A universal strategy for visually guided landing

Abstract

Landing is a challenging aspect of flight because, to land safely, speed must be decreased to a value close to zero at touchdown. The mechanisms by which animals achieve this remain unclear. When landing on horizontal surfaces, honey bees control their speed by holding constant the rate of front-to-back image motion (optic flow) generated by the surface as they reduce altitude. As inclination increases, however, this simple pattern of optic flow becomes increasingly complex. How do honey bees control speed when landing on surfaces that have different orientations? To answer this, we analyze the trajectories of honey bees landing on a vertical surface that produces various patterns of motion. We find that landing honey bees control their speed by holding the rate of expansion of the image constant. We then test and confirm this hypothesis rigorously by analyzing landings when the apparent rate of expansion generated by the surface is manipulated artificially. This strategy ensures that speed is reduced, gradually and automatically, as the surface is approached. We then develop a mathematical model of this strategy and show that it can effectively be used to guide smooth landings on surfaces of any orientation, including horizontal surfaces. This biological strategy for guiding landings does not require knowledge about either the distance to the surface or the speed at which it is approached. The simplicity and generality of this landing strategy suggests that it is likely to be exploited by other flying animals and makes it ideal for implementation in the guidance systems of flying robots.rchestrating a safe landing is one of the greatest challenges for flying animals and airborne vehicles alike. Although some progress has been made toward unraveling the cues that flying animals might use for triggering landings (1-10), we do not yet have a good understanding of how these or other possible cues are used to control the landing process once it has been initiated.To achieve a smooth landing, it is essential to control deceleration in such a manner that the approach speed decreases to a value close to zero at the time of touchdown. An obvious way to achieve this would be to measure flight speed and distance to the target simultaneously and to use this information to reduce speed progressively, in a moment-to-moment fashion. However, this strategy is computationally demanding and unsuitable for animals such as flying insects, whose close-set, fixed-focus eyes prevent them from using stereopsis or accommodation to measure the distances to surfaces directly (11-13).When performing a grazing landing on a horizontal surface, honey bees use a technique that allows them to overcome the limitations of their relatively simple nervous systems. Instead of measuring the distance to the surface directly, they hold constant the magnitude of optic flow (the speed of image motion on the retina) that is generated by the ground beneath them (4, 5). This automatically ensures that the speed of flight is reduced as the ground is approac...

Science, technology and the future of small autonomous drones

“…To safely fly in cluttered environments, insects instead rely on image motion, also known as optic flow 38,39 , generated by their own displacement relative to the surroundings 40 . It has been experimentally shown that their neural system reacts to optic flow patterns 41,42 to produce a large variety of flight capabilities, such as obstacle avoidance 40,43 , speed maintenance 44 , odometry estimation 45 , wall following and corridor centring 46 , altitude regulation 47,48 , orientation control 49 and landing 50,51 . Optic flow intensity is proportional to the distance from objects only during translational movements, but not during rotational movements when it is proportional to the rotational velocity of the agent.…”

Touchdown to take-off: at the interface of flight and surface locomotion

“…Exploiting the principles of claws, van der Waals forces and wet adhesion, animals have evolved to generate the required attachment forces (figure 2) [60][61][62][63][64][65][66][67][68]. This enables animals to negotiate and exploit complex surfaces with a combination of effective aerial approaches, contact strategies, surface locomotion techniques and take-off manoeuvres of which the dynamics are not well understood [1,[69][70][71][72][73][74][75][76][77][78][79][80][81][82][83][84][85]. By contrast, aerial robots are just starting to implement some of these successful perching and locomotion strategies.…”

“…All of these animals depend on their eyes for situational awareness and navigation [69,70,[82][83][84]97], which is enhanced by echolocation in many bats and some cave dwelling birds [98,99]. The majority of the flying animals rely thus on vision to land on a surface, and their visual feedback allows them to slow before perching.…”

“…Several birds, including pigeons and hummingbirds, have been shown to keep the time derivative of t approximately constant during the approach [69,70]. Similarly, insects, including flies and bees, have been shown to use image expansion and optic flow on the retina to trigger deceleration, though with less precision [82][83][84].…”

Wind alters landing dynamics in bumblebees

“…Previous studies in honey bees (Apis mellifera) and fruit flies (Drosophila melanogaster) have demonstrated that the visual system plays an important role in controlling approach speed to landing surfaces (Evangelista et al, 2010;Srinivasan et al, 2000), as well as triggering key landing behaviors (van Breugel and Dickinson, 2012). In particular, recent work has shown that insects regulate flight speed during landing by the relatively simple strategy of maintaining a constant rate of image expansion (Baird et al, 2013), causing flight speed to decrease smoothly to zero as the landing target draws closer.…”