A Dutch researcher has come up with a new theory that will allow drones to estimate distances with a single camera using similar methods to flying insects.
Although insects such as honey bees have two-facet eyes, they are so close together that the bees cannot use stereo vision to estimate distances for navigation purposes. They therefore rely heavily on the ‘distanceless’ cue of optical flow which looks at the speed with which objects move through a field of view.
Assistant Professor Guido de Croon from TU Delft’s Micro Air Vehicle Laboratory has found that drones approaching an object with this vision strategy become unstable at a specific distance from the object. Turning this weakness into a strength, drones can actually use the timely detection of that instability to estimate distance.
“This research was born of frustration with being unable to recreate fast, smooth optical flow landings. The drones would always start to oscillate up and down close to the end of the landing,” said de Croon.
“At first I thought it had to do with issues such as the computer vision algorithms not working well enough when close to the ground, but later I discovered that the effect is still there even when you have perfect vision.”
He said theoretical analysis of drone control laws showed that a robot that is trying to keep optical flow constant starts to oscillate at a specific distance from the landing surface. The oscillations are induced by the robot itself, because movements close to the ground have a much larger and faster effect on optical flow than at greater distances. The key idea, then, is that timely detection of such oscillations tells the drone how far it is from the surface.
“What really makes me smile is that the robot exploits the oncoming instability of its control system to see distances so that it can, for instance, determine when to switch off its propellers. The last few months, I’ve been receiving strange looks in the flight arena while cheering a flying robot that seemed to be on the verge of losing control,” says de Croon.
A closer look at the biological literature showed that flying insects do exhibit some behaviours that are triggered at specific distances, for example honey bees always start to hover at a certain distance from a landing surface. The new theory provides a hypothesis as to how insects might see the distances that induce such behaviours.
Drone image courtesy: TU Delft