Cable-Tethered Drones Morph to Tackle Complex Aerial Tasks

For most drones, the only physical contact they make with the world around them happens during landing. They can inspect a bridge, fly through a warehouse, or survey a disaster area, but actually interacting with the environment requires bulky grippers or other specialized hardware. Researchers have now developed a drone platform that approaches this problem in a very different way. By using a cable stretched between two small aircraft, the platform can grasp, pull, perch, and even transform into a quadcopter without adding additional hardware.

The platform is built from two nearly identical bicopters connected by a tether. Each bicopter carries two brushless motors with tilting propellers, while one includes a motorized winch that adjusts the cable length and the other measures cable tension with an integrated force sensor. Magnets mounted underneath the aircraft allow the pair to lock together when they are pulled close enough.

These components enable three completely different flight modes. With the cable hanging slack, the bicopters fly independently, allowing them to inspect different viewpoints while remaining connected. Tightening the tether puts the system into a coordinated flight mode where both aircraft maintain tension in the cable. Instead of treating the tether as a nuisance, the researchers use it as the robot's tool for interacting with the environment. Finally, reeling the cable all the way in brings the two aircraft together, where magnets join them into a single quadcopter that can better navigate narrow spaces before separating again when needed.

Using the cable as the working tool eliminates the need for dedicated grippers or manipulators. By carefully controlling cable tension, the platform can apply forces to external objects, transport suspended payloads, or wrap the tether around structures for perching. This approach simplifies the aircraft while still enabling meaningful physical interaction with its surroundings.

To make the transitions possible, the team developed separate control systems for each operating mode along with algorithms that manage the changeovers while the aircraft remains airborne. During testing, the prototype successfully flew as two independent bicopters, transitioned into tethered operation, reeled itself together into quadcopter mode, then reversed the entire sequence in midair while maintaining stable flight. Additional experiments showed the platform transporting a 40-gram suspended payload and operating when complete position feedback was unavailable for one of the aircraft.

Each 165-gram bicopter uses two 6,000-kV brushless motors, two servo motors for thrust vectoring, a Crazyflie Bolt 1.1 flight controller, a 4-in-1 electronic speed controller, and a 500 mAh 2S LiPo battery. One aircraft adds the cable winch and encoder, while the other carries the cable tension sensor.

Rather than adding a collection of specialized mechanisms to a drone, this design gets several jobs out of the same few components. The cable connects the aircraft, changes their configuration, and gives them a way to manipulate their surroundings. That could make this type of transformable drone particularly useful for missions where getting to a difficult location is only the first part of the job.