing two drones together during flight, Dai says. The GNC system must be precise enough to bring them together and connect their electrical contacts, but computationally light enough to run on their small microcontrollers. 

They can’t rely on GPS alone -- GPS is only accurate to within 3 meters (10 feet), and urban environments with tall buildings can cause additional variance.

The smaller working drone only needs to control its descent rate during the docking process.

The supply drone, with its ample power and size, can support a camera and the heavier computational load of visual processing software. Dai’s team programmed it to hover at a steady elevation and align itself with the specific markings on the bottom of the smaller drone. 

The small worker drone prepares to fly autonomously to a set location to dock with the larger drone and recharge its battery. [Alan Cesar/Purdue University]

For the final inches of approach, the team constructed an in-air landing platform for the top of the supplier. It’s essentially a big, 3D-printed fun-nel that matches the support on the receiver drone. 

When it reaches the bottom of the funnel, magnets maintain a solid electrical connection.magnets maintain a solid electrical connection.

“We use fast DC charging that can be completed in two to three minutes, so they can resume their original mission,” Dai says.

An autonomous charging network

This individual pairing is just one part of a larger, in-air power logistics network. Dai and her team have conceived a whole apparatus that would in-volve many supply drones and working drones, each conducting their tasks autonomously and communicating when they need recharging. 

Work on a UAV project like this would be much more difficult without research spaces like Purdue’s UAS Research and Test facility (PURT). It’s the largest indoor motion-capture facility in the world, with tools for spoofing GPS and simulating other sensor inputs. PURT is the perfect controlled environment to speed up GNC development.

“We don’t need to worry about weather effects, and we don’t need FAA approval to fly inside there,” Dai says. “Without PURT, it would be much harder to do this work.”

Her team’s developments on this new and novel system will be published and presented at AIAA’s SciTech annual convention in January 2025.