How flying insects and drones can discern up from down

Scientists have developed a theory that can explain how flying insects determine the gravity direction without using accelerometers. It also forms a substantial step in the creation of tiny, autonomous drones. 

Scientists have discovered a novel manner for flying drones and insects to estimate the gravity direction. Whereas drones typically use accelerometers to this end, the way in which flying insects do this has until now been shrouded in mystery, since they lack a specific sense for acceleration. In an article published today in Nature, scientists from TU Delft, the Netherlands, and Aix Marseille Université / CNRS, France, have shown that drones can estimate the gravity direction by combining visual motion sensing with a model of how they move. The study is a great example of the synergy between technology and biology. 

On the one hand, the new approach is an important step for the creation of autonomous tiny, insect-sized drones, since it requires fewer sensors. On the other hand, it forms a hypothesis for how insects control their attitude, as the theory forms a parsimonious explanation of multiple phenomena observed in biology. 

The importance of finding the gravity direction

Successful flight requires knowing the direction of gravity. As ground-bound animals, we humans typically have no trouble determining which way is down. However, this becomes more difficult when flying. Indeed, the passengers in an airplane are normally not aware of the plane being slightly tilted sideways in the air to make a wide circle. When humans started to take the skies, pilots relied purely on visually detecting the horizon line for determining the plane’s “attitude”, that is, its body orientation with respect to gravity. However, when flying through clouds the horizon line is no longer visible, which can lead to an increasingly wrong impression of what is up and down – with potentially disastrous consequences.

Also drones and flying insects need to control their attitude. Drones typically use accelerometers for determining the gravity direction. However, in flying insects no sensing organ for measuring accelerations has been found. Hence, for insects it is currently still a mystery how they estimate attitude, and some even question whether they estimate attitude at all. 

The theoretical analysis in the article shows that finding the gravity direction with optic flow works almost under any condition, except for specific cases such as when the observer is completely still. “Whereas engineers would find such an observability problem unacceptable, we hypothesize that nature has simply accepted it”, says Guido de Croon. “In the article we provide a theoretical proof that despite this problem, an attitude controller will still work around hover at the cost of slight oscillations – reminiscent of the more erratic flight behavior of flying insects.”

”We expect that novel biological experiments, specifically designed for testing our theory will be necessary for verifying the use of the proposed mechanism in insects”, adds Franck Ruffier.

The article shows how the synergy between robotics and biology can lead to technological advances and novel avenues for biological research.