Gauging distance with one eye is tricky. That also applies to robots with one camera and to bees with eyes set right next to each other. By understanding more precisely how drones land, researchers have now also gained more insights into bee behaviour. Guido de Croon of the Micro Air Vehicle lab at the faculty of Aerospace Engineering published on this topic last January.
If you approach an object with only one eye open, you can see it grow bigger, but you don’t know exactly how close you are to that object. As you approach, the object’s apparent size grows more quickly; this is also known as optical flow. By keeping this optical flow constant, a drone automatically slows down as it gets closer to its objective.
Researcher Dr Guido de Croon wanted to use a constant optical flow to make drones land automatically, but that turned out to be trickier than expected. “This study originated from frustration, because it turned out to be impossible to execute a fast, smooth landing using optical flow. By the end of the approach, the drones kept on oscillating up and down”, De Croon explains. “At first I thought that the imaging software did not perform well enough close to the ground, but later on I discovered that the effect was also present when using perfect imaging.”
Theoretical analyses showed that the oscillations were caused by the much greater effect on the optical flow from movements close to the ground than from a greater distance. This led to a new angle for De Croon: he decided to make use of the apparently inevitable oscillations as an indicator to initiate the final landing. De Croon: “What I like so much about it is that the robot actually uses the instability of its own control system as a way to gauge distances, so that it can determine when to shut down the propellers, for instance. Over the past months, people have been giving me odd looks whenever I cheered in the lab because a flying robot was on the verge of losing control”, De Croon says.
By changing the amplification during the descent, the drone will approach the ground automatically while oscillating until it shuts down its engines at a height of a few decimetres and then plops onto the mat. And that looks suspiciously similar to what bees do: they also hover briefly at a certain distance from their landing spot. The new theory that De Croon and his co-workers at the MAV lab have published in the journal Bioinspiration & Biomimetics (7 January 2016) offers a hypothesis on how and why a bee behaves like that.