Foldable Drone Changes Its Shape in Mid-Air

Foldable Drone Changes Its Shape in Mid-Air

Quadrotors are fast, cheap, and capable, and they’re getting smarter all the time. Where they struggle a little bit is with adaptation. Many other kinds of robots can change their structure to better perform different tasks: Humanoids do it all the time, with all those conveniently placed limbs. Hey, wouldn’t it be cool if drones had movable limbs too? Yes, it would. Someone should figure out how to do that.

We’ve seen some drones in the past that can alter their shape in flight, but this Folding Drone (developed by roboticists from University of Zurich and EPFL) is different in several important ways. Each of its four arms has a servo motor at the base that can rotate one propeller independently, with some height differences between the arms making sure that the Folding Drone doesn’t immediately blenderize itself. While there are some arm location combinations that are particularly handy, like completely unfolded (“X” morphology), completely folded (“O”), straight line (“H”), and partly folded (“T”), the drone isn’t limited to those shapes, and it remains fully stable and controllable wherever its arms happen to be, even if the configuration is asymmetric. It’s not easy to do this—it requires “an adaptive control scheme able to cope in real-time with the dynamic morphology of the vehicle.”

While the Folding Drone doesn’t have quite as many degrees of freedom as that crazy flying dragon robot from ICRA, it’s much less complex and expensive, and can transform very quickly, as you can see in the video. It may not be able to wrap itself around anyone’s neck and slowly strangle them, but we’ll let that slide just this once. The (relatively) simple design helps the Folding Drone maintain both efficiency and versatility, allowing it to operate autonomously with onboard sensing and computing. It doesn’t sound like the drone can autonomously decide how to reconfigure itself to get past an obstacle yet, but the researchers are definitely working on that.

They’re also working on some other cool things, and for more about that, we spoke with Davide Scaramuzza at the University of Zurich via email.

IEEE Spectrum: Why is a drone that transforms like this better than a drone that’s simply small enough to pass through small gaps?

Davide Scaramuzza: Small drones have several disadvantages: They are less stable in case of external disturbances (e.g., wind), they have very limited flight time, and cannot transport large payloads.

Why did you choose these particular degrees of freedom for the transforming ability?

We chose these degrees of freedom because they allowed us not to increase the complexity of the vehicle both from in terms of mechanical design and from a control perspective. At the same time, rotating the arms around the main body leads to very compact shapes, reducing the overall size of the platform significantly.

What are the disadvantages of a drone that can morph?

The main disadvantage of morphing is the potential reduction of stability and flight time compared to the standard X configuration. Some morphologies are significantly less stable than others, and reduce the flight time significantly. Therefore, morphing comes at the cost of sacrificing this two aspects. The amount of such sacrifice depends on the desired morphology. For example, the O morphology allows the robot to become very compact (each dimension becomes roughly 20 percent smaller than in the X morphology), but can produce lower angular accelerations (which translates into less robustness against external disturbances) and a flight time approximately 60 percent lower than in the X shape.

The T and H configurations, instead, are capable of providing higher angular accelerations around one of the body axes, but sacrifice their agility around the other axis. This shows that the standard X morphology is the most efficient and therefore should be used as long as a different morphology is not strictly required by the task at hand.

The paper mentions that you’re working on “exploitation of the morphology for improved flight at high-speed, and novel, bio-inspired mechanical designs.” Can you elaborate?

At the moment, we exploit the morphing to adapt the vehicle’s size to tasks such as traversing gaps, inspecting surfaces or transporting objects. However, we believe that a morphing quadrotor can tailor its shape to more dynamic tasks than those shown in the paper, as for example flying at high speed, where it can improve its performance by folding to change its aerodynamic properties. This would allow very fast flight in time-critical scenarios, allowing the vehicle to navigate through large areas in very little time.

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