Breakthrough in research to save multicopters from crashing

Our wise advice? Please make sure that your drone does not get entangled in a knotty situation that will leave it with a dead rotor.

Because chances are, you will lose it.

Or that was the situation before researchers at Beihang University in China found a way to make sure that a quadcopter flying on only three propellers finds a way to land without crashing and shattering into several fragments.

The researchers, whose area of focus is aeronautical and astronautical research, designed an algorithm to stabilise the drone and keep it flying autonomously after one, two or even three rotors suddenly give out.

But why the focus on multirotor drones?

Because these drones usually have nothing protecting them in case of equipment failure will they are still airborne. There are unmanned aerial aircraft systems like those made by Wingcopter and Avy that have redundancy systems that will automatically kick in to protect the aircraft from crashing if a rotor or a fixed wing fails.

Multirotor drones do not enjoy the luxury of redundancy. Readers might remember our article about the a DJI Phantom drone that got intercepted by an eagle and ended up at the bottom of Lake Michigan.

When a multirotor loses one of its propellers, the high-speed rotational motion will cause controllers to fail and once it starts spinning, the drone can no longer estimate its position in the sky and eventually crashes.

The new software developed by the Beihang researchers will ensure that the drone at least lands safely should one or more of its rotors give out.

To be fair though, this is not the first research into what engineers call fault tolerance – the ability of a system to maintain proper operation in the event of failures or faults in one or more of its components.

The gist of fault tolerance is that if a system’s (in this case a multirotor drone) operating quality decreases at all, the decrease is proportional to the severity of the failure, as compared to a naively designed system, in which even a small failure can lead to total breakdown.

Before focus turned to drones, fault tolerance was particularly sought after in high-availability, mission-critical, or even life-critical systems.

The ability of maintaining functionality when portions of a system break down is referred to as graceful degradation. Thus; researchers into UAS are looking to ensure the graceful degradation of quadcopters that would have lost propelling power.

“When one rotor fails, the drone begins to spin on itself like a gyro,” said lead researcher Quan Quan, a professor at the university.

Professor Quan’s team solved the issue by bypassing the conventional approach of controller switching and instead employed a sophisticated technique known as “uniform passive fault-tolerant control.”

This study proposes a uniform passive fault-tolerant control (FTC) method for a quadcopter that does not rely on fault information subject to one, two adjacent, two opposite, or three rotor failure.

The uniform control implies that the passive FTC is able to cover the condition from quadcopter fault-free to rotor failure without the need for controller switching. To achieve the purpose of passive FTC, the fault of rotors is modelled as a lumped disturbance acting on the virtual control of the quadcopter system.

The estimated disturbance is used directly in the passive FTC. At the same time, a modified controller structure is designed to achieve the passive FTC ability for two- and three-rotor failure.

To avoid the control allocation switching from the fault-free control to the FTC, a dynamic control allocation is used. In addition, the closed-loop stability is analysed in the presence of up to three-rotor failure.

To validate the proposed uniform passive FTC method, outdoor experiments are performed for the first time, which have demonstrated that the hovering quadcopter is able to recover from one rotor failure using the proposed controller and resume its mission even if two adjacent, two opposite, or three rotors fail, without the need for any rotor fault information or controller switching.

The paper, which appeared in the international journal IEEE Transactions on Robotics and has a lot of equations in it, reads in part.

The algorithm will enable the onboard computer to control the drone as it flies and spins, even if three propellers fail.

As further explained by Professor Quan; “The algorithm we have developed enables a functional propeller to generate the entire lift of the drone after it starts spinning — it’s akin to engaging in a game of table tennis where an adept player possesses the ability to serve, dash across to the opposing side, and skilfully catch the ball, thus seamlessly completing the game single-handedly.”

Ke Chenxu, a primary participant in the research and a Ph.D. student at the university, said that the method can be applied to multi-rotor drones with six or eight rotors.

These findings could hold a significant key in salvaging multicopter drones, which are the most widely used drones in the world for various important commercial and humanitarian applications; but whose only saving grace thus far had been a parachute mounted onboard to help with crash landings.


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