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New algorithm makes quadrocopters safer

Researchers at ETH Zurich have developed a novel control algorithm that allows quadrocopters to continue to fly in spite of multiple motor or propeller failures. This algorithm makes these vehicles safer and may allow them to be used for instance in delivery services.

Quadcopter flying with only three propellers  
Even after loss of a propeller, a quadrocopter remains airborne thanks to a new control algorithm. (Photo: Mark Mueller / ETH Zurich)

Drones like quadrocopters have become very popular with hobbyists due to their simplicity and agility. But they are also increasingly being used for commercial applications such as aerial photography and cinematography, inspection and transportation of cargo. Only a few days ago, Amazon announced plans for a new Prime Air service using drones to deliver small packages. Since then the US package delivery company UPS has announced similar plans.

One of the primary concerns currently limiting the widespread use of quadrocopters is safety. At the moment regulations severely restrict the commercial use of drones in most countries, including the EU and the US. However, regulators including the Federal Aviation Administration in the US are presently drafting suitable rules for civil unmanned aircraft systems potentially allowing drone delivery services like Amazon's Prime Air to become commercially viable as soon as 2015. When Jeff Bezos, CEO of, recently presented their research in using flying machines for package delivery, he pointed out one of the major concerns: "Look, this thing can’t land on somebody’s head while they’re walking around their neighborhood".

If a quadrocopter loses a propeller, it can no longer control its flight with the normal control software, and may crash in a public area. ETH researchers have now developed a failsafe technology that makes quadrocopters safer and may thus pave the way for these vehicles to be used in safety-critical applications.

Safety is the key to future operations

Sequence: Quadrocopter loss of propeller and stabilization  
(A) shows the quadrocopter in normal operation. In (B) the propeller detaches due to vibrations, and the quadrocopter starts pitching over in (C) - (E). In (F) the vehicle has regained control, and is flying stably. (Photo: Mark Mueller / ETH Zurich)

"When our new software detects a propeller failure, it uses the remaining propellers to cause a continuous rotation of the vehicle,” explains Mark Mueller, doctorate student at the Institute of Dynamic Systems and Control of ETH Zurich. The failsafe algorithm then controls the vehicle's movement by tilting this rotation, and varying the total produced thrust of the remaining propellers.

Until now, designers have had to use more than four propellers (for instance six propellers for a hexacopter, and eight for an octocopter) to allow the vehicles to survive the loss of a propeller. But hexa- and octocopters tend to be less efficient than quadrocopters, because each additional propeller adds structural weight, complexity, and increases the probability of a single motor failing.

Easy retrofit for multicopters

"Our failsafe technology requires no additional hardware, it's a software-only upgrade. This means that it could be easily implemented on existing multicopters." says Mueller. "The software allows multicopters to remain airborne and perform a controlled emergency landing with as little as three, two, or even just one motor intact."

The technology builds on, and extends, previous results from the Flying Machine Arena - a research testbed at ETH Zurich. Here, the researchers continuously work on developing new capabilities for flying machines, with a specific focus on quadrocopters.

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  • Tim 10 December 2013 13:39

    That is pretty impressive. As a quad pilot I do appreciate the complexity behind this undertaking. Will the algorithm be released to the public domain so that open source developers can benefit from it?

    • Mark Mueller 19 December 2013 13:06

      Dear Tim, thank you for your interest. We have applied for patent protection for the system, and are interested in commercial aspects. The algorithm itself is, however, described in a publication, which we expect to be published in June of next year.

  • Felix Rising 6 January 2014 08:05

    At least mathematical algorithms aren't patentable. It would be good to see a commercial compatible OSS license utilised wherever sample code is published.

  • Thomas Lewis 14 March 2014 22:19

    This could help develop smaller 4 rotor single piloted aircraft,that are not only cheap to build and fly,but extremely safe.With the addition a ballistic chute,it could be one of the safest aircraft ever built.

    • mijewen 22 August 2014 23:50

      Hm, the pilot would certainly get pretty dizzy and disorientated!

  • Nils Stolzlechner 14 June 2014 18:19

    Hi Mark, when will the software upgrade be available for downloads to upgrade the software on the flight controllers such as the one for DJ?

  • mijewen 23 August 2014 00:06

    I had an idea some time ago for recovery of a quadcopter that has lost one motor - and it doesn't involve spinning the craft.
    If the F=Front and R=Rear, L=Left and R=Right, then the four corners are FL, FR, RL, RR. On my little Hubsan, the clockwise motors are on FL and RR. If this was changed so that FL and FR were clockwise and RL and RR were anti-clock, then if FL died (for example), the craft could be hung, without rotating, on FR and RL, with RR used to balance the craft by spinning it clockwise or anticlockwise. Of course, unless the two remaining motors had the capacity to support the craft, there would be a rate of fall, and the objective would be to get onto the ground ASAP, but with two motors working at full throttle, the craft should be able to get down without major damage. This solution also would be purely a firmware reload.

    Losing a motor in flight is certainly unfortunate, but losing more than one could be seen as careless.

  • Fabrizio Schiano 2 February 2015 11:25

    Will it be published at the next ICRA? Or which conference?

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