In the skies above, unmanned aerial vehicles (UAVs) have become a more common sight as the months and years have gone by. These vehicles have been deployed in applications ranging from infrastructure inspections and mapping to precision agriculture. But by and large, drones take a hands-off approach in their work. They may be equipped with cameras and other sensors to collect vast amounts of data that would otherwise be very difficult to gather, but they generally cannot directly interact with the world around them.
Wouldn’t it be nice if the same UAV that performs an infrastructure inspection in a nearly inaccessible location could also fix the problems it finds? This capability would be very useful, so researchers and developers have experimented with adding robotic arms to drones. But to date, the solutions have not really caught on because of the trade-off between weight and manipulator capabilities. To be useful, the arm must have many degrees of freedom, but each degree of freedom adds weight, which limits flight time.
An overview of the system (📷: R. Peng et al.)
A group of engineers at the University of Hong Kong has proposed a possible solution to this present dilemma. Inspired by the flexibility and agility of an elephant’s trunk, they have developed what they call the Aerial Elephant Trunk (AET). It is a continuum manipulator — meaning that it can move any which way — that does not pack on the pounds to support all of those degrees of freedom. These features make the AET practical for real-world missions, and also capable of performing precise manipulations.
Unlike traditional rigid-link robotic arms, the AET uses a tendon-driven continuum design. This design allows the arm to bend and twist like a living appendage, giving it nearly infinite degrees of freedom. Its compliance allows it to maneuver through complex environments like pipelines or wreckage-filled disaster sites. Just like an elephant’s trunk, it can wrap around, grasp, and manipulate objects of various shapes and sizes, from rigid beams to flexible cables.
To solve the weight issues that plagued past designs, the AET’s motors are placed outside of the arm, significantly reducing its inertia. This clever design choice means that even lightweight quadrotors can carry the AET without sacrificing much flight time or agility. In tests, the AET demonstrated not only precise grasping and manipulation but also complex aerial maneuvers such as threading through narrow gaps and winding around irregularly shaped objects.
To support the control system, the team developed a suite of algorithms for state estimation, shape prediction, and whole-body motion planning. These include the use of an Extended Kalman Filter for drone position tracking, as well as a minimum-jerk planning approach for smooth, predictable movement. Furthermore, the researchers overcame the common problem of tendon slacking, which plagues most tendon-driven systems, by designing a lightweight, yet robust actuation system.
In the future, AET could help clear debris from power lines, maintain high-altitude structures like cross-sea bridges, or assist in rescue missions by accessing areas too dangerous or tight for humans. Using this novel manipulator, UAVs may soon evolve from passive observers to active agents.