# Drone quadracopters throw and catch inverted pendulum

## Let the spear-throwing killbot FUD commence

Kids' stuff … now that we have the evidence below of drones throwing and catching an inverted pendulum.

That trick has been accomplished by boffins at ETH Zurich’s Institute for Dynamic Systems and Control, where a “Flying Machine Arena” exists and is used to train flying 'bots.

The Institute and Arena scored plenty of attention back in 2011 with this video of quadracopters juggling, after a fashion.

Now Masters student Dario Brescianini has taught the drone 'quadracopters an even tougher trick: throwing and catching inverted pendulum, essentially a pole with its centre of mass above its pivot point. Because the inverted pendulum used has two weights on its mid-section, its motion is rather more complex than that of a ball. Teaching a quadracopter to toss and catch one is therefore a rather harder problem.

Here's how the team solved it, according to Robohub:

“First, a state estimator was used to accurately predict the pendulum’s motion while in flight. Unlike the ball used in the group’s earlier demonstration of quadrocopter juggling, the pendulum’s drag properties depend on its orientation. This means, among other things, that a pendulum in free fall will move sideways if oriented at an angle. Since experiments showed that this effect was quite large for the pendulum used, an estimator including a drag model of the pendulum was developed.”

“Second, a fast trajectory generator was needed to quickly move the catching quadrocopter to the estimated catching position.”

“Third, a learning algorithm was implemented to correct for deviations from the theoretical models for two key events: A first correction term was learnt for the desired catching point of the pendulum. This allowed to capture systematic model errors of the throwing quadrocopter’s trajectory and the pendulum’s flight. A second correction term was learnt for the catching quadrocopter’s position. This allowed to capture systematic model errors of the catching quadrocopter’s rapid movement to the catching position.”

The results can be seen in the video below, which at 1:34 features a puff of smoke as the flour-filled bag that is the pendulum’s rudimentary shock absorber bursts.

Watch Video

The video is undoubtedly impressive, but it is also worth noting that military drones have vast area viewing and target acquisition capabilitie. BAE's "Gorgon Stare", for example, can track multiple objects within a 4KM radius beneath a drone.

But as any modern story story about semi-autonomous flying vehicles has to worry about cheaper models and how they will fill suburbia with privacy invading aerial cameras, we'll do so by imagining a pendulum with a sharp tip and quadracopters capable of identifying a human target at which to lob them.

Were such a rig to be developed, and it doesn't look like a massive stretch given the ingenuity on display, we'd have flying, spear-throwing, killbots to contend with. Which makes even sharks with lasers look kind of lame. ®

Thanks to Jack Clark for the Gorgon tip.