“As the crow flies” is a common idiom referring to the shortest distance between two points, but the Laboratory of Intelligent Systems (LIS), led by Dario Floreano, in EPFL’s School of Engineering has taken the phrase literally with RAVEN (Robotic Avian-inspired Vehicle for multiple ENvironments). Designed based on perching birds like ravens and crows that frequently switch between air and land, the multifunctional robotic legs allow it to take off autonomously in environments previously inaccessible to winged drones.
“Birds were the inspiration for airplanes in the first place, and the Wright brothers made this dream come true, but even today’s planes are still quite far from what birds are capable of,” says LIS PhD student Won Dong Shin. “Birds can transition from walking to running to the air and back again, without the aid of a runway or launcher. Engineering platforms for these kinds of movements are still missing in robotics.”
RAVEN’s design is aimed at maximizing gait diversity while minimizing mass. Inspired by the proportions of bird legs (and lengthy observations of crows on EPFL’s campus), Shin designed a set of custom, multifunctional avian legs for a fixed-wing drone. He used a combination of mathematical models, computer simulations, and experimental iterations to achieve an optimal balance between leg complexity and overall drone weight (0.62kg). The resulting leg keeps heavier components close to the ‘body’, while a combination of springs and motors mimics powerful avian tendons and muscles. Lightweight avian-inspired feet composed of two articulated structures leverage a passive elastic joint that supports diverse postures for walking, hopping, and jumping.
“Translating avian legs and feet into a lightweight robotic system presented us with design, integration, and control problems that birds have solved elegantly over the course of evolution,” Floreano says. “This led us to not only come up with the most multimodal winged drone to date, but also to shed light on the energetic efficiency of jumping for take-off in both birds and drones.” The research has been published in Nature.
Better access for deliveries or disaster relief
Previous robots designed to walk have been too heavy to jump, while robots designed to jump did not have feet suitable for walking. RAVEN’s unique design allows it to walk, traverse gaps in terrain, and even to jump up onto an elevated surface 26 centimeters high. The scientists also experimented with different modes of flight initiation, including standing and falling take-off, and they found that jumping into flight made the most efficient use of kinetic energy (speed) and potential energy (height gain). The LIS researchers teamed up with Auke Ijspeert of EPFL’s BioRobotics Lab, and with Monica Daley’s Neuromechanics Lab at University of California, Irvine, to adapt bird biomechanics to robotic locomotion.
In addition to elucidating the costs and benefits of powerful legs in birds that frequently transition between air and ground, the results offer a lightweight design for winged drones that can move on rough terrain and take off from restricted locations without human intervention. These capabilities enable the use of such drones in inspection, disaster mitigation, and delivery in confined areas. The EPFL team is already working on improved design and control of the legs to facilitate landing in a variety of environments.
“Avian wings are the equivalent of front legs in terrestrial quadrupeds, but little is known about the coordination of legs and wings in birds – not to mention drones. These results represent just a first step towards a better understanding of design and control principles of multimodal flying animals, and their translation into agile and energetically efficient drones,” Floreano says.
